Investigation of weak hydrogen bond using terahertz time-domain spectroscopy and mid-IR absorption

We have investigated THz and mid-IR absorption features of weak hydrogen bond formed between dimethylsulfoxide (DMSO) molecules in cyclohexane. The IR band due to the S=O stretching mode shows two bands in the region from 1050～1100 cm-1, the ratios of which are dependent on the concentration of DMSO. These bands are well described two bands at 1072 and 1083 cm-1, which are assigned to the S=O stretching modes of a DMSO monomer and a DMSO dimer, respectively. In ab initio quantum calculation we obtained stable structures of DMSO dimers, which have IR-active intermolecular vibrational modes originating in weak hydrogen bond of S=O…H-C. We observed that the broad THz absorption band is present in DMSO solution and that the THz absorption is enhanced as concentration increase of DMSO solution. Therefore, we directly show that weak hydrogen bonds are formed between DMSO molecules in solution by THz spectroscopy