The pyrimidine⋯NH3 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 is assignable to a single stable conformer in which the ammonia molecule binds via two hydrogen bonds within the plane of the ring, in a location which minimises repulsion between the ammonia nitrogen lone pair and that of the second, more remote pyrimidine nitrogen in the 3 position on the opposite side of the ring. Ground state estimated CCSD(T) interaction energies were extrapolated to the complete basis set limit: these calculations found the dissociation energy of the most stable conformer, σB, to be 20% larger than that of a second in-plane conformer, σA, in which the ammonia forms a similar pseudo five-membered ring, bridging the nitrogen at the 1 position with the carbon at the 2 position. This conformation in turn was found to have a dissociation energy 35% larger than that of a π-complex in which the ammonia binds above the plane of the aromatic ring. The results of multidimensional Franck–Condon simulations based on ab initio ground and excited state CASSCF and RICC2 geometry optimisations and vibrational frequency calculations showed good agreement with experiment. It is postulated that longer-range electrostatic interactions between the ammonia lone pair and the more distant of the two ring nitrogens on the pyrimidine, play a key role in determining which of the two in-plane structures is the more stable and which, therefore, is responsible for all of the spectral features observed in the R2PI spectrum.

The 4-fluorotoluene-ammonia van der Waals complex has been studied using IR-UV depletion and hole-burning spectroscopies with assignments supported by ab initio and DFT calculations of ground state binding energies and intramolecular vibrational frequencies. The experimental IR-UV depletion and hole-burning spectra presented here provide unequivocal empirical evidence that the 4FT-NH3 complex exists in two almost equally stable conformational forms. Both isomers contribute intensity to the experimental R2PI spectrum with one responsible for bands appearing to the red of the 4FT band origin position, and the other for those appearing immediately to the blue. On the basis of comparison of computed NH stretching frequencies with those obtained from the IR-UV spectra, the red-shifted bands are assigned to a π-proton-acceptor complex featuring an NH⋯π-hydrogen bond, and the blue-shifted bands are assigned to an in-plane σ-complex in which the ammonia binds in the plane of the ring forming a double-hydrogen-bonded six-membered ring with the fluorine atom. Ground state interaction energies computed at the M06-2X/cc-pVTZ level were found to compare favourably with those obtained at the CCSD(T) CBS level, although the former resulted in overbinding of the π-complex compared with the in-plane conformer, a characteristic shared with MP2 level calculations. The observation of a π-complex in addition to a σ-complex is consistent with the conclusion that the electron-donating power of the methyl group is sufficiently large to counter-balance the electron-withdrawing power of the fluorine atom.

The pyrimidine⋯NH3 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 is assignable to a single stable conformer in which the ammonia molecule binds via two hydrogen bonds within the plane of the ring, in a location which minimises repulsion between the ammonia nitrogen lone pair and that of the second, more remote pyrimidine nitrogen in the 3 position on the opposite side of the ring. Ground state estimated CCSD(T) interaction energies were extrapolated to the complete basis set limit: these calculations found the dissociation energy of the most stable conformer, σB, to be 20% larger than that of a second in-plane conformer, σA, in which the ammonia forms a similar pseudo five-membered ring, bridging the nitrogen at the 1 position with the carbon at the 2 position. This conformation in turn was found to have a dissociation energy 35% larger than that of a π-complex in which the ammonia binds above the plane of the aromatic ring. The results of multidimensional Franck–Condon simulations based on ab initio ground and excited state CASSCF and RICC2 geometry optimisations and vibrational frequency calculations showed good agreement with experiment. It is postulated that longer-range electrostatic interactions between the ammonia lone pair and the more distant of the two ring nitrogens on the pyrimidine, play a key role in determining which of the two in-plane structures is the more stable and which, therefore, is responsible for all of the spectral features observed in the R2PI spectrum.

The 4-fluorotoluene-ammonia van der Waals complex has been studied using IR-UV depletion and hole-burning spectroscopies with assignments supported by ab initio and DFT calculations of ground state binding energies and intramolecular vibrational frequencies. The experimental IR-UV depletion and hole-burning spectra presented here provide unequivocal empirical evidence that the 4FT-NH3 complex exists in two almost equally stable conformational forms. Both isomers contribute intensity to the experimental R2PI spectrum with one responsible for bands appearing to the red of the 4FT band origin position, and the other for those appearing immediately to the blue. On the basis of comparison of computed NH stretching frequencies with those obtained from the IR-UV spectra, the red-shifted bands are assigned to a π-proton-acceptor complex featuring an NH⋯π-hydrogen bond, and the blue-shifted bands are assigned to an in-plane σ-complex in which the ammonia binds in the plane of the ring forming a double-hydrogen-bonded six-membered ring with the fluorine atom. Ground state interaction energies computed at the M06-2X/cc-pVTZ level were found to compare favourably with those obtained at the CCSD(T) CBS level, although the former resulted in overbinding of the π-complex compared with the in-plane conformer, a characteristic shared with MP2 level calculations. The observation of a π-complex in addition to a σ-complex is consistent with the conclusion that the electron-donating power of the methyl group is sufficiently large to counter-balance the electron-withdrawing power of the fluorine atom.