RAPID COMMUNICATIONS IN MASS SPECTROMETRY, VOL. 9, 1185-1188 (1995)

Unimolecular Reaction of the (H3C0)3C+ Cation Viet Nguyen, Henri Edouard Audier, Arielle Milliet* and Philippe Mourgues Laboratoire des MCcanismes RCactionnels, DCpartement de Chimie, Ecole Polytechnique, 91128 Palaiseau Cedex, France

From metastable ion studies, Fourier transform ion cyclotron resonance experiments and semi-empirical calculations, the unimolecular reaction of the (H$O)$+ cation 1 is discussed. The first step of the reaction is an irreversible, 1,3-CH: transfer giving the H3COC(0)O(CH3): ion, 2. This ion either cleaves or isomerizes into the ion-neutral complex [H3COCO+, CH30CH3], 3. Since the H3COCO+ cation is a very good CH: donor, a CH: transfer then occurs within the complex, 3, to yield the predominant fragment (CH,)@+.

Methylation in the gas phase is a reaction of current interest'-' which can be performed either: (i) directly by reaction with a methyl cation generated

by protonation-dissociation of a methyl halide, or: (ii) by reaction with a CH: donor cation such

as (CH3)30+,7,10 CH30CH:,9 CHaCH: (X= ha10gen)~ or H3COC0+.4

In this last case, the methylation reaction depends on the relative methyl cation affinity (MCA)6 of the neutral and the molecule corresponding to the demeth- ylated ion (i.e. CH20 for CH,OCH;), for instance:

+CH,0CH3 + OC(CH,),+ CH20 + CH,O+C(CH,), CH30H + CH30CO+-+ CH30H+CH3 + C 0 2

These reactions are useful in order to easily prepare selectively labelled ions in the gas phase. They are also important for the study of unimolecular reactions which are complex mediated. In this case, when the cationic moiety is an alkyl donor cation, an alkyl transfer can be observed within the complex.

In this vein it has been shown' that the loss of C02 from phenylalkylcarbonates involves the intermediacy of a [C&O', ROCO'] complex in which a stereospeci- fic substitution reaction yields an ionized phenylalkyl ether fragment:

C,H,OC(O)OR+'-+ [C6H50', ROCO+] -+ C,H,OR+' + COz

Similarly, the different steps in the reaction of the CH,OCH,OC(R): cation have been studied both by experiment and calculation^.^

CH,OCH2O+C(R),-+ [ +CH20CH3,OC(R)2] +CH,O+C(R), + CH20

The study of the unimolecular reactions of the ion (H3C0),C+, giving another example of a CH: transfer reaction within a complex, is presented in this work.

EXPERIMENTAL. Unimolecular reactions were studied with a chemical ionization source, by mass-analysed ion kinetic energy (MIKE) spectrometry in the second field-free region of a VG ZAB-2F mass spectrometer (Manchester, UK), under standard conditions: ionizing energy 70 eV and accelerating potential 8 kV. Collision-induced dissocia- tions (CID) were examined after collision with helium; normally, the collision gas pressure was adjusted so as

Author for correspondence.

to reduce the intensity of the main beam signal by 50%. The kinetic energy releases (KER) were determined after correction of the width of the main beam and calculated from the width at half-height ( Commercially available chemicals were used without further purification. (CD30)3C+ and (CD30)(CH30)2C+ were prepared in the source of the spectrometer by a mixture of (CH,O),CH and CD,OH. CD30(CO)OCH3 was prepared by refluxing CD30H with ClCOOCH, and (CD30)zC0 was made by reflux- ing CD30H with C1,CO. The products were distilled at (CH30)2C0 boiling point. The (CH30),C+ ions were generated in the ion source by loss of H' from ionized (CH30)3CH. Ion cyclotron resonance (ICR) experi- ments were performed on a Bruker CMS 47X FT-ICR (Fallenden, Switzerland). ICR experiments were per- formed by injecting reactant ions into a cell containing 5 x lo-' Torr of neutral reactant from an external source, followed by specific ejection of unwanted ions and then by a burst of argon from a pulsed valve to relax the ions prior to reaction. Calculations were performed using AM1" as implemented in the MOPAC version 6.0 program.12 All structural para- meters were optimized by minimizing the energy with respect to all geometrical variables. All structures were characterized as stationary points and true minima on the potential energy surface using the key word FORCE. The criterion for a minimum is that all eigenvalues of the Hessian matrix are positive; for the transition states, only one of these values is negative.

