Hydrogen Bonding

Abstract

Infra-red spectroscopy has found extensive use for studies of hydrogen bonds. A proton attached to a more electronegative atom X (e.g. Cl, O, N, P, Br, F, S) forms a partial bond with a neighbouring electronegative atom Y (e.g. CI, O, N, P, S, Br, F) or pi bond (e.g. C=0, C=C, aromatic ring). Atom Y or the pi bond provides two electrons in an asymmetric orbital for the hydrogen bonding to develop. The strongest hydrogen bonds are formed where the atomic centres X, H and Y are collinear. In the infra-red spectrum hydrogen-bonded protons are characterized by shifts to lower frequencies (higher wavelengths) of the X—H stretching vibration mode, coupled with a marked increase in intensity of this absorption. Hydrogen bonding (association) involving —O—H groups causes the largest shifts, with lesser ones observed for —NH— groups. Only weak hydrogen bonds develop with S—H and P—H groups. Infra-red spectroscopy offers a simple method for distinguishing between intramolecular hydrogen bonding, intermolecular hydrogen bonding, and chelation (very strong intramolecular hydrogen bonding) such as that occurring in ß-diketones. Individual factors, namely temperature, concentration and the sample diluent, can substantially alter the bonded O—H or N—H vibration absorption frequency positions. Therefore, standardization of operating conditions is a critical factor in comparative studies.