On anomalous H/D isotopic effects for νXH and νXD band integral intensities in IR spectra of cyclic hydrogen-bonded dimeric systems

• Published in: Journal of molecular structure Vol. 443; no. 1; pp. 265 - 271
• Main Authors: Flakus, Henryk T., Rogosz, Krystyna
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.1998
• Subjects: Anomalous H/D isotope effect; Band intensities; Hydrogen bonding; Infrared spectroscopy; Infrared spectroscopy; Hydrogen bonding; Band intensities; Anomalous H/D isotope effect
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/S0022-2860(97)00393-1

In this paper we propose a new explanation for the abnormal isotopic H/D effects concerning the ν XH and ν XD band integral intensities in the infrared for cyclic systems of hydrogen bonds. In our approach we take into account two different but parallelly acting mechanisms of generating the band contours. One of them is governed by the symmetry-allowed transition to the A u state of the nontotally symmetric protonic vibrations, while the other corresponds to the vibronically activated forbidden transition to the A g state of the totally symmetric vibration of the protons. As the latter mechanism was found to be much more dependent on hydrogen atom mass compared with the allowed transition mechanism, the ν XH to ν XD band intensity ratio could noticeably exceed the expected value of 1.41 (which characterizes the symmetry-allowed substransitions forming the ν XH and ν XD band contours) and approach 1.9. No similar situation takes place for chain systems of hydrogen bonds, for which the forbidden band promotion mechanism cannot play a dominant role, with the isotopic effect being more regular and the band intensity ratio being close to 1.41. The proposed hypothesis was verified experimentally by investigating the isotopic effect for NH⋯S bonded cyclic dimeric systems of 2-thiapyridone and mercaptobenzothiazole. For the IR spectra of these two compounds the forbidden components of their dimeric ν ND bands in the infrared are practically absent, so that the ν NH to ν ND band intensity ratios were expected to exceed 1.9. For both cases the measured band intensity ratios were equal to 2.6 ± 0.3, which generally remains in good agreement with the predictions of the assumed model of the spectral phenomenon.