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The dicoordinated borinium ion, dihydroxyborinium, B(OH) 2 + is generated from methyl boronic acid CH 3 B(OH) 2 by dissociative electron ionization and its connectivity confirmed by collisional activation. Neutralization–reionization (NR) experiments on this ion indicate that the neutral B(OH) 2 radical is a viable species in the gas phase. Both vertical neutralization of B(OH) 2 + and reionization of B(OH) 2 in the NR experiment are, however, associated with particularly unfavorable Franck-Condon factors. The differences in adiabatic and vertical electron transfer behavior can be traced back to a particular π stabilization of the cationic species compared to the sp 2 -type neutral radical. Thermochemical data on several neutral and cationic boron compounds are presented based on calculations performed at the G2 level of theory.

Tandem mass spectrometry based experiments on the decarbonylation products of ionized methyl-β-hydroxypyruvate (MHP) and dimethyloxalate (DMO) show that the hydrogen-bridged radical cation (HBRC) CH2O ⋯ H ⋯ OC–OCH3·+ is a stable species in the gas phase. Its low energy dissociation products are protonated methylformate, HOC(H)OCH3+, and the formyl radical, HCO·. The HBRC isomers HOCH2C(O)OCH3·+ (ionized methylglycolate) and (CH3O)2CO·+ (ionized dimethylcarbonate) show the same dissociation characteristics. Deuterium labeling experiments dictate that loss of HCO· from the title HBRC cannot be formulated as a simple H shift from the formaldehyde moiety to the C atom of the OC·–OCH3 group. Ab initio molecular orbital (MO) calculations support the proposal that this dissociation proceeds via sequential transfers of a proton, electron, and another proton within ion–dipole complexes. The first step in this rearrangement process is a 1,2-proton shift catalyzed by a formaldehyde dipole. This yields an ion/dipole complex, CH2O ⋯ H–C(O)OCH3·+, that is in the correct configuration for electron transfer to occur at the energetic threshold dictated by experiment. The resulting intermediate triggers the transfer of yet another proton from the formaldehyde unit, thereby generating another stable H-bridged radical cation viz. HCO ⋯ H ⋯ OC(H)OCH3·+. This final intermediate dissociates with little or no activation energy into HOC(H)OCH3+ and HCO·. It is further predicted by the calculations that ionized methylglycolate isomerizes into the title HBRC by a fairly high barrier that makes the communication between ionized methylglycolate and dimethylcarbonate via the title ion quite unlikely; instead an alternative route for this communication is proposed.

The decarbonylation reactions of ionized 2-acetylpyridine, 2-acetylpyrazine, and 2-acetylthiazole have been investigated using the multiple collision technique of neutralization–reionization/collision-induced dissociation mass spectrometry and related techniques. The resultant heterocyclic C6H7N·+, C5H6N2·+, and C4H5NS·+ ions were identified as 2-methylene-1,2-dihydropyridine, 6·+, 2-methylene-1,2-dihydropyrazine, 9·+, and 2-methylene-2,3-dihydrothiazole, 12·+, respectively. This result refutes proposals in the older literature that the decarbonylation would involve a methyl transfer yielding the 2-methyl species 2-methylpyridine (2·+), 2-methylpyrazine (8·+), and 2-methylthiazole (11·+). Literature proposals for a methyl transfer in the dissociation of ionized dimethyl-2,3-pyridinedicarboxylate and methyl-4-pyridinecarboxylate were also examined, but could not be substantiated either. However, the proposed gas phase synthesis of the N-methylpyridinium ylide, 1·+, from ionized methyl-2-pyridinethiocarboxylate did provide evidence for a genuine methyl migration. To reinforce these conclusions, the dissociation characteristics of isomers structurally related to 6·+ (3·+– 5·+ and 7·+), to 9·+ (10·+) and to 12·+ (13·+– 15·+) were also considered. Exploratory quantum chemical calculations (at the B3LYP/6-31G∗ level of theory) indicate that 6·+ and 12·+ are among the most stable isomers in the C6H7N·+ and C4H5NS·+ systems, lying 24 and 19 kcal/mol lower in energy than 2·+ and 11·+, respectively. Their neutral counterparts, however, are considerably less stable: 6 is calculated to be 27 kcal/mol higher in energy than 2. Nevertheless, from neutralization–reionization experiments it follows that the neutral counterparts of the ionized decarbonylation products 6, 9, and 12 are stable molecules on the microsecond time scale. Significant 1,3-hydrogen shift barriers hinder the interconversion of both the ions and the neutrals into their 2-methyl substituted counterparts, thus accounting for their stability in the dilute gas phase.

Ab initio molecular orbital calculations at the Gaussian-2 level of theory on a set of isodesmic, atomization, and substitution type reactions have been used to deduce the enthalpy of formation of the methoxycarbonyl ion as ΔHf298[CH3O−CO+] = 130 ± 2 kcal/mol. From the G2 computed ionization energy (IEa = 7.32 eV) and ΔHf298 (−40 kcal/mol) of the parent radical CH3O−C•O, we arrive at 129 kcal/mol for its ionic counterpart. Combining these theoretical findings with a reevaluation of existing experimental data (appearance energy measurements) yields 129 ± 2 kcal/mol as our recommended value for ΔHf298[CH3O−CO+], a large upward revision of the current literature value of 120 kcal/mol. By use of the new value as the anchor point, G2 derived ΔHf298 values for the isomers HOCH2−CO+, +CH2−H···OCO, +CH(OH)-C(O)H, +CH2−O−C(O)H, and +CH2O−C−OH have been calculated as 147, 131, 157, 144, 144, 140, and 177 kcal/mol, respectively.

