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Enolate ion MeOCOCHCH2CO2Me fragments through the intermediacy of two ion complexes [ MeOCO (CH2=CHCO2Me)] and [ MeO-(O=C=CHCH2CO2Me)]. The major fragmentations produce CH2=C=C( OMe )O-+HCO2Me and O=C=CH- CH=C( OMe ). Similar ion complexes are formed from methyl-substituted succinates , but their fragmentations are often different from those outlined above: e.g. MeOCOC (Me)C(Me)2CO2Me - [ MeOCO (M2C=CmeCO2Me)] → CH2(Me)C=C(Me)CO2- Me+HCO2Me. The corresponding enolate from hexane-2,s-dione (MeCOCHCH2COMe) fragments through the acetyl anion complex [ MeCO (CH2=CHCO2Me)].

The major collision-induced fragmentations of deprotonated primary and secondary amines are best rationalized as proceeding through the intermediacy of ion complexes. For example, the characteristic fragmentation of deprotonated ethylamine isMeCH2NH- → [Me-(CH2=NH)] → CH2N- + CH4Secondary alkylamines behave in a similar fashion. The occurrence of proton transfer as a prelude to fragmentation is rare: the only example observed in this study is the probable reaction PhNEt →→ PhNHCHMe → [(PhNH=CH)Me-] → C7H6N- + CH4which is preceded or accompanied by proton transfer between the methylene and phenyl substituents. Deprotonated aniline undergoes specific elimination of CNH from the 1-position to form the cyclopentadienyl anion. Finally, retro reactions are observed for the piperazine anion, e.g. HN(CH2CH2)2N → -CH2CH2N=CH2+CH2NH

Treatment of laevulinic acid with aniline yields 5-methyl-2,3-dihydro- 1H-1-benzazepin-2-one and 5-(2-methyl-5-oxo-1-phenylpyrrolidin-2-yl)-4- oxo-N-phenylpentanamide In contrast, the reaction between β- benzoylpropionic acid and aniline yields 4-(2-oxo-1,5-diphenyl-2,3- dihydropyrrol-3-ylidene)-N,4-diphenylbutanamide, whose structure is confirmed by an independent synthesis. The yield of the latter type of reaction is enhanced if p-methoxyaniline is used, whereas the desired benzazepine is obtained if m-methoxyaniline is utilized. For example, the reaction between β-benzoyl-propionic acid and m-methoxyaniline yields 8-methoxy-5-phenyl-2,3-dihydro-1H-1-benzazepine-2-one as the major cyclized product.

The benzoate anion reacts with acetic anhydride to yield an intermediate which decomposes by loss of ketene with accompanying proton transfer. The proton transfer is involved in the rate- determining step of the elimination reaction. The abundance of the product ion in the reaction of RC6H4CO2- with acetic anhydride increases as the electron-donating properties of R increase. Collision-induced charge stripping of the ions RC6H4CO2- yields the corresponding cations which fragment primarily by elimination of CO2 (or O+CO). Fragmentation through the carboxyl centre precedes the fragmentation of R (e.g. when R = Me, MeO, Cl or Br).

Kinetic energy release values [T(50%) values] between 0.05 and 0.8 eV have been noted for the unimolecular decomposition M-? → (M-NO?)-? of the molecular anions of variously substituted nitrobenzenes. Large T values are observed when the substituent is strongly electron-withdrawing. Composite peaks are obtained for m-nitrobenzoic acid and m- nitroacetophenone, showing the occurrence of two distinct processes in these cases.

Kinetic energy release values T(50%) for the collision-induced formation of the p-nitrobenzoate and p-nitrophenoxide anions from the molecular anions of p-O2NC6H4COOC6H4R-p and p-O2NC6H4- OC6H4R-p respectively are small and relatively insensitive to the nature of the substituent R.T (50%) values for the elimination of R? (R = alkyl) from O2NC6H4OR molecular anions range from 0.4 to 0.01 eV, and are dependent upon the nature of R and upon its number of active oscillators.

