X-H center dot center dot center dot pi(phenyl) interactions - Theoretical and crystallographic observations

X-H...pi (phenyl) is the interaction of a hydrogen atom with the Pi-system of an aromatic ring in either an intermolecular or an intramolecular fashion. Although the existence of these types of interaction has gained recent attention in the literature, the geometry is poorly understood. This paper attempts to investigate the preferred geometry and interaction strength for O-H, N-H, N-H+, S-H, sp(2) C-H and sp(1) C-H interactions with phenyl rings. This has been done through searches of the Cambridge Structural Database, combined with semi-empirical and ab initio molecular orbital calculations. It is found that the classical image of T-shaped geometry is rarely adopted and that the preferred geometry involves direct interaction of X-H with the carbon atoms of the phenyl ring. The binding energy associated with the interaction decreases with the electronegativity of the donor atom X.