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International Journal of Mass Spectrometry 359 (2014) 44– 54

Contents lists available at ScienceDirect

International Journal of Mass Spectrometry

jou rn al h om epage: www.elsev ier .com/ locate / i jms

ntracluster reactions in negatively charged aggregates ofiquaternary amines – Gemini surfactants with bromide and formateounterions

oguslaw P. Pozniak ∗, Edyta Kuliszewska1

nstitute of Heavy Organic Synthesis, Kedzierzyn-Kozle, Poland

r t i c l e i n f o

rticle history:eceived 1 October 2013eceived in revised form6 December 2013ccepted 19 December 2013vailable online 25 December 2013

eywords:on-covalent clustersurfactant aggregatesnergy-resolved mass spectrometryemini surfactants

a b s t r a c t

Three series of the gemini surfactants (diquaternary amines) anionic clusters with formate or bromideanions were studied in the gas phase by ER-MS on triple quadrupole mass spectrometer. The gemini serieswere: alkanediyl-�,�-bis-(N,N-dimethyl-N-dodecyl ammonium), alkanediyl-�,�-bis-(N-hydroxyethyl-N-methyl-N-dodecyl ammonium) and oligo(oxa)ethyl-�,�-bis-(N,N-dimethyl-N-dodecyl ammonium),where the alkyl spacer size was from 2 to 12 methyl groups, and the oxaethyl range was from 2 to 8units. The clusters were formed by one dication and three anions with formulas: [MBr3]−, [MBr2HCOO]−,[MBr(HCOO)2]− and [M(HCOO)3]−. Collisions induced internal reactions: nucleophilic substitutions SN2at nitrogen �-carbons, eliminations E2, and hydrogen transfer from hydroxy group to the anion. Byquantitative determination of the amounts of fragments in each dissociation channel and by plottingthe ratios in function of the spacer lengths it was found that the clusters with short and long spacersformed two distinct reaction patterns. By comparisons to the cationic cluster reactions it was found that

ntracluster reactionsiquaternary ammonium salts

crowding inside the anionic cluster caused by the extra anions makes them to reacts through pathwaysthey avoided in cationic clusters. Mixed anion clusters were determined to produce the same sets offragments as homogeneous clusters in the amount which roughly corresponds to the increment of a givenanion; on these bases, it was decided that anions are mobile in the cluster prior to dissociation reaction.When ethyloxy group in the mixed cluster was one of the nitrogen substituents a slight preference was

duct

toward formate anion pro

. Introduction

Surfactants has been studied by mass spectrometry since thencept of the modern soft ionization techniques [1,2]. Nonetheless,ne [3] of the recent excellent reviews [4,5] stressed a scarcity ofata relating to surfactant assemblies and expressed a surprise thatuch area of great practical importance is left mostly unexploited.

Diquarternary ammonium salts with long alkyl chains are sur-actants. They are called gemini (dimeric) [6] surfactants becausenstead of one they have two ionic head groups connected by anlkyl or other chain, which is called spacer. Gemini surfactants areuperior to common single head surfactants because they deliver

he same surface action with fewer molecules, which is an issuef great technological and environmental significance. The spacerength and its nature determine many of the surfacting properties

∗ Corresponding author at: Institute of Heavy Organic Synthesis “Blachownia”, ul.nergetykow 9, 47-255 Kedzierzyn-Kozle, Poland. Tel.: +48 77 487 3169.

E-mail addresses: pozniak.b@icso.com.pl, boguslawpozniak@yahoo.comB.P. Pozniak), kuliszewska.e@icso.com.pl (E. Kuliszewska).

1 Tel.: +48 77 487 3346.

387-3806/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ijms.2013.12.016

s was observed, which was explained by formation of hydrogen bonding.© 2013 Elsevier B.V. All rights reserved.

[7,8]. Gemini surfactants are dissociated into dications and anions,but multiple ion pairing and other specific interactions betweenions affect the surfacting properties [7–12]. Electrospray ionizationby transferring into the gas phase whole assemblies of anions andcations allows investigating these ionic interactions in the assem-blies in the absence of solution effects [13].

In the previous publication [14] we presented a study of intr-acluster reactions in the cationic clusters which consisted of onebromide or one formate anion and one dication. This publica-tion presents a study of intracluster reactions in anionic clusterswhich consist of three anions and one gemini dication. The study isintended as a stepwise manner approach for identification interac-tions inside bigger assemblies of surfactants and their counterionswhich exist in the liquid phase and at the interphase region [15–17].Although a correlation between gas and liquid phases is not trivialand does not lead to exact answers we hope it is able to give manyhelpful hints. The method of studying the cationic clusters then wasenergy resolved mass spectrometry (ER-MS). The same method is

used now.

An anionic cluster consists of three anions and one dication sothe total charge is negative one. The cluster can be homogeneousthat is consisting of same three anions or heterogeneous, consisting

Journal of Mass Spectrometry 359 (2014) 44– 54 45

otwobFaebaacatpdtct

itsesna

tbim

(

(

;

(

2

tbod

tattalIf(

50403020100

0

200

400

600

800

1000

ch. D

ch. B2/F

ch. C/E

ch. A, ch. B1

inte

nsity

[103 c

ps]

collisio n ener gy [a.u]

Fig. 1. Raw data of fragment ion intensities in function of collision energy. Note:

B.P. Pozniak, E. Kuliszewska / International

f different anions. The list of combinations amounts to four clusterypes: [MBr3]−, [MBr2HCOO]−, [MBr(HCOO)2]− and [M(HCOO)3]−

here M denotes the gemini surfactant dication. In the previ-us study of the cationic clusters the main issue was competitionetween reactions of substitution SN2 and elimination E2 [18,19].or the anionic clusters additional topics are: the influence of extranions on the above competition and competition between differ-nt anions. In the homogeneous anionic cluster there is competitionetween three anions of the same kind but of a different locality. In

mixed anionic cluster there is also competition between kinds ofnions. The heterogeneous anionic clusters allow observing suchompetition directly, unveiling a role of nucleophilicity, basicitynd structure of the anion. Testing the two types of gemini surfac-ant head groups (with or without oxyethyl) is meant to probe thoseroperties even more specifically. Thus, the reactions and break-own curves of the homogeneous anionic clusters are discussed inerms how their internal reactions compare to the correspondingationic clusters and the mixed anionic clusters are discussed inerms of competition bromide vs. formate.

