H-bond bending

Finally, vib represents the internal vibrational entropy of the water molecules. These vibrations are represented mainly by H-bonding bendings and vibrations normal to the surface [Fig. 6.85(c)]. Their variation with qM is shown in Fig. 6.84(c). 

The application of Stepanov s theory to intramolecular F bonded systems has been criticized [42], In this case the low frequency vibration described above as vXH Y is also partly constrained by a more nearly harmonic vibration involving skeletal bending motions of the rest of the molecule, and the X, H, and Y atoms are not collinear. These factors would seem to suggest that (I) the vXll Y type of vibration will be of higher frequency than in the usual case (perhaps 200-300 cm"1 rather than 100-200 cm"1) so that the sub-bands will be more widely spaced and may not be recognised as part of the rXH band (2) the motion of the H atom will have less effect on rXY and (3) H-bond bending vibrations may also couple considerably with vXH. The observation of rather smaller frequency shifts for vXR and narrower absorption bands w such cases are in reasonable agreement with this picture,... 

This argument underscores the important role played by- the low frequency H-bond bending mode for determining the shape and width of the main vt band. One will recall in this respect the additional argument put forward by Bertie and Falk28) which was mentioned in Section 3 a. Still another piece of supporting evidence for this was found by Bevan, Martineau and Sandorfy 411 who did gas phase work on the first overtone of v, of the ether. HF complex (Sect. 3.3). Theoreticians should take account of this in their calculations. 

A similar variation occurs with position in the periodic table when acetylene is bonded in organometallics C—C distance in the bound state increases for a zero-valent Pt complex C—H bond bending has been predicted by calculation the C—C stretching frequency (2100 cm ) is reduced to 1775 cm" when FaCC—CCFa is bonded to Pt or Ir. The decrease is smaller when HC=CH is bonded to Pd (1838) or Mn (1919), i.e. a 3d metal. 

Benzene (Figure 13.35c) has only 577 -hybridized carbons, and all of its peaks for C—H stretching lie above 3000 cm CC stretching gives a weak peak at 1478 cm The most intense peak in benzene (667 cm ) results from a vibration in which one of the C—H bonds bends out of the plane of the ring. 

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