Albert J. Gotch

Multiphoton ionization studies of clusters of immiscible liquids. I. C6H6–(H2O)n, n=1,2

Resonant two‐photon ionization (R2PI) time‐of‐flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S0–S1 origin transition and the presence of a splitting at 610 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 610 transitions of the isotopomers confirms this picture and also determines a ground state center‐of‐mass separation between C6H6 and D2O of 3.32±0.07 A, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best‐fit structure in which the two water molecules reside on ...

The spectroscopy and dynamics of π hydrogen‐bonded complexes: Benzene–HCl/DCl and toluene–HCl/DCl

The benzene–HCl/DCl and toluene–HCl/DCl complexes have been studied using both fluorescence and multiphoton ionization detection. These complexes are prototypical of π hydrogen‐bonded complexes involved in the chemically important process of electrophilic aromatic attack. Laser‐induced fluorescence (LIF) etalon scans of the 610 rotational band contour are used to determine the S1 state geometry of the benzene–HCl as one in which HCl is on the sixfold axis with a center‐of‐mass separation of 3.64±0.03 A. The lack of significant van der Waals’ intensity points to the complex having a hydrogen‐bonded geometry similar to that found in the ground state. Dispersed fluorescence scans are used to put crude bounds on the S0 and S1 binding energies of the benzene–HCl complex of 1.8≤D‘0 ≤ 3.8 kcal/mol and 1.5≤D’0≤3.5 kcal/mol. The fluorescence lifetimes of bound levels of the complexes are factors of 7–12 times shorter than the corresponding levels of the free molecules. In contrast, the C6H6–CH3Cl complex, which ha...

The structure and photophysics of clusters of immiscible liquids: C6H6(H2O)n

Abstract Mixed clusters of C 6 H 6 and H 2 O type of C 6 H 6 (H 2 O) n where n = 1–5 have been studied using one-color resonance-enhanced two-photon ionization coupled with time-of-flight mass spectrometric detection. The 1:1 complex is seen to have an electronically forbidden origin with intensity at least 1000 times less than that at 6 1 0 . The H 2 O molecule thus lies on the sixfold axis of benzene and is undergoing free radical rotation about this axis. The blue-shift and high-efficiency of fragmentation of the 1:1 complex argue for hydrogen bonding of the H 2 O molecule to the benzene π cloud. The 1:2 complex induces a strong origin with intensity 14% of that at 6 1 0 . The rotational band contour yields a geometry in which the two water molecules bind to benzene on one side of the benzene ring, with a H 2 OH 2 O separation close to that in the water dimer. Higher 1: n clusters show spectra which suggest structures involving a network of hydrogen bonded water molecules building up away from the benzene ring, consistent with a microscopic immiscibility of C 6 H 6 and H 2 O.

Dipole-dipole transfer between acetone solvates of chlorophyll a and chlorophyll a dihydrate dimers in water/acetone mixtures. A model for P680 sensitized excitation

Abstract We describe a simple model for P680 sensitized excitation in photosynthesis. Chl a fluorescence quenching effects observed when water is added to Chl a solutions in acetone are shown to be the result of resonant transfer between acetone solvates of monomeric Chl a , Chl a ·Ac, and dimers of Chl a dihydrate. The presence of (Chl a ·2H 2 O) 2 is evidenced by a 678 nm difference absorbance (Δ A band obtained on conversion of a 680 nm absorption shoulder to polycrystalline Chl a precipitate, (Chl a ·H 2 O) n . The equilibration between (Chl a ·2H 2 O) 2 and Chl a ·Ac as a principal mechanism for Chl a ·Ac fluorescence quenching is supported by theoretical fits of the data.

The Spectroscopy and Photophysics of π Hydrogen-BondedComplexes: Benzene–CHCl3

A vibronic level study of the spectroscopy and photophysics of the C6H6–CHCl3 complex has been carried out using a combination of laser-induced fluorescence and resonant two-photon ionization (R2PI). In C6H6-CHCl3, the S1–S0 origin remains forbidden while the 1610 transition is weakly induced. Neither 610 nor 1610 are split by the presence of the CHCl3 molecule. On this basis, a C3vstructure is deduced for the complex, placing CHCl3 on the six-fold axis of benzene. The large blue-shift of the complex’s absorption relative to benzene (