Nitrosomethane

In the presence of potassium fluoride in dimethylformamide, trifluoroacetyl nitrite converts perfluoroisobutylene to tris(trifluoromethyl)nitrosomethane via the carbanion generated by the nucleophilic attack of fluoride ion on perfluoro-isobutylene [5] (equation 5)... 

Spectroscopic Data. Freshly prepd nitrosomethane has an IR absorption band at 6.3—6.4 p which slowly decays and is replaced by a band at 11 y characteristic of formaldehyde oxime (Ref 8). The visible absorption Amax is 287my in eth and at 266—7m p in w (Ref 4)... 

The concoitradon of CHjNO decreased monotonically with temperature, but even at 800 °C a small concentration could be detected. The decrease could be due to unimolecular reactions, to biomolecular reactions with either CH,- or NO, or to a reverse reaction, CH3NO - CHj + NO. It is reasonable to assume that similar gas phase reactions would occur at much higher pressures however, we have not detected CH3NO during our catalytic experiments over the Sr/LajOj catalyst. Perhaps at much larger concentrations of NO the reaction CHjNO + NO - N O + CHjO- rapidly removes the nitrosomethane. 

Peril uoro-teri-nitrosobutanc (Tris(trifluoromethyl)nitrosomethane)... 

For the first time, the primary nitrone (formaldonitrone) generation and the comparative quantum chemical analysis of its relative stability by comparison with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described (357). Both, experimental and theoretical data clearly show that the formal-donitrones, formed in the course of collision by electronic transfer, can hardly be molecularly isomerized into other [C,H3,N,0] molecules. Methods of quantum chemistry and molecular dynamics have made it possible to study the reactions of nitrone rearrangement into amides through the formation of oxaziridines (358). 

The Hel PE spectrum of nitrosomethane (Figure 13) was first studied by Bergmann and Bock125,127. This compound as well as several other aliphatic and aromatic C-nitroso compounds were investigated by Pfab and coworkers126,128, however several of them were dimers. 

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

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

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 n+ and it ionization energies are lowered by 1.8 and 1.7 eV, respectively. 

C-nitroso compounds exhibiting an a-H, such as nitrosomethane and nitrosoethane, may isomerize to the corresponding oxime, as evidenced by mass spectrometry116,117. Unsaturated C-nitroso compounds may undergo electrocyclic ring closure as discussed below. 

The authors of primary Reference 80 present their own and selected literature values for the R—NO bond enthalpies for the hydrocarbyl cases of Me, Et, t-Bu, allyl and benzyl, as well as mixed fluorinated, chlorinated methyl radicals. We now wish to compare nitroso species with the corresponding amino and nitro compounds. Choosing what we consider the most reliable and relevant nitroso compound data, and accompanying them with the corresponding radical data, we derive enthalpies of formation of gaseous nitrosomethane, 2-methyl-2-nitrosopropane and o -nitrosotoluene81 to be 65 2, —29 4 and 174 7 kJmol-1. (By comparison, the earlier values recommended8 for nitrosomethane and 2-methyl-2-nitrosopropane were 70 and —42 kJmol-1 respectively.)... 

The photoelectron spectra of nitrosomethane, 2-methyl-2-nitrosopropane and perhalogeno nitro someth anes has been re-examined and re-assigned on the basis of ab initio SCF-CI calculations139. Photoionization quantum yields140 have been measured for 2-methyl-2-nitrosopropane at wavelengths 147, 123, 105 and 107 nm. The results show that photoionization at energies up to 1.5 eV above threshold is of low probability. The data have been compared with those of recent photoelectron spectroscopy. 

The quintessential C-nitroso compound nitrosomethane, CH3-NO (1), was first isolated by Coe and Doumani in 1948 while already in 1936 Staveley and Hinshelwood found that addition of small quantities of nitric oxide to the reaction vessel during the... 

SCHEME 1. Isomers of nitrosomethane. Besides 4 and 5 deprotonation of 1, 2 and 3 would lead to the formation of a nitrosomethanide, while in case of 4 and 5 only an isomerization process following the deprotonation can give nitrosomethanide... 

While highly reactive formaldonitrone (3) was detected only recently, both the nitrosomethane (1) and the formaldehyde oxime (2) are also reactive compounds in the condensed phase 2 undergoes facile oligomerization , whereas 1 forms a dimer and isomerizes to 2 . Deprotonation of any of these isomers (1, 2, 3) would result in the corresponding simplest nitrosomethanide anion [H2C=N—O]. ... 

Nitrosomethane (1) is known to be less stable than its isomer formaldoxime 2 and original attempts to isolate this species failed owing to its facile isomerization to the oxime 2. Already Bamberger and Seligman considered in 1903 that it would be difficult to isolate nitrosomethane after oxidation of methylamine due to its rapid isomerization to 2. Hence, 2 is always present in the synthesis of the nitrosomethane. Nitrosomethane is produced in the pyrolysis or photolysis of tcrf-butyl nitrite and by the reaction of methyl radicals with nitric oxide. Early results were confusing since the final product obtained is dimeric nitrosomethane. It was first isolated in 1948 by Coe and Doumani from the photolysis of gaseous ferf-butyl nitrite according to the overall reaction shown in equation 2. 

Batt, Gowenlock and Trotman carried out a detailed study of the pyrolysis and photolysis of terf-butyl nitrite and established that dimeric nitrosomethane exists in two isomeric forms, cis and trans (Scheme 5). Monomeric nitrosomethane could be generated by heating the dimer in the gas phase (the activation energy for dissociation was found to be ca 90 kJmoH )". Also ultraviolet irradiation dissociates the dimer, leaving monomeric 1. Vibrational analysis of monomeric 1 is summarized in Table 4. 

TABLE 4. Experimental and calculated vibrational frequencies (cm ) of monomeric nitrosomethane, IR intensities in parentheses (kmmoR )... 

For the photolysis of ferf-butyl nitrite a possible reaction mechanism (Scheme 6) consists of the production of ferf-butoxy radicals (equation 3), followed by their decomposition to give acetone and methyl radicals (equation 4). The latter are trapped by the nitric oxide liberated in the first step (equation 5). However, the absence of ethane production in the actual experiments suggested that an intramolecular formation of nitrosomethane is unlikely ". ... 

SCHEME 6. Possible reaction mechanism for the formation of nitrosomethane in a free radical reaction... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

Since formaldoxime 2 is stable in the gas phase it is more easily studied experimentally than nitrosomethane. Experimental structural data are only available for formaldoxime 2... 

The dimer of nitrosomethane (azo-dioxide), (CHsNO) , exists at ambient temperatures in two isomeric forms, cis and trans (Figure 8), which undergo isomerization relatively easily. rrawi-(CH3—NO)2, having a planar azo-dioxide moiety, crystallizes in the orthorhombic space group Cmcm with Z = 4. The experimentally determined bond lengths are rf(N—O) = 1.25 and rf(N—C) = 1.57 A, respectively (calculated data, see Figure 8). ... 

FIGURE 9. Resonance scheme for dimeric trans nitrosomethane... 

---