title: Eight-membered-ring solid-state conformational interconversion via the atom-flip mechanism, a CPMAS 13C NMR and crystallographic stereochemical study.
abstract: 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 P2(1)/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 iodide) increase the void distance between the 2(1)-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 A, 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 2(1)-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 A close intermolecular (TCC)Me(eq)-H...H-Me(ax)(TCC) distance between adjacent 2(1)-screw symmetry ammonium cations. The 203 K low-temperature CPMAS 13C NMR spectrum of the methiodide salt showed two slow exchange limit (SEL) delta 57.91 [BB] and delta 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.
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel. glaser@bgumail.bgu.ac.il
authors.forename: R
authors.initials: R
authors.lastname: Glaser
authors.forename: A
authors.initials: A
authors.lastname: Novoselsky
authors.forename: D
authors.initials: D
authors.lastname: Shiftan
mesh_terms: D018712:Analgesics, Non-Narcotic / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D009340:Nefopam / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*
chemical_list: D018712:Analgesics, Non-Narcotic; D000644:Quaternary Ammonium Compounds; D009340:Nefopam