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Due to ligand non-innocence and reversible one-electron-transfer processes dithiolene complexes have been intensively studied both experimentally and computationally. While the substitution of the ligating sulfur atoms by selenium provides a means to delicately tune the behavior of dithiolene compounds, diselenolene complexes have not been as thoroughly examined. Yet, the search for such ligands has been ongoing since the 1970s. Thus, we have looked at several metal-bisdiselenolene complexes and have compared key properties of these complexes with their bisdithiolene analogues to determine the effect of substituting the chalcogen atom. The results herein show that substitution of the sulfur atoms by selenium within these complexes only subtly changes the thermodynamics and kinetic reactivity of bisdithiolene complexes while not significantly affecting the geometries of the complexes. The significance being that the relatively minor structural changes that occur upon redox is a key feature of dithiolene complexes. Due to ligand non-innocence and reversible one-electron-transfer processes dithiolene complexes have been intensively studied, however, diselenolene complexes have not. First-principles calculations show that substitution of the sulfur atoms by selenium within the investigated complexes does offer the ability to subtly tune the thermodynamics and kinetic reactivity of bisdithiolene complexes, while not significantly affecting the geometries of the complexes. © 2015 Wiley Periodicals, Inc.

Predicting the correct ground-state multiplicity for iron(II) porphyrin, a high-spin quintet, remains a significant challenge for electronic-structure methods, including commonly employed density functionals. An even greater challenge for these methods is correctly predicting favorable binding of O2 to iron(II) porphyrin, due to the open-shell singlet character of the adduct. In this work, the performance of a modest set of contemporary density-functional approximations is assessed and the results interpreted using Bader delocalization indices. It is found that inclusion of greater proportions of Hartree-Fock exchange, in hybrid or range-separated hybrid functionals, has opposing effects; it improves the ability of the functional to identify the ground state but is detrimental to predicting favorable dioxygen binding. Because of the uncomplementary nature of these properties, accurate prediction of both the relative spin-state energies and the O2 binding enthalpy eludes conventional density-functional approximations.

We performed an assessment of 10 common DFT functionals to determine their suitability for calculating the reduction potentials of the ([M(S2C2H2)2](0)/[M(S2C2H2)2](1-)), ([M(Se2C2H2)2](0)/[M(Se2C2H2)2](1-)), ([M(S2C2H2)2](1-)/[M(S2C2H2)2](2-)), and ([M(Se2C2H2)2](1-)/[M(Se2C2H2)2](2-)) redox couples (M = Ni, Pd, and Pt). Overall it was found that the M06 functional leads to the best agreement with the gold standard CCSD(T) method with an average difference of only +0.07 V and a RMS of 0.07 V in calculated reduction potentials. The variability in calculated reduction potentials between the various DFT functionals arise, in part, from the multireference character of these systems, which was determined by the T1 diagnostic values. Thus, the bisdiselenolene complexes show similar multireference character as the bisdithiolene complexes, which were previously shown to have such character. In particular, for the Ni-, Pd-, and Pt-bisdiselenolene complexes the average T1 values are 0.05, 0.03, and 0.02, respectively. For the CCSD(T) calculations the similarities in the reduction potentials between analogous bisdithiolene and bisdiselenolene redox couples, which appear to be independent of the metal, is a result of the noninnocence of the dithiolene and diselenolene ligands. Thus, the reduction potential is more dependent on the ligand than the metal.

Organotin compounds are very important in material design as well as in biomedical and biochemical applications. However, little is known about their BDEs experimentally or computationally. Thus, a variety of common quantum chemical methods in combination with several approaches to treating relativistic effects of the tin core electrons were used to calculate the BDEs of organotin compounds. Our results show that the BDEs are very sensitive to the choice of the computational method and to the treatment of relativistic effects.

