Unraveling the internal dynamics of the benzene dimer: a combined theoretical and microwave spectroscopy study

Melanie Schnell,Undine Erlekam,Gerard Meijer,Ad van der Avoird,Gert von Helden,Jens-Uwe Grabow,P. R. Bunker

doi:10.1039/c3cp51181b

Abstract

We report a combined theoretical and microwave spectroscopy study of the internal dynamics of the benzene dimer, a benchmark system for dispersion forces. Although the extensive ab initio calculations and experimental work on the equilibrium geometry of this dimer have converged to a tilted T-shaped structure, the rich internal dynamics due to low barriers for internal rotation have remained largely unexplored. We present new microwave spectroscopy data for both the normal (C6H6)2 and partially deuterated (C6D6)(C6H6) dimers. The splitting patterns obtained for both species are unraveled and understood using a reduced-dimensionality theoretical approach. The hindered sixfold rotation of the stem can explain the observed characteristic 1 : 2 : 1 tunneling splitting pattern, but only the concerted stem rotation and tilt tunneling motion, accompanied by overall rotation of the dimer, yield the correct magnitude of the splittings and their strong dependence on the dimer angular momentum J that is essential to explain the experimental data. Also the surprising observation that the splittings are reduced by 30% for the mixed (C6D6)(C)(C6H6)(S) dimer in which only the cap (C) in the T-shaped structure is deuterated, while the rotating stem (S) monomer is the same as in the homodimer, is understood using this approach. Stark shift measurements allowed us to determine the dipole moment of the benzene dimer, μ = 0.58 ± 0.051 D. The assumption that this dipole moment is the vector sum of the dipole moments induced in the monomers by the electric field of the quadrupole on the other monomer yields a calculated value of μ = 0.63 D. Furthermore, the observed Stark behavior is typical for a symmetric top, another confirmation of our analysis.

Highlights

The benzene dimer, bound by dispersion forces, is a prototypical system in the study of the interactions between aromatic species that play an important role in biomolecular systems.1–6 It exhibits two competing equilibrium structures, a T-shaped one and a parallel-displaced one
Rotational spectrum and internal dynamics of (C6H6)2 the rotational spectrum of the tilted T-shaped structure of the benzene dimer is expected to be that of a near-prolate asymmetric top with A c B > C, we unambiguously observed the rotational spectrum of a prolate symmetric top for (C6H6)2, in agreement with the findings of Arunan and Gutowsky in 1993.9 As stated in ref. 9, several more transitions having singlet and doublet structures have been observed, which we did not attempt to assign yet
In our combined theoretical and microwave spectroscopic study, we unravel the internal dynamics of the benzene dimer, a benchmark system for studying dispersion forces

Summary

Introduction

The benzene dimer, bound by dispersion forces, is a prototypical system in the study of the interactions between aromatic species that play an important role in biomolecular systems. It exhibits two competing equilibrium structures, a T-shaped one (edge-to-face arrangement) and a parallel-displaced one (stacked arrangement). Allowing for all internal rotation tunnelings, the permutationinversion (PI) symmetry group that describes the symmetry of the bound states has 576 elements in this case; it is called G576.23,24 What made the calculations on the benzene dimer demanding is that some of the barriers between the minima are very low and allow delocalization by tunneling between equivalent minima, whereas the barriers in other degrees of freedom are much higher so that the internal states are localized in these directions This implies that the internal rotor basis used in the calculations had to be extremely large, in order to allow sufficient localization and converge the tunneling splittings. We construct a reduced dimensionality approach based on a two-dimensional (2D) cut of the same ab initio potential as used in the full six-dimensional (6D) calculations This approach involves only the sixfold hindered stem rotation and tilt tunneling, and accounts for the coupling between these internal motions. A brief report of our results is given in ref. 25; the present paper gives a more complete description of the experiments and of our theoretical approach

Experimental

Reduced-dimensionality approach

Experimental results

Stark effect measurements

Intensities

Summary and conclusions