Blue-shifting hydrogen bonds

In this paper we put forward the idea that the various improper, blue-shifting hydrogen bond systems discussed in the literature are all of essentially the same nature and occur because of three necessary circumstances: (i) the presence of a negative dipole moment derivative, dmu(0)/dr(XH), for the isolated H-bond donor molecule; (ii) the interaction between such a molecule and any electron density concentration on the H-bond acceptor ( pi-system density, lone-pair density, ionic charge,...) which at large intermolecular distances gives rise to a field-dominated, modest vibrational blue shift; (iii) an additional blue shift due to electronic exchange overlap. The negative dipole moment derivative is a necessary but not sufficient condition for the formation of a blue-shifted H-bond: thus, the blue-shifting CH4, F3CH, and Cl3CH molecules and their relatives can also give rise to normal, red-shifted H-bonds. This is a logical extension of the blue-shifting property and occurs when the electric field from the acceptor is sufficiently strong at the intermolecular equilibrium distance (e.g., for F- and Cl- acceptors).