Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0023 92

Supplement A3: The Chemistry of Double-Bonded Functional Groups. Edited by Saul Patai Copyright 1997 John Wiley & Sons, Ltd.

ISBN: 0-471-95956-1

CHAPTER 2

Mass spectra of double-bonded groups

¨

TINO GAUMANN

Institute of Physical Chemistry, Federal Institute of Technology, 1015 Lausanne, Switzerland

Fax: C41 21 6933092: e-mail: [e-mail]fl.ch

I. ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		23
II. AN INTRODUCTION, TO WARM UP . . . . . . . . . . . . . . . . . . . . . . .		24
III. THE LOCALIZATION OF THE CDC DOUBLE BOND:
	AN ANALYTICAL PROBLEM . . . . . . . . . . . . . . . . . . . . . . . . . . . .	25
IV. ETHYLENE; AN OLD FAITHFUL . . . . . . . . . . . . . . . . . . . . . . . . .		30
V. THE PENTENE STORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		36
VI. HEPTENE: THE OTHER STORY . . . . . . . . . . . . . . . . . . . . . . . . . .		38
VII. ALKENES: TO FILL SOME GAPS . . . . . . . . . . . . . . . . . . . . . . . . .		41
VIII. ACETONE: SEEMINGLY SIMPLE . . . . . . . . . . . . . . . . . . . . . . . . .		45
IX. OTHER KETONES; THE MORE IT CHANGES, THE MORE IT IS
	THE SAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	55
X. DIELS ALDER AND RETRO-DIELS ALDER REACTIONS . . . . . . .		63
XI. ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		80
XII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		80
	I. ABBREVIATIONS
CI	chemical ionization
CID	collisional-induced dissociation
CR	charge reversal
CT	charge transfer
DA	Diels Alder
EI	electron impact
FAB	fast atom bombardment
FI	Þeld ionization
FIK	Þeld ionization kinetics
ICR	ion cyclotron resonance

23

24					Tino Gaumann¬
NRMS	neutralization				reionization mass spectrometry
PEPICO	photoelectron				photoion coincidence spectroscopy
PES	photoelectron spectrum
RDA	retro-Diels		Alder

II. AN INTRODUCTION, TO WARM UP

It is probably difÞcult to Þnd a mass spectrometric fragmentation where neither a doublebonded nor a three-electron-bonded CDC, CDO or CDN group is present. In addition, in most ions, other functional groups containing a heteroatom may localize the charge; this might have a larger inßuence on the fragmentation pathway than a double bond. Thus it is not practicable to cover the whole Þeld within a review of this size and the author has concentrated on a few Þelds that offer in his view a special interest and may serve as a pars pro toto in this Þeld to guide the ideas. Whenever available, much weight is given to data including labeled compounds and the measurement of the time dependence over a wide range. It is the authorÕs opinion that much information is hidden in such data that often invalidate conclusions drawn too hastily. Several new applications of mass spectrometry have been described by Splitter and Turecekÿ1.

Mass spectrometric fragmentation within the ion source covers roughly the time scale up to ca 1 s. This might seem a short lapse of time, but is very long compared to the few hundred femtoseconds it takes for an elementary reaction to take place2. Many isomerizations will take place already within the ion source. Field ionization kinetics (FIK)3 6 is a method that allows one to sample the time range from 10 ps to 1 s. Unfortunately, the experimental difÞculties are such that this interesting method cannot count on many adherents, but it is a Þeld where unexpected surprises are programmed in advance. Infrared excitation can be used to excite additionally the neutral before Þeld ionization takes place in order to get additional fragmentation. The time window around ca 10 s is deÞned by ßight time in magnetic and quadrupolar mass spectrometers and has received very much attention and delivered much new information about ions and short-lived neutrals. The metastable decay and the different modiÞcations of collisionalinduced dissociation (CID) fall in this Þeld. However, it should not be forgotten that the structure of the ions whose properties and structure are elucidated during this time span rarely correspond to those of the ions initially produced. The time window for the study of reactions and properties of ions has been enlarged to ca 1 s by the ion cyclotron resonance (ICR) technique. This increase is of importance, e.g. for the study of reactions of vibrationally relaxed ions. The large majority of ions is being produced with much internal vibrational energy. It so happens that the time range of the ICR instruments falls within the range of vibrational emission7 9. A review has been given by Dunbar10. Even slow metastable decays can be observed within this time range11,12. The recent developments in the applications of the theoretical calculations by RRKM-QET have been reviewed by Lifshitz13.

