Author:
Allen F. H.,Bird C. M.,Rowland R. S.,Raithby P. R.
Abstract
The hydrogen-bond acceptor ability of sulfur in C=S systems has been investigated using crystallographic data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. The R1R2C=S bond lengths span a wide range, from 1.58 Å in pure thiones (R
1 = R
2 = Csp
3) to 1.75 Å in thioureido species (R
1 = R
2 = N) and in dithioates —CS^{-}_2. The frequency of hydrogen-bond formation at =S increases from 4.8% for C=S > 1.63 Å to more than 70% for C=S > 1.70 Å in uncharged species. The effective electronegativity of S is increased by conjugative interactions between C=S and the lone pairs of one or more N substituents (R
1
R
2): a clear example of resonance-induced hydrogen bonding. More than 80% of S in —CS^{-}_2 accept hydrogen bonds. C=S...H—N,O bonds are shown to be significantly weaker than their C=O...H—N,O analogues by (a) comparing mean S...H and O...H distances (taking account of the differing non-bonded sizes of S and O and using neutron-normalized H positions) and (b) comparing frequencies of hydrogen-bond formation in `competitive' environments, i.e. in structures containing both C=S and C=O acceptors. The directional properties and hydrogen-bond coordination numbers of C=S and C=O acceptors have also been compared. There is evidence for lone-pair directionality in both systems, but =S is more likely (17% of cases) than =O (4%) to accept more than two hydrogen bonds. Ab
initio calculations of residual atomic charges and electrostatic potentials reinforce the crystallographic observations.
Publisher
International Union of Crystallography (IUCr)
Subject
General Biochemistry, Genetics and Molecular Biology,General Medicine
Cited by
192 articles.
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