Inversion barrier, double nitrogen

It is interesting to compare tert-butylbenzyl methyl amine (35) with the tetrahedral intermediate 36 derived from N-benzyl-N-methylacetamide which has a similar degree of substitution. The rotation barrier for the (CH3)3C — N bond and the nitrogen inversion barrier in 35 have been found identical and estimated at 6.2 kcal/mol (30). The higher value of 8.0 kcal/mol for the intermediate 36 must be a consequence of the double-bond character of the C —N bond (nitrogen atom has one secondary electronic effect (n-o )). 

In aromatic amines, the partial double bond character of the C-N bond will tend to enhance the rotation barrier, while the inversion is found to be reduced, as compared to aliphatic amines. For example, the rotational barrier in IV-methylaniline is just above 7 kcal/mol, while the nitrogen inversion barrier is 1.6 kcal/mol [88]. 

The potential energy curve for the nitrogen inversion process in NX 3 is a symmetrical double minimum curve with an energy barrier Fmax (Fig. 1). 

Before discussing structural effects on barrier heights, it is necessary to distinguish between the two processes planar nitrogen inversion and rotation about the C=N bond, which may both lead to interconversion of the isomers. Rotation about a non-activated C=N double bond is expected to be hindered by an energy barrier similar to the ethylene barrier, i.e. of the order of 50—60 kcal/mole 121.132) Thus, alkyl- and presumably also aryl-substituted imines, which show interconversion barriers below 30 kcal/mole (Table 6), undergo nitrogen inversion ). 

In the case of the carbon-nitrogen double bond, the reaction of isomerization can occur via rotation about the double bond, and by nitrogen inversion (XLVII). The latter process as a rule is strongly favored over rotation, resulting in a lowered barrier of overall isomerization as compared to ethylenes. For many imines (XLVII, c = H), this barrier is in the range 20-30 kcal/mol. Electronegative substituents on the nitrogen atom increase stability toward inversion, as evidenced by the relative stability of oximes (XLVII, c = OH) and hydrazones (XLVII, c = NRR ) [56]. 

The lone pair on trivalent nitrogen or carbon readily conjugates with substituents able to accept electrons. Thus N-phenylaziridines have lower inversion barriers than N-alkylaziridines, and attached double bonds can render carbon sp2 (see 1 and 2). In these... 

Introduction of a second adsorbed ammonia molecule in the neighboring position generates a weak potential barrier in this rotation (broken line in Fig. 7b), which decreases with increase of the distance between the two adsorbed species. The most influenced intramolecular vibration after the adsorption is the umbrella, connected with molecular inversion of the three N-H bonds in gas-phase ammonia. Stronger attraction of the nitrogen atom to the siuface and repulsion of the hydrogens converts the symmetric double-well inversion potential of the gas molecule into a distorted single well (Fig. 8). This change in the potential ciuve modifies the distances between the vibrational levels for this mode. 

Inversion of planar nitrogen sites also occurs in a double minimum potential and thus the above discussion applies as well. However, barriers... 

Isomerization at a nitrogen-nitrogen double bond (azo derivatives XLVIII) occurs by inversion at one of the nitrogen atoms. The observed isomerization barrier of azobenzene (XLVIII, a = c = phenyl) is about 23 kcal/mol, as compared with 18 kcal/mol for the analogous imine (XLVII, a = c = phenyl, b = H) and with 43 kcal/mol for diphenylethylene (see above) [56]. 

Other applications are studies of rotation about double bonds 136>, ring inversion in cyclooctatetraene 137>, the insertion of carbon into ethylene and. fraws-2-butene to give allenes 133), the barrier height to inversion of nitrogen in hydrazine and alkylamines 133>, the Cope... 

In aqueous solutions, protonation of one of the nitrogen atoms — preferably at the N = N double bond — causes a considerable reduction in the activation barrier. Inversely, hydroxide ions decrease the reaction rate. To study thermal Z—> isomerization rates of unprotonated azobenzenes in aqueous solutions, the pH of the solution should be clearly above the threshold where protonation influences the rate. The dependence of Iq of 4-(phenylamino)-4 -nitroazobenzene on H5O concentration in acetone/ water (1 1, v/v) mixtures corresponds to a titration curve (see Figure 89.6). - The flash-photolytic preparation of thermally unstable Z-isomers offers the interesting possibility of determining their pKj-data by means of kinetic experiments. 

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