Raman study of a 4‐nitropyridine <i>N</i>‐oxide single crystal. 1—Lattice vibrations

Abstract

Abstract High‐quality single crystals of 4‐nitropyridine N oxide (NPO) and 4‐nitropyridine‐ d 4 N ‐oxide (NPO‐ d 4 ) of adequate size were prepared by slow recrystallization from an acetone solution. Low‐frequency polarized Raman spectra were recorded in the range 6–200 cm −1 . The external vibrations were assigned using group theory and assuming that the libration frequencies are inversely proportional to the square root of the moments of inertia; in the spectra of NPO, the A g ‐B 2g components appear at 37–43 cm −1 for R y and at 51–61 cm −1 for both T x and T z , because pseudo‐degeneracy occurs; the B 1g ‐ B 3g components appear at 51–35 cm −1 ( R x ), 89–110 cm −1 ( R z ) and 152–163 cm −1 ( T y ). Two internal vibrations which appear below 200 cm −1 are also assigned: the NO 2 twisting at 79 cm −1 and an out‐of plane bending vibration of the N ‐oxide and NO 2 groups at 121–145 cm −1 . As expected, the NPO‐ d 4 spectra show little difference from that of NPO. The influence of the intermolecular bonds on the frequencies, splittings, widths and intensities of the lines is discussed.