http://hdl.handle.net/2005/12 2013-05-04T19:27:24Z http://hdl.handle.net/2005/1937 Title: Theoretical Studies Of The Thermodynamics And Kinetics Of Selected Single-Molecule Systems Authors: Chatterjee, Debarati Abstract: This thesis is a report of the work I have done over the last five years to study thermodynamic and kinetic aspects of single-molecule behavior in the condensed phase. It is concerned specifically with the development of analytically tractable models of various phenomena that have been observed in experiments on such single-molecule systems as colloids, double-stranded DNA, multi-unit proteins, and enzymes. In fluid environments, the energetics, spatial conformations, and chemical reactivity of these systems undergo fluctuations that can be characterized experimentally in terms of time correlation functions, survival probabilities, mean first passage times, and related statistical parameters. The thesis shows how many of these quantities can be calculated in closed form from a model based on simple Brownian motion, or generalizations of it involving fractional calculus. The theoretical results obtained here have been shown to agree qualitatively or quantitatively with a range of experimental data. The thesis therefore demonstrates the effectiveness of Brownian motion concepts as a paradigm of stochasticity in biological processes. 2013-02-24T18:30:00Z http://hdl.handle.net/2005/1980 Title: Electrochemical Supercapacitor Investigations Of MnO2 And Mn(OH)2 Authors: Nayak, Prasant Kumar Abstract: Electrical double-layer formed at the electrode/electrolyte interface in combination with electron-transfer reaction can lead to many important applications of electrochemistry, including energy storage devices, namely, batteries, fuel cells and electrochemical supercapacitors. Electrochemical supercapacitors are characterized by their higher power density as compared to batteries and higher energy density than the conventional electrostatic and electrolytic capacitors. Thus, supercapacitors are useful as auxiliary energy storage devices along with primary sources such as batteries or fuel cells for the purpose of power enhancement in short pulse applications. These are expected to be useful in hybrid devices together with batteries or fuel cells, in electric vehicle propulsion systems. Among the various materials studied for electrochemical supercapacitors, carbonaceous materials, transition metal oxides and conducting polymers are important. Carbon in various forms is used as a double-layer capacitor material, which stores charge by electrostatic charge separation at the electrode/electrolyte interface. The specific capacitance (SC) of high surface area activated carbon is about 100 F g-1 in aqueous electrolytes. Transition metal oxides have attracted considerable attention as electrode materials for supercapacitors because of the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and convenience in handling. Hydrated RuO2 prepared by sol-gel process exhibited a SC as high as 720 F g-1. However, high cost, low porosity and toxic nature of RuO2 limit its commercialization in supercapacitors. On the otherhand, MnO2 is an attractive electrode material as it is electrochemically active, cheap, environmentally benign, and its resources are abundant in nature. In an early report on the capacitance properties of MnO2 by Lee and Goodenough [J. Solid State Chem. 144 (1999) 220], amorphous hydrous MnO2 synthesized by co-precipitation method exhibited a SC of 203 F g-1 in 2 M KCl electrolyte. According to the charge-storage mechanism of MnO2 involving MnO2 + M+ + e- ↔ (MnOO)-M+ (where M+ = Li+, Na+, K+ etc.), a SC of 1110 F g-1 is expected over a potential window of 1.0 V. However, SC values in the range of 100-200 F g-1 are reported in the literature. The low values of SC are because of the charge-storage is confined to surface region of MnO2 particles or films. It is desirable to enhance the SC of MnO2 to a value close to the theoretical value. In view of this, attempts are made to enhance the SC of MnO2 by adopting different synthetic procedures such as electrochemical method for depositing MnO2 and also nanostructured mesoporous MnO2 by polyol route, hydrothermal route and sonochemical method in the present studies. As the charge-storage mechanism of MnO2 involves the surface insertion/deinsertion of cations from the electrolyte during discharge/charge processes, respectively, the capacitance properties of MnO2 are studied in various aqueous electrolytes containing monovalent (Na+), bivalent (Mg2+, Ca2+, Sr2+ and Ba2+) and trivalent (La3+) cations. The mass variation occurring at the electrode during the charge/discharge of MnO2 is examined by electrochemical quartz crystal