STRUCTURAL MEASUREMENTS OF HYDROGEN-BONDED VAN-DER-WAALS DIMERS AND TRIMERS BY ROTATIONAL COHERENCE SPECTROSCOPY - 2,5-DIPHENYLOXADIAZOLE WITH ARGON, METHANE, WATER, AND ALCOHOLS
T. Troxler et al., STRUCTURAL MEASUREMENTS OF HYDROGEN-BONDED VAN-DER-WAALS DIMERS AND TRIMERS BY ROTATIONAL COHERENCE SPECTROSCOPY - 2,5-DIPHENYLOXADIAZOLE WITH ARGON, METHANE, WATER, AND ALCOHOLS, The Journal of chemical physics, 100(2), 1994, pp. 797-811
Picosecond time-resolved polarized fluorescence experiments involving
time-correlated single-photon counting have studied rotational coheren
ce phenomena of hydrogen-bonded and other molecular aggregates in thei
r lowest excited singlet states. The experiments are supported by deta
iled simulations. Using the molecule 2,5-diphenyl-1,3,4-oxadiazole (PP
D) as a host species, experiments have compared van der Waals aggregat
es 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-dimen
sional structures, having out-of-plane distances for the attached spec
ies of 3.3-3.5 Angstrom. The 1:2 aggregates involving argon and methan
e exhibit additive spectral shifts and nearly additive rotational recu
rrence times. This shows that the sites for addition of consecutive sp
ecies are equivalent. Calculations of rotational constants confirm the
se findings. All except the Ar 1:2 cluster exist close to the prolate
symmetric top limit. On the other hand, the excitation spectra of comp
lexes involving hydrogen-bonding species ah-show small complexation sh
ifts at the 1:1 level and disproportionately larger shifts at the 1:2
level. Similar nonadditive behavior is seen for the rotational recurre
nce transients. Hydrogen-bonded species differ from the nonpolar cases
, since they show both prominent C-type and J-type transients. This sh
ows that these species all differ significantly from prolate symmetric
tops. Detailed simulations reveal that all of the hydrogen-bonding sp
ecies produce aggregates that involve a single hydrogen bond to one of
the PPD nitrogen atoms. This, imposes a planar type of structure on t
he 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 ou
t-of-plane twist, consistent with the increasing influence of dispersi
ve 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 addit
ion, the water and methanol 1:2 aggregates-both contain hydrogen-bonde
d dimer units that resemble the free dimers of each species as identif
ied 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.