3.1 Introduction to Hydrogen Bonds

In basic terms, a hydrogen bond (or H-bond) is an attractive interaction
between two elctronegative atoms, a donor and an acceptor (Latimer and
Rodebush 1920; Huggins 1971; Baker and Hubbard 1984; Ippolito, Alexander at
al 1990; Stickle, Presta et al 1992).  A hydrogen atom lies aligned between
them and covalently bound to the donor.  The donor attracts the electron on
the hydrogen from its orbital towards the donor itself.  This leaves a
partial positive charge on the hydrogen, which is electrostatically
attracted towards the elctronegative acceptor.  The interaction is
energetically favourable in a number of ways, including polarisation energy
and covalent energy, but particularly the electrostatic energy.

Some studies (eg Levitt and Perutz 1988) have suggested that the pi
electron shells of aromatic rings may act as weak hydrogen bond acceptors.
Because the pi electron shells are perpendicular to the plane of the
aromatic ring rather than coplanar with it, the angles at the acceptor are
formed by the perpendicular to the plane rather than the other covalent
bonds (not including those to hydrogens) with the acceptor.