Polaron formation in poly(G)-poly(C) cations is investigated with density functional theory (DFT) and molecular mechanics (MM) employing a two-layer ONIOM method. In these calculations, the high layer, composed of all complementary base pairs, is treated by a DFT method, while the low layer, which includes the sugar-phosphate backbone, counterions and water molecules, is described by the AMBER force field. The high layer is the model system in which the charge transfer takes place. According to our calculations, three or four guanines move in a paddle-like fashion when an electron is removed from the neutral model system. In the cation model system, about 80% of the charge is delocalized onto the guanine residues, and the remaining charge is delocalized onto the cytosine residues. This happens because guanine has a lower ionization potential (IP) than cytosine. The counterions and water molecules in the low layer are important in the geometry optimization. The optimized geometry of the model system is closer to the standard B-form structure when counterions and water molecules are included than when they are omitted. Comparison of the optimized neutral and cationic model systems reveals a polaron in poly(G)-poly(C) cations extending from the first to the third guanine. It is demonstrated that the position of counterions and the number of surrounding water molecules can affect polaron formation.

The structures of 19 alpha-helical alanine-based peptides, 13 amino acids in length, have been fully optimized using density functional theory and analyzed by means of the quantum theory of atoms in molecules. Two types of N-H...O bonds and one type of C-H...O bond have been identified. The value of the electron density at hydrogen bond critical points corresponding to N-H...O interactions is higher than that of C-H...O interactions. The effect of amino acid substitution at the central position of the peptide on the hydrogen bond network of the alpha-helix has been assessed. The strength of the hydrogen bond network, measured as the summation of the electron density over the hydrogen bond critical points, may be used to explain experimental relative helix propensities of amino acids in cases where solvation and entropic effects cannot.

Triply hydrogen-bonded complexes of the form AAA-DDD are shown to have the strongest interaction when the complex is substituted with electron withdrawing groups on the donor molecule (DDD) and electron donating groups on the acceptor molecule (AAA). In particular, the largest effects are observed when the withdrawing groups act through resonance. This serves to flatten the entire system resulting in more linear, and consequently stronger, hydrogen bonds. Furthermore, the present calculations show that the binding energy correlates with the electron density at the bond critical points and inversely with the hydrogen bond lengths.

In this chapter, we discuss the influence of an anisotropic protein environment on the reaction mechanisms of saccharopine reductase and uroporphyrinogen decarboxylase, respectively, via the use of a quantum mechanical and molecular mechanical (QM/MM) approach. In addition, we discuss the importance of selecting a suitable DFT functional to be used in a QM/MM study of a key intermediate in the mechanism of 8R-lipoxygenase, a nonheme iron enzyme. In the case of saccharopine reductase, while the enzyme utilizes a substrate-assisted catalytic pathway, it was found that only through treating the polarizing effect of the active site, via the use of an electronic embedding formalism, was agreement with experimental kinetic data obtained. Similarly, in the case of uroporphyrinogen decarboxylase, the effect of the protein environment on the catalytic mechanism was found to be such that the calculated rate-limiting barrier is in good agreement with related experimentally determined values for the first decarboxylation of the substrate. For 8R-lipoxygenase, it was found that the geometries and energies of the multicentered open-shell intermediate complexes formed during the mechanism are quite sensitive to the choice of the density functional theory method. Thus, while density functional theory has become the method of choice in QM/MM studies, care must be taken in the selection of a particular high-level method.

Theoretical calculations have been performed on three model reactions representing the reduction of hydrogen peroxide by ebselen, ebselen selenol, and ebselen diselenide. The reaction surfaces have been investigated at the B3PW91/6-311G(2df,p) level, and single-point energies were calculated using the 6-311++G(3df,3pd) basis set. Solvent effects were included implicitly with the conductor-like polarizable continuum model and in one case with explicit inclusion of three water molecules. Mechanistic information is gained from investigating the critical points using the quantum theory of atoms in molecules. The barriers for the reduction of hydrogen peroxide with the ebselen, ebselen selenol, and ebselen diselenide models are 56.7, 53.4, and 35.3 kcal/mol, respectively, suggesting that ebselen diselenide may be the most active antioxidant in the ebselen GPx redox pathway. Results are also compared to that of the sulfur analogues of the model compounds.

A systematic comparison of the optimized geometries of five organotin compounds, Cl(n)Sn(CH(3))(4-n), n = 0-4, with the available gas-phase electron diffraction results is reported. All optimizations were carried out with the B3LYP density functional method. Comparison of 10 basis sets and three effective core potentials leads to the conclusion that the combination of the SDB-aug-cc-pVTZ basis set and the LANL2 effective core potential for tin, together with the 6-31G(d,p) basis set for the other atoms, is recommended for the prediction of the geometries of organotin compounds.

