Molecular structure of p-cyclohexylaniline. Comparison of results obtainedby 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 rho -cyclohexylaniline (pCHA) in the electronic gro und and the first electronically excited state are reported. The experiment al investigations are performed for the crystalline phase by X-ray diffract ion 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 a dequate description of the rotational constants as obtained by the gas phas e experiments. Thus, a detailed comparison of the ab initio structure for t he ground state with the X-ray structure is performed in order to ascertain differences in the molecular geometry between the gas and crystalline phas e. In particular, the size of the aromatic and cyclohexyl ring, their mutua l orientation, and the conformation of the NH2 group are affected. The latt er can be ascribed to intermolecular hydrogen bonding forming chainlike str uctures in the crystalline phase. Moreover, the results of the ab initio ca lculations allow for a discussion of the structural changes of pCHA upon ph otoexcitation in the gas phase.