http://hdl.handle.net/123456789/1312 Thesis submitted by MP -2013 batch as part of their course 2026-04-04T07:56:27Z http://hdl.handle.net/123456789/2123 Title: Light-induced Spatio-temporal Localization of Molecular Wave Packets on Saddle points of potential energy surfaces Authors: Kumar, Naveen; Balanarayan, P. 2021-08-01T00:00:00Z http://hdl.handle.net/123456789/2122 Title: Computational and matrix isolation studies of heterocyclic radicals and azoheteroarene photoswitches Authors: Sah, Chitranjan; Venkataramani, Sugumar Abstract: Free radicals are regarded as one of the important classes of highly reactive, short-lived intermediates containing an unpaired electron. The utility of these free radicals has been proposed by experimentalists and theoreticians in the field of organic synthesis, combustion chemistry, atmospheric chemistry, interstellar chemistry, and biological implications. Several approaches such as conjugation, spin delocalization, and introduction of heteroatoms have been adapted to enhance the stability as well as tuning the reactivity of radicals. In recent times, nitrogen-based heterocyclic radicals have gained immense importance for their various roles in reactive oxygen species (ROS), material chemistry, biofuels, and in constructing organic molecular-based magnets. Among the various heterocyclic radicals studied thus far, all of the radicals potentially resulted in two center-three electrons (2c-3e) interactions except for boryl and cyclopentadienyl radicals as shown in Scheme A.1. Scheme A.1. Five-and six-membered heterocyclic radicals exhibiting (2c-3e) interactions [exceptions are boryl and cyclopentadienyl radicals]. In this regard, the detailed investigation of the electronic structure and reactivity aspects were carried out using quantum chemical calculations. Through space (TS) and through-bond (TB) interactions prevailing between lone-pair and the radical electron provided valuable information regarding the thermodynamic stability of the isomeric radicals. Whereas, spin density value played a crucial role in predicting the bond cleavage in unimolecular decomposition channels aswell as in explaining the reactivity trend. Moreover, attempts have also been made towards the experimental characterization of 2- and 3-dehydropyridine-N-oxide radicals using matrix isolation (MI) technique assisted with infrared spectroscopy and computations. Based on the observations under photochemical conditions, we were able to identify the formation of 3- dehydropyridine-N-oxide radical in an inert gas matrix at 4 K. However, our attempts were not successful in the generation of 2-dehydropyridine-N-oxide radical, but, we did observe a few interesting species due to photoirradiation. Scheme A.2. 2-Hydroxyphenylazo-3,5-dimethylisoxazole (HPAI) undergoing E-Z photoisomerization in an argon matrix at 4 K. Recently, azoheteroarenes have been intensely studied due to their excellent photoswitching properties and exceptional thermal stability of the photoswitched state. Typically, photoisomerization of azo compounds is well known in the solution phase but their studies are limited under cryogenic conditions in the rigid matrix. In this regard, we explored azoheteroarene based photoswitch (i.e. 2-hydroxyphenylazo-3,5-dimethylisoxazole, HPAI) and successfully confirmed the E-Z photoisomerization in an argon matrix at 4 K as shown in Scheme A.2. The sharp spectral features at low temperature in combination with computations allowed us to understand the conformations for both E- and Z-isomers. Overall, theoretical insights on five-and six-membered heterocyclic radicals, and matrix isolation studies of 2- and 3-iodopyridine-N-oxide, 2-hydroxyphenylazo-3,5-dimethylisoxazole will be presented. 2021-12-01T00:00:00Z http://hdl.handle.net/123456789/1731 Title: Matrix-isolation infrared spectroscopy and computational studies of diazine radicals Authors: Saraswat, Mayank; Venkataramani, Sugumar Abstract: Free radicals, in general, are highly reactive and short-lived chemical entities (intermediates) containing one or more unpaired electrons. Their importance is felt in many fields such as organic synthesis, biochemistry, medicinal chemistry, polymer chemistry, atmospheric chemistry, and interstellar chemistry, etc. Various strategies and methods have been adapted to tune and control the stability and in turn, reactivity of radicals that include the introduction of various heteroatoms that can be stabilizing/destabilizing the radicals. Recently, the studies on nitrogen-based heterocyclic radicals gained importance, as these moieties constitute the main building block of several biological systems (like nucleobases, amino acids, etc.), biologically active drug molecules, and are also potential precursors in prebiotic chemistry. Among the various N-heterocycles, diazines are those containing two nitrogen atoms that can be classified as pyridazine (1,2-diazine), pyrimidine (1,3-diazine), and pyrazine (1,4-diazine) depending on the relative position of the two nitrogens. Of all these diazine derivatives, pyrimidine (1,3-diazine) moiety has a significant role in biological compounds