The dramatic increase in computing power has allowed one to calculate ions (and neutrals) containing many atoms. It can be estimated that semiempirical self-consistent Þeld methods allow calculations of structures that contain about 100 atoms, whereas semiempirical methods increase this limit by a factor of ten14 16. A short review of the use of MINDO/3 in the Þeld of ketones has been presented by Parker and collaborators17. The accuracy has reached a point where often it compares favorably with the experimental results. Many new structures have been conÞrmed or discovered. The price to pay is that many different isomeric structures may have energies that fall within the reach of internal energies of the ions and it is often an academic discussion to assign a given structure to a given intermediate.

2. Mass spectra of double-bonded groups

25

Neutralization-reionization mass spectrometry (NRMS) is a new technique that has increased considerably our knowledge about the structure of intermediate cations, anions and neutrals18 24. It is based on the collisional-induced dissociation (CID), a technique of utmost importance in mass spectrometry25; it has recently been reviewed by Cooks26 and Wesdemiotis and McLafferty27. In NRMS one seeks to neutralize an ion by a (soft) charge exchange without accompanying isomerization or isotopic scrambling28,29 in the Þeld-free region between two sectors. The efÞciency of the process is governed by the Franck Condon overlap between the projectile ion, its neutral counterpart and those of the target species30. The neutral will in principle keep its high kinetic energy and can according to the instrumental conditions be dissociated in a high-energy collision process. Reionization of the fragments then provides a mass spectrum negative31,32 and positive ions are feasible that characterizes the original ion structure, but allows also conclusions about the structure of the neutral intermediate33 43. A special application introduced by Turecekÿ and colleagues44 46 is the survivor ion mass spectrometry which is based on the simultaneous neutralization and reionization of all stable ions produced in the ion source followed by selective monitoring of the nondissociating species. This procedure achieves isomer differentiation in a single spectrum while providing information on all stable isomers, as opposed to the conventional tandem mass spectrometry, where a matrix of spectra must be acquired in order to characterize two or more precursor ions47. These evolved applications necessitate multisector instruments48 54. The applications involving multisector instruments are summarized by Hoffmann55.

III. THE LOCALIZATION OF THE C=C DOUBLE BOND:

AN ANALYTICAL PROBLEM

The location of a CDC double bond in mass spectrometry is not an easy problem, because the energy for its delocalization within the ion is often not very high, as will be shown in later sections. This is particularly true for shorter chains. The problem is somewhat different, when other functional groups with a stronger tendency to localize the charge are present, such as acids, esters, alcohols etc. In this case a charge-remote fragmentation might yield fragments that are typical for the position of the CDC double bond (among other criteria). The analytical possibilities of charge-remote fragmentations have been reviewed up to 1989 by Adams56,57; some applications are summarized by Gross58 and newer results resumed up to 1991 by the same author59,60. Jensen, Tomer and Gross demonstrated on several examples with negative ions (unsaturated61 and polyunsaturated62 fatty acids) the feasibility of the concept (see later). The collision-induced allylic cleavage reactions of deuterium-labeled [M H C 2Li]C and [M H] ions were investigated by Adams, Gross and coworkers63. They demonstrated the usefulness of speciÞc labeling for the elucidation of such processes; this fact of no visible H/D randomization is also an indirect proof that the charge is not involved in the fragmentation.

There are several possibilities to circumvent the problem of isomerization. Field ionization (FI) seems to be an ideal solution, since the fragmentation is faster than the isomerization. Effectively, it has been shown by Levsen and coworkers64 for 19 oleÞns from C4 to C8 that the fragments thus obtained are typical for the location of the double bond. However, the difÞculty to obtain reproducible results with the FI technique prevented widespread use. All methods that induce either a fragmentation that is faster than a shift of the double bond or use ionization energies at the threshold of the ionization energy are potentially useful candidates. One possibility is the secondary fragmentation of the ions that did not fragment after a few s, as is done in NRMS and the related techniques. The possibility of a preceding isomerization of these ions is a problem, unless the excess of internal energy in ionization is very small. Photoionization at the lowest-energy

