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.

We have recorded the S1 <-- S0 electronic spectra of Biphenylene and its Ar and O2 van der Waals complexes inside helium nanodroplets using beam depletion detection. In general, the spectrum is similar to the previously reported high-resolution REMPI spectrum. The zero phonon lines, however, are split similar to the previously reported tetracene case. The calculated potential energy surface predicts that helium atoms can simultaneously occupy all equivalent global minima positions. Therefore, it appears that the splitting cannot be explained either by different isomers or by tunneling. Furthermore, surprisingly the splitting is retained for the Ar van der Waals complexes (and possibly for the O2 complex as well). This case suggests that the current models of the origin of zero phonon line splitting and the helium solvation are incomplete.

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We have recorded the electronic spectra of three polycyclic aromatic hydrocarbons (acenaphtylene, fluoranthene, and benzo(k)fluoranthene) containing a five-member ring and their van der Waals complexes with argon and oxygen with a molecular beam superfluid helium nanodroplet spectrometer. Although the molecules, which differ by addition of one or two fused benzene rings to acenaphtylene, have the same point group symmetry, the spectral lineshapes show distinct differences in the number of zero phonon lines and shapes of the phonon wings. Whereas the smallest molecule (acenaphtylene) has the most complicated line shape, the largest molecule (benzo(k)fluoranthene) shows different lineshapes for different vibronic transitions. The van der Waals complexes of fluoranthene exhibit more peaks than the theoretically allowed number of isomeric complexes with argon/oxygen. The current models of molecular solvation in liquid helium do not adequately explain these discrepancies.