**Principal Investigator**
Gad Fischer **Project** u54,u01

Chemistry Department **Machine** VP

The Faculties

**Structures and Vibrational Frequencies of Pure
Halocarbon Clusters and Mixed Clusters with Water. (u54)**

**Energies and Potential Surfaces of the Excited
Electronic States of the Triazines. (u01)**

On the one hand the halocarbons have proved to be beneficial in raising the standard of living in many parts of the world, but on the other hand they have been found to be detrimental to the environment. Knowledge of the structures, potential energy surfaces and vibrational frequencies of the halocarbon monomers and clusters can assist in understranding their atmospheric chemistry and in determining the extents of their impacts on the environments. The experimental measurement of infrared absorption frequencies has been complemented by ab initio molecular orbital calculations of the absorption frequencies and intensities. Initial work has been concentrated on some cyclic fluoroalkanes. Similar computations have been carried out for a number of azines for both the ground and excited electronic states. The azines are the parent molecules for many molecules of biological and medical importance. They are characterized by a large number of close-lying excited electronic states. We are interested in the intramolecular interactions between these electronic states. It is these interactions which govern processes such as photochemistry and the non-radiative dissipation of energy.

**What are the basic questions addressed?**

Ab initio calculations are carried out to determine molecular structures, the fundamental absorption frequencies and transition intensities of a range of halocarbon molecules and clusters. In the case of the halocarbons the computations are for the electronic ground state. For the azines less sophisticated calculations are also carried out for the excited electronic states. The aim is to determine possible structures for the cluster molecules, to help in the analysis of experimentally measured spectra, and to support intensity determinations. The long term goal is to improve our understanding of their atmospheric chemistry.

**What are the results to date and future of the
work?**

Calculations have been carried out using the Gaussian 94 suite of programs. For the ground state most of these calculations have been carried out at the MP2/6-31G* level. For perfluorocyclobutane density functional calculations were also carried out at the B3LYP/6-31G* and B3P86/6-31G* levels. For the excited states of the azines the calculations have been carried at the CIS/6-31G* level. In some cases imaginary frequencies have been obtained indicative of shallow double-minimum potentials. These double-minimum potentials have been investigated by two computational methods.

**What computational techniques are used and why
is a supercomputer required?**

The calculations were carried out using the Gaussian 94 programs, in particular RHF/6-31G* and MP2/6-31G*. These involve many iterations and are most suitably performed on a supercomputer. Excited state calculations have been carried out at the CIS/6-31G* level and more sophisticated calculations at the CASSCF level using the MOLPRO program.

**Publications**

*The electronic spectrum of phthalazine*,
*Theory and Experiment.* G. Fischer and P. Wormell, Chem.
Phys. **198** 183-206 (1995)

*Ab initio calculations and jet-cooled vibrational
spectrum of perfluorocyclobutane,* G. Fischer,
R. L. Purchase and D. M. Smith, J. Mol. Spectrosc. in press