Electrical anharmonicity for the OH stretching vibration of methanol and its hydrogen bonded complexes

Abstract

Listen

Translate article

We have calculated the first and second dipole moment derivatives for a number of hydrogen bonded systems involving methanol and a variety of organic bases. Herman and Shuler's [1] expressions for the intensities of fundamental and first overtone are employed for the calculations. Both ++ and +− combinations of experimental values of transition moment integrals R 1 0 and R 2 0 are considered. Given below are the typical values of μ 1 and μ 2 for the two sign combinations in the case of dilute solutions of CH 3OH in CCl 4 representing free OH and CD 3OH in DMSO-d 6 as well as self associated CD 3OH in CD 3OD. It is found that for all the systems considered both for ++ and +− combinations μ 1 as well as μ 2 increase on hydrogen bonding. Often a reference is made in the literature that the intensity of the first overtone band of XH stretching vibration decreases on hydrogen bonding whereas that of the fundamental increases. On the basis of these observations a decrease in the overtone intensity Methanol in ++ comb. +−comb. μ 1 × 10 20 μ 2 × 10 20 μ 1 × 10 20 μ 2 × 10 21 CCl 4 4.567 1.017 4.770 8.094 DMSO-d 6 20.443 1.641 20.750 3.943 CD 3OD 23.545 2.033 23.952 4.936 can be explained on the basis of the fact that the negative contribution to the overtone intensity from an increase in the first derivative of the dipole moment supersedes the positive contribution from the increase in the second derivative of the dipole moment. This explanation is quite justified as it can be noticed from the above values that whereas μ 1 increases about 4 to 5 times on hydrogen bonding in the cases considered in these studies, μ 2 only increases by 50 to 10%. This observation is similar to the prediction made by Di Paolo, Bourderon and Sandorfy [2] on the basis of their theoretical model calculations on the influence of Electrical Anharmonicity on infrared intensities in hydrogen bonded systems. However, contrary to their predictions it is rather clear from these studies that the decreasing intensities of first overtones on hydrogen bonding can be explained by taking +− combinations as nicely as from ++ combinations of transition moment values.