Methylene, singlet states

Basch in 1970 carried out one of the first MCSCF calculations on a polyatomic system216 in an investigation of the least-motion, coplanar approach of two methylenes to give C2H4. For two closed shell, singlet state methylenes, the reaction path is purely repulsive, and the reaction occurs for two appropriately oriented bent triplet methylenes. 

CHa + CH2— C2H4.—-The coplanar approach of two methylenes to form ethylene was investigated by Basch268 using the MCSCF method. The states of bent methylene that correlate with the ground state of ethylene are the triplet states. It is found that for two closed-shell singlet-state methylenes, the reaction path is purely repulsive. 

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. 

As well known, the methylene lowest state is a triplet, with electronic configuration (Ifli) (2ai) (162) (3 i) (l ), which lies somewhat below the fundamental singlet state, Mj. In addition, the companion singlet state, fii, is also known. To... 

The nature of the photochemically produced methylene has been the subject of considerable study. Evidence tends to indicate that this species is produced in its singlet state photochemically. Photolysis of diazomethane in the gas phase or in solution in the presence of excess cis- or trans-2-butene produces cyclopropane products due to methylene insertion in which the... 

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. 

Quantum-chemical calculations of the transition state of the enantiomerization of 6, that is, for the interconversion of (M)-6 and (P)-6, support the diradical 6-D [13, 18] (Scheme 6.4). Most probably, its singlet state is a few kcal mol-1 more stable than the triplet state. The zwitterions 6-Zi and 6-Z2 are excited states [1], However, if a methylene group directly attached to the allene system in 6 is replaced by an appropriate heteroatom, either 6-Za or 6-Z2 may be strongly stabilised (see Section 6.3.5). 

Carbenes with two orbitals, occupied by one electron each, have a total spin number of s = 1 the multiplicity is therefore 2s -f-1 =3, i.e., a triplet state. This is the usual ground state of a carbene. If however, the two orbitals are not degenerate, the multiplicity can become 1, i.e., a singlet state is possible. Methylene — and other carbenes — can, in principle, be linear or bent. The binding is achieved by using the 15 hydrogen Is and the 2s and 2p carbon orbitals. 

Sensitizers have also been used to induce inters5retem crossing to the triplet The insertion reaction of methylene into C—H bonds in solution is widely acknowledged to be a concerted process resulting from the singlet state. Photolysis of... 

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... 

A highly reactive compound containing a neutral divalent carbon with two nonbondmg electrons (ie.,. CR2 or a substitution derivative). The nonbonding electrons can have parallel spins (triplet state) or antiparallel spins (singlet state). The parent species, iCRz, is also known as methylene. A number of carbene derivatives have been used as photoaffinity labels of proteins. Irradiation of 3 -0-(4-benzoyl)benzoyl-ATP will cause 70% inactivation of mitochondrial Fi-ATPase. ... 

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. 

Carbenes and related compounds are among those reactive intermediates for which gas-phase experimental data exist. Some of those are compared to calculated geometries in Table 5-17, drawn from a larger collection provided in Appendix A5 (Tables A5-42 to A5-49). Except for methylene (CH2), where both singlet and triplet states have been considered, only singlet-state molecules have been examined. The usual theoretical models have been assessed. Mean absolute errors in bond lengths and angles based on the full data set have also been provided. 

These are not easy questions to answer Indeed, the chemical world argued for some decades over them. Finally, it became clear from increasingly sophisticated theoretical and experimental work that, at least for methylene the bent triplet (3) was the overall minimum, with a H—C—H angle of 137°. The bent singlet state... 

It is also possible to induce intersystem crossing from an initially formed singlet state to a lower energy triplet by forcing the singlet state to suffer collisions with an inert medium (no easy task for a species as reactive as methylene ). 

This fact was first brought to the attention of experimentalists, interested in RIs, by two different computational predictions, made in the 1970s, about methylene. Triplet methylene was computed to have a bent geometry, with an H—C—H bond angle of 135° and the best calculations predicted the triplet to lie 10 kcal/mol below the lowest singlet state. Both of these computational results, when published, were in apparent conflict with experiments, which had been interpreted as indicating that triplet methylene is linear and that it lies >20 kcal/mol below the lowest singlet state. However, subsequent experiments proved the calculations to be correct. 

In the light of experimental difficulties associated with the identification of intermediates, a MINDO/3 quantum-mechanical study of the singlet state ( a2) of the cyclobutylidene to methylenecyclopropane rearrangement has been carried out. It has been proposed that the whole process is initiated by electrophilic attack from the C3 methylene group of cyclobutylidene at the empty p atomic orbital on the Cl carbene site, so that a shift of electron density towards Cl can take place to give the bicyclobutane-like nonclassical carbene intermediate 4. Finally, the bicyclobutane intermediate 4 undergoes a symmetry-allowed conrotatory bond-fission process to generate methylenecyclopropane. The activation enthalpy calculated for the two steps is 8 kcal mol-1.2... 

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