microbalance (EQCM) study. In addition to this, the kinetics of electrodeposition and capacitance properties of Mn(OH)2 are studied by employing EQCM. Also, properties of asymmetric capacitors assembled with Mn(OH)2 as the positive electrode and carbon as the negative electrode are studied and compared with symmetric Mn(OH)2 capacitors. Furthermore, attempts are made to increase the potential window of Co(OH)2 in alkaline and neutral electrolytes. The contents of the thesis by Chapter-wise are given below. Chapter 1 introduces the importance of electrochemistry in energy storage and conversion, basics of electrochemical power sources, importance of some electroactive materials in electrochemical energy storage, different synthetic procedures for MnO2 and its application in electrochemical supercapacitors. Transition metal oxides are widely studied because of their variable oxidation states, high electrochemical activity, abundance in nature and environmental compatibility. Various reports appeared in the form of open publications on supercapacitor studies of transition metal oxides such as RuO2, MnO2, Fe3O4, Co(OH)2, Ni(OH)2, NiO, etc., are briefly reviewed. The chapter ends with statements on objectives of the studies carried out and reported in the thesis. Chapter 2 provides experimental procedures and methodologies used for the studies reported in the thesis. Different experimental routes adopted for synthesis of MnO2, Mn(OH)2 and Co(OH)2 used for the studies are described. Also included are brief descriptions of various physicochemical and electrochemical techniques employed for the investigations. In Chapter 3, MnO2 samples synthesized by various routes such as electrochemical method, polyol route, hydrothermal route and sonochemical method are studied. MnO2 and Mn(OH)2 are simultaneously electrodeposited on the anode and the cathode, respectively, in a galvanostatic electrolysis cell consisting of aqueous Mn(NO3)2 electrolyte. MnO2/SS and Mn(OH)2/SS electrodes are used as the negative and the positive electrodes, respectively, in an asymmetric Mn(OH)2//MnO2 supercapacitor. MnO2 samples are prepared at room temperature and in hydrothermal method at a temperature of 140 ◦C by reduction of KMnO4 with poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) or P123 as a reductant. Also, MnO2 is prepared from KMnO4 by hydrothermal method without using any reducing agent. This procedure requires a temperature of 180 ◦C and 24 h duration. MnO2 is also synthesized with an ultrasonic aided procedure. The electrochemical capacitance properties of MnO2 samples synthesized by various routes are investigated. A maximum SC of 264 F g-1 is obtained at a current density of 0.5 mA cm-2 (1.0 A g-1) for MnO2 prepared by sonochemical method. The capacitance properties of MnO2 are generally studied in neutral aqueous Na2SO4 electrolytes. In Chapter 4, electrolytes of NaNO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2 and also La(NO3)3 are studied and the results are compared with Na2SO4 electrolyte. Among the alkaline earth salt solutions, higher SC values are obtained in Mg(NO3)2 and Ca(NO3)2 electrolytes than in the rest of the electrolytes. Furthermore, MnO2 exhibits capacitance behaviour in La(NO3)3 solution with enhanced SC in comparison with NaNO3 and Mg(NO3)2 solutions. The SC increases with an increase in charge on the cation (Na+, Mg2+ and La3+). The values of SC measured in Na+, Mg2+ and La3+ electrolytes are 190, 220 and 257 F g-1, respectively at a c.d. of 0.5 mA cm-2 (1.0 A g-1). Rate capabilities are also found to be different in different electrolytes. Specific energy and specific power are calculated and presented as Ragone plots. The presence of divalent and trivalent cations inserted onto MnO2 is identified by X-ray photoelectron spectroscopy. EQCM is employed to monitor the increased mass variations that accompany reversible adsorption/desorption of Na+, Mg2+ and La3+ ions onto MnO2. In Chapter 5, EQCM has been used to study the kinetics of electrochemical precipitation of Mn(OH)2 on Au-crystal and its capacitance properties. From the EQCM data, it is inferred that NO3- ions get adsorbed on Au-crystal, and then undergo reduction resulting an increase in pH near the electrode surface. Precipitation of Mn2+ occurs as Mn(OH)2, resulting an increase in mass of the Au-crystal. On charging, Mn(OH)2 undergoes oxidation to MnO2, which exhibits electrochemical supercapacitor behaviour on subjecting to cycling in aqueous Na2SO4 electrolyte. EQCM data indicates the mass variations corresponding to surface insertion/extraction of Na+ ions during discharge/charge cycling of Mn(OH)2 in aqueous Na2SO4 electrolyte. In Chapter 6, Mn(OH)2 synthesized by precipitation of MnSO4 with