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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.

A new semiquantitative measure of the difference between two electron density distributions is proposed. Specifically, the natural orbitals of an accurate electron density, such as those given by a configuration interaction method, are used to expand the Kohn–Sham orbitals of an approximate density functional method. The difference between the two density distributions is then reduced to a single number, the orbital-based density difference index (DDI). With the reference densities calculated from quadratic configuration interaction calculations including single and double substitutions, DDIs were obtained for four diatomic molecules using three basis sets and seven approximate functionals. Results are also included for an additional six small molecules with a flexible extended basis set and the same set of functionals. These results show that the DDI leads to conclusions that are consistent with those obtained previously by visual comparison of density difference plots. The orbital-based density differen...

The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti’s general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is...

The molecular quadrupole moments of benzene and naphthalene are calculated using a range of ab initio and density functional methods, with basis sets as large as aug-CC-PVTZ for benzene and CC-PVTZ for naphthalene. The quadrupole moments calculated using Hartree-Fock wavefunctions are lower than the experimentally determined values, and all DFT values are higher than the experimental values. The use of the hybrid B3P86 and B3PW91 functionals, with a large basis set, produce quadrupole moments which are within experimental error for these two small aromatic molecules.

The interactions between polycyclic aromatic hydrocarbons (with up to four rings) and dinitrobenzene have been calculated using semiempirical methods and shown to give poor correlation with experimental stability constants. Quadrupole moments for these molecules have been calculated at the HF/6-31G(d,p) and BLYP/6-31G(d,p) levels of theory. The interaction between polycyclic aromatic hydrocarbons and nitrated benzenes is found to be dependent on the quadrupole moment. Dispersion interactions are estimated to be considerably smaller than quadrupole interactions and not a significant influence on the binding energy. The two methods used show the same trends in the calculated quadrupole moment, and it is shown that a method involving electron correlation may not be necessary for prediction of stability constants.

Ab initio MO calculations indicate that protonation of XHn to XHn+1+ increases bond dissociation energies (BDEs) for homolytic cleavage of CX bonds in CH3XHn, CH3−CH2−XHn, CH2CH−XHn, and CH⋮C−XHn compounds (X = N, O, F, P, S, and Cl). Deprotonation of XHn to XHn-1- (X = C, N, O, Si, P, and S) in saturated species generally results in small decreases in CX BDEs; in unsaturated substances, as a result of resonance, deprotonation yields large increases in the CX BDEs, X = Si being the exception. For adjacent CC bonds, protonation of XHn increases the CC BDEs because it produces larger electronegativity differences between bonded groups; deprotonation decreases the BDEs as a result of resonance effects. Two types of correlation between bond lengths and homolytic BDEs are observed, with second-row elements exhibiting bond length changes to a much lesser extent than first-row ones. Firstly, bond lengths of adjacent CC bonds increase as their BDEs decrease. Secondly, and apparently anomalously, CX bond lengths a...

Dialkylstannylene acetals are organotin intermediates widely used to facilitate regioselective monofunctionalization of diols or polyols by electrophiles. Alkylation is both the slowest and the most useful reaction of these intermediates, and this reaction is markedly accelerated by the addition of nucleophiles to the reaction media, usually cesium fluoride in dimethylformamide (DMF) or tetrabutylammonium iodide or bromide in toluene. The regioselectivity may be influenced by aggregation of the dialkylstannylene acetals into dimers and higher oligomers, and by the addition of these nucleophiles. The stabilities and the geometries of the species potentially involved in these processes were examined by using theoretical chemistry methods with di-n-butyldialkoxytin derivatives as examples and fluoride as the nucleophile. Geometry optimizations were performed at B3LYP/6-31G(d,p) level, and single point energies obtained at the MP2/6-311G(2d,p) level with diffuse functions added for fluorine. The LANL2DZdp basis set with diffuse and polarization functions and its effective core potential were used to describe tin. The addition of fluoride to monomeric di-n-butyldialkoxytin derivatives to give fluoridated monomers is predicted to be strongly exothermic, by 187 to 209 kJ/mol, depending on the alkoxyl group. The fluoridated monomers are calculated to react with monomers exothermically to give monofluoridated dimers, except for the di-t-butoxy derivative. Dimer formation on average is about 20 kJ mol(-1) more exothermic than for the nonfluoridated monomers alone. Monofluoridated monomers strongly prefer to exist as monomers because the difluoridated dimers are estimated to be 209 to 278 kJ mol(-1) less stable at 298 K.

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.

A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies of the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree–Fock/3‐21G(d,p) level. Good overall agreement is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanical (QM/MM) coupling schemes are investigated using gas‐to‐liquid vibrational frequency shifts as a probe. The results suggest, somewhat surprisingly, that the method with the weakest QM/MM coupling gives the best overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme overestimates the shifts towards the red direction due to overestimation of the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in describing the O–H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail.