In the previous publication [14] we have shown that differencesn internal reactions of dodecyl and hexadecyl gemini cationic clus-ers are discernible but compared to other factors small, so thistudy is limited to dodecyl chain geminis. It has to be added, how-ver that although the influence of the alkyl chain length on thetudied reactions was subtle the influence of alkyl chain size can-ot be slighted when comparisons to the liquid phase propertiesre to be made, so the issue is put off to a separate study.

Three series of gemini dications were studied. In each of themhe spacer length “n” was systematically varied in the range aselow. Because each element of each series produced four clusters

t amounted to the total of 84 clusters being studied (see supple-entary data).

a) alkanediyl-�,�-bis-(N,N-dimethyl-N-dodecyl ammonium) dimeric cations;C12H25

+N(CH3)2 (CH2)n+N(CH3)2 C12H25; where “n”

is 2, 3, 4, 6, 8, 10 and 12; short name G12-n.b) alkanediyl-�,�-bis-(N-hydroxyethyl-N-methyl-

N-dodecyl ammonium) dimeric cations;C12H25

+N(CH3)(C2H4OH) (CH2)n+N(CH3)(C2H4OH) C12H25

where “n” is 2, 3, 4, 6, 8, 10 and 12; short name G12(EtOH)-n.c) oligo(oxa)ethyl-�,�-bis-(N,N-dimethyl-

N-dodecyl ammonium) cations orC12H25

+N(CH3)2 (C2H4 O)n CH2+N(CH3)2 C12H25;

where “n” is 1, 2, 3, 4, 5, 6 and 7 which corresponds to 5,8, 11, 14, 17, 20 and 23 atoms in the spacer respectively; shortname G12-EO-n.

. Experimental

All gemini samples were the same as in the previous publica-ion [14]. They were synthesized in our lab as described in detailsefore [14]. The samples were carefully checked by fragmentationf doubly charged positive ions that they are pure, that is a givenication has both alkyl chains of equal length.

Surfactant samples were dissolved in 90/10 (v/v) mixture of ace-onitrile and water (both HPLC grade, J. T. Baker – HPLC analyzed)nd diluted to ca. 300 �M, which means they were approximatelywenty times more concentrated than solutions used to producehe cationic clusters. Such high concentration was necessary tossure good signal to noise ratio. Because geminis were crystal-

ized as bromides they naturally formed bromide anionic clusters.n order to produce efficiently formate and mixed anion clustersormic acid (Sigma–Aldrich purris p.a.) was added in big excessfinal conc. ca. 0.5 M). The concentration formic acid caused no

channels A and B1 are equal at all energies (black – ch. A, red – ch. B1, orange – ch.B2, blue – ch. C/E, magenta – ch. D, gray – ch. X1, X2, X3).

other effect than change in the intensity of the formate and mixedclusters. No attempt to quantify this relationship was done at thistime.

All experiments were done on AB Sciex Q-TRAP 4000 serieshybrid quadrupole mass spectrometer, equipped with electrosprayion source. Sprayed liquid was fed with a syringe pump (HarvardApparatus) with 5 �L/min. Flow of spray gas, drying gas temper-ature, entrance potential and declustering potential were set atzeros. All other instrumental settings: spray voltage, drying gasflow, were kept at the lowest possible values to avoid fragmentationof ions in the source.

Ions were selected in the first quadrupole, fragmented in thesecond quadrupole and analyzed in the third one. Collision gas wasnitrogen. Base pressure in the instrument was 0.9 × 10−5 Torr, dur-ing fragmentation it increased to 2.4 × 10−5 Torr. The data wereacquired with automatic energy ramp, typically in steps of 0.1 unitsof energy in the instrument scale. After finding the maximum ofthe intensity of product ions on the energy scale, the extra run wasmade at exactly that energy, typically averaging 100–300 scans.Because the instrument is not calibrated in terms of energy and theinitial energy content in the population of ions is not known, inorder to avoid false numbers or impressions Fig. 1 is drawn in anarbitrary scale.

Fig. 1 presents the idea of data reading. It shows the raw data –intensities of all fragment ions in function of fragmentation energy.Such data were exported to Origin Pro v. 8 graphical software andusing fitting routine (Gauss function) the maximum and its posi-tion on energy axis was determined. The ratios of fragmentationchannels were determined at the energy at which the combinedintensity of all ions shows a maximum. The intensities of the peaksat maxima were determined by integration using seven pointsaround centroid maximum. At the maximum bromide or formateintensities were either zero or very small; that amount was notincluded toward the final ratios. The ion intensity data were notcorrected for NCE (normalized collision energy) [20–23] nor in anyother fashion because the whole concept is in any instance basedon relative comparisons. Although, when the maxima were plottedagainst the spacer size (that is mass of the cluster) they formed astraight line.