NH4OH solution is studied for capacitance properties. A SC of 141 F g-1 is obtained for the Mn(OH)2 at a c.d. of 0.66 A g-1 in 1.0 M Na2SO4 electrolyte in the potential range of 0-1.0 V vs. standard calomel electrode (SCE). Also, carbon electrode made from high surface area carbon exhibits a SC of 158 F g-1 at a c.d. of 0.81 A g-1 in the potential range of 0 to -1.0 V vs. SCE. Asymmetric capacitors are assembled by combining Mn(OH)2 as the positive and carbon as the negative electrodes. The asymmetric capacitor has a SC of 39 F g-1 at a c.d. of 0.42 A g-1 in the operating voltage of 1.8 V. However, a symmetric capacitor consisting of two Mn(OH)2 electrodes provides a SC of 11 F g-1 only at a c.d. of 0.24 A g-1 in an operating voltage of 1.2 V. In Chapter 7, MnO2 synthesized by reduction of KMnO4 using ethylene glycol is used for fabrication of large area electrodes. Stainless steel (SS) mesh of 3 cm x 3 cm with geometrical area of 18 cm2 is used as current collector. Three symmetrical electrochemical supercapacitors (capacitance of about 100 F per each at a current of 0.2 A) are assembled, each with 11 electrodes positioned in parallel. Six alternate electrodes are stacked as the negative terminal and the other five as the positive terminal. The electrochemical properties of MnO2 supercapacitors are studied by galvanostatic charge-discharge cycling and ac impedance in 1.0 M Na2SO4 electrolyte. Also, the capacitors are combined in parallel as well as in series and the capacitance is evaluated. The practical application of the electrochemical supercapacitors is shown by demonstrating the running of a toy fan connected to the charged capacitor as well as the glowing of LED cell connected to charged supercapacitors connected in series. A parallel combination of batteries and capacitors is also demonstrated. Capacitor studies of Co(OH)2 over a limited potential window in alkaline electrolytes are reported in the literature. A high potential window of a capacitor material is desirable for using in a device. In Chapter 8, experiments are conducted to understand the reason for a low potential window for Co(OH)2 as a capacitor material and also to increase its potential window. Experiments are conducted in aqueous NaOH and Na2SO4 electrolytes of various concentrations using electrochemically precipitated Co(OH)2 on stainless steel current collectors in an aqueous Co(NO3)2 electrolyte. Based on the potential window, specific capacitance and specific energy, it is found that 0.05 M NaOH electrolyte is more appropriate for capacitor properties of Co(OH)2 than the rest of the electrolytes studied. Using a Co(OH)2 electrode with a specific mass of 1.0 mg cm-2 in 0.05 M NaOH, a SC of about 380 F g-1 is obtained with a potential window of 0.85 V at a charge-discharge c.d. of 10 A g-1 (10 mA cm-2). The work presented in this thesis is carried out by the candidate as a part of Ph. D. training program and most of the results have been published in the literature. A list of publications of the candidate is enclosed below. It is hoped that the studies reported here will constitute a worthwhile contribution. 2013-04-29T18:30:00Z http://hdl.handle.net/2005/1657 Title: A Computational Study Of Nucleophilic Attacks In Organometallic Complexes Authors: Dinda, Shrabani Abstract: A wide variety of computational methods are available for exploring molecular structures and reactivity in chemistry. These range from molecular mechanics calculations allowing determination of the geometry of a molecule to ab initio calculations for the electronic structure of compounds. Electronic structure calculations can be carried out with sufficient rigor so that the results are now comparable with experimental results in many cases. Density Functional Theory (DFT) with hybrid functional like B3LYP, for example, is very popular especially for studies on organometallic molecules and their reactions. Traditional ab initio approaches including Hartree-Fock (HF) and post-HF methods that include configuration interaction, such as MP2 and MP4 continue to be used, often for comparison with DFT based methods. Semi-empirical methods now appear to have only limited use except in large systems, in combination with molecular mechanics (MM) calculations. A relatively new use of MM for large systems is in hybrid calculations where the reactive center of the system is treated at a higher level leaving the remainder to be treated at the MM level. These hybrid QM/MM (quantum mechanics/molecular mechanics) calculations, such as ONIOM (our own n-layered integrated molecular orbital and molecular mechanics developed by Morokuma and co-workers) enable one to treat the steric bulk of the big system effectively and computationally efficiently. They