Conformational fluxionality of long-chain alkene clusters in the gas phase evidenced from a combined experimental and theoretical approach

Clusters bound by weak, non-covalent forces, such as van der Waals interactions and hydrogen bonds, are ubiquitous in dilute media ranging from aerosols to molecular fluids and biological structures, their interest being not only fundamental as in astrochemistry but also more applied as in organic electronics. Neutral clusters of up to six 1-hexene molecules produced by supersonic expansion of a gas mixture were ionized, mass selected, and spectroscopically characterized using synchrotron-based VUV photoelectron photoion coincidence technique. Ionization energies inferred from these measurements show decreasing trends as the cluster size increases, by about 0.5 eV over the range of 1-6 molecules. Dedicated theoretical DFT-based calculations were performed to unravel the possible structures of these clusters and determine their vertical and adiabatic ionization energies. Our computational search for stable structures considered the possible chirality effects associated with most conformers of the monomer having enantiomers, in an approach with a broad structural sampling employing classical force fields followed by systematic re-optimization using an efficient quantum chemical method. Vertical and adiabatic ionization energies obtained using wavefunction-based methods exhibit significant dispersion due to conformational flexibility already in the monomer, but these effects are magnified in clusters due to their fluxionality at the experimental temperature of about 130 K. Overall, the trends obtained for the calculated vertical ionization energies agree well with the measured data and suggest that possible chiral recognition effects that could stabilize specific structures are likely to be hampered under the present experimental conditions.