*Article* **Spectroscopic Identification of Hydrogen Bond Vibrations and Quasi-Isostructural Polymorphism in N-Salicylideneaniline**

**Łukasz Hetma ´nczyk 1, Eugene A. Goremychkin 2, Janusz Waliszewski 3, Mikhail V. Vener 4,5, Paweł Lipkowski 6, Peter M. Tolstoy 7,\* and Aleksander Filarowski 2,8,\***


**Abstract:** The *ortho*-hydroxy aryl *Schiff* base 2-[(E)-(phenylimino)methyl]phenol and its deuteroderivative have been studied by the inelastic incoherent neutron scattering (IINS), infrared (IR) and Raman experimental methods, as well as by Density Functional Theory (DFT) and Density-Functional Perturbation Theory (DFPT) simulations. The assignments of vibrational modes within the 3500–50 cm<sup>−</sup><sup>1</sup> spectral region made it possible to state that the strong hydrogen bond in the studied compound can be classified as the so-called quasi-aromatic bond. The isotopic substitution supplemented by the results of DFT calculations allowed us to identify vibrational bands associated with all five major hydrogen bond vibrations. Quasi-isostructural polymorphism of 2-[(E)-(phenylimino)methyl]phenol (**SA**) and 2-[(E)-(phenyl-D5-imino)methyl]phenol (**SA-C6D5**) has been studied by powder X-ray diffraction in the 20–320 K temperature range.

**Keywords:** *Schiff* bases; inelastic incoherent neutron scattering; hydrogen bond; isotopic effect
