Aaron W. Garrett

Resonant ion‐dip infrared spectroscopy of benzene–H2O and benzene–HOD

Resonant ion‐dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large‐amplitude motions present in these π hydrogen‐bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher‐energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two‐dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double‐minimum potentials with large‐amplitude excursions for HOD over nearly 180°.

Multiphoton ionization studies of clusters of immiscible liquids. II. C6H6–(H2O)n, n=3–8 and (C6H6)2–(H2O)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)n with n=3–8 and (C6H6)2–(H2O)1,2. Due to limitations imposed by the size of these clusters, a number of vibronic level arguments are used to constrain the gross features of the geometries of these clusters. Among the spectral clues provided by the data are the frequency shifts of the transitions, their van der Waals structure, the fragmentation of the photoionized clusters, and the complexation‐induced origin intensity and 610 splitting. In the 1:3 cluster, simple arguments are made based on the known structures of the 1:1 and 1:2 clusters which lead to the conclusion that all three water molecules reside on the same side of the benzene ring.Three structures for the 1:3 cluster are proposed which are consistent with the available data. Of these, only one is also consistent with the remarkable similarity of the 1:4 and 1:5 spectra to those of the 1:3 cluster. This struc...

Multiphoton ionization studies of C6H6–(CH3OH)n clusters. I. Comparisons with C6H6–(H2O)n clusters

Resonant two‐photon ionization (R2PI) scans of the S0–S1 spectra of C6H6–(CH3OH)n clusters with n=1–5 have been recorded. These scans provide an interesting comparison with earlier spectra from our laboratory on C6H6–(H2O)n clusters. A variety of vibronic level arguments are used to constrain the geometries of the C6H6–(CH3OH)n clusters. The 1:1 and 1:2 clusters possess vibronic level features which are very similar to their aqueous counterparts. The 1:1 cluster places the methanol molecule in a π hydrogen‐bonded configuration on or near the sixfold axis of benzene. The spectral characteristics of the 1:2 cluster are consistent with both methanol molecules residing on the same side of the benzene ring as a methanol dimer. Higher C6H6–(CH3OH)n clusters show distinct differences from the corresponding C6H6–(H2O)n clusters. Vibronic level arguments lead to the following conclusions: the methanol molecules in the 1:3 cluster show the strongest hydrogen bonding to the π cloud of any of the clusters and attach ...

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.

REMPI fragmentation as a probe of hydrogen bonding in aromatic-X clusters

Abstract The complexes of C 6 H 6 with a series of molecules of varying hydrogen-bonding abilities (HCl, CHCl 3 , H 2 O, and CH 3 Cl) are studied in one-color resonance-enhanced two-photon ionization. The degree of fragmentation of the ionic complex is seen to be extremely high in several of these complexes, even under very mild laser power conditions. Quantitative measurements show the degree of fragmentation to be well correlated with the hydrogen bonding ability of the complexing molecule. A π-hydrogen-bonded neutral complex will efficiently fragment upon photoionization by virtue of the repulsive geometry formed for the ionic complex in Franck-Condon excitation from the S 1 state.

Multiphoton ionization studies of C6H6-(CH3OH)n clusters. II: Intracluster ion-molecule reactions

The neutral C6H6–(CH3OH)n clusters, which have been spectroscopically characterized in Paper I, serve here as precursors for the study of intracluster ion chemistry initiated by resonant two‐photon ionization. Resonant enhancement allows ion chemistry product yields to be determined as a function of cluster size by selective excitation of a single size cluster with the laser. Most of the work presented here uses one‐color resonant ionization via the 610 transition of the C6H6 chromophore in the cluster. No ion chemistry is observed for the 1:n clusters with n≤2. At n=3, dissociative electron transfer (DET) to form C6H6+M+3 (M=CH3OH) is observed with a product yield of 6%. The remaining 94% of ionic products result from fragmentation of the 1:3 cluster by loss of a single CH3OH molecule. The unprotonated M+3 product ion is unusual in that electron bombardment or photoionization of pure methanol clusters yields exclusively protonated methanol cluster ions. The attachment of a C6H6 molecule to the methanol c...