Excited state enol-keto tautomerization in salicylic acid: A supersonic free jet study

Abstract

Excited state enol-keto isomerization in salicylic acid ͑SA͒ monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S 1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching ͑3230 cm Ϫ1 ͒, and in-plane bending of the carboxylic group ͑240 cm Ϫ1 ͒. The spectra appear to consist of two components, normal ͑UV͒ and tautomer ͑BLUE͒ emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm Ϫ1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ϳ1 ns experimental time resolution. A nonradiative decay process with an activation energy of ϳ1100 cm Ϫ1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UV fluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S 1 state.

Spectroscopic Properties of Quercetin-3-O-rhamnoside and Quercetin-3-O-rutinoside in Aerosol-OT Reverse Micelles

“…5,6 It has been known that excited state intramolecular proton transfer (ESIPT) by tautomerization has been observed in many molecules having this kind of intramolecular hydrogen bonding, such as o-hydroxybenzaldehyde, salicylic acid, 9-hydroxyphenalenone, polyhydroxyflavone etc. [40][41][42][43][44][45][46][47][48][49][50] Furthermore, in 5-hydroxyflavone, direct evidence of the ESIPT between this -OH at the C-5 and O-11 was observed. 40 Since QCRM and QCRT are also 5-hydroxyflavones, it can be assumed that a similar ESIPT will occur in these molecules.…”

“…The relative stability of these two forms was predicted using "nodal plane" model. 41,43 To verify the appearance of ESIPT in QCRM and QCRT, this "nodal plane" model was applied to quercetin. The schematic representation of S o , S 1 (π)…”

Photolysis of sulfosalicylic acid in aqueous solutions over a wide pH range

“…At pH > 12, the absorption spectrum exhibits a new band with a maximum at 260 nm (ε = 1.56 × 10 4 M −1 cm −1 ), which corresponds to acid tri-anion (SSA 3− ). In non-polar solvents at high concentration of SAD formation of dimeric species is observed [11,25]. In aqueous solutions of SSA no evidence for formation dimeric species was found in the range 10 −5 -10 −2 M of SSA.…”

Ground-State Proton-Transfer Tautomer of the Salicylate Anion

“…[6][7][8][9][10][11][12][13][14] It is evident that the polarity and H-bonding strength of solvents affect the stabilization of ground-state tautomers. Recently 15,16 Bisht et al interpreted the pressureinduced increase of the emission intensity of salicylic acid in nonpolar methylcyclohexane as partly due to increased formation of the ground-state tautomer with pressure. They deduced a partial molar volume change of -4.3 cm 3 /mol for N f T tautomerization.…”