BOND CONTRACTION AND SPECTRAL BLUE-SHIFT IN HYDROGEN-BONDED DIMERS - AN ATOM-BASED MOLECULAR MECHANICS ANALYSIS

HX bonds are often elongated upon forming a hydrogen-bonded complex. Concomitantly, the HX stretching frequency undergoes a red-shift. Ab initio calculations for several dimers of HF suggest that an opposite behavior may sometimes arise for the proton acceptor, namely, the bond that is adjacent to the hydrogen bond may contract, with a concomitant blue-shift. Ab initio calculations of the forces in the various systems and an atom-based molecular mechanics analysis of these forces indicate that part of the effect stems from charge flux.