RESULTS AND DISCUSSION Unimolecular reaction of metastable ion (H,CO),C+, 1 In the second field-free region of the mass spectro- meter, metastable ion 1, fragments into two ions: C202H:, mlz 59 (28%) and C30H;, rnlz 61 (72%) (Fig. 1). Formation of these fragment ions is accompa- nied by a significant kinetic energy release (KER): To.5=37meV for mlz59 fragment ion and To,5= 70 meV for mlz 61. Upon collision the abundance of mtz 59 fragment ion increases.

The fragmentation of (D3C0)3C+ ion leads to CZOzD: mlz 62 and C30D9+ mlz 70. This strongly sug- gests that these fragment ions have respectively the D,COCO+ and the (CD3)@+ structures. This is con- firmed by their MIKE-CID spectra: D,COCO+ yields CD; (mlz 18) as the dominant fragment ion, while (CD,),O+ eliminates CD, (to give mlz 50).7

The spectra of the other labelled ions (Table 1) indicate that the dissociations are not preceded by any

CCC O951-4198/95/121185-O4 (CJ 1995 by John Wiley & Sons, Ltd.

Received 3 August 1995 Accepted 10 August 1995

1186 MASS SPECTROMETRY AT PALAISEAU

M+ mlr 6 1

Figure 1. MIKE spectrum of (CH30),Ct

hydrogen atom exchange and that isotope effects are negligible.

From these results, we propose the pathway shown in Scheme 1:

-the reaction begins with a 1,3-CH; transfer giving the isomeric ion 2

-ion 2 can either dissociate or isomerize into the [H,COCO+, CH30CH3] ion-neutral complex 3

-since the H,COCO+ cation is a CH; donor, the (CH,),O+ fragment ion is formed by methyl transfer within the complex 3.

Of course, none of these results tell anything about the relative stability of 1 and 2 ions and about a possible reversibility of the reaction 1+2. These two matters can be addressed by the study of ion/molecule reac- tions.

FT-ICR study In order to determine the relative stability of ions 1 and 2, the preferential site of methylation of H3COC(0)OCH3 was determined. A soft methylation agent , CD,OCD:, generated by fragmentation of D3COCD2CD20CD:' in the ion source, was used (Scheme 2). The structure of the so-formed product ions was studied by CID.

(i) If the CD; was only transferred onto the oxygen of the carbonyl group, the cation (CH30),C+OCD3 l a would be formed and would yield the H3COCO+ and D,COCO+ frag- ments in a 2 : 1 ratio.

(ii) If the CD; was specifically transferred on the other oxygen the H3COC(0)O+(CH3)CD3 ion 2a so formed would yield H3COCO+ as the only fragment-ion.

Since the product ion, from the reaction of H3COC(0)OCH3 with CD,OCD: in the cell, gives, upon CID, H3COCO+ and D,COCO+ ions in a ratio close to 2: 1,13 one concludes that the CD: transfers onto the carbonyl oxygen atom.

The reaction of CH30CH: on CD30C(0)OCD3 as

Table 1. MIKE spectra of labelled ions 1 miz 59 61 62 61 67 70

28 72 -

(CH30)2C+OCD3 16 - 10 74 -

(CD,O),C+OCHj 1 - 18 - 75 - (CD@)K+

- - -

- (CH@)3Ct

76 - 24 - - -

O/CH3 0 I1

1 H3C\~/c\0+-CH3 H3COCOf, CH30CH3

/ '7H3

I I H3C,0/C:0 -

I I CH3 1 CH3

O+ H3C' 'CH,

m/z 61

\ H3coCO' m/a 59

Scheme 1

well as methylation of CD30(CO)OCH,, lead to the same conclusion.

The second question concerns the reversibility of the 1-2 reaction. In order to solve this problem, the reaction of the H3COCO+ cation (generated by frag- mentation of H,COC(O)OCH:. in the ion source) with CD30CD3 was performed (Scheme 3).

-Either this reaction is irreversible and only the H30CO+ fragment ion should be formed;

-or this reaction is reversible and an exchange between CH3 and CD3 groups should take place before fragmentation leading to both H,OCO+ and D,OCO+ fragment-ions formation.

