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Dive into the research topics where P. Lakshminarasimhan is active.

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Featured researches published by P. Lakshminarasimhan.


Chemical Communications | 1998

Energy transfer, proton transfer and electron transfer reactions within zeolites

V. Ramamurthy; P. Lakshminarasimhan; Clare P. Grey; Linda J. Johnston

The chemistry of olefins in zeolites illustrates both the potential complexity and utility of zeolites as reaction media. Of particular interest are the changes in product selectivity that result from carrying out oxidation reactions in the constrained space of the zeolite cavity. Since zeolites are capable of promoting proton and electron transfer reactions one needs to be particularly careful in the choice of a zeolite as a reaction medium.


Journal of Photochemistry and Photobiology A-chemistry | 2002

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: A comparison between solution and zeolite as reaction media

P. Lakshminarasimhan; R.B Sunoj; S Karthikeyan; Jayaraman Chandrasekhar; Linda J. Johnston; V. Ramamurthy

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization.


Tetrahedron Letters | 2001

Controlling the reactive state through cation binding: photochemistry of enones within zeolites

Sundararajan Uppili; Shinsuke Takagi; R. B. Sunoj; P. Lakshminarasimhan; Jayaraman Chandrasekhar; V. Ramamurthy

The nature of the lowest triplet state of enones is altered by the cations present within Y zeolites. Alkali metal ions, such as Li+, are predicted to interact with the carbonyl unit of enones in a collinear fashion and significantly lower both the p-type n and π-2 orbitals. Excited state energies, estimated at the CIS(D)/6-31+G* level, show that the lowest triplet is n-π* in character for the enones, but switch to π–π* on coordination with Li+. Observed product distribution within zeolite is consistent with this theoretical prediction.


Journal of Computational Chemistry | 2001

Configuration interaction and density functional study of the influence of lithium cation complexation on vertical and adiabatic excitation energies of enones

R. B. Sunoj; P. Lakshminarasimhan; V. Ramamurthy; Jayaraman Chandrasekhar

The changes in the excited state energies of representative cyclic enones (cyclopentenone and cyclohexenone) induced by lithium ion coordination have been examined using ab initio and DFT methods. Quantitative estimates of the vertical triplet state energies were obtained using configuration interaction calculations at the CIS and CIS(D) levels with the 6‐31+G(d) basis. Inclusion of perturbative doubles corrections has a marked effect on the relative energies of the n–π* and π–π* triplet states. At both CI and CIS(D) levels, lithium complexation is predicted to raise the energy of the n–π* triplet state much more than the π–π* triplet. The trends obtained at the CIS(D) level are reproduced using B3LYP/6‐31+G(d) calculations. Adiabatic excitation energies were also computed by carrying out geometry optimization of the triplet states at the B3LYP level. While the separation between the geometry optimized n–π* and π–π* triplet states is very small for the parent enones, the π–π* triplet is clearly favored in the lithium complexes. These results suggest the possibility of reversing the reactive photoexcited state in enones through cation complexation. The conclusions provide a rationale for interesting variations in product distributions observed for enones in cation exchanged zeolites.


Tetrahedron Letters | 1997

Cation radical and carbocation mediated reactions within Ca Y zeolite: 1-phenyl 3,4-dihydronaphthalene

K. Pitchumani; P. Lakshminarasimhan; G. Turner; M.G. Bakker; V. Ramamurthy

Reactions of carbocations and radical cation of 1-phenyl 3,4-dihydronaphthalene have been investigated within Ca Y.


Chemical Communications | 1998

Detection of low levels of Brønsted acidity in Na+Y and Na+X zeolites

V. Jayathirtha Rao; Deborah L. Perlstein; Rebecca J. Robbins; P. Lakshminarasimhan; Hsien-Ming Kao; Clare P. Grey; V. Ramamurthy

By using retinyl acetate, retinol and retinyl Schiff base as probes, zeolites NaY and NaX are demonstrated to possess a small number of Bronsted acidic sites; the color test employed here is potentially simple and may be universally applied.


Journal of the American Chemical Society | 2000

Cation−π Interaction Controlled Selective Geometric Photoisomerization of Diphenylcyclopropane

P. Lakshminarasimhan; R. B. Sunoj; Jayaraman Chandrasekhar; V. Ramamurthy


Nano Letters | 2002

Spectroscopic Probe of the Surface of Iron Oxide Nanocrystals

Nicholas J. Turro; P. Lakshminarasimhan; Steffen Jockusch; Stephanie G. Grancharov; Franz X. Redl


Journal of Physical Chemistry A | 1998

Activation Conditions Play a Key Role in the Activity of Zeolite CaY: NMR and Product Studies of Brønsted Acidity

Hsien-Ming Kao; Clare P. Grey; Kasi Pitchumani; P. Lakshminarasimhan; V. Ramamurthy


Organic Letters | 2003

Stereochemical Features of the Physical and Chemical Interactions of Singlet Oxygen with Enecarbamates

Thomas Poon; Nicholas J. Turro; Jessica Chapman; P. Lakshminarasimhan; Xuegong Lei; Steffen Jockusch; Roberto Franz; Ilyas Washington; Waldemar Adam; Sara G. Bosio

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R. B. Sunoj

Indian Institute of Science

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