Tentative vibrational assignments for 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) between 4000 and 600 cm−1 have been made. Complementary infrared and Raman studies indicate that the two amino groups of minoxidil are not equivalent in the solid state. One of them is a free amino group, while the other is hydrogen bonded to the N-oxide. Vibrational spectroscopic analyses of minoxidil in different molecular environments, such as in polar solvents, aprotic solvents, and deuterium oxide, as well as in the pH = 1.5-12.5 range, have been performed to probe the structural variations generated by these conditions. Spectral comparison of minoxidil and its derivatives reveals that most spectral changes involve the amino and N-oxide functional groups. For instance, the N-oxide stretching vibration occurs at 1210 cm−1 in the solid state, but it shifts to 1193 cm−1 in polar solvents. The amino hydrogens of minoxidil and desoxyminoxidil (2,4-diamino-6-piperidinopyrimidine) are readily exchanged with deuterium, as reflected in the ND2 stretching region (2600-2300 cm−1) in the infrared spectra of deuterated crystalline minoxidil and desoxyminoxidil. The similarity of band contours in the NH2 stretching region suggests that there are no structural alterations on deuteration for either minoxidil or desoxyminoxidil.
R. S. Chao, T. J. Thamann, and K. C. Vail, "Vibrational Spectroscopic Analysis of Minoxidil (2,4-Diamino-6-Piperidinopyrimidine-3-Oxide) and Related Analogs," Appl. Spectrosc. 42, 236-242 (1988)