J . Phys. Chem. 1988, 92, 6465-6469 6465

Energetics of Activation of Fe(CO),- and Cr(CO),- in Gaseous Collisions

Vicki H. Wysocki, Hilkka I. Kenttamaa,* and R. Graham Cooks*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: February 19, 1988; In Final Form: May 17, 1988)

Internal energy distributions associated with low-energy (eV) and high-energy (keV) collision-activated dissociation of Fe(CO), and Cr(CO)5- have been estimated from the known thermochemistry and the measured abundances of decarbonylation products of these ions and are compared with the distributions estimated for the cation radicals Fe(C0)5+, Fe(C0)4+, Cr(C0)6+, and Cr(CO)5+. It is shown that the average amount of internal energy deposited by collisional activation is of similar magnitude for the negative and positive ions. However, the results obtained for the negative ions indicate that collision-activated electron detachment may compete with or follow collision-activated dissociation in both collision energy regimes. Further support for electron detachment in high-energy collisions is obtained from charge inversion experiments which record the positively charged fragment ions arising from Fe(CO),- and Cr(CO),-.

Introduction

Negative ion mass spectrometry of organic and metal-organic compounds has recently been reviewed.'+ While there have been many advances in the field, particularly in development of methods to generate negative ions,I4 the amount of information available on collisional activation of negative ions is still very limited. This is in stark contrast with the large body of data available on collision-activated dissociation of positive

The possibility of deriving structural information from collisional activation experiments is especially important for negative ions since the preferred methods for negative ion formation (e.g., chemical ionization,'+ fast-atom bombardment8v9) frequently do not impart enough energy to cause appreciable fragmentation of the parent ions. It has recently been demonstrated that collisional activation of negative ions can provide structural as well as fun- damental information analogous to that obtained for positive ions.l**4 For example, collision-activated dissociation of negative

~ ~~

(1) Bowie, J. H. Mass Spectrom. Reu. 1984, 3, 161. (2) Budzikiewicz, H. Mass Specrrom. Reu. 1986, 5 , 345. (3) Squires, R. R. Chem. Reu. 1987, 87, 623. (4) Gregor, I. K.; Guilhaus, M. Mass Spectrom. Rev. 1984, 3, 39. ( 5 ) Cooks, R. G. In Collision Spectroscopy; Cooks, R. G., Ed.; Plenum:

(6) Levsen, K.; Schwarz, H. Mass Spectrom. Reu. 1984, 2, 77. (7) McLafferty, F. W. Tandem Mass Spetrometry; McLafferty, F. W.,

(8) Cochran, R. L. Appl. Spectrosc. Rev. 1986, 22, 137. (9) Fenselau, C.; Cotter, R. J. Chem. Reu. 1987, 87, 501. (10) McClusky, G. A,; Kondrat, R. W.; Cooks, R. G. J . Am. Chem. SOC.

(1 1) Hunt, D. F.; Shabanowitz, J.; Giordani, A. B. Anal. Chem. 1980.52,

(12) Bowie, J. H. Acc. Chem. Res. 1980, 13, 76. (13) Sallans, L.; Lane, K. R.; Squires, R. R.; Freiser, B. S. J. Am. Chem.

(14) Frcelicher, S. W.; Lee, R. E.; Squires, R. R.; Freiser, B. S. Org. Mass

(15) Stringer, M. B.; Bowie, J. H.; Holmes, J. L. J . Am. Chem. SOC. 1986,

(16) Young, A. B.; Harrison, A. Org. Mass Spectrom. 1987, 22, 622. (17) Lyon, P. A.; Stebbings, W. L.; Crow, F. W.; Tomer, K. B.; Lippstreu,

(18) Jensen, N. J.; Torner, K. B.; Gross, M. L. Anal. Chem. 1985, 57,

(19) Bambigiotti, A. M.; Coran, S. A.; Giannellini, V.; Vincieri, F. F.;

