Methylene singlet-triplet splitting

Table 1 Methylene singlet-triplet splitting obtained with MR methods (in kcal/mor)...

Table 2 Methylene singlet-triplet splitting at the CCSD(T)/6-311-1— -G(2d,2p) geometry (in kcal/mol )...

Analogous negative ion PES experiments reveal that the singlet-triplet splitting of methylene (CH2) is only 9.05 kcal/mol.This difference is easily understood on the basis of a simple molecular orbital (MO) diagram. 

Abstract The singlet-triplet splittings of the di-radicals methylene, trimethylene-methane, ortha-, meta- and para-benzynes, and cyclobutane-l,2,3,4-tetrone have become test systems for the applications of various multi-reference (MR) coupled-cluster methods. We report results close to the basis set limit computed with double ionization potential (DIP) and double electron attachment (DBA) equation-of-motion coupled-cluster methods. These diradicals share the characteristics of a 2-hole 2-particle MR problem and are commonly used to assess the performance of MR methods, and yet require more careful study unto themselves as benchmarks. Here, using our CCSD(T)/6-311G(2d,2p) optimized geometries, we report DIP/DEA-CC results and single-reference (SR) CCSD, CCSD(T), ACCSD(T) and CCSDT results for comparison. 

Carbenes are a fascinating class of chemical species My own interest in this area was first inspired by Shi Shavitt s seminar on methylene in 1979—the material presented was subsequently published as a review article in Tetrahedron [1]. It is a comprehensive summary of the most important experimental and theoretical determinations of the geometry and singlet-triplet splitting in methylene to date, i.e. 1984.1 have read that paper many times. Its clarity is typical of Shavitt s publications, where pioneering research was beautifully presented and discussed. It is an honour to contribute a piece of work on carbenes to this memorial issue that honours the life and scientific contributions of Isaiah Shavitt. 

P. J. Reynolds, M. Dupuis, and W. A. Lesteii J. Chem. Phys., 82, 1985 (1985). Quantum Monte Carlo Calculation of the Singlet-Triplet Splitting in Methylene. 

Lengel. R.K. and Zare. R.N. (1978) Experimental determination of the singlet-triplet splitting in methylene. Journal of the American Chemical Society. 100, 7495-7499. 

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 ... 

The ground state zero point heats of formation are known for all species in 5), as are the excitation energies X A2 A Ax for methyl and a Ax b Bx for methylene.Taking the singlet-triplet splitting in CH2 to be 9.5 kcal/mol, one obtains AE = -94 kcal/mol for reaction (5), so reaction (1) is clearly favored over (4). In fact, using the appropriate states, reaction (4) is slightly endothermic (by about 0.5 eV). 

Hayden, C.C., D.M. Neumark, K. Shobatake, R.K. Sparks, and Y.T. Lee (1982), Methylene singlet-triplet energy splitting by molecular beam photodissociation of ketene, J. Chem. Phys., 76, 3607-3613. 

The spectrum of butyl methyl ether is shown in Figure 3.43. The absorption of the methyl and methylene hydrogens next to the oxygen are both seen at about 3.4 ppm. The methoxy peak is unsplit and stands out as a tall sharp singlet. The methylene hydrogens are split into a triplet by the hydrogens on the adjacent carbon of the chain. 

D. C. Comeau, I. Shavitt, P. Jensen, and P. R. Bnnker, An ab initio Determination of the Potential Energy Snrfaces and Rotation-Vibration Energy Levels of Methylene in the Lowest Triplet and Singlet States and the Single-Triplet Splitting, J. Chem. Phys. 90, 6491-6500 (1989). 

The —OH proton of a primary alcohol RCH2OH is vicinal to two protons and its sig nal would be expected to be split into a triplet Under certain conditions signal splitting of alcohol protons is observed but usually it is not Figure 13 21 presents the NMR spec trum of benzyl alcohol showing the methylene and hydroxyl protons as singlets at 8 4 7 and 2 5 respectively (The aromatic protons also appear as a singlet but that is because they all accidentally have the same chemical shift and so cannot split each other)... 

The salts of some enamines crystallize as hydrates. In such cases it is possible that they are derived from either the tautomeric carbinolamine or the amino ketone forms. Amino ketone salts (93) ( = 5, 11) can serve as examples. The proton resonance spectra of 93 show that these salts exist in the open-chain forms in trifluoroacetic acid solution, rather than in the ring-closed forms (94, n = 5, 11). The spectrum of the 6-methylamino-l-phenylhexanone cation shows a multiplet at about 2.15 ppm for phenyl, a triplet for the N-methyl centered at 7.0 ppm and overlapped by signals for the methylene protons at about 8.2 ppm. The spectrum of 93 ( = 11) was similar. These assignments were confirmed by determination of the spectrum in deuterium oxide. Here the N-methyl group of 93 showed a sharp singlet at about 7.4 ppm since the splitting in —NDjMe was much reduced from that of the undeuterated compound. 

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