The negative ion mass spectra derived from the three dinitrobenzenes show NO2-, [M-?-NO?]- and M-? as major ions. The i.c.r. spectrum of o- dinitrobenzene shows the formation of the adducts [M+NO2-]-, [M+(M-?-NO?)- ]- and 2M-?, but the m- and p-dinitrobenzenes only produce the adduct [M+NO2-]-. The nucleophile Cl- adds to each of the three dinitrobenzene neutrals in the abundance ratio o >> m ≈ p. Evidence is presented which suggests that the neutral products formed by the reaction between NO2- and o-dinitrobenzene are dinitro- and trinitro-benzenes, whereas those resulting from the interaction of Cl- with o-dinitrobenzene are chloronitrobenzene and a chloro-dinitrobenzene.

Acetic anhydride[α,α?-D2] has been synthesized in near-quantitative yield by the reaction between acetic acid[α-D] and dicyclohexylcarbodiimide. The gas-phase reaction between the acetate anion and acetic anhydride[α,α-D2] yields a 1 : 1 adduct which decomposes by loss of CH2CO and CHDCO so that kH/kD is 1.2 when the ion- transit time is 10-3-10-4 s, and 2.0 at 10-1-10-2 s.

The positive ion mass spectra of long-chain alcohols and acids are either devoid of molecular ions or contain such species in low abundance. The negative ion spectra of p-nitrophenyl esters of these compounds exhibit pronounced molecular anions, showing the technique to be a useful analytical method for these classes of compounds.

Possible structures for some rearrangement ions in the negative ion mass spectra of 3- and 4-nitro- phthalic anhydride are suggested from an examination of the + E spectra obtained by the conversion of the negative ions to positive ions by charge-stripping reactions. The reactivities of the non-decomposing negative ions have been investigated by observation of their ion-molecule reactions with the neutral anhydride in the cell of an ion cyclotron resonance spectrometer.

The gas phase reaction between the acetate anion and acetic anhydride yields a 1 : 1 adduct, which may decompose by loss of ketene. The extent of these reactions depend upon both the sample pressure and the transit time of ions in the cell. Power absorption maxima are obtained for electron beam energies of 0 eV, 4 eV and values in excess of 10 eV.

The gas phase reaction between the trifluoroacetate anion and perfluoroacetic anhydride produces a 1 : 1 adduct. The decompositions of this adduct suggest a tetrahedral species rather than a loose association complex. If this is so, this constitutes the first direct observation of the formation of a tetrahedral intermediate produced by anionic displacement at a carbonyl centre.

Positive ion-molecule reactions of representative alkyl and aryl cyanides have been studied by ioncyclotron resonance spectroscopy. The major secondary ion produced from alkyl cyanides is an [MH]+ species. Neutral alkyl cyanides react preferentially with even-electron ions produced from alkyl cyanide molecular ions by the general process +R1 + R2CN → R2-C=N+-R1 but the corresponding aryl cyanides react with both odd- and even-electron ions. The characteristic reaction of acetonitrile,'s2 viz. CH2CN+ + CH3CN → C3H4N+ + HCN does not occur for higher homologues, but a mixture of acetonitrile and propionitrile gives the analogous reaction CH2CN+ + C2H5CN → C4H6N+ + HCN

Collisional activation of alkyl and aryl siloxide ions produces a variety of ions and neutrals containing double bonds to silicon. Sila enolate ions and sila acyl ions RSi =O are common features of the spectra of alkylsiloxide ions; ab initio calculations suggest these ions to have considerable multiple bond character. It is proposed that most reactions proceed through ion complex intermediates. Specific proton transfer reactions often occur for ions Me3SiCHR (R = C3H5, C3H3 or Ph), i.e. Me3SiCHR → -CH2(Me)2SiCH2R. In the particular case of the formation of PhCH2- from Me3SiCHPh, i.e. Me3SiCHPh → -CH2(Me)2SiCH2Ph → PhCH2- +Me2Si=CH2, the proton transfer competes with scrambling of all hydrogens in the decomposing ion.

The mechanisms of formation of the major product ions produced by collision activation of the enolate negative ion of 3,3-dimethylheptan-4-one have been studied by deuterium labelling. H2 loss involves 1,2-elimination from the 6- and 7-positions. Methane elimination is complex involving three competitive rearrangement processes. Ethene loss produces the most abundant fragment ion and occurs by elimination of the C1-C2 ethyl group with concomitant proton transfer from the 1-position. In the case of the enolate ion from CD3CH2CMe2COPr, the expected elimination of C2H2D2 competes with loss of C2H4 from positions 6 and 7. Other decompositions are as follows: loss of C3H8 involves the methyl and ethyl substituents at positions 7, 1 and 2 respectively; loss of C4H8 occurs by two processes, ( i ) successive losses of two C2H4 units (from the 1, 2 then 6, 7 positions), and (ii) loss of CH2=CMe2 (from the 2 and 3 positions); and loss of C5H12 produces Et-C=C-0-. .

