Disclosed herein is a molecular crystal form of (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.