We report a study of the inclusion complex of p-chlorophenol inside α-cyclodextrin (α-CD) by the theory of atoms in molecules (AIM). We use a quantitative comparison of some AIM properties of isolated p-chlorophenol (PCP) and the inclusion complex (PCP-CD) and we characterize some weak interactions within the host–guest complex. Furthermore, we compare the electrophilic aromatic substitution on the p-chlorophenol in the isolated state and inside α-CD. The analysis of the bond critical points of PCP shows that there is no trend in the effect on the AIM properties of PCP due to inclusion in the α-CD.

A type of cage critical point (CCP) that has remained so far only a ‘mathematical possibility’ is reported for the first time. A CCP enclosed by two ring surfaces is found in a single ring in each of three derivatives of 1,12-difluorobenzo[c]phenanthrene. The two fluorine nuclei are linked by a bond path which results in the closure of a seven-membered ring in the ‘bay’ region. The crowding of the fluorine atoms forces an α-helical (staircase) twisting of the molecule causing the ring surface to split into two giving rise to a CCP. The Poincare–Hopf relationship is satisfied in each case.

A bond path linking two saturated fluorine atoms is found to be ubiquitous in crowded difluorinated aromatic compounds. The bond path is shown to persist for a range of internuclear distances (2.3-2.8 A) and a range of relative orientations of the two C-F internuclear axes. The F. . .F bonding is shown to exhibit all the hallmarks of a closed-shell weak interaction. The presence of such a bond path can impart as much as 14 kcal/mol of local stabilization to the molecule in which it exists, a stabilization that can be offset or even overwhelmed by destabilization of other regions in the molecule. Several other weak closed-shell interactions were also found and characterized including F. . .C, F. . .O, and C. . .C interactions, hydrogen bonding, dihydrogen bonding, and hydrogen-hydrogen bonding. This study represents another example of the usefulness and richness of the bond path concept and of the theory of atoms in molecules in general.

Electron-withdrawing groups (EWGs) on the nitrogen atom of pyrroles have significant effects on the properties of the pyrrole. It has been suggested that the experimental 13C chemical shifts show a general increase in deshielding effect with the increase of N-EWGs strength [A. Thompson, S. Gao, G. Modzelewska, D.S. Hughes, B. Patrick, D. Dolphin, Org. Lett., 2, 3587 (2000)]. However, recently observed 15N chemical shifts of pyrroles do not correlate with the N-EWG strength. To elucidate the relationship between the electronic structures of pyrroles and their nitrogen and carbon chemical shifts, density functional theory calculations were performed on pyrroles with various substituents. A correlation between the paramagnetic shift and the 15N chemical shift was observed for the pyrroles, indicating that the nitrogen chemical shift trend for the pyrroles arises entirely from variations of the paramagnetic shift contribution. However, a general correlation between the 15N chemical shifts and the EWG strength...

The performance of the DFT functionals B3LYP, B3PW91 and PBE1PBE in conjunction with selected basis sets for the prediction of 15N and 13C nuclear shieldings for small nitrogen-containing molecules is assessed. For molecules containing only singly bonded nitrogen and carbon atoms, the PBE1PBE/6-311+G(2df,p) level of theory is shown to provide reliable 15N and 13C nuclear shieldings. For the triply bonded nitrogen-containing molecules N2, CH3CN and HCN, all three levels of theory are much less satisfactory.

The equilibrium geometries of 54 small molecules containing linear or near-linear N-H...N bonds (sample M) have been optimized at the MP2/6-31G(d,p) level and the values of p' and p" of the parameters p c at the bond-critical points (p' in the N-H, p" in the H...N bond) have been computed from the results of these optimizations. Because the N-H and the H...N part of an N-H...N bond system have different character, the trends of p' and of p" in M are described by different functions. With the p c as descriptors (the electron density ρ c , the curvatures λ c , i , the Laplacian * 2 c , the kinetic energy densities G c and K c , and the potential energy density V c ), we have searched for correlations of p' and p" (homocorrelations) in M. A high degree of correlation has been found for all the parameters. With the exception of the linear ρ',ρ" correlation the homocorrelations of the other p c are nonlinear and some of them nonmonotonic. The homocorrelations permit estimates of the p c values, p s , in symmetric N-H-N bonds, where estimates from experiment are not without problems. They also answer some of the questions concerning limiting values of the p c . With the exception of G c , correlations between unlike p c 's (heterocorrelations, p',q' and p",q") will be reported in a subsequent paper, now in preparation. The heterocorrelations involving G c are included here because of the prominence of G c in recent discussion of hydrogen bonds in the literature.

