Bernd Reimann

ANTI-HYDROGEN BOND BETWEEN CHLOROFORM AND FLUOROBENZENE

Abstract Accurate theoretical calculations (ab initio MP2/6-31G * counterpoise-corrected gradient optimization, harmonic and anharmonic vibrational analysis) on the fluorobenzene⋯chloroform complex predict a new type of bonding, termed the anti-hydrogen bond. This bond distinguishes itself by the contraction of the C–H bond of chloroform and a blue shift of the corresponding stretching frequency, i.e. features opposite to those characteristic for a hydrogen bond. The predicted blue shift was confirmed experimentally by double-resonance infrared ion-depletion spectroscopy. The calculated blue shift of the chloroform C–H stretching frequency (12 cm −1 ) agrees with the experimental value of 14 cm −1 . The anti-hydrogen bond originates from the dispersive interaction between molecules (contrary to the hydrogen bond which is of electrostatic origin). It plays a significant role in benzene-containing molecular clusters and is expected to be of consequence for the structure of biomolecules.

Hydrogen bonding in (substituted benzene)·(water)n clusters with n≤4

Abstract Infrared ion-depletion spectroscopy, a double resonance method combining vibrational predissociation with resonant two-photon ionization (R2PI) spectroscopy, has been applied to study mixed clusters of the type (substituted benzene)·(H 2 O) n with n ≤4. The UV chromophores were p -difluorobenzene, fluorobenzene, benzene, toluene, p -xylene and anisole. From the IR depletion spectra in the region of the OH stretching vibrations it could be shown that the water molecules are attached as subclusters to the chromophores. Size and configuration of the subclusters could be deduced from the IR depletion spectra. In the anisole·(H 2 O) 1 and 2 complexes the water clusters form an ordinary hydrogen bond to the oxygen atom of the methoxy group. In all other mixed complexes a π-hydrogen bond is formed between one of the free OH groups of a water subcluster and the π-system of the chromophore. According to the strength of this interaction the frequency of the respective absorption band exhibits a characteristic red-shift which could be related to the total atomic charges in the aromatic ring.