Methylene triplet

Examine the highest-occupied molecular orbital (HOMO) of singlet methylene. Where is the pair of electrons, inplane or perpendicular to the plane Next, examine the electrostatic potential map. Where is the molecule most electron rich, in the o or the 7t system Where is the most electron poor Next, display the corresponding map for triplet methylene. Which molecule would you expect to be the better nucleophile The better electrophile Explain. Experimentally, one state of methylene shows both electrophilic and nucleophilic chemistry, while the other state exhibits chemistry typical of radicals. Which state does which Elaborate. 

Fig. 5. Electron-impact efficiency curves at (a) m/e = 41 and m/e = 13, and (b) m/e = 14, recorded at a lab angle of 30° during the CMB study of the reaction 0(3P) + C2H2 at Ec = 12.6kcalmol 1. The thick arrow marks the electron energy (17eV) used for product angular and TOF distribution measurements. In (b) the thin arrow marks the literature value50 of the IE of triplet methylene (see text).

Fig. 11. Center-of-mass translational energy distribution of the CH2 + H product channel of CH3 photodissociation at 193 nm. Arrows indicate the thermodynamic maximum available energies for formation of singlet and triplet methylene. (From North et al.112)...

Fig. 12. Internal energy spectrum of the CH2 fragment from photolysis of the CH3 radical at 216.3 nm. The combs above the figure indicate the expected TOFs of H atoms formed, in association with triplet methylene CH2(X3Bi) or singlet CH2( i1Ai) respectively as a function of V2, the vibrational quantum number for their respective bending mode. (From Wilson et al,113)...

Similarly, the direct [Eq. (11.24)] and photosensitized [Eq. (11.25)] decomposition of diazomethane in the presence of cyclohexene yielded product distributions indicative of greater selectivity in the triplet methylene addition ... 

Methylene insertion into C—H bonds is believed to be concerted for the singlet species and stepwise for the triplet.<164,156) The C—H insertion of methylene into the 14C-labeled isobutylene shown below results in 92% unrearranged isopentenes and 8% rearranged isopentene [Eq. (11.22)]. Assuming that an additional 8% of the unrearranged isopentene arises from the stepwise addition, it is clear that 84% of the insertion products result from insertion by singlet methylene and 16% by triplet methylene ... 

Additional evidence that photolysis of diazomethane leads to singlet methylene was obtained by Kopecky, Hammond, and Leermakers.(21) These workers observed that triplet methylene produced by energy transfer de-... 

Phenylcarbene (la). Just as in triplet methylene (CH2), in triplet phenylcarbene (3A"-la) one electron occupies the p-jr atomic orbital on the carbene carbon and one electron occupies the in-plane a hybrid orbital. However, in the lowest singlet state of CH2 and of phenylcarbene ( A -la), both electrons occupy the hybrid a orbital, because this orbital is substantially lower in energy than the p-jt AO. 

Early spectroscopic experiments on the structure of methylene were interpreted to show that the 3Bj state was linear or nearly linear (Herzberg, 1961). However, theoretical calculations and eventually observation of the epr spectrum of triplet methylene (Bernheim et al., 1970 Wasserman et al., 1970) led to a reinterpretation of the earlier experiments (Herzberg and Johns, 1971). Now there is universal agreement that the 3Bj state of methylene is bent with a bond angle (0) of approximately 135° (see Fig. 1.)... 

The values obtained were >=0.6844 cm i and E =0.0034 cm. A species with a greater motional freedom was assigned to the values of D =0.6634 cm and <0.002. From these data an HCH angle of 136° was deduced, in excellent agreement with the latest calculations. However, the values of Shell which are practically identical with the above, were interpreted differently. Similiar data obtained for HCD and DCD are additional evidence that the bond angle of triplet methylene is in fact 136°. 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

However, coUisional deactivation in solution is so effective that no vibration-ally excited species is present. The reaction of photochemicaUy generated methylene with 2-methylpropene-l-)- C yields, 2-methyl-butene, which is formed by allylic insertion. In the liquid phase 2 % of the rearranged product labeled in the 3-position are formed, whereas in the gas phase 8% of this olefin can be isolated. This can be interpreted as follows 4% of 2-methyl-butene in solution and 16% of 2-methyl-butene in the gas phase are formed by an abstraction-recombination mechanism involving triplet methylene 96). 

Photolysis studies of diazomethane/isopentane mixtures in the presence and absence of oxygen, gave a calculated figure of 15—20% triplet methylene 97). 

PhotochemicaUy generated triplet methylene (from CH2N 2) abstracts hydrogen from ethers almost exclusively at the a-position. The insertion/abstraction ratio was used to calculate the percentage of triplet methylene it was about 10%. 

Fluorinated hydrocarbons favoring intersystem crossing by dilution increase the proportion of triplet methylene up to about 56%... 

Sensitization, which can populate the triplet manifold, was used in a number of instances. Sensitization with benzophenone was used in the photolysis of diazomethane to generate triplet methylene. The triplet methylene thus produced, however, failed to abstract much hydrogen from alkanes (cyclohexene), but... 

Hoffmann was the first to apply the concept of orbital symmetry to the cycloaddition of carbenes to olefins. This concept, which is based on EH-calculations was demonstrated for the [H-2]-cyclo-addition of triplet methylene to ethylene. 

This technique has been repeatedly utilized. Photolysis of diazomethane in the liquid phase yields methylene in the singlet state, as is shown by the practically stereospecific addition to cis- or trans-butene. Dilution with perfiuoropropane reduces the degree of stereospecifity as well as the amount of C—H-insertion, indicating that triplet methylene is involved A similar effect has been reported for CF3CH which, on dilution with perfluoro-diethylether (in the gas phase), adds in a non-stereospecific manner owing to the presence of the triplet... 

In the photosensitized reaction of diazomethane which jdelds triplet methylene, a loss of stereospedfity is observed. However, with trans-2-butene cyclo-addition occmrs only to a limited extent. 

It was demonstrated in the previous section that calculated IR spectra are able to reproduce the experimental spectra of highly reactive molecules to the point that they are extremely useful to the experimentalist in confirming the synthesis of such species. As will be seen in this section, the same holds true for reactive intermediates. We undertook the calculation of IR spectra of both singlet and triplet methylene in part to test the reliability of computed ab initio spectra of reactive intermediates such as carbenes. Among the known carbene intermediates, methylene is perhaps the most studied. It represents the simplest carbene, consisting... 

At long distances the first excited A state, on the other hand, is a combination of triplet ethylene and ground state (triplet) methylene. The important tableaux in the wave function are shown in Table 16.4, and in this case the principal tableau of... 

Triplet methylene is known to be bent with a bond angle of approximately 136°. This is closely reproduced by all Hartree-Fock models (except for STO-3G which yields a bond angle approximately 10° too small), as well as local density models, BP, BLYP, EDFl and B3LYP density functional models and MP2 models. Semi-empirical models also suggest a bent structure, but with an HCH angle which is much too large. 

All three semi-empirical models yield similar results, indicating that triplet methylene is much more stable than the corresponding singlet than is actually the case. 

The origin of the difference lies in the fact that triplet carbenes are biradicals (or diradicals) and exhibit chemistry similar to that exhibited by radicals, while singlet carbenes incorporate both nucleophilic and electrophilic sites, e.g., for singlet and triplet methylene. 

Figure 9.3. Change in the relative energy of singlet and triplet methylene with respect to

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