Singlet and Triplet Methylene

Methylene (CH2) has six valence electrons. Four are needed for the two CH bonds. Possibilities for the other two include  

The first two arrangements are singlet states (all electrons are paired), while the last is a triplet state (with two unpaired electrons). Experimentally, the ground state of methylene is a triplet, although much of methylene s chemistry (and that of substituted methylenes) is due to the singlet state. 

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. 

Electrostatic potential map for singlet methylene shows negatively-charged regions (in red) and positively-charged regions (in blue). 

HOMO of singlet methylene shows location of the molecule s highest energy pair of electrons. 

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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)...

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... 

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

On the other hand, in the reaction of methylene with CCI4, a considerable fraction of CI3CCH2CI NMR signals is found to be polarized both in direct and triplet-sensitized photolysis. It is deduced that both singlet and triplet methylene appear capable of abstracting chlorine atom from CCI4 (Eqs. 20 and 21). 

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. " ... 

It has been argued128-124 that singlet methylene adds directly to the carbon double bond to form a cyclopropane ring by a three-center mechanism involving essentially simultaneous formation of two carbon-carbon bonds. We believe, however, that the detailed mechanism must include formation of a short lived diradical as the initial step for both singlet and triplet methylene. The diradical may undergo ring closure or struc-... 

In subsequent work, Bayes (9) found that formation of allene was suppressed by NO and O2, well-known triplet scavengers, at wavelengths above 260 nm. No suppression was observed below 260 nm. From this observation he concluded that C2O in a low-lying singlet state was the photodecomposition product below 260 nm, but that triplet C2O was formed above this wavelength. This behavior with 2 and NO is similar to that found with singlet and triplet methylene. The relative Importance of the two channels of reaction 1 which lead to singlet or triplet C2O is in some doubt near 260 nm, however, since Cundall et al. (13) found that butene-2 suppressed the major part of processes which yield CO at 253.7 nm while Bayes found that 2 and NO had little effect at 254 nm on allene formation. 

Figure 2. Molecular orbitals of singlet and triplet methylene.

At short wavelengths, the methylene formed is almost entirely in its singlet state, whereas at long wavelengths the photolysis of ketene produces predominantly triplet methylene (methods for the estimation of singlet and triplet methylene fractions are discussed on p. 393). The intervention of a triplet state of ketene is indicated by quenching experiments with biacetyP and, more conclusively , ci5-butene-2. 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole" ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures", which are thought to ensure complete collisional deactivation of excited reaction products. 

Relative rates of reaction of a number of olefins with singlet and triplet methylene have been determined in the gas phase Singlet methylene was generated by direct photolysis of ketene at 2600 A and the triplet by mercury-photosensitized decomposition of ketene. The evaluation of relative... 

Singlet and triplet methylene have been observed in the gas phase and their eleetronie speetra recorded. [10] Unfortunately, gas phase speetroseopy of ear-benes is limited to very simple species. Arylcarbenes have never been speetros-eopieally deteeted in the gas phase despite numerous attempts. This is unfortunate beeause high resolution gas phase speetroseopy could, in principle, yield important struetural information about these molecules. 

For a summary of the physical properties of singlet and triplet methylene, see W.T. Borden and E.R. Davidson Ann. Revs. Phys. Chem. 30, 128ff. (1979). 

According to [527] an excited singlet oxygen atom readily inserts into the C—H bond (in contrast to 0( P) the addition of which requires violation of the spin conservation rule). The same difference is observed for singlet and triplet methylene [36]. The insertion of methylene into C—H bonds can be considered to be reliably established. 

Reuter W, Engels B, Peyerimhoff SD (1992) Reaction of singlet and triplet methylene with ethene - a multireference configuration-interaction study. J Phys Chem-Us 96 6221-6232... 

The electronic spectroscopy of ketene is of particular interest because the upper state is photochemically active, leading to the formation of singlet and triplet methylene. There has been much controversy over the wavelength dependence of the relative yields of the two species of methylene, which to this day has not been satisfactorily resolved. For such a relatively simple molecule, the difficulties in unraveling its spectroscopy continue to be both surprising and challenging. 

The first direct experimental measurements of the energy difference between the equilibrium geometries of singlet and triplet methylene (7e = 19.5 kcal mol" )contradicted both theory T = 11 2 kcal mol" ) and indirect experiments (Te = 8.5 0.9 kcal mol" ). However, it turned out that the 19.5 kcal mol" estimate was due to hot bands. " McKellar et al. published the most accurate determination of the singlet-triplet splitting of methylene to date ... 

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