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ions has been successfully utilized in structural studies of anionic surfactants,17J8 fatty acid methyl ester^,'^,^^ peptides,2's22 and e n o l a t e ~ . l ~ * ' ~ , ~ ~ * ~ ~ A related method of obtaining structural data for negative ions involves detection of the dissociation products obtained for metastable positive ions generated by removal of two electrons from the negative ions upon ~ o l l i s i o n . ~ ~ ' ~ ~ ~ ~ Charge inversion is a highly endothermic process and typically leads to extensive, structurally diagnostic f r a g m e n t a t i ~ n . ~ . ~ ~ , ~ ' Charge inversion spectra have been utilized to solve problems of ion chemistry and ion structure in cases in which conventional daughter spectra obtained by collision-activated dissociation yield insufficient data.9,26,28-31

Fragmentation induced by collision produces daughter spectra, the features of which are controlled by the distribution of internal energies, P(E), of the activated i o n ~ . ~ Z ~ ) Knowledge of and control over these energies are important objectives in mass spectrometry. An approximate method based on the energy requirements for fragmentation and measured fragment ion abundances has been successfully sed^^*^^ for estimating P ( E ) for positive ions. The information has been extremely useful in understanding the positive ion chemistry of gaseous hydrocarbon^^^,^' and other system^.^^,^^ We previously presented35 an apparent P(E) dis-

(22) Bradley, c . V.; Howe, I.; Beynon, J. H. Biomed. Mass Spectrom. 1981, 8, 85.

(23) (a) Eiching, P. C. H.; Bowie, J. H. Org. Muss Spectrorn. 1987, 22, 103. (b) Prorne, J. C.; Aurelle, H.; Prome, D.; Savagnec, A. Org. Mass Spectrom. 1987, 22, 6. (c) Bowie, J. H.; White, P. Y.; Blumenthal, T. Org. Mass Spectrom. 1987, 22, 541.

(24)-Froelicher, S. W.; Freiser, B. S.; Squires, R. R. J . Am. Chem. SOC. 1984, 106, 6863.

(25) Ast, T. Adv. Mass Spectrom. 1980, 8A, 555. (26) Bowie, J. H.; Blumenthal, T. J. Am. Chem. SOC. 1975, 97, 2959. (27) Holmes, J. C. Org. Mass Spectrom. 1985, 20, 169. (28) Bursey, M. M.; Harvan, D. J.; Parker, C. E.; Pedersen, L. G.; Hass,

(29) Zakett, D.; Ciupek, J. D.; Cooks, R. G. Anal. Chem. 1981, 53, 723. (30) Burinsky, D. J.; Cooks, R. G. J. Org. Chem. 1982, 47, 4864. (31) Burinsky, D. J.; Cooks, R. G. Org. Mass Spectrom. 1983, 18, 410. (32) Forst, W. Theory of Unimolecular Reactions; Academic: New York,

1973. (33) Levsen, K. Fundamental Aspects of Organic Mass Spectrometry:

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1984, 64, 79. (35) Wysocki, V. H.; Kenttamaa, H. I.; Cooks, R. G. Int. J . Mass Spec-

trom. Ion Processes 1987, 75, 181. (36) Vainiotalo, P.; Kenttamaa, H. I.; Mabud, Md. A,; OLear, J.; Cooks,

R. G. J. Am. Chem. SOC. 1987, 109, 3187. (37) Pachuta, S. J.; Kenttamaa, H. I.; Sack, T. M.; Cerny, R. L.; Tomer,

K. B.; Gross, M. L.; Pachuta, R. R.; Cooks, R. G. J . Am. Chem. SOC. 1988, 110, 657.

J. R. J . Am. Chem. Soc. 1979, 101, 5489.

(38) Kenttamaa, H. I. Org. Mass Specrrom. 1985, 20, 703.

