Molecular Energetics Research Articles

Experimental and computational study on the molecular energetics of the three monofluoroanisole isomers

The standard ( p ∘ = 0.1 MPa) molar enthalpies of formation in the liquid phase of three isomers of fluoroanisole were derived from the standard molar energies of combustion, in oxygen, to yield CO 2(g) and HF · 10H 2O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization of these compounds, also at T = 298.15 K, were determined using Calvet microcalorimetry. - Δ c U m ∘ ( l ) / ( kJ · mol - 1 ) - Δ f H m ∘ ( l ) / ( kJ · mol - 1 ) Δ l g H m ∘ / ( kJ · mol - 1 ) 2-Fluoroanisole 3629.7 ± 1.0 301.9 ± 1.5 52.2 ± 1.1 3- Fluoroanisole 3617.1 ± 1.1 314.5 ± 1.6 48.1 ± 1.1 4- Fluoroanisole 3622.4 ± 1.2 309.2 ± 1.6 48.7 ± 1.1 The standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived from the former two experimental quantities. These values are also compared with estimates based on two different methodologies: one using the empirical scheme developed by Cox and the other one based on high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311++G(d,p) basis set. The computed values and the estimated values using the Cox method compare well with the experimental results obtained in this work.

Thio- versus oxo-derivatives of DNA bases: theoretical study on possible mutagenic effect of sulfur atom

The molecular structures, energetics and mechanisms of proton transfer in 9-methyl-2N-substituted guanines and thioguanines have been studied at the gas phase (QCISD/cc-pVDZ//B3lyp/6-31G(d,p)) and polar solvent (IEFPCM//B3LYP/6-31G(d,p)) environments. Calculations predict the height of the proton transfer barrier for the thione → thiol reactions to be approximately 2–3 times lower than that for the keto → enol process. Similarly, presence of sufficiently long 2N substituent with the terminal OH group reduces the barrier. The polar media (water) significantly stabilizes thione and keto forms. Tautomeric concentration ratio and rate constants have also been calculated for all the considered processes.

Gas-phase molecular structure and energetics of anionic silicates

The gas-phase stabilities of linear, branched and cyclic silicates made of up to five silicon atoms were studied with density functional theory (DFT). The starting geometries for the DFT calculations at the B3LYP/6-311+G(2d,2p) level of theory were obtained from classical molecular dynamics simulations. We have observed that geometric parameters and charges are mainly affected by the degree of deprotonation. Charges on Si atoms are also influenced by their degree of substitution. The enthalpy of deprotonation of the neutral species was found to decrease with the size of the molecule, while the average deprotonation enthalpy of highly charged compounds increased with molecular size. Furthermore, the formation of rings in highly charged silicates is enthalpically preferred to chain growth. These observations result from two competing effects: the easier distribution of negative charge in silicates with low charge density and the strong intramolecular repulsions present in silicates with high charge density. As a consequence, highly charged silicates in the gas phase tend to be as small and as highly condensed as possible, which is in line with experimental observations from solution NMR.

Selective Influence on Contextual Memory: Physiochemical Properties Associated with Selectivity of Benzodiazepine Ligands at GABAAReceptors Containing the α5 Subunit

Ligands that bind to the benzodiazepine binding site on the GABA A receptor can attenuate or potentiate cognition. To investigate this property, the chemical determinants favoring selective binding or selective activation of the alpha5beta2gamma2 and alpha1beta2gamma2 GABA A receptor isoforms were examined. A 3D-pharmacophore, developed from a diverse set of BDZR ligands, was used as an initial basis for multivariate discriminant, fragment, and 3D-quantitative structure-activity relationship analyses, which formed the criteria for selection of additional compounds for study. We found that the electrostatic potential near the ligands' terminal substituent correlated with its binding selectivity toward the alpha5beta2gamma2 versus alpha1beta2gamma2 isoform; while the fragment length and frontier molecular orbital energetics correlated with a compounds influence on electrophysiological activity. Compounds with promising alpha5 profiles were further assessed for their ability to attenuate scopolamine-induced contextual memory impairment in mice. Surprisingly, both weak inverse agonist and antagonists that display binding selectivity toward the alpha5beta2gamma2 isoform were able to attenuate contextual memory impairment.

