ON ANOMALOUS H D ISOTOPIC EFFECTS FOR NU(X-H) AND NU(X-D) BAND INTEGRAL INTENSITIES IN IR-SPECTRA OF CYCLIC HYDROGEN-BONDED DIMERIC SYSTEMS/

In this paper we propose a new explanation for the abnormal isotopic H /D effects concerning the nu(X-H) and nu(X-D) band integral intensitie s in the infrared for cyclic systems of hydrogen bonds. In our approac h we take into account two different but parallelly acting mechanisms of generating the band contours. One of them is governed by the symmet ry-allowed transition to the A(g) state of the nontotally symmetric pr otonic vibrations, while the other corresponds to the vibronically act ivated forbidden transition to the A(g) state of the totally symmetric vibration of the protons. As the latter mechanism was found to be muc h more dependent on hydrogen atom mass compared with the allowed trans ition mechanism, the nu(X-H) to nu(X-D) band intensity ratio could not iceably exceed the expected value of 1.41 (which characterizes the sym metry-allowed subtransitions forming the nu(X-H) and nu(X-D) band cont ours) and approach 1.9. No similar situation takes place for chain sys tems of hydrogen bonds, for which the forbidden band promotion mechani sm cannot play a dominant role, with the isotopic effect being more re gular and the band intensity ratio being close to 1.41. The proposed h ypothesis was verified experimentally by investigating the isotopic ef fect 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 nu(N-D) bands in the infrared ar e practically absent, so that the nu(N-H) to nu(N-D) band intensity ra tios were expected to exceed 1.9. For both cases the measured band int ensity ratios were equal to 2.6 +/- 0.3, which generally remains in go od agreement with the predictions of the assumed model of the spectral phenomenon. (C) 1998 Elsevier Science B.V.