Kerin Scanlon

Tests of approximation schemes for vibrational energy levels and partition functions for triatomics: H2O and SO2

Various approximate schemes are used to obtain vibrational energy levels and partition functions for H2O and SO2. These results are then compared with accurate quantum mechanical values computed for the same original quartic force fields. The neglect of interaction force constants in internal coordinates and the use of a Morse model to describe stretching anharmonicity are both shown to provide reasonably accurate perturbation theory energy levels and partition functions, while the neglect of interaction force constants in normal coordinates is found to yield much less accurate results. In addition, the method of Pitzer and Gwinn to calculate partition functions without first calculating energy levels is shown to provide a good approximation to the accurate partition functions.

The absolute intensities of the binary combination bands in the infrared spectrum of SF6

Abstract The intensities of all the fundamentals and binary combination bands in the infrared spectrum of SF 6 have been measured. Estimates of the cubic normal-coordinate force constants previously determined from an anharmonic Urey—Bradley force field have been used to compute the contributions to the combination intensities from the first derivatives of the dipole moment. These, in turn, have been used to obtain experimental values of the second derivatives of the molecular electric dipole moment with respect to the normal coordinates.

The anharmonic force field of CClF3 and the determination of the Coriolis coefficients and anharmonic coefficients from the infrared spectrum

The spectroscopic parameters of CClF3 have been calculated from an assumed anharmonic Urey–Bradley force field. These calculated values have been used as an aid to the interpretation of the observed infrared spectrum. The Coriolis splitting of the K subbands of the parallel transitions to the combination states formed from two degenerate fundamentals, e.g., n4+n5, has been observed and used in the determination of the Coriolis coefficients.

The Determination of Absolute Infrared Intensities with a Fourier Transform Spectrometer

In principle it is apparent that Fourier transform infrared spectroscopy offers a number of significant advantages in the determination of infrared intensities integrated over the rotational fine structure in vibration-rotation bands. One expects that there should be no problem with photometric accuracy; yet, before embarking on a systematic program of study, we believe it important to establish that the method will work in practice. As a test case we have chosen the V2 fundamental of CO2 at 673 cm−1 since this is, in our experience, as difficult a band as any when it comes to measuring integrated intensities. We have, using a Digilab FTS-20 spectrometer, determined a value of the integrated intensity, Γ2 = 7901 ± 105 cm2/mol which should be compared with the value previously determined on a dispersive instrument, Γ2 = 7882 ± 110 cm2/mol. We have also determined a value of Γ3 = 25,300 ± 570 cm2/mol for the integrated intensity of the antisymmetric stretching fundamental.

Vibrational averages of molecular properties in polyatomic molecules; the average geometry of CF4

Abstract A formalism is developed for the convenient calculation of first-order anharmonic vibrational averages of molecular properties which depend on nuclear position coordinates. This formalism is used to calculate the average geometry of CF 4 in vibrationally excited states.

An ab initio calculation of first and second derivatives of the electric dipole moment function for certain infrared-active modes of SF6

Abstract Certain first and second derivatives of the dipole moment function for SF 6 have been calculated from ab initio molecular orbital theory in the finite difference approximation. All calculations were carried out in normal coordinates with an experimental force field and geometry. A double-zeta basis with d -functions on the sulfur was employed. Calculated harmonic frequencies in cm −1 are 1084 (ω 3 ) and (648(ω 4 ) and calculated intensities in km/mole are 1612 (ν 3 ) and 209 (ν 4 ). Second derivatives of the dipole moment function for ν 1 + ν 3 x and ν 2 a +ν 3 x in units of e-amu −1 A are 0.0552 and −0.0119, respectively. A comparison with the values determined from an STO-3G basis is presented together with a comparison of the derived quantities from experiment.