appear to be very standard methods particularly in studies relating to reactions of organometallic systems and structures of large biomolecules. A short description of these methods is given below. • ab initio: a wide variety of programs that calculate the electronic structure of molecules using the Schrödinger equation, the values of the fundamental constants and the atomic numbers of the atoms present (Atkins, 1991). Molecular structures, optimized as a function of the electronic structure, are valuable starting points for many studies. • Density Functional Theory (DFT): a theoretical model in which the energy of an N-electron system is described as a functional of the density. • Semi-empirical techniques use approximations to evaluate the overlap, repulsion and exchange integrals in solving the Schrodinger equation. Often, these integrals are not evaluated but estimated to reproduce experimental data. • Molecular mechanics uses classical physics to explain and interpret the behavior of atoms and molecules. • Molecular dynamics (MD): Newton’s laws of motion are used to examine the time-dependent behavior of systems, including vibrations and Brownian motion, using a classical mechanical description. When combined with DFT, it leads to the Car-Parrinello method. • QM/MM method: It is a molecular simulation method that combines the strength of both QM (accuracy) and MM (speed) calculations, thus resulting in an extremely powerful tool for the study of bigger systems like chemical process in solution, interaction of drugs with biomolecules etc. Several commercial and educational packages in computational chemistry include a suite of programs that enable study of organic and organometallic molecules in an integrated fashion. While no list can be comprehensive, those that are more popular and useful are listed in several websites URL (http://www.ccl.net/chemistry/links/software/index.shtml). In the early days of computational chemistry up to 1980's, detailed studies were only carried out on small organic compounds or empirical studies were carried out on transition metal containing organometallics. However, in recent times, significant advancements in theoretical methods and computer capability (hardware and software), have led to the acceleration of theoretical and computational studies of complex systems including compounds containing transition metal elements. Computational and theoretical studies of organometallic complexes and their reactions have gained immense popularity and the numbers of papers including theoretical studies are dramatically increasing. One reason for this popularity is that organometallic complexes exhibit unusual geometries, bonding, and reactivity which often do not fall into the domain of inorganic or organic chemistry making them difficult to understand. Catalysis is one of the most extensively studied areas in organometallic chemistry where computational studies already make a real and valuable contribution to the analysis and interpretation of experimental data. However, what might be called ‘in silico’ catalyst screening and design, has rarely been achieved. One might say that successful prediction of catalyst performance is still a dream. A recent review summarizes the current state of the art in computational chemistry as applied to organometallic catalysis, covering both calculated ligand property descriptors and mechanistic studies of catalytic cycles.1 Some of the widely studied catalytic reactions of current interest, that provide huge scope for computational and theoretical analysis, are allylic alkylation (Pd),2 hydrogenation (Rh),3 hydroformylation (Rh),4 alkene metathesis (Ru),5 cross-coupling (Pd),6 C–H activation (Pd)7 and amination (Pd).8 There are many more examples where computational studies appear to be very useful for analysis of crystal structures and NMR structures or prediction of structures where no experimental data are available for complicated organometallic systems. There are a number of studies on drug-DNA/nucleobases interactions using QM/MM-MD simulations where people have investigated the interactions of metal complexes with double stranded (ds) DNA/nucleobases and the effects of their binding on the local and the global structure of DNA. QM/MM methods are also very helpful for studying catalytic reactions, interpretation of structure of large systems (proteins) and understanding reactions in biological systems. Scope of the Thesis In this thesis an attempt is made to use computational chemistry to understand organometallic reactions that are of significance from biological and synthetic view points, such as the action of organometallic complexes on DNA and the mechanism of some catalytic reactions. In many of these