P. Lakshminarasimhan
Tulane University
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Featured researches published by P. Lakshminarasimhan.
Chemical Communications | 1998
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
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
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
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
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
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
P. Lakshminarasimhan; R. B. Sunoj; Jayaraman Chandrasekhar; V. Ramamurthy
Nano Letters | 2002
Nicholas J. Turro; P. Lakshminarasimhan; Steffen Jockusch; Stephanie G. Grancharov; Franz X. Redl
Journal of Physical Chemistry A | 1998
Hsien-Ming Kao; Clare P. Grey; Kasi Pitchumani; P. Lakshminarasimhan; V. Ramamurthy
Organic Letters | 2003
Thomas Poon; Nicholas J. Turro; Jessica Chapman; P. Lakshminarasimhan; Xuegong Lei; Steffen Jockusch; Roberto Franz; Ilyas Washington; Waldemar Adam; Sara G. Bosio