Shruti Maheshwari

Bio: Shruti Maheshwari is an academic researcher. The author has contributed to research in topics: Deprotonation & Molecular orbital. The author has an hindex of 1, co-authored 1 publications receiving 110 citations.

Ground and Excited State Intramolecular Proton Transfer in Salicylic Acid: an Ab Initio Electronic Structure Investigation

Abstract: Energetics of the ground and excited state intramolecular proton transfer in salicylic acid have been studied by ab initio molecular orbital calculations using the 6-31G** basis set at the restricted Hartree -Fock (RHF) and configuration interaction-single excitation (CIS) levels and also using the semiempirical method AM1 at the RHF level as well as with single and pair doubles excitation configuration interaction spanning eight frontier orbitals (PECI ) 8). The ab initio potential energy profile for intramolecular proton transfer in the ground state reveals a single minimum corresponding to the primary form. In the first excited singlet state, however, there are two minima corresponding to the primary and tautomeric forms, separated by a barrier of 6 kcal/mol, thus accounting for dual emission in salicylic acid. Electron density changes with electronic excitation and tautomerism indicate no zwitterion formation. Changes in spectral characteristics with change in pH, due to protonation and deprotonation of salicylic acid, are also accounted for, qualitatively. Although the AM1 calculations suggest a substantial barrier for proton transfer in the ground as well as the first excited state of SA, it predicts the transition wavelength in near quantitative accord with the experimental results for salicylic acid and its protonated and deprotonated forms.

Theoretical Studies of Excited State Proton Transfer in Small Model Systems

Abstract: Ab initio calculations that address the problem of excited-state proton transfer across an intramolecular hydrogen bond are reviewed. Small molecules, such as malonaldehyde, containing such a H-bond are first examined. This work reveals that in comparison to the ground state, the H-bond is strengthened and the transfer barrier reduced in the1ππ* state; opposite trends are noted in the triplet ππ* as well as nπ* states. Replacement of the H-bonding O atoms of malonaldehyde by N has only a small effect upon these results, as does enlargement or reduction of the malonaldehyde ring, coupled with anionic charge. The transfer barrier is linearly related to the equilibrium length of the H-bond in the various states of each system. Attachment of a phenyl ring to malonaldehyde introduces a fundamental asymmetry into the proton transfer potential, as the enol and keto tautomers are inequivalent. Whereas the enol is more stable in the ground and nπ* states, a reversal occurs in the ππ* states, which may be understoo...

Controlling Excited State Single versus Double Proton Transfer for 2,2'-Bipyridyl-3,3'-diol: Solvent Effect

Abstract: In this work, we theoretically investigate the sequential excited state double proton transfer (ESDPT) mechanism of a representative intramolecular hydroxyl (OH)-type hydrogen molecule 2,2′-bipyridyl-3,3′-diol (BP(OH)2). We mainly adopt three kinds of different polar solvents (nonpolar cyclohexane (CYH), polar acetonitrile (ACN), and moderate chloroform (CHCl3)) to explore solvent effects on this system. Two intramolecular hydrogen bonds of BP(OH)2 are testified to be strengthened in the S1 state, which provides possibility for ESDPT process. Explorations of charge redistribution and potential energy surfaces (PESs) reveal ESDPT process. Searching transition state (TS) structures in different polar aprotic solvents, we successfully regulate and control the stepwise ESDPT behaviors of BP(OH)2 through solvent polarity.

Fluorescence Studies of Salicylic Acid Doped Poly(vinyl alcohol) Film as a Water/Humidity Sensor

Abstract: Photoluminescence of salicylic acid (SA) and its sodium salt in poly(vinyl alcohol) (PVA) film and its quenching by water/moisture has been studied by steady-state and time domain fluorescence measurements. The results suggest that salicylic acid is completely ionized and present as a monoanion in PVA film, having a molar extinction coefficient (max) of 3545 M -1 cm -1 , and its emission shows a large Stokes shifted (8300 cm -1 ) fluorescence band with a quantum yield (f) of 0.34 and a decay time (Uf) of 6.7 ns. This emission band is due to excited-state intramolecular proton transfer (ESIPT) and is found to be sensitive to moisture and water contents in organic solvents. The films dipped in organic solvents, viz. dioxane, ethanol, and acetonitrile, containing water shows Stern-Volmer type fluorescence quenching. It is shown that a linear quenching of fluorescence intensity and decay time of SA doped PVA film in the presence of moisture or water in the organic solvents can be useful as an optical sensor for determination of humidity in the range 5%-85% and water contents in these solvents in the range 1%-60% with an accuracy of (0.2%. The response time of the sensor film is about 2 min, and recovery time is less than 1 min.

Spectroscopic Study and Simulation from Recent Structural Models for Eumelanin: I. Monomer, Dimers

Abstract: Spectroscopic simulations of a leading structural model for melanin, the pigment responsible for coloration and photoprotection in humans and animals, were done. We performed density functional theory (DFT) calculations using both the local density approximation (LDA) and the generalized gradient approximation (GGA) on a recent structural model for eumelanin based on higher oligomers of the monomer of neutral 5,6-indolequinone and its reduced forms, semiquinone and hydroquinone. This paper reports on our semiempirical spectroscopic simulations for the monomer and dimer energy-minimized structures. Our second study (part II) extends this approach to higher oligomers (tetramers through hexamers) and points out that the known optical spectra of eumelanins can be adequately explained on this basis.