Water–Hydrocarbon Interactions in Anionic Pyrene Monohydrate

Abstract

Interactions between water and polycyclic aromatic hydrocarbons are essential in many aspects of chemistry, from interstellar and atmospheric processes to interfacial hydrophobicity and wetting phenomena. Despite their growing importance, the intermolecular potentials of the water−hydrocarbon interactions are underdeveloped compared to the water−water potentials, and there are similarly few experimental probes that are sensitive to the details of the water−hydrocarbon potential. We present a combined experimental and computational study of anionic pyrene monohydrate, one of the simplest water/hydrocarbon clusters. The action spectrum in the OH region of the mass-selected cluster ion provides a rigorous benchmark for intermolecular potentials and computational methodologies. We identify missing intermolecular interactions and shortcomings in conventional dynamics calculations by comparing experimental data to density functional theory and classical molecular dynamics calculations. Kinetic trapping is prevalent, even for one water molecule and one pyrene molecule, leading to slow equilibration in conventional molecular dynamics calculations, even on nanosecond time scales and at low temperatures (50 K). At constant energy, temperature fluctuations for the pair of molecules are substantial. Immersing the system in a bath of soft spheres and employing parallel tempering alleviates kinetic trapping and dampens temperature fluctuations, bringing the system closer to the thermodynamic limit. With such augmented sampling, a simple, flexible water model reproduces the line width and the asymmetric broadening of the symmetric OH stretching mode, which we assign to spectral diffusion. In the OH stretching region, dynamics calculations predict a more intense antisymmetric peak than experiments observe but do not predict the bimodal split symmetric peak that the experiments show. Our work suggests that electronic polarization, missing in the empirical force field, is responsible for the first discrepancy and that quantum nuclear effects, captured neither in density functional theory nor in classical dynamics, may be responsible for the second.

Competition between Water–Water Hydrogen Bonds and Water–π Bonds in Pyrene–Water Cluster Anions

“…3710 cm –1 ), , corroborating this characterization. The calculated spectra of both minima are compatible with the experimental spectrum, but it is unclear which of these structures exist or are populated. The experimental spectrum merely allows for the qualitative identification of the water–Py interactions, not the position of the water molecule on the Py – frame.…”

“…Infrared photodissociation action spectra of Py – ·(H 2 O) n ·Ar 2 with n indicated in each panel. Data for n = 1 have been taken from ref .…”

“…The spectrum of the Py monohydrate anion has been described in detail in recent work, and we will only provide a brief discussion here. The OH stretching region of the photodissociation spectrum of Py – ·H 2 O·Ar 2 is reproduced in Figure .…”

“…Infrared photodissociation spectrum of Py – ·H 2 O·Ar 2 (data taken from ref ). The labels ω s and ω a are attached to the features of the symmetric and antisymmetric OH stretching vibrations of the water molecule, respectively.…”

“…Cluster ions of the form Py – ·(H 2 O) n ·Ar m were prepared in a supersonic entrainment source described previously. ,, We entrained Py vapor from a copper oven ( T = 400 ± 10 K) and water vapor (at room temperature) from a pulsed valve (General Valve Series 9) into a pulsed supersonic expansion of argon (Even–Lavie valve, stagnation pressure of 1.5 bar). An electron beam (800 eV kinetic energy) injected into the high-density region of the expansion created a plasma, where the attachment of low-energy secondary electrons to molecules and clusters created negative ions.…”