MP2/6-31G method

The performance of several semi-empirical (modified neglect of diatomic overlap (MNDO), AMI, PM3, and SAMI) and ab initio (Hartree-Fock (FIF) and MP2/6-31G ) methods for determining structural and electronic factors of a series of isothiazolo[5,4-b]pyridines was compared by Martinez-Merino et al. <1996T8947>. They found that most of the semi-empirical methods calculated reasonable molecular structures when compared to the actual X-ray structures (compounds 3-5) (see, for example. Table 1 for selected bond lengths of compound 3). Flowever, the dipole moments were not reproducible using these methods. 

Valence tautomers, benzene oxide 1 and oxepine 2 (Equation 1), as well as relative tautomeric systems, benzene sulfide-thiepine and o-xylene-2,7-dimethyloxepine, have been studied by a post-Hartree-Fock (HF) ab initio QCISD(r)/6-31G //MP2/6-31G method. In particular, the enthalpy calculated for a benzene oxide-oxepine system is 0.59 kj moF1 <1997PCA3371>. The calculated molecular orbital (MO) energies are in linear relationship to those from the photoelectron (PE) spectra <1996JCF1447>. Barrier to tautomerization for a benzene oxide-oxepine system is 29.4 kj mol-1. Protonation stabilizes the oxide form versus the oxepine <1997PCA3371>. 

The gas-phase unimolecular elimination reactions of 2-substituted ethyl N,N-dimethylcarbamates23,24 and several heterocyclic carbamates25 have been studied using the Moller-Plesset MP2/6-31G method. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition-state structure. 

Anderson and his coworker carried out a series quantum chemistry studies of oxygen reduction reactions.52-57 Anderson and Abu first studied reversible potential and activation energies for uncatalyzed oxygen reduction to water and the reverse oxidation reaction using the MP2/6-31G method. The electrode was modeled by a non-interacting electron donor molecule with a chosen ionization potential (IP). The primary assumption is that when the reactant reaches a point on the reaction path where its electron affinity (EA) matched the donor IP, an electron transfer is initialized. The donor s IP or reactant s EA was related to the electrode potential by,... 

Protonation enthalpies for the N- and 0-protonated forms of oxazole were also calculated by the DF and MP2/6-31G methods <86JOCii05>, and are compared with an experimentally determined... 

In the case of O3H+, MP2/6-31G method is unable to distinguish between the two 0-0 lengths. Valence angles are, instead, of the same order in all the calculations. 

In the course of calculating the potential energy surface for OH addition to toluene with the MP2/6-31G method [14], we have found a significant minimum corresponding to addition of the OH radical at C, i.e., the methyl-substituted carbon atom. In fact, this adduct is more stable, by about 0.6 kcal/mol, than the one with the OH radical added at the ortho position, and the activation energy for its formation is only... 

The interaction of the methyl cation with disilazanes was modeled by quantum chemical methods. The equilibrium geometry and electronic structure of the methyl cation-disilazane adduct were obtained by the Hartree-Fock and MP2/6-31G methods. It was shown that the interaction between the highest occupied molecular orbital (HOMO) of the H3SiNHSiH3 molecule with the lowest unoccupied molecular orbital (LUMO) of the methyl cation is prohibited by symmetry, but the interaction between the next to the highest occupied molecular orbital of the silazane molecule (with the lower energy) and the methyl cation LUMO is possible. So, the gap between these molecular orbitals increases and the overlap probability decreases. Thus, the condensation channel for the HsSiNHSiHs molecule becomes less realizable and as a consequence the probability of the proton transfer channel increases. In the case of the aminosilane molecule the same situation is not true because of the absence of symmetry prohibition. 

The MP2/6-31G method is used. The H (aq) was modeled with H30 (H20)2 cluster. The electron transfer was assumed to occur when the electron affinity of the reaction complex equaled the ionization potential, IP, of the electrode. 

Calculations on one-electron-bonded radical cations necessitated data for the radicals from which they derive, and thus ab initio molecular orbital calculations give values for the ionization energy for BH2 of 175 kcal/mol (6.72 eV) and 179.3 kcal/mol (7.78 eV), when HF/6-31G and MP2/6-31G methods are used, respectively [2]. BH2 as one of three test molecules was used to calculate partition functions from ab initio methods involving sums of states of molecules in their spatially degenerate lowest electronic states [3]. 

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