Spectroscopic features of the low-lying singlet states of some N-alkyl retinylnitrone model systems and their involvement in oxaziridine formation

2015

RSC Adv.

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This comprehensive spectroscopic analysis of a-(2-naphthyl)-N-methylnitrone has proposed its photochemical oxaziridine formation and thermal E-Z isomerization mechanisms. The activation energy for the conversion of the unstable non-planar E isomer to the stable planar Z-isomer is found to be 23.7 kcal mol À1 at the CASSCF/6-31G* level of calculation. A transition state with a negative frequency of 350 cm À1 is likely to be responsible for this process. Both CASSCF and ONIOM-based studies have revealed that the nitrone-oxaziridine photochemical conversion involves non-radiative decay channels which include biradicaloid conical intersection (CI) points through Hula-twist and terminal one-bond-flip motions, situated at 35-40 kcal mol À1 below the first excited singlet state (S 1 ). Following the directions of their gradient-difference vectors, the optimized oxaziridine geometries are obtained. The nature of the low-lying singlet-singlet transitions of these a-naphthyl N-methylnitrones is found to be similar to that of the conjugated non-polar polyenes, and differs appreciably from our previously studied longchain conjugated nitrone systems. The fluorescent S 1 state with a radiative lifetime of nanosecond order is populated by the weak upward S 0 -S 1 transition (transition moment: 0.3 Debye) and through the decay of the S 2 state, which eventually gets involved in the S 0 /S 1 conical intersections.

Section: Introductionmentioning

confidence: 97%

A comprehensive spectroscopic investigation of α-(2-naphthyl)-N-methylnitrone: a computational study on photochemical nitrone–oxaziridine conversion and thermal E–Z isomerization processes

Saini

2015

RSC Adv.

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“…Photochemical reactions of nitrones have been thoroughly investigated by our group in the last few years . Various categories of acyclic and cyclic nitrones have been analyzed through quantum mechanical studies.…”

Section: Introductionmentioning

confidence: 99%

“…Photochemical reactions of nitrones have been thoroughly investigated by our group in the last few years. [1][2][3][4][5][6] Various categories of acyclic and cyclic nitrones have been analyzed through quantum mechanical studies. Experimental results reported for these nitrones were clearly justified by these abinitio-based calculations.…”

Section: Introductionmentioning

confidence: 99%

A Computational Study on the Photochemistry of 2,4,4‐Trimethyl‐1‐pyrroline 1‐Oxide and Investigation on the Reaction Paths of Its Photoproduct Oxaziridine

Sen

2019

ChemistrySelect

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This computational study reveals the reaction pathways of the experimentally reported photo‐conversion of 2,4,4‐trimethyl‐1‐pyrroline 1‐oxide to oxaziridine and the subsequent reactions (thermal and photochemical) of this photo‐product. The latter involve the oxaziridine to N‐acetyl azetidine, pyrrolidone and pyrroline formation paths. The photo‐excitation of the cyclic nitrone gives weakly allowed S0‐S1 and strongly allowed S0‐S2 transitions with transition moment values of 0.12 D and 3.16 D, respectively. The low‐lying conical intersection (S0/S1) situated at 80 kcal / mol (at CASSCF level) above the ground state nitrone geometry is found to be responsible for the oxaziridine formation. This bicyclic compound has to overcome a barrier (∼ 50 kcal / mol) to form N‐acetyl azetidine. The transition state involved in this process has an imaginary frequency of 909i cm−1. A parallel photochemical path has been also predicted for the same reaction. The thermal oxaziridine‐pyrrolidone conversion involves a transition state with an imaginary frequency of 1136i cm−1 which indicates the breaking of the C–CH3 bond adjacent to the nitrogen atom. This cyclic amide compound is almost 20–25 kcal/mol more stable than the 4‐membered cyclic azetidine and both are lower in energy than oxaziridine.

“…The chemopreventive N-methyl retinylnitrone 6 w a sf o u n dt oc o n v e r t slowly to stable oxaziridine under the exposure to room light. Recently our group [7][8][9] has also identified oxaziridine as the primary photoproduct of these types of nitrones from computational analysis. In fact, it is now well-known from experimental studies that N-alkyl group on nitrones stabilizes the oxaziridine while the N-aryl or electron-withdrawing groups (EWG) on nitrogen have an opposite effect.…”

Section: Introductionmentioning

confidence: 99%

“…Another reason for choosing the model 2,4-pentadiene-nitrone system with N-electron withdrawing group is to compare this system with our earlier reported structurally similar nitrone system with electron-donating N-alkyl substituents. 8 Now let us focus on the reason for choosing trifluoromethyl group as an EWG on the nitrogen atom in this conjugated nitrone system. It has already been stated that presence of N-substituted phenyl rings in nitrones always lead to non-isolable oxaziridines; on the other hand, N-sulfonyl, [11][12][13] and perfluorinated 14 oxaziridines are never reported to be prepared from the photoirradiation of their corresponding nitrones.…”

Section: Introductionmentioning

confidence: 99%

A computational investigation of the photochemical oxaziridine and amide conversion process of open-chain conjugated nitrone with electron-withdrawing trifluoromethyl group on nitrogen

Saini

2015

J Chem Sci

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This computational study investigates the photo-excitation process and subsequent photoproduct formation steps through non-radiative deactivation channels in open-chain conjugated N-substituted nitrone systems (model compounds of corresponding retinylnitrones) having electron-withdrawing groups on nitrogen. Calculations mostly based on CASSCF/6-31G* and CASMP2/6-31G* level of theories on a representative system with N-trifluoromethyl substituent have predicted initial photo-excitation to a planar singlet state. This photochemical path is subsequently followed by a barrierless non-radiative channel towards the lowest-energy conical intersection (CI) geometry having a terminal CNO kink, and situated at 30 kcal/mol below the planar excited state. Following the direction of its gradient difference (GD) vectors, an oxaziridine-type species (R C−O = 1.38 Å, R N−O = 1.53 Å, < CNO= 55.8 •) appears at 3-6 kcal mol −1 below the ground state nitrone system through a transition state (along its reverse direction of minimum-energy path), situated on the reaction pathway. This species with an elongated NO bond seems to be heading towards an amide geometry. On the other hand, in the opposite GD vector direction a proper oxaziridine geometry has been obtained with a much shorter NO bond distance (R N−O = 1.42 Å).