Rotational spectroscopy of (C6H6)2 and (C6H6)S(C6D6)T, Stark effect mea-surements of (C6H6)2and permutation-inversion group theory are performed in order to get insight in the internal dynamics of the benzene dimer.
The observed rotational transitions are split in quartets and can be assigned toJ+ 1, K←J, K transitions. The quartets have a regular splitting pattern of 1 : 2 : 1 and can be fitted to a symmetric top Hamiltonian, which is supported by Stark effect measurements on the assigned (C6H6)2 transitions.
The very regular 1 : 2 : 1 quartet splitting can be reproduced when assuming aC6torsion tunneling of one benzene subunit with a barrier≥20 cm−1and the second benzene subunit being rigid in a (distorted) T-shaped structure. As the barrier for the "top"C6 torsion is expected to be below 10 cm−1, the observed
quartet splitting is probably due to the "stem"C6 torsion. TheC6 torsion in the "top" should occur as well, however with a much larger separation of the four torsional levels, due to its lower barrier. In this picture both tunneling processes split the rotational states in four components which are each split again in quartets. From the four expected quartets for each rotational level J, K only one is found. The three other quartets might be shifted considerably, as the splitting of the "top"C6torsion is expected to be very large.
The intensity pattern of the quartets in the (C6H6)2spectrum is 3 : 2 : 2 : 1.
Spin statistical weight calculations for the torsional levels assuming several tunneling pathways ("top" and "stem"C6torsion, "stem" bending, "top" turnover and "top"-"stem" interchange), however, cannot reproduce the experimental intensity pattern.
In order to get more insight in the internal dynamics of the benzene dimer, more experimental studies on isotopically labeled benzene dimers are necessary.
The splitting patterns (absolute splitting width and relative intensities) of different isotopologues can yield valuable additional information as the absolute values of the splittings also depend on the isotopic composition of the tunneling benzene ring. The analysis of the absolute splittings of the dimers (C6H6)2, (C6H6)S(C6D6)T and (C6D6)S(C6H6)T, for example, can allow one to deter-mine in which benzene moiety, if not in both, the tunneling is localized. The spectral broadening induced by quadrupole coupling of the deuterons, however, complicates the spectra. This difficulty can be avoided using a mixed dimer like (12C6H6)(13C6H6). For this isotopologue, however, no preliminary work on the abundance of the different isomers exists yet. Furthermore, dimers of benzene isotopologues, such as CH5D or CH3D3, can also contribute to a deeper understanding of the benzene dimer, as they have a different molecular symmetry and the consequences should be spectroscopically observable.
Provided that both subunits internally rotate about theirC6 axes, three more quartets must exist for each rotational level. Proving their existence experimentally would support the current concept on the internal dynamics of the benzene dimer.
Control and manipulation of conformational
interconversion
4.1 Introduction
Since the pioneering work of Becker and Bier in 1954 [147] and due to the development of new experimental facilities, supersonically expanding molecular beams have become standard tools for gas phase experiments (see for example section 1.2). Techniques exist to generate pulsed or continuous molecular beams that can be used to investigate the entrained molecules, or as a probe to investigate various targets via molecular beam scattering techniques [3, 148, 149].
Often the molecules of interest are seeded in a carrier gas. Via collisions with the carrier gas in the beginning of the supersonic expansion the internal energy of the molecules drops. The cooling efficiency depends on the collision cross sections and also on the efficiency of energy transfer between the molecule and the rare gas atoms. While larger atoms or molecules are best suited to cool the rotational and vibrational degrees of freedom, small rare gas atoms better reduce the spread of the translational energy. At a certain distance from the nozzle of the valve the molecular beam provides a collision free environment which allows for experiments on isolated molecules. In this "zone of silence" only the lowest vibrational and rotational levels of the molecules are populated, giving rise to a less congested spectrum. The isolated species can then experience well defined perturbations (laser radiation, collisions with species of a second beam etc.) and their effects can be observed in a controlled way. Furthermore, due to the internal cooling in the expansion, very weakly bound complexes with small dissociation energies can be formed.
Molecular beam techniques provide perfect conditions to investigate the intrinsic properties of isolated molecules and clusters. Many molecules or molecular complexes can adopt various conformational structures that differ in potential energy. In molecular beam experiments these different conformations are often found to co-exist [150]. As an example of biological relevance, amino acids and nucleobase(pair)s, which are the building blocks of proteins and DNA strands, are often very flexible, and many low-energy conformers can exist [9, 151–156]. In particular, structures can be observed that have high potential energies, and the observed structural distributions are therefore frequently not in thermal equilibrium with the other degrees of freedom [150, 151, 157–163]. This can be rationalized by the height of the barriers that separate potential energy minima which might be hard to overcome under the conditions prevailing in the molecular beam experiments. The study of the potential energy landscape of gas phase molecules in the electronic ground state and the development of experimental methods to manipulate the conformational distribution of these species currently is an active field of research [164–166]. Exploration of their vibrational dynamics is of particular interest because it can help to get insight into the complex potential energy surface and to rationalize the observed conformational distribution.
There has been a variety of studies on the dependence of the conformational distribution in a molecular beam on the experimental parameters. The question how the composition of the carrier gas influences the relative population of different conformers in a supersonic expansion has been addressed by Ruoffet al.
in 1990 [158]. In that work, microwave absorption measurements were performed (Trot ≈ 3 K) to deduce relative populations of the possible conformational structures of a variety of molecules in molecular beams. In such measurements, the effect of changing the rotational temperature and the effect of changing the relative abundance of a specific conformer can appear the same, and it is non-trivial to distinguish between these effects. Nevertheless, one of the important observations of that work is that conversion between conformers, separated by barriers of less than 350 cm−1, appears to involve relatively long-range polarization effects and to take place relatively late in the expansion region. A molecular dynamics simulation addressing the problem of thermodynamic versus kinetic control of isomers of dihalogen - rare gas complexes in a supersonic expansion has been presented by Bastidaet al. [167]. In a more recent theoretical study, the conformational changes of glycine by collisions with rare-gas atoms, with collision energies ranging from 100 to 1000 K, have been investigated [168]. In that work it is shown that attractive interactions between the colliding atoms and the glycine molecule can lower the barrier between conformers, i.e.
that these interactions can catalyze conformer conversion. In 2006, Suhm and co-workers have investigated the isomerism of jet-cooled, isotopically mixed methanol dimers (CH3OH·CH3OD) in a supersonic jet expansion [163]. They use Fourier Transform Infrared (FT-IR) absorption spectroscopy in the 3-4µm (O-H and O-D stretch) region to study the degree of conformer conversion (or:
donor/acceptor isomerism) in their 10-20 K jet expansion. A complication in the latter studies is that it is difficult to spectroscopically distinguish the mixed dimers from their homodimer counterparts. For the mixed methanol dimer system spectra are only reported in a pure He expansion, and it is concluded that relaxation to the lowest energy conformer is incomplete in this case.