Molecular Structure of p-Cyclohexylaniline. Comparison of Results Obtained by X-ray Diffraction with Gas Phase Laser Experiments and ab Initio Calculations

The results of a combined experimental and theoretical investigation of the molecular structure of p-cyclohexylaniline (pCHA) in the electronic ground and the first electronically excited state are reported. The experimental investigations are performed for the crystalline phase by X-ray diffraction for the first time and related to former gas phase results obtained by time-resolved rotational laser spectroscopy. The theoretical results, from new ab initio calculations at the MP2/6-31+G(d) and CIS/6-31+G(d) level of theory for the electronic ground and excited state, respectively, give an adequate description of the rotational constants as obtained by the gas phase experiments. Thus, a detailed comparison of the ab initio structure for the ground state with the X-ray structure is performed in order to ascertain differences in the molecular geometry between the gas and crystalline phase. In particular, the size of the aromatic and cyclohexyl ring, their mutual orientation, and the conformation of the NH₂ group are affected. The latter can be ascribed to intermolecular hydrogen bonding forming chainlike structures in the crystalline phase. Moreover, the results of the ab initio calculations allow for a discussion of the structural changes of pCHA upon photoexcitation in the gas phase.