A Universal Approach to Solvation Modeling

Continuum mean-field models that have been carefully designed to address the various electrostatic and nonelectrostatic interactions that develop between a molecule and a surrounding medium are particularly efficient tools for studying the effects of condensed phases on molecular structure, energetics, properties, spectra, interaction potentials, and dynamics. The SM8 model may be combined with density functional theory or Hartree-Fock theory to describe a solute's electronic structure and its self-consistent-field polarization by a solvent. A key feature is the use of class IV charge models to obtain accurate charge distributions (either in the vapor phase or in solution), even when using small basis sets that are affordable for large systems. A second key feature is that nonelectrostatic effects due to cavity formation, dispersion interactions, and changes in solvent structure are included in terms of empirical atomic surface tensions that depend on geometry but do not require atom-type assignments by the user. Use of an analytic surface area algorithm provides very stable energy gradients that allow geometry optimization in solution. The SM8 continuum model, the culmination of a series of SMx models (x = 1-8), permits the modeling of such diverse media as aqueous and organic solvents, soils, lipid bilayers, and air-water interfaces. In addition to predicting accurate transfer free energies between gaseous and condensed phases or between two different condensed phases, SMx models have been useful for predicting the significant influence of condensed phases on processes associated with a change in molecular charge, including acid/base equilibria and oxidation/reduction processes. In this Account, we provide an overview of the algorithms associated with the computation of free energies of solvation in the SM8 model. We also compare the accuracies of the SM8 model with those of other continuum solvation models. Finally, we highlight applications of the SM8 models to compute ionic solvation free energies, oxidation and reduction potentials, and pK(a) values.

Standard molar enthalpies of formation of dimethylbenzophenones

AbstractThe standard (p0 = 0.1 MPa) molar enthalpy of formation of 3,4‐dimethylbenzophenone was derived from the standard molar energy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The Calvet high temperature vacuum sublimation technique was used to measure the enthalpy of sublimation of the compound. From these experimental parameters, the standard molar enthalpy of formation of 3,4‐dimethylbenzophenone, in the gaseous phase and at T = 298.15 K, was derived as −(17.1 ± 2.9) kJ mol−1. Density functional theory was used to investigate the gas‐phase molecular energetics of the 12 dimethylbenzophenones. Molecular geometries and vibrational frequencies were computed at the B3LYP/6‐31G(d) level of theory. The larger 6‐311+G(2d,2p) basis set was used to compute the energy of all dimethylbenzophenones and of the other compounds that were considered for the estimation of the standard molar enthalpies of formation at T = 298.15 K. The calculations show that the 2,2′‐ and 4,4′‐dimethylbenzophenones are the least and most stable isomers, respectively. Finally, the calculated enthalpy of formation of the benzophenone that was also studied experimentally, 3,4‐dimethylbenzophenone, is −16.7 kJ mol−1, which is in excellent agreement with the experimental result. Copyright © 2008 John Wiley & Sons, Ltd.

π and σ-Phenylethynyl Radicals and Their Isomerso-,m-, andp-Ethynylphenyl: Structures, Energetics, and Electron Affinities

