Geometries of H-Bonds at Equilibrium

In this chapter we focus on the equilibrium values go 0 (p of the three coordinates 

Some authors have postulated the existence, beside ordinary H-bonds, of bifurcated H-bonds or three-centre H-bonds that can be occasionally encountered and consist of a single X-H donor group that simultaneously interacts with two acceptor groups Y and Y. These multicentred H-bonds lose the properties of directionality of ordinary H-bonds. These authors are mainly crystallographers (2 pp. 136-146) who measured, by X-ray or neutron diffraction, distances X- Y and X- -Y that are nearly the same in an appreciable number of systems. The existence of such H-bonds has never been experimentally put into evidence by other techniques, particularly IR spectroscopy, the most sensitive method to observe H-bonds (Chs. 4 and 5). It has been occasionally used as an ad hoc explanation of some 

Enthalpies A// and eqniUbrinm distances Qq of some isolated H-bonds in gases H-bond X-H- B (kJ mol ) Go (A) 

For H-bonds with NHj as the acceptor, values for All are calculated (14) while values for Go are experimental  

Enthalpies of formation AH and equihbrium distances Go selection of H-bonded dimers in  

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