Suresh Dua

Ethylenedione: An intrinsically short-lived molecule

Ethylenedione C2O2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques, This is surprising, since theoretical studies predicted the triplet state of C2O2 to be stable towards spin-allowed dissociation and hence long-lived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization -reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive, Both the singlet and triplet states of C2O2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

A comparison of skeletal rearrangement reactions of even-electron anions in solution and in the gas phase

Abstract There is significant correspondence between certain skeletal rearrangement processes of close-shell organic anions in the condensed and gas phases. Examples of such correspondence include the acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, Smiles and Wittig rearrangements. In contrast, there are some rearrangements observed in the condensed phase, which are either minor or do not occur at all in the gas phase. The Favorskii, Tiemann and Carroll rearrangements fall into this category. Finally, there are some gas phase rearrangements which have no condensed phase analogy: for example the negative ion pinacol/pinacolone and Beckmann rearrangements. These, and related processes are discussed in this Review.

Mass spectrometric studies of the oxocarbons CnOn (n = 3–6)

Abstract The anion radicals CnOn−· (n = 3–6) can be generated by ionization of cyclic carbonyl compounds in the negative ion mode. The ions as well as the corresponding neutral counterparts are probed by means of different mass spectrometric techniques. The results suggest that oxocarbons, i.e. cyclic polyketones, are formed under conservation of the skeletons of the precursor molecules. At least for n = 3, however, the experimental findings indicate partial rearrangement of the expected cyclopropanetrione structure to an oxycarboxylate for the anion, i.e. ·O–C≡C–CO2−. For n=4 and 6 almost complete dissociation of the neutral polyones into carbon monoxide is found, whereas for n = 5 a distinct recovery signal indicates the generation of genuine cyclopentanepentaone.

THE METHOXYMETHYL CATION CLEAVES PEPTIDE BONDS IN THE GAS PHASE

Methoxymethyl cations and simple N-acyl amino acids and dipeptides react in the gas phase to form [M + MeOCH 2 ] + ions which fragment via a number of pathways including amide bond cleavage.

Search for charge-remote reactions of even-electron organic negative ions in the gas phase. Anions derived from disubstituted adamantanes

The collision induced decompositions of 3-substituted adamantanecarboxylate anions have been studied with a view to uncover charge-remote fragmentations of the 3-substituent. The 3-substituent is chosen so that it cannot approach the anion site and therefore any fragmentations of that substituent should proceed independently of the charged centre. (i) Charge-remote radical losses are observed from a 3-CH(Et)2 substituent [e.g. Et˙ and ˙CH(Et)2 losses], but the classical Adams–Gross charge remote loss of an ethene plus dihydrogen is not observed. (ii) Charge-remote loss of MeOD is observed from a 3-C(CD3)2(OMe) substituent together with a number of charge-remote radical losses [e.g. Me˙, MeO˙ and ˙C(CD3)2(OMe)]. (iii) The 3-substituent C(CD3)2 (OCHO) undergoes charge-remote loss of HCO2D for both the carboxylate anion and its corresponding cation, a neutral reaction analogous to both the McLafferty rearrangement of radical cations and the Norrish II diradical rearrangement of aliphatic ketones. (iv) The charge-remote radical losses of MeO˙ and ˙CO2Me occur from a 3-CO2Me substituent.

The negative ion mass spectra of deprotonated 2,5-diketopiperazines

Abstract Deprotonation of 2,5-diketopiperazine (with HO − ) can occur either on N (position 1(4)) or on carbon (position 3(6)). The two depotonated forms are interconvertible on collisional activation. The major collision-induced fragmentations of (M  H) − ions of substituted 2,5-diketopiperazines are (i) characteristic side-chain losses (e.g. Me . for Ala, PhCH 2 . for Phe, and O  C 6 H 4  CH 2 for Tyr), which identify the particular 2,5-diketopiperazine, and (ii) an unusual loss of RCHO (R is the substituent, e.g. Me for Ala), which involves initial 1,2 migration of R . to the carbon of the adjacent carbonyl group.

