AbstractA detailed analysis of the 13C NMR spectra of trans‐stilbene and ten deuteriated trans‐stilbenes has been undertaken. Some unusual deuterium isotope effects on carbon–hydrogen spin–spin coupling constants could not be explained by the ordinary primary and secondary isotope effects. The positive and negative changes of nJ(CH) were interpreted in terms of a steric effect, the vibrational influence of the CD bond and the para‐effect induced by deuterium. In this respect, deuterium behaves as a real substituent with electronic properties different from those of hydrogen. The deuterium isotope effects on 13C NMR chemical shifts and carbon–deuterium coupling constants have also been determined.
Deuterium-and N-15-induced isotope effects on H-1 and C-13 chemical shifts of the imino moiety of 16 trans-N-benzylideneaniline (tBA) isotopomers were determined and analysed. These effects appear to be a sensitive probe of molecular conformation and its changes. All determined deuterium isotope effects on the imino-proton are small and negative and are spread in the range from -0.88 ppb in 4-H-2-tBA to -4.97 ppb in H-2(10)-tBA. The effects on the imino-carbon are either positive or negative and have much larger values. Negative sign of isotope effects is primarily governed by lone-pairs, while magnitude and non-additivity depend on the site of isotopic substitution, molecular conformation and distribution of rotamer populations. All N-15 isotope effects on both H-1 and C-13 chemical shifts are positive. A comparison of isotope effects in tBA with those in related isoelectronic compounds is made. (C) 1998 Elsevier Science B.V. All rights reserved.
1,2,4,6-Cycloheptatetraene: the key intermediate in arylcarbene interconversions and related C7H6 rearrangements
作者:Robert J. McMahon、Christopher J. Abelt、Orville L. Chapman、Jeffery W. Johnson、Curtis L. Kreil、Jean Pierre LeRoux、Anne M. Mooring、Paul R. West
AbstractA detailed analysis of the 13C NMR spectra of trans‐stilbene and ten deuteriated trans‐stilbenes has been undertaken. Some unusual deuterium isotope effects on carbon–hydrogen spin–spin coupling constants could not be explained by the ordinary primary and secondary isotope effects. The positive and negative changes of nJ(CH) were interpreted in terms of a steric effect, the vibrational influence of the CD bond and the para‐effect induced by deuterium. In this respect, deuterium behaves as a real substituent with electronic properties different from those of hydrogen. The deuterium isotope effects on 13C NMR chemical shifts and carbon–deuterium coupling constants have also been determined.