Because bromine has two isotopes of almost equal intensity

the intensities of different isotopes were added when necessary.Clusters made of different isotopic compositions of bromine wereexamined. None isotope related instrumental artifacts were found

4 Journal of Mass Spectrometry 359 (2014) 44– 54

(Rts

3

tbcc

cbil

mIaeai[ficaeq

bioglTiAp

3

safmibgt

nfwStOSnaET

N

OH

Y-

Y-

Y-

Y-

ch. B1

ch. B2

ch. C

ch. AN

HOC9H19

C9H19

++

N

OY-H

H

H

H

Y-

Y-

Y-

ch. F

ch. G

ch. D

ch. E

N

HO

C9H19

C9H19

++

Scheme 1. Places of attack of nucleophile Y− on the example of G12(EtOH)-4. Upper

6 B.P. Pozniak, E. Kuliszewska / International

the true isotope effect would be within experimental error).epeatability of data was very good, the results agreed from runo run in better than 5%. No degradation of signal due to storage ofamples was observed.

. Results and discussion

In Fig. 2a is shown a mass spectrum of four anionic gemini clus-ers, each made by a different combination of three formate andromide anions. In order to see comparable intensities of formateontaining clusters it was necessary to use hundreds of times higheroncentration of formic acid than the concentration of bromide.

During dissociation of anionic clusters three processes must beonsidered [24–26]: electron detachment, loss of noncovalentlyonded anion and internal rearrangement which may result in

nternal reaction between ions, which next may be followed by aoss of a neutral.

In the dissociation of the cationic clusters the recovery of frag-ent ions in moderate range of energies was practically complete.

n the similar conditions for the anionic clusters the recovery wast best 20% but often even less. This low recovery is due to loss oflectron from the anionic cluster. Electron affinities of small anionsre typically less then bond energies e.g. formate electron bind-ng energy is 80.3 kcal/mol and bromide is even less 77.4 kcal/mol27,28]. However in the case of the cluster the electron is not lostrom the anion alone but from the whole cluster. Electron is bondedn the molecular orbital of the whole cluster so its binding energyan be significantly different; lower or higher than that of a simplenion [24]. There are techniques which allow capturing detachedlectrons and by this to quantify such dissociation channel but ouruadrupole instrument is unfit for this [29].

A decomposition of the anionic cluster into original formate orromide anions has never been observed. Formate and bromide

ons as products of dissociation have always been observed butnly when the collision energy was very high, above typical ener-ies at which internal reaction products were observed. So, mostikely these anions were the products of secondary fragmentations.hey have been easily observed as such in MS/MS experiments orn quasi-MS experiments (first fragmentation is in the ion source).ctually, CID experiments on those ions produced exclusively sim-le bromide or formate anions.

.1. Reaction pathways (channels)

Differences between bromide and formate reaction pathwaystem from their different nucleophilicity and basicity. Bromidenion gas phase basicity is 1331.8 kJ/mol (318.3 kcal/mol) whileormate is more basic 1419 kJ/mol (339.1 kcal/mol) [27,28]. Bro-

ide anion is also expected to be more nucleophilic than formaten which the charge is distributed over two oxygens. It haseen shown before [14,30] that in the cationic clusters of theeminis with all spacer sizes the formate anion prefers elimina-ion.

The list of all observed in the anionic clusters reaction chan-els is in Table 1. There are four (nucleophilic) substitution and

our elimination channels. Names of the channels are consistentith those previously used [14,30]. The reactions are illustrated in

cheme 1; relevant formulas are in reaction Scheme 2, which illus-rates the fragmentation pattern of the most complicated example.ther cases are simpler in that fewer channels are actually present.ubstitution reaction always occurs at �-carbon next to quaternary

itrogen; competing with it is elimination reaction [31–34]. Therere two pairs of reaction channels, which give identical ion product.ach pair is made of one substitution and one elimination channel.he first is formed by channels C and E, the second by channels B2

panel shows four elimination reactions; lower panel four substitution reaction. Forclarity nonparticipating hydrogens are omitted. Nonparticipating anions are alsoomitted.

and F. Although the third pair could be formed by channels A andD it is not, because relevant reactions are taking place at the spacerand this leads to different ion products.

3.1.1. Channels A and D (reactions at spacer; SN2 vs. E2)Distinguishing between elimination and substitution channels

is an old problem of ion-molecule reaction studies. There is no gen-eral method. A number of specific methods based on identificationof neutrals have been tried [18,25,35]. Another and more versatilemethod was developed by Gronert who called it the ionic platformmethod [18,19,36]. One relevant study [37] involved internal reac-tions inside anionic clusters consisting of one doubly charged anionand one tetraalkylammonium singly charged cation. Both substitu-tion and elimination reactions were identified and additionally byisotopic labeling it was possible to establish “the true, concerted E2mechanism”. In our clusters distinguishing between reaction chan-nels A and D is based on a similar to the Gronert’s ionic platformtrick but is made á rebours. The essence is in this that a nucleophileis reacting between the permanent changes of the doubly chargedcation. If the reaction is substitution it leads to a loss of neutral tri-amine, if the reaction is elimination it leads to loss of triamine andacid molecule. In each case the remaining ion has a different mass.One difference from Gronert’s studies is that in our experimentsthe reactive complex does not have to be made by collisional asso-ciation in the gas phase. It is made ready by the electrospray, and assuch is already sitting in the first of the two potential energy wellsaccording to the established double well model of ion-molecular

reactions [38]. The second difference is the reactants in the clustermade by electrospray can have opposite charges, so a true ion-ionreaction is taking place, what is more corresponding to chemistryin the liquid phase.