cases, the key step involved a nucleophillic attack. Specifically four such problems have been addressed where experimental results are not sufficient to provide a complete mechanistic picture of the reaction. Hence, the thesis contains four chapters with each having an independent brief introduction. The first chapter deals with the substitution reaction where water replaces chloride ion in the piano stool type ruthenium (II)-arene complexes and subsequently coordination of Ru to guanine/adenine occurs in these complexes. These steps have been studied using density functional theory at the B3LYP level. The complexes have promising anticancer activity. These nucleophilic substitution reactions are very important for activating these complexes so that they can interact with DNA, because DNA is thought to be primary target for their anticancer activity. In this chapter, both associative and dissociative pathways have been explored in the gas phase, as well as in the presence of other solvents for substitution reactions. Among the associative paths, a variety of possibilities can exist for the hydrolysis based on the direction of the nucleophilic attack by a water molecule. The proposed theoretical model for hydrolysis provides new insight into the hydrolysis process in half sandwich ruthenium complexes. The second chapter deals with the QM/MM calculations to investigate the structural and electronic properties of drug-DNA interactions, where DNA acts as nucleophile towards the metal complex. A series of piano-stool type ruthenium (II)-arene complexes were selected for the present study. These interactions were analyzed using the two layer ONIOM method. The importance of this study lies in the detailed understanding of factors that govern DNA binding and reactivity which is clearly of great pharmacological interest, as it may provide the basis for designing better anticancer agents. Experimental results that explore the structural feature of DNA-metal complexes at a molecular level are very limited. Thus theoretical calculations of molecular and electronic structure represent a valuable complement to experiments. They provide an alternative way to explore structure-activity relationships, and the drug binding mechanism, in detail. The third chapter reports the use of QM/MM methods in understanding the reaction mechanism and enantioselectivity in an organic transformation. In this section, a computational investigation of the enantioselectivity observed in the allylation of cinnamaldehyde, catalyzed by chiral platinum phosphinite complexes, have been carried out. The catalysts are ascorbic acid based phosphinite complexes where enantioselectivity depends on the substitution of benzyl groups on the chiral phosphinite ligands. From the experiment, it is not clear how the effect of an ancillary ligand can make such a big impact on enantioselectivity. To find out the origin of stereoselectivity, a computational study was taken up. A reaction mechanism was established where the nucleophilic attack determines the rate of the reaction and the corresponding enantioselectivity. A screening process has been utilized to select relevant reactant adducts and corresponding transition states from approximately 200 theoretically possible conformers using MM calculations. Finally with the help of QM/MM calculations, the numbers of contributions of these conformers were estimated. This approach correctly predicts the enantioselectivity in these reactions catalyzed by these complexes especially when the experimental enantioselectivity is very high. The fourth chapter of the thesis discusses the use of computational techniques to study the nucleophilic attack of an imine on a Ti-olefin complex. The reaction of Grignard reagents with imines mediated by stoichiometric amounts of titanium isopropoxide has been reported recently. On the basis of deuterium labeling experiments, nucleophilic attack of an imine on a Ti-olefin complex was believed to be a key step. Effect of deuterium labeling on the ratio of products formed is not easy to understand from experiments. Hence a computational study was performed using the DFT method to establish the mechanism of substitution and to understand the role of deuterium labeling. The thesis also includes a study of Cu-Cu interactions using Atoms in Molecules (AIM) theory in copper complexes with reasonably short Cu-Cu distances. The concept of bond critical points (BCP) from AIM analysis is employed to investigate the CuI-CuI bonding interactions in ligand unsupported copper complexes where the CuI-CuI contacts are shorter than the sum of their van der Waals radii. There is extensive debate about the nature of interactions between d10 "closed shell" systems in copper (CuI) complexes, which is known as cuprophilicity. In this study, an attempt has been made to compute the electron density between the two CuI centers and examine the nature of this “interaction”. As this falls outside the main theme of nucleophilic interactions in metal complexes, it has been relegated to an appendix. 