Anant D. Kulkarni

Performance of Density Functional Theory and Møller-Plesset Second-Order Perturbation Theory for Structural Parameters in Complexes of Ru.

We assess the performance of density functional theory (DFT) and Møller-Plesset second-order perturbation theory (MP2) for predicting structural parameters in Ru complexes, in particular, a Ru(IV) allyl dicationic complex with the formula [Ru(η(5)-Cp*)(η(3)-CH2CHCHC6H5)(NCCH3)2](2+) and the molecules RuO4 and Ru(C2O4)2(H2O)2(-), where Cp* denotes C5Me5 and Me denotes methyl. The density functionals studied are B3LYP, B3PW91, M05, M06, M06-L, MOHLYP, MPW3LYP, PBE0, PW6B95, SOGGA, τHCTHhyb, ωB97X, and ωB97X-D, in combination with three different basis sets, namely, LANL2DZ, def2-SVP, and def2-TZVP. The theoretically computed Ru-C distances corresponding to the phenylallyl complex are especially well predicted by the SOGGA (pure DFT) and ωB97X-D (DFT plus an empirical molecular mechanics term) methods. This contrasts with an article in this Journal [ Calhorda , M. J. , Pregosin , P. S. , and Veiros , L. F. J. Chem. Theory Comput. 2007 , 3 , 665 - 670 ] in which it was found that DFT cannot account for these Ru-C distances. Averaging over four Ru-C distances in the allyl complex and three unique Ru-O distances in RuO4 and Ru(C2O4)2(H2O)2(-), the SOGGA and ωB97X-D methods have both a smaller mean unsigned error than MP2 and the same maximum error. The M06, PW6B95, PBE0, M06-L, and ωB97X density functionals also have a smaller or the same mean unsigned error as MP2.

H⋯π complexes of acetylene–benzene: a matrix isolation and computational study

Abstract Hydrogen bonded H⋯π complexes of C 2 H 2 and C 6 H 6 were studied both computationally and experimentally. Computationally, C 2 H 2 –C 6 H 6 complexes of 1:1 and 2:1 stoichiometries were identified. The molecular structure and stabilisation energies of the complexes were calculated at the HF, MP2, MP2(Full) and B3LYP levels of theory employing basis sets ranging from 6-31G(d,p) and 6-31++G(d,p) while the frequency calculations were performed at HF, B3LYP and MP2 levels using 6-31G(d,p) and 6-31G++(d,p) basis sets. Using matrix isolation infrared spectroscopy, we observed a 1:1 adduct in an argon matrix. Formation of the adduct was evidenced by shifts in the vibrational frequencies of the acetylene and benzene submolecules in the complex. Though our computations showed two types of 1:1 complexes—one where the acetylene is the proton donor and another where the benzene is the proton donor, experimentally, we observed only the complex, where acetylene acts as a proton donor to the π cloud of benzene.

Many-body interaction analysis: Algorithm development and application to large molecular clusters

A completely automated algorithm for performing many-body interaction energy analysis of clusters (MBAC) [M. J. Elrodt and R. J. Saykally, Chem. Rev. 94, 1975 (1994); S. S. Xantheas, J. Chem. Phys. 104, 8821 (1996)] at restricted Hartree-Fock (RHF)/MA Plesset 2nd order perturbation theory (MP2)/density functional theory (DFT) level of theory is reported. Use of superior guess density matrices (DMs) for smaller fragments generated from DM of the parent system and elimination of energetically insignificant higher-body combinations, leads to a more efficient performance (speed-up up to 2) compared to the conventional procedure. MBAC approach has been tested out on several large-sized weakly bound molecular clusters such as (H(2)O)(n), n=8, 12, 16, 20 and hydrated clusters of amides and aldehydes. The MBAC results indicate that the amides interact more strongly with water than aldehydes in these clusters. It also reconfirms minimization of the basis set superposition error for large cluster on using superior quality basis set. In case of larger weakly bound clusters, the contributions higher than four body are found to be repulsive in nature and smaller in magnitude. The reason for this may be attributed to the increased random orientations of the interacting molecules separated from each other by large distances.