B.P. Pozniak, E. Kuliszewska / International Journal of Mass Spectrometry 359 (2014) 44– 54 47

820810800790780770760750740730720710700690680670660650m/z, Da

0.0

2.0e7

4.0e 7

6.0e 7

8.0e7

1.0e 8

1.2e 8

1.4e 8

1.6e8

1.8e8

2.0e8

2.2e 8

2.4e8

2.6e 8

2.8e8

Inte

nsity

, cps

850800750700650600550500450400350300250m/z, Da

0.00

1.00e6

2.00e6

3.00e6

4.00e6

5.00e6

6.00e6

7.00e6

8.00e6

9.00e6

1.00e71.04e7

Inte

nsity

, cps

MBr3 781.6

ch. B1 687.5

ch. B2 657.5

ch. A 538.3

ch. C/E 533.3

ch. X1 440.3 ch. D

458.3 ch. X2 414.2

ch. X3 290.2

MBr3 783.6

MBr2(HCOO) 747.6

MBr(HCOO)2 713.6

M(HCOO)3 677.6

Fig. 2. G12(EtOH)-4. Upper panel – mass spectrum of four anionic clusters. Lower panel – fragmentation spectrum of MBr3 cluster m/z 781 (79Br79Br 81Br) at collisionale on ch

awletta

nergy a little above the maximum. Product ions are marked by names fragmentati

A problem appears, however in an ability to distinguish between true elimination channel (E2) and two fast consecutive reactionshich gives the same ion product as E2: a substitution being fol-

owed by an elimination of an acid. It was often seen that the same

nergy products corresponding to the elimination reaction and tohe substitution reaction were present. To make things worse it wasested by MS/MS that for the cationic clusters [14] elimination of thecid is often a favored fragmentation pathway of the substitution

annels. Both spectra display proper bromine isotopic composition.

product of channel A. Thus the distinction between mechanisms byER-MS method is not airtight. It is based on prima facie results ofthe intensities of the two different ion products. If the eliminationand the substitution products are present at the same collisional

energy and if corresponding ion intensity peaks are fairly close toeach other on the energy scale then the distinction has ground. Ifhowever, the elimination channel ion products appears at visiblehigher energies in ER-MS plots then it should be suspected that it

48 B.P. Pozniak, E. Kuliszewska / International Journal of Mass Spectrometry 359 (2014) 44– 54

Table 1List of all reaction channels and their neutral products. Channel names are consistent with those previously used [14,30]. Channel G is hydrogen transfer but it is included inelimination section.

G12-n, G12-EO-n channel G12(EtOH)-n channel Bromide neutral product(s) Formate neutral product(s)

SubstitutionA – C12H25N(CH3)2 C12H25N(CH3)2

– A C12H25N(CH3)C2H4OH C12H25N(CH3)C2H4OHB B1 BrCH3 HCOOCH3

– B2 BrCH2CH2OH HCOOCH2CH2OHC C BrC12H25 HCOOC12H25

EliminationD – HBr + C12H25N(CH3)2 HCOOH + C12H25N(CH3)2

– D HBr + C12H25N(CH3)C2H4OH HCOOH + C12H25N(CH3)C2H4OH

iot

obfsrTtr

St

– E

– F

– G

s rather a two step process with the second step being eliminationf the acid. Our experimental data were processed accordingly tohis distinction.

It has consequences for ability to tell a difference between ratiosf elimination and substitution for bromide and formate anions,ecause most of the difference is earned by a difference betweenormate favoring elimination channel D and bromide favoring andubstitution channel A. These two are SN2 and E2 reactions which

equire an altogether different geometry of the transition state.hus on the basis of all experimental data [14,30] the statementhat in geminis more basic formate favors elimination should beeplaced by a more cautious conclusion that the overall reaction

cheme 2. Fragmentation pathway of G12(EtOH)-4 homogeneous MBr3 cluster. Formulriamine.

HBr + C12H24 HCOOH + C12H24

HBr + CH3CHO HCOOH + CH3CHOHBr HCOOH

ion product corresponds the elimination reaction ion product butit may or may not be the true E2 process.

Distinguishing between elimination and substitution channelscomplicates further if other factors favoring elimination over sub-stitution are present. It has been determined in Aime’s study [30]that a shorter spacer in geminis favors elimination. Specifically, itfavors channel D over channel A. But the above statement does notadequately stress that a shorter spacer favors all reactions occur-

ring at the spacer between nitrogens. It can be seen from summingup ratios of channels A and D for either anion and plotting the totalin function of spacer size. Considering the issues raised above, whatis being seen experimentally is more the preference for the specific

a(s) of the lost neutral fragment(s) are next to arrows; symbol “ N ” designates

Journal of Mass Spectrometry 359 (2014) 44– 54 49

rse

3

cnbnbocaitbtsit[mT

3

misbelbw

popbrdbptcttnnfbl

3

itwShobolt

0

25

50

75

100

ch. D

ch. A

ch. A

ch. D

ch. C/E

ch. C/E

ch. B

ch. B

ratio

[%]

201612840

0

25

50

75

100M(HCOO )3

MBr3

ratio

n [%

]

numbe r of atoms in sp acer

Fig. 3. Homogenous clusters. Gemini series G12-n (filled symbols) and G12-EO-n(open symbols) fragmentation ratios in percentages of total. Upper panel is for MBr3

cluster, lower panel for formate cluster M(HCOO)3. Symbols and colors: red square– ch. B, black circle – ch. A, blue up-triangle – ch. C/E, magenta down-triangle – ch.

B.P. Pozniak, E. Kuliszewska / International

eaction locus, than a choice of a specific mechanism. Again, it isafer to formulate a simpler conclusion that a short spacer breaksasier regardless.

