The aim of this work has been to measure the integrated absorption intensities of both halocarbons of topical interest and a range of chloroethanes and ethanes to observe the effects of fluorination. Difficulties were encountered in obtaining new halocarbons thought to have commercial applications, resulting in the syntheses of two
halopropanes. l-Chloro-2,2,3,3-tetrafluoropropane (HCFC244ca) and 1-chloro- 2,2,3,3,3-pentafluoropropane (HCFC235cb) were synthesised and used in subsequent spectroscopic measurements. The consistency of the resultant integrated band intensities confirmed that the compounds were of a purity comparable with the other halocarbons used.
In an attempt to obtain reproducible results for the absorption intensities, the - errors in FT1R spectroscopy were examined closely, and results from different instruments were compared. A comprehensive set o f procedures for recording and analysing infrared spectra was determined in an attempt to reduce errors and
discrepancies in the data. Band intensities in the ranges 3500-450cnr1, 1250-833cnr1, and 1300-700cm 1 were measured for 25 compounds with an estimated error o f i4 % .
In order to investigate the effects on the vibrational absorbance of fluorine atoms in a molecule, 19 ethane-type compounds were selected for theoretical studies. Several different methods o f modelling chemical substances may be found in the literature, and in order to compare experimental and theoretical results, two approaches were used in this work: a normal coordinate analysis using a Urey-Bradley force field; and ab initio
calculations. The UB force field gives a relatively simple mechanistic model, without reference to quantum theories and methods. In contrast, ab initio methods are based on Schrödinger1 s wave equation and a number o f mathematical approximations.
The calculations enabled some comparisons of fundamental frequencies, force constants, intensities and molecular geometries to be made. The results from three different sources were combined; those from both experiments reported in the literature and experiments earned out as part of this work; from the normal coordinate analyses; and from ab initio methods. Using these results, literature assignments o f the
fundamental frequencies have been confirmed and some new assignments tentatively made for the 19 compounds studied.
While the discrepancies between the experimental frequencies and the UB force field were larger than those for the ab initio results, the errors were such that the resultant force constants may be considered reasonable, and the group force constants may be used as a starting point for frequency calculations for similar molecules. As far as possible, the UB and ab initio force constants were compared and found to be in reasonable agreement, in that the trends across a group of molecules with increasing numbers of fluorine atoms were generally consistent. C-H stretching force constants from the two methods of calculation were not in agreement, however, variation was also seen in the geometries. Problems occur when using the simple UB force field for the C-H group (Shimanouchi 1963), and have been reported for C-H stretching frequencies and force constants when using ab initio methods (Fogarasy and Pulay
1985).
The theoretical methods and basis set used for ab initio calculations were the most sophisticated available for this work, and have produced satisfactory results. Comparisons o f HF and MP2 calculations using the basis set 6-31G* show similar results with the judicious use of scaling factors.
It was not possible to calculate intensities as part o f the UB force field, however, intensity calculations were obtained from the ab initio calculations. Comparison
between the experimental and theoretical results have shown surprisingly good agreement, considering that calculated intensities have been assumed to be only
qualitative (Foresman and Frisch 1993). The derivation o f a scaling factor of 0.734 for intensities in the range 3500-450cm_1 has enabled comparisons to be made between the experimental and calculated results for the C-F stretching region o f approximately 1350-
1000cm-1 and the C-H stretching region of 3200-2800cnr1. A satisfactory agreement was found in the results for the C-F stretching region, but not for the C-H region, due, in part, to the problems associated with calculating frequencies and force constants for the C-H bond.
A relationship has been observed between the number of fluorine atoms and the absorption intensity of the molecule in the mid-infrared region. Initially, the
calculations for the HCFCs were not carried out as there is insufficient relationship between the HCFC molecules used in the experimental part o f this work. However, the usefulness of combining experimental and theoretical results has become apparent for studies of frequencies and intensities, and so it would be o f interest to complete
calculations for the HCFCs used. Barton et al. (1993) suggested that the combination of theoretical calculations and experimental measurements in the study of vibrational spectroscopy is much more powerful than if either technique is used alone. This has proved to be the case in this work, where the assignment o f fundamental modes has been made easier by the use of both observed and calculated values. Calculations
involving all halopropanes containing fluorine, chlorine and hydrogen atoms would enable further investigation of the effects on intensities of the position of the fluorine atoms within the molecules, as well as facilitate the assignments of fundamental frequencies should the relevant spectra become available.
The experimental results of this work may be useful in the calculations of global wanning potentials. The frequencies, force constants and intensity calculations carried out suggest that these methods of calculation, when applied to fluorocarbons thought to have commercial uses, may be used to predict infrared spectra with an accuracy suitable for use in climate modelling techniques. Should further studies o f the relationship between the intensities and the number o f fluorine atoms be undertaken to confirm the findings made in chapter seven, it may be possible to estimate the intensity of new halopropanes in the C-F stretching region using graphs such as those in figures 7.1,7.2 and 7.3, without recourse to synthesis or expensive ab initio calculations.