Structural measurements of hydrogenhyphen;bonded van der Waals dimers and trimers by rotational coherence spectroscopy: 2,5hyphen;diphenyloxadiazole with argon, methane, water, and alcohols

摘要

Picosecond timehyphen;resolved polarized fluorescence experiments involving timehyphen;correlated singlehyphen;photon counting have studied rotational coherence phenomena of hydrogenhyphen;bonded and other molecular aggregates in their lowest excited singlet states. The experiments are supported by detailed simulations. Using the molecule 2,5hyphen;diphenylhyphen;1,3,4hyphen;oxadiazole (PPD) as a host species, experiments have compared van der Waals aggregates with Ar1,2, (CH4)1,2, (H2O)1,2, (CH3OH)1,2, (C2H5OH)1, and (1hyphen;C3H7OH)1. Bare PPD, and the argon and methane aggregates all exhibit prominentJhyphen;type recurrences. The resulting sums of rotational constants (B+C) are consistent with centerhyphen;ofhyphen;masshyphen;bound, threehyphen;dimensional structures, having outhyphen;ofhyphen;plane distances for the attached species of 3.3ndash;3.5 Aring;. 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 hydrogenhyphen;bonding species all 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. Hydrogenhyphen;bonded species differ from the nonpolar cases, since they show both prominentChyphen;type andJhyphen;type transients. This shows that these species all differ significantly from prolate symmetric tops. Detailed simulations reveal that all of the hydrogenhyphen;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 outhyphen;ofhyphen;plane twist, consistent with the increasing influence of dispersive interactions.Hydrogen bond distances (N...Hndash;O) are found to be in the range 2.7ndash;2.9 Aring;, and the hydrogenhyphen;bond angles (Nndash;N...Hndash;O), relative to the PPD long axis, range from 115deg; to 130deg;. In addition, the water and methanol 1:2 aggregates both contain hydrogenhyphen;bonded dimer units that resemble the free dimers of each species as identified by infrared and microwave techniques. For example, we find the (O...Hndash;O) distance in the methanol dimer complex to be ape;2.7 Aring;.