Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups / 0159 204

FIGURE 13. PE spectra of nitrosomethane (b) and its cis-dimer (a) displaying the temperaturedependent decomposition of cis-(MeNO)2 into MeNO. Reproduced (modified) with permission from Reference 125

TABLE 12. Ionization potentials IP (eV) and orbital energies ε (eV) of nitrosomethane

a CNDO127. bAb initio126. c AM126.

d OVGF(AM1)26.

FIGURE 14. MOs of nitrosomethane (AM1 results)

In the second band of monomeric nitrosomethane two vertical IPs (13.7 and 14.1 eV, Table 12) can be determined, and according to Koopmans’ theorem17 they should be assigned to (2a00 ) and n (10a0), respectively126. However, the inverse assignment, implying a breakdown of Koopmans’ theorem, has been established by comparison with the spectra of isoelectronic molecules and with ab initio calculations for MeNO and the corresponding radical cations126. The incipient vibrational structure on the high-energy side of this composite band supports this conclusion. As is indicated by the data displayed in Table 12, this assignment is also supported by AM1 and OVGF(AM1) calculations.

In Table 13 the ionization potentials of some more C-nitroso compounds are collected. The spectrum of monomeric t-nitrosobutane126 exhibits a well separated band at 9.05 eV. The following ionizations show maxima at 11.85 and 12.46 eV. The spectrum is dominated by a strong composite band from 12.9 14.5 eV. The spectrum can be assigned by comparison with nitrosomethane. The substitution of Me by t-Bu lowers the n ionization energy of the nitroso group by 0.7 eV, whereas the nC and ionization energies are lowered by 1.8 and 1.7 eV, respectively.

In the perhalogenonitrosomethanes126, which do not form dimers, the first IP is again assigned to n , which is stabilized relative to that of MeNO proportional to the electronegativity of the halogen substituent and, as expected, the largest shift is observed for the trifluoro derivative. The next bands following at higher energies can all be assigned to halogen lone-pair ionizations126. Higher ionizations at 14.5 17.1 and 16.1 18.4 eV are assigned to IP(nC ) and IP( ), respectively.

The electronic structure of aliphatic C-nitroso compounds is thus characterized by unusually strong lone-pair interactions in the nitroso group giving rise to n /nC splittings up to 6 eV, a high-lying antibonding MO (n ) followed by strongly bonding and closely adjacent NDO and nC MOs.

PE spectroscopic studies on nitrosoalkanes at variable temperatures were performed by Bergmann and coworkers125 and by Pfab and coworkers126. Below about 65 °C spectra of the two isomeric dimers of MeNO are recorded, from 70 °C upwards the cis dimer is accompanied by rearrangement to the trans isomer supporting direct cis trans isomerization (Figure 13). Above 85 °C only the monomer is observed in the spectrum125. The PE spectrum of monomeric t-BuNO was scanned at temperatures above 150 °C. At lower temperatures the spectrum of the trans dimer could be recorded and began to change above 120 °C126. To our knowledge, the spectra of C-nitroso dimers were not analysed in detail.

B. Nitrosobenzene and Related Compounds

The PE spectrum of nitrosobenzene has been investigated by Rabalais and Colton129 and by Green and coworkers128. Unfortunately, the published IP values are considerably at variance. From the characteristic IPs of C-nitroso compounds only IP n D 8.51 eV could be assigned as the first IP. IP2 and IP3 were identified as ionizations of electrons from the benzene MOs 2 and 3 (Table 13).

Some IP data have been published for p-substituted nitrosobenzenes with substituents like Me, Cl, NMe2 and OMe128. Four IP(n ) values between 7.78 (4-nitroso-N,N- dimethylaniline, 65) and 9.02 eV (4-chloronitrosobenzene, 66) were observed. The corresponding value of 1-methyl-3-nitroso-2-phenylindole (67) was found to be 7.50 eV130.

N O

N

Me

anO of morpholine oxygen atom.

C.Miscellaneous Nitroso Compounds

Several other nitroso compounds have been investigated by PES. The nitrosyl halides125, Hal-NO, are basic inorganic compounds. Of particular interest for organic chemists are nitrosamines, R2N NO, and nitrites, RO NO. The NO group occurs as a ligand in transition metal complexes and PE spectra of complexes of chromium, manganese, iron, cobalt and nickel have been measured131.

The PE spectrum of methylnitrite has been investigated by Bergmann and Bock127. Some alkyl nitrites have been studied as precursors of radicals which are generated by thermolysis from the former compounds132.

The main feature in the PE spectra of aliphatic nitrosamines45,133 138 are two very distinctive bands in the region 8.5 10.0 eV which are assigned to a -type orbital 2 mainly localized on the amino nitrogen atom and an nNO orbital. IPs of some nitrosamines are summarized in Table 14.