0022-3654/88/2092-6465$01.50/0 0 1988 American Chemical Society

6466

tribution for Fe(CO),- after multiple low-energy collisions in a triple-quadrupole mass spectrometer. Curiously, this negative ion P ( E ) differs in shape from those obtained for positive ions under the same experimental conditions. It is of interest to understand the origin of this result and to obtain further information con- cerning the energetics and other features of collisional activation of polyatomic anions. In the present study, we apply this me- thod3,3j5 of estimating P(E) to collision-activated dissociation of Fe(CO),- and Cr(CO)5- under single- and multiple-collision conditions, as well as in the low (eV) and high (keV) collision energy regimes. The results show that while the energetics involved in collisional activation of positive and negative poiyatomic ions are probably similar, electron detachment processes may signif- icantly reduce the amount of analytically useful information available from collisional activation experiments in the case of negative ions.

Experimental Section Daughter spectra resulting from low-energy collision-activated

dissociation of mass-selected ions were obtained with a Finnigan triple-quadrupole i n ~ t r u m e n t . ~ ~ Positive ions were generated by 70-eV electron ionization. Negative ions were generated by chemical ionization in the negative ion mode with argon as moderating gas at a nominal source pressure of 0.3 Torr. Argon was used as the target gas in the collision chamber (middle quadrupole). Argon pressure was monitored with an ionization gauge situated outside the gas-tight collision chamber. The in- dicated pressure was less than 3 X 10" Torr when operating under single-collision conditions; a linear increase in fragment ion abundances as a function of target pressure was taken as the criterion for single-collision condition^.^' To access multiple- collision conditions, argon pressure was raised until the indicated pressure was about 5 X Torr. (The pressure in the collision chamber was ca. 2 mTorr as read with a Hastings ionization gauge.) A laboratory kinetic energy of up 10 30 eV is available on this instrument.

High-energy (7-keV) collision-activated dissociation and charge inversion experiments were performed with a reversed geometry instrument of the MIKES type.42,43 Positive ions were generated by charge exchange with Ar". Negative ions were generated by chemical ionization with argon as the moderating gas. The nominal source pressure for these experiments was 0.2 Torr. Argon or xenon was used as the collision gas, at an indicated pressure of less than 2 X Torr as measured with an ionization gauge situated outside the collision chamber. Spectra obtained with the two different target gases were similar, provided that the target pressure was adjusted so that the same attenuation of the main beam was achieved. For the data discussed, the main beam was attenuated to reach approximately 65 or 85% of the original intensity. At the 15% attenuation value, the relative abundances of the lowest energy fragments are increased. This results in P(E) distributions of the same general shape as those presented in the Figures 5 and 6 but with increased probability in the low-energy region.

The MIKES instrument is not equipped with a conversion dynode. To ensure that the results were not affected by detector discrimination against low-mass (low translational energy) negative fragment ions, the collision-activated dissociation spectrum of Fe(COj4- was compared with that obtained on a Finnigan MAT 8200 equipped with a conversion dynode. (The acceleration voltage was 3 kV, and the postacceleration voltage was 18 kV.) The spectra obtained with the two instruments were similar, thus

The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 Wysocki et al.

(39) Vincenti, M.; Homing, S. R.; Cooks, R. G. Org. Mass Spectrom., in

(40) Slayback, J. R. B.; Story, M. S . Ind. Res. Deo. 1981, 129. (41) Nystrom, J. A.; Bursey, M. M.; Hass, J. R. Int . J . Mass Spectrom.

(42) Beynon, J. H.; Cooks, R. G.; Amy, J . W.; Baitinger, W. E.; Ridley,

(43) Kruger, T. L.; Litton, J . F.; Kondrat, R. W.; Cooks, R. G. Anal.

(44) Compton, R. Pi.; Stockdale. J A D. Inr. J . Mass Spectrom. Ion Phys.

press.

Ion Processes 1984, 55, 263.

T. Y . Anal. Chem. 1973, 45, 1023A.

Chem. 1976, 48, 21 13.

1976. 22. 47.