Generation of neutrals from ionic precursors in the gas phase. The rearrangement of CCCCCHO to HCCCCCO

The neutrals HCCCCCO and CCCCCHO have been studied by experiment and by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. Neutral HCCCCCO has been made by one-electron reduction of [HCCCCCO]+ in the dual collision cell of a VG ZAB 2HF mass spectrometer. The isomer CCCCCHO is also formed in the dual collision cell, but this time by one-electron oxidation of the anion [CCCCCHO]-. Comparison of the CID and +NR+ mass spectra of [HCCCCCO]+ indicates that neutral HCCCCCO, when energised, retains its structural integrity. If the excess energy of HCCCCCO is > or = 170 kJ mol-1, decomposition can occur to give HCCCC and CO (calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory). The situation with the isomer CCCCCHO is different. Comparison of the -CR+ and -NR+ spectra of [CCCCCHO]- shows that both neutral and cationic forms of CCCCCHO partially rearrange to a species which decomposes by loss of CO. The peak corresponding to loss of CO is more pronounced in the -NR+ spectrum, indicating that the rearrangement is more prevalent for the neutral than the cation. Theoretical calculations suggest that the species losing CO could be CCCCHCO or HCCCCCO, but that HCCCCCO is the more likely. The lowest-energy rearrangement pathway occurs by successive H transfers, namely CCCCCHO-->CCCCHCO-->CCCHCCO-->HCCCCCO. The rearrangement of CCCCCHO to HCCCCCO requires CCCCCHO to have an excess energy of > or = 94 kJ mol-1. The species HCCCCCO formed by this exothermic sequence (214 kJ mol-1) has a maximum excess energy of 308 kJ mol-1: this is sufficient to effect decomposition to HCCCC and CO.

The formation of neutral CCC and its radical cation from the CCC radical anion in the gas phase. A joint experimental and theoretical study

The radical anion [CC13C]−˙ has been produced by treatment of [(CH3)3SiCC13C(NNHSO2C6H4-p-CH3)Si(CH3)3] with HO−/F− in the ion source of a mass spectrometer. The stable anion undergoes vertical two-electron oxidation [charge reversal (−CR+)] in a collision cell to give [CC13C]+˙ which cyclises to the more stable [cyclo-CC13C]+˙ over a barrier of only 11 kJ mol−1 [calculated at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory], effectively scrambling the three carbon atoms of the cation radical. One-electron Franck–Condon oxidation of [CC13C]−˙ yields neutral CC13C. Theoretical calculations suggest that neutral CCC may undergo a degenerate rearrangement through a cyclic C3 transition state if the excess energy of CCC is ≥104 kJ mol−1 (at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory). It is likely that at least a proportion of the CC13C neutrals formed from [CC13C]−˙ should have sufficient energy to effect this reaction, resulting in the scrambling of the 13C label. The neutralisation/reionisation (−NR+) spectrum of [CC13C]−˙ ([CC13C]−˙ → CC13C → [CC13C]+˙) shows a pronounced peak corresponding to the parent cation, confirming that neutral CC13C is stable for the time of the NR experiment (10−6 s). However due to total scrambling of the label in the cation, possible scrambling in the neutral CCC molecule cannot be probed by this experiment. The corresponding −NR− experiment of [CC13C]−˙ showed a recovery signal but the sensitivity of the instrument was not sufficient to detect the decomposition fragments of the final radical anion.

Potential interstellar molecules. Formation of neutral C6CO from C6CO−· in the gas phase

Computations at the RCCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory indicate that neutral C(6)CO is a stable species. The ground state of this neutral is the singlet cumulene oxide :C=C=C=C=C=C=C=O. The adiabatic electron affinity and dipole moment of singlet C(6)CO are 2.47 eV and 4.13 D, respectively, at this level of theory. The anion (C(6)CO)-* should be a possible precursor to this neutral. It has been formed by an unequivocal synthesis in the ion source of a mass spectrometer by the S(N)2(Si) reaction between (CH(3))(3)Si-C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-CMe(3) and F(-) to form (-)C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-Me(3) which loses Me(3)C* in the source to form C(6)CO(-)*. Charge stripping of this anion by vertical Franck-Condon oxidation forms C(6)CO, characterised by the neutralisation-reionisation spectrum (-NR(+)) of C(6)CO(-*), which is stable during the timeframe of this experiment (10(-6) s).

The unusual neutral OCOCO and possible charged analogues. A theoretical investigation

Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory indicate that singlet neutral OCOCO is unstable with respect to dissociation to CO2 and CO. In contrast, triplet OCOCO is a stable species provided it can be formed with excess energy of less than 41 kJ mol−1 [the process 3OCOCO → 1CO + 3CO2 is endothermic by only 9 kJ mol−1, but the barrier for this process is 41 kJ mol−1]. Triplet OCOCO is not accessible by one-electron oxidation from [OCOCO]−˙ or one-electron reduction from [OCOCO]+˙ because neither of these charged species is stable at the level of theory used for these calculations. A report by Cooper and Compton indicates that dissociative electron capture by maleic anhydride results in loss of the elements of C2H2 yielding an anion C2O3−˙. Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level of theory suggests that the ion radical C2O3−˙ may be the stable species [O2C–CO]−˙ provided that the dissociating maleic anhydride radical anion has excess energy of at least 260 kJ mol−1.