Hydrogen-bonding in the pyrimidine⋯NH3 van der Waals complex: experiment and theory

Abstract

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, σ_B, to be 20% larger than that of a second in-plane conformer, σ_A, 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.