.1.2. Channels C and E (reactions at alkyl chain)At alkyl chain �-carbon the substitution reaction (channel C) is

ompeting with the elimination reaction (channel E). These chan-els cannot be distinguished; however a following argument cane made. If the spacer is long and the second charge on the otheritrogen is far away and the transition state structure is open (seeelow) then the nucleophile sees alkyl chain side and spacer sidef the nitrogen atom as fairly symmetric. Indeed, for the cationiclusters with spacers longer than six atoms ratios of the reactiont the spacer side and at the alkyl chain side were the same. Thus,f an anion in such symmetric environment reacts through substi-ution on one side of the quaternary nitrogen it is a circumstantialut a sound evidence that it does the same on the other. The abilityo distinguish between substitution and elimination on the spaceride allow us, by analogy, claim that the channel designated as C/Es actually the substitution channel C. When the spacer is shorterhan six atoms such arguments are not valid. Gronert has shown19] that when charges on a doubly charged ion are separated by

ore than 15 A then each reaction center behave independently.he previous [14] and current publication support that.

.1.3. Channels B, F and G (reactions at short substituent)In gemini series G12-n and G12-EO-n both short substituents are

ethyls. There, only the substitution reaction is possible resultingn a loss of BrCH3 (channel B). In the gemini series G12EtOH-n oneubstituent is methyl and the other is oxyethyl. Four reactions cane observed: substitution at methyl (B1), substitution at EtOH (B2),limination at EtOH (F) and transfer of proton to the anion (G). Theast one is an acid–base proton exchange and as such is driven byasicities of donor and acceptor. The elimination (F) is competingith substitution reaction (B2) from which it is not distinguishable.

In the G12EtOH-n cationic clusters with bromide the amount ofroducts at the EtOH side (B2/F) was almost equal to the amountf products at the methyl side (B1), while formate gave exclusivelyroducts of the reaction at the EtOH side (B2/F and G). When forromide sum of reactions of both sides was added it was close toatio of channel B in G12-n. Because the presence of hydroxy groupid not make any change the in the total it was concluded thatromide reacts through substitution but because for formate it theresence of EtOH made such a difference the output was believedo be split into either elimination and hydrogen transfer. This wasredited to greater basicity of formate ion and also and more likelyo formation of strong hydrogen bonding [39]. In the anionic clus-ers, however hydrogen transfer channel (channel G) is practicallyot observed; neither in the homogeneous formate anionic clustersor in the heterogeneous ones. It is not a even a favored reaction

or the geminis with short spacers. The presence of other anionslocks the hydrogen transfer almost completely. This is also most

ikely because of hydrogen bonding (see also below).

.1.4. Channels X1, X2 and X3 (internal ring closing)After an internal reaction in the cluster a product is smaller but

t is still a cluster of ions. All such smaller clusters decomposedhrough a loss of the simple anion. There was only one exception,hich we called then [14] channel X (or X1, X2 and X3; see reaction

cheme 2). The reaction is present [14,18] in geminis with spacersaving exactly four atoms (five is too many) and it occurs whenne of nitrogens, after losing one of the substituents and a charge

ecomes a nucleophile itself. It then attacks the spacer at �-carbonf the second, still quaternary nitrogen forming a ring followed by aoss of triamine; the same triamine as in the channel A fragmenta-ion. This secondary ring closing reaction was described and proven

D. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of the article.)

by Williams’ group work [31]. Here we also see that when nitrogenhas three different substituents (the fourth is the spacer) then thering closing reaction can be a follow up reaction to the loss of anyof them.

In the anionic cluster the ring closing reaction is occurring amida different ionic environment. In the cationic cluster the only avail-able anion (nucleophile) is gone after the first substitution step. Thering closing reaction engages the only available nucleophile, whichis the tertiary nitrogen. In the anionic cluster two other anions arestill present and are potentially available. Thus, without additionalevidence, it can be argued that the observed in the anionic geminiclusters ring closing is a mere second substitution of anion. Suchdoubts are not baseless because a second anion substitution can beobserved. For example, we have observed a sequential attack of twoanions in a different kind of gemini anionic clusters [40], but thenthe reaction was observed for the whole series of clusters with allspacers’ lengths. Because the reaction is observed only in geminiswith spacers of four atoms, which together with nitrogen form anunstrained five membered ring, strongly suggests that it is indeedthe ring closing.

3.2. Homogeneous bromide clusters

The results for the gemini series G12-n and G12-EO-n are sosimilar that they are presented together in Fig. 3. It is clear that thepresence of etheric oxygen in the spacer does not change reactionpathways. When plots for the anionic clusters are compared with

50 B.P. Pozniak, E. Kuliszewska / International Journa

0

25

50

75

100

ch. D

M(HCOO)3

ch. Dch. G

ch. C/E

ch. C/E

ch. B2 /F

ch. B2

ch. A

ch. B1

ch. B1ch. A

MBr3

ratio

[%]

12108642

0

25

50

75

100

ratio

[%]

numbe r of atoms in sp acer

Fig. 4. Homogenous clusters. Gemini series G12(EtOH)-n fragmentation ratios inpercentages of total. Upper panel is for MBr3 cluster, lower is for M(HCOO)3. Symbolsand colors: red square – ch. B1, orange square – ch. B2, black circle – ch. A, blueuit

top

“2sstsbew

act5btoccp

tc

p triangle – ch. C/E, magenta down triangle – ch. D, green diamond – ch. G. (Fornterpretation of the references to color in this figure legend, the reader is referredo the web version of the article.)

hose for the cationic clusters they appear similar, nonetheless a lotf quantitative shifts in ratios can be identified and credited to theresence of extra anions in the cluster.

In all figures from 3 to 6 we can easily notice “short spacer” andlong spacer” reaction regimes. The geminis with short spacers like

or 3 atoms have an entirely different pattern of reactions. Whenpacer grows to 5, 6 or more atoms then the reactions assume ateady pattern which does not change with any further increase ofhe spacer length up to such high values as 23 atoms. A distance ofix methylene groups means that nitrogens are separated in spacey about 7 A. This distance appears to be large enough to liberatenough space around nitrogen so the transition state in a clusterith any long spacer is similar.

