Basis sets have been developed for carrying out G2 calculations on bromine-and iodine-containing molecules using all-electron ͑AE͒ calculations and quasirelativistic energy-adjusted spin-orbit-averaged seven-valence-electron effective core potentials ͑ECPs͒. Our recommended procedure for calculating G2͓ECP͔ energies for such systems involves the standard G2 steps introduced by Pople and co-workers, together with the following modifications: ͑i͒ second-order Mo "ller-Plesset ͑MP2͒ geometry optimizations use polarized split-valence ͓31,31,1͔ basis sets for bromine and iodine together with 6-31G(d) for first-and second-row atoms; ͑ii͒ single-point higher-level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6-311G and McLean-Chandler 6-311G bases for first-and second-row atoms, respectively; and ͑iii͒ first-order spin-orbit corrections are explicitly taken into account. An assessment of the results obtained using such a procedure is presented. The results are also compared with corresponding all-electron calculations. We find that the G2͓ECP͔ calculations give results which are generally comparable in accuracy to those of the G2͓AE͔ calculations but which involve considerably lower computational cost. They are therefore potentially useful for larger bromine-and iodine-containing molecules for which G2͓AE͔ calculations would not be feasible.

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A sudden transition in a system from an inanimate state to the living state—defined on the basis of present day living organisms—would constitute a highly unlikely event hardly predictable from physical laws. From this uncontroversial idea, a self-consistent representation of the origin of life process is built up, which is based on the possibility of a series of intermediate stages. This approach requires a particular kind of stability for these stages—dynamic kinetic stability (DKS)—which is not usually observed in regular chemistry, and which is reflected in the persistence of entities capable of self-reproduction. The necessary connection of this kinetic behaviour with far-from-equilibrium thermodynamic conditions is emphasized and this leads to an evolutionary view for the origin of life in which multiplying entities must be associated with the dissipation of free energy. Any kind of entity involved in this process has to pay the energetic cost of irreversibility, but, by doing so, the contingent emergence of new functions is made feasible. The consequences of these views on the studies of processes by which life can emerge are inferred.

High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out for the six non-identity nucleophilic substitution reactions, Y- + CH3X → YCH3 + X-, for Y, X = F, Cl, Br, and I. Central barrier heights (ΔH ⧧ cent) for reaction in the exothermic direction vary from 0.8 kJ mol-1 for Y = F, X = I up to 39.5 kJ mol-1 for Y = Cl, X = Br (at 0 K), and are in most cases significantly lower than those for the set of identity SN2 reactions X- + CH3X → XCH3 + X- (X = F−I). Overall barriers (ΔH ⧧ ovr) for reaction in the exothermic direction are all negative (varying from −68.9 kJ mol-1 for Y = F, X = I to −2.3 kJ mol-1 for Y = Br, X = I), in contrast to the overall barriers for the identity reactions where only the value for X = F is negative. Complexation enthalpies (ΔH comp) of the ion−molecule complexes Y-···CH3X vary from 30.4 kJ mol-1 for Y = F, X = I to 69.6 kJ mol-1 for Y = I, X = F (at 298 K), in good agreement with experimental and earlier computational studies. Complexation enthalpies in the reaction series Y- + CH3X (Y = F−I, X = F, Cl, Br, I) are found to exhibit good linear correlations with halogen electronegativity. Both the central barriers and the overall barriers show good linear correlations with reaction exothermicity, indicating a rate−equilibrium relationship in the Y- + CH3X reaction set. The data for the central barriers show good agreement with the predictions of the Marcus equation, though modifications of the Marcus equation that consider overall barriers are found to be less satisfactory. Further interesting features of the non-identity reaction set are the good correlations between the central barriers and the geometric looseness (%L ⧧), geometric asymmetry (%AS), charge asymmetry (Δq(X−Y)), and bond asymmetry (ΔWBI) of the transition structures.