26 Tino Gaumann¬

level is another method. For mixtures, it has the disadvantage that for best results the photon energy must be carefully adjusted. Negative ions often show a smaller tendency for fragmentation which can be an advantage for collisionally activated fragmentation without a preceding shift of the double bond. CI (chemical ionization) is a very soft ionization or derivatization method. In mixtures, practically all these methods have to be preceded by a separation either by gas or liquid chromatography. Analytically, the most promising method where applicable is the derivatization of the double bond in the neutral molecule before separation and fragmentation. A review of the mass spectrometric methods for the structural determination and analysis of fatty acids up to 1986 has been prepared by Jensen and Gross65. A particularly useful ionization method in this Þeld is the fast atom bombardment (FAB), explained in the preceding review of this series by Mruzek25, possibly coupled with collision-induced dissociation66. Positive ions, containing alkali metals, as well as negative ions are observed. Contado and Adams67 elucidated

the mechanism of the charge-remote fragmentations of the [M	C	Li]C ions of fatty esters;
		68
a detection limit of 25 ng of methyl palmitate is obtained. Gross and collaborators

studied a series of homoconjugated octadienoic acids with FAB. The carboxylate ion [M H] , the dilithiated species [M H C 2Li]C or the bariated species [M H C Ba]C allow one to localize the charge. The Ba as metal has the additional advantage to shift the fragments into mass regions where only little overlap with other fragments, i.e. little Ôchemical noiseÕ, is present. In addition, the Ba has a typical isotopic peak distribution. Other alkaline earth metals can also be used69. Electrospray ionization has been exploited by Wheelan, Zirrolli, and Murphy70 for tandem mass spectrometry of polyhydroxy unsaturated fatty acids. Last but not least, it should be remembered that other methods such as infrared spectroscopy71 or 13C NMR72 are often in competition with mass spectroscopy, albeit with less sensitivity.

The mass spectrometry of oleÞns is discussed in Sections V VII. The possibility to distinguish geometrical (E)- and (Z)-isomers by different mass spectral methods has been reviewed up to 1990 by Vairamani and Saraswathi73. It is not possible to formulate general rules for the fragmentation of unsaturated compounds, but a mathematical treatment of the intensities of the different peaks can give some indication of the location of the double bond. Thus Brakstad74 proposes the use of a least-squares correlation between the normalized spectral intensities of the electron impact mass spectrum as the independent variable and the CDC bond position as the response. Using 12 straight-chain monounsaturated fatty methyl esters, ranging in double-bond position from C(5) to C(11), incorporating both cis- and trans-isomers and chain lengths from C16 to C20, the author obtained the true position with a reproducibility of š0.46 at a 95% conÞdence interval. Sanchez and Kowalski75 use the advantage of the tandem mass spectrometry, thus obtaining two matrices to optimize. The method works, at least for lower oleÞns. An extended and modern use of the possibilities of the computer is the application of the fuzzy logic for the interpretation of the spectra, as has been done by Yuan, Horiike and coworkers. The procedure has been tested, partly in mixtures separated by gas chromatography, with tetradecenols76,77, hexadecenols78, dodecenols79,80, unsaturated acetates81 83 and in a practical application on insect pheronomes84.

For the separate determination of at least high boiling alkenes, a gas chromatographic separation before identiÞcation by mass spectrometry is necessary. With this combination Ramnas,¬ Oestermark, and Petersson separated 52 acyclic and 11 cyclic C5 C7 alkenes either in petrol85 or in the air emitted from petrol86. Different complex hydrocarbon mixtures were analyzed by Revill, Carr, and Rowland87. In such problems a heavy load is taken by the gas chromatographic separation, in particular by the choice and the quality of its column. Sojak,« Kraus and collaborators managed to separate

2. Mass spectra of double-bonded groups

27

C17 and C18 alkanes and alkenes88 and Þnally all 17 (cis- and trans-) isomers of nonadecenes on a mesogenic stationary phase and to identify them by mass and infrared (cis/trans!) spectroscopy89. The separation between two peaks in the gas chromatogram was often barely more than one Kovats« index unit. The determination of monounsaturated C12 to C18 acetates, aldehydes, alcohols and carboxylic acids was performed after gas chromatographic separation by Leonhardt, DeVilbiss, and Klun90 and Lanne, Appelgren, Bergstrom¬ and Lofstedt¬91. In both cases the correlation with certain mass fragment ratios was necessary to determine the position of the double bond within a certain probability. Polyunsaturated fatty acids from microalgae in the C16 to C22 range with up to six double bonds were determined by Bousquet, Sellier and GofÞc92. Hori, Sahashi, and Koike93 separated polyunsaturated fatty acids in triglycerides. In all these investigations, the main importance of the mass spectrometer is its high sensitivity and the possibility to determine the molecular weight. The combination of gas chromatography with tandem mass spectrometry (MS/MS) was used to analyze pentacyclic triterpenes by Hazai, Galvez-Sinibaldi, and Philp94. In this work a library search enabled the components to be identiÞed.

