Dobrzyńska, D. & Jerzykiewicz, L.B. Journal of Chemical Crystallography (2004) 34: 51. doi:10.1023/B:JOCC.0000014688.79954.ca
Quinoline-2-carboxylic acid (quinaldic acid), C10H7NO2, crystallizes in the monoclinic space group P21/c, a =9.724(1) Å, b = 5.937(1) Å, c = 27.545(2) Å, β = 90.15(1)°, z = 4, and occurs in the crystal in two tautomeric forms: the neutral molecule and the zwitterion, with molecular ratio 1:1. The compound can be named quinoline-2-carboxylic acid ⋅ quinolinium-2-carboxylate. The crystal structure is built up of tautomeric pairs consisting of C9H6NCOOH and C9H6NH+COO− moieties, held up together by hydrogen bonds.