The influence of mechanical and electrical anharmonicity on the intensity of X—H stretching vibrations is investigated by means of model calculations in relation to hydrogen bonding. It is found that while both kinds of anharmonicities have a significant effect on the intensity, they cannot explain the characteristic increase in intensity of the fundamental and the equally characteristic decrease in the intensity of the first overtone. It is shown that not only the first but both these phenomena are due to the large value of the first derivative of the dipole moment in hydrogen bonded systems. This is linked to the possibility that the effect of electrical anharmonicity (d2μ/dQ2) might be cancelled by a high value of (dμ/dQ)c in the expression of the overtone intensity.
The long-standing problem of the assignment of the first overtone of the OH stretching vibration of hydrogen bonded alcohols is examined for the cases of methanol, ethanol, and tertiary butanol. The spectra of the related deuterated alcohols were measured and isotopic isolation experiments were carried out at temperatures ranging from 25° to − 190°C. Dilute solutions were studied as well as the pure liquids. For the latter the second overtones were also measured. The first OH overtones, although overlapped by CH + OH combination bands were definitely identified and the anharmonicity constants were determined. They show a moderate but appreciable increase with respect to free OH stretching vibrations. Bands due to simultaneous excitation or coupling of two contiguous OH groups bound together by hydrogen bonds are also present.
of H20-substantially more negative than AH for the solvation of water monomer.
Concluding RemarksAn important point to be made from this study is that to describe hydrogen bonding between water and the propylene carbonate solvent, wre have had to use a continuum model, wffiereas for the hydrogen bonding between water molecules in dilute solution, we have had to use a specific chemical association model. The combined infrared and nmr data conclusively eliminate cyclic trimer and suggest that an open dimer is the only
Publication costs assisted by Université de Montréal An attempt has been made to demonstrate the presence of oligomers and monomers in liquid methanol, by a study of the OH stretching fundamental, and its first and second overtones. The key to the solution of this problem is the previously made observation that a decrease in the intensity of the polymer overtones gives the bands of the less associated species a chance to appear.
It is shown that despite the complexity of the related parts of the spectrum, vibrational overtones give evidence of the presence of free OH groups in liquid water.Malgre la complexitt du spectre on montre que l'examen des harmoniques apporte des arguments en faveur de la pr6sence de OH libre dans l'eau liquide.
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