Structural, spectroscopic and theoretical studies of the 1:1 complex of N-methylpiperidine betaine with squaric acid

The 1:1 complex of N-methylpiperidine betaine, MPB, with squaric acid, H2SQ, has been characterized by single-crystal X-ray analysis, FTIR and NMR spectroscopies, and by DFT calculations. The crystals are orthorhombic, space group Pbca. Two non-equivalent OH···O hydrogen bonds of 2.472(1) and 2.508(2) Å link H2SQ with MPB. The piperidinium ring has a chair conformation with the methyl group in the equatorial position and the CH2COO− substituent in the axial one. The geometry of the optimized structures of the MPB·H2SQ complex at the B3LYP/6-31G(d,p) level of theory, in a vacuum and solvated by water, have been used to calculate harmonic IR frequencies and to predict the chemical shifts in H2O.

► Squaric acid, H2SQ, and N-methylpiperidine betaine, MPB, form a 1:1 complex.
► Two OH protons of H2SQ are engaged in hydrogen bonds with two oxygen atoms of MPB carboxylate group.
► The bulky CH2COO− group is in the axial position in MPB.
► MPB are surrounded by four H2SQ molecules involved the CH⋯O contacts.
► 1H and 13C NMR spectra confirm the conformation of MPB in the complex with H2SQ.