Disclosed herein is a molecular crystal form of the compound (R)-(+)-7-(3-amino-2,3,4,5,6,7-hexahydro-1H-azepin-1-yl)-1-cyclopropyl-8-chloro-6-fluoro-1,4-dihydro-4-oxoquinoline-3-carboxylic acid. The molecular crystal is characterized by at least one of: (a) an X-ray powder diffraction (“XRPD”) spectrum that comprises peaks at 2θ angles of 10.6, 15, 19.7, 21.1, and 22°±0.2°; (b) a DSC melting peak at 288° C.; (c) a 13C NMR spectrum having peaks at 23.3, 27.7, 41.1, 54.5, 116.6, and 153.5 ppm; and (d) pKa values of 5.65 and 9.91. The compound belongs to the class of fluoroquinolones and is useful as an antibacterial agent.