LOW-FREQUENCY TORSIONAL OSCILLATIONS IN ASYMMETRICALLY SUBSTITUTED HALOPROPANES

Using force constants derived from gas-phase data, the torsional frequencies in the 84 molecules (X = F,Cl,Br,I), XH2C-CH2-CH3, X2HC-CH2-CH3, XH2C-SCH-CH3, (XH2C)2CHX, X3C-CH2-CH3, X2HC-XCH-CH3, XH2C-CX2-CH3, X3C-XCH-CH3, X2HC-CH2-CH2X, X2HC-CX2-CH3, X2HC-XCH-CH2X, X3C-CH2-CH2X, (X2HC)2CHX, X2HC-CX2-CH2X, X3C-CX2-CH3, X3C-CH2-CHX2, X3C-XCH-CH2X, X3C-CX2-CH2X, X3C-XCH-CHX2, X3C-CX2-CHX2 and (X3C)2CHX have been estimated by normal-coordinate analysis. It is shown that the expected regions of these wavenumbers (cm-1) are 66-161 (fluorides), 53-141 (chlorides), 34-138 (bromides) and 25-138 (iodides). The lowest bending modes have wavenumbers in the ranges (cm-1) 133-332 (fluorides), 81-298 (chlorides), 51-281 (bromides) and 37-209 (iodides). Generally the lowest torsional frequency in a conformer has a wavenumber below the lowest bending mode in the same conformer. Torsional CH3 frequencies in the range 194-270 cm-1 are obtained. Simple formulas are derived for calculating approximately correct values of the lowest wavenumbers if the torsional force constants are known. Systematic trends among the wavenumbers are pointed out. These trends can be used in estimating frequencies without doing the actual normal-coordinate analysis, and without knowing the torsional force constants. Including all halopropanes, 25 spectroscopic observations of wavenumbers in the low-frequency region 50-160 cm-1 have been recorded. The calculated and experimental values agree within 10% error limits. The experimentally estimated CH2F torsion in 1-fluoropropane (anti) does not fit this pattern. Since the torsional force constants were not adjusted to fit the observations, our calculated wavenumbers may be considered an independent estimate for these molecules.