Singlet state of methylene

An adequate theoretical description of the lowest-lying singlet state of methylene is not trivial owing to the similar energies of the 3a 1 and lb MOs. In linear methylene these orbitals are degenerate and their occupation results in a Ag state with two degenerate components. The electron configuration of the Ag state is then ... 

In contrast, the B state can be described with one CSF, and the HF wavefunction is thus expected to be a reasonable approximation to the correct wavefunction. As a consequence, HF theory treats the triplet and singlet states of methylene in an unbalanced way. The triplet state is described better than the singlet state, resulting in singlet-triplet energy separations which are too large. 

The importance of hyperconjugative effects on A sx was rejected on the basis of various GVB approaches and Mulliken population analyses (see Electronic Wavefunctions Analysis) for methylcarbene. It was concluded that it is not the singlet state stabilization by the methyl group but rather the triplet state destabilization which results in a smaller singlet-triplet separation with respect to methylene. The GVB Mulliken population analyses show that the hybridizations of the a orbitals are almost identical in the singlet states of methylene and methylcarbene. For the triplet states, however, Mulliken population analyses indicate a decrease in p character when... 

Figure 3 Schematic depiction of the wave function for the lowest singlet state of methylene, showing how the presence of the. .. tt ) configuration results in rabbit ear GVB orbitals that partially confine the two electrons of the lone pair to different regions of space...

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. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

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. 

We examine the two lowest singlet states of cyclopropane as one of the CH2 groups is pulled away from the other two. Figure 16.2 shows the basic arrangement of the molecule with the three C atoms in the x-y plane. The C atom on the right is on the j-axis, and Ri is its distance to the midpoint of the other two Cs. R2 is the distance between the two Cs that will become part of ethylene and is the angle out of planarity. We have labeled the C atoms 1, 2, and 3 to identify the three different methylenes for designating orbitals. 

On this basis a substantial number of not wholly conclusive attempts have been made to treat methylene theoretically. Both VB and MO approximations have been used, but, in particular, there has been no consensus of theoretical opinion as to whether the ground state of methylene is a singlet or a triplet. The literature up to 1964 has been critically reviewed by Gaspar and Hammond (1964). 

The photolysis of diazomethane at 4360 and 3650 A was investigated by Nor-rish, who found (N2) 4 and methylene was postulated as the intermediate. Spin conservation rules require that methylene be formed in a singlet state, but rapid decay to the ground triplet occurs and much of the controversy in the literature centers on the spin state of methylene prior to chemical reaction with a double bond . The benzophenone or Hg( Pi) sensitized decompositions of diazomethane yields triplet methylene directly, presumably via electronically-excited (triplet) diazomethane . Recently it has been shown that triplet methylene can be formed in the photolysis of diazomethane at longer wavelengths by intersystem crossing , viz. 

The oxidation reactions of 2,4,5-triphenylimidazole (lophine) have received considerable attention. With chromic acid it gives benzamide and benzanilide, but even more interest has centred on its involvement in the phenomenon of chemiluminescence. Some of this material has been discussed earlier (Sections 4.06.3.6, 4.07.1.2.1 and 4.07.1.2.3). The oxidative decomposition of lophine in the presence of air is accompanied by the emission of light, and it is the excited singlet state of the diaroylarylamidine (12 Scheme 2) which is the light emitter. The radical (46) derived from oxidation of lophine with aqueous ferricyanide and ethanolic KOH forms a hydroperoxide with hydrogen peroxide with consequent luminescence. When 2,4,5-tri- and 1,2,4,5-tetra-phenylimidazoles are oxidized in dilute methanol solution in the presence of methylene blue, the dibenzoylbenzamidine is also formed under circumstances in which hydroperoxides cannot be intermediates (B-76MI40701). 

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