Homologous series of dihydroxamic acids [HONHCO(CH 2 n CONHOH with n D 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 Nf 1 Hg and 13 Cf 1 Hg) 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.

Twelve para-and meta-substituted benzhydroxamic acids were subjected to exhaustive silylation with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (with 1% of tert-butyldimethylsilyl chloride as a catalyst). In all cases only one product was isolated. According to the assigned NMR spectra ( Si shifts of the two silicon atoms are almost equally sensitive to the substituent effects despite their different distances from the substituent.

29Si-13C couplings were measured in para substituted silylated phenols, X--C6H4--O--SiR1R2R3 (X = NO2, CF3, Cl, F, H, CH3, CH3O). The SiR1R2R3 silyl groups included trimethylsilyl (Si(CH3)3, TMS), tert-butyldimethylsilyl (Si(CH3)2C(CH3)3, TBDMS), dimethylsilyl (SiH(CH3)2, DMS), and tert- butyldiphenylsilyl (Si(C6H5)2C(CH3)3, TBDPS). Previously developed (Si,C,Si)gHMQC methods and narrow 29Si lines allowed the determination of coupling constants over up to five bonds. Besides the number of intervening bonds between the silicon and carbon atoms, all the measurable couplings depend also on the nature of the substituents on the silicon. The two- and three-bond couplings are not affected by ring substitution in the para position. These properties render the 29Si-13C couplings suitable for line assignment in the spectra of silylated polyphenols. The experimental results are in reasonable agreement with theoretical calculations. The calculations show, in agreement with the data reported in the literature for couplings between other nuclei, that the two-bond and three-bond couplings, which are of similar magnitudes, are of opposite signs. If the signs of these geminal and vicinal couplings could be determined experimentally, they would greatly facilitate the line assignment. The four- and five-bond couplings are affected by the substituent X in a nontrivial manner.

(29)Si-(13)C spin-spin couplings over one, two, and three bonds as well as other NMR parameters [delta((29)Si), delta((13)C), delta((1)H), (1)J((13)C-(1)H), and (2)J((29)Si-C-(1)H)] were calculated and measured for a series of trimethylsilylated alcohols of the types Me(3)Si-O-(CH(2))(n)CH(3) and Me(3)Si-O-CH(3-n)R(n)(n = 0-3; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2-bond and 3-bond (29)Si-O-(13)C couplings are of opposite signs ((2)J > 0 and (3)J < 0), and their relative magnitudes depend on the extent of branching at the alpha-carbon.

Crystal structure of benzohydroxamic acid and its ring-substituted derivatives RC6H4C(O)NHOH (R = 4-Me, 4-NO2, 4-Cl, 3-Cl and 2-Cl) was determined by single-crystal X-ray diffraction. In all the compounds, the hydroxamic group is in the planar amide form and the structures differ mainly in the tilt of the aromatic and hydroxamic acid planes. For the 2-chloro derivative, the dihedral angle of the two planes is 46.1° which corresponds to the intramolecular van der Waals contact of the ortho-substituents. In other compounds, the tilt originates from intermolecular hydrogen bonding and varies between 3.5 and 22.0°; four crystallographically independent molecules present in the structure of benzohydroxamic acid also differ significantly in this tilt, as well as three independent molecules of the 4-nitro derivative do. Although there are only two types of hydrogen bonding in all the compounds, a short one between OH and O-N and a second longer between NH and O=C, bonded network in the crystal is of three different types. In unsubstituted acid, its 4-Me, 4-Cl and 3-Cl derivative, the molecules are assembled into hydrogen-bonded layers stacked loosely along the largest cell parameter. As a result of the large tilt of the molecular planes in the 2-Cl compound, its molecules are linked into chains with unusual, strongly bent orientation of the aromatic groups. The self-assembly of the remaining 4-nitro compound is quite unique, consisting of pseudohexagonal, partly interpenetrating stacks. In several cases, the hydrogen bonding is supported by π-interaction of the aromatic rings.