Triplet methylene, detection

Although singlet and triplet imidogen have similar absorption spectra, singlet and triplet methylene do not. In fact, most carbenes have rather poor chromo-phores for UV-vis detection by LEP and must be visualized by trapping with pyridine to form ylides. The exceptions are arylcarbenes, which have n n transitions localized on the aromatic n system. " ... 

The long-awaited ESR detection of the parent triplet methylene, CH2, was recently accomplished by Bernheim,... 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

The idea of bent triplet carbenes has been strengthened by the detection of the two possible geometric isomers of bent naphthyl-methylenes.41 When a-naphthyldiazomethane was irradiated at 77°K, the resulting ESR spectrum revealed two separate triplets with somewhat different D values. 

When an alkene molecule loses an electron, a cation radical is formed. The very reactive cation radical (CH3)2C—CHJ is generated from 2-methyl-propene in light in the presence of TiCl4. It can be detected by ESR in the frozen parent compound at 123 K [172], We assume that at higher temperatures these formations are dimerized to dications. The existence of a donor-acceptor complex is a necessary condition for the mechanism generating cation radicals (see Chap. 3, Sect. 5). a-Methylstyrene is cationically polymerized when illuminated in the presence of tetracyanobenzene in methylene chloride. From the two compounds, of which a-methylstyrene is the donor (D) and tetracyanobenzene the acceptor (A), the donor-acceptor complex is generated in the singlet and triplet states it dissociates to solvated ion radicals [173]... 

In a second similar experiment 1Z2) the benzyl radical, PhCHs could be detected after the photodissociation of PhCHjCl, adsorbed on silica gel, under experimental conditions similar to the previous case. The EPR spectrum of this radical in the adsorbed state is more complicated (Fig. 20b) consisting of a triplet, with components split additionally into quadruplets. The spectrum could be satisfactorily interpreted by a delocalization of the lone electron of the methylene group, which is partly injected into the benzene ring. 

At slightly higher energies there exists a singlet state. Whereas singlet methylene can be detected by LIF, only the triplet state responds to LMR. Hence, it will be evident that the detection techniques are often complementary, and that no one technique is ideal for all transients. The reaction of 3CH2 with ethene,... 

H-MA-MMA at 90°C, along with its simulation, is shown in Fig. 23. A 12-Une spectrum with a very small quartet in each spectroscopic line was detected. The 12-line spectrum is caused by a quartet of triplets (4 X 3) from three equivalent methyl protons and two methylene protons. An additional very small splitting of a doublet (0.80 G) due to the y-proton in the MA moiety can also be observed. The signal intensity of each spectroscopic line displays a temperature dependent change due to hindered rotation around the Ca- Cp bond. At 90°C, the bond rotates freely and the simulated spectrum is shown in Fig. 23. The ESR spectra of such model dimeric radicals, with well-defined structures, have not been observed before. Analyses of the values obtained from the hyperfme coupling constants clearly show the dimeric radicals have the stractures indicated in Fig. 22. 

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