Effect of additional hydrogen peroxide to H2O2⋯(H2O)n, n=1 and 2 complexes: Quantum chemical study

Hydrogen peroxide, H2O2, acts as a particularly strong reactant in aqueous environment. It has been demonstrated earlier that agglomerates with a single peroxide interacting with one and two water molecules manifest in several stable conformers within a narrow energy range. In the present study we seek structural changes brought out by adding an extra H2O2 to these systems at molecular level employing ab initio quantum chemical methods, viz., restricted Hartree-Fock and the second order Moller-Plesset perturbation theory. These clusters exhibit consistent trends in energy hierarchy at both the levels. Further, a many body interaction energy analysis quantifies the strength and cooperativity of hydrogen bonding in the (H2O2)2...(H2O)n, (n=1 and 2) clusters, bringing out structuring/destructuring effects attributed to attachment of water and hydrogen peroxide molecules.

Methanol clusters (CH3OH)n, n = 3–6 in external electric fields: Density functional theory approach

Structural evolution of cyclic and branched-cyclic methanol clusters containing three to six molecules, under the influence of externally applied uniform static electric field is studied within the density functional theory. Akin to the situation for water clusters, the electric field is seen to stretch the intermolecular hydrogen bonds, and eventually break the H-bonded network at certain characteristic threshold field values of field strength in the range 0.009-0.016 a.u., yielding linear or branched structures with a lower energy. These structural transitions are characterized by an abrupt increase in the electric dipole moment riding over its otherwise steady nonlinear increase with the applied field. The field tends to rupture the H-bonded structure; consequently, the number of hydrogen bonds decreases with increasing field strength. Vibrational spectra analyzed for fields applied perpendicular to the cyclic ring structures bring out the shifts in the OH ring vibrations (blueshift) and the CO stretch vibrations (redshift). For a given field strength, the blueshifts increase with the number of molecules in the ring and are found to be generally larger than those in the corresponding water cluster counterparts.

Microsolvation of methyl hydrogen peroxide: Ab initio quantum chemical approach

Methyl hydrogen peroxide (MHP), one of the simplest organic hydroperoxides, is a strong oxidant, with enhanced activity in aqueous ambience. The present study investigates, at the molecular level, the role of hydrogen bonding that is conducive to cluster formation of MHP with water molecules from its peroxide end, with the methyl group remaining hydrophobic for up to five water molecules. Ab initio quantum chemical computations on MHP...(H(2)O)(n), [n=1-5] are performed at second order Møller-Plesset (MP2) perturbation theory employing the basis sets 6-31G(d,p) and 6-311++G(2d,2p) to study the cluster formation of MHP with water molecules from its peroxide end and hydrophobic hydration due to the methyl group. Successive addition of water molecules alters the hydrogen bonding pattern, which leads to changes in overall cluster geometry and in turn to IR vibrational frequency shifts. Molecular co-operativity in these clusters is gauged directly through a detailed many-body interaction energy analysis. Molecular electrostatic potential maps are shown to have a bearing on predicting further growth of these clusters, which is duly corroborated through sample calculations for MHP...(H(2)O)(8). Further, a continuum solvation model calculation for energetically stable clusters suggests that this study should serve as a precursor for pathways to aqueous solvation of MHP.

Response of Scalar Fields and Hydrogen Bonding to Excited-State Molecular Solvation of Carbonyl Compounds

An attempt has been made to understand the mechanism of excited-state molecular solvation and its effect on hydrogen bonding in carbonyl compounds in aqueous solution. The correlation between solvation and electronic transitions has been investigated by comparing results obtained either with a supermolecular description in terms of hydrogen-bonded clusters or with a combined method embedding such clusters with a polarizable continuum dielectric mimicking the bulk water. Popular scalar fields such as molecular electrostatic potential and molecular electron density have been used as useful tools to probe the changes in the hydrogen bonding passing from ground to excited states in the gas as well as solvent phase.