Isotopic localization of the partially deuterated methyl group in solid methanol and methyl iodide

Abstract

Heat capacity measurements were made down to 0.35 K for the isotopic modifications of methanol, CHDOH, and methyl iodide, CHDI, (n = 0, 1, 2, 3) to determine the orientation of the partially deuterated methyl group in the solid phase. The mono-deuterated modifications favor the symmetric conformation, whereas the di-deuterated ones favor the asymmetric conformation. Infrared spectroscopy demonstrates that some vibrational modes change in intensity depending on temperature, which supports the energy scheme obtained by calorimetry. Zero-point kinetic energies were obtained by single molecule density functional theory calculations. Although the favorable conformations of CHDOH and CHDOH were confirmed, the energy difference between symmetric and asymmetric conformations was twice as large as that determined experimentally, which indicates that intermolecular forces significantly decrease the energy difference. For CHDOH, the conversion between the two asymmetric conformations becomes very slow at low temperature and results in a residual entropy of R ln 2.