Hydrogen bonding Part 45. Thermodynamic and IR study of the hydrates of N-methylmorpholine oxide and quinuclidine oxide. Effect of hydrate stoichiometry on strength of HOH⋯ON hydrogen bonds; implications for the dissolution of cellulose in amine oxide solvents

Abstract

N-Methylmorpholine oxide forms two hydrates containing 2.5 and 1 mole H 2O respectively. Equilibrium dissociation vapor pressure measurements show that the strength of HOH⋯ON hydrogen bonds in the higher hydrate (15.83 kcal mol −1) is greater than in the monohydrate (12.01 kcal mol −1). The failure of cellulose to dissolve in N-methylmorpholine oxide when the H 2O content is greater than 2 moles H 2O per mole of amine oxide may arise because H 2O is more tightly bound to the NO group at higher H 2O content, rather than because the amine oxide oxygen is “saturated” with two or more hydrogen bonds. Quinuclidine oxide forms a crystalline monohydrate; no other stable hydrates are observed at 25°C. The similarity of the IR spectra for bound H 2O in N-methylmorpholine oxide monohydrate and quinuclidine oxide monohydrate and the similarity of thermodynamic parameters for the dissociations of these two hydrates suggests that they have the same ribbon-type hydrate structure.