Molecular structures, energetics, vibrational frequencies, and electron affinities are predicted for the phenylethynyl radical and its isomers. Electron affinities are computed using density functional theory, -namely, the BHLYP, BLYP, B3LYP, BP86, BPW91, and B3PW91 functionals-, employing the double-zeta plus polarization DZP++ basis set; this level of theory is known to perform well for the computation of electron affinities. Furthermore, ab initio computations employing perturbation theory, coupled cluster with single and double excitations [CCSD], and the inclusion of perturbative triples [CCSD(T)] are performed to determine the relative energies of the isomers. These higher level computations are performed with the correlation consistent family of basis sets cc-pVXZ (X = D, T, Q, 5). Three electronic states are probed for the phenylethynyl radical. In C2v symmetry, the out-of-plane (2B1) radical is predicted to lie about 10 kcal/mol below the in-plane (2B2) radical by DFT methods, which becomes 9.4 kcal/mol with the consideration of the CCSD(T) method. The energy difference between the lowest pi and sigma electronic states of the phenylethynyl radical is also about 10 kcal/mol according to DFT; however, CCSD(T) with the cc-pVQZ basis set shows this energy separation to be just 1.8 kcal/mol. The theoretical electron affinities of the phenylethynyl radical are predicted to be 3.00 eV (B3LYP/DZP++) and 3.03 eV (CCSD(T)/DZP++//MP2/DZP++). The adiabatic electron affinities (EAad) of the three isomers of phenylethynyl, that is, the ortho-, meta-, and para-ethynylphenyl, are predicted to be 1.45, 1.40, and 1.43 eV, respectively. Hence, the phenylethynyl radical binds an electron far more effectively than the three other radicals studied. Thermochemical predictions, such as the bond dissociation energies of the aromatic and ethynyl C-H bonds and the proton affinities of the phenylethynyl and ethynylphenyl anions, are also reported.

Direct Observation of Hierarchical Folding in Single Riboswitch Aptamers

Riboswitches regulate genes through structural changes in ligand-binding RNA aptamers. With the use of an optical-trapping assay based on in situ transcription by a molecule of RNA polymerase, single nascent RNAs containing pbuE adenine riboswitch aptamers were unfolded and refolded. Multiple folding states were characterized by means of both force-extension curves and folding trajectories under constant force by measuring the molecular contour length, kinetics, and energetics with and without adenine. Distinct folding steps correlated with the formation of key secondary or tertiary structures and with ligand binding. Adenine-induced stabilization of the weakest helix in the aptamer, the mechanical switch underlying regulatory action, was observed directly. These results provide an integrated view of hierarchical folding in an aptamer, demonstrating how complex folding can be resolved into constituent parts, and supply further insights into tertiary structure formation.

Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport

Structural and electronic criteria for ambient stability in n-type organic materials for organic field-effect transistors (OFETs) are investigated by systematically varying LUMO energetics and molecular substituents of arylene diimide-based materials. Six OFETs on n+-Si/SiO2 substrates exhibit OFET response parameters as follows: N,N'-bis(n-octyl)perylene-3,4:9,10-bis(dicarboximide) (PDI-8): mu = 0.32 cm2 V(-1) s(-1), Vth = 55 V, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis(dicarboximide) (PDI-8Br2): mu = 3 x 10(-5) cm2 V(-1) s(-1), Vth = 62 V, I(on)/I(off) = 10(3); N,N'-bis(n-octyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl4): mu = 4 x 10(-3) cm2 V(-1) (s-1), Vth = 37 V, I(on)/I(off) = 10(4); N,N'-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): mu = 4.7 x 10(-3) cm2 V(-1) s(-1), Vth = 28, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2): mu = 0.13 cm2 V(-1) s(-1), Vth = -14 V, I(on)/I(off) = 10(3); and N,N'-bis(n-octyl)-2,6-dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN2): mu = 0.15 cm2 V(-1) s(-1), Vth = -37 V, I(on)/I(off) = 10(2). Analysis of the molecular geometries and energetics in these materials reveals a correlation between electron mobility and substituent-induced arylene core distortion, while Vth and I(off) are generally affected by LUMO energetics. Our findings also indicate that resistance to ambient charge carrier trapping observed in films of N-(n-octyl)arylene diimides occurs at a molecular reduction potential more positive than approximately -0.1 V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest that, at E(red1) < -0.1 V, the interfacial trap density increases by greater than approximately 1 x 10(13) cm(-2), while, for semiconductors with E(red1) > -0.1 V, the trap density increase is negligible. OFETs fabricated with the present n-type materials having E(red1) > -0.1 V operate at conventional gate biases with minimal hysteresis in air. This reduction potential corresponds to an overpotential for the reaction of the charge carriers with O2 of approximately 0.6 V. N,N'-1H,1H-Perfluorobutyl derivatives of the perylene-based semiconductors were also synthesized and used to fabricate OFETs, resulting in air-stable devices for all fluorocarbon-substituted materials, despite generally having E(red1) < -0.1 V. This behavior is consistent with a fluorocarbon-based O2 barrier mechanism. OFET cycling measurements in air for dicyanated vs fluorinated materials demonstrate that energetic stabilization of the charge carriers results in greater device longevity in comparison to the OFET degradation observed in air-stable semiconductors with fluorocarbon barriers.