CI as a soft ionization tool, also capable of producing characteristic adducts, possibly coupled with a preceding gas chromatographic separation, is an efÞcient tool to produce speciÞc ions that can be characterized by collisional activation. Early reviews have been given by Ferrer-Correia, Jennings and Sen Sharma95 and Budzikiewicz96. Vairamani, Mirza and Srinivas97 reviewed up to 1988 unusual positive ion reagents in CI. The use of acetone as reagent ion by Vairamani and collaborators98 is described in Section VIII. A simple method is to let the molecular ion react with its neutral, a method not always possible in an analytical application. Ferrer-Correia, Jennings and Sen Sharma99 applied this technique to determine the position of double bonds. Einhorn, Kenttamaa¬ and Cooks100 enlarged the method for linear alkenes with 6 to 23 carbon atoms. Budzikiewicz and Busker101 studied various reactant gases (CH4, i-C4H10, NO, N2O, amines, ethers, Me2Si) for their usefulness in localizing double bonds, but only isobutane, NO and MeNH2 proved to be useful. Isobutane has been a reagent for CI of long standing. Doolittle, Tumlinson and Proveaux102 demonstrated that it can be used in different functionalized conjugated dienes in the C12 to C18 range, as long as the double bonds are not near or conjugated to the functional groups such as aldehydes, alcohols, formates and acetates. Einhorn and coworkers103 enlarged the series in the C12 to C16 range. Munson and coworkers104 concentrated on molecules of smaller molecular weight and formulated rules concerning the type of ions formed. The optimal conditions for the use of methyl vinyl ether have been studied by Jennings and collaborators95,105,106. A mixture of methyl vinyl ether (and N2) has been proposed by Chai and Harrison107, giving cleaner spectra and well deÞned adducts. DiMe ether was employed by Keough108 for the characterization of alkanes and alkenes. Formaldehyde and diethyl ether were found to be less useful. Dichlorocarbene, :CCl2, produced by pyrolysis from sodium trichloroacetate directly in the methane CI source, proved to be a good reagent to determine the geometric conÞguration of an added unsaturated compound such as stilbene, furmaric and maleic acid (Yang and coworkers109). It was found that the E-isomer of the alkene formed a more stable E-substituted dichlorocyclopropane ion than that of the corresponding Z-isomer, which more easily gives fragment ions. Acetonitrile, proposed by Traldi and coworkers110, has the advantage, that the [M C CH2CN]C cation seems to be very stable, independent of the manifold temperature, allowing one to obtain very reproducible results.

Nitric oxide as a reagent gas for the CI has been proposed by Hunt and Harvey111. The ions m/z 30 (NOC ) and m/z 60 [NO Ð NO]C react with internal oleÞns and dienes to produce M C NO C , MC and M 1 C . Terminal oleÞns produce in addition a series of fragments derived from the Markownikoff addition of NOC to the oleÞn linkage. This

28	Tino Gaumann¬

system has been studied in detail by Einhorn, Malosse and collaborators: in long-chain alcohols, acetates and aldehydes112 the double bond assignment is mostly provided by the presence of an acylium ion CxC2H2xC3OC formed from the alkyl side of the molecule. Similar to isobutane as reagent gas mentioned above, the fragmentation scheme changes when the double bond is located near a functional group. The study has been extended to alkenoic acids, esters and alkenes113. The reproducibility of the results is inßuenced by instrumental conditions. Not only the position of a double bond, but also the location of a cyclopropane ring is possible by this method114. However, according to Budzikiewicz and coworkers115 the reaction of aliphatic double bonds with NOC can be governed in a wide range by the remote functional group. This may explain certain discrepancies observed by other authors (compare Traldi110 cited above). The mechanism of the cleavage of the CDC double bond after the insertion of NOC is a complex process, as is demonstrated by Bukovits and Budzikiewicz116 by deuterium labeling. Homoconjugated dienoic acids do not give characteristic fragmentation patterns with NOC , as found by Brauner, Budzikiewicz and Francke117. Budzikiewicz, Blech and Schneider118 showed that also the position of the double bond close to the hydrocarbon end of an aliphatic diene functionalized at C(1) can readily be determined by CI with NOC , but the localization of the other double bond may be difÞcult owing to low abundance of characteristic fragments.

