Methylene triplet geometry

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. 

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. 

The possibility of using I3C-NMR spectroscopy to assist in assigning the geometry of bis-phosphine transition metal complexes has attracted the attention of several groups (80, 86-90). The examination of the PMR spectra of such complexes and analysis of the resulting AA XnXJ, spectra has proved valuable (91) but has the disadvantage that it is limited to systems in which the phosphorus atom is attached to methyl, methoxy, or methylene or to related groups in which the protons couple to phosphorus but not to other protons. The condition for the observation of a 1 2 1 triplet (usually associated with trans phosphine molecules) in the proton resonance spectrum is... 

According to the Pauli Exclusion Principle, two electrons in the same orbital must have opposite spins. Thus, the two electrons of triplet (spin-unpaired) methylene must occupy different orbitals. In triplet methylene, sp-hybridized carbon forms one bond to each of two hydrogens. Each of the two unpaired electrons occupies a p orbital. In singlet (spin-paired) methylene the two electrons can occupy the same orbital because they have opposite spins. Including the two C-H bonds, there are a total of three occupied orbitals. We predict sp2 hybridization and planar geometry for singlet methylene. 

In 1970, Bender and Schaefer " reported afc initio computations of triplet methylene. Employing the CISD/DZ method, they computed the energy of triplet methylene at 48 different geometries, varying the C-H distance and H-C-H angle. Fitting this surface to a quadratic function, they predicted that the H-C-H angle is 135.1°, and emphatically concluded that the molecule is not linear. 

Shavitt, I. Geometry and singlet-triplet energy gap in methylene. A critical review of experimental and theoretical determinations, Tetrahedron 1985,41,1531-1542. 

Russo, N. Sicilia, E. Toscano, M. Geometries, singlet-triplet separations, dipole moments, ionization potentials, and vibrational frequencies in methylene (CH2) and halocarbenes (CHF, CF2, CCI2, CBr2, and CCI2), J. Chem. Phys. 1992, 97, 5031-5036. 

In comparison with acyclic and cyclic substrates, polycyclic systems are far more reactive. The di-ir-methane unit par excellence is benzonorbornadiene, which affords on triplet sensitization the tricyclic product (15) (equation 13). ° The favorable geometry of this substrate, which by virtue of the methylenic bridge possesses the two Tr-bonds fixed at an optimal interacting distance, is largely responsible for the... 

Shavitt, I., Geometry and Singlet Triplet Energy Gap in Methylene a Critical Review of Experimental and Theoretical Determinations, Tetrahedron 1985, 41, 1531 1542. 

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