Collect. Czech. Chem. Commun. 1987, 52, 1985-1991

Assignment of 29Si NMR lines and determination of 29Si-13C coupling constants in pertrimethylsilylated lignin-related phenol-carboxylic acids by selective heteronuclear INADEQUATE method

Jan Schramla, Jaan Pastb, Jüri Puskarb, Tõnis Pehkb, Endel Lippmaab and Robert Brežnýc

a Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6-Suchdol, Czechoslovakia
b Institute of Chemical Physics and Biophysics, Estonian Academy of Sciences, 200001 Tallinn, U.S.S.R.
c Institute of Chemistry, Slovak Academy of Sciences, 842 38 Bratislava, Czechoslovakia


Selective heteronuclear (Si-C) INADEQUATE method has been successfully used to assign all 29Si chemical shifts in six pertrimethylsilylated lignin model compounds. Empirical assignment of the lines in the region δ = 23-25 to trimethylsilyl esters of carboxylic compounds has been confirmed. Selectivity of the method permits unambiguous assignment to trimethylsiloxy groups which differ by their position on the benzene ring only. The assignment is possible despite that 29Si-13C two-bond coupling constants (2·1-3·0 Hz) are of comparable magnitude with the three-bond coupling constants (1-2·3 Hz). The chemical shifts of trimethylsiloxy groups on the benzene ring are sensitive to substituent effect to such an extent that their assignment cannot rely on an empirical rule but requires experimental method such as the one used here.