A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet α-naphthoflavone

Abstract

The hydrogen abstraction (HA) reaction by the triplet of alpha-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet excited state of 1, in acetonitrile, has an absorption maximum at 430 nm and lifetime of 10 micros. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, substituted phenols and amines were determined. The low reactivity of this ketone with respect to HA from 1,4-cyclohexadiene is in accord with a pi,pi* excited state. HA from phenols in acetonitrile is proposed to occur in a diffusion controlled reaction from free phenol based upon the determination of the Abraham beta(H)(2) value for acetonitrile and correction of the quenching rate constants for hydrogen bonding of the phenols to acetonitrile. A molecular orbital analysis of the triplet (SOMO and SOMO-1) of 1 reveals contributions from the carbonyl oxygen atom, but principally from the alpha-carbon and the associated pi-bond network, consistent with a pi,pi* excited state. From a thermodynamic point of view, the triplet HA from phenol to oxygen of the carbonyl group is 17 kcal mol(-1) less demanding than the transfer to the alpha-carbon, consistent with the acidic nature of the phenolic hydrogen atom. DFT and AIM analysis of the hydrogen abstraction reaction reveals that the transition state (TS) is pseudo-symmetrically polarized and that HA in the hydrogen bonded exciplex occurs in a concerted manner but not necessarily by simultaneous electron and proton transfer.

Theoretical Study of Photochemical Hydrogen Abstraction by Triplet Aliphatic Carbonyls by Using Density Functional Theory

“…Therefore, it is possible to use density functional theory (DFT) to calculate only the lowest-lying triplet state of molecular systems. Our previous studies, in comparison with experimental results, validate this statement. − …”

“…We have previously applied this method to compare the theoretical results with experimental results for the phenolic hydrogen abstraction by the triplet state of cyclic αdiketones. 68,70 Additionally, the DFT method has also been employed to investigate the photoreduction of 2-benzoylthiophene by phenol 84,85 and the photoreduction of orthonaphthoquinones. 86 3.1.…”

Determinação da constante de velocidade absoluta para a reação de abstração de hidrogênio fenólico pelo estado excitado triplete de xantona em acetonitrila e no líquido iônico hexafluorfosfato de 1-butil-3-metil-imidazólio [bmim.PF6]

“…43 Assim, o mecanismo proposto no processo de abstração de hidrogênio fenólico por xantona envolve a formação de um exciplexo triplete, estabilizado por ponte de hidrogênio, seguida por uma transferência acoplada elétron/próton, levando ao par de radicais cetila/fenoxila como produtos finais (Esquema 1). 30,[32][33][34][35][36][37][38][39][44][45][46][47][48] Tal mecanismo tem sido recentemente corroborado por cálculos do tipo DFT através dos quais foi claramente demonstrado que a eletrofilicidade da cetona no estado excitado triplete é a força motriz para o processo de abstração de hidrogênio fenólico. 4,6,7,38,39 A irradiação a 355 nm de uma solução de xantona em acetonitrila quando em presença de excesso de fenol, ou de seus derivados contendo substituintes polares, levou à formação de um novo transiente com absorções máximas a 380, 480 e 580 nm (Figura 4), atribuídas ao radical cetila derivado da xantona.…”

“…[32][33][34][35][36][37][38][39] Estudos por fotólise por pulso de laser de nanossegundo, assim como cálculos teóricos de DFT para a reação de cetonas e α-dicetonas com fenóis nos levaram a concluir que, diferentemente do mecanismo conhecido como tipo alcoxila operante na abstração de hidrogênio alquílico, para o caso de fenóis como doadores de hidrogênio a reação se passa via uma transferência acoplada de elétron e próton (PCeT).…”

A photochemical and theoretical study of the triplet reactivity of furano- and pyrano-1,4-naphthoquionones towards tyrosine and tryptophan derivatives

“…The hydrogen abstraction from phenol by the NQF and NQP triplets was theoretically investigated in a similar manner to previous investigations. 65,74,85,109,110 Ground state S 0 van der Waals (S 0 -VDW) complexes between the quinones and phenol were calculated so as to investigate the differences in energy of the four possible hydrogen bonded complexes (Fig. 9).…”