The purpose of this overview is to highlight the broad scope and utility of current applications of density functional theory (DFT) methods for the study of the properties and reactions of biomolecules. This is illustrated using examples selected from research carried out within our research group and in collaboration with others. The examples include the hyperfine coupling constants of amino acid radicals, the use of an amino acid as a chiral catalyst for the formation of carbon–carbon bonds in the aldol reaction, hydrogen-bond mediated catalysis of an aminolysis reaction, radiation-induced protein–DNA cross-links, and the mechanism by which an antitumor drug cleaves DNA. We demonstrate that DFT-based methods can be applied successfully to a broad range of problems that remain beyond the scope of conventional electron-correlation methods. Furthermore, we show that contemporary computational quantum chemistry complements experiment in the study of biological systems.

Density functional theory is used to study a series of model iron porphyrins in the gas phase. In the first part of this study, three range-separated hybrid density functionals developed by Chai and Head-Gordon were assessed; ωB97, ωB97X, and ωB97XD. The effects of including full Hartree-Fock exchange at long-range and dispersion corrections are reported with respect to the geometries and binding energies of oxygen to the iron porphyrin systems. The functionals all correctly predict the quintet ground state for the deoxy-iron porphyrins, where typically hybrid functionals fail and predict a triplet ground state. Including dispersion in ωB97XD is shown to give the best results for the O2 binding energy and geometrical parameters. The second part of the study employs ωB97XD to study iron porphine systems with different amino acids in the axial position. Geometrical parameters are reported and compared to experimental data, where available. Binding energies of the systems with oxygen are also reported and discussed.

The geometries, interaction energies, and vibrational properties of (HF)2, (HCl)2, and (HBr)2 have been investigated using a variety of hybrid density functional methods and the 6-311+G(2df,p) basis set. Although most of the density functional methods predict geometries in reasonable agreement with experiment, methods containing the LYP correlation functional yield geometrical parameters in slightly better agreement. The (HF)2 interaction energy, predicted by the hybrid density functional methods, is in reasonable agreement with experiment, but the (HCl)2 and (HBr)2 interaction energies are underestimated substantially. The frequency shift for the XH donor bond correlates linearly with the elongation of the XH bond upon complexation, and is predicted reasonably well by methods containing the LYP functional. Overall, the hybrid density functional methods adequately predict the properties of the halide dimers. It is interesting to note that the BHandHLYP and B1LYP density functional methods offer a competitive alternative to the popular B3LYP method. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1590–1597, 2001

In the present study, ab initio methods have been used to study the OH hydrogen-abstraction reaction from two substituted aldehydes: FCHO and ClCHO. A complex mechanism in which the overall rate depends on the rates of two competitive reactions, a reversible step where a reactant (or prereactive) complex is formed, followed by the irreversible hydrogen abstraction to form the products, is corroborated. This mechanism was previously shown to describe accurately the kinetics of the OH hydrogen-abstraction reaction from formaldehyde and acetaldehyde. Classical transition state theory (TST) rate constants calculated with tunneling corrections, assuming an unsymmetrical Eckart barrier, agree very well with experimental upper bound values. Activation energy barriers and enthalpies of reaction have been estimated through CCSD(T) single point calculations using MP2 geometries and frequencies and the 6-311++G(d,p) basis set.

In an attempt to discover and analyze trends in distance relationships and properties at the bond critical point (BCP) in linear or near-linear N−H···N hydrogen bonds, the geometry of such bonds in a large number of suitable simple chemical species was optimized at the RHF/6-31G** level. The results for 67 of these are reported here; the geometry of 19 of them was optimized also at the MP2/6-31G** level. Correlations between the internuclear N−H, H···N, and N···N separations as well as between the N···BCP and H···BCP distances for these data sets and for different model functions are described in detail. The special case of symmetric N−H−N bonds is discussed; comparison with available experimental evidence shows that the correlation functions derived from the ab initio data have useful predictive value for crystallographic determinations involving short N−H−N bonds. Analysis of the correlation between the d(N−H) distance and the electron density ρc at the BCP has shown that although acceptable d,ρc repres...

