Molecular Clusters and Complexes

Morphing Potentials

Molecular Clusters are ideal systems for testing strategies to devise and refine intermolecular interactions. In a first step an interaction potential from fit to experimental data together with a physically motivated parametrized form or from ab initio electronic structure theory can be used. Then, such potentials can be successively refined by morphing the potential[1]. An example of an original and its morphed potential is shown in the Figure below. The potential energy surface describes the interaction of Ne with HF(v=1). The morphed potential reproduces all known experimental observables to better than 0.1%.

 

A particular class of molecular clusters are proton bound clusters. The intermolecular interactions in such clusters are particularly strong due to electrostatic and induced interactions. In the past clusters composed of rare gases and ionic cores such as N2H+, COH+, and OH+ were investigated in some detail. Both, the XH+-Rg dimers and larger XH+-(Rg)n clusters were treated with a number of different techniques. 

Proton Transfer in Small Clusters

More recently, the proton  transfer reaction along protonated ammonia chains was investigated. The problem of proton migration along "mole Protonated Ammonia Dimer cular wires" has been of interest since the observation of the "Zundel continuum in liquid water. Our interest arose from the suggestion that proton transfer along an am as beemonia chain may be possible in the electronic ground state of 7-Hydroxyquinoline (NH3)6 [2]. Here, an ammonia chain links the hydroxy-end with the ring-nitrogen atom as shown in the Figure. Upon proton transfer from the hydroxy-H to the nearest ammonia molecule an ion pair forms with a negatively charged 7-HQ and a protonated ammonia chain. The central part of such a chain consists of the protonated ammonia dimer, shown to the left. High-resolution experimental characterization could not unequivocally determine the minimum energy "structure" of the proton bound complex. From the ab initio calculations however it appears that the the transferring proton (H*) is asymmetrically bound with a very low barrier to proton transfer.

Experimental observables were calculated with both, classical molecular dynamics and at the quantum level. For the quantum mechanical calculations propagation techniques (BOUND) and Diffusion Monte Carlo simulations are used.

References:

[1]    M. Meuwly and J. M. Hutson, J. Chem. Phys., 110, 8338 (1999)

[2]    M. Meuwly, A. Bach, S. Leutwyler, J. Am. Chem. Soc., 123, 11446 (2001)

Classical and Quantum Dynamics of Clusters

Clusters are also ideal systems to study the influence of treating the nuclear dynamics at a classical or quantum mechanical level. Since experimental studies are always carried out at finite temperature, the inclusion of temperature-dependent dynamics becomes essential. This is straightforward in classical dynamics simulations but less simple if the dynamics should capture quantum mechanical effects (tunneling). Our group is involved in investigating the effects of quantum effects on the infrared spectroscopy of small gas phase clusters. For this, Fourier Path integral techniques are used and the results are compared with classical dynamics.