Andrew Komornicki

An efficient abinitio method for computing infrared and Raman intensities: Application to ethylene

A new and very efficient method for the calculation of the infrared and Raman spectral intensities of polyatomic molecules using ab initio Hartree–Fock theory is described. The utility of the method is exemplified by an evaluation of the dipole and polarizability derivatives of the ethylene molecule employing a Gaussian basis set of double zeta quality, augmented by two sets of polarization functions. The predicted values of the vibrational intensities and the depolarization ratios are nearly within the experimental uncertainty which indicates that Hartree–Fock theory is capable of correctly describing these phenomena. The new method enables the efficient calculation of these properties by evaluating the dipole and polarizability derivatives as the first and second order contributions to the potential energy gradient of a molecule in the presence of a finite perturbing electric field. The calculations are thus performed at a single nuclear geometry and the molecular integrals are evaluated only once. The ...

An ab initio investigation of the structure, vibrational frequencies, and intensities of HO2 and HOCl

The infrared spectral intensities for HOC1 and HO2 have been calculated using a new ab initio technique. Theoretical results for the geometries, vibrational frequencies, and the dipole moments of these species are also reported. All of the calculations were performed at the SCF level using near Hartree–Fock quality basis sets. Our results for the molecular geometries and the vibrational frequencies are in good agreement with available experimental data. We believe that our computed intensities are accurate to at least 50%. Our results should be helpful in attempts to determine the stratospheric abundance of HOC1 and HO2 by in situ infrared spectroscopic measurements.

Nitrogen-nitrogen bond in dinitrogen tetroxide

Dinitrogen tetroxide is a weakly bound dimer of NO/sub 2/, whose geometrical and electronic structure has been the subject of numerous experimental and theoretical investigations over the past 30 years. The molecule is characterized by two NO/sub 2/ groups whose geometrical parameters are virtually identical with those found in free NO/sub 2/ and a very long, 1.78 A N-N bond. Previous calculations have been unable to reproduce this geometrical structure. The results of a series of CAS-SCF calculations which employ a double-xi plus polarization quality basis set are reported. The most extensive CAS-SCF calculations which include 18 electrons correlated in 12 valence orbitals not only reproduce the known experimental geometry but also show that a proper description for N/sub 2/O/sub 4/ is associated with a correct description of the individual NO/sub 2/ fragments.