A-H stretch frequency

Spectroscopic methods are useful for characterization. The A H stretching frequency in the IR spectrum typically falls on hydrogen bond formation and the AH proton resonance also shifts to high field. Where A or B has a nuclear spin (e.g. F), the coupling constant between A or B and H can be used as long as exchange is not too rapid. For example, in the gas phase, HF has an H,F coupling constant near 600 Hz, but in amine H-F adducts, this value falls to about 450 Hz. ... 

In some cases, the so-called blue-shifted hydrogen bond, the shift of the A-H stretching frequency is toward higher wave numbers. The reasons for this exceptional behavior have been now properly rationalized [39]. 

The major contribution to the kinetic isotope effect lies in the loss of the zero-point energy associated with the original A—H stretching frequency, since the contributions of bending frequencies tend to cancel... 

The majtH spectral changes that occur when a weak or moderate hydrogen bond forms are 1. the A-H stretching frequency (Vj) decreases, 2. the A-H bending frequency (ir,) increases, and 3. the bandwidth and intensity of the A-H stretching frequency (Vs) increase, see Figure 3. 

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