The Interplay of Theory and Experiment in the Study of Phenylnitrene

Borden, Weston Thatcher; Gritsan, Nina P.; Hadad, Christopher M.; Karney, William L.; Kemnitz, and Carl R.; Platz, Matthew S.

doi:10.1021/ar990030a

Cited by 237 publications

(274 citation statements)

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“…51,52 The fact that calculations could predict accurately the experimentally measured value of E ST in PhN gave credence to the calculated description of the lowest singlet state of PhN as having an unpaired electron localized in a  orbital on N and an electron of opposite spin largely localized in the  system of the benzene ring. However, replacing the lone pair of electrons, which occupies a nominally sp hybridized AO in NH, with an sp CH bond has an important consequence.…”

Section: Nitrenes -Hn H 3 Cn and Phnmentioning

confidence: 73%

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: 73%

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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“…As with multireference calculations on NPh, 30 variations are seen in the CASSCF-optimized bond lengths between the ipso carbon and the nitrene nitrogen (Table 2) (Table 2) for the different electronic states. The nitrene nitrogen-ipso carbon distance for the 1 A′ ground state is ∼0.06 Å longer than the analogous bond in the 3 A′′ state.…”

Section: S -Symmetric Copper Nitrene Model Complexesmentioning

confidence: 83%

“…35 Conversely, a triplet copper nitrene complex is expected to result from a weaker, less covalent coppernitrene bond. However, given the intricate multireference character revealed by Borden and co-workers 30 for metalfree NPh, quantitative calculations on copper nitrenes are essential to support/refute such predictions. Given the steady progression in research on 3d late-transition-metal nitrene complexes from Fe f Co f Ni, the time seems most propitious for a proactive computational chemistry study of copper nitrene complexes.…”

mentioning

confidence: 99%

Bonding and Structure of Copper Nitrenes

2008

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Copper nitrenes are of interest as intermediates in the catalytic aziridination of olefins and the amination of C-H bonds. However, despite advances in the isolation and study of late-transition-metal multiply bonded complexes, a bona fide structurally characterized example of a terminal copper nitrene has, to our knowledge, not been reported.In anticipation of such a report, terminal copper nitrenes are studied from a computational perspective. The nitrene complexes studied here are of the form ( -diketiminate)Cu(NPh). Density functional theory (DFT), complete active space self-consistent-field (CASSCF) electronic structure techniques, and hybrid quantum mechanical/molecular mechanical (QM/MM) methods are employed to study such species. While DFT methods indicate that a triplet (S ) 1) is the ground state, CASSCF calculations indicate that a singlet (S ) 0) is the ground state, with only a small energy gap between the singlet and triplet. Moreover, the ground-state (open-shell) singlet copper nitrene is found to be highly multiconfigurational (i.e., biradical) and to possess a bent geometry about the nitrene nitrogen, contrasting with the linear nitrene geometry of the triplet copper nitrenes. CASSCF calculations also reveal the existence of a closed-shell singlet state with some degree of multiple bonding character for the copper-nitrene bond.

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“…The phenyloxenium ion 1 is isoelectronic to phenylnitrene 4; it is thus interesting to contrast the relative orderings of the electronic states between the phenyloxenium ion 1 ( 1 A 1 < 3 A 2 < 1 A 2 ) and its more highly studied isoelectronic nitrogen counterpart ( 3 A 2 < 1 A 2 < 1 A 1 ). 30 state of the phenyloxenium ion 1 can be well described by a single closed-shell determinant (closed-shell reference weight = 86%), the 1 A 1 state of the phenylnitrene 4 must be described as a linear combination of two closed-shell determinants (Figure 3). This suggests that the degeneracy of the orthogonal p orbitals on the electron-deficient heteroatom is much more strongly broken in phenyloxenium ion than in phenylnitrene.…”

Section: Comparison Of the Electronic States Of Phenyloxeniummentioning

confidence: 99%

Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

Hanway

Winter

2011

J. Am. Chem. Soc.

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The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph−O + ) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ( 1 A 1 ) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ( 3 A 2 ). Open-shell singlet configurations ( 1 A 2 , 1 B 1 , 1 B 2 , 2 1 A 1 ) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet−triplet energy gap (ΔE ST ) is found to have a positive linear free energy relationship with the Hammett-like σ + R /σ + substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and pcyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have lowenergy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar−NH + ) than their isoelectronic arylnitrene (Ar−N) counterparts. Disciplines Materials Chemistry | Other Chemistry | Physical Chemistry CommentsReprinted (adapted) with permission from J. Am. Chem. Soc., 2011, 133 (13) Oxenium ions are hypovalent species of formula R-O þ . These reactive intermediates are isoelectronic with nitrenes, with a formally monovalent oxygen containing two nonbonding electron pairs. Like nitrenes, the electron-deficient nature of these intermediates makes them powerful electrophiles, further amplified by the formal positive charge on the electronegative oxygen. These species are increasingly being proposed as intermediates in useful umpolung synthetic transformations, such as the oxidative Hosomi-Sakurai reaction, 1,2 the oxidative Wagner-Meerwin transposition, 3 electrochemical oxidations of phenols and phenolates, 4,5 alkane oxidations, 6 and a wide variety of other phenolic oxidations and tautomerization reactions. 7-11 These reactions convert phenols and alkanes, many of which are derived from petrochemical feedstocks, into value-added products such as cyclohexadienones, substituted phenols, and oxidized alkanes, as well as petrochemicals like poly(1,4-phenylene ether), a high-va...

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The Interplay of Theory and Experiment in the Study of Phenylnitrene

Cited by 237 publications

References 70 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

Bonding and Structure of Copper Nitrenes

Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

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