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"The role of the methyl group in stabilising the weak N–H π hydrogen bond in the 4-fluorotoluene–ammonia complex" M. P. Gosling, I. Pugliesi, M. C. R. CockettPhys. Chem. Chem. Phys12, 132 (2010) Abstract: 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 π-bound structure involving one bond between an ammonia hydrogen and the π-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 π-complex about 10% larger than that of the in-plane, σ-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 π-complex in addition to a σ-complex supports the intuitive expectation that electron-donating groups should help to increase π-density and hence stabilise π-proton acceptor complex formation. In this case, this occurs in spite of the presence of a strongly electron-withdrawing fluorine atom. BMO authors (in alphabetic order): Igor Pugliesi Order Hardcopy  WWW-Version |