STRUCTURAL DEPENDENCE OF ISOTOPE EFFECTS IN H-1 AND C-13 NUCLEAR-MAGNETIC-RESONANCE SPECTRA OF THE TRANS-N-BENZYLIDENEANILINE IMINO GROUP

Deuterium-and N-15-induced isotope effects on H-1 and C-13 chemical sh ifts of the imino moiety of 16 trans-N-benzylideneaniline (tBA) isotop omers were determined and analysed. These effects appear to be a sensi tive probe of molecular conformation and its changes. All determined d euterium isotope effects on the imino-proton are small and negative an d 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 ne gative and have much larger values. Negative sign of isotope effects i s primarily governed by lone-pairs, while magnitude and non-additivity depend on the site of isotopic substitution, molecular conformation a nd distribution of rotamer populations. All N-15 isotope effects on bo th 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.