In this work, the OH + formaldehyde and OH + acetaldehyde reactions have been characterized using accurate ab initio methods with large basis sets. The results clearly indicate that the reaction occurs by hydrogen abstraction, and that the OH addition channel is unfavorable. Close to zero (for formaldehyde) and negative (for acetaldehyde) activation energy values are obtained, which are in excellent agreement with the experimentally observed values. The reaction rate constants, calculated using the classical transition-state theory as applied to a complex mechanism involving the formation of a prereactive complex, reproduce very well the reported experimental results. Consideration of the prereactive complex is shown to be essential for the determination of the height of the energy barrier and thus for the correct calculation of the tunneling factor.

Density-functional theory (B3LYP functional) is used to investigate the ionization potentials and electron affinities of the DNA and RNA nucleotide bases. For the first time, anions lying lower in energy than the neutral species have been calculated for both thymine and uracil (i.e., positive adiabatic electron affinities). Additionally, the calculations show that anion formation leads to significant geometrical changes to the nucleobases. This is a very important finding as previous calculations have indicated that the anions are very similar in geometry to the neutral species and reported negative valence adiabatic electron affinities.

Density functional theory was employed to investigate the radicals that have been proposed to be formed upon irradiation of glycine crystals. The present theoretical study suggests that the radicals are R1: +NH3C•HCOO-; R2: •CH2COOH; R3: NH2C•HCOOH; and R4: NH2CH2COC•HCOOH. A Cs structure for R1, obtained using the Onsager model, gives hyperfine coupling constants in agreement with experiment. Hyperfine coupling constants computed for R2 are in agreement with the unassigned experimental data of Teslenko, V. V. et al. (Mol. Phys. 1975, 30, 425). The computed hyperfine coupling constants for R4 are in good agreement with the experimental data assigned to the zwitterionic form +NH3CH2COC•HCOO-. It is shown that the structure of R3 is influenced significantly by the glycine crystal environment. Protonation of R3 gives rise to hyperfine couplings similar to the experimental values assigned to one conformer of R3.

The effects of the successive addition of alkyl substituents (methyl and reduced rings) on the excited states of naphthalene are reported. The calculated electronic states of all the reduced derivatives with two, three, and four substituents are compared with the excited states of their methylated analogues. The excited states of several reduced derivatives with seven and eight substituents are also studied. The AM1 method was used to optimize the geometry of 57 naphthalene derivatives, and excited states were calculated with the ZINDO/S (INDO/S) method. ZINDO/S calculations on naphthalene gave excited states in better agreement with experimental results than with results of other semiempirical (CNDO/S and CNDOL) and ab initio (CIS, TD-HF, and TD-DFT) methods. Successive alkyl substitutions are accompanied by bathochromic displacements of the UV-visible bands, since the occupied orbitals are raised in energy more than the unoccupied orbitals. However, not all available substituent positions in naphthalene alter its orbital energy distribution in the same way when they are occupied by alkyl substituents. Distortion from planarity of the naphthalene skeleton of some reduced derivatives is the cause of “anomalous” bathochromic displacements of the absorption bands.

Density-functional theory is used to investigate the radiation products (radicals R1, •CH(CH3)COOH; R2, H3N+C•(CH3)COO-; and R3, H2NC•(CH3)COOH) of l-α-alanine at 295 K. Four conformers were found for R1 and R3. A planar structure of R2 in a zwitterionic form was obtained with the Onsager model. The relative energies of each of the four conformers of R1 and R3 show that structures with intramolecular hydrogen bonding are more stable. The computed hyperfine couplings are shown to be in good agreement with the accurate results obtained from the electron paramagnetic resonance (EPR), electron−nuclear double resonance (ENDOR), and EIE (ENDOR-induced EPR) experiments performed by Sagstuen et al. [Sagstuen, E.; Hole, E. O.; Haugedal, S. R.; Nelson, W. H. J. Phys. Chem. A 1997, 101, 9763]. The effects of rotation about the NC2 bond on the HFCCs of the amino protons in R2 support the previous suggestion that the amino protons are fixed by intermolecular hydrogen bonding in l-α-alanine crystals. Moreover, a good c...

