Molecular force fields and thermodynamic properties of electronically excited triatomic and tetraatomic molecules

Abstract

Electronically excited molecules play an important role in photochemical reactions. Up to the present time, thermodynamic properties for the molecules in their excited states have not been reported in the literature. During the past few years, a sufficient number of detailed spectroscopic measurements have been carried out on the rotational and the vibrational states of triatomic and tetraatomic excited species so that calculations of their thermodynamic properties with the methods of statistical mechanics can now be carried out. Certain simple polyatomic molecules, such as H₂O, D₂O, ClO₂, SO[superscript 16, subscript 2], SO[superscript 18, subscript 2], NO₂, CH₂O, CD₂O, and CHDO for which electronic spectra are thoroughly studied and satisfactorily analysed, were selected for this study to carry out statistical calculations of the thermodynamic properties. To predict the unobserved fundamental frequencies and molecular force fields, a normal coordinate analyses were carried out using the most general quadratic potential function of the valence force type. For formaldehyde, Schachtschneider's perturbation program for 709 digital computer was used. For the water molecule, Wilson's FG matrix method employing data for the isotopic species was used. For remaining bent triatomic molecules, it was established that the modified method of operation of high and low frequencies may be successfully employed. Finally, assuming the rigid rotor harmonic oscillator approximation, the thermodynamic properties such as enthalpy of formation, heat capacity, entropy, and Gibbs energy of formation for these excited molecules were predicted and compared with those in the ground states.