In the long spacer reaction regime the anionic clusters produce higher ratio of BrCH3 loss (ch. B) – about 80% then analogousationic clusters in which it was only 50%. In the cationic clusterhe ratio was plainly statistical because two methyl groups made0% of substituent choices. In the anionic the number 80% cannote arrived by any elementary statistics even including the fact thathree anions can attack two reaction centers. The remaining 20%f the total is divided almost equally between channel C/E andhannel A, with a small preference for the former. Their ratios arelose which tells that both �-carbon atoms are on almost equivalent

ositions, as it was in the cationic clusters.

The results for the series G(EtOH)-n are presented in Fig. 4. Forhis series also the overall reactivity pattern is similar to the cationiclusters. Among the differences is the ratio of substitution at the

l of Mass Spectrometry 359 (2014) 44– 54

spacer carbon (A) which for the anionic clusters is smaller, but theratio of substitution at alkyl chain (C/E) is greater. The substitutionat methyl group carbon (B1) is favored to a larger extent over thesubstitution at oxyethyl group (B2/F) than it was in the cationicclusters where these two channels were almost equal. Here theyare not, but still the sum of both (B1 and B2) makes 60%, which isthe same amount as in the cationic clusters.

The cationic clusters in the ground state of energy form a saltbridge; the anion is placed between two positive charges on nitro-gens. A fully developed salt bridge [30], the one in which the anglecation–anion–cation is close to 180◦ is possible only when thespacer is long enough, at least 5–6 atoms. For any internal reac-tion in the cationic cluster to occur a dissolution of the salt bridgeis necessary because there is only one anion available and as long asit is bonded within the salt bridge no proper SN2 or E2 configurationcan be achieved. In the anionic clusters there are two other anionswhich can achieve proper reaction configuration, so dissolution ofthe salt bridge is not necessary. This is not an possible for all reac-tions, specifically it is not for E2 reactions. Whether the salt bridgein the transition state is open or not will affect more strongly theelimination reactions: channels D, E and F than substitution reac-tion: channels B and C (channel A is an exception). Thus a biggerratio of channel B and a preference of ch. C/E (alkyl chain) over ch. A(spacer) may be caused by reaction pathways which do not requiredissolution of the salt bridge in the transition state. This is not tosay that the transition state is never open, but that some pathwaysare of this kind and this effect shifts the experimental ratios towardabove mentioned preferences.

The anionic clusters with the short spacers (2–5 atoms) reactdifferently. A shorter spacer imposes a different geometry in theground state; the salt bridge is bent. However the transition stateconfiguration does not have to be drastically different from that inclusters with longer spacers especially in those cases when in thetransition state the salt bridge is open. In the clusters with shortspacers differences between the cationic clusters and anionic clus-ters in some substitution ratios are very big. In the cationic clustersthe ratio of the substitution reaction at the methyl group (ch. B)was very low, almost zero. But in the anionic clusters this chan-nel is present with ca. 50%. The ratio of the substitution at alkylchain (ch. C/E) is present at ca. 17%, while in the cationic cluster itwas close to zero. The explanation why shorter spacers favor elim-ination given in the Aime and al. work [30] related to the cationicclusters was such that it is the behavior of the reaction enthalpywhich in case of the substitution does not depend on the size of thespacer while in case of the elimination it goes down fast with thesize. This explanation is not sufficient in case of the anionic clusterswhere substitution reactions are visibly present even in clusters ofgeminis with the short spacers. It again suggests that in the anionicclusters there are transition states which do not involve dissolutionof the salt bridge.

In the cationic clusters with short spacers the reactions at thespacer were the dominating ones. Short spacers broke apart eas-ily because of the electrostatic repulsion between positive chargeslocated permanently on nitrogens. In the anionic clusters this repul-sive force is moderated by the presence of extra anions in thecluster, which causes that in the anionic clusters there are alsopresent products of reactions at other nitrogen substituents. A sim-ple electrostatic reasoning tells that when we have two positiveand three negative charges of equal size the minimum of energyis achieved when between charges of the same polarity are placedcharges of opposite polarity; very much the way as simple inorganicsalt crystals are made. For the anionic cluster this would suggest

that in the ground state and in the transition state one anion islocated somewhere between nitrogens and two others are on theopposite ends. The first is well positioned to react by nucleophilicsubstitution at the spacer, but at the same time two other anions can

Journal of Mass Spectrometry 359 (2014) 44– 54 51

rerfttrio

3

nlrapttcee

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3

tmtp

0

25

50

75

100

all Br-

ch. A

all HCOO -

all HCOO -

all Br-

ch. A

MBr(HCOO )2

MBr2(HCOO )

ratio

[%]

201612840

0

25

50

75

100

ratio

[%]

numbe r of atoms in space r

Fig. 5. Mixed clusters. Gemini series G12-n and G12-EO-n. Sum of ratios of all frag-ments coming from a given anion. Channel A is unresolved. Upper panel is forMBr2HCOO clusters; lower panel for MBr(HCOO)2. Symbols and colors: olive square– bromide products, violet diamond – formate products, black circle – channel A. (For

B.P. Pozniak, E. Kuliszewska / International

eact at other substituents, notably at the alkyl chain. Because thelectrostatic repulsion between charges on nitrogens is partiallyeduced by screening presence of the anions the main driving forceor the reaction at the spacer is no longer that strong, so it leadso reduction in ratios related to reactions there. The screening ofhe charges also causes that even in geminis with short spacers theeactive centers around quaternary nitrogens behave to a degreendependently, a feature which in the cationic clusters was presentnly when the spacer was sufficiently long – six atoms or more.