The geometries, spin density distributions, and hyperfine coupling constants (HFCC) in possible radiation products of guanine are studied through the use of density functional theory. Numerous hydrogenated, dehydrogenated, and hydroxylated radicals are ex

The geometries, relative energies, and hyperfine coupling constants of dehydrogenated, hydrogenated, and hydroxylated radiation products, as well as the anion and the cation, of thymine (T), 1-methylthymine (1-MeT), and uracil (U) are calculated through the use of density functional theory. The results for T and 1-MeT are subsequently compared to accurate ENDOR experiments performed by Sagstuen and co-workers (Sagstuen, E.; Hole, E. O.; Nelson, W. H.; Close, D. M. J. Phys. Chem. 1989, 93, 5974 and references therein). The theoretical coupling constants support the experimental assignment of the observed radicals. It is suggested that the hydrogen added to form the O4-hydrogenated product is located at an angle of 50° out of the molecular plane in T. Unfavorable interactions of this hydrogen with the N3 and the methyl hydrogens are shown to lead to this out-of-plane position for O4−H. For the similar radical in 1-MeT, hydrogen bonding in the crystal leads to an in-plane hydrogen position. Notable geometry ...

The cleavage reactions of the 1-butene and 4,4-dimethyl-1-pentene molecules and their cations, to form neutral and charged hydrocarbon products, are investigated using hybrid Hartree-Fock/density functional theory. In comparison with previous theoretical results (Du et al.), the density functional cleavage and ionization energies, including zero-point vibrational energy, lie in better agreement with experimental and thermochemical data. Assuming vertical ionization processes the mean absolute deviation (MAD) compared with experiment is 3.4 kcal/mol for the reaction sequences studied. Using adiabatic ionization processes instead gives a MAD of 5.2 kcal/mol. The largest deviation from experiment occurs for the cleavage reactions of the neutral parent molecules, where the difference between theory and experiment is up to 12.8 kcal/mol. In addition to reaction energies we also report optimized ground-state structures, and for the radicals studied, isotropic hyperfine coupling constants that are compared to experimental data. It is found that the experimental hyperfine properties of the 1-butene cation can be explained by rotational averaging caused by the flat potential surface for rotation about the C2—C3 bond.

The structures and relative stabilities of the four different forms of hydroxypyridine and hydroxyquinoline (normal, protonated cation, deprotonated anion and zwitterion) in the gas phase have been calculated at the Moller-Plesset (MP) perturbation and coupled-cluster theory levels with basis sets up to 6–31+ G ∗∗ . The solvation free energies of the reactions were treated with the SMx series of aqueous solvation models by Cramer and Truhlar. By combining the PM3-SM3 model for solvation with the MP 2/6–31+ G ∗∗ level treatment for the gass-phase species, it is found that the free energy changes of the proton transfer reactions can be predicted reasonably well.

Properties of the bond critical points, including the electron density and its Laplacian, and distances to bonded nuclei, for all species (reactants, transition states, and products) in the reactions of the hydroxyl radical with the series C2HnF6−nn = 1–6 were calculated using Bader's atoms-in-molecules methodology. The electron density and its Laplacian at the bond critical points correlate with bond strength, as measured by bond dissociation energies. The positions of the bond critical points vary with the electronegativity of surrounding atoms. Charge development in the course of the reactions was monitored using Mulliken population analysis at the HF/6-31G(d), HF/6-311G(d,p)//HF/6-31G(d), MP2/6-311G(d,p)//HF/6-31G(d), and HF/6-311G(d,p)//MP2/6-31G(d,p) levels of theory, and natural population analysis and Bader population analysis at the highest common level of theory, MP2/6-311G(d,p)//HF/6-31G(d). In general, there is a buildup of charge in the transition states, concentrated near the reaction centre, which dissipates somewhat as the reaction proceeds to products. The description of charge transfer varies somewhat with the three methods. Key words: topological properties, atomic charges, charge development, reactions of the hydroxyl radical with fluorinated ethanes.

