Molecular dynamics in solid riboflavin as studied by 1H NMR.

Spin-lattice relaxation times Tl and Tld as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B2) in the temperature range 55-350 K. The broad and flat Tl minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson-Cole distribution of correlation times for this motion is assumed.