An XAS study of the structure and thermodynamics of Cu(I) chloride complexes in brines up to high temperature (400 °C, 600 bar)

The transport and deposition of copper in saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride. Despite their role in the formation of most hydrothermal copper deposits, the nature and stability of Cu(I) chloride complexes in highly saline brines remains controversial. We present new X-ray absorption data ( P = 600 bar, T = 25–400 °C, salinity up to 17.2 m Cl), which indicate that the linear CuCl x 1 - x ( x = 1, 2) complexes are stable up to supercritical conditions. Distorted trigonal planar CuCl 3 2 - complexes predominate at room temperature and at high salinity (>3 m LiCl): subtle changes in the XANES spectrum with increasing salinity may reflect geometric distortions of this CuCl 3 2 - complex. Similar changes were observed in UV–Vis data [Liu, W., Brugger, J., McPhail, D.C., Spiccia, L., 2002. A spectrophotometric study of aqueous copper(I) chloride complexes in LiCl solutions between 100 °C and 250 °C. Geochim. Cosmochim. Acta 66, 3615–3633], and were erroneously interpreted as a new species, CuCl 4 2 - . Our XAS data and ab-initio XANES calculations show that this tetrahedral species is not present to any significant degree in our solutions. The stability of the CuCl 3 2 - complexe decreases with increasing temperature; under supercritical conditions and in brines under magmatic-hydrothermal conditions (e.g., 15.58 m Cl, 400 °C, 600 bar), only the linear Cu(I) chloride complexes were observed. This result and the instability of the CuCl 4 2 - complex are also consistent with the recent ab-initio molecular dynamic calculations of Sherman [Sherman D. M.(2007) Complexation of Cu + in hydrothermal NaCl brines: ab-initio molecular dynamics and energetics. Geochim. Cosmochim. Acta 71, 714–722]. This study illustrates the power of the quantitative nature of XANES and EXAFS measurements for deciphering the speciation of weak transition metal complexes up to magmatic-hydrothermal conditions. The systematic XANES data are used to retrieve the formation constant for CuCl 3 2 - at 150 °C, which is in good agreement with the reinterpretation of the UV–Vis data of Liu et al. (Liu et al., 2002). At high temperatures (≫400 °C), the solubility of chalcopyrite in equilibrium with hematite–magnetite–pyrite and K–feldspar–muscovite–quartz calculated with the new properties is lower than that calculated using the previous model, and the calculated solubilities are at the lower end of the range of values measured in brine inclusions from porphyry copper systems.

Controlling dynamics in diatomic systems

Controlling molecular energetics using laser pulses is exemplified for nuclear motion in two different diatomic systems. The problem of finding the optimized field for maximizing a desired quantum dynamical target is formulated using an iterative method. The method is applied for two diatomic systems, HF and OH. The power spectra of the fields and evolution of populations of different vibrational states during transitions are obtained.

Molecular Energetics of Cytosine Revisited: A Joint Computational and Experimental Study

A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T = 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p degrees = 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as -(221.9 +/- 1.7) kJ.mol(-1). This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ.mol(-1). Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer [J. Chem. Soc., Dalton Trans. 1984, 75], (155.0 +/- 3.0) kJ.mol(-1), results in a value for the gas-phase standard molar enthalpy of formation for cytosine of -66.9 kJ.mol(-1). A similar value, -65.1 kJ.mol(-1), has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as -60.7 kJ.mol(-1) and -57.2 kJ.mol(-1) for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C-H, N-H, and O-H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies.

Experimental Vibrational Zero-Point Energies: Diatomic Molecules

Vibrational zero-point energies (ZPEs), as determined from published spectroscopic constants, are derived for 85 diatomic molecules. Standard uncertainties are also provided, including estimated contributions from bias as well as the statistical uncertainties propagated from those reported in the spectroscopy literature. This compilation will be helpful for validating theoretical procedures for predicting ZPEs, which is a necessary step in the ab initio prediction of molecular energetics.

