TRESA VARGHESE, HEMA, et al. “Vibrational Spectroscopic Studies and Ab Initio Calculations of Pyridine-3-Sulfonic Acid”. Asian Journal of Chemistry, vol. 19, no. 4, Mar. 2010, pp. 2627-32, https://asianpubs.org/index.php/ajchem/article/view/13354.