The singlet and triplet states of phenyl cation. A hybrid approach for locating minimum energy crossing points between non-interacting potential energy surfaces

Harvey, Jeremy N.; Aschi, Massimiliano; Schwarz, Helmut; Koch, Wolfram

doi:10.1007/s002140050309

Cited by 841 publications

(614 citation statements)

References 13 publications

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“…For the distal oxygen atom attack, the barrier height is computed to be 91.7 kJ mol À1 ( 5 I d -ts1) at the quintet surface while the barrier height in the triplet surface is computed to be twice as much (Figure 1). This difference demands a spin inversion [15] and suggests that the antiferromagnetic exchange observed for this species hinders its reactivity. The optimized structure of 5 I d -ts1 along with the computed spin density plot is shown in Figure 2.…”

Section: Fementioning

confidence: 96%

CH Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Ansari¹,

Jayapal²,

Rajaraman³

2014

Angewandte Chemie

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Metal-superoxo species are ubiquitous in metalloenzymes and bioinorganic chemistry and are known for their high reactivity and their ability to activate inert CÀH bonds. The comparative oxidative abilities of M-O 2 C À species (M = Cr III , Mn III , Fe III , and Cu II ) towards CÀH bond activation reaction are presented. These superoxo species generated by oxygen activation are found to be aggressive oxidants compared to their high-valent metal-oxo counterparts generated by O···O bond cleavage. Our calculations illustrate the superior oxidative abilities of Fe III -and Mn III -superoxo species compared to the others and suggest that the reactivity may be correlated to the magnetic exchange parameter.Mononuclear metalloenzymes with coordinated oxygen at the metal center have applications in biology, industry, and the laboratory.[1] Oxygenated metal intermediates like oxo, peroxo, hydroperoxo, and superoxo species play a vital role in catalytic reactions such as hydrogenation, halogenation, hydroxylation, olefin epoxidation, and C À H bond activation.[2] In the last decade many high-valent metal-oxygen species have been studied to understand the fundamental structural, functional, and mechanistic aspects of their enzymes and their counterparts. [2a,d, 3] Apart from the metaloxo species, oxidation of CÀH bonds by several superoxometal complexes is also reported.[4] Unlike the metal-oxo species, the reactivity of M-O 2 C À species and their competing oxidative abilities are relatively less explored, although nature utilizes both species to carry out efficient catalysis. [5] Among other factors, the nature of the transition metals in M-O 2 C À species is also important, as it determines the electronic structure and the reactivity of these species. Over the years, the synthesis, structure, and reactivity of superoxo species containing copper, [6] iron, [5,7] and manganese [8] have been reported along with other metals.[9] An important addition to this class is the report of the crystal structure of the end-on Cr-O 2 C À species and kinetic studies to probe its ability to perform C À H bond activation in hydrocarbons. [10] Metal-superoxo species are generally transient in nature and are generated at the first step of the oxygen activation both in enzyme catalysis and in biomimetic chemistry. [7c, 10b] As these species are key intermediates in iron and copper catalysis, it suggests that the M-O 2 C À species perhaps play a larger role as an oxidant in enzyme catalysis. [6,11] In the M-O 2 C À species the unpaired electrons on the metal and the radical center are strongly coupled and the electronic configuration of the metal ions dictates the nature of the magnetic coupling (J) and this may in turn correlate to the CÀH bond activation. Here we have undertaken a detailed theoretical study to specifically address the following questions 1) probing the mechanism of C À H bond activation by Cr-O 2 C À and its comparative oxidative ability to high-valent metal-oxo species, 2) establishing the comparative oxidativ...

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Section: Fementioning

confidence: 96%

CH Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Ansari¹,

Jayapal²,

Rajaraman³

2014

Angewandte Chemie

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“…In particular, the nature of the ground state multiplicity for the parent and substituted phenyl cations has attracted considerable attention. The parent system is known to have two low-energy minima which corresponds to the 1 A 1 and 3 B 1 states, with the intersection between the lowest energy singlet and triplet hypersurfaces lying close to the triplet geometry and energy [7]. Coupled with nontrivial spin-orbit coupling between these two states near the crossing point [8], the triplet phenyl cation is a shortlived intermediate that rapidly decays [7] to the ground singlet state 102 kJ/mol below the triplet minimum [9].…”

