Michael J. Frisch

Self‐consistent molecular orbital methods 25. Supplementary functions for Gaussian basis sets

Standard sets of supplementary diffuse s and p functions, multiple polarization functions (double and triple sets of d functions), and higher angular momentum polarization functions (f functions) are defined for use with the 6‐31G and 6‐311G basis sets. Preliminary applications of the modified basis sets to the calculation of the bond energy and hydrogenation energy of N2 illustrate that these functions can be very important in the accurate computation of reaction energies.

The vinylidene-acetylene isomerization barrier

Abstract The barrier to rearrangement of vinylidene to acetylene has been determined using Moller-Plesset perturbation theory and an extended polarized basis set. With the inclusion of zero-point vibrational corrections, the barrier is found to be only 0.9 kcal mole , suggesting that vinylidene will have an extremely short lifetime.

A systematic study of the effect of triple substitutions on the electron correlation energy of small molecules

Abstract The importance of triple substitutions to the electron correlation energy in fourth-order perturbation theory is demonstrated in a systematic study of a number of small molecules. Some approximate methods to calculate the contribution of triple substitutions are also discussed.

Hydrogen bonds between first‐row hydrides and acetylene

The structures and association energies of the complexes of NH3, H2O, and HF with acetylene have been determined using Hartree–Fock and Mo/ller–Plesset theories. HF is found to act as a proton donor to acetylene, while H2O and NH3 act as proton acceptors.

Ab initio potential energy curve for CH bond dissociation in methane

Abstract Ab initio calculations on CH 4 → CH 3 + H were performed at the MP4/6-31G** level including all single, double triple and quadruple excitations. Although triple excitations have little effect on the dissociation energy, they are very important in the 2–3 A range. The potential curve appears to rise more sharply than a Morse curve in this region. Correlation effects are not important for the HCH angle optimization.

The ground-state singlet potential surface for C2H4

Abstract Calculations with an extended polarized basis set and Moller-Plesset perturbation theory including triple substitution correlation corrections in the fourth-order treatment indicate that singlet ethylidene (CH3CH:) is not a local minimum on the C2H4 potential energy surface. Rearrangement to ethylene occurs without actuation. Barriers for hydrogen scrambling and for 1,1-hydrogen elimination are estimated.

Cyclic C3 structures

Abstract Alternative cyclic C3 structures are not competitive energetically with the linear 1∑g+ ground state, 1. The D3h triplet, 4, is a local minimum on the potential energy surface. Cyclopropynylidene, a C2v singlet (3) is a saddle point for the degenerate isomerization which permutes the order of carbon atoms in 1. The activation energy for this transformation is predicted to be 29 kcal mole .

The stability of fluorovinylidene and difluorovinylidene

Abstract The barriers to rearrangement of fluorovinylidene and difluorovinylidene to the corresponding acetylenes have been calculated using Moller-Plesset perturbation theory. Difluorovinylidene is found to have a barrier of 36.3 kcal/mole, and hence should have significant lifetime, in agrrement with experiment. Fluorovinylidene should rearrange via hydrogen migration without a barrier.

Hydrogen bonds between hydrogen halides and unsaturated hydrocarbons

Abstract Ab initio molecular orbital theory is used to study geometrics and energies of hydrogen-bonded complexes between hydrogen fluoride, hydrogen chloride (as proton donors) and acetylene, ethylene (as proton acceptors). Symmetrical T-shaped structures are found to be equilibrium structures for all four complexes. The strengths of the hydrogen bonds are found to be less than for conventional hydrogen bonds involving lone pairs of electrons.

Unusual low-energy isomers for simple radical cations

Abstract The radical cations of formaldimine, methylamine, formaldehyde, methanol, diazene, hydrazine, nitroxyl, hydroxylamine and hydrogen peroxide, and of isomers derived formally from these systems by means of a 1,2-hydrogen shift have been studied using ab initio molecular orbital theory, including electron correlation. For the ions of formaldimine, methylamine and methanol, evidence is presented that the 1,2-hydrogen-shifted species lie lower in energy than the conventional isomers.