Split-valence 6-31G Research Articles

Theoretical studies of C 2H 4O isomers : Part 1. Acetaldehyde ground, triplet [formula omitted] and triplet n → 3 s electronic states

The ground, triplet n → π ∗ and triplet Rydberg n → 3 s states of acetaldehyde have been studied at both the SCF and CI levels. The minimal STO-3G, split valence 3-21G and polarized 6-31G ∗ basis sets have been used in conjunction with both spin unrestricted (UHF) and restricted (RHF) wavefunctions. The CI calculations included all single and double excitations from the SCF reference configuration, excluding three core and the six highest-lying virtual orbitals, using the 3-21G basis set. The Rydberg calculations used the 3-21G basis, augmented by a diffuse s function, plus CI. The agreement between theory and experiment is very good where corresponding data exist. The ground-state methyl rotation barrier is calculated to be 4.3 kJ mol −1 at the 6-31G ∗/SCF level and 5.2 kJ mol −1 at the 3–21G/CI level, with the syn conformer lowest. The triplet n → π ∗ state is pyramidal, and lies 264 kJ mol −1 above the S 0 state while the vertical excitation energy is 362 kJ mol −1 The main effect of either an RHF versus a UHF wavefunction, a larger basis, or CI, is to increase barrier and excitation energies somewhat. The CO bond length is reduced below the experimentally determined value at the 6-31G ∗ level. The preferred Rydberg state conformation is syn, in agreement with the recent experimental result. The methyl rotation barrier is computed to be 9.0 kJ mol −1 and the excitation energy 595 kJ mol −1 (602 kJ mol −1 vertical).

Structures of C 3H 6O species in the triplet state: Methyloxirane, propanal, acetone, methyl vinyl ether and propenol

The geometries of thirteen triplet C 3H 6O species, plus methyloxirane and propene in the ground state, have been completely optimized by analytic gradient (force) techniques at both the minimal STO-3G and split valence 3-21G basis set levels. Single calculations using the 6-31G* polarized basis set were performed for each 3-21G optimum structure. With the exception of bond angles at oxygen and the pyramidality at carbon radical centers, the geometries obtained with the two basis sets differ little. Except for the propene +O( 3 P) atom, the addition of d orbitals has little effect on the relative energies compared to the 3-21G energy differences (Δ E), but both were markedly different from the STO-3G results. Whereas the latter Δ E values covered a range of almost 40 kcal mol −1, this range was reduced to 24.5 and 22.4 kcal mol −1 at the 3-21G and 6-31G −* levels, respectively.

A unimolecular reaction ABC→A+B+C involving three product molecules and a single transition state. Photodissociation of glyoxal: HCOHCO→H2+CO+CO

Following an earlier proposal [Y. Osamura and H. F. Schaefer, J. Chem. Phys. 74, 4576 (1981)], the unimolecular reaction HCOHCO→H2+CO+CO has been examined via nonempirical molecular electronic structure theory. Specifically, the constrained symmetric (point group C2v) transition state for this ABC→A+B+C reaction has been located at several levels of self-consistent-field (SCF) theory. Four different basis sets of contracted Gaussian functions were used: an STO-3G minimum basis, the small split valence 3-21G basis, the standard C(9s 5p/4s 2p) double zeta (DZ) set, and a double zeta plus polarization (DZ+P) basis. Vibrational analyses of the four stationary point structures (all of which are geometrically similar) yield a remarkable variety of results. The STO-3G stationary point has three imaginary vibrational frequencies, 3-21G has one imaginary frequency (and thus is a genuine transition state), while the DZ and DZ+P structures yield two imaginary vibrational frequencies. For the latter two cases, one of the two imaginary vibrations is a very small bending frequency, while the larger frequency clearly connects glyoxal with the three products H2+CO+CO. This suggests the existence of a slightly nonplanar true transition state. To our knowledge such a unimolecular transition state is without precedent. Configuration interaction (CI) suggests that the barrier for this ABC→A+B+C reaction is competitive with that for HCOHCO→H2CO+CO.