Nanoporous carbon membranes for separation of nitrogen and oxygen: Insight from molecular simulations

Here we summarize some of our recent work on using molecular simulations to understand the key mechanism that result in large differences in the reported permeation rates of O 2 and N 2 in nanoporous carbon membranes. Two different representation of the amorphous nanoporous membrane structure are used; the hypothetical C 168 Schwarzite and a single wall carbon nanotube with a constriction. By comparing the results obtained from empirical planar graphite potential and an ab initio-based potential, the effect of carbon curvature and the presence of non-hexagonal carbon rings in C 168 Schwarzite is also investigated. It is found using either force field, that the energetic effect alone cannot explain the experimental observations. However, simulations performed using carbon nanotube with a constriction show that the size or entropic effect can be dominant. In particular, it is shown that an appropriate size constriction can result in large transport resistance to nitrogen while letting oxygen to pass through at a much higher rate, even though these gases have very similar molecular sizes and interaction energetics.

Chiral Separation: Mechanism Modeling in Two-Dimensional Systems

Fluid phase separations of racemates are difficult because the subtle, short-ranged differences in intermolecular interactions of like and unlike pairs of chiral molecules are typically smaller than the thermal energy. A surface restricts the configurational space available to the pair of interacting molecules, thus changing the effective interactions between them. Because of this restriction, a surface can promote chiral separation of mixtures that are racemic in bulk. In this paper, we investigate chiral symmetry breaking induced by an achiral surface in a racemate. A parallel tempering Monte Carlo algorithm with tempering over the temperature domain is used to examine the interplay between molecular geometry and energetics in promoting chiral separations. The system is restricted to evolve in two dimensions. By controlling the balance between electrostatic and steric interactions, one can direct the surface assembly of the chiral molecules toward formation of small clusters of identical molecules. When molecular shape asymmetry is complemented by dipolar alignment, chiral micellar clusters of like molecules are assembled on the surface. We examine the case of small model molecules for which the two-dimensional restriction of the pair potential is sufficient to induce chiral segregation. An increase in molecular complexity can change the balance of intermolecular interactions to the point that heterochiral pairs are energetically more favored. In this case, we find conditions in which formation of homochiral micelles is still achieved, due to a combination of multibody and entropic effects. In such systems, an examination of the pair potential alone is insufficient to predict whether the multimolecular racemate will or will not segregate.

Analyzing Molecular Current-Voltage Characteristics with the Simmons Tunneling Model: Scaling and Linearization

Use of the Simmons model for analyzing tunneling transport across molecular junctions is reviewed, and its inherent limitations are examined, specifically for cases where there are no molecular length-dependent data (to extract a decay parameter), be it for experimental reasons or because of changes of the molecular energetics or packing with molecular length. The potential barrier across a molecular junction is shown to be strongly bias-dependent, much more so than is assumed in the commonly used version of the Simmons model. The means to distinguish true tunneling from conduction via pinholes (or hot spots) are also considered. Power expansion to the Simmons model shows that I/V vs V2 plots should be linear over that range, thus providing a simple and standardized parameter extraction. From such plots, we can extract values for the equilibrium conductance and for the “shape factor”, a complementary parameter that describes the shape of the I−V relations. The applicability of these two parameters for describing actual transport is illustrated by analyzing data for three different types of molecular junctions. The linearity of the I/V vs V2 plots can be used to evaluate if, and if so, at which bias (and if at all) direct tunneling occurs and under which conditions this is not the case. The extracted equilibrium conductance and the “shape factor” provide an empirical method for quantifying electronic transport across practical molecular junctions where the exact packing of the molecules is rather uncertain. As such, the analysis can be used to weigh and classify the effect of chemical modifications to molecular junctions or compare contacting methods, so as to allow a deeper understanding of transport via molecular junctions.

