A velocity map ion imaging study of difluorobenzene-water complexes: Binding energies and recoil distributions

Abstract

The binding energies of the p-, m-, and o-difluorobenzene-H2O complexes have been measured by velocity map ion imaging to be 922±10, 945±10, and 891±4cm−1, respectively. The lack of variation provides circumstantial evidence for water binding to the three isomers via the same interaction, viz. an in-plane O–H⋯F hydrogen bond to one of the fluorine atoms on the ring, with a second, weaker interaction of the water O atom with an ortho hydrogen, as determined previously for the p-difluorobenzene-H2O complex [Kang et al., J. Phys. Chem. A 109, 767 (2005)]. The ground state binding energies for the difluorobenzene-H2O complexes are ∼5%–11% larger than that for benzene-H2O, where binding occurs to the π electrons out-of-plane. However, in the S1 state the binding energies of the o- and p-difluorobenzene-H2O complexes are smaller than the benzene-H2O value, raising an interesting question about whether the geometry at the global energy minimum remains in-plane in the excited electronic states of these two complexes. Recoil energy distributions for dissociation of p-difluorobenzene-H2O have been measured from the 31¯, 52¯, and 3151¯ levels of the excited electronic state. These levels are 490, 880, and 1304cm−1, respectively, above the dissociation threshold. Within the experimental uncertainty, the recoil energy distributions are the same for dissociation from these three states, with average recoil energies of ∼100cm−1. These recoil energies are 60% larger than was observed for the dissociation of p-difluorobenzene-Ar, which is a substantially smaller increase than the 400% seen in a comparable study of dissociation within the triplet state for pyrazine-Ar, -H2O complexes. The majority of the available energy is partitioned into vibration and rotation of the fragments.

Binding energy determination in a π-stacked aromatic cluster: the anisole dimer

“…The first experimental determinations of the binding energy of molecular clusters were related to the benzene dimer 14,15 or clusters formed by aromatic molecules and rare gas atoms. [16][17][18][19][20] Only a few of these studies were probing the ground electronic state properties while most of them were related to potential energy surfaces of the lowest electronically excited states. 21,22 While studying photodissociation a crucial issue is the determination of the recoil energy: time of flight methods were the preferential approach for its measurement.…”

“…Only in the last years velocity map imaging methods (VMI) have been applied to study the photodissociation dynamics in clusters formed by aromatic molecules with rare gas atoms or water. [18][19][20] Most recently also more complicated systems have been studied with a similar approach. 23,24 Also, it is worth to remember here the studies on the dissociative photodetachment of anionic clusters that allow probing dissociation dynamics on the ground neutral potential energy surface of the system even though not starting from the ground equilibrium conformation.…”

Experimental and Theoretical Determination of Dissociation Energies of Dispersion-Dominated Aromatic Molecular Complexes

“…Since 2000, the groups of Lawrance ,,, and Becucci have applied the velocity map imaging (VMI) technique to determine the D 0 values of aromatic M·S complexes and self-dimers M 2 . VMI is an enhanced resolution variant of ion imaging, for a detailed description, see ref.…”

“…From the spectral shift of the S 0 → S 1 0 0 0 band (for a) or the AIE (for b) the ground-state dissociation energy D 0 ( S 0 ) is obtained via thermochemical cycles. The Lawrance group has studied para-difluorobenzene ( p -DFB) with solvents S = Ar, Kr and H 2 O. ,,, Becucci and co-workers have applied VMI to the anisole dimer . (Anisole) 2 is a Type-III complex which is expected to undergo large geometry changes after S 0 → S 1 excitation; indeed the D 0 measured by VMI has recently been questioned and revised, based on calculations .…”

“…Since 2000, the groups of Lawrance 16,17,34,35 Figure 8. In Figure 8( [38][39][40][41] Similar to the ionization-based appearance potential, ion breakdown, MATI and VMI methods described in Sections 3.2 -3.4, the method is based on the detection of the M + ·S ion and is mass-(and isotope-) specific.…”