Davydov Collective Vibrational Modes and Infrared Spectrum Features in Aniline Crystal: Influence of Geometry Change Induced by van der Waals Interactions

Intermolecular interactions have significant influences on molecular crystals, oligomers, and various van der Waals clusters. They are essential in determining the quantum optics, quantum transport, and chemical properties of complex molecular systems. In this work, we investigate the infrared spectra of aniline crystal via the density functional theory. Then we identify four intriguing collective modes that are featured by NHHN wagging vibrations and four other collective modes that are featured by NH2 wagging vibrations. All these eight collective modes are due to Davydov splitting. To clarify the origin of such vibrational pattern, we further simulate aniline molecule and oligomers, and thoroughly analyze the spectra differences on some key vibrational modes, such as N–H wagging and torsional vibrations. Our results reveal that the chain structure of aniline crystal significantly enhances the van der Waals forces among adjacent molecules, and the intermolecular interactions are responsible for those NHHN wagging collective modes. Our study provides insights in intermolecular interactions and collective motions in aniline crystal and also establishes a standard protocol for the theoretical investigation of other van der Waals clusters.