A theoretical and NMR lanthanide-induced shift (LIS) investigation of the conformations of lactams.

Molecular mechanics (MM) with MMFF94 and MMX force fields and ab initio (RHF/6-31G*,RHF/6-311G**, and B3LYP/6-311G**) calculations are used with lanthanide-induced shift (LIS) to investigate the conformations of N-methyl-2-pyrrolidone 1, N-methyl-2-piperidone 2, ε-caprolactam 3, γ-valerolactam (1,5-dimethyl-2-pyrrolidone) 4, 2-azetidinone 5, 4-methyl azetidinone 6, 4-phenyl azetidinone 7, and N-methyl-4-phenyl azetidinone 8. The Yb(fod) paramagnetic induced shifts of all the H and C nuclei are measured and the corresponding diamagnetic complexation shifts obtained by the addition of Lu(fod) . The complexation model (two-, three-, or four-site) used depends on the relative rates of the processes involved.

Reversibly changing a painkiller structure: a hot topic for a cold case--ibuprofen lysine salt.

Ibuprofen lysine salt can undergo a fully reversible, thermally induced phase transition into a different enantiotropically related polymorphic form. The structures of both the high and low temperature phases were solved using state-of-the-art X-ray powder diffraction methods, showing many similarities both in the molecular conformation and in the crystal packing. The full structural analysis and comparison of the two crystal structures allowed to understand the mechanism of the phase transition and explain its reversible nature in what appears to be a rare case of isosymmetric temperature-driven phase transformation of an organic solid.

A structural powder diffraction study of two polymorphic forms of nortriptyline hydrochloride.

Nortriptyline hydrochloride, a tricyclic antidepressant, appears in two different polymorphic forms, only one of which (hereafter, form β) has been previously characterized by single-crystal analysis. Form β is monoclinic, P2(1)/c, with a = 5.070(2), b = 34.

Polymorphism of linezolid: a combined single-crystal, powder diffraction and NMR study.

Linezolid (S)-N-[[3-(3-fluoro-4-morpholinylphenyl)-2-oxo-5-oxazolidinyl]methyl] acetamide is one of the first commercially available (and most widely used) oxazolidinone antibiotics. It was selectively prepared as two anhydrous polymorphic forms, labelled form II and IV in accordance with preliminary reports in the patent literature. Form II has been characterized by single-crystal X-ray diffraction methods (orthorhombic, P2 1 2 1 2 1, a=6.