Molecular structures of gas-phase neutral morpholine and its monohydrated complexes: experimental and theoretical approaches

Morpholine (NH(CH2CH2)2O) is a typical six-membered aliphatic heterocyclic compound. Herein, infrared plus vacuum ultraviolet (IR/VUV) single photon “soft” ionization spectroscopy was employed to study the structures of neutral morpholine and its monohydrated clusters. Theoretical calculations revealed that the structures containing equatorial-chair and axial-chair conformations were the most stable conformers in the gas phase, and this was confirmed by IR spectral analysis. Analysis of the observed and calculated spectra of the monohydrated clusters suggested that multiple conformers may co-exist in the molecular beam, and that the water molecule acts as a hydrogen donor. In the most stable structure, the hydrogen atom of the water molecule is bound to the NH group of the equatorial-chair conformer of morpholine. Moreover, the water molecules simultaneously serve as hydrogen bond donors for the NH group and interact with the CH group weakly. It is suggested that the weak intermolecular CH/O interaction is also responsible for the molecular stability.