Quantum Effects in the Threshold Photoionization and Energetics of the Benzene−H₂O and Benzene−D₂O Complexes:  Experiment and Simulation

Abstract

The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D 0 (C 6 H 6 -H 2 O) ) 106 ( 4 meV and D 0 (C 6 H 6 -D 2 O) ) 116 ( 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C 6 H 6 + -H 2 O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutral ground state wave function for the intermolecular vibrational motion. Using classical molecular dynamics simulations in order to sample this region, it has been shown that the neutral conformations involved in the production of stable ions at the ionization threshold exhibit a strong geometry change compared to the neutral equilibrium conformation; i.e., the water molecule is strongly shifted off the benzene C 6 axis and is also flipped over backward the benzene ring. The difference in the ionization energy of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes, which cannot be explained by the difference in the neutral binding energies alone, supports this result.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol–Ar cluster near the ionization threshold

“…The PIE spectrum of Bz-H 2 O shows only a very slow monotonic rise. 38 Such a slow rise corresponds well to the broad PIE signal observed in the View Article Online present study for PhOH-Ar, although sharp steps are superimposed from additional ionization to the p-bound local minimum. Bz + -H 2 O does not have such a local minimum at the Franck-Condon region.…”

“…A good example for such a cluster is the benzene-water dimer, Bz-H 2 O. [36][37][38][39][40] In the neutral state the water molecule binds above the aromatic p ring forming an OH-p hydrogen bond, while in the cationic state the water is located in the plane of the benzene ring and forms two bifurcated CH-O hydrogen bonds. As a consequence, vertical ionization of Bz-H 2 O brings the cluster to a very high energy part of the interaction potential, because the positively charged hydrogen atoms of the water molecule point toward the positive charge generated by the ionization of benzene.…”

Photochemical R2PI study of chirality and intermolecular forces in supersonic beam

“…B3LYP/6-31G** calculations point to just one stable conformer for the E R molecule in agreement with the experimental observations. Measurement of the experimental IP of this molecule is 8.89 eV in fair agreement with the adiabatic calculated value [10].…”

A Systematic CCSD(T) Study of Long-Range and Noncovalent Interactions between Benzene and a Series of First- and Second-Row Hydrides and Rare Gas Atoms

“…This is in good agreement with the most recent experimental value of -2.44 kcal/mol. 31 For the other complexes, ZPVEs can be estimated by taking the difference between experimental (D 0 ) and theoretical (D e ) binding energies, yielding the ZPVE ranges presented in Table 8. However, for genuine de novo prediction of vibrationally averaged binding energies, it is necessary to calculate ab initio zero-point vibrational energies for each complex.…”