The gas-phase reactions of MeO ---- HOMe with acetone and methyl acetate to form solvated enolate anions have rates of 2.0×10-9 and 4.8×10-11 cm3 molecule-1 s-1. It is proposed that the latter reaction is slow because internal barriers (transition states more negative in energy than reactants) cause the reactants to be entropy- favoured. Rates of reaction decrease dramatically as the extent of solvation of the nucleophile is increased; e.g., for the reactions of MeO -( HOMe )n(n = 1- 3) with acetone, the rates for n = 1-3 are 2.0×10-9, 1.5×10-10 and 1.6×10-12 cm3 molecule-1 s-1, respectively.

The gas phase reactions of the systems Me2C=+OH/ROH (R = Me and Pri ) have been investigated by using a combination of experimental [ion cyclotron resonance ( i.c.r .) and flowing afterglow ( f.a .)] and ab initio (4-31G, GAUSSIAN 82) methods. A stabilized adduct, thought to correspond to Me2C=+OH...O(H)R, is observed when R = Pri but not for Me. Decomposing forms of adducts eliminate water; 18O labelling shows the major (overall) reaction to be Me2C=+OH+R18OH → Me2C=+(18O)R+H2OAb initio calculations indicate ( i ) that the product ion in this reaction is produced by two channels, both proceeding through Me2(HO)C-+O(H)R; one directly, the other indirectly through Me2C=+OH...O(H)R, and (ii) that the transition state for the key proton transfer reaction Me2(HO)C-+O(H)Me → Me2( MeO )C-+OH2 is of higher energy than reactants. The latter result suggests that precursor ion Me2C=+O needs to be vibrationally hot for the elimination of water to occur: a comparison of i.c.r . and f.a . experiments confirms this to be the case.

Treatment of laevulinic acid with N- methylaniline yields 1,5-dimethyl- 1H-1-benzazepin-2(3H)-one and 5-methyl-5-[4-( methylamino )phenyl]-1- phenylpyrrolidin-2-one. The structure of the latter is confirmed by a single-crystal X-ray study. The yield of benzazepinone is increased if either m- methoxy-N-methylaniline or m-methyl-N- methylaniline is allowed to react with laevulinic acid. Treatment of the benzazepinones with phosphoryl chloride in pyridine produces 2-chloro-1,5-dimethyl-1H-1- benzazepines in quantitative yields, but these compounds are highly reactive under acidic conditions, undergoing nucleophilic displacement at C2. In contrast, treatment of benzazepinones with neat phosphoryl chloride yields yellow dimers , assigned as 2′-chloro-1,1′,5,5′- tetramethyl-2,3′-bi-1H-1-benzazepines, and with stabilities only marginally greater than those of the 2-chloro-1,5-dimethyl-1H-1- benzazepines.

The ion [F-- - -HOMe] may be produced by the reaction of CF3O- with methyl formate, a reaction of a type widely used for the production of 'solvated' negative ions. The possible reaction mechanism for the model system F-/HCO2H has been explored by ab initio calculations. The reactions of acetaldehyde and acetone with [F-- - -HOMe] yield stable [M + F-] ions which are thought to correspond to 'solvated' enolate negative ions. Their formation in terms of a mechanistic pathway, recently proposed to account for the formation of [M + RO-] ions from [RO-- - -HOR]/ > CH-CO- systems, is supported by ab initio calculations.

Simple alkyl aldehydes, ketones and esters containing the unit >: CH-CO-react with alkoxide-alkanol negative ions [RO----HOR] to form stable [M+RO-] negative ions (neutral denoted by M). The initial intermediate in the reaction sequence is produced by the formation of a hydrogen bond between the negatively charged oxygen of [RO----HOR] and the hydrogen of the > >CH-CO- unit. This intermediate decomposes to the stable product ion by two reaction sequences: (i) by direct elimination of ROH, and (ii) by rearrangement to a decomposing 'doubly-solvated' negative ion in which the central hydrogen of [RO----HOR] and the hydrogen of the >CH-CO unit become identical. As both reaction pathways are complex, all intermediates and certain transition states have been properly defined by using ab initio calculations at the 4-31G level.

