Chemistry Department, Machine VP
Co-Investigator Marilyn P Olliff
Chemistry Department, The Faculties
Theoretical Studies of Vibrational Frequencies of Selected Halocarbons of Topical Interest
Halocarbons have proved to be beneficial in raising the standard of living in many parts of the world. Since the early 1930s halocarbons have been manufactured for commercial use and, once used, are released into the atmosphere. Due to their molecular stability, halocarbons accumulate in the lower atmosphere and slowly diffuse into the upper atmosphere. Recent studies have highlighted some serious disadvantages in the use of fluorine- and chlorine-containing compounds with regard to the effects they may have on the environment. These include destruction of the ozone layer and contribution to greenhouse warming and consequent climate change. The present work is a theoretical investigation of the molecular vibrations of a range of fluorine-containing halocarbons, motivated by the requirement for further research in this area.
Halocarbons are nonflammable, odourless, and stable man-made compounds with many uses in the areas of refrigeration, air conditioning, cleaning and plastic foam manufacture. The three most commonly found groups of these substances are known as the chlorofluorocarbons (CFCs), the hydrofluorocarbons (HFCs), and the hydrochlorofluorocarbons (HCFCs). These molecules all consist of some combination of carbon and fluorine atoms.
It is important to investigate the physical and chemical properties of halocarbons in order to increase our understanding of the role played by these chemicals in global warming and ozone depletion. It has been found that many halocarbons absorb radiation strongly in the atmospheric window region. This is a region of the spectrum where the natural substances in the earth's atmosphere do not absorb. The accumulation of halocarbons in the troposphere may, therefore, reduce the loss of heat through radiation from the earth's surface, subsequently increasing the ambient temperature. The ab initio calculations carried out here are part of a research project to measure the infrared absorption intensities, and therefore their Greenhouse Warming Potentials, of a range of halocarbons, with emphasis on some new HFCs and HCFCs which have been proposed as suitable replacements for the more damaging CFCs currently being phased out.
What are the basic questions addressed?
Ab initio calculations are carried out to determine the fundamental absorption frequencies and transition intensities of a range of halocarbons. The aim is to help in the analysis of experimentally measured spectra, to support intensity determinations, and to investigate theoretically any possible relationships between the numbers and locations of fluorine and/or chlorine atoms, and the intensities of absorption.
What are the results to date and the future of the work?
Calculations have been carried out using the Gaussian 92 suite of programs. Most of these calculations have been carried out at the RHF/6-31G* level, and reasonable agreement has been achieved with experimental results. Some interesting relationships have been found between the numbers of fluorine atoms and the variations in absorption intensities. Theoretical interpretations of these relationships have yet to be undertaken. However, uncertainty exists on the validity of some of the experimentally determined results. For this reason future work will involve conducting ab initio calculations at higher levels, such as MP2, and more sophisticated basis sets incorporating more diffuse functions, in order to assess the reliability of the calculated frequencies and intensities.
What computational techniques are used and why is a supercomputer required?
The calculations were carried out using the Gaussian 92 programs, in particular RHF/6-31G* and a few MP2/6-31G*. These involve many iterations and are most suitably performed on a supercomputer.