Assignment techniques for 29Si NMR chemical shifts in pertrimethylsilylated products. Tests of empirical assignment rules by a modified 29Si indor

Abstract

A modification of the 29Si INDOR technique is described that monitors instead of the usual Si(CH3)3 proton line the line of CHnXSi (n = 1, 2; X = O, S, NH) protons while the 29Si NMR spectrum is being scanned. Though less sensitive and accurate than the routine INDOR, the modified method permits assignment of 29Si chemical shifts providing that the CHn proton lines in 1H NMR spectrum are well separated and assigned. The modified INDOR is used here to assign the 29Si chemical shifts in pertrimethylsilylated 1,6-anhydro-β-D-glucopyranose (1) and methyl β-D-xylopyranoside (2). The values of chemical shifts determined by this technique agree with those determined by standard 29Si NMR methods but the assignment suggested for 1 on empirical grounds previously must be partially revised, the empirical assignment in the spectrum of 2 (based on additivity rule) is fully confirmed.