Laser-induced fluorescence spectroscopy of ethyl and methyl p-aminobenzoate van der Waals complexes with non-polar solvents (CH4, C2H6 and CF4)

Abstract

Both low- and high-resolution nanosecond-pulsed laser-induced fluorescence spectra of methyl and ethyl p-aminobenzoate van der Waals complexes (MAB and EAB) have been characterised following jet-expansion investigations. A combined MNDO–Lennard-Jones computation method has been used to determine the potential-energy surface minima in the complexes between the hosts MAB and EAB and the solvents CH₄, C₂H₆ and CF₄. Whilst this approach yielded satisfactory results for bare hosts, the method was found to be unsatisfactory for computing the complex equilibrium geometries consistent with the experimental rotational contours of the observed complexes. However, the method is still a valid tool for predicting possible complex conformers and relative host–solvent angles. Substitution of the Lennard-Jones computation by a trial and error grid-search geometry, followed by simulation of the geometry rotational band structure and fitting to experimental rotational contours leads to a successful determination of the geometry of the complexes. With the exception of the MAB–CH₄ complex, equilibrium geometries, quantitatively close to those observed, have been simulated for all the complexes studied, including some conformers. No simple general picture emerges from the present intermolecular force interpretation of the complexes that are described.