Section: Introductionmentioning

confidence: 99%

“…The parent system is known to have two low-energy minima which corresponds to the 1 A 1 and 3 B 1 states, with the intersection between the lowest energy singlet and triplet hypersurfaces lying close to the triplet geometry and energy [7]. Coupled with nontrivial spin-orbit coupling between these two states near the crossing point [8], the triplet phenyl cation is a shortlived intermediate that rapidly decays [7] to the ground singlet state 102 kJ/mol below the triplet minimum [9]. The phenyl cation has been isolated and characterized in argon [10,11] and LiCl matrices [12] and in the gas phase [13], and indirectly detected in aqueous solution with a lifetime of <500 ps [14].…”

Section: Introductionmentioning

confidence: 99%

Singlet–triplet excitation energies of substituted phenyl cations: a G4(MP2) and G4 theoretical study

Rayne¹,

Forest

2016

Theor Chem Acc

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“…Thus it was found that in the case of phenyl cation the singlet-triplet splitting is relatively small and with a right choice of substituents at the phenyl ring can even be reversed [19]. The geometry for the cross-point between singlet and triplet has been calculated as well as spin orbit coupling for this transient structure [20]. Other reactive intermediates that were extensively studied by Squires and coworkers are o-, m-, and p-isomeric benzyne anions [21][22][23].…”

mentioning

confidence: 99%

Homolytic cleavages in pyridinium ions, an excited state process

Denekamp

Tenetov

Horev

2003

J. Am. Soc. Mass Spectrom.

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The favored fragmentation pathway for protonated and alkylated pyridinium cations of the general formula p-XC 6 H 4 CH 2 CH 2 CH¢CH Py ϩ R (R¢H, Me; Py¢pyridine) is a C™C homolytic cleavage. The tendency to form radicals is higher for alkylated pyridinium cations than for the protonated ones that can also afford closed-shell products. Theoretical calculations show that the singlet-triplet gap for transient structures with an elongated benzylic C™C bond is very low and the formation of radicals may result from mixing of these states. In addition to the notable substituent effect on the fragmentation efficiency of the cations under study, calculated results show a clear substituent effect on the singlet-triplet transitions. We also observe that triphenylphosphonium cations behave notably different. Thus, the pyridinium system that contains a p-chloro benzyl moiety loses a benzyl radical readily while the analogous triphenylphosphonium cation is very stable under the same conditions. O rganic closed shell ions typically dissociate into closed shell ionic and neutral products. Nevertheless, there are exceptions to this rule that may result from lack of selectivity under extremely energetic conditions, unusual stability of some cation radicals or specific kinetic effects. It is reasonable that highly energetic conditions promote simple bond cleavage, either heterolytic or homolytic. Some radicals, however are generated under mild conditions and their formation can only be attributed to their relative stability.Different classes of organic closed-shell ions undergo C™C and C™N cleavages that are not necessarily charge driven [1][2][3][4][5]. Furthermore, it has been suggested that these reactions occur through a concerted mechanism or are mediated by the formation of radicals [6 -9]. The development of electrospray ionization allowed the formation of various classes of closed shell ions under very mild conditions and hence it has been unambiguously shown that at least in some cases homolytic C™C and C™H cleavages are involved in the gas phase decomposition of cationized organic compounds [10,11].One class of organic cations that are known to undergo homolytic cleavages is quaternary ammonium and pyridium cations [8,[12][13][14][15][16][17]. Fisher and Veith studied the gas phase fragmentation of ammonium and pyridinium ions, generated with field desorption and reported the formation of nitrogen based radical cations under collision induced dissociation (CID) [12,13]. Harrison showed that the formation of radicals and cations from quaternary ammonium ions is dominated by the relative ionization potential of the appropriate radicals and neutrals, showing that the reaction products are the ones thermodynamically favored [14,15]. Katritzky and coworkers studied homolytic cleavages in pyridium cations that were designed in order to show the substituent effect on the relative stability of radicals. They propose the term "merostabilization" to describe stabilization that results from the presence of both an electron-attracti...

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The singlet and triplet states of phenyl cation. A hybrid approach for locating minimum energy crossing points between non-interacting potential energy surfaces

Cited by 841 publications

References 13 publications

CH Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

CH Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Singlet–triplet excitation energies of substituted phenyl cations: a G4(MP2) and G4 theoretical study

Homolytic cleavages in pyridinium ions, an excited state process

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