.3. Homogenous formate clusters

The ratios of the formate anionic clusters for the series of G12- and G12-EO-n are plotted in Fig. 3. At first appearance, the plot

ooks similar as for the cationic clusters. Quantitative shifts in theatios follow the trends for the bromide anionic clusters. So againre seen: an increase in the methyl substitution ratio (ch. B), a smallreference toward the substitution at the alkyl chain (ch. C/E) overhe substitution at the spacer (ch. A) and a decrease in reactions athe spacer (ch. A and ch. D). Reduction of the repulsion of the twoharges caused by the presence of extra anions and availability ofxtra anions not bound in the salt bridge are also here offered asxplanation.

However, when we consider the anionic clusters of geminis withtOH group (Fig. 4 lower panel) and compare their ratios with theationic clusters we see that shifts in ratios are a lot bigger. In theationic clusters 100% of the observed reactions of formate anionere at EtOH substituent. It was either elimination (channel B2/F)

r hydrogen transfer to formate anion (channel G). In the anioniclusters still the strongest is channel B2/F, but hydrogen transferch. G) is, surprisingly, totally absent. Substitution channels A, B1nd C/E which were completely absent in the cationic clusters butn the formate anionic clusters they are easily observed.

Such shifts in ratios are attributed to hydrogen bonding betweenormate anion an EtOH group. There are two EtOH groups and threenions. Two anions makes hydrogen bonding to two of the oxyethylroups and the third anion remains unengaged. It is free to reactt either alkyl chain or methyl. Because differences in enthalpiesetween these two substitution reactions are not big [30] bothroducts are almost equally present. That reactivity pattern is pos-ible only when the barrier for a reaction of the nonbonded formates lower than of the one with the hydrogen bond. It also explainshe total absence of channel G (hydrogen transfer). The transfer ofydrogen from OH to HCOO− makes a zwitterion with the chargen oxygen. When the spacer is long the negative oxygen end cane attracted to the second quaternary nitrogen and making also aort of a salt bridge. When there are two other anions this placeor a perspective salt bridge is already taken so it tilts the energet-cs toward elimination at �-carbon of EtOH group and subsequentoss of formic acid and acetic aldehyde. Thus in the formate anioniclusters in addition to sheer crowding of anions affecting electro-tatic interactions, there is also a specific and directional effect ofydrogen bonding, which by saturation of EtOH reaction centersllows for other reactions to occur, those which were absent in theationic clusters, but it disallows for hydrogen transfer, the reactionhannel which was prominently present in the cationic clusters. Inonclusion the hydrogen bonding causes the formate anion in thenionic clusters to substitute instead of eliminating.

.4. Mixed clusters

A mixed cluster contains bromide and formate anions. Such clus-

er forms an environment in which formate and bromide anions

ay compete directly. The reaction products of the mixed clus-ers are all products which were seen before and no new reactionroduct was observed.

interpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)

Noncovalent clusters may exist in many structural isomers. Asshown in the studies of anionic clusters [24,39] the energetic min-ima are often close so in a population of ions many isomers coexist,but when the internal energy is raised the isomers can rearrange.Because we have found no evidence to the contrary we say thatin the anionic gemini clusters there is a similar situation, whichmeans anions are free to rearrange and exchange positions priorto the reaction. The experimental results do not split into series ofresults each corresponding to a given isomer. The amount of bro-mide products from a cluster containing one bromide anion is thesame. We conclude this on the basis of comparison of the ratios ofreactions inside the anionic clusters to those in the cationic clusters.The reaction ratios for the anionic clusters reproduce well those forthe cationic clusters where there is only one anion. The results forthe mixed clusters are presented in Figs. 5 and 6. In these figuresratios of all channels for a given anion are summed up. If the channelratios were not summed up but each channel is plotted separatelyfor a given anion (like channel B bromide, channel B formate, etc.)the plot would look as the plot for the homogeneous cluster onlydivided by a number. For example, if we take only bromide productsin MBr(HCOO)2 and plot those then they would make an appear-ance of a plot for the homogenous bromide cluster divided by 3. Thechannel ratios remain in the same proportions regardless whetherin the cluster is one, two or three bromide ions. Such repetition of

individual increments of channels for each anion is also an argu-ment supporting that the anions are free to exchange positions inthe cluster the prior to the reaction. The other option would require

52 B.P. Pozniak, E. Kuliszewska / International Journal of Mass Spectrometry 359 (2014) 44– 54

0

25

50

75

100MBr2(HCOO )

ratio

[%]

12108642

0

25

50

75

100

ch. A

ch. A

all HCO O-

all HCO O-

all Br-

all Br-

MBr(HCO O)2

ratio

[%]

numbe r of at oms in sp acer

Fig. 6. Mixed clusters. Gemini series G12(EtOH)-n. Sum of ratios of all fragmentscoming from a given anion. Channel A is left unresolved. Upper panel is forMBr2HCOO clusters; lower panel for MBr(HCOO)2. Symbols and colors: olive square– bromide products, violet diamond – formate products, black circle – channel A. (Forinterpretation of the references to color in this figure legend, the reader is referredt

tp

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0

25

50

75

100

M(H COO)3 MBr(HCO O)2 MBr2(HCOO)

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6

4

3

2

6

4

2

G12( EtOH)- n

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0

25

50

75

100

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4

6

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2

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Fig. 7. All clusters. Ratios of all fragments coming from a given anion plotted infunction of cluster composition. Unresolved channel A is also shown. For clarityonly formate products are shown; bromide products are the reminder to 100%.Number next to the line designates the size of the spacer, number 6 means spacercounting six or more atoms. Symbols and colors: violet/purple diamond – formate

ity of cases a nucleophilic substitution at �-carbons of nitrogen’s

o the web version of the article.)

hat the initial population of is made exactly in some very specificroportions for which there is no evidence.

