Gary D. Bent

The electronic structure of the formyl radical HCO

Many‐body perturbation theory (MBPT) and couple‐cluster doubles (CCD) calculation are reported for the formyl radical and for points on the potential energy surface corresponding to decomposition of the radical to hydrogen plus carbon monoxide. The predicted equilibrium structure (rCH=2.1 b, rco=2.245 b, and ϑ=124°) and dissociation energy (De=16.6 kcal mol−1) are in excellent agreement with experimental data. An analysis of the saddle‐point region of the hypersurface provides a structure for the activated complex (rCH=3.35 b, rco=2.15 b, ϑ=115°) and predicts the critical energy (E0=18.5 kcal mol−1). Comparison of MBPT and CCD results for the dissociation energy and barrier height shows that equivalent results are obtained. A RRKM prediction of the decomposition rate coefficient for HCO→H+CO is also given.

Many‐body perturbation theory electronic structure calculations for the methoxy radical. I. Determination of Jahn–Teller energy surfaces, spin–orbit splitting, and Zeeman effect

Many‐body perturbation theory calculations of the electronic structure are reported for C3v and Jahn–Teller distorted conformations of the methoxy radical CH3O. The Jahn–Teller distortion reduces the energy relative to the minimum energy for the C3v structure by −0.64 kcal/mol. Furthermore, the dynamic Jahn–Teller effect reduces the calculated spin–orbit splitting from 78 to 37 cm−1. An analysis of the Jahn–Teller energy surface yields the e mode vibrational frequencies (ν4 = 2314, ν5 = 1066, ν6 = 792 cm−1) and Coriolis coupling coefficients (ζ4 = 0.065, ζ5 = −0.152, and ζ6 = 0.186) for the ground state. The orbital g factor g0 = 0.647 was calculated and used to determine the components of the g tensor for free methoxy and matrix‐trapped methoxy. For free methoxy, g∥ = 2.645 and g⊥ = 0; for the matrix‐trapped radical, experimental data was used to calculate the splitting 1.7 kcal/mol of the methoxy energy level caused by its site environment. This splitting quenches g∥ to a value of 2.096.

Formaldehyde: Electronic structure calculations for the S0 and T1 states

Many‐body perturbation theory (MBPT) and coupled‐cluster method calculations are reported for the S0(X 1A1) and T1(a 3A″) electronic states of formaldehyde. The structural parameters for the S0 minimum (RCH = 1.102 A, RCO = 1.211 A, HCH = 116.2°) and the T1 minimum (RCH = 1.085 A, RCO = 1.327 A, HCH = 118°, ’’out‐of‐plane’’ angle = 37° 12′) agree well with experimentally deduced values. Calculated heats of reaction for dissociation to radical products and molecular products agree well with literature values. The energy barriers for dissociation to molecular products and rearrangement to hydroxycarbene are presented. Vertical and adiabatic transition energies are reported for S0→T1, while a vertical transition energy for S0→S1 is reported.

Calculation of dissociation energies using many-body perturbation theory☆

Abstract Dissociation energies for the step-wise removal of hydrogen from methanol, CH 3 OH→CH 3 O→CH 2 O→CHO→CO, are obtained by many-body perturbation theory. The heat of formation of CH 3 O is predicted as 2 ± 3 kcal mole −1 . Computed geometries are in excellent agreement with experiment, where available, and provide a prediction for CH 3 O.

Ab initio (HeH2)+ energy surfaces and nonadiabatic couplings between them

The energy surfaces of the three lowest adiabatic states of the (HeH2)+ triatomic molecular system have been calculated ab initio as functions of all three variables describing the triatomic geometry, using the BRLJHU set of quantum chemistry programs. The procedure is described by the acronym SA‐MCSCF/CI, for state‐averaged multiconfiguration self‐consistent‐field calculation, followed by a full configuration interaction calculation. In addition the nonadiabatic matrix elements which couple these adiabatic states have been calculated. Results have been obtained on a sufficiently fine mesh for interpolation by a spline‐fit program to produce energy differences and nonadiabatic coupling matrix elements over the full mesh required for collisional excitation problems of He+ on H2 and H+2 on He involving these states.

