Structural and Spectroscopic Characterization of Two Nitropyridine Amino N-Oxide Derivatives

Two new nitropyridine amino N-oxide derivatives were synthesized and characterized: 3-N-methylamino-4-nitropyridine N-oxide (NOPCH₃) and [(4-nitropyridine-3-yl)amino]propanoic acid N-oxide (NOPCOOH)1. Their molecular and crystal structures were determined by single-crystal X-ray diffraction, and the intermolecular contact patterns were evaluated using Hirshfeld surface analysis. Vibrational properties were investigated by FT-IR and Raman spectroscopy and interpreted with the support of quantum-chemical calculations (DFT). Electronic absorption was examined by UV-Vis measurements, while photoluminescence spectra and decay profiles were recorded under femtosecond excitation and interpreted conservatively in terms of relaxed excited-state emission. Solid-state ¹³C CP-MAS and ¹H WPMLG NMR spectroscopy provided additional support for the chemical constitution of both derivatives, with ¹H spectra interpreted conservatively because of the limitations inherent to solid-state homonuclear-decoupled measurements. A comparative analysis shows that replacing the methyl substituent (NOPCH₃) with the carboxylic-acid side chain (NOPCOOH) modifies the hydrogen-bonding landscape and the local electronic environment of the NO₂ group, which is reflected in the positions and intensities of selected NO₂-related vibrational bands and in the near-UV electronic transitions. Overall, the combined structural and spectroscopic dataset provides a consistent structure-property picture for both N-oxide derivatives and a basis for further studies of their coordination and photophysical behavior.