The role of the methyl group in stabilising the weak N-H ... pi hydrogen bond in the 4-fluorotoluene-ammonia complex

The 4-fluorotoluene-ammonia van der Waals complex has been studied using a combination of resonant two-photon ionisation (R2PI) spectroscopy, ab initio molecular orbital calculations and multidimensional Franck-Condon analysis. The R2PI spectrum shows two sets of features assignable to two distinct conformers: one in which the ammonia binds between the hydrogen meta to the methyl group and the fluorine atom in a planar configuration and the other a pi-bound structure involving one bond between an ammonia hydrogen and the pi-system and another between the ammonia lone pair and the slightly acidic hydrogens on the methyl group. Ground state estimated CCSD(T) interaction energies were computed at the basis-set limit: these calculations yielded very similar interaction energies for the two conformers, whilst zero point energy correction yielded a zero point binding energy for the pi-complex about 10% larger than that of the in-plane, sigma-complex. The results of multidimensional Franck-Condon simulations based on ab initio ground and excited state geometry optimisations and vibrational frequency calculations showed good agreement with experiment, with further improvements achieved using a fitting procedure. The observation of a pi-complex in addition to a pi-complex supports the intuitive expectation that electron-donating groups should help to increase pi-density and hence stabilise pi-proton acceptor complex formation. In this case, this occurs in spite of the presence of a strongly electron-withdrawing fluorine atom.