Eight-membered-ring solid-state conformational interconversion via the atom-flip mechanism, a CPMAS ¹³C NMR and crystallographic stereochemical study

Nefopam methohalide (chloride, bromide, and iodide) medium-ring quaternary ammonium salts of the non-narcotic analgesic tertiary amine drug give crystals belonging to the identical monoclinic P2₁/C space group, and all of these pseudopolymorphs exhibit the same packing motif. A singular boat-boat (BB) more compact conformation is observed in the nefopam methochloride crystal. Larger halide anions (bromide and iodidei increase the void distance between the 2₁-screw axis related adjacent ammonium cations to accommodate void-size dependent equilibrium quantities of the twist-chair-chair (TCC) more extended conformation. The BB:TCC occupancy factors are 0.961(5):0.039(5) [193 K], 0.780(5):0.220(5) [293 K], and 0.755(6):0.245(6) [343 K] for the methobromide crystal, while values of 0.657(5):0.343(5) [193 K] and 0.592(7):0.408(7) [293 K] were measured for the methiodide. Above a minimum of ca. 2.53 Å, the occupancy factors were found to be linearly correlated to the intermolecular (TCC)Me(eq)-H···H-Me(ax)(TCC) distance between abutting methyl group protons in 2₁-screw axis related neighbors. Temperature-dependent occupancy factors for the two conformers are interpreted in terms of a medium ring atom-flip facile interconversion between the two low energy conformations in crystals containing the appropriate size intercation void. A BB/TCC atom-flip interconversion in the methochloride unit cell would result in van der Waals interactions due to an estimated 2.31 Å close intermolecular (TCC)Me(eq)-H···H-Me(ax)(TCC) distance between adjacent 2₁-screw symmetry ammonium cations. The 203 K low-temperature CPMAS ¹³C NMR spectrum of the methiodide salt showed two slow exchange limit (SEL) δ 57.91 [BB] and δ 63.10 [TCC] OCH₂CH₂N peaks. A variable low-temperature CPMAS NMR investigation of the solid methiodide showed complex dynamic behavior that cannot be interpreted solely on the basis of an atom-flip conformational interconversion. Local magnetic fields from the gem-dimethyl rapidly rotating proton magnetic dipoles provide a distance-dependent T₁ relaxation mechanism for neighboring carbons in the solid-state.