The 2Σ+ states of HBeO, HMgO, and HCaO have been studied at the CASSCF level with a basis set of at least triple‐zeta plus double polarization quality. The correlation treatment includes the nine valence electrons and ten orbitals resulting in an active space denoted by (622). The optimized structures of HMO (M=Be, Mg, and Ca) are all linear and the energy of HMO lies around 250 kJ/mol (2.6 eV) above that of MOH. The M–O bond length in HMO is predicted to be longer than that in the corresponding MOH by amounts ranging from 0.01 A to 0.05 A. The M–O stretching frequencies are very similar for the two structures. The H–M bond lengths and stretching frequencies in HMO are very similar to the same properties in the corresponding diatomic molecule. Analysis of the electronic structures suggests that HBeO has two polarized covalent bonds formed from the sp hybrids of Be; HMgO has one covalent bond (between H and Mg) and one ionic bond and can be viewed as (HMg)+O−, HCaO has two ionic bonds represented by H−Ca2+O−.

The energies and structures of the 28-electron tetraatomic molecules, composed of the first row nonmetallic elements: N4 (1), CN2O (2), BFN2 (3), C2O2 (4), B2F2 (5), CBFO (6), C2FN (7), and BNO2 (8) have been studied uniformly by ab initio methods including coupled-cluster theory. New estimates of the stability of the N4 isomers, tetraazatetrahedrane and tetraazacyclobutadiene, are presented, and a new triplet NNNN open-chain isomer has been established computationally. Potential energy surfaces of the nonpolar 1 and the polar 2 are compared. Three-membered cyclic C2v fluorodiazaboririne has been found to be the most stable isomer similar to diazirinone (Chem. Phys. Lett. 1994, 227, 312). Linear triplet CO and BF dimers, OCCO, FBBF, and OCBF, are the most stable forms of 4, 5, and 6, respectively. The singlet cyanofluoromethylene, NCCF, the global energy minimum of 7, is 7 kcal/mol more stable than isomeric CNCF and 10 kcal/mol lower in energy than 3-fluoroazacycloprop-2-ylidene. The singlet and triplet ...

Approximate Fukui indices have been derived from the atomic volumes of molecules as calculated within the framework of Bader's atoms-in-molecules theory. Parameters of the bond critical points {∇ρ(rc)n = 0} have been explored in calculating the hardness (η) and electronegativity (χ) of a selection of typical chemical groups, widely considered as transferable entities. Resulting η and χ indices demonstrate strong dependence on the nature of neighboring atoms and on the bond system vicinal to the group; however, they are not directly related to the group charges.

The internal motion of benzene is studied in the gas phase at 30 and 300 K and in the liquid phase at 300 K by means of a molecular dynamics method based on a hybrid of quantum and molecular mechanical force fields. The fluctuations of bond lengths, bond angles and torsional angles are calculated and compared with available experimental data. A conformational analysis for the planar, boat, chair and twisted structures of benzene is carried out and liquid benzene is found to be effectively a planar hexagon at room temperature.

A new class of calcium antagonists (dibenzotricyclic compounds) is studied by means of reaction field ab initio calculations and molecular dynamics simulations. The central ring of these tricyclic molecules is found to be more important to the calcium antagonistic potency than the two phenyl rings. The central ring with antagonistic potency shows hydrophobic character, thus the interaction between the drug and the binding sites is assumed to be dominated by hydrophobic interactions. Variation of the flexure angle, the angle between the two phenyl rings, does not change the hydrophobic property of the central ring significantly, therefore it is not expected to affect the interaction between the drug and binding site directly. The effect of the flexure angle on calcium antagonistic potency, the relation between drug affinity of these tricyclic molecules and their ionization energies, and the interaction of calcium ions with the central ring are discussed. © 1994 John Wiley & Sons, Inc.

