Autobiography of Krishnan Raghavachari

Autobiography of Krishnan Raghavachar

Autobiography of Krishnan Raghavachari

Autobiography of Krishnan Raghavachar

Autobiography of Krishnan Raghavachari

Autobiography of Krishnan Raghavachar

Investigating the Stacking Interactions Responsible for Driving 3D Self-Association of Tricarb Macrocycles

We have investigated the noncovalent forces that play a crucial role in the three-dimensional (3D) self-association of the tricarb macrocycle (composed of alternating triazoles and carbazoles) to understand the multilayer stacks observed through electron microscopy. To explore this idea quantitatively, we have investigated a stacked dimer model of tricarb, where we consider homochiral as well as heterochiral forms of the dimer. We have computed the rotational potential energy surface of the dimer by conducting an angle-dependent scan between the two macrocycles using different levels of theory including the RI-MP2 ab initio method. We observe that dimers oriented at an angle of 60° exhibit the highest stability, while a secondary minimum is observed at an angle of 30°. While density functional theory (DFT) describes the behavior of both minima very close to that obtained with RI-MP2, semiempirical and MM models appear to obtain only a shoulder instead of the second minimum. To further understand the underlying interactions responsible for stabilizing the self-assembly of the macrocycles, we employed energy decomposition analysis (EDA) using SAPT0. This quantitative assessment allowed us to identify the major contributing noncovalent interactions, including electrostatic, exchange-repulsion, dispersion, and induction. Finally, we expanded our study to evaluate the accuracy of the MIM (molecules-in-molecules) fragmentation methodology in capturing the crucial π-stacking interactions. Our benchmarking results using the MIM method accurately replicated the angle-dependent PES results. This shows the efficacy of MIM in predicting the noncovalent interactions responsible for the construction of 3D and other higher-order nanoarchitectures for tricarb and related compounds

Hydroxyl Migration in Heterotrimetallic Clusters: An Assessment of Fluxionality Pathways

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H₂ liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W₃O₆^– core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W₂TcO₆^– (<b>2</b>) and W₂OsO₆^– (<b>3</b>) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H₂ liberation from H₂O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium

Dimers of Dimers (DOD): A New Fragment-Based Method Applied to Large Water Clusters

We have developed a dimer-based two-body interaction model, denoted “dimers of dimers” (DOD), for the study of water clusters using a fragmentation approach. By including a second layer at a lower level of theory (e.g., Hartree–Fock), our DOD2 model accurately reproduces the total energies of water clusters ranging in size from 16 to 100 molecules using the MP2 method with a variety of basis sets. The mean absolute error in the calculated DOD2 total energy for a broad range of calculations is only 0.83 kcal/mol. The size of the fragments is independent of the cluster size, and the number of fragments grows fairly slowly with the cluster size. Our results suggest that our method should be applicable for the study of large water clusters

H₂S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions

Understanding the mechanistic aspects of heterogeneous reactions on supported metal catalysts is challenging due to several disparate factors, among which the dynamic nature of the surface is a major contributor. In this study, the dynamic aspect of a surface has been probed by choosing small metal clusters as illustrative models. Two anionic hetero-trimetallic clusters, namely, W₂TcO₆^– and W₂OsO₆^–, were reacted with H₂S gas to exhibit splitting of the gas molecule and complete oxygen–sulfur exchange in the metal core. During this atom-exchange process, the core exhibits remarkable fluxionality to augment a thiol proton migration from one metal center to another, as well as a rapid interchange of the terminal and bridging oxygens. The fluxional nature of the core is further evidenced by two oppositely oriented oxo groups working in concert to accomplish the proton transfer, upon introduction of sulfur inside the core. These fluxional processes in the small hetero-trimetallic cores closely resemble the dynamic nature of the surface in a heterogeneous reaction. Throughout the fluxional processes investigated in this study, two-state reactivity and multiple instances of spin crossover are observed in our computational studies. Interestingly, the neutral hetero-trimetallic cores can also undergo complete oxygen–sulfur exchange reaction with H₂S. The investigated metal clusters are promising materials, since they not only can liberate dihydrogen from water (reported in <i>J. Phys. Chem. A</i>, <b>2014</b>, 118, 11047) but also can completely strip the sulfur from environmentally hazardous H₂S gas

Analysis of Different Fragmentation Strategies on a Variety of Large Peptides: Implementation of a Low Level of Theory in Fragment-Based Methods Can Be a Crucial Factor

We have investigated the performance of two classes of fragmentation methods developed in our group (Molecules-in-Molecules (MIM) and Many-Overlapping-Body (MOB) expansion), to reproduce the unfragmented MP2 energies on a test set composed of 10 small to large biomolecules. They have also been assessed to recover the relative energies of different motifs of the acetyl­(ala)₁₈NH₂ system. Performance of different bond-cutting environments and the use of Hartree–Fock and different density functionals (as a low level of theory) in conjunction with the fragmentation strategies have been analyzed. Our investigation shows that while a low level of theory (for recovering long-range interactions) may not be necessary for small peptides, it provides a very effective strategy to accurately reproduce the total and relative energies of larger peptides such as the different motifs of the acetyl­(ala)₁₈NH₂ system. Employing M06-2X as the low level of theory, the calculated mean total energy deviation (maximum deviation) in the total MP2 energies for the 10 molecules in the test set at MIM­(<i>d</i>=3.5Å), MIM­(η=9), and MOB­(<i>d</i>=5Å) are 1.16 (2.31), 0.72 (1.87), and 0.43 (2.02) kcal/mol, respectively. The excellent performance suggests that such fragment-based methods should be of general use for the computation of accurate energies of large biomolecular systems

Prediction of Accurate Thermochemistry of Medium and Large Sized Radicals Using Connectivity-Based Hierarchy (CBH)

Accurate modeling of the chemical reactions in many diverse areas such as combustion, photochemistry, or atmospheric chemistry strongly depends on the availability of thermochemical information of the radicals involved. However, accurate thermochemical investigations of radical systems using state of the art composite methods have mostly been restricted to the study of hydrocarbon radicals of modest size. In an alternative approach, systematic error-canceling thermochemical hierarchy of reaction schemes can be applied to yield accurate results for such systems. In this work, we have extended our connectivity-based hierarchy (CBH) method to the investigation of radical systems. We have calibrated our method using a test set of 30 medium sized radicals to evaluate their heats of formation. The CBH-rad30 test set contains radicals containing diverse functional groups as well as cyclic systems. We demonstrate that the sophisticated error-canceling isoatomic scheme (CBH-2) with modest levels of theory is adequate to provide heats of formation accurate to ∼1.5 kcal/mol. Finally, we predict heats of formation of 19 other large and medium sized radicals for which the accuracy of available heats of formation are less well-known