2012-04-22T18:30:00Z http://hdl.handle.net/2005/1656 Title: Time Resolved Resonance Raman Spectroscopic Studies Of Heterocyclic Aromatic Systems Authors: Sahoo, Sangram Keshari Abstract: Benzophenone (BP) and substituted BPs constitute a major class of aromatic ketones and are of potential interest in various areas of excited state solution phase photochemistry and photobiology. High triplet state energy, faster rate of intersystem crossing (ISC) and higher triplet state quantum yield enables BP systems as potential photosensitizers via triplet energy transfer mechanism. The short lived triplet state of BP systems are highly reactive and acts as potential electron acceptor and interesting photochemical behavior have been observed for photoinduced electron transfer reactions in various solvent media, in particular for donor-bridgeacceptor (D-B-A) family. Though detailed spectroscopic studies of BP and substituted BP are documented, not much attention are given to its heterocyclic analogue. Substitution of aromatic ring carbon with one or more heteroatom (N and S) results in drastical change in photochemical properties and excited state reactivity. In solution phase and in nanosecond time domain heteroaromatic ketones form the triplet excited state that upon subsequent photoreactions, leads to formation of short lived species viz. radicals, ions and radical ions. Therefore exploring the trends in excited state reactivity with the variation with functional group and ring substitution and solvent medium is of considerable interest. The complete reaction mechanism of a photoreaction can be understood by studying reactivity of various short lived intermediates formed. In solution phase, the reactivity of a certain species or rate of a chemical reaction can be well understood by correlating to its structure. This approach requires accurate reproducible techniques for the excited state structural determination. Wide range of time resolved (TR) spectroscopies spanning over whole electromagnetic spectrum have been developed over decades and successfully applied to study excited state phenomena. In a typical two beam experiment, the pump pulse excites the molecular system to higher electronic state and the probe pulse records the spectrum of intermediate species at variable delay time with respect to the pump. The data from different TR techniques used to be complementary in nature and the combination helps in a deeper understanding of excited state reaction mechanism. Though time resolved absorption (TRA) is the most popular and oldest technique to study the excited state photoreactions, no structural information and the poor spectral resolution of the broad and overlapping absorption bands are the limitations towards predicting the reactive intermediates with accuracy. However time resolved resonance Raman (TR3) spectroscopy is a very sensitive technique to obtain vibrational structural information of short lived intermediates. The position and intensity of highly resolved Raman bands provide information about the structural and kinetics parameters respectively. From a set of Raman spectra along various delay time, structure of multiple intermediates evolved for parallel photoreactions can be predicted accurately. We have employed TRA, TR3 and density functional theoretical (DFT) calculation to address few fundamental questions about effect of solvent and ring substitution on the excited state structure and energetics of heterocyclic ketones, hence the reactivity. Comparing the experimental findings with the theoretical output not only makes the data more accurate but also several additional conclusions can be drawn that could not be performed only with the experimental modality. In chapter 1 of the thesis, we have presented a general summary of photophysical phenomena and measured properties and parameters of heterocyclic ketones. Typical photoreactions involving various related aromatic ketones obtained from literature are discussed. This is followed by a brief account of theory of resonance Raman spectroscopy and density functional theoretical calculation. The objectives of the present investigation are highlighted. The detailed assembly of experimental techniques employed for present investigation is discussed in chapter 2. The lasers, spectrometers, collection optics, detection systems and data collection and analysis procedures are briefly illustrated for individual