1415of the constituent diabatic surfaces as well as that of the curve of intersection can be easily rationalized by using simple M O energy expressions.
IV. ConclusionIn this paper we have analyzed the cycloaddition of two ethylene molecules in terms of diabatic surfaces. We have shown that this analysis provides a clear understanding of the index and origin of the various critical points. We have also shown that the behavior Registry No. Ethylene, 74-85-1.Abstract High field deuterium NMR provides a new and simple method for establishing the degree of aromaticity of a compound. The procedure is based on the determination of the magnetic susceptibility anisotropy from quadrupolar deuterium couplings of molecules in solution aligned by the magnetic field. The technique is illustrated for some deuterated dialkylbenzenes (Zd, 3e, 4e) and applied to [8,1 1-2H2] [5]metacyclophane (Id). Surprisingly, Id is found to be fully aromatic in spite of its strongly bent benzene ring.
Lme positions in high-resolution NMR spectra may be affected by dipolar couplings and anisotropic shift contributions, which arise from the partial orientation of the molecules by the magnetic field of the spectrometer. It is shown that, depending on the system studied, these effects can be larger than 0.01 Hz at moderate fields (ca 7 X lo4 G), and should therefore be considered when claiming accuracies of 10 mHz or better.
In order to study the aromatic ring currents in triptycene and tribenzotriptycene, the magnetic susceptibility anisotropies of these molecules have been determined by the NMR analogue of the Cotton-Mouton effect: AX(triptycene) = 24.0 X m3 (1.91 X lo-= emu), AX(tribenz0triptycene) =44.5 X m3 (3.54 X lo-= emu).These values are well predicted from a model of independent fragment molecules, showing that the ring currents in the rings of triptycene and tribenzotriptycene are essentially the same as in planar aromatic molecules.
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