Conformational assignments in molecular beam experiments are often based on relative energies, although there are many other relevant parameters, such as conformer-dependent oscillator strengths, Franck-Condon factors, quantum yields and vibronic couplings. In the present contribution, we investigate the conformational landscape of 1,3-dimethoxybenzene using a combination of rotationally resolved electronic spectroscopy and high level ab initio calculations. The electronic origin of one of the three possible planar rotamers (rotamer (0,180) with both substituents pointing at each other) has not been found. Based on the calculated potential energy surface of 1,3-dimethoxybenzene in the electronic ground and lowest excited state, we show that this can be explained by a distorted non-planar geometry of rotamer (0,180) in the S state.

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The molecular structures of guaiacol (2-methoxyphenol) and mequinol (4-methoxyphenol) have been studied using high resolution electronic spectroscopy in a molecular beam and contrasted with ab initio computations. Mequinol exhibits two low frequency bands that have been assigned to electronic origins of two possible conformers of the molecule, trans and cis. Guaiacol also shows low frequency bands, but in this case, the bands have been assigned to the electronic origin and vibrational modes of a single conformer of the isolated molecule. A detailed study of these bands indicates that guaiacol has a vibrationally averaged planar structure in the ground state, but it is distorted along both in-plane and out-of-plane coordinates in the first electronically excited state. An intramolecular hydrogen bond involving the adjacent   -OH and   -OCH3 groups plays a major role in these dynamics.

We report on the short-time dynamics in colloidal mixtures made up of monomers and dimers highly confined between two glass-plates. At low concentrations, the experimental measurements of colloidal motion agree well with the solution of the Navier-Stokes equation at low Reynolds numbers, which takes into account the increase of the drag force on each particle due to wallparticle hydrodynamic forces. We find that the ratio of the short-time diffusion coefficients of the monomer and that of the center of mass of the dimer remains independent of both the total packing fraction and the dimer molar fraction up to concentrations near to the crystallization transition. The same physical scenario is observed for the ratio between the parallel and perpendicular components of the short-time diffusion coefficients of the dimer. This dynamical behavior is corroborated by means of Molecular Dynamics computer simulations that explicitly include the particle-particle hydrodynamic forces induced by the solvent. Thus, our results point out toward that the effects of the particle-particle hydrodynamic interactions on the diffusion coefficients are identical and, thus, factorable in both species.Introduction. Many phenomena observed in colloidal dispersions resemble those of atomic systems. However, the colloidal dynamics exhibits special features due to the solvent-mediated forces typically known as hydrodynamic interactions (HI) [1]. Contrary to direct particleparticle interactions, HI can be tuned, but never completely screened or switched off. In a simple physical picture, HI can be understood as follows. The motion of a given colloidal particle induces a flow field in the solvent which is felt by the surrounding colloids. Thus, the motion of one colloidal particle causes a solvent-mediated force on the neighboring colloidal particles.In contrast to the static counterpart, the colloidal dynamics is far from being completely understood. The reason is partially related with the fact that the colloidal dynamics extends over a wide range of temporal and length scales due to the enormous difference in size and mass between the colloids and the solvent molecules, giving rise to complex and long-ranged HI [1,2]. The latter ones lead to non-trivial coupling among colloids that extends over many mean-interparticle distances [1,2]. The understanding of HI is thus of relevance not only in physics, but also in several branches of science, such as biology, since phenomena like hydrodynamic synchronization in either biological systems (sperm, cilia, flagella) [3,4] or active fluids [5], and the dynamics of microswimmers [6,7] can only be explained in terms of hydrodynamic forces.During the last few decades, the study of the hydrodynamic coupling between two or three spherical colloids [8,9] or a spherical colloid near a wall [10][11][12] has been

Rotationally resolved fluorescence excitation spectra of the origin bands in the S1 ← S0 transition of 2-tolunitrile (2TN) and 3-tolunitrile (3TN) have been recorded in the collision-free environment of a molecular beam. Analyses of these data provide the rotational constants of each molecule and the potential energy curves governing the internal rotation of the attached methyl groups in both electronic states. 2TN exhibits much larger barriers along this coordinate than 3TN. Interestingly, the electronic transition dipole moment in both molecules is markedly influenced by the position of the attached methyl group rather than the position of the cyano group; possible reasons for this intriguing behavior are discussed.