Ab initio molecular orbital (MO) calculations support the proposal that the key processes in the rearrangement of HOCH2CH2OH•+ and HOCH2CH(CH3)OH•+ (ionized 1,2-ethanediol and 1,2-propanediol) are ...

The intriguing gas-phase ion chemistry of β-hydroxypyruvic acid (HPA), HOCH2C(==O)COOH, has been investigated using tandem mass spectrometry (metastable ion (MI) and (multiple) collision-induced dissociation (CID) experiments, neutralization-reionization mass spectrometry (NRMS),18O and D isotopic labelling on both the acid and its methyl ester) in conjunction with computational chemistry (ab initio MO and density functional theories). HPA does not enolize upon evaporation, but it retains its keto structure. When ionized, decarbonylation occurs and, depending on the internal-energy content, this dissociation reaction proceeds via two distinct routes. The source-generated, high-energy ions lose the keto C==O, not via a least-motion extrusion into ionized glycolic acid, HOCH2COOH.+, but via a rearrangement that yields the title H-bridged radical cation CH2==O...H...O==C-OH.+for which Δ Hf0= 99 ± 3 kcal/mol. The long-lived low-energy ions enolize prior to decarbonylation and lose the carboxyl C==O. Again, this is not a least-motion extrusion (which would produce the most stable isomer, HOC(H)==C(OH)2.+Δ Hf0= 73 kcal/mol) but a rearrangement yielding the ion-dipole complex HOC(H)C==C==O.+/H2O. The methyl ester of HPA behaves analogously, yielding CH2==O...H...O==C-OCH3.+and HOC(H)C==C==O.+/ CH3OH upon decarbonylation of the high- and low-energy ions, respectively. Decarboxylation into the ylidion CH2OH2.+characterizes the dissociation chemistry of both the title H-bridged ion and its glycolic acid isomer HOCH2COOH.+. A computational analysis of this reaction (which satisfies the experimental observations) leads to the proposal that the decarboxylation of the acid occurs via CH2-O(H)...H...==C==O.+as the key intermediate, whereas the title H-bridged ion follows a higher energy route that involves ion-dipole rotations leading to the ionized carbene HO(H2)CO-C-OH.+and the distonic ion H2O-C(H2)-O-C==O.+as key intermediates.Key words: tandem mass spectrometry, hydrogen-bridged radical cation, hydroxypyruvic acid, ab initio calculations, keto-enol tautomerization,18O labelling.

Tandem mass spectrometry based experiments on commercial oxalacetic acid (OAA) samples confirm the indirect evidence from the elegant study of Flint et al. (J. Org. Chem. 57 (1992) 7270) that solid OAA exists solely in the (Z)-enol form of HOOCC(H)C(OH)COOH. It is further shown that: (1) the samples contain a minor impurity in the low percent range, assigned as a dehydration product of 4-hydroxy-4-methyl-2-ketoglutaric acid; (2) careful evaporation of solid OAA samples yields mass spectra representative of the (Z)-enol form. The (E)-enol is not present in the solid, nor is its ion generated in the gas phase via isomerization of the ionized (Z)-enol form. However, even under gentle sample introduction conditions, a partial ketonization of the neutral (Z)-enol takes place. The resulting keto OAA molecules are either ionized intact or decarboxylate to yield a mixture of α-hydroxyacrylic acid, CH2C(OH)COOH and its keto isomer pyruvic acid, CH3C(O)COOH. From computational quantum chemistry (CBS-4) and experimental data, ionic enthalpies of formation of 95 and 109 kcal/mol were derived for CH2C(OH)COOH·+ and CH3C(O)COOH·+, respectively; and (3) under less controlled sample introduction conditions, thermal dehydration may take place to yield C4H2O4 molecules whose mass spectral characteristics are indistinguishable from those of ionized hydroxymaleic anhydride.

The fascinating unimolecular chemistry of ionized 1,2-propanediol, CH 3 C(H)OHCH 2 OH ·+ , 1 , has been re-examined using computational chemistry (ab initio MO and density functional theories) in conjunction with modern tandem mass spectrometric and 13 C labelling experiments. The calculations allow a considerable simplification of a previously proposed complex mechanism (Org. Mass Spectrom., 23 (1988) 355). Again, the central intermediates are proposed to be stable hydrogen bridged ion—dipole complexes, but our present calculations indicate that the key transformation now is the rearrangement CH 3 C(H)OH + ···O(H)-CH 2 . → CH 3 C(H)OH + ··· . OCH 3 , which can best be viewed as the cation-catalyzed 1,2-hydrogen shift . CH 2 OH → CH 3 O . , a rearrangement which does not occur so easily in the unassisted system. Another important process is the electron transfer CH 3 C(H)O···CH 3 OH ·+ → OCH(CH 3 ) ·+ ···O(H)CH 3 which allows proton transfer to generate CH 3 OH 2 + + CH 3 CO . . Other dissociation processes (loss of CH 3 . , H 2 O, H 2 O + CH 3 . , H 2 O + CH 4 ) are interpreted in terms of Bohme's ‘methyl cation shuttle’ (J. Am. Chem. Soc., 118 (1996) 4500) taking place in ion-dipole complexes. The most stable intermediate is the hydrogen bridged ion-dipole complex CH 2 CHOH .+ ···O(H)CH 3 , which is the reacting configuration for loss of methanol.