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Structure determination of dihydroxamic acids and their trimethylsilyl derivatives by NMR spectroscopy
Author(s) -
Schraml Jan,
Kvíčalová Magdalena,
Blechta Vratislav,
Soukupová Ludmila,
Exner Otto,
Boldhaus HansMichael,
Erdt Frank,
Bliefert Claus
Publication year - 2000
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/1097-458x(200009)38:9<795::aid-mrc732>3.0.co;2-a
Subject(s) - chemistry , chemical shift , conformational isomerism , magic angle spinning , nmr spectra database , nuclear magnetic resonance spectroscopy , carbon 13 nmr , amide , trimethylsilyl , spectral line , crystallography , stereochemistry , computational chemistry , molecule , organic chemistry , astronomy , physics
Homologous series of dihydroxamic acids [HONHCO(CH 2 ) n CONHOH with n = 0, 1, 2, 3, 4 and 6] were prepared and trimethysilylated [ 1 ( n ) and 2 ( n )]. The solution NMR spectra ( 1 H, 13 C, 15 N) of 1 ( n ) show that the hydroxamic end groups assume Z–Z and Z–E combinations of conformers. An exception is oxalodihydroxamic acid, which assumes only one combination. 13 C cross‐polarization magic angle spinning reproduces the solution chemical shift in this compound and indicates the Z–Z combination as determined earlier by x‐ray diffraction. The trimethylsilylation produces compounds with a hydroximic structure on both ends, both groups being disilylated. Z–Z , Z–E and E–E isomer combinations are visible in the spectra and their ratio can be determined. Again, oxalodihydroximic acid derivatives are an exception: only one silylated product was found and its geometry could not be determined. Selective decoupling experiments ( 15 N{ 1 H} and 13 C{ 1 H}) are an inexpensive alternative to 15 N enrichment used to identify E and Z conformers. To differentiate hydroxamic and hydroximic structures, the most reliable parameter is the 15 N chemical shift, which differs in the two classes of compounds by about 120 ppm. To differentiate E and Z hydroxamic conformers 13 C chemical shifts of C O groups are preferable to 15 N chemical shifts but for distinguishing E and Z isomers of the hydroximic structure both 15 N and 13 C NMR of the C N group are useful. 17 O NMR data are of no practical value in this respect. Copyright © 2000 John Wiley & Sons, Ltd.