set up. The theory of methods of DFT calculations is also discussed. The effect of substitution of N atom in the aromatic rings on excited state structure and reactivity (hydrogen abstraction reaction) for isomeric (2, 3, 4) benzoylpyridines (BzPy) in various solvents is studied using the above experimental and theoretical methodologies and is presented in Chapter 3. In neutral solvents viz. acetonitrile and carbon tetrachloride the photogenerated lowest triplet state (T1) is observed to be formed that follow monoexponetial decay. In the presence of hydrogen donating solvents like methanol and isopropanol the triplet state is found to undergo hydrogen abstraction reaction to form a ketyl radical and solvent radical. The lifetime and absorption and Raman features of triplet state and ketyl radicals are entirely different from each other and lack any overlapping characteristics. The observed enhanced reactivity of BzPy in comparison to BP is believed to be because of the introduction of the N hetero atom in one of the phenyl ring. From the theoretical data, it was clear that more planarity is attained in case of BzPy as compared to BP and contributes to the enhanced reactivity. The spin density calculation shows that one third of the spin is localized in the phenyl ring in case of BP. The total spin density on Phenyl ring is 0.62 and on carbonyl group is 1.45. In case of BzPy the spin density on phenyl ring is 0.45 and on carbonyl group is 1.59. This indicates that in the excited state the spin is localized more on the carbonyl group. Also from charge density calculation using DFT it is clear that in the triplet state of BzPy the oxygen atom of C=O group is more positive than in case of BP which makes it more electrophilic. Among the three isomeric BzPy the trend in charge density is dependent on the position of nitrogen and found to be in the order of 2-BzPy>3-BzPy>4-BzPy. This can be explained on the basis of -I and –M effect of N atom and the extent depends on its position. So the trend for case of photoreduction follows the order 2-BzPy>3-BzPy>4-BzPy. The hydrogen abstraction reaction used to be considerably fast that produces a substrate ketyl radical and solvent radical (donor radical). These radicals further can dimerise to form various photoproducts viz. Pinacols or can form a stable complex between them. The fate of the radicals formed as a result of hydrogen abstraction of 4-BzPy and the accurate characterization of the adduct is explained in Chapter 4. In the present case the cross coupling reaction of the radicals is observed at longer delay time to form a light absorbing transient (LAT) which is the dominant pathway over other parallel reactions. The exact position of the donor radical in the complex is predicted by correlating the experimental Raman bands and theoretically obtained structural parameters and vibrational frequency. The adduct formed as a result of cross coupling reaction was identified as p-LAT, 2-[4-(hydroxylpyridylmethylene)cyclohexa-2,5dienyl]propan-2-ol. In case of benzoylthiophenes (BzTh), the effect of substitution of S atom on the excited state structure and reactivity towards various hydrogen donors viz. phenol and indole in different solvents are presented in Chapter 5. The difference in rate and mechanism of photoreaction for both the hydrogen donors are compared. For TPK the T1 state is of ππ* character and the T2 state is of nπ* character as is confirmed by flash photolysis and low temperature phosphorescence spectra in EPA matrix. The CO bond length for the triplet state species is more than that of ground state. In case of the ππ* triplet prominent structural changes in thienyl ring are observed and the phenyl ring remains much unaltered. The reaction of the triplet state species with phenol in two different solvents shows a relatively faster rate of reaction. If only ππ* triplet has been taking part in reaction, it might have resulted in slow reaction rate. Because the reaction rate is fairly high, It is concluded that not only ππ* triplet is involved in reaction but there is a contribution from the little higher energy T2 state having nπ* character. The reactivity trends towards hydrogen transfer reaction for three isomeric dithienyl ketones with respect to the position of heteroatoms in the ring are presented in Chapter 6. Energetically close lying (ππ* and nπ*) triplet states are observed to undergo state switching with the change in position of heteroatom in the ring and thus define the characteristics of the triplet state and plays important role in predicting the reactivity trend. Brief summary of the present investigation along with important possible extensions of the present work in described in Chapter 7. 2012-04-22T18:30:00Z