Non-empirical approximate calculations for the ground states of H2 and H3 including complete configuration interaction

Calculations are reported for the ground states of H2 and H3 in which the original atomic orbital basis is transformed to a sysmmetrically orthonormal atomic orbital basis. Complete configuration interaction is carried out and the results are shown to be in good agreement with ab initio calculations

The acid-catalyzed rearrangement CH3Oo --> oCH2OH and its involvement in the dissociation of the methanol dimer radical cation; A Quid pro Quo reaction

The barrier for the radical isomerization CH3Oo --> oCH2OH is calculated by CBS-QB3 to be 29.7 kcal mol-1 and lies higher (by 5.7 kcal mol-1) than the dissociation limit CH2O+Ho. Hence, CH3Oo does not isomerize to the more stable oCH2OH on its own. However, this barrier is reduced to 15.8 kcal mol-1 when the CH3Oo radical is coordinated with protonated methanol (CH3-Oo...H-O(H)-CH3+) and the CH3Oo -->oCH2OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation oCH2-O(H)...H-O(H)-CH3+ can be viewed as an acid catalyzed rearrangement. The ion CH3-Oo...H-O(H)-CH3+ represents the most stable form of the methanol dimer radical cation. The ion oCH2-O(H)...H-O(H)-CH3+ can fragment directly to CH3OH2+ + oCH2OH or it can rearrange further to produce the hydrogen-bridged radical cation oCH2-O+(CH3)-H...OH2, which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or tho CH2=O...H+...OH2+CH3o via an SN2 type reaction. Alternatively, oCH2-O+(CH3)-H...OH2 may undergo "proton-transport catalysis" to produce the complex ion CH3-O-CH3o+...OH2 which then dissociates. Our calculations confirm for the most part recent experimental findings on the methanol dimer radical cation [Y.-P. Tu, J.L. Holmes, J. Am. Chem. Soc. 112 (2000) 3695] but they also provide a different mechanism for the key isomerization reaction observed in that study

The remarkable dissociation chemistry of 2-aminoxyethanol ions NH2OCH2CH2OH+studied by experiment and theory

Low-energy 2-aminoxyethanol molecular ions NH2OCH2CH2OH+ exhibit a surprisingly rich gas-phase ion chemistry. They spontaneously undergo five major dissociations in the microsecond timeframe, yielding ions of m/z 61, 60, 46, 32 and 18. Our tandem mass spectrometry experiments indicate that these reactions correspond to the generation of HOCH2CH(OH)+ (protonated glycolaldehyde), HOCH2C(O)H+ (ionized glycolaldehyde), HC(OH)NH2+ (protonated formamide), CH2OH2+ (the methylene oxonium ion) and NH4+. A mechanistic analysis of these processes using the CBS-QB3 model chemistry shows that the molecular ions undergo a 1,4-H shift followed by a facile isomerization into the ion–molecule complex [HOCH2C(O)H+][NH3] which acts as the reacting configuration for the five exothermic dissociation processes. Analysis of the D-labelled isotopomer ND2OCH2CH2OD+, in conjunction with our computational results, shows that proton-transport catalysis may be responsible for the partial conversion of the m/z 60 glycolaldehyde ions into the more stable 1,2-dihydroxyethene isomer HOC(H)C(H)OH+

Multireference coupled electron-pair approximations to the multireference coupled cluster method. The MR-CEPA1 method

An extension of the CEPA1 method to the multi-reference case is presented, the MR-CEPA1 approach. The method takes the variationally included terms into account as in MRDCEPA and corrects for the exclusion principle violating terms as in closed shell CEPA1. It is shown that this method yields potential energy curves that are close to those of the multi-reference coupled cluster method and parallel the full CI results quite well. The size consistency of the method is as good as the MRDCEPA method and much beter than approaches that ignore the VI terms like MR-ACPF and MR-AQCC. A simpler method, where the EPV terms are not corrected on an individual basis for the doubly occupied orbitals, dubbed the multi-reference averages CEPA1 method (MR-ACEPA), which is akin to methods previously suggested by Szalay et al., is comparable in performance

Formaldehyde mediated proton-transport catalysis in the ketene-water radical cation CH2=C(=O)OH2•+

Previous studies have shown that the solitary ketene-water ion CH2=C(=O)OH2+ (1) does not isomerize into CH2=C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2=O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2C=O+. A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH2C(=O)O(H)-H...OCH2]+ in which the CH2O molecule catalyzes the transformation 1->2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol-1, is reduced to mere 0.6 kcal mol-1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis

The loss of NH2O• from the N-hydroxyacetamide radical cation CH3C(=O)NHOH•+: An ion-catalysed rearrangement

A previous study [Ch. Lifshitz, P.J.A. Ruttink, G. Schaftenaar, J.K. Terlouw, Rapid Commun. Mass Spectrom. 1 (1987) 61] shows that metastable N-hydroxyacetamide ions CH3C(=O)NHOH+ (HA-1) do not dissociate into CH3C=O++NHOH by direct bond cleavage but rather yield CH3C=O++NH2O. The tandem mass spectrometry based experiments of the present study on the isotopologue CH3C(=O)NDOD+ reveal that the majority of the metastable ions lose the NH2O radical as NHDO rather than ND2O. A mechanistic analysis using the CBS-QB3 model chemistry shows that the molecular ions HA-1 rearrange into hydrogen-bridged radical cations [O=C-C(H2)-H...N(H)OH]+ whose acetyl cation component then catalyses the transformation NHOH->NH2O prior to dissociation. The high barrier for the unassisted 1,2-H shift in the free radical, 43 kcal mol-1, is reduced to a mere 7 kcal mol-1 for the catalysed tranformation which can be viewed as a quid-pro-quo reaction involving two proton transfers

Dissociation of protonated oxalic acid [HOOC-C(OH)2]+ into H30++CO+CO2: an experimental and CBS-QB3 computational study

The predominant dissociation process observed for metastable protonated oxalic acid ions HOOC-C(OH)2+ (generated by self-protonation) leads to H3O++CO+CO2. We have traced the mechanism of this intriguing reaction using the CBS-QB3 model chemistry. Our calculations show that a unique ter-body complex, O=C=O ...H3O+...CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H2O...H...O=C=O]+ and [H2O...H...CO]+ which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer O=C=O...H+...CO which is formed by water extrusion from the ter-body complex. Argments are provided that the ter-body complex is also generated in the ion source by the collision encounter between neutral and ionized oxalic acid

The acrylonitrile dimer ion: A study of its dissociation via self-catalysis, self-protonation and cyclization into the pyrimidine radical cation

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH2CHCN+, into their more stable hydrogen-shift isomers CH2CCNH+ or CHCH–CNH+. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/z 54 ions are ions CH2CHCNH+ generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH2CHCNH–C(CN)CH2]+ and [CH2CHCNH–C(H)C(H)CN]+; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH2CCNH+ or CHCH–CNH+ by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/z 53 ions are CH2CHCN+ ions, generated by back dissociation; (iii) the peaks at m/z 79, 80 and 105 correspond with the losses of HCN, C2H2 and H, respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH2CHCN–C(H2)C(H)CN]+; experiments indicate that the m/z 79 (C5H5N) and m/z 105 (C6H6N2) ions have linear structures but the m/z 80 (C4H4N2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule