The direct reaction between carbohydrates and CO2 has recently attracted attention in the context of cellulose dissolution and derivatization as well as carbon capture applications. We have directly demonstrated the formation of cellulose carbonate upon the introduction of CO2 into a non-aqueous cellulose solution by nuclear magnetic resonance spectroscopy. Comparison of the observed spectra with accurate electronic structure calculations of the changes in chemical shifts upon reaction allowed us to confirm the expectation that CO2 reacts with the hydroxyl group on carbon 6 of the cellulose but not exclusively this hydroxyl group. We found good agreement between predicted and measured chemical shifts using a simple computational method.