A shortening of the C-H bond lengths and a blue shift of the C-H stretching frequencies for the C-F small middle dot small middle dot small middle dotH-C groups indicates that anti-hydrogen bonds are present the difluoromethane dimer. The most stable conformer has three such interactions (shown schematically).

The free jet millimeter wave spectra of the 1:1 complex between pyridazine and three isotopomers of water (H2O, D2O, and H2 18O) have been assigned. The derived moments of inertia are consistent with a planar structure of the adduct in which one hydrogen of the water molecule is bound to the nitrogen of the aromatic ring, and the “free” water hydrogen is entgegen to the ring. Information on the motions of water with respect to the ring is obtained.

The free jet millimeter wave spectra of five isotopomers of the 1:1 complex between ethylene oxide and water have been assigned. The water molecule lies in the plane of symmetry of ethylene oxide perpendicular to the ring; the water hydrogen involved in the hydrogen bond points toward the ring oxygen, while the “free” hydrogen is entgegen to the ring. The H bond parameters are r(Oring···H) = 1.92 Å, ∠(Oring···H−O) ≅ 163°, and ∠(ring−Oring···H) ≅ 103°, respectively. The large differences of the two angles with respect to those of 1,4-dioxane−water and tetrahydropyran−water are mainly due to dipole−dipole interaction energy effects. The MP2 method using a 6-311++G(d,p) is shown to be a reliable approach for this kind of study. An experimental measure of the difference in zero-point energy between the O···D−O−H and the O···H−O−D species is given by the intensity ratio 3/1 of corresponding lines when a sample of water 50% enriched in D is used.

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We have recorded the electronic spectra of benzo[g,h,i]perylene and coronene and their van der Waals complexes with argon and oxygen with a helium-nanodroplet depletion spectrometer. These molecules differ by the addition of one and two fused benzene rings to perylene, which was previously studied in helium. The coronene spectrum is similar to a previously reported jet-cooled laser-induced fluorescence (LIF) spectrum. The van der Waals complexes with argon and oxygen show different complexation sites and maximum number of adsorbants. We report a vibronically resolved benzo[g,h,i]perylene S(1) <-- S(0) spectrum. The spectral lines are split in a similar way to that of several molecules studied before. However, surprisingly, while the van der Waals complexes with argon are free of the splitting, the complexes with oxygen retain the splitting, with increased linewidth and splitting. We could also observe the S(2) <-- S(0) origin transition of benzo[g,h,i]perylene which was previously observed by cavity ring down spectroscopy. While in general the two spectra are quite similar, the relative intensities and spectral shifts of several lines are different.