The microwave spectrum of the weakly bound complex benzene–HCl was studied in the gas phase using Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity with a pulsed supersonic nozzle as the molecular source. Several R-branch a-dipole transitions have been observed for benzene–H 35Cl, benzene–D 35Cl, benzene–H 37Cl, and benzene(d6)–H 35Cl. The spectrum was characteristic of a symmetric top, indicating that the time averaged displacement of the H and Cl atoms from the benzene C6 axis is zero. Deuterium substitution of HCl demonstrated that the acidic proton lies between the Cl atom and the benzene ring. The chlorine nuclear quadrupole coupling constant χClaa, was measured for all four isotopic species and is interpreted in terms of a projection of the chlorine quadrupole coupling constant in free HCl, averaged over two degenerate vibrational ground state bending modes involving the angles between the benzene C6 axis and the HCl bond axis. The spectroscopic constants for benzene–HCl are: aNumbers in parentheses represent one standard deviation in the fit.

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Explains the effect of optical parameters and concentration on fluorescence and provides an experimental procedure that illustrates this relationship.

Structure, properties, and inversion of the ArSO2 van der Waals moleculeThe infrared spectrum of a gaseous mixture of oxygen and argon at 93 oK reveals descrete features which are attributed to absorption by the 02-Ar van der Waals molecule. The spectrum occurs near the infrared inactive vibration of O 2 , One band, VI' corresponds to the stretching frequency of O 2 within 02-Ar. An analysis of the P and R envelope of this band provides an approximate intermolecular distance for 02-Ar of R e = 3.5 A. Other absorption features extend as much as 50 cm-I to the high and low frequency of the VI fundamental. These features are assigned as libration or hindered rotation of O 2 within the van der Waals molecule in combination with the VI fundamental. A model is developed to account for these rotatory energy levels in which the anisotropic intermolecular interaction in 02-Ar is approximated by a simple sinosoidal potential. The internal rotatory energy levels are then obtained from a quantum mechanical calculation. Using this model the infrared data are shown to be consistent with an angular interaction which favors a "T" orientation with a barrier to internal rotation of 30 cm-I (85 cal/mole). Thus a picture of 02-Ar emerges in which molecules with rotatory energy less than the barrier height undergo librational motion and will appear semirigid or "locked" into a "T" configuration. van der Waals molecules with rotatory energies greater than the barrier height will undergo hindered internal rotation. This study, in addition to other recent work, indicates the power of spectroscopy to provide a mapping of intermolecular interactions of small molecules.

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