The relationship between the rate–equilibrium coefficient α and transition state properties: a second-order Møller–Plesset perturbation study of SN2 reactions

Abinitio calculations including electron correlation are used to study rate–equilibrium relationships in gas-phase SN2 reactions. The difference between the "intrinsic" α and "group" α is emphasized. In general, the "group" α cannot be used as a measure of the transition state structure. The relationships between the "intrinsic" α and other properties, such as reaction endothermicity, geometry change, and the charge transfer, are discussed. A geometry change parameter Rα, defined by analogy with the definition of the "intrinsic" α, is shown to be linearly related to the "intrinsic" α. The charge transfer at the transition state is related not only to energy changes but also to the electronegativities of the entering nucleophile and leaving group in the product and reactant, respectively, and to the electronic structures at the transition state. Thus, the charge transfer parameter Qα, unlike the "intrinsic" α and Rα, is affected by the electronegativities of the groups involved in the reaction. The systems studied are SN2 reactions of the type N− + CH3X → CH3N + X−, where X = H, NH2, OH, OOH, F, CCH, CN, NC, PH2, SH, and Cl when N = H, and where X = H, NH2, OH, F, CN, NC, PH2, SH, and Cl when N = F. Keywords: SN2 reactions, rate–equilibrium relationships, transition state properties.

Theoretical study of nitro–nitrite rearrangement of CH3NO2

Calculations designed to characterize the transition state and determine the barrier height for rearrangement of nitromethane to methyl nitrite are reported. Structures of CH3NO2, CH3ONO, dissociation products, CH3 + NO2, and CH3O + NO, and the transition state for nitro–nitrite rearrangement have been optimized at the MCSCF/4-31G level. The geometry of the transition state may be approximately described as separated CH3 and NO2 species with extremely long C—N and C—O bond lengths, 3.396 and 3.654 Å, respectively. Energies have been obtained by large-scale multireference single- and double-excitation CI calculations (6-31G* basis). The transition state is calculated to lie 56.7 kcal/mol above nitromethane (with zero-point energy). A C—N bond dissociation energy of 51.7 kcal/mol is obtained. Results are compared with the infrared multiphoton dissociation experiment of Wodtke, Hintsa, and Lee. Keywords: nitromethane, abinitio calculations, transition state, rearrangement, dissociation.

Abinitio study on radical anions of nitro compounds

The results of abinitio molecular orbital calculations on some nitro compounds, RNO2 (R = CH3, CF3, CH2F, BH2, BF2, BHF, C6H5) and on the corresponding radical anions are reported. The geometries of the neutral and charged species were optimized at the SCF and MP2 levels of theory employing the 6-31G* and the 6-31 + G* basis sets. The rotation about the CN and BN bonds reveals distinct conformations for each molecule and vibrational frequencies were determined for such structures. All the employed levels of theory predict structural differences between neutral and charged molecules. The major geometrical changes occurring by electron adding to RNO2 consist in the lengthening of the NO bonds and in the shortening of the CN and BN bonds. The radical anions are calculated to be pyramidal at nitrogen in the stable conformers of CH3NO2−, CF3NO2−, CH2FNO2−, while, for the π-electron-accepting substituents (BH2, BHF, BF2, C6H5), the anion is planar. Electron correlation energy corrections in the framework of Møller–Plesset perturbation theory were included to determine relative stabilities between different conformations. At the MP4/6-31G*//MP2/6-31G* level, an easy rotation of the NO2 group is predicted for all the radical anions and neutral molecules with the exception of BH2NO2−, BF2NO2−, and BHFNO2−, which show high torsional barriers about the BN bond. Keywords: nitro compounds, radical anions, abinitio calculations, molecular structure.

Benzoxathiete and related structures: experimental and quantum chemical studies

The products, plausible intermediates, and their mechanisms of formation in the aprotic diazotization of 2-[(2-acetoxyethyl)-sulfinyl (and sulfonyl)]anilines with isoamyl nitrate have been investigated by experimental and quantum chemical methods. Oxidation of 1,2,3-benzthiadiazole with hydrogen peroxide in acetic acid affords 1,2,3-benzthiadiazole-1-oxide, 13. The thermal stability of 13 up to 135 °C, together with EPR evidence which disfavors an aryldiazenyl radical precursor, discount 13 as an intermediate in the formation of benzoxathiete 14 or its valence tautomer 17 during aprotic diazotization of the sulfinylanilines. Electron paramagnetic resonance evidence in conjunction with spin trapping indicates an intermediate arylaminyl radical. This evidence, taken together with abinitio calculations of optimized geometries energies and energy differences for biradical intermediate 15, favors a mechanism of formation of benzoxathiete 14 via aryldiazoate anhydride 5 thence to rapid ring closure of carbon-centered radical 8. Further oxidation of 1,2,3-benzthiadiazole-1-oxide, 13, with hydrogen peroxide in methanol – acetic acid affords biphenylene and dibenzo-1,4-oxathiane-S-oxide, 38. Rose bengal sensitized photooxidation of 1,2,3-benzthiadiazole affords 13, biphenylene, and 38. Formation of the latter, in which one of the original S—O bonds has been broken, requires the formation of benzoxathiete-S-oxide, 34, and its rapid valence tautomerism to ketosulfine 36 and (2 + 4) cycloaddition of 36 to the simultaneously generated dehydrobenzene to give 38. Both ring closure of singlet biradical 35 to 34 and valence tautomerism of the latter to 36 are predicted by abinitio calculations to be facile and exothermic. In contrast to the aprotic diazotization of 2-[(2-acetoxyethyl)sulfinyl]anilines, the reaction of isoamyl nitrite with the corresponding sulfonyl anilines may plausibly follow a mechanism via 1,2,3-benzthiadiazole-1,1-dioxide 33 owing to the thermal instability of the latter and supported by abinitio treatments of the energetics of the processes involved. In addition EPR evidence, in conjunction with spin trapping of carbon centered radicals, support the viability of the pathway via 23, 25, 28, and 16 to biphenylene 20. The abinitio calculations of the energy differences between the reaction intermediates and estimates of the activation energies elucidated several aspects of these novel reactions. Keywords: benzoxathiete, abinitio calculations, valence tautomerism.

A study on the backbone/side-chain interaction in N-formyl-(L)serineamide

The N-formyl-serineamide (For-Ser-NH2), a model diamide for the conformational behaviour of the protein backbone at serine residue, has been gradient optimized for selected conformations at the abinitio 3-21G level. Previous molecular mechanics ECEPP/2 calculations suggested that the optimal side-chain (χ1, χ2) and backbone [Formula: see text] conformations are strongly coupled, due to intramolecular H-bonding formed between the side-chain hydroxyl group and the amide moiety. Four different side-chain geometries (I–IV) were considered at each of the four backbone conformations (α, β, β′, γ) giving a total 16 different critical points on the coupled [Formula: see text] surface. The very fact that upon changing the [Formula: see text] values the global minimum of (χ1, χ2) surface was shifted indicated clearly that the two subspaces are strongly coupled. The present theoretical results were compared to experimental peptide and protein geometries taken from the Cambridge Structure Database and from the Brookhaven Protein Data Bank, respectively. Keywords: conformation of a serinediamide, backbone side-chain interactions, topological analysis of potential energy surfaces, abinitio calculations.