Nitrogen inversion energy barrier

In cyclic amines rigidity of the ring and a small angle between the C-N bonds characteristically result in a relatively high energy barrier to inversion of configuration at the nitrogen atom. The effect is most marked for aziridine derivatives, for which the kinetics of inversion processes are conveniently studied by variable-temperature NMR... 

Preliminary studies of nitrogen substituent inversion processes have been reported for several naphthalen-l,4-imine derivatives. The syn and anti invertomers of the A-chloroamine (117) equilibrate in solution to a mixture in proportion 3 2. The process can be followed kinetically by NMR spectroscopy starting from the pure anti compound the inversion is relatively slow ki = 2.6 x 10 sec at 23°), and the free-energy barrier to inversion is as high AF = 23.5 kcal mole ) as values found for inversion in aziridines. (A-Chloroaziridine derivatives, for which the energy barrier is even higher, have also been resolved into diastereoisomeric invertomers. )... 

Although unsynunetrically substituted amines are chiral, the configuration is not stable because of rapid inversion at nitrogen. The activation energy for pyramidal inversion at phosphorus is much higher than at nitrogen, and many optically active phosphines have been prepared. The barrier to inversion is usually in the range of 30-3S kcal/mol so that enantiomerically pure phosphines are stable at room temperature but racemize by inversion at elevated tempeiatuies. Asymmetrically substituted tetracoordinate phosphorus compounds such as phosphonium salts and phosphine oxides are also chiral. Scheme 2.1 includes some examples of chiral phosphorus compounds. 

Adenine as an isolated molecule has no symmetry elements and therefore might mathematically be considered chiral however, as in the case of glycine (Section 1.2.1), this description is not useful in chemistry since the enantiomers differ only by inversion through the weakly pyramidal nitrogen atom of the amine functionality, the main body of the molecule being planar. The inversion corresponds to a low-frequency vibration and a low-energy barrier such that single enantiomers... 

However, non-quaternary nitrogen, although tetrahedral, is not chiral. There is a rapid inversion that converts one enandomer into the other effectively, the lone pair does not maintain its posi-don. The energy barrier to interconversion is about 25 kJ mol , which is sufficiently low that inversion... 

Aromatic cyclic 7r-electron delocalization does indeed stabilize the planar structure with bond equalization (84ZOR897)—the problem is that, in addition to that effect, there may exist some others that may eventually overshadow it. Thus, the foregoing warrants the conclusion that the preference of a planar or nonplanar geometry of heterocycle depends on a number of factors including aromaticity (antiaromaticity), which may not even be the most important. In any case, this factor should not be disregarded if one wishes to obtain a correct overall energy balance. For example, aromaticity is reflected in the values of inversion barriers. Thus, for antiaromatic 2-azirine the nitrogen inversion barrier is, as was mentioned earlier, 37.7 kcal/mol, whereas in the case of its saturated... 

Hie large energy barrier to conversion of the Z to E isomer is spoken of casually with respect to rotation about the N-N bond, but it could equally well be considered an inversion of the nitroso nitrogen. Hie diastereomeric nature of these two forms of unsymraetrically substituted nitrosamines is clearly illustra-... 

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