Although the final outcome of the internal reactions in the mixedluster is a linear combination of contributions of reactions tracedo particular anions it does not mean that coefficients of such com-ination are always equal to the number of given anions in theluster. This issue is addressed in Fig. 7. Fig. 7 is a transformationf Figs. 3–6 but plotted from a composition perspective. It com-ares the total output for the formate ion in function of the clusteromposition (bromide ion contribution is the reminder to 100). Thepper panel shows the results for geminis with two methyl groupsn nitrogen G12-n. The line for geminis with spacers longer than

atoms is almost straight, meaning that if there are 2 bromides inhe cluster they will give 2/3 of the reaction products. Such plotsould be made for any separate reaction channel and they wouldook as those in Fig. 7.

It is seen in Fig. 7 (upper panel) that for geminis with a shorterpacer like 2 or 3 atoms there is a preference for the formate anion.f there is at least one formate anion in the cluster its products take

share bigger than its count. This favoring of formate products isven more pronounced in the clusters with EtOH group (Fig. 7 loweranel). Even in the clusters with longer spacers some preference forormate is visible. Because such preference is earned chiefly by theigh presence of channel D (elimination reaction at the spacer) not

y channel G (hydrogen transfer) it rather suggests that it ought toe credited more to its ability to make hydrogen bonding then to

ts higher basicity.

products, black circle – channel A. Upper panel is for geminis G12-n series lowerpanel for G12(EtOH)-n series. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of the article.)

Products of reactions of each anion can be identified by theirunique mass. However in one case – channel A our two anionsproduce the ion product having the same mass but not the samestructure. In one instance bromide is covalently bonded to the car-bon in the other it is formate. I cannot be determined even byMS/MS experiment how much each anion contributes to this chan-nel because such secondary cluster will fragment exclusively withthe loss of the remaining noncovalently boded original anion (bro-mide or formate). This problem however might be addressed in anindirect way by analysis of ratios of products across cluster compo-sition as in Fig. 7 where channel A was plotted separately insteadof being included into a given anion pool. It is seen that regardlessof whether the cluster contains one, two or three formate anionsthe ratio of channel A is almost the same. Only in the homogeneousbromide cluster this ratio is a lot bigger. Such trend means that bro-mide is disfavored in the middle position between two nitrogens;this position is favored by formate anion which then does not reactthrough substitution (ch. A) but through elimination (ch. D). Thisdoes not contradict the statement that anions are allowed to freelyexchange positions prior to the reaction but it only means that acertain position is favored by a given ion.

4. Conclusions

Pathways of the internal reactions in the anionic clusters of gem-ini surfactants are in general nature similar to the reactions insidethe cationic clusters. That is, the reaction is in prevailing major-

substituents. The extra two anions in the cluster do not produceany entirely new reaction pathway but they influence ratios of thereaction channels. Quantitative differences are caused to a large

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B.P. Pozniak, E. Kuliszewska / International

egree by crowding inside the cluster but in part are caused bypecific interactions like hydrogen bonding. Partial screening ofhe permanent charges located on nitrogens makes the electro-tatic interaction less intense and results in a weaker preferenceor the reaction to take place between nitrogens. Presence of twoxtra anions leads to more reactions at the alkyl chain and at theethyl groups. This is suggested to be caused by reaction pathwayshich do not involve dissolution of the salt bridge. In the homo-

eneous formate clusters the extra formate anions are causing thenion to react by substitution, which was the reaction pathwayotally absent or only weakly present in the cationic clusters. Whenromide and formate anions are put in direct competition in theixed cluster they largely preserve the pattern of reaction chan-

els they showed in the homogeneous clusters. A given anion giveshe amount of products as tells its count in the cluster although inhe clusters of short spacer geminis formate anion reactions are ait privileged and the amount of formate anion reaction products

s a little higher than its share. It is suggested that this preserva-ion of the reactivity pattern is caused by mobility of anions in theluster and rapid internal rearrangements prior to reaction and dis-ociation. In the experimental data there is no indication that thenions are not mobile and that each of possible isomeric structuresorms its own set of products. On the contrary it seems that uponctivation an equilibrium is being established and anions in theluster change positions and compete for the reaction site. Thushe method of studying reactions inside the mixed clusters turnedut to be fruitful to compare nucleophilic properties of anions ineactions with cations, thus a true ion–ion reactions which to aegree are more relevant to the solution chemistry.

Conclusions of this study are beyond mass spectrometry. Inemini surfactant studies have been noticed that the size of thepacer is a factor in many surfacting properties. In particular, a fea-ure which matters is how the length of the spacer measures againsthe thickness of the alkyl chain. If the alkyl chain can bent in such

way that it slides between two nitrogens then nucleophilic prop-rties are diminished. It has been shown in this work that suchredictions can be related and traced by reactivities inside clus-ers, by demonstrating that there is a clear division between shortnd long spacer reactivity patterns, where the division falls at aboutve atoms in the spacer. No further extension of the spacer beyondix atoms changes the reactive properties. Also the similarity ofhe results for series G12-n and G12-EO-n is meaningful for surfac-ant chemistry, because the similarity of methylene chain behavioro ethoxy kind of chain gives needed flexibility in the applicationelated issues.

cknowledgement

This research project was supported by the Statutory Fund ofnstitute of Heavy Organic Synthesis “Blachownia” No. 18-90.

ppendix A. Supplementary data

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.ijms.2013.12.016.

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