TABLE I: Thermochemical Data (eV)

ion m l z Eoa EA" ref Eo ref EA Cr(CO)<- 192 0.0 48 49 Cr(C0);- 164 0.5* Cr(C0); 136 2 9 Cr(CO)* 108 4.4 Cr(C0)- 80 5.9 Cr- 52 6.8

Fe(C0); 168 0.0 2.4 f 0.3 44 Fe(CO)< 140 0.3b 1.8 f 0.2 Fe(C0)2- 112 2.8 1.22 f 0.02 Fe(C0)- 84 3.7 1.26 f 0.02 Fe- 56 5.9 0.164 f 0.035

Cr(CO)5+ 192 0.0 Cr(C0)4+ 164 0.2 Cr(CO)3+ 136 1 . 1 Cr(CO),+ c r i c o j ; cr+ CrC+

Fe(C0)4+ Fe(C0)3+ Fe(CO)*+ Fe(CO)+ Fe' FeC+

08 1.7 80 3.1 52 4.4 64 > I 1.5

68 0.0 40 1.0 12 2.2 84 4.4 56 6.4 68 14.5

51

52-55 (av).

50

a The activation energies, E,, and the electron affinities, EA, are from the references listed in the last two columns of the table. Recent determinations of the first activation energy for fragmentation of Fe- (C0);- and Cr(CO)S'- have yielded somewhat higher values than those listed here.s7 However, this does not change the conclusions presented in this paper. 'See also ref 56.

confirming that significant discrimination of the low-mass frag- ments does not occur in the instrument used in this study.

The compounds were obtained commercially and used without purification. Evaporation of the samples into the mass spec- trometers was controlled by mixing them with powdered activated charcoal prior to placement on the direct insertion probe.

Metal carbonyl ions whose major fragmentation pathway consists of several consecutive reactions with known activation energies (Ei) (Table I) and similar entropy requirements were used in estimating the P(E) distribution^.^^^^^ The relative abundance of each fragment ion, [F,'], divided by the energy interval where formation of the fragment dominates (Ei+, - Ei) gives a P(E) value which was placed at ' / * ( E i + Note that the final point in each of the positive ion P ( E ) distributions has an uncertain pla~ement.)~ The final points for positive ions were placed midway between the activation energies for formation of the bare metal ion and FeC' or CrC'. For the negative ions, the energy interval for the highest energy fragment (Cr- or Fe-) was assumed to be similar to that found for the previous fragment (CrCO- or FeCO-) in the sequence.

The internal energy distributions of Figures 2 and 3 were obtained under multiple-collision conditions, a circumstance where isomerization of the excited but yet unfragmented parent ions can be expected to have maximum p r ~ b a b i l i t y . ~ ~ However, the use of metal carbonyls with their extremely simple fragmentation sequence^^^,^' effectively eliminates this possibility. Under

(45) Kenttamaa, H. I.; Cooks, R. G. J . Am. Chem. Soc. 1985, 107, 1881. (46) Litzow, M. R.; Spalding, T. R. Mass Spectrometry of Inorganic and

Organo-Metallic Compounds; Elsevier: Amsterdam, 1973; pp 475-490, and references therein.

(47) Dillard, J . G. Chem. Reo. 1973, 73, 589. (48) Pignataro, S.; Foffani, A,; Grasso, F.; Cantone, P. Z . Phys. Chem.

(Munich) 1965, 47, 106. (49) A regular increase in electron affinity with increasing number of CO

ligands has been measured for chromium carbonyls: Leopold, D. G.; Murray, K. K.; Lineberger, W. C., private communication of unpublished results quoted in ref 3.

(50) Engelking, P. C.; Lineburger, W. C. J . Am. Chem. SOC. 1979, 101, 5569.

(51) Das, P. R.; Nishimura. T.; Meisels, G. G. J . Phys. Chem. 1985, 89, 2808.