The use of Fe(I) CI has been explored by Peake and Gross. The authors60,119 propose the formation of an Fe oleÞn (or alkyne) intermediate complex, followed by the insertion of FeC into the allylic C C bond, followed by ˇ-H transfer to produce bis(oleÞn) complexes. The collisional-induced spectra allow the determination of the double or triple bond. The procedure has been extended to mixtures of oleÞns and alkynes, separated by gas chromatography120,121. The approach is successful over a 1:10 dynamic concentration range. Hydrocarbons with more than twelve carbon atoms pose problems because of successive fragmentations. Ni and Harrison122 studied the singly charged transition metal ions from ScC through CuC with six acyclic C5H8 isomers. ScC , TiC and VC , produced by FAB ionization of the solid metal chlorides, give distinctly different spectra for all isomers investigated. CI spectra with MeNH2 as reagent gas have been studied by Budzikiewicz, Laufenberg and Brauner123. The use of MeNH2 as reagent gas is not limited (as, e.g., i-C4H10 or NO) to straight-chain Z-alkenes. Addition to the double bond occurs also with E-di-, triand tetra-substituted and cyclic oleÞns. Isomers differing in their double bond position do in most cases give distinguishable spectra.

The concept of a charge-remote fragmentation is also useful in the domain of negative ions, especially coupled with collision-induced fragmentations, and contains practical structural information. The loss of CnH2nC2 from the alkyl terminus of unsaturated fatty acid carboxylate anions is such a case, as is demonstrated by Gross and coworkers61,124. Even the double bonds in polyunsaturated fatty acids were located by combination of FAB with MS/MS by these authors62. Bambagliotti, Traldi and collaborators125 produce [M H] anions from six C18 fatty acid methyl esters CI with OH as reagent gas. The positions of chain branching and the double bond are clearly recognized. The collisioninduced fragmentation of [M H] ions of isomeric decenyl and dodecenyl acetates were systematically studied by Takeuchi, Weiss and Harrison126. With the exception of double bonds in the 3 and 4 positions, the spectra are characteristic for the position of the double bond. This might be another example of the above-mentioned proximity effect of the charged functional end group. Stearic, oleic, linoleic and arachidonic acids have been recorded by GrifÞths and coworkers127, using electrospray ionization and collision of the pseudomolecular [M H] parent ion with Xe, using a new type of combined mass spectrometer. The double bonds are clearly characterized. The collision-induced spectra of hydrogenated cyclic fatty acids, analyzed as their pentaßuorobenzyl esters by

C5H10133

and C5H8134

2. Mass spectra of double-bonded groups

29

Le Quer«e« and colleagues128, furnish again characteristic fragmentations. The resonance electron capture furnishes an additional parameter for the analysis of different systems and is studied by Vionov, Elkin and Boguslavskiy129,130. The capture of electrons by fatty acids, their methyl esters and pyrrolides takes place in three electron energy regions: ca 0 eV, 1 2.5 eV and 7 eV. It was established that at low energy, fatty acids form carboxylate anions and ester carbanions by the loss of an H from C(2). In the high energy region neither of these ions is generated, but the fragment spectra contain the information about the structure of the neutrals131. A molecular orbital study for some of the measured ions gives some information about the mechanism132.

Photodissociation is also used to differentiate among different isomeric alkenes. isomers as examples are detailed in Section V. Photodissociation photoionization mass spectrometry is used to determine the sites of branching and

unsaturation in small (C5 C8) aliphatic compounds, as is shown by van Bramer and Johnson135. The same group extend this research also to higher oleÞns136. Van der Hart137 and Tecklenburg and Russell138 reviewed some aspects of the Þeld up to 1988 and 1989, respectively.

