NMR, crystal structure and FAB mass spectral studies of morpholinomethylphosphonic acid

Abstract

The pD behaviour of morpholinomethylphosphonic acid has been investigated using multinuclear NMR (¹H, ¹³C and ³¹P) techniques in D₂O solvent. The molecule, found to exist in a zwitterionic form, consisting of a weakly deuteriated basic nitrogen and strongly acidic phosphonic acid groups, showed a dramatic change in the NMR spectra as the nitrogen was dedeuteriated. At low pD when the ring is in a state of slow exchange, complex coupling patterns for the (ABCD)₂ spin system are observed. However as the pD was raised broad featureless spectra were obtained and finally at higher values of pD deceptively simple coupling patterns for a new spin (AA′BB′)₂ system were observed, corresponding to a conformationally mobile ring in fast exchange. The zwitterionic nature of the molecule was also highlighted in the X-ray crystal structure of the monohydrate which revealed hydrogen-bonded dimers, and was also observed in the FAB mass spectrum, where aggregates of molecules were noted at masses of (M + 1)⁺, (2M + 1)⁺, (3M + 1)⁺ and (4M + 1)⁺. In the crystal structure, further hydrogen bonding involving the water of crystallisation produces a three-dimensional network.