Geometry and singlet-triplet energy gap in methylene: a critical review of experimental and theoretical determinations

Shavitt, Isaiah

doi:10.1016/s0040-4020(01)96393-8

Cited by 192 publications

(77 citation statements)

References 111 publications

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“…9 provides a value 72 of E ST = 9.0  0.1 kcal/mol for CH 2 , a result that agrees well with the best values available from calculations. [53][54][55][56] Moreover, all of the extensive vibrational structure in the triplet spectrum could now be definitively assigned as a bending progression in ground state triplet methylene. 40 The energy scale was chosen so as to emphasize the major improvements to the experimental apparatus since the 1976 study.…”

Section: Nitrenes -Hn H 3 Cn and Phnmentioning

confidence: 99%

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

Lineberger

Borden

2011

Phys. Chem. Chem. Phys.

106

107

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This Perspective describes research, carried out in the authors' labs over the past forty years, aimed at understanding, predicting, and measuring the singlet-triplet energy differences ( ST ) in diradicals. A theory for qualitatively predicting the ground states of diradicals and the use of Negative Ion Photoelectron Spectroscopy (NIPES) for measuring  ST are described. The application of this theory, ab initio calculations, and NIPES to the prediction and measurement of  ST in a wide variety of organic diradicals is detailed. Among the diradicals that are discussed in this Perspective are HN, CH 3 N, PhN, CH 2 , trimethylenemethane (TMM), oxyallyl (OXA), meta-benzoquinodimethane (MBQDM), meta-benzoquinone (MBQ), tetramethyleneethane (TME), 1,2,4,5-tetramethylenebenzene (TMB), and D 8h cyclooctateraene (COT). All of these diradicals have been studied in one and, in most cases, in both of the authors' laboratories. The studies of OXA and D 8h COT were, in fact, collaborations between the research groups of the authors. These two projects both took advantage of the ability of NIPES to provide information about transition states. Transition-state spectroscopy was used to measure the cabonyl stretching frequency in the singlet state of OXA and to establish that D 8h COT violates the strictest version of Hund's rule. However, in some cases, it is not qualitative theories but the results of high-level calculations that motivate experimentalists to test how quantitatively accurate the calculations are. In other cases calculations are performed to see how well they reproduce experimental results that have already been obtained. When the results of calculations and experiments are found to be at odds with each other, additional experiments and/or calculations almost inevitably follow.Investigations of the energy differences between the lowest singlet and triplet state (E ST ) of organic diradicals 1 have provided many illustrations of the above types of synergies. Qualitative theories have resulted in computational and/or experimental tests, and calculations and experiments have not only stimulated each other but, in some cases, also led to the development of qualitative theories.For example, in response to the results of experiments and calculations on cyclobutadiene and trimethylenemethane, one of the coauthors of this review (WTB) helped to develop a qualitative theory, which allows the qualitative prediction of the approximate size and sign of

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Section: Nitrenes -Hn H 3 Cn and Phnmentioning

confidence: 99%

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

Lineberger

Borden

2011

Phys. Chem. Chem. Phys.

106

107

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“…The experimental value corrected for the effects of zeropoint vibrational energy and relativity is 9.12Ϯ 0.20 kcal/ mol. 35 The difference between the experimental and computed CBS values should be ascribed to the higher-order dynamical correlation contributions that are neglected in CASPT2. The use of larger active spaces helps to capture these effects.…”

mentioning

confidence: 99%

Communication: Second-order multireference perturbation theory with explicit correlation: CASPT2-F12

Shiozaki

Werner

2010

The Journal of Chemical Physics

120

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“…The chemistry of carbenes in solution is well documented (1,2) and methylene, the simplest carbene, is a well-understood compound (3). The electronic and molecular structures of singlet and triplet methylene and the energy difference between these states (AEs,) has been determined both experimentally (4) and theoretically (5).…”

Section: Resultsmentioning

confidence: 99%

Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

Suh¹,

Pole²,

Warkentin³

et al. 1996

Can. J. Chem.

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Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH,O-C-OCH,CF,'+, I", are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-~-1 ,3,4-oxadiazoline I. Neutralization-reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of I'+, shows no recovery ion signal and thus 1 is not a viable species within the ks time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis -mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF,CH,C(=O)OCH,, l a . This technique was also used to study the related dialkoxycarbenes C,H,O-C-OCH,CF,, 2, CH,O-C-0C2H,, 3, and CH,O-C-OCH(CH,),, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-A3-1 ,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF,CH,, as compared to CH, and CH,CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing p-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group.Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.Resume : On peut gCnCrer proprement les cations radicaux mtthoxy-(2.2.2-trifluoroCthoxy)carbknes, CH30-C-0CH2CF,'+, 1'+, en procCdant ? I une ionisation Clectronique (IE) dissociative de la 2-mCthoxy-5,5-dim~thyl-2-(2,2,2-trifluoroCthoxy)-~3-1 ,3,4-oxadiazoline, I. La spectroscopie par neutralisation-rkionisation (NR) du carbkne neutre, 1, gCnCrC par une rCduction un electron du 1". ne prksente pas de signal correspondant ii de la rCcupCration d'ion et le composC 1 n'est donc pas une espkce approprike ii I'tchelle de temps, ks, de I'expCrience: La spectromCtrie de masse du composC I avec pyrolyse ii trks basse tension de vapeur (VLVP), rCalisCe de concert avec des expkriences de (multiples) collisions ont montrC que le composC 1 s'isomkrise complktement, par le biais d'un dCplacement-1,2 d'un groupe trifluoroCthyle, en un 3,3,3-trifluoropropionate de mCthyle, CF,CH,C(=O)OCH,, la. On a aussi utilisk cette technique pour Ctudier des dialkoxycarbknes apparentis : C2HjO-C-OCH,CF,, 2, CH,O-C-OC,H,, 3, et CH,O-C-OCH(CH,),, 4, gCnCrCs a parti des 2,2-dialko~~-5,5-dirnCth~l-~~-1,3,4-oxadiazolines correspondantes. Le carbkne 2 prCparC d'une f a~o n py...

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Geometry and singlet-triplet energy gap in methylene: a critical review of experimental and theoretical determinations

Cited by 192 publications

References 111 publications

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

Communication: Second-order multireference perturbation theory with explicit correlation: CASPT2-F12

Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

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