Basis sets barrier height

An accurate MO study of the inversion barrier in dimethyl sulphoxide9 showed that the height of the calculated barrier is much more sensitive to the overall quality of the basis set, and to geometry optimization, than to the presence of 3d functions. This study predicts an S—O bond lengthening to 1.55 A, and the best estimate of the barrier is 39.9 kcal mol 1. This was the difference in energy between optim um planar and pyramidal... 

Table 13-4. Complexation energy (AEC) and barrier heights (Al .oh and Al .h. see text) for the gas phase bimolecular Sn2 identity reaction CE + CH3C1 —> C1CH3 + Cl" [kcal/mol]. HF and DFT calculations were done with the 6-311+G(d,p) basis set and include zero-point vibrational contributions. 

Table 13-7. Computed classical barrier heights AE [kcal/mol] for the reaction H2 + H — [ II II 111 —> H2 + H (6-311++G(,3pd) basis set) data compiled from Johnson et al., 1994, and Csonka and Johnson, 1998.

For most molecules studied, modest Hartree-Fock calculations yield remarkably accurate barriers that allow confident prediction of the lowest energy conformer in the S0 and D0 states. The simplest level of theory that predicts barriers in good agreement with experiment is HF/6-31G for the closed-shell S0 state (Hartree-Fock theory) and UHF/6-31G for the open-shell D0 state (unrestricted Hartree-Fock theory). The 6-31G basis set has double-zeta quality, with split valence plus d-type polarization on heavy atoms. This is quite modest by current standards. Nevertheless, such calculations reproduce experimental barrier heights within 10%. 

We performed CBS-4M single point energy calculations at these stationary points. The barrier height for the first proton transfer and the relative energy of the first dienolate are quite sensitive to the level of theory and basis set employed (Table 4.10 and Fig. 4.14). The initial Asp" (i.e. formate ion) carries the full negative charge in... 

Unimolecular pyrolysis of the tautomers of monothioformic acid (two conformers of thiol- and two conformers of thiono-) have been studied by ab initio methods with STO-3G and 6-31 G basis sets. The barrier heights for dehydrogenation (via a four-centre transition state) and dehydrogensulfldation (via a three-centre transition state) of thiol formic acid are 67.47 and 67.09 kcalmol" respectively. Dehydration of 5-cw-HCSOH occurs via a three-centre transition state with an activation energy of 81.18 kcalmoG this is much greater than for dehydration of the s-trans form, which occurs via a four-centre transition state with a barrier of only 68.83 kcalmol" ... 

There have been three other studies of the lowest singlet PE surface of CH4. Two of these were SCF studies using a minimum basis set.61 62 Both predict a barrier height for the reaction (2) of > 209 kj mol-1. This is in agreement with orbital sym-... 

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