Derivatization, i.e. the transformation of the double bond (or the terminal functional group), can be due to two reasons: Either it substitutes the double bond by a group that is more easily recognizable in the mass spectrometric fragmentation, or/and it makes the transfer into the gas phase easier. The price to pay are additional chemical reactions on the neutral and, coupled with it, probably some loss of sensitivity. An overview of the different possible strategies up to 1987 was presented by Anderegg139. A comparison with 13C NMR is given by Schmitquiles, Nicole and Lauer140. DiMe disulÞde seems to be a very favorable choice. The addition reaction of disulÞdes to alkenes was studied by Caserio, Fisher and Kim141. Francis and Veland142 worked with alkenes between C11 and C16, Bhatt, Ali and Prasad143 with mixtures of dehydrogenated parafÞns between undecenes and tetradecenes, but containing many aromatic compounds, only partly separated by GC-MS. Hexa-, heptaand octadienes were studied by Pepe and coworkers144. Carlson and collaborators145 derivatized natural and synthetic long-chain alkenes, alkadienes and alkatrienes (C25 C37) and investigated the problem of their separation and identiÞcation in mixtures by GC-MS. Attygalle, Jham and Meinwald146 determined the double-bond position in unsaturated terpenes and other compounds branched at the double bond. In contrast to the spectra of the derivatives containing initially the CHDCH double bond, which show two predominant fragment ions, most of the branched compounds showed only one predominant fragment ion, arising from that part of the molecule which possesses the more substituted carbon of the double bond. A few nanograms furnished already good spectra. Long-chain acetates (C11 C18) were investigated by the groups of Buser147 and of Vincenti148. The double-bond locations in fatty acid esters were determined by Pepe and coworkers149,150. The separation and double-bond determination on nanogram quantities of aliphatic mono-unsaturated alcohols, aldehydes and carboxylic acid methyl ester were effectuated by Leonhardt and DeVilbiss151. Beside the position of the double bond, also cis trans isomers are distinguished. The use of oxazoline derivatives of unsaturated fatty acid esters is compared (and combined) with an infrared technique by Wahl and coworkers152. The infrared spectrum is less sensitive, but allows an easy distinction between cis/trans isomers. A combination of both techniques seems to be very useful.

ÿ	153	to identify polyunsaturated fatty acids from
The same derivatives were use by Rezanka

slime moulds. The derivatization of oleÞns by cycloaddition of halocarbenes seems to be a promising method, studied by Schlunegger, Schuerch and collaborators154; the use of bromoßuorocarbene offers some advantages155. A ßow-chart type procedure of analysis is proposed.

30	Tino Gaumann¬

IV. ETHYLENE: AN OLD FAITHFUL

Ethylene is a relatively simple ion that allows measurements and comparison with calculations that are not possible for more complex ions. The ground state conÞguration

of ethylene is 1ag21b21u2ag22b21u1b23u3ag21b22g1b22u. The highest occupied orbital is of theCC type and therefore concentrated in the carbon p orbitals. Several electronic levels of

ethylene have been determined by Ohno and collaborators156 by Penning ionization by collision with HeŁ in the 23S state. They improved the resolution compared with their earlier work157 (see also Kimura and colleagues158) and measured Þve bands (in parentheses the orbital character and the state): 10.51 eV ( CC, 1b2u); 12.85 eV ( CH, 1b2g);

14.66 eV ( CC, 3ag); 15.87 eV ( CH, 1b3u); 9.10 eV (C2s, 2b1u). The cross section for the Þrst state decreases with increasing collision energy, indicating that the interaction poten-

tial between the He atom and the -orbital of ethylene is attractive. This could have been expected, since the -orbital behaves as a Lewis acid to form hydrogen bonds with electron acceptors159, but the authors put a question mark to this explanation. However, this decrease seems to be a general trend for unsaturated hydrocarbons157. The cross section of the other bands increases with increasing collision energy (contrary to the isoelectronic HCHO molecule), which is explained as a repulsive potential between these orbitals and He. Higher energy levels have been determined and compared with theoretical ab initio calculations with the help of the Auger spectra up to 64 eV by Liegener160. Two of the vibrational levels of ethylene ion and its completely deuterated isotopomer have been reviewed and calculated by Somasundram and Handy161. Both are lowered compared to the neutral molecules. Ab initio molecular orbital calculations on the 1,2-H shift on ethylene, allene and propyne by van der Hart162 show that CH3CHCž is not a stable structure, whereas CH2CHCHCž obtained from the two other ions is only stable because the ion can relax to a more stable form, which could be described as an allyl cation with one of the terminal hydrogen atoms removed.

