Benzhydroximic acids — NMR study of trimethylsilyl derivatives

Assignment of chemical shifts in benzohydroxamic acid and structure of its silylated derivatives by 15N enrichment

“…The shifts in the range 210 to 220 confirm the hydroxamic nature of 1 and 4 and the shifts in the range 70 to 90 demonstrate the hydroximic structure of 2, 3 and 5. 1 The 29 Si NMR spectra, in addition to reproducing the already reported chemical shifts (for 2, 4 and 5), 11,12,17 provided the values of 29 Si, 15 N coupling constants. In the disilylated derivative 2 the lines at υ 27.29 and 23.13 have been tentatively assigned earlier to Si-1 and Si-4.…”

“…In the disilylated derivative 2 the lines at υ 27.29 and 23.13 have been tentatively assigned earlier to Si-1 and Si-4. 17 This assignment leads to a relation 2 J 15 N,O-29 Si > 3 J 15 N,C-O-29 Si), but there are in sufficient data in the literature to confirm the validity of this relationship. Although the 29 Si chemical shifts in 3 differ substantially from those in 2 (owing to substitution of two methyl groups by phenyl groups), all the coupling constants of the silicon nuclei confirm the assignment of the signal at υ 2.67 to Si-1 and that at υ 3.34 to Si-4.…”

Derivatives of hydroxamic acids

“…The correlations described and other details 22,23 indicate that the ring substituent has no significant effect on the geometry of the hydroximic moiety; its structure and torsion angle with the benzene ring do not vary in the series.…”

“…The detailed NMR data have been published 22,23 but we shall mention here briefly only some interesting results concerning simple Hammett-type correlations; a more complicated treatment (DSP) can be found in the references cited. 22,23 Linear correlation of 15 N chemical shifts with Hammett substituent constants in the silyl derivatives of benzhydroximic acids studied confirms the hydroximic structure, as the slope (TMS derivatives; b = 7.73, n = 12, r = 0.966) is intermediate between those found in analogous correlations for N,N-dimethylbenzamides (C-N; b = 3.79, n = 12, r = 0.951) and benzonitriles (CN; b = 10.5, n = 7, r = 0.993).…”

Ring‐substituted benzohydroxamic acids: 1H, 13C and 15N NMR spectra and NHOH proton exchange

“…Correlations of 44 about half of that in silylated benzohydroximic acids, R-C 6 H 4 -C(OSi) 15 N-OSi (7.7-7.9 ppm) 45 or benzamidoximes, R-C 6 H 4 -C NH 2 15 N-OH (8.6 ppm) 46 and about one-fifth of that in benzaldoximes, R-C 6 H 4 -CH 15 N-OH (15.1 ppm). 47 The 2-chloro derivative is anomalous in several respects.…”