13C CP MAS NMR, FTIR, X-ray diffraction and PM3 studies of some N-(ω-carboxyalkyl)morpholine hydrohalides

Abstract

N-(ω-carboxyalkyl)morpholine hydrochlorides, OC4H8N(CH2)nCOOH·HCl, n=1–5, were obtained and analyzed by 13C cross polarization (CP) magic angle spinning (MAS) NMR, FTIR and PM3 calculations. The structure of N-(3-carboxypropyl)morpholine hydrochloride (n=3) has been solved by X-ray diffraction method at 100K and refined to the R=0.031. The crystals are monoclinic, space group P21/c, a=14.307(3), b=9.879(2), c=7.166(1)Å,β=93.20(3)°, V=1011.3(3)Å3,Z=4. In this compound the nitrogen atom is protonated and two molecules form a centrosymmetric dimer, connected by two N+–H⋯Cl− (3.095(1)Å) and two O–H⋯Cl− (3.003(1)Å) hydrogen bonds. 13C CP MAS NMR spectra, contrary to the solution, showed non-equivalence of the ring carbon atoms. The PM3 calculations predict a molecular dimer without proton transfer for an HCl complex, while for an HBr complex an ion pairs with proton transfer, and reproduces correctly the conformation of both dimers but overestimates H-bond distances. Shielding constants calculated from the PM3 geometry of ion pairs gave a linear correlation with the 13C chemical shifts in solids.