The photoelectron photoion coincidence mass spectrometry has allowed one to elaborate the fragmentation mechanism at the lower energy limit. Stockbauer and Inghram163 determined the ionization potential (10.517/10.528 eV), the appearance energy for the loss of H/D (13.22/13.41 eV), of H2/D2 (13.14/13.24 eV), of 2H/2D (17.86/17.91 eV) and of CH2/CD2 (18.04/18.13 eV), where the second energy refers to the deuterated compound. Bombach, Dannacher and Stadelmann164 calculated the breakdown diagram for energies up to 18 eV and compared it with the experimental values of Stockbauer and Inghram. A very good coincidence was found. Tsuji and collaborators165 used the chargetransfer reaction from ArC to measure the branching fraction at an energy of 15.76 eV and obtained 4% C2H4Cž , 76% C2H3C and 20% C2H2Cž . The fraction of available energy deposited into internal energy of the ions is estimated to be 85 95%. They obtained similar results for CH4, C2H6, C3H6 and C3H8, but a much smaller value (38%) for C2H2. The Balmer emission of the hydrogen atom from ethylene, ionized by 70-eV electron ionization, allowed Beenakker and de Heer166 to draw conclusions about the ionization process at this energy. The investigation of the nonresonant multiphoton ionization at two wavelengths (532 nm and 355 nm) allowed Martin and OÕMalley167 to study the fragmentation of acetylene, ethylene, ethane, propene, propane, isobutene and cis-2-butene as a function of the laser energy. The values for the bond dissociation energies have been reviewed by Berkowitz and collaborators168,169. An experimental and theoretical study of the photoionization of vinyl chloride allowed Li and collaborators170 to draw conclusions about possible isomeric structures for the polyatomic fragments. The gas phase acidity of ethylene has been determined by Graul and Squires171 and DePuy, Bierbaum and collaborators172. Both groups arrived at an identical value of 1703 š 4 kJ mol 1.

2. Mass spectra of double-bonded groups

31

The isotope effect of the fragmentation of the ethylene ion has been treated experimentally and theoretically by several authors. Gordon, Krige and Reid173 observed a strong isotope effect and an H/D scrambling for trans CHDCHD and C2HD3. Hvistendahl and Williams174 deduced from the isotopic substitution for molecular hydrogen elimination that the two C H bonds in the transition state must be synchronously stretched. All possible isotopomers of ethylene have been synthesized and studied by Vial, Nenner and Botter for dissociation within the source as a function of the electron energy175 or in the metastable range176. They can explain their results with the quasiequilibrium theory by allowing for the difference in zero-point energies; no tunneling was needed.

The concentration of doubly charged ions in the 70-eV mass spectrum of ethylene is only ca 1% of all ions, making their study difÞcult. A theoretical investigation of the outer valence doubly ionized states of ethylene was undertaken by Ohrendorf, Sgamellotti and coworkers177, who showed that the ground state of the dication is nonplanar with a torsional angle of 90°. Auger spectroscopy permitted the authors to determine many double ionization potentials from 29.46 eV up to 47 eV and to attribute the orbitals involved178. The ionization and appearance energies of several doubly charged ions of ethylene, dißuoroethylene and a few other compounds was the subject of a study of Brehm, Frobe¬ and Neitzke179. For ethylene they measured the following energies: C2H4CC: 31.4 eV;

C2H3CC 35.7 eV; C2H2CC 36.8 eV; C2HCC 50.1 eV; for dißuoroethylene C2H2F2CC 28.5 eV; C2H2FCC 33.4 eV; C2HFCC 35.5 eV; C2H2CC 42.0 eV; C2HCC 51.8 eV. The

dissociation of doubly charged CH2CD2 and CH2CF2 ions by single photon excitation of the valence electrons with photons of 37 and 75 eV was evaluated by Ibuki and collaborators180. The branching ratios for a large number of fragments were determined. It seems that the double ionization releasing at least one CC electron seems to give a large contribution to the central CDC bond cleavage, while the double ionization of CC,CH and C2s orbital electrons occurring at photon energies above 37 eV results in bond Þssion to form two smaller fragment ions. By using OHC as projectiles, GrifÞths and Harris181 populated mainly triplet electronic states, thus sorting out the values of 31.4, 34.9, 38.2, 40.3 and 42.9 eV from the large number of values determined or calculated by the authors cited above. A theoretical and experimental study on tetraßuoroethylene dication has been undertaken by Schwarz and coworkers182. It is accessible by charge stripping from C2F4Cž . According to the calculations the planar (D2h) form is 15 kJ mol 1 more stable than the perpendicular (D2d) isomer, in distinct contrast to the analogous ethylene dication (see above).