Molecular aspects on the interaction of phenosafranine to deoxyribonucleic acid: Model for intercalative drug–DNA binding

The mode, mechanism and energetics of interaction of phenosafranine, the planar, cationic and rigid phenazium dye to calf thymus DNA was investigated from absorption, fluorescence, circular dichroism, isothermal titration calorimetry, thermal melting, and viscosity. The study revealed non-cooperative binding of the dye to DNA with an affinity in the range (3.81–4.22) × 10 5 M −1 as observed from diverse techniques and obeying neighbor exclusion principle. The stoichiometry of binding was characterized to be one phenosafranine molecule per two base pairs. The binding was characterized by strong stabilization of DNA against thermal strand separation, large intrinsic circular dichroic changes of DNA by itself and the generation of induced circular dichroism for the optically inactive phenosafranine molecules. Hydrodynamic and fluorescence quenching studies revealed strong evidence that the phenosafranine molecules are intercalated between every alternate base pairs of calf thymus DNA. Isothermal titration calorimetry studies suggested that the binding was exothermic and favoured by both negative enthalpy and positive entropy changes. This study for the first time presents the complete molecular aspects and energetics of phenosafranine complexation to DNA as model for intercalative drug–DNA interaction.

A Simple Model System for the Study of Carbohydrate−Aromatic Interactions

A molecular scaffold was identified which enables the establishment of intramolecular interactions between a monosaccharide and a nearby phenyl ring. A group of molecules containing four different monosaccharides (glucose, galactose, N-acetyl-glucosamine, and N-acetyl-galactosamine) was synthesized and used to investigate the extent and nature of this carbohydrate-arene interaction, as well as the effect on the overall 3D structure of the molecules involved. The sugar-aromatic distance was evaluated by rigorous NMR studies supported by molecular modeling and found to be constant throughout the series, independent of the nature of the sugar and of the conformational behavior of the fragment connecting the two elements. Ab initio calculations at the B3LYP/DZV(2d,p) level of theory enable the analysis of the electronic nature of the interaction. The study shows that, given the opportunity, persistent intramolecular aromatic-sugar interactions can be established and can significantly influence overall molecular shape and energetics. These results have important implications in the design of structural mimics of oligosaccharides.

Multiscale modeling of polymer composite properties from nano- to macro

Some distinctive computer technologies, which lead to multiscale computational experiments and investigations of peculiarities of micromechanical behavior of heterogeneous composite media, taking into consideration of atomic-molecular formations, were discussed. The results calculated by Monte-Carlo approach were considered as a perspective method for description of the important features of atomic and molecular texture and energetics of heterogeneous polymer media. The quantum-mechanical approach was discussed as the method for solutions of top problems of micromechanics of polymer composites, namely, an investigation of the interaction of soot model particle with non-terminated and H-terminated surface with segments of polymer chain with different chemical structures. The parallel technologies of calculations and supercomputer were used. Optimization of viscoelastic behavior of composite media such as rubbers leads to the procedure of identification. Validity of reinforcement effect on the basis of the relaxation properties analysis of materials seems very perspective.

Polarizabilities and energetics of boroxopyrrole homologues studied by ab initio and density functional methods

Molecular geometries, static polarizabilities and energetics have been calculated for the boroxopyrrole (iso-electronic to pyrrole) homologues including boroxopyrrole (B 2 O 2 H 3 N), boroxophosphorylpyrrole (B 2 O 2 H 3 P), boroxoarsenylpyrrole (B 2 O 2 H 3 As) and boroxoantimonylpyrrole (B 2 O 2 H 3 Sb) at the Hartree-Fock by ab initio and Density Functional Theory (DFT) by choosing combined exchange correlation potentials B3LYP and B3PW91. All the calculations utilized the 6-311G basis set for lighter atoms and 3-21G for heavier atoms The influence of substitution with heavy atoms on molecular properties of the homologues was examined theoretically. The calculated results here, show that substitution of the heteroatom with heavy atoms, systematically, increases the individual components of static polarizability, a. This property has been discussed in term of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energies. Further, to check the adequacy of the method and the suitability of the basis set, the normal mode frequencies in C 2v symmetry was examined theoretically for B 2 O 2 H 3 N and compared with available experimental data.