Since in the FT-ICR experiment no D,COCO+ for- mation is observed, even after a long reaction time, the conclusion is that the 1+2 reaction is irreversible and therefore that the 1,3-CH: migration involves a signifi- cant energy barrier.

Finally, the (CH,)O+ cation is unreactive toward CO,. This means that the [(CH,)@+, CO,] encounter- complex 4 gives only the (CH,)@+ fragment and that the final state (of (CH3)30+ + CO,) lies in energy under (H,COCO+ + CH30CH3). This qualitative energy dia- gram will now be confirmed by semi-empirical calcula- tions.

Calculations: energy profile The AM1 calculations (Fig. 2) first confirm the great stability of ion 1 compared to ion 2 which is in agree- ment with the respective MCA of CO, and CH30CH3. However, the semi-empirical calculations lead to a difference which is smaller than that given by the experimental methyl cation affinities.6 Furthermore the transition state TS (1+2), corresponding to 1,3-CH; transfer is rate determining in the formation of the (CH3)30+ fragment ion. This step is therefore irrevers- ible.

Iodneutral complexes 3 and 4 are minima on the potential energy surface. The complex [(CH3),0+, CO,], 4, is weakly bonded and only yields the (CH,)@+ fragment ion. The complex [H3COCO+, CH,OCH,], 3, corresponds to a shallow minimum and is the reactive entity for the last CH: migration. The semi-empirical calculations are not precise and reliable enough to show that 3 which geometry is most favor- able for the complex and correspond most probably to several stable structures which allow the cation to turn

Scheme 2

MASS SPECTROMETRY AT PALAISEAU 1187

p 3

CO2

[ H3c00'C+ ' 0 .:"'I 3 CD3

o+ + H3C' 'CD3 ' m/z67

I

2 CH3

$ o F D 3

I

m/z 59

Scheme 3

d z 59 92kcaVmol

I / TS 86 kcaVmol - O Y O + O \

67 kcaVrnol - - d z 6 1 58 kcaVmol

(%,+,CH, 56 kcaVmol

0 + C% -

CH,

- 0

?' C' 4

Figure 2. Energy (kcal/mol) profile of (CH,O),C' unimolecular fragmentation.

35 kcaVmol

-0' '0 1 1

around the neutral within the complex. This phenome- non has been recently exemplified' by ab inztio calcula- tions for the (CH,OCH:, CD2 = 0) complex (Scheme 4)-

Finally, the (H,COCO++CH,OCH,) final state lies

only a few kcal/mol above the transition state. This process corresponds, in the unimolecular reaction, to the fragmentation of the most energetic population of ions 1. Since the fragment results from simple cleavage of 3, the KER is not significant. By contrast the final state ((CH,),O+ + CO,) lies very low in energy and this process is favoured. Since its reverse energy is substan- tial, the corresponding kinetic energy release is large.

CONCLUSION

1. A new example of complex mediated alkyl+ mig-

2. The 1,3-CH; shift 1+2 is the rate-determining step of the unimolecular reaction of ion 1.

I 0 [ H3C~o'CHZ+-..0=cD2 ] L [ +H&' 'CH3 ---O=CD2

1 CH,=O---H$' 'CD2+ C_ [H2C=O---+CH3---O=CD2] ration is demonstrated here.14

TS " 1

Scheme 4

1188 MASS SPECTROMETRY AT PALAISEAU

3 . The alkyl-OCO+ cations are among the most efficient alkyl' donors. Ab-initio calculations per- formed by Morton show that these ions are, in fact, weakly bonded complexes between an alkyl cation and a CO, molec~ le . ' ~

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13. CID were performed by exciting the isolated product ion (Energy of center of mass 56 eV to 88 eV) and letting it collide with argon under conditions giving a few collisions. (Argon pressure 2 x lo-' mbar, collision time 20 ms). Longer collision times result in secondary reactions of the fragment ions with (CH,O)&O, increasing the ratio CH,OCO+/CD,OCO+.

14. A preliminary communication has been prepared about this work: V. NGuyen, T. H. Morton, H. E. Audier, P. Mourgues and A. Milliet, Proceedings of the '11"' JournCes FranGaises de Spectrometrie de Masse' Rouen, France (1994).

15. T. H. Morton (personal communication).