Nitrogen ionization potentials

The axial C—H bonds are weaker flian the equatorial C—H bonds as can be demonstrated by a strongly shifted C—H stretching frequency in the IR spectrum. Axial C-2 and C-6 methyl groins lower the ionization potential of the lone-pair electrons on nitrogen substantially more than do equatorial C-2 or C-6 methyl groups. Ehscuss the relationship between these observations and provide a rationalization in terms of qualitative MO theory. 

The ionization potential of lone pair AO s is easily obtained by PES35. Consequently, this method can be a delicate probe of n—a conjugative interactions. For example, consider the ionization energy of the nitrogen lone pair AO as a function of the substituent X in the model system shown below ... 

Experimental support for the above analysis is found in 2 substituted pyridines where X=CH3 and X=Si(CH3)3. Specifically, the ionization potential of the nitrogen lone pair in the former case is 9—lOeV and 8.5eV in the latter355. ... 

It is well known that a change in the H—N—H valence angle of NH3 is important for the energy of the mn orbital. In the transition from the pyramidal to planar conformation, this orbital destabilizes appreciably with decreasing contribution of the nitrogen 2s orbital. This is also reflected in the very low ionization potentials of planar amines (see below). 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

Higher ionization potentials in amines The large difference in ionization potential between the nitrogen lone pair and the electrons of alkyl groupings has made it comparatively easy... 

Other experimental evidence leads to essentially the same conclusion regarding the n ionization of pyridine. El Sayed and Kasha (1961) have detected Rydberg series in the absorption spectrum similar to those in benzene and ascribable to n orbitals (9-266 e.v., 02 11-56 e.v., 62) and, in addition, reported a fragmentary series leading to a third ionization potential of 10-3 e.v. which they ascribed to the nitrogen lone pair. Similar values are found by photoelectron spectroscopy which also indicated the 10-3 e.v. (10-54 e.v.) level to be only weakly bonding. 

An upsurge of interest in the N-methylborazines in the early 1970 s was coupled with a convenient method of synthesis and purification for these compounds The photoelectron spectrum of N-trimethylborazine has been reported. Table 6 summarizes the theoretical and experimental data comparing the location of the molecular orbitals of N-trimethylborazine with those of borazine. The HOMO is predicted and observed to be an e" (w) orbital as in borazine The methyl substitution on nitrogen destabilizes the e" and the a2 jr-orbitals, but does not signiBcantly effect the e (a) orbital. The result is a lowering of the ionization potential for electrons in the two TT-orbitals. This effect, predicted in the dieoretical calculations, was also verified experimentally. 

This last path has experimental support in the case of the reaction with acridine in the isolation of 9-acyl-9,10-dihydro derivatives. This behavior can be correlated with the fact that the protonation of the heterocyclic nitrogen and the presence of an electron-withdrawing group (R-CO) causes a relatively high ionization potential of the o-complex... 

The comparison with adiabatic ionization potentials [147] indicates that the latter decrease as electronic charge builds up on nitrogen, as one would normally expect, thus suggesting that the charges calculated here are in the right order. (These ionization potentials correspond to the suppression of an electron of the lone pair on... 

Figure 5 Range distribution in nitrogen for 200-eV electrons obtained via the convolution method at 110 eV [38]. The final energy is about twice the ionization potential. See text for details.

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