Ethylene cation forms easily clusters with neutral ethylene. The question if the ethylene cation preserves its structure in such a polymer or whether it undergoes an ÕinternalÕ ion molecule reaction to form ions of higher molecular weight has found much interest; e.g. Ono and collaborators183 cited, in 1983, 30 references on the subject without being complete. The basic question is under which conditions does an ion molecule reaction take place, because the reverse reaction, the loss of an ethylene neutral, is one of the main fragmentations of alkenes, at least in the metastable range (see later). The cluster properties can be regarded from two sides: either the ethylene cation collects its neutrals by some sort of ion molecule reaction, which demands a minimum pressure that can also collisionally stabilize the freshly formed clusters, or the clusters are preformed, preferentially in a molecular beam, and subsequently ionized. In this case the problem consists of separating the neutral clusters of different size. This can be done by elastically scattering with He and using angular-dependent mass spectra. Buck and collaborators184 showed that ionization of the neutral dimer yields the fragmentation spectrum of the monomer: the internal energy is redistributed and results in the evaporation of the second ethylene. A small contribution of higher masses than m/z 28 is attributed to internal ion molecule reaction. For trimers

32 Tino Gaumann¬

and tetramers again a large probability to form the monomer ion is observed. However, C3H5C and C4H8Cž are now becoming the prominent ions. The structure of the latter ion forms one of the main problems in this kind of research. It can be concluded that the fragmentation of such clusters is dominated by ion molecule reaction of a single ionized C2H4Cž ion within the cluster. In Þeld ionization conditions Beckey and coworkers3,185 showed that C2H5C is the main reaction product of ethylene. Since the protonation of ethylene cannot be achieved by a Þeld reaction of ethylene ions, the reaction must take place in a physically adsorbed layer.

The current interest in clusters stems from their unique position as an aggregate state of matter between the gas and the condensed phase. Photoionization is probably one of the best methods to investigate the behavior of neutral van der Waal clusters. Ceyer and coworkers186 chose their expansion conditions in a way to produce mainly the neutral dimer, which is subsequently photoionized by removing an electron from the 1b3u ( - bond) of the ethylene, the vibrational Þne-structure near the threshold serving as evidence. They determine a well depth of 76 š 2 kJ mol 1 for the dimer and of 18 š 3 kJ mol 1 for the trimer ionic complex. The authors propose structures and energy diagrams for the solvation of the fragments. The molecular beam photoionization method was also used by Tzeng and collaborators to study the behavior of ethylene dimers183 and trimers187. They made the observation that at nozzle expansion conditions, where the trimer and heavier clusters produced in the beam are higher, the appearance energies for the C3H5C and C4H7C fragments from the dimer are shifted from 10.21 š 0.04 eV and 10.05 š 0.04 eV, respectively, to lower values, indicating that trimers and tetramers can give rise to the same product ions, conÞrming earlier values183. The ionization energy of the dimer is found to be 9.84 š 0.04 eV. The group of Sieck and Ausloos188 photoionized different isotopomers of ethylene containing two D atoms and determined the distribution of the deuterium in the C3(H,D)5C ion as C3HD4C 8%, C3H2D3C 39%, C3H3D2C 40% and C3H4DC 11% a nearly random distribution, independent of the initial position of the D atoms. Tzeng and coworkers187 observed several fragmentation pathways for the trimer, yielding the following product ions: C3H6C , C3H7C , C4H7C , C4H8Cž , C5H9C and C6H11C . The fact that these channels are similar to those observed in the unimolecular decomposition of (C3H6)2C and (c-C3H6)2C is consistent with the interpretation that these loose complexes rearrange to similar stable C6H12C ions prior to fragmenting. The ionization energies of the trimer and the tetramer were determined as 9.46 š 0.04 and 9.29š0.03 eV and the binding energies for successive ethylene units for (C2H4)2C C2H4

and (C2H4)3C		C2H4	as 38	š	4 and 19	š	4 kJ mol 1, respectively. The precision of
								189	because of the
some of these values has been questioned by Baer and collaborators

gradual onset of the photoionization efÞciency curves. Meisels and coworkers190 stated that the angular momentum can have an inßuence in ion molecule reactions and can affect the branching ratios when the products are selected by an angle-sensitive method. Electron ionization is used by the group of Garvey191 to investigate clusters of ethylene, 1,1-dißuoroethylene and propene as a function of expansion and ionization conditions. For ethylene and dißuoroethylene, a peaking is observed for clusters containing four molecules. For propene, it changes from three to four and then to six, with increasing expansion pressure and lowering of the electron energy. This is explained by intracluster ion molecule reactions, i.e. the formation of covalent bonds between the single units.

The structures and isomerization of C4H8Cž ions in connection with the problem of ethylene clusters has been the subject of many studies. Doepker and Ausloos192 studied the photolysis of cyclobutane, its deuterated isotopomer and mixtures thereof, and in their detailed product analysis they found cis-2-butene, trans-2-butene and 1-butene as major ionic products in the approximate ratio of 1:1:2. Lias and Ausloos193 determined