Ab initio calculations on the nonplanarity of adducts with spin traps H-N=O, CH 3 -N=O, and CH 3 -CH 2 -N=O indicate that the degree of nonplanarity at the N correlates weakly with the electronegativity of the adding radical. The degree of nonplanarity depends on the level of theory used: extending the basis set beyond the 6-31G(d) level within the Hartree-Fock framework reduces the nonplanarity slightly, and including the effects of electron correlation at the MP2/6-31 G(d,p) level reduces it considerably more. However, the trends within a given theory level and basis set are essentially the same. Formation energies indicate that in general the adducts are strongly favored over the free radical and the spin trap

Density functional theory calculations of isotropic couplings of the atoms B to O are carried out with a variety of functional forms and basis sets. It is shown that the atomic isotropic coupling constants are very dependent on functional form, auxiliary basis set and orbital set. The calculations on the molecular systems NH 2 and H + 3 show less dependence for isotropic couplings and almost independence for anisotropic couplings. It is found that the combination of Perdew and Wang's non-local exchange functional and Perdew's correlation correction functional with a [7s, 6p, 2d]/(5, 2; 5, 2) orbital/auxiliary basis set gives results in semiquantitative agreement with experiment and MRSDCI calculations.

The electronic and hyperfine structures of the three smallest neutral nitrogen oxides, NO, NO 2 , and NO 3 , are investigated by means of ab initio and density functional (DFT) methods. The emphasis of the present work is on the performance of the DFT approach and the computation of hyperfine parameters. The geometries of the different species are optimized at up to the ab initio MP4(SDTQ) level as well as local and gradient corrected DFT levels, after which the isotropic hyperfine coupling constants are calculated using configuration interaction and DFT techniques. The results are compared with experimental and previous theoretical data. The DFT-optimized geometries of NO and NO 2 are in very good agreement with experiment

A variety of density functional theory (DFT) methods are paired with Pople basis sets of varying sizes and evaluated for use with organoselenium compounds. The ability of each method to predict reliable geometries and energies is determined through comparison with quadratic configuration interaction with single and double excitations (QCISD) results. The recommended procedure for accurate prediction of energies and geometries is to use the B3PW91 functional with the 6-311G(2df,p) basis set. The B3PW91/6-31G(d,p) level of theory gives almost identical geometries as larger basis sets, so geometries can be predicted at this level for computational efficiency.

A systematic study of the active space in MCSCF calculations, using N 2 and the stationary points on the HCN→CNH and HCCH→CCH 2 potential hypersurfaces as model systems, is reported. The different sets of active orbitals are tested with a variety of standard basis sets. The optimized geometries of the ground states and transition states are compared with experiment and other theoretical studies. It is concluded that MCSCF calculations with two occupied and two virtual orbitals (MCSCF(2+2)), in which the active space is formed from the 1π u 4 and 1 π g * orbitals, provide highly accurate descriptions of the equilibrium ground-state geometries of N 2 , HCN, CNH, and HCCH. In particular, the results in which a split-valence + polarization basis is employed are very close to the experimental values

Theoretical electron density and transition state studies have been carried out on monosubstituted cyclohexanones and heterocyclohexanones. The electron density studies reveal no significant difference in the extent of charge depletion on the two sides of the carbonyl plane. The transition state studies show that for the 4-axially-substituted cyclohexanones the increased preference for axial attack is a result of the large barrier for equatorial attack. The transition state stabilization model cannot explain these results. The results show that the electrostatic field difference between the two sides of the carbonyl plane affects the stereoselectivity

The influence of up to six Ne atoms on the isotropic hyperfine (hf) coupling constants of the hydrogen atoms in CH+4 is investigated using density functional theory. It is found that at very short NeH distances (R = 1.8 A), the hf structure is strongly affected, whereas at the more realistic van der Waals distance (R ≈ 2.5 A), the methane radical experiences a very weak field from the surrounding Ne atoms. At larger distances, the effects of the Ne matrix are negligible. It is furthermore found that at small R, the changes in aiso (H) are strongly dependent on the relative positions of the Ne atoms.

The radical cations of methane, ethane, propane, n-butane, ethene, propene, and 1- and 2-butene are optimized at the first principles LCGTO-DFT level, using a DZP basis set and including nonlocal corrections to the exchange and correlation functionals. The optimized geometries are reported and compared with previously reported structures, obtained at the ab initio UHF or MP2 levels. The calculations show a general agreement between the DFT and MP2 optimized structures, although the DFT results tend to overestimate the C-H bond lengths by 0.02 A. The structures of the carbon frameworks and all bond angles are, however, in very good agreement between the two methods