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Structure and vibrations of phenol(H 2 O) 7,8 studied by infrared-ultraviolet and ultraviolet-ultraviolet doubleresonance spectroscopy and ab initio theory J. Chem. Phys. 110, 9898 (1999); 10.1063/1.478863Infrared-depletion spectroscopy study on hydrogen-bonded fluorobenzene-methanol clusters Laser spectroscopy, i.e., resonant two-photon ionization ͑R2PI͒, IR/R2PI ion depletion and hole burning spectroscopy have been applied in an experimental study of heterogenous clusters consisting of fluorobenzene ͑FB͒ or p-difluorobenzene ͑pDFB͒, respectively, microsolvated by up to three methanol molecules. Their infrared ion depletion spectra were taken in the region of the OH and CH stretches of methanol. In these complexes the methanol molecules form subclusters, which are weakly hydrogen bonded to either the aromatic -system ( OH ) or to the fluorine substituent ( F ) and a CH group ( CH ). In FB•͑MeOH͒ 1 , pDFB•͑MeOH͒ 1 , pDFB•͑MeOH͒ 2 and one isomer of FB•͑MeOH͒ 2 the methanol subunits exclusively exhibit F and CH H-bonds. A further isomer of FB•͑MeOH͒ 2 exhibits a OH type interaction. For FB•͑MeOH͒ 3 and pDFB•͑MeOH͒ 3 ͑1:3͒ complexes the methanol subcluster may take on either a chainlike or a ringlike conformation. In the chainlike isomer of FB•͑MeOH͒ 3 the methanol trimer interacts with the chromophore via a OH H-bond while with the corresponding pDFB cluster a F H-bond is formed. High level ab initio calculations, still feasible for molecular clusters of this size, have also been carried out to supplement the experimental investigations. These calculations include complete geometry optimizations of FB•͑MeOH͒ n ͑nϭ1-2͒ and pDFB•͑MeOH͒ 1 at the MP2/6-31ϩG* and B3LYP/ 6-31ϩG* levels of theory. The minimum energy structures of the microsolvates together with their harmonic vibrational spectra allow a reliable assignment of the experimentally observed spectra and the determination of structure of the complexes.

The infrared depletion spectrum of the aniline dimer, formed in a supersonic jet, has been recorded in the N−H stretch region by combining infrared laser excitation and resonant two-photon ionization/time-of-flight mass spectrometry. Only two bands have been found at 3394.0 and 3465.9 cm-1 (±0.5 cm-1) in the region 3130−3530 cm-1. These are red-shifted by 27.8 and 42.3 cm-1 from the symmetric and asymmetric NH stretching vibrations of the aniline monomer, respectively. A configuration with mutual NH2···π bonds and phenyl groups stacked in parallel is suggested for the ground-state aniline dimer.

Infrared-depletion spectroscopy, a double resonance method combining infrared- with resonant two-photon ionization (R2PI)-spectroscopy has been applied to mixed molecular aggregates of fluorobenzene⋅(methanol)n (Fb⋅MeOH) with n⩽4. From the IR spectra in the region of the OH stretching vibration of methanol it can be shown that the solvent moiety forms subclusters on one side of the aromatic ring. For Fb⋅(MeOH)3 the methanol trimer exhibits a linear as well as a cyclic structure. The different shifts of the UV bands of these two isomeric clusters reveal the diverse character of the hydrogen bonds involved in their formation. Furthermore it can be shown that for both isomeric forms the product anisole+ is formed in equal quantities by an intracluster SN2 reaction following the photoionization of the chromophore. Whereas for Fb⋅(MeOH)2 this reaction is in competition with evaporative fragmentation it is the dominant reaction channel for the Fb⋅(MeOH)3 cluster.

This combined experimental and computational study is aimed at elucidating the structure and reactivity of heterogeneous molecular clusters. We report results for the system para-chlorofluorobenzene/methanol (pClFB/MeOH). Particularly, three different van der Waals (vdW) isomers of the neutral (1:1) aggregate (π,σF,σCl) have been assigned by comparison of experimental infrared frequencies in the O–H and C–H stretch regions measured by IR/R2PI depletion spectroscopy with calculated frequencies at MP2/6-31+G(d) and B3LYP/6-31+G(d) level. The isomers are weakly hydrogen-bonded complexes with methanol’s OH group coordinated toward the aromatic π-electron cloud, the fluorine or the chlorine substituent, respectively. This assignment is corroborated by the UV and IR spectra of the corresponding monosubstituted benzene/methanol complexes. After resonant photoionization of pClFB⋅(MeOH)n, for n=1 besides vdW fragmentation, no chemical reactivity was observed for any isomer. The investigation of aggregates with n⩾5 indicated a substitution reaction taking place to produce chloroanisole+, emphasizing the importance of neutral precursor cluster size over cluster structure for intracluster ion–molecule reactions.