Anti-hydrogen bond between chloroform and fluorobenzene

Accurate theoretical calculations (ab initio MP2/6-31G* counterpoise-correc ted gradient optimization, harmonic and anharmonic vibrational analysis) on the fluorobenzene ... chloroform complex predict a new type of bonding, te rmed the anti-hydrogen bond. This bond distinguishes itself by the contract ion of the C-H bond of chloroform and a blue shift of the corresponding str etching frequency, i.e. features opposite to those characteristic for a hyd rogen bond. The predicted blue shift was confirmed experimentally by double -resonance infrared ion-depletion spectroscopy. The calculated blue shift o f the chloroform C-H stretching frequency (12 cm(-1)) agrees with the exper imental value of 14 cm(-1). The anti-hydrogen bond originates from the disp ersive interaction between molecules (contrary to the hydrogen bond which i s of electrostatic origin). It plays a significant role in benzene-containi ng molecular clusters and is expected to be of consequence for the structur e of biomolecules. (C) 1999 Elsevier Science B.V. All rights reserved.