AbstractThe lanthanide shift reagent (LSR)/1H NMR study of some 7‐chloro‐5‐phenyl‐2,3‐dihydro‐1H‐1, 4‐benzodiazepines shows that these compounds exist in CDCI3solution as two pseudo‐boat conformers, rapidly interconverting at room temperature. Computer simulated lanthanide induced shifts (LIS) are consistent with LSR complexation by the imine nitrogen atom. The lanthanide shifts the conformational equilibrium towards conformer (a), with a pseudoequatorial 1‐substituent: this effect may be the result of the greater basicity of 4‐N when the 1‐N lone pair is pseudoaxially directed, thus permitting an extended electron delocalization from 1‐ to 4‐N through the fused benzene ring. The detection of H‐9/1‐Me through space spin‐spin coupling in medazepam (7‐chloro‐1‐methyl‐5‐phenyl‐2,3‐dihydro‐1H‐1,4‐benzodiazepine) supports the observed higher availability of conformer (a) to LSR. A LIS computer calculation to predict the population ratio results in reasonable agreement with the conformational changes based on the response of3J(HH) to lanthanide addition. The electronic and steric effects which determine the greater stability of conformer