Energetics of Activation of Fe(C0)4- and Cr(CO)5-

a

b

Figure 1. Comparison of (a) the photoelectron spectrum of Cr(C0)6 and (b) the P(E) distribution of Cr(C0)6t generated and activated by 70-eV electron ionization. Spectrum a is redrawn from ref 58.

a b

eV e v

Figure 2. Apparent P ( E ) distributions obtained for (a) Fe(C0)4+ and (b) Fe(C0)4- activated by using 25-eV ion kinetic energy and multiple- collision conditions with Ar target gas.

multiple-collision conditions, the fragment as well as parent ions can undergo collision-activated dissociation. This means that the internal energy distributions derived for multiple-collision con- ditions are only approximate measures of the degree to which the original parent ions were excited.35 In fact, these P(E) distributions indicate the apparent energies which, if deposited in the parent ion population in one activation step, would result in the same product ion distribution that is actually obtained under multi- ple-collision conditions. Provided experimental conditions are maintained constant, the data can be used to compare the behavior of negatively and positively charged ions.

Results and Discussion Metal carbonyls yield anions and cations, both of which frag-

ment principally via a sequence of CO lo~ses.''~*~ This makes these systems ideal for directly comparing the behavior of positively and negatively charged ions. Collision-activated dissociation spectra of Fe(C0)4-, Cr(CO),-, Fe(CO),+, Fe(C0)4+, Cr(C0)6+, and Cr(CO),+ were recorded, and P(E) distributions were estimated from the spectra by using the method described earlier.34,35 The appearance energies for fragmentation of the ions are given in Table I. Note that even though some of the ions do not have stable neutral counterparts, their P ( E ) distributions can be estimated. Comparison of the internal energy distribution of Cr(C0)6+ generated and activated by 70-eV electron ionization (Figure 1 b) with the photoelectron spectrum58 of Cr(C0)6 (Figure la) provides

(52) Winters, R. E.; Kiser, R. W. Inorg. Chem. 1964, 3, 699. (53) Junk, G. A,; Svec, H. J. Z . Naturforsch. B 1968, 23B, 1. (54) Bidnosti, D. R.; McIntyre, N. S. Can. J . Chem. 1967, 45, 641. (55) Foffani, A,; Pignataro, S . Z . Phys. Chem. (Munich) 1965, 45, 79. (56) Distefano, G. J . Res. NatI. Bur. Stand., Sect. A 1972, 74A, 233. (57) Wang, D.; Squires, R. R., unpublished results.

The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6467

a b

e v eV

Figure 3. Apparent P(E) distributions obtained for (a) Cr(CO)St and (b) Cr(CO)S- activated by using 25-eV ion kinetic energy and multi- ple-collision conditions with Ar target gas.

a

P(E

b

Figure 4. Apparent P ( E ) distributions obtained for (a) Cr(CO)4- and (b) Fe(CO)S- activated by using 25-eV ion kinetic energy and single- collision conditions.

support for the reliability of the method of estimating P ( E ) . Photoelectron spectra, which are based on a vertical ionization process as is electron ionization, are thought to approximate the general features of the P(E) distributions for electron i~nization.)~ From Figure 1 it is apparent that the gross features of the pho- toelectron spectrum are well-described by the P(E) distribution determined here, although the fine structure is not observable.

Figure 2a,b shows the internal energy distributions obtained for Fe(C0)4+ and Fe(C0)4- activated by multiple gas-phase collisions in the low collision energy range. Figure 3a,b shows the corresponding distributions for Cr(CO)S+ and Cr(CO)<. The P(E) distributions obtained for Cr(CO)5- and Fe(C0)c by using single-collision conditions are presented in Figure 4. The energy distributions obtained for the positive ions (Figures 2a and 3a) are similar to those presented in earlier paper^^^,^, for other positive ions. Indeed, in earlier the general features of the dis- tributions of internal energy associated with collisional activation of positive ions wefe found to be relatively independent of the particular ion examined. The energy distributions obtained in this work for the negative ions (Figures 2b, 3b, and 4a,b) resemble those of the positive ions in their main features. This result suggests that similar amounts of internal energy are deposited when positive and negative ions of similar size are activated under identical conditions. There is good reason, if these results are typical, to expect that low-energy collision-activated dissociation can be of comparable importance of negative ions and positive ions.

