DISTRIBUTION OF OBSERVED C-H BOND LENGTHS IN NEUTRON CRYSTAL-STRUCTURES AND TEMPERATURE-DEPENDENCE OF THE MEAN-VALUES

An analysis is given of C-H bond lengths and their apparent shortening by thermal vibrations in neutron crystal structures. D atoms were excluded. Mean bond lengths and the spread of observed values were compiled for six different chemical connectivities in three temperature ranges (room temperature, 80 less-than-or-equal-to T < 125 and T < 30 K). At low temperatures (T < 30 K), the mean values agree very well with spectroscopic data. Owing to thermal vibrations, the average observed C-H bond length in methyl groups at room temperature (1.057 angstrom) is shortened by 0.03 angstrom compared with that at T < 30 K (1.088 angstrom). The shortening reduces only slowly upon cooling and is still significant at temperatures around 100 K. For the other connectivities, in which the C-H orientations are more rigidly confined, the average shortening at room temperature is 0.01 to 0.02 angstrom, and at T approximately 100 K the mean observed bond lengths have already (or almost) reached the spectroscopic values. The distributions of the observed bond lengths are asymmetric with the maxima at distances greater than the average. The positions of the maxima are less affected by temperature changes than the average values, implying that at room temperature many bond lengths are only slightly affected by thermal motions.