Nucleophilic attack of a ketone, carboxylic acid or ester at the electrophilic centre of the trimethylsilyl cation (Me,Si+) produces a 1:1 adduct. This adduct does not decompose in the case of ketones. Acid adducts decompose primarily by loss of methane, but a minor pathway exists which involves elimination of a ketene. Ester adducts fragment primarily by this latter process through a four-membered transition state. The transfer of hydrogen was shown to arise solely from the acyl group and was shown to proceed with a small deuterium isotope effect. Further decomposition of the resulting ion by loss of methane provides unequivocal proof that esters react predominantly through the alkyl oxygen with the Lewis acid Me3Si+.

Nucleophilic attack of an alcohol (ROH) at the electrophilic silicon centre of the trimethylsilyl cation (Me3Si+) produces the 1 : 1 adduct Me3-O+(H)R(1). The adduct may fragment by loss of methane to yield Me2Si+-O-R; and this elimination is most pronounced when R = Me. When R ≥ C2H5, the major decomposition pathway of (1) involves elimination of the alkene [R-H] to produce Me3Si-O+H2, which may undergo further reaction with the neutral alcohol to reform (1). The proton transfer which accompanies the elimination of [R- H] from (1) originates predominantly from C2 of the alcohol; this suggests the intermediacy of a four-membered transition state in this reaction.

Abundant peaks produced by retro-Diels-Alder cleavages are observed in the dissociative + E spectrum of a negative ion, whenever such peaks are seen in the corresponding positive-ion spectrum of that compound.

Ion cyclotron resonance allows the sensitive measurement of doubly charged anions at low cell pressures. Evidence is presented which suggests the formation of X-- (X = halogen) ions from alkyl halides.

Molecular anions derived from Ar-CH=NOH systems fragment by the process [MI- → [M-H2O]- to yield pronounced peaks corresponding to [ArCN]-. Molecular anions from Ar(CH2)nC(NOH)Ph (n = 0-2) also eliminate H2O, but the mechanisms are often complex. Possible mechanisms for some of these eliminations are suggested after consideration of the + E spectra of non-decomposing [M?H2O]- ions. A characteristic cyclization/retro process of o-O2NC6H4-CH=CH-C(NOH)R molecular anions produces CHO-CR=N-O- as the major product anion.

+ E spectra derived from the molecular anions of flavones and quinones are used to illustrate and compare the resolution and sensitivity of spectra obtained by both magnetic and electric sector scan techniques. Some fragmentations are noted which do not occur in the conventional positive-ion spectra of these compounds.

Collision-induced decompositions of molecular anions of the general formula aryl-CO-alkyl yield aryl- ions. The aryl-CO-aryl group is stable under these conditions. Parent anions from α-dicarbonyl systems do not generally fragment but, in the case of o-nitrobenzil, fragment anions are produced after cyclization of the nitro group at both carbonyl centres.

The C7H7O- ions formed by deprotonation of benzyl alcohol, norbornadien-7-ol and quadricyclin-7-ol are discrete species which all fragment by competitive losses of H, H2, CH2O and C6H6 on collisional activation. The isomeric ion from norbornen-2-one behaves differently undergoing retro cleavage to form HC2O- and C5H5- as product ions. The three deprotonated cresols are also distinct species. Anisole is deprotonated (by amide ion) to form both PhOCH2- and (C6H4)-OMe, ions which equilibrate prior to elimination of formaldehyde.

Siloxide ions undergo O/S exchange reactions with suitable sulfur-containing neutrals, e.g. H3SiO-+CS2 → H3SiS-+COS. Silamide ions similarly undergo NR/O and NR/S exchange reactions together with nucleophilic displacement reactions, e.g. Me3SiNMe + CO2→ Me3SiO-+MeNCO →No simple correlation between rate and mechanism is observed for all the studied reactions.

Ab initio calculations indicate that the collisional induced losses of molecular hydrogen from the ethoxide negative ion and methane from the t- butoxide negative ion to be stepwise processes in which the key intermediates are [H-… MeCHO ] and [Me-…Me2CO] respectively. Deuterium kinetic isotope effects observed for these and other alkoxide negative ions are in accord with the operation of a stepwise reaction.

