Quantum Chemical Investigations of the Thermal and Photoinduced Proton-Transfer Reactions of 2-(2‘,4‘-Dinitrobenzyl)pyridine

Abstract

Quantum chemical calculations of the proton-transfer system 2-(2‘,4‘-dinitrobenzyl)pyridine (DNBP) and of derivatives with larger aromatic bases are presented. These systems undergo photochemical isomerizations from a stable form to two different photoisomers. Ab initio methods are employed to investigate the isomerization of methylimine to vinylamine as a model system for the proton-transfer reaction of DNBP. The ground and excited states of both isomers and of the transition state for the isomerization are studied with Hartree−Fock and MRD-CI (multireference singles and doubles CI) methods. The isomerizations of the larger system DNBP and its derivatives are investigated with semiempirical PM3-SCF and PM3-MRD-CI methods. The intrinsic reaction coordinate formalism is used for calculating the reaction pathways from the optimized transition states. The relative stabilities of the various isomers and the mechanism of the intramolecular proton-transfer reaction are discussed. The ortho-nitro group assists the proton transfer from the methylene group to the pyridine ring. The experimentally observed reaction barriers are reproduced by taking crystal effects into account.

Ultraviolet spectrum of vinylamine

“…With observations 1-6 above strongly indicating a vinylamine assignment for the UV band system, we are in a position to compare the present spectroscopic results with predictions available from a previously reported computational study [8] of vinylamine electronic states. In that study, Frank et al used an ab initio (MRD-CI) method to calculate vertical and adiabatic excitation energies for the lowest 1 ðp; p Ã Þ state.…”

“…We have recorded the ultraviolet (UV) absorption spectrum of vinylamine in the wavelength region 235-270 nm and have observed about 20 vibronically resolved bands. The electronic excitation energy inferred from the spectrum is about 2 eV lower than predicted (from ab initio calculations [8]) for the lowest 1 ðp; p Ã Þ state of vinylamine. This finding most likely rules out a p à p electronic assignment for the observed band system.…”

“…However, the lowest-energy 1 ðp; p Ã Þ and 3 ðp; p Ã Þ excited states have been studied computationally [8]; these states play a central role in the photoinduced hydrogentransfer reaction that converts vinylamine into its tautomer ethanimine (CH 3 CH@NH). The excited-state energetics calculated for this reaction have been used to understand enamine-imine photoisomerization reactions of larger analogs [8].…”

Crystal structure of 2-(2',4' -diphenylsulfonylbenzyl)pyridine, C24H19NO4S2

“…The proton transfer process from the methylene carbon atom towards the nitrogen atom of the pyridine has been characterized for a-DNBP [4][5][6], and assumed to be assisted by the o-nitro group. To test this assumption, it was synthesized the 2-(2' ,4' -diphenylsulfonylbenzyl)-pyridine [ 1 ], a new a-DNBP derivative in which the electron withdrawing nitro groups have been replaced by sulfone groups, and shown that this derivative is photochromic in solution and in the crystal.…”

Photoinduced intramolecular proton transfer in 2- and 4-(2,4-dinitrobenzyl)pyridine

“…Because a photoinduced intramolecular proton transfer (PIPT) does not require large molecular rearrangements, the reaction time of PIPT is expected to be very fast and the reaction can take place for many cycles even in the solid state [2,3]. As a result of this property, photochromic compounds accompanied by PIPT might be promising candidates for fast photomemory and molecular-protonic devices.…”