Many‐body perturbation theory electronic structure calculations for the methoxy radical. II. Hyperfine coupling coefficients

The methoxy radical undergoes the dynamic Jahn–Teller effect which gives rise to diagonal and off‐diagonal hyperfine coupling constants. The off‐diagonal constants are between the degenerate vibronic wave functions of the lowest spin–orbit level. The experimental hyperfine coupling constants were measured in terms of spherical tensors. In this paper the spherical tensors are expanded in Cartesian matrix elements. These expansions are used to calculate the hyperfine coupling constants, both the isotropic constants (also known as the Fermi contact terms) and the anisotropic constants (also known as the dipolar terms). The diagonal hyperfine coupling constants could be calculated by standard methods, but the off‐diagonal hyperfine coupling constants had to be calculated between nonorthogonal orbitals. The calculated dipolar hyperfine terms, both diagonal and off‐diagonal, are in fair agreement with the known experimental values. The calculated diagonal Fermi contact terms are in good agreement with experiment, while the calculated off‐diagonal Fermi contact terms are in terrible agreement with experiment.

Approximate calculation of the Jahn–Teller energy and Ham reduction factor for CH3S

The difference in the spin–orbit splittings of CH3S and CD3S, the value of ν5 for CH3S, and the Teller–Redlich rule for CH3S/CD3S and CH3O/CD3O are used to make an approximate calculation of the Jahn–Teller energy and Ham reduction factor p for CH3S. The results are E JT=95 cm− 1 and p=0.79. The results indicate that the unpaired electron in CH3S is localized on the sulfur atom.

The multielectron, hidden crossings method for inelastic processes in slow ion/atom–atom collisions

A new method is described for studying collision dynamics in slow ion/atom–atom collisions. It is a generalization of the single-electron, two-center hidden crossings method to multielectron systems. This approach derives from the analytic properties of energy surfaces and wave functions of the adiabatic electronic Hamiltonian when the internuclear distance is extended into the complex plane. The collision dynamics in the adiabatic limit is determined by the topology of the unique multivalued electronic energy surface, particularly by its singular points, the square-root branch points. The surfaces described here have been studied using a complex version of the unrestricted Hartree–Fock and configuration interaction method with all single electron excitations, based on a bivariational principle. Although various inelastic processes can be calculated, the method is especially useful for the description of ionization. We have illustrated this through the calculation of cross sections for ionization of heliu...

Calculation of Potential Energy Surfaces for HCO and HNO Using Many-Body Methods

It has been recognized, since the work of Eyring and Polanyi,1 that an understanding of detailed reaction dynamics requires a knowledge of the topographical features of the potential energy hyper-surfaces pertinent to the chemical reaction. The task of the theoretician is to refine electronic structure calculations so that electronic energy differences on an energy surface may be predicted with “chemical accuracy”, or about 2 kcal/mol. Recent publications, by several different groups, establish the importance of electron correlation effects in studying even qualitative aspects of potential energy surfaces,2-5 and experience to date indicates that neglect of the effects of electron correlation can lead to substantial errors in predicted heats of reaction.

Method of measuring the Ham reduction factor q in C3v molecules

This paper applies to molecules which have (i) a doubly degenerate electronic state in C3v symmetry, (ii) a single unpaired electron, (iii) a dynamic Jahn–Teller effect, (iv) spin–orbit coupling less than the Jahn–Teller active vibrational frequency, and (v) nonzero magnetic moments for the equivalent nuclei. Two Fermi contact terms can be measured in the hyperfine interaction of such molecules. A proof is given that the ratio of the Fermi contact terms is the Ham reduction factor q. Recent measurements for CH3O and CH3S are compared to the predictions of this paper.