High-energy single-collision data for Fe(C0)4- and Cr(C0)5- are compared with the corresponding positive ion data in Figures 5 and 6 . It is evident that, for the high collision energy regime, the energy distributions obtained for the negative ions are dra- matically different from those t y p i ~ a l ' ~ , ~ ~ , ~ ~ for positive ions. Specifically, P ( E ) of the negative ions show no high-energy tail (Figures 5a and 6a), a well-documented characteristic of the corresponding positive ion energy distribution^.^,^^^^-^^^^^ Closer

( 5 8 ) Turner, D. W.; Baker, C.; Baker, A. D.; Brandle, C. R. Molecular

(59) Kim, M. S.; McLafferty, F. W. J. Am. Chem. Soc. 1978, 100, 3279. Photoelectron Spectroscopy; Wiley-Interscience: New York, 1970.

6468 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988

a

P ( E ) -

Wysocki et al.

i 9 I2 15

eV 0 3 T6

Eo( Fe- ' I

b

Figure 5. Apparent P ( E ) distributions obtained for (a) Fe(C0)4- and (b) Fe(C0)4+ activated by collision at 7 keV. Xe was used as target gas, and the parent ion beam was attenuated to 65% of its original value.

a b

EOICr- ) eV 9V

Figure 6. Approximate P(E) distributions obtained for (a) Cr(CO)< and (b) Cr(C0)5* activated by collision at 7 keV. Xe was used as target gas, and the parent ion beam was attenuated to 65% of its original value.

examination of the P(E) distributions obtained for the negative ions by using the low collision energy regime (Figures 2b and 3b) reveals that in these experiments, also, the probability of depositing high energies seems to be lower than is typical for positive ions.34*35 For example, the full width a t half-maximum (fwhm) of the negative ion energy distributions is less than 3 eV while positive ions usually a fwhm of at least 4.5 eV under the same experimental conditions.

The differences between the negative and positive ion data described above may or may not be due to different probabilities of energy deposition upon collisional activation. The m e t h ~ d ~ ~ , ~ ~ used to estimate P(E) is based on the relative abundances of ionic fragmentation products of the ion of interest, and it does not allow for differential removal of ions by other processes. Therefore, the resu!ts (energy distributions) obtained will be erroneous if the ions also undergo other competing reactions. For metal carbonyl ions, the consecutive loss of carbonyls is the only dominant

a 168

190 170 150 I30 110 90 70 50 m l z

140 120 100 BO 60 40 mlr 160

c 140

160 140 120 100 BO 60 40 mfr

d 84 56

1 112 I I

160 140 120 100 80 60 40 mIr

Figure 7. Comparison of (a) the high-energy daughter spectrum of Fe(CO)5+, (b) the high-energy daughter spectrum of Fe(CO),+, (c) the high-energy daughter spectrum of Fe(CO)L, and (d) the charge inversion spectrum of Fe(C0)4-.

that found for positive ions. Thus the difference between the behavior of negative and positive ions seems to be independent of the excitation mechanism (electronic excitation for high-energy collisional ac t iva t i~n ,~* ' t~~ vibrational excitation for low-energy collisional activation).' This finding strongly suggests that our experimental results do not reflect the internal energy distributions of the negative ion population immediately after activation but that the data are affected by a process that selectively removes either the parent ions of high internal energy or their fragmen- tation products. In the case of negative ions, the most likely process competing with dissociation is collision-activated electron detachment. Electron detachment following collisional activation has been suggested by other authors although it has not been demonstrated u n a m b i g u ~ u s l y . ' ~ ~ ' ~ ~ ~ ~ ~ It should be pointed out here that vibrational activation of molecular anions by infrared photons has been found to cause electron d e t a ~ h m e n t . ~ ~ b ~ This process-just like low-energy collision-activated electron

fragmentation pathway. However, it i s possible that additional processes occur which change the charge of the ions (e.g., charge exchange for positive ions, electron detachment and charge in- version for negative ions). These processes cannot be directly 1670. observed since their products are not detected in a conventional collision-activated dissociation experiment.

