engleski

Deuterium Isotope Effects on C-13 NMR Chemical Shifts in Unprotonated and Protonated trans-Azobenzene

Differently deuteriated trans-azobenzenes (tAB) were prepared and studied by C-13 NMR spectroscopy. A number of long-range deuterium isotope effects (LRDIE) on C-13 chemical shifts was observed. At the 4' (para') position the LRDIE through ten bonds in 4-D-tAB ((10)D = 3.3 ppb) and the cumulative or total effect in (D_5)-tAB (tD = 11.1 ppb) were determined. These effects are of similar magnitude and extent as those observed in isoelectronic trans-stilbene and cis-stilbene, although in solution trans-azobenzene and trans-stilbene have nearly planar conformation, while cis-stilbene is highly distorted. The protonation of trans-azobenzene was performed using CF3COOH and spectra were measured in C_6D_6 solution. The most striking change in protonated tAB is a shielding effect at C-1,1' by 8.7 ppm (from 153.5 to 144.8 ppm). The parallel behaviour on protonation was also found for deuterium isotope effect at the quarternary carbons (C-1,1') in (D_10)-tAB. This effect is by 27 ppb greater in the protonated than in unprotonated species (83.5 ppb), which means a ca. 30% increase of the effect on protonation. The isotope effect at C-4 is also greater, by ca. 10%, while effects at C-2,6 and C-3,5 are lower by ca.15-20%. The changes in magnitude, sign and extent of isotope effects in unprotonated and protonated trans-azobenzenes depend on pi-electron delocalization, which is in the case of protonated species disturbed by protonation. The importance of steric effects and lone-pair interactions in transmission of isotope effects is also discussed.

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