Nonlinear hydrogen bonds

A nice example of this teclmique is the detennination of vibrational predissociation lifetimes of (HF)2 [55]. The HF dimer has a nonlinear hydrogen bonded structure, with nonequivalent FIF subunits. There is one free FIF stretch (v ), and one bound FIF stretch (V2), which rapidly interconvert. The vibrational predissociation lifetime was measured to be 24 ns when excitmg the free FIF stretch, but only 1 ns when exciting the bound FIF stretch. This makes sense, as one would expect the bound FIF vibration to be most strongly coupled to the weak intenuolecular bond. 

Legon, A. C., Nonlinear hydrogen bonds O-H—C and the role of secondary interactions. The rotational spectrum of the oxirane "acetylene complex, Chem. Phys. Lett. 247, 24-31 (1995). 

There is still no certain information obtainable from infrared and NMR measurements on cyclic dimers or multimers. The dimer band may arise from a cyclic species involving nonlinear hydrogen bonds, and one might expect that if a cyclic trimer exists, the OH stretching frequency would be similar to and slightly lower than that of a cyclic dimer. This would account for the broadening of the dimer band in pentamethylethanol at high concentrations. 

FIGURE 2.6 A comparison of linear and nonlinear hydrogen bonds. Nonlinear bonds are weaker than bonds in which all three atoms lie in a straight line. 

This nonlinear hydrogen bonded species does not show any dimer absorption both for the C-C stretching (1/4) and the CH3 rocking (1/7) mode. For the next larger clusters n = 3 and n = 4 single peaks are observed which are slightly shifted to the... 

Yang Y, Meuwly M (2010) A generalized reactive force field for nonlinear hydrogen bonds hydrogen dynamics and transfer in malonaldehyde. J Chem Phys 133 064503... 

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