Abstract:

The OH stretching (ν_OH) band of methanol observed in condensed phase has been analyzed in terms of hydrogen-bonding patterns. Quantum chemical calculations for methanol clusters have revealed that broadening of the ν_OH envelope is reasonably reproduced by considering nearest and next-nearest neighbor interactions through hydrogen bonding. Because the hydrogen bond formed between donor (D) and acceptor (A) is cooperatively strengthened or weakened by a newly formed hydrogen bond at D or A, we have proposed the following notation for hydrogen-bonding patterns of monohydric alcohols: a_DDAd_Aa_A, where a is the number of protons accepted by D (a_D) or A (a_A), and d_A is the number of protons donated by A. The indicator of the hydrogen-bond strength, which is given by M_OH = a_D + d_A − a_A, is correlated well with the ν_OH wavenumber of the methanol molecule D participating in the a_DDAd_Aa_A pattern. The correlation between M_OH and the hydrogen-bonding energy of the a_DDAd_Aa_A pattern has also been deduced from the calculation results for the clusters. The ν_OH bands of methanol measured in the CCl₄ solution and pure liquid have been successfully analyzed by the method proposed here.