The thermally induced decomposition of ethylene, C2H4, to acetylene, C2H2, on the Ni(111) surface has been treated theoretically: potential-energy curves were computed within a modified extended-Huckel framework and reaction rates estimated via a master equation approach. As a preliminary step, adsorbate geometries and binding energies were optimized for one and two molecules of each hydrocarbon species chemisorbed in various sites on rigid bulk cluster models of the Ni substrate. A fragmentation pathway is proposed, eliciting the concerted tunnelling of intermolecular H's from a transient bimolecular complex comprised of two rotated C2H4's, which rationalizes the observed second-order decomposition kinetics.

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 electron densities of H2, Li2, N2, O2, and F2 have been calculated by density functional theory (DFT) methods with various exchange and correlation functionals such as SVWN, BLYP, B3LYP, BP86, B3P86, and B3PW91 and compared with the results from quadratic configuration interaction calculations including all single and double substitutions (QCISD) using Dunning's correlation consistent polarized valence double- and triple-ζ basis sets augmented with the corresponding diffuse functions (aug-CC-PVDZ and aug-CC-PVTZ). The DFT methods yield electron densities and Laplacians of the densities in good agreement with the QCISD results. The gradient-corrected functionals improve upon the densities calculated by use of the local spin density approximation (SVWN) relative to the corresponding QCISD densities. All gradient-corrected functionals are generally insensitive to the grids used in the study. The basis set, however, has a significant effect on the electron density and shows a strong dependence on the choi...

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 ...

The solvent dependence of the tautomeric equilibria of 2-, 3- and 4-hydroxypyridines is investigated by means of self-consistent reaction field (SCRF) theory. The tautomeric equilibrium constants (KT) in cyclohexane, chloroform, and acetonitrile are predicted at both the Hartree−Fock (HF) and the second-order Moller−Plesset perturbation theory (MP2) levels. It is found that log KT is linearly dependent upon the solvent polarity as measured by the Kosower Z parameter, in agreement with earlier experimental findings. Based on this relation, log KT is predicted to be 1.72, 0.86, and 5.65 for 2-, 3-, and 4-hydroxypyridines, respectively, in aqueous solutions.

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 electron densities of CH3F, CO, CO2, H2CO, H2O, HCN, HF, and NH3 have been calculated by density functional theory (DFT) methods with various exchange and correlation functionals and are compared with QCISD (quadratic configuration interaction method including single and double substitutions) results. The DFT methods yield electron densities and Laplacians of the densities in good agreement with the QCISD results. As expected, the gradient-corrected functionals including B3LYP, B3P86, BP86, and BLYP improve the densities calculated by the local spin density approximation relative to the corresponding QCISD densities. Relative to his 1988 functional, Becke's three-parameter exchange functional clearly improves the results for equilibrium geometries, vibrational frequencies, dipole moments, and electron densities. For all gradient-corrected functionals, the results are generally insensitive to the numerical accuracy of the grid. The basis set, however, has a significant effect on the electron density, w...

The energy surface of CaOH has been studied and an isomer of the form of HCaO with an electronic state of 2Σ+ symmetry has been found. It is linear with H–Ca and Ca–O bond lengths of 2.021 and 2.002 A, respectively, at the CISD level with a basis set of triple‐zeta plus double polarization quality. The overall electronic structure is largely ionic and can be described as H(1s2)Ca(4s0)O(2s22p2x2p2y2p1z) (H−Ca2+O−). The minimum on the energy surface is well defined and the harmonic vibrational frequencies have been calculated. The O–H bond length and stretching frequency and the bending frequency of CaOH have also been calculated and compared with experiment.

The thermal motion of CD+4 in the gas phase at 4 and 40 K is studied by means of a molecular dynamics method using the MP2/6‐31G(d,p) force field. The thermal motion of CD+4 in the temperature range from 4 to 40 K does not involve a deuterium permutation on the picosecond time scale. The electronic energy fluctuations of CD+4 due to thermal motion are less than 0.7 kcal/mol at 4 K and 1.6 kcal/mol at 40 K, which is less than the energy barrier (3.4 kcal/mol at MP2 level) required for the permutation between two energy minimum C2v structures. On the basis of these calculations it is suggested that the observation of four equivalent hydrogen atoms in the ESR spectrum of CH+4 can be attributed to fast quantum tunneling, with an estimated rate of 4.3×106 s−1 for CH+4 and 1.6×103 s−1 for CD+4.

The effect of using ‘geometry-adapted’ atomic charges in computer simulations is studied by carrying out molecular dynamics simulations of trans-stilbene (tS) in methanol solution. The atomic charges on tS have been calculated at the Hartree-Fock/6-31G(d, p) level according to the Merz-Singh-Kollman scheme as a function of the two torsional angