Fluoronaphthyridines and -quinolones as antibacterial agents. 3. Synthesis and structure-activity relationships of new 1-(1,1-dimethyl-2-fluoroethyl), 1-[1-methyl-1-(fluoromethyl)-2-fluoroethyl], and 1-[1,1-(difluoromethyl)-2-fluoroethyl] substituted derivatives.