IV. NITRO COMPOUNDS

The nitro group is of high importance in organic chemistry, in particular in aromatic compounds because of its strong electron acceptor capacity. In accordance with this property, the nitro group has low-lying occupied and unoccupied orbitals, and the characteristic IPs of nitro compounds are usually found higher than 10 eV, which may lead to problems in the analysis of PE spectra.

The PE spectra of the two basic organic nitro compounds, nitromethane and nitrobenzene, were first analysed by Rabalais139. These and other nitro compounds have been the

194 Paul Rademacher

The orbitals 2b1 ( 1) and 1a2 ( 2) are the totally bonding and the non-bonding orbital, respectively, of the nitro group. The orbitals 8a1 (nOC ) and 5b2 (nO ) are essentially the in-phase and the out-of-phase combination, respectively, of oxygen lone-pair orbitals; nO has some bonding character, nOC is slightly antibonding and 2 is nonbonding.

In Figure 16 the PE spectrum of nitromethane is depicted. The relevant data are summarized in Table 15.

The PE spectra of nitromethane and other nitroalkanes (Figure 17) present a severe problem, which has been investigated most thoroughly both experimentally and theoretically: There are two overlapping bands between 10 and 13 eV which have to be assigned to three ionizations, namely to the three non-bonding orbitals of the nitro group ( 2, nOC , nO ). A breakdown of Koopmans’ theorem has to be considered as even more probable than in the case of nitrosomethane (Section III.A). Most likely is the assignment given recently by Huang and coworkers143,144, who included HeII and Penning ionization electron spectra.

The first band shows a vibrational spacing of 480 š 70 cm 1 while the second exhibits

a progression of 540 š 40 cm 1, both relating to the symmetric bending of the nitro group139,143. For the first band the adiabatic IP is lower than the vertical IP, but for the second they are the same, indicating appreciable geometrical reorientation in the former case, and little in the latter, upon ionization. The first band is more intense than the second; the intensity ratio was found to be 1.41. These features of band structure and intensities are consistent with the first band being due to the overlap of two components attributable

(a)

FIGURE 16. PE spectrum of nitromethane: (a) full spectrum, (b) left part expanded

(b)

FIGURE 16. (continued)

aAb initio [6-31G]143. bAM126.

c OVGF (AM1)26.

dC2vCs symmetry.

to the removal of electrons from two orbitals with some bonding or anti-bonding character (nOC and nO ), while the second band is associated with ejection of an electron from an essentially non-bonding orbital ( 2).

As indicated by the ab initio results143 (Table 15), the orbitals nOC and nO are almost degenerate. However, the AM1 results show 2 and nOC to be very close in energy, while

for the OVGF(AM1) results this holds for 2 and nO , which is not consistent with the assignments given by Huang and coworkers143. But these results cannot be considered as very significant because of the shortcomings of semi-empirical methods in dealing with lone-pairs on neighbouring atoms.

The HeI and HeII spectra of some simple nitroalkanes are depicted in Figure 17. IP values are collected in Table 16. The assignment of the characteristic IPs is analogous to those of nitromethane (Table 15). Comparison of the data reveals that the orbitals are destabilized in a manner parallel to the electron-donating power of the alkyl groups143. The gap between the first two ionizations remains essentially constant, between 0.41 and 0.44 eV, in the series.

B. Nitrobenzene and Related Compounds

Nitrobenzene has 64 electrons of which 18 are 1s core electrons of the non-hydrogen atoms. In the point group C2v the occupied MOs factorize as 16a1, 2a2, 3b1 and 11b2. A schematic diagram of some MOs is shown in Figure 18.

The orbitals associated mainly with the nitro group are: 1b1 1 , 1a2 2 , 11b2nO and 16a1nOC .

The PE spectrum of nitrobenzene (Figure 19) has been investigated repeat-

edly21,139 142,144.

In Table 17 the relevant data from the most recent investigation144, which is also based on Penning ionization electron spectra, are summarized. The characteristic ionizations of

TABLE 17. Ionization potentials IP (eV) and orbital energies ε (eV) of nitrobenzene

a Ab initio [6-31G]144.

FIGURE 18. Schematic diagram for MOs of nitrobenzene. Reproduced with permission from Reference 144

the nitro group are assigned to the composite band in the region 10.5 12 eV with maxima at 11.1 eV nOC , nO and at 11.23 eV 2 . As expected, the ionizations of the aromatic

orbitals are shifted by 0.6 1.1 eV to higher energies compared to benzene.

The PE spectra of several substituted nitrobenzenes like the nitrotoluenes, fluoro-

nitrobenzenes and dinitrobenzenes have been investigated141. Some data are summarized in Table 18. For the characteristic IPs of the nitro group nOC , nO , 2 one or two ionizations, which are close to those found for unsubstituted nitrobenzene, have been assigned.