Structural analysis and IR-spectroscopy of a new anilinium hydrogenselenite hybrid compound: A subtle structural phase transition

• A subtle and unusual structural behavior was detected and characterized.
• The structural phase transition is correlated with the molecular rotation.
• The vibrations of the ammonium group could be the origin of the phase transition.

An interesting structural behavior has been detected and characterized in a new anilinium based compound by single crystal X-ray diffraction measurements and infrared spectroscopy. The temperature dependent structural investigation reveals that the studied compound undergoes a subtle non-centrosymmetric to centrosymmetric structural phase transition. At room temperature the crystal structure is non-centrosymmetric and is characterized by an important disorder in the organic part where the independent aromatic rings are rotated around the C1—C4 and C7—C10 molecular axis by 52.06(6)° and 56.25(4)° respectively. By decreasing the temperature to 100 K the organic cation is less disordered and the rotation angle of the aromatic rings changes to 54.73(2)°. As a consequence, the low temperature structure becomes centrosymmetric. The infrared spectra recorded on cooling and heating the sample in the temperature range of 300–9 K support this analysis. The ammonium group (NH3) is significantly affected by the temperature change. This effect results, when cooling the sample below 140 K, in the appearance of a new vibrational band corresponding to the (NH3)-wagging modes at 705 cm−1. The phenomenon is completely reversible and the new vibrational band disappears upon heating the sample above 140 K. Moreover, the ν(C–N)-stretching and the (NH3)-scissoring modes are also affected by the temperature variation as can be seen in the spectral ranges of 1250–1400 cm−1 and 1610–1635 cm−1 respectively. The spectroscopic results are complementary to the structural analysis and indicate that the structural phase transition is correlated with the molecular rotation of the aromatic moieties around the C1—C4 axis that affect the different vibrational modes of the ammonium group.

A new anilinium hybrid compound was characterized by the combination of variable temperature single crystal X-ray diffraction and FT-IR spectroscopy. This new compound undergo a subtle phase transition correlated with the molecular rotation of the organic cation even at low temperature. Such a phase transition is a consequence of atypical molecular rotation of the aromatic rings that affect the vibrational modes of the ammonium group.Figure optionsDownload as PowerPoint slide