SCF Gradient Research Articles

Structures and conformations of cyclopentane, cyclopentene, and cyclopentadiene

The structures and conformations of cyclopentane, cyclopentene, and cyclopentadiene have been investigated by SCF gradient ab initio computation with a double zeta basis set. For cyclopentane the half-chair ( C 2) and the envelope ( C s) forms were found to be isoenergetic, consistent with the experimentally observed free pseudorotation. The barrier to coplanarity of the ring is 4.3 kcal mol −1. Cyclopentene has a puckered C s equilibrium conformation with a very low barrier to planarity. Cyclopentadiene has a planar ring. The compounds with 5, 3 and 1 methylene groups, respectively, show a decreasing tendency to form puckered rings as interaction between adjacent methylene groups becomes of less significance compared with the influence of angle strain which favors ring planarity. A corrected geometrical optimization of the half-boat ( C s) form of cyclohexene is also presented.

Structure and tunneling dynamics of malonaldehyde. A theoretical study

The geometry and harmonic vibrational frequencies of equilibrium malonaldehyde and of the transition state for the symmetric intramolecular hydrogen atom transfer have been determined at the SCF level of theory (using a slightly better than double-basis set). All geometrical parameters were fully and simultaneously optimized by using SCF gradient techniques. Comparison of the equilibrium structure with the structure determined from microwave spectra shows good agreement in most respects, although there are a few differences. At the C/sub s/ equilibrium geometry and the C/sub 2//sub v/ transition state large scale configuration interaction (all single and double excitations) calculations were carried out to determine the barrier height for the symmetric hydrogen atom transfer; including a correction for quadruple excitations, this gives a classical or ''bare'' barrier height of 8.0 kca./mol. A one-dimensional model for the tunneling dynamics of H-atom transfer leads to a tunneling splitting in the ground vibrational state of approx. 18 cm/sup -1/, in relatively good agreement with the experimental value. It is noted, though, that the value of the splitting is sensitive to the potential surface parameters (i.e., bare height, frequencies) and that there is also uncertainty about the accuracy of the one-dimensional description of the tunneling.

MC SCF gradient optimization of the H 2CO→H 2 + CO transition structure

A compact algorithm is presented to obtain the two-electron integral derivative contribution to MC SCF gradients that avoids the storage of the integral derivatives or the back-transformed two-electron density matrix. The MC SCF gradients are applied to the optimization of the H 2CO→H 2 + CO transition structure at the 6–31G* basis-set level using a complete CI in a four-electron, four-orbital valence space.

Structural relaxation upon internal rotation in cis-methyl nitrite, CH 3 ONO

The ab initio SCF gradient method has been used to obtain changes of bond lengths and valence angles upon internal rotation of CH 3 and NO groups in cis-methyl nitrite. The data for methyl torsion are confirmed by comparison of calculated and observed shifts of rotational constants in the first methyl torsional state.

Ab initio studies on the electronic structure of the FSO radical

The geometries of the FSO radical in the ground ( 2A″) and first excited ( 2A′, n → π*) state have been calculated with the ab initio UHF SCF gradient method. The geometry in the first excited state is predicted to be r so = 1.69 3 Å, r SF = 1.60 g Å, and <FSO = 95.5°. The force constants, vibrational frequencies and dipole moments have also been calculated. The predicted 2A′ ← 2A″ transition energy, calculated at the optimized geometries with an UHF-NO-CI method, is 3.9 eV for the vertical excitation, 0.9 eV for the vertical emission and 2.6 eV for the electronic term value. The oscillator strength is also calculated. A brief discussion is given on the role of bond functions on the calculated geometry.