Methylene, singlet

Photolysis of the parent compound (44) yields singlet methylene, as evidenced by its stereospecific addition to ( )-butene. The cyclopropane is formed together with the characteristic insertion products of methylene (62MI50800, 64PAC(9)527)... 

The lowest energy molecular orbital of singlet methylene looks like a Is atomic orbital on carbon. The electrons occupying this orbital restrict their motion to the immediate region of the carbon nucleus and do not significantly affect bonding. Because of this restriction, and because the orbital s energy is very low (-11 au), this orbital is referred to as a core orbital and its electrons are referred to as core electrons. 

Singlet methylene also possesses unoccupied molecular orbitals. The unoccupied orbitals have higher (more positive) energies than the occupied orbitals, and these orbitals, because they are unoccupied, do not describe the electron distribution in singlet methylene. Nevertheless, the shapes of unoccupied orbitals, in particular, the few lowest energy unoccupied orbitals, are worth considering because they provide valuable insight into the methylene s chemical reactivity. 

For electron movement to occur, the donor and acceptor molecules must approach so that the donor HOMO and acceptor LUMO can interact. For example, the LUMO of singlet methylene is a 2p atomic orbital on carbon that is perpendicular to the molecular plane. Donors must approach methylene in a way that allows interaction of the donor HOMO with the 2p orbital. 

Draw a Lewis structure for singlet methylene, CH2 (all the electrons in singlet methylene are spin-paired). Ho many electrons remain after all bonds have been formei Where are the extra electrons located, in the plane the molecule or perpendicular to the plane Examine t highest-occupied molecular orbital (HOMO) of methyle to tell. 

Hydrocarbons normally have very small dipole momen Why (Hint Consider the relationship betwe electronegativity differences and dipole momer established above for hydrogen halides.) Does sing methylene possess a small dipole moment Explain. W1 direction do you expect singlet methylene s dipole point Explain. In what direction does it point ... 

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. 

Compare electrostatic potential maps for the more stable form of CH2N2 and singlet methylene. Describe similarities and differences between the two. 

Step through the sequence of structures representing dissociation oiketene to methylene and carbon monoxide. Plot energy (vertical axis) vs. carbon-carbon bond distance (horizontal axis). Would you describe ketene as a weak complex between singlet methylene and carbon monoxide Explain. (A table of CC and CO bond lengths is found at left.) Is there an energy barrier to the dissociation ... 

A/E) (Roth, 1972b). In contrast, no polarization is observed in the absence of a sensitizer, despite the fact that ethylbenzene is produced. This is consistent with a direct insertion of singlet methylene into the C—H bond, but it could also arise from an abstraction-recombination mechanism if the lifetime of the intermediate radical-pair were too short to permit a significant amount of Tq-S mixing. 

Given the zwitterionic natnre of single carbenes, the possibility exists for coordinating solvents such as ethers or aromatic compounds to associate weakly with the empty p-orbital of the carbene. Several experimental stndies have revealed dramatic effects of dioxane or aromatic solvents on prodnct distribntions of carbene reactions. Computational evidence has also been reported for carbene-benzene complexes. Indeed, picosecond optical grating calorimetry stndies have indicated that singlet methylene and benzene form a weak complex with a dissociation energy of 8.7kcal/mol. ... 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

Acetylene also plays an important role as intermediate in soot formation in flames, because the reverse of reaction (la) forms singlet methylene,... 

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

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-... 

Let us turn now to a reaction surface that has been studied in more detail, that is, the surface for the addition of methylene to ethylene (11). Figure 5 shows the various approaches of the two fragments, b) is the most symmetric approach, but the correlation diagram shows that the reaction is symmetry-forbidden for the ground configuration singlet methylene along this path. In Fig. 5 c the levels have been classified as symmetric or antisymmetric with respect to the xz plane, which is the relevant symmetry element for use of the symmetry conservation rales. 

Although concerted C=C and C-H insertion into benzene by methylene is possible, several experiments on the effect of various solvents on the product ratio suggest the intermediacy of a complex between methylene and benzene.67-69 Based on the obtained kinetic and thermodynamic data, the transient is believed to be a complex formed between singlet methylene and benzene. 

Exercise Estimate the bond angle of singlet methylene (CH2). 

The stereospecificity of the above reaction is due to the addition of singlet methylene as follows in which case both the bonds are formed simultaneously. 

Photolysis of diazomethane in carbon-tetrachloride in the presence of benzophenone yields 1,1,1,2-tetrachloroethane showing an enhanced absorption due to the triplet carbene. The direct photolysis of diazomethane proceeds via singlet methylene CIDNP-studies of the photolysis of methyl-diazoacetate, for which a radical pair mechanism was suggested, were recently challenged 2). 

Table 1 Equilibrium geometries, harmonic vibrational frequencies (cm ), and adiabatic excitation energies (eV) for singlet methylene (i, and states) ...

Some years ago a short description of a more restricted version of the problem in the last section was published[39]. Using an STO-3G basis, the earlier calculation examined the two lowest Ai energies as a singlet methylene approached an ethylene molecule. In this case, however, the ethylene was not allowed to relax in its geometry. The curves are shown in Fig. 16.5. The important point is that we see... 

Figure 16.5. The two lowest A states showing the attack of singlet methylene on a rigid ethylene. These energies were obtained using an STO-3G basis, with which we obtain a barrier of about 0.8 eV.

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