Multiplicity of the ground state and magnitude of the T1-S0 gap in substituted carbenes

Baird, N. Colin; Taylor, Kathleen F.

doi:10.1021/ja00473a001

Cited by 142 publications

(77 citation statements)

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“…Also, the low temperature stabilization and direct esr.detection of a series of triplet state ketocarbenes from the photolysis of a-diazo ketones has been reported (1 3 -18). These observations are in agreement with molecular orbital predictions of a triplet ground state for these species, with a small singlet-triplet energy separation (19).…”

supporting

confidence: 89%

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

Torres¹,

Ribo²,

Clement³

et al. 1983

Can. J. Chem.

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This prrper is dedictrteil to Prqf'e~sor Hnrry E. Gut~tlitzg 011 the occ.n.siotz of' his 65th b i r t h~l q MANUEL TORRES, JUAN RIBO, ALLAN CLEMENT, and O R 0 P. STRAUSZ. Can. J. Chem. 61, 996 (1983). The conformational control of the Wolff rearrangement (WR) has bcen studied by Incans of a comparative study of the low temperature photolysis of argon-matrix isolated a-diazo ketones, 1, and of vinylcne thioxocarbonates, 2. Under these conditions thc s-E, s-Z conformational equilibrium of 1 will be hindered whereas 2 has a fixed s-Z conformation. The results indicate that the conformational control observed in the WR of a-diazo ketones is not per se a proof for a concerted mechanism and that a mechanism involving a ketocarbene intermediate is the most reasonable.MANUEL TORRES, JUAN RIBO, ALLAN CLEMENT ct ORO P. STRAUSZ. Can. J. Clicm. 61, 996 (1983). On a CtudiC le contrble conformationnel de la transposition de Wolff (TW) au moyen d'une Ctude comparative de la photolyse i basse tempkrature des a-diazo cCtones 1 et des thioxocarbonates de vinylhe 2, isol6s dans une matrice d'argon. Dans ces conditions, I'Cquilibre conformationnel s-E, s-Z du composC 1 sera emp&chC tandis que le composC 2 a une conformation s-Z fixe. Les rtsultats indiquent que le contrble conformationnel observe dans la TW des a-diazo cCtones n'est pas en soi une preuve de I'existence d'un mecanisme concert6 et qu'un mCcanisme impliquant un citocarbkne comme intermidiaire est plus raisonnable.[Traduit par le journal]The Wolff rearrangement (WR) of a-diazo ketones and a-diazo esters to ketenes can be brought about either photolytically, thermally, or catalytically and ketocarbenes are generally thought to be implicated as short-lived reactive intermediates in the photochemical and thermal reactions (1 -12).Unambiguous evidence for the intermediacy of ketocarbenes and carbalkoxycarbenes came from the detection of oxirenes as reactive intermediates in the WR (4, 10, 11) kinetic studies (7) as well as from the effects of the substituent upon nitrogen elimination (6). Moreover, molecular orbital computations predict a prohibitively high energy requirement for concerted nitrogen elimination (12). Also, the low temperature stabilization and direct esr.detection of a series of triplet state ketocarbenes from the photolysis of a-diazo ketones has been reported (1 3 -18). These observations are in agreement with molecular orbital predictions of a triplet ground state for these species, with a small singlet-triplet energy separation (19).A concerted mechanism for the WR would be in clear contradiction to the now well established transient formation of oxirene in a large number of photochemical WR (for a recent review, see ref. 20). Recently, however, Kaplan and Mitchell (21) have restated an earlier suggestion (22) of an alternative concerted mechanism for the decomposition WR sequence of those a-diazo ketones in which the intermediacy of oxirene was not detected. The a-diazo ketones may exist as two rotational isomers of the type s-E and s-Z and the propose...

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supporting

confidence: 89%

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

Torres¹,

Ribo²,

Clement³

et al. 1983

Can. J. Chem.

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“…If the substituent X is a m--donor then, as in carbocations, the singlet structure is stabilised by donation into the "vacant" p-orbital on the carbene carbon. This interaction has been studied previously at various levels of theory (17)(18)(19)(20) and while our calculations used a larger basis set than in most studies, the overall trends and conclusions which we find are in agreement with the literature. At the 6-3 1G*//6-31G* level the singlet-triplet splitting (S-T gap) is negative for the two strongest m--donors, NHZ and OH, indicating that the singlet states are lower in energy (Table 3).…”

Section: (B) Carbenessupporting

confidence: 90%

“…Reaction [4] has been used to estimate substituent stabilisation of carbenes over and above any stabilisation in methane (17). All substituents are stabilising in both the singlets and the triplets, but the only large effects are for T-donors in the singlets (see Table 3 sation energies by between 2 and 10 kcal/mol.…”

Section: E(s) -E(t)mentioning

confidence: 99%

“…Carbenes, CHX, also contain an electron-deficient carbon atom and a-substituents having a lone pair of electrons are known to stabilise the singlet state (13)(14)(15)(16)(17)(18)(19)(20). Here there is no positive charge to assist in attracting the lone pair to the carbene centre and substituent effects are therefore smaller than in the carbocations.…”

mentioning

confidence: 99%

“…Here there is no positive charge to assist in attracting the lone pair to the carbene centre and substituent effects are therefore smaller than in the carbocations. Carbenes have previously been examined at various levels of theory (15)(16)(17)(18)(19)(20) and we have included them here to permit comparison with the carbocations.…”

mentioning

confidence: 99%

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Substituent effects in carbocations CX⁺, CHX^+•, and CH₂X⁺, and in singlet and triplet carbenes CHX. Proton affinities of singlet carbenes

Hopkinson¹,

Lien²

1985

Can. J. Chem.

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Abinitio molecular orbital calculations with a 6-31G* basis set have been used to optimise structures for CX+, CHX+•, CHX (both singlets and triplets), H2CX+ and H3CX (X = H, CH3, NH2, OH, F, CN, and NC). Single point calculations were then performed on the 6-31G* optimised structures using a configuration interaction method involving all single and double excitations from the valence shell. Stabilisation energies for the carbocations are compared with those already reported for H2CX+. For the carbenes the single-triplet energy gap is examined as a function of substituent and the stabilisation energies of the singlet carbenes are compared with those of the isoelectronic carbocations H2CX+. Proton affinities are reported for the singlet carbenes and are compared with the proton affinities of similarly substituted ethylenes, H2C=CHX.

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C3H4: Theoretical study of structures and stabilities of isomers

Kakkar

Padhi

1998

Int. J. Quantum Chem.

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The various isomers including stable structures, carbenes, and diradicals on the C,H, surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR) spectra. The r electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH,=CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. 0 1996 John Wiley & Sons, Inc.

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Multiplicity of the ground state and magnitude of the T1-S0 gap in substituted carbenes

Cited by 142 publications

References 1 publication

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

Substituent effects in carbocations CX⁺, CHX^+•, and CH₂X⁺, and in singlet and triplet carbenes CHX. Proton affinities of singlet carbenes

C3H4: Theoretical study of structures and stabilities of isomers

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Multiplicity of the ground state and magnitude of the T1-S0 gap in substituted carbenes

Cited by 142 publications

References 1 publication

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

Substituent effects in carbocations CX+, CHX+•, and CH2X+, and in singlet and triplet carbenes CHX. Proton affinities of singlet carbenes

C3H4: Theoretical study of structures and stabilities of isomers

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About

Substituent effects in carbocations CX⁺, CHX^+•, and CH₂X⁺, and in singlet and triplet carbenes CHX. Proton affinities of singlet carbenes