specifically in nucleobases such as cytosine, thymine, and uracil, which are the key building blocks of DNA and RNA. Given their biological implications and their relevance in radical damage, systematic studies on diazine-based radicals are quite insightful, and equally intriguing from the fundamental point of view. Since all these radicals have one unpaired electron and two nitrogen lone pairs, the nature and strength of possible “3-centered – 5 electrons” (3c-5e) interactions may play a crucial role in their existence or non-existence, and also their inherent reactivity. To address these, we investigated the electronic structure and unimolecular reactivity aspects of these six-isomeric dehydrodiazine radicals using quantum chemical calculations, which provided the importance of through space (TS) and through bond (TB) interactions between the unpaired electron and the lone pair(s). Attempts have also been made towards the experimental characterization of such transient species using the matrix-isolation (MI) technique in combination with infrared spectroscopy and computations. Under photochemical conditions, we have successfully generated dehydro-diazine radicals (2-dehydropyrimidineand 2-dehydropyrazine radicals from their respective iodo-precursors) in an inert gas matrix at cryogenic conditions (4 K). Moreover, the photochemistry of these diazine radicals has led to the ring-opening and ring-fragmented products, which have relevance in astrochemistry. Interestingly, several nitrogen rich ring-opened products with a molecular formula C 4 H 2 N 2 have been characterized, which are relevant to aza-enediyne antibiotics. Similarly, 4,6- diiodopyrimidine and 3,6- diiodopyridazine have been used to generate didehydrodiazines, which essentially led to ring-opened products. To shed further light on this, electronic structure of didehydrodiazine biradicals (4c-6e interaction) has also been investigated. Overall, the theoretical insights on the dehydro- and didehydrodiazine radicals, and the experimental results including the design of matrix isolation experimental set up will be presented. 2021-07-28T00:00:00Z http://hdl.handle.net/123456789/1705 Title: Cluster mean field approach to low dimensional quantum magnets Authors: Singhania, Ayushi; Kumar, Sanjeev Abstract: Collective behaviour of large number of interacting particles results in fascinating phe- nomena ranging from as simple as freezing of water to as complex as appearance of su- perconductivity. Magnetism is a remarkable example of how quantum physics can spring up new surprises even in cases where relevant particles remain immobile. It covers wide scale of complexity, from magnets sticking to our household fridges to the exotic quantum spin liquid phases that define the forefront of current research in quantum magnetism. Low dimensional spin-1/2 magnetic systems are ideal candidates for observing and un- covering mysteries of quantum physics as the combination of low dimensionality and low spin quantum number enhances quantum fluctuations. Motivated by their importance in understanding fundamental aspect of quantum mechanics and potential applications, a plethora of low dimensional magnetic materials have been discovered and studied experi- mentally. However, strictly 1D or 2D magnets are almost never realized in real materials, as contribution from spins in neighboring chains or planes affect the magnetic ordering. This often leads to unexpected ordering and phase transitions. This thesis attempts to understand such low temperature behaviour of real materials, in terms of quasi-1D and 2D model spin Hamiltonians studied using cluster mean field theory (CMFT). The key idea of CMFT is to treat all interaction links located within the cluster exactly, and to make use of the conventional mean field decoupling for interaction links connecting the cluster and the environment. The approach allows for an accurate treatment of short range spatial correlations, as well as thermodynamic behavior, in the mean field spirit. The technique captures the subtle competition between different possibilities of magnetic ordering at the level of finite-size calculations. CMFT successfully explains the origin of low-temperature peak observed in specific heat data reported in the experiments per- formed on CuInVO 5 . For the frustrated ferromagnet β−TeVO 4 , CMFT is able to uncover multiple phase transitions in the absence of applied field. In presence of field, it identifies complex orders such as quadrupolar and vector chiral orders along with specific anomalies like re-entrant transition similar to experimental observations. Furthermore, a problem of disordered antiferromagnetic spin chain with anisotropic impurities is explored. CMFT analysis reveals that a fraction of anisotropic impurities is capable of inducing a Néel type ordering. In addition to providing a satisfactory understanding of observations on CuInVO 5 and β-TeVO 4 , this thesis highlights that CMFT can become a powerful tool in understanding the nature of magnetic order emerging at low temperatures in frustrated as well as disordered magnets. 2021-07-28T00:00:00Z