Alkanol-alkoxide negative ions [RO----HOR] react with ketene to form stable species H-C≡C-O----HOR. The initial intermediate in the reaction sequence is produced by the formation of a hydrogen bond between the negatively charged oxygen of [RO----HOR] and a hydrogen of ketene. This intermediate decomposes by competing reactions, viz. ( i ) a minor pathway involving direct elimination of ROH, and (ii) a major pathway involving a rearrangement sequence. Intermediates and product ions have been defined by using ab initio calculations at the 4-31G level. The differences between the reactions of [RO----HOR]/ketene and [RO---- HOR]/acetaldehyde systems are discussed.

Thioglycollates and β-thiopropionates with SH and/or CO2H groups yield pronounced (M-H)- ions in their negative ion spectra, not molecular negative ions as we previously reported. Compounds containing R1-S-CH2- functionality (R1 = alkyl or aryl) undergo α cleavage to S to yield R1S- and -S-CH2-ions. Hydrogen rearrangements are rare but thioglycollates R1SCH2CO2H characteristically eliminate R1SH to yield CH2CO2-. Fragment negative ions generally do not contain enough excess energy to allow decomposition, so we have studied their collision induced dissociations. Simple cleavage reactions are usual: for example BuSCH2CO2- decomposes to yield both BuS- and BuSCH2-. Group migration to sulfur is also observed; for example, ions -SCH2CO2R2 (R2 = alkyl or aryl) eliminate carbon dioxide to produce R2SCH2- as the major product.

α-Dicarbonyl compounds form stable parent negative ions by capture of low-energy electrons. The parent negative ions undergo a variety of collision induced dissociations, the most characteristic being β cleavage to a carbonyl group, i.e. see diagram in paperCyclic α-dicarbonyl parent negative ions may also eliminate molecules of carbon monoxide, for example the fragmentations M- → [M-(R + CO)]- M- →[M - (R-2CO)]- and M- → [M-2CO]- occur in a number of cases.

Ab initio LCAO-MO-SCF calculations suggest that reaction of HO-(or MeO-) with silane should proceed through a stable trigonal bipyramidal adduct. Reaction of alkoxide anions with a variety of alkylsilanes in an i.c.r. spectrometer leads to (i) nucleophilic displacement reactions, (ii) stable adducts, (iii) decomposing adducts which collapse to tetrahedral anions by elimination of a neutral molecule, and (iv) proton abstraction reactions. The nucleophilic displacement reaction between MeO- and Me3SiR has been used to produce a variety of R- species, including the allyl, propargyl and benzyl anions. The reactions of these ions with methyl nitrite have been used to show that (i) the benzyl anion does not undergo benzylic hydrogen/phenyl hydrogen scrambling prior to reaction, (ii) the allyl anion reacts as a symmetrical species, and (iii) that hydrogen scrambling does not occur in the propargyl anion, and that C3H3- may either react as HC≡C-CH2- or -HC=C=CH2.

(i) The i.c.r. spectrum of the CD3O-/acetic anhydride system shows the occurrence of the negative ion McLafferty rearrangement [reaction (1)] and the characteristic elimination shown in reaction (2) (R = COMe) CD3O-+ (MeCO)2O → (CD3O)(Me)(HO)C-O-+ CH2CO (1) CD30-+ MeCO2R → -CH2C02CD3+ ROH (2) (ii) The i.c.r. spectra of CD3O-/alkyl ester systems show major peaks due to [ester-H+]- ions, together with small peaks corresponding to [ester+CD3O-]- adducts. Carboxylate and alkoxide anions (derived from the ester) are also observed in certain spectra; peaks due to these ions increase in intensity with elaboration of alkyl substituents. Reaction (2) is characteristic of all esters studied (illustrated above for acetates) which have at least one hydrogen substituent on the carbon atom α to the carbonyl group. The reaction must occur by nucleophilic attack of the alkoxide anion at the carbonyl group of the ester. Negative ion McLafferty rearrangements do not occur in alkyl ester systems. (iii) The H transfer reactions (1) and (2) show deuterium isotope effects kH/kD of 3.0 and 1.5 respectively (when the carbon adjacent to the carbonyl group contains one deuterium substituent). Isotope effect calculations suggest that reaction (1) proceeds through a 'near symmetrical' six membered transition state, whereas (2) goes through a 'reactant-like' but distorted four-centred transition state.