Conformations of Morpholine in Liquid and Adsorbed on Gold Nanoparticles Explored by Raman Spectroscopy and Theoretical Calculations

Morpholine is a typical six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O. In this work, the conformations of Morpholine in liquid and adsorbed on the surface of gold nanoparticles were studied by means of Raman spectroscopy and theoretical calculations. Ab initio calculations indicate that the energy of the chair conformers of Morpholine is ca. 7.5 kcal/mol lower than the skew-boat conformers, which implies that the chair conformers would be favorable in liquid Morpholine. Comparison of the observed Raman spectra of liquid Morpholine, its solution, and the predicted spectra of the chair conformers (equatorial and axial) revealed that both of the chair conformers coexist in its liquid, and the content of the equatorial-chair conformer may reduce in the solution. Considering the concentration-dependent Surface-Enhanced Raman Scattering (SERS) spectral profile, the surface selection rule, and the theoretical calculations, it has been inferred that at higher concentrations Morpholine is vertically chemisorbed on gold nanoparticles through the N atom of the ring, and the dominant conformation adsorbed is the axial-chair conformer. However, at the dilute concentrations, Morpholine is gradually flatly chemisorbed on the gold nanoparticles through the N atom, and the Morpholine may be deprotonated. Furthermore, the predicted spectra agree with the experimental ones very well, which confirms the results above.