Intermolecular bonding and vibrations of the carbazole⋅B complexes (B=H2O, D2O, NH3)

Abstract

Carbazole⋅B complexes (B=H2O, D2O, NH3) were synthesized and cooled in pulsed supersonic nozzle beams. The intermolecular hydrogen‐bond vibrations and dissociation energies were studied by several laser‐spectroscopic techniques (fluorescence excitation and emission, resonance‐two‐photonionization with mass‐specific detection). The following results were obtained for both S 0 and S 1 electronic states: (1) determination of the structural symmetry of the complexes, (2) measurement of the intermolecular stretching (νσ) and bending (νβ) frequencies, (3) determination of stretching force constants, (4) hydrogen‐bond dissociation energiesD 0 for carbazole⋅H2O/D2O, and (5) electronic spectral shifts δν̃ relative to the bare carbazole molecule. The latter are large (500–710 cm− 1) and reflect an increase of the hydrogen‐bond energy by ≈40% upon electronic excitation. Fermi resonance couplings between the intermolecular νσ and an intramolecular b 1 vibration of carbazole are observed and partially analyzed. To complement the experimental work, extensive a b i n i t i o quantum chemical calculations of the same complexes were performed at the Hartree–Fock level. The calculated complex structures are consistent with the experimental information. Intermolecular potential‐energy curves for the stretching vibrational coordinate were calculated for a number of increasingly flexible basis sets (STO‐3G, 4‐31G, 6‐31G, 4‐31G*); anharmonic vibrational frequencies were then obtained numerically. Excellent agreement with the experimental intermolecular stretching frequencies was found for all three complexes using the 4‐31G* basis set. Good agreement with experiment was also found for the calculated 4‐31G* hydrogen‐bond dissociation energy.