On the conformation of the cellulose solvent N-methylmorpholine-N-oxide (NMMO) in solution

The N-oxide group of the cellulose solvent N-methylmorpholine-N-oxide showed a strong preference for the axial position compared with N-methyl as determined by NMR experiments and computational studies. In solvents with negligible solvent-solute interaction, about 95% of the NMMO molecules obtained a typical chair conformation with an axial N-O (1) while 5% had an equatorial N-O (2) at room temperature (25 °C). Other conformations (boat and twist) are energetically largely disfavored. N-Benzylmorpholine-N-oxide was prepared as reference compound possessing 100% axial N-O. Aprotic solvents of increasing polarity slightly shifted the conformation equilibrium towards the more polar conformer 2. The effect of protic solvents, capable of forming H-bonds, was more pronounced, with water increasing the percentage of 2 to 25% of the total population. Addition of sugar model compounds reversed this effect, so that formation of 2 was suppressed and exclusively 1 was found, indicating a strong interaction between the latter conformer and the carbohydrate.