In both collision energy regimes, the apparent probability of depositing higher energies in negative ions is reduced relative to

(60) Froelicher, S. W.; Freiser, B. S.; Squires, R. R. J . Am. Chem. SOC.

(61) McLuckey, S. A.; Glish, G. L.; Kelley, P. E. Anal. Chem. 1987, 59,

(62) Graul, S. T.; Squires, R. R. J . Am. Chem. SOC. 1988, 110, 607. (63) Meyer, F. K.; Jasinski, J. M.; Rosenfeld, R. N.; Brauman, J. I. J . Am.

(64) Tumas, W.; Salomon, K. E.; Brauman, J. I. J . Am. Chem. SOC. 1986,

1986, 108, 2853,

Chem. SOC. 1982, 104, 663.

108, 2541.

Energetics of Activation of Fe(C0)4- and Cr(C0)5- The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6469

detachment-requires conversion of the vibrational excitation energy into electronic energy of the parent a n i ~ n . ~ ~ . ~ ~

To obtain evidence for electron detachment of the negative ions studied, charge inversion showing the positive fragment ions derived from Fe(C0)4- and Cr(CO)5- were recorded. The charge inversion spectrum of Fe(C0)4- is shown in Figure 7, together with high-energy collision-activated dissociation spectra of Fe(CO)S+, Fe(C0)4+, and Fe(CO),-. The exact sequence of events leading to the formation of each of the positive ions in the charge inversion spectrum is not known. For example, the ion Fe+ ( m / z 56) could be formed by detachment of two electrons from Fe- produced by collision-activated dissociation of Fe(C0)4- ( m / z 168) (Le., 168-- 56-- 56 - 56+), by fragmentation of Fe(C0)4+ produced by collision-activated charge inversion of Fe(CO);, (Le., 168-- 168 - 168'- 56+), or by various other pathways not discussed further.

The charge inversion spectra of Fe(C0)4- and Cr(CO)5- are useful for the purposes of this study for two reasons. (i) The charge inversion experiments confirm that some of the negative ions of interest do indeed suffer electron loss during the high-energy experiments.6' In addition to the ions undergoing charge inversion, ions are almost certainly lost during the experiment by the more probable, lower energy process of simple electron detachment (removal of a single electron). Note that the positive ions which are produced in the greatest abundance by charge inversion of Fe(C0)4- (Figure 7d) correspond to those negative ions which are of the lowest relative abundance or absent in the collision- activated dissociation spectrum of Fe(C0)4- (Le., Fe(C0)-, Fe-; Figure 7c). (ii) The charge inversion experiments provide data that prove the existence of a high-energy tail in the high-energy P(E) distributions of negative ions. For example, the presence of Cr'+ in the charge inversion spectrum of Cr(CO)5- (supple- mentary material) indicates that a portion of the activated parent ions Cr(CO)5-do in fact acquire at least 14 eV following collisional activation (Scheme I; thermochemical estimations are based on the data shown in Table I or found in the references of Table I).

SCHEME I 6.8 eV 0.66 eV 6.8 eV

Cr(CO)5*- - Cr*- - Cr - Cr*+ (-5CO)

estimated total activation energy = 14.2 eV

2.1 eV 8.0 eV" 4.4 eV Cr(C0)S'- - CI-(CO)~ - Cr(CO)S*+ (-sco1 Cr'+

estimated total activation energy = 14.5 eV

The interpretation of the differences in the negative and positive ion collision-activated dissociation data on the basis of electron detachment competing with collision-activated dissociation for the negative ions is further supported by predictions concerning the ions which are most likely to undergo electron detachment on the basis of the available thermochemistry. This process is expected to dominate for those ions that have a low electron binding energy

(65) Simons, J. J . Am. Chem. SOC. 1981, 103, 3971. (66) Acharya, P. K.; Kendall, R. A.; Simons, J . J . Am. Chem. SOC. 1984,

106, 3402. (67) In these experiments, collision-activated electron detachment cannot

be easily verified by chemical trapping of the released electrons because of the possible simultaneous occurrence of kinetic energy driven endothermic charge exchange with the target molecules. Moreover, inefficient conversion of parent ions to fragments does not provide sufficient evidence for the oc- currence of electron detachment since the inability to detect fragments can be due to ion loss by scattering.

(68) Assumed to be equal to the ionization energy of Cr(C0)6.

and that require relatively large amounts of energy to fragment. For example, Fe(CO),- is not likely to undergo electron detach- ment since the corresponding neutral molecule has a relatively high electron affinity (EA = 1.2 eV), and the lowest activation energy for dissociation of Fe(CO),- is 0.9 eV. In contrast, loss of CO from Fe(C0)- has an activation energy of 2.2 eV while detachment of an electron requires only 1.26 eV (see Table I ) . Thus, electron detachment may compete efficiently with disso- ciation of this ion to Fe- if the conversion of vibrational energy to electronic energy is not very slow. Moreover, electron de- tachment from Fe- (EA of Fe is 0.16 eV; Table I) will further reduce the abundance of this dissociation product in the final product distribution. As a conclusion, the datum point on the P(E) curve corresponding to the high-energy product Fe- (e.g., at 7 eV in Figures 2b and 5a) is expected to have an artificially low probability value.

Our attempts to fragment a number of large organic negative ions by low-energy collisions suggest that electron detachment also occurs for some organic ions. For example, the collision-activated dissociation spectrum of the stearate ion [CH3(CH2)16C0~, m / z 283-1 shows no fragment ions although the parent ion abundance decreases dramatically when collision gas is added to the middle quadrupole of the triple-quadrupole i n s t r ~ m e n t . ~ ~ ~ ~ ~ A likely explanation60s62 for this behavior is that, upon collision, the parent anion loses C 0 2 to generate an alkyl anion which spontaneously detaches an electron. However, the stearate anion produces de- tectable dissociation products upon high-energy collisional acti- vation.I8 Apparently, some dissociation reactions that require high excitation energies and are therefore only accessible in the high collision energy regime can compete with electron detach- ment.34,35.59

Conclusions The average amount of internal energy deposited by collisional

activation is of similar magnitude for negative and positive metal carbonyl ions. This similarity holds for experiments done at high as well as low collision energy and under single- as well as multiple-collision conditions. Collision-activated electron de- tachment evidently competes with or follows collision-activated dissociation of Fe(C0)4- and Cr(CO)5- in the low as well as the high collision energy regimes. This may seriously reduce the analytically useful information content of collision-activated dissociation spectra of some negative ions, as evidenced by low- energy collision-activated dissociation of the stearate anion. However, in the high-energy range, it is possible to take advantage of the high energy deposition possible and to use charge inversion data to complement the collision-activated dissociation data ob- tained for negative ions.

Acknowledgment. Marco Vincenti is thanked for the data recorded on a Finnigan-MAT 8200. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society.

Registry No. Fe(CO),-, 51222-96-9; Cr(CO)5-, 39586-86-2.

Supplementary Material Available: Cr(CO)5- charge inversion spectrum (1 page). Ordering information is given on any current masthead page.

(69) Unexpected lack of fragmentation of long-chain carboxylate anions following activation by low-energy collisions has also been observed by other authors: Barnbagiotti, A. M.; Coran, S. A,; Vincieri, F. F.; Petrucciani, T.; Traldi, P. Org. Mass Speclrom. 1986, 21, 485.

(70) Note also contrasting results presented in ref 16 and: Chowdhury, S.; Harrison, A. G. J . Am. Chem. SOC., in press.