Simulation of a Liquid State Photoinduced Enol-Keto Tautomerization Involving Long-Range Proton Transfer

Abstract

Photoexcited 7-hydroxy-8-(N-morpholinomethyl)quinoline (HMMQ) has previously been shown to transform from the excited enol form E* into the excited keto form K* in a number of consecutive elementary processes involving formation of an excited zwitterionic form Z* and rotational Brownian motion of the morpholino group which acts as a proton carrier and gets into motion only if its initial intramolecular H-bond is broken thermally. The latter motion is not restricted to cases in which HMMQ is dissolved in polar solvents, but may proceed also in nonpolar media which contain proton accepting solvents such as 1,4-dioxane. Molecular dynamics simulations of the motion of the morpholino group in the forms E and Z* in liquid cyclohexane and in liquid 1,4-dioxane are presented. The simulation of Z* in cyclohexane reveals that an intramolecular hydrogen bond between the protonated N atom in the morpholino group and the deprotonated O atom inhibits the rotational motion of the proton carrier during the whole period of simulation. In the case of Z* in 1,4dioxane, an intermolecular H-bond is formed at the expense of the intramolecular hydrogen bond and then the protonated carrier drags the solvent molecule along its path in the rotational Brownian motion. The main conclusions are summarized at the end of the publication.

Elementary Processes in Photoinduced Proton Transfers in 2-Hydroxy-1-(N-morpholinomethyl)naphthalene and 7-Hydroxy-8-(N-morpholinomethyl)quinoline in Liquid Solutions

“…The various positions taken by the proton on the morpholino group relative to the electronically excited aromatic moiety in HMMQ yield different spectra and cause spectral broadening, due to variation of the interaction between the two subsystems. The observed broadening provides experimental support for the procedure, introduced in simulations of the photoinduced tautomerization of HMMQ, in which the electron distribution of the excited chromophoric part of the molecule is adapted to the position of the protonated morpholino group. , As time proceeds the spectrum sharpens gradually, and at t = 45 ps a narrower band can be seen around 550 nm. The sharpening can be attributed to the buildup of the keto form K *, whose spectrum is not expected to be strongly dependent on the position of the morpholino group, because the two interaction subsystems are now electrically neutral.…”

“…The observed broadening provides experimental support for the procedure, introduced in simulations of the photoinduced tautomerization of HMMQ, in which the electron distribution of the excited chromophoric part of the molecule is adapted to the position of the protonated morpholino group. 7,16 As time proceeds the spectrum sharpens gradually, and at t ) 45 ps a narrower band can be seen around 550 nm.…”

Some new views on the tautomerisation mechanism

“…By contrast, an intermolecular hydrogen bond is formed in 1,4-dioxane, and the morpholine nitrogen atom serves as a proton carrier. 15 The tautomeric equilibrium in methyl esters of para-substituted benzoylacetic acids was studied by 1 H NMR spectroscopy 16 and the dependences of the equilibrium constants K T on the solvent and the nature of the substituent were found.…”

Influence of ultraviolet absorbers on the ultraviolet-irradiation behaviour of rigid poly vinyl chloride

“…The short-wavelength bond λ 1 was due to a local transition within the benzotriazole moiety, for both the polar and the distorted (no IMHB) molecule and the long-wavelength bond λ 2 corresponded to the conformation with rings much nearer planarity and containing an intramolecular N•••H bond. 1,7,8,[27][28][29] The two conformations were present in roughly equal amounts. λ 2 was the basis of the intramolecular hydrogen transfer process and therefore of the UVA properties.…”