STRUCTURAL MEASUREMENTS OF HYDROGEN-BONDED VAN-DER-WAALS DIMERS AND TRIMERS BY ROTATIONAL COHERENCE SPECTROSCOPY - 2,5-DIPHENYLOXADIAZOLE WITH ARGON, METHANE, WATER, AND ALCOHOLS

Picosecond time-resolved polarized fluorescence experiments involving time-correlated single-photon counting have studied rotational coherence phenomena of hydrogen-bonded and other molecular aggregates in their lowest excited singlet states. The experiments are supported by detailed simulations. Using the molecule 2,5-diphenyl-1,3,4-oxadiazole (PPD) as a host species, experiments have compared van der Waals aggregates with Ar-1,Ar-2, (CH4)(1,2), (H2O)(1,2), (CH3OH)(1,2), (C2H5OH)(1), and (1-C3H7OH)(1). Bare PPD, and the argon and methane aggregates all exhibit prominent J-type recurrences. The resulting sums of rotational constants (B+C) are consistent with center-of-mass-bound, three-dimensional structures, having out-of-plane distances for the attached species of 3.3-3.5 Angstrom. The 1:2 aggregates involving argon and methane exhibit additive spectral shifts and nearly additive rotational recurrence times. This shows that the sites for addition of consecutive species are equivalent. Calculations of rotational constants confirm these findings. All except the Ar 1:2 cluster exist close to the prolate symmetric top limit. On the other hand, the excitation spectra of complexes involving hydrogen-bonding species ah-show small complexation shifts at the 1:1 level and disproportionately larger shifts at the 1:2 level. Similar nonadditive behavior is seen for the rotational recurrence transients. Hydrogen-bonded species differ from the nonpolar cases, since they show both prominent C-type and J-type transients. This shows that these species all differ significantly from prolate symmetric tops. Detailed simulations reveal that all of the hydrogen-bonding species produce aggregates that involve a single hydrogen bond to one of the PPD nitrogen atoms. This, imposes a planar type of structure on the 1:1 water and methanol complexes. On the other hand, the aggregates methanol 1:2, ethanol 1:1, and propanol 1:1 all involve a distinct out-of-plane twist, consistent with the increasing influence of dispersive interactions. Hydrogen bond distances (N...H-O) are found to be in the range 2.7-2.9 A, and the hydrogen-bond angles(N-N...H-O), relative to the PPD long axis, range from 115 degrees to 130 degrees, In addition, the water and methanol 1:2 aggregates-both contain hydrogen-bonded dimer units that resemble the free dimers of each species as identified by infrared and microwave techniques. For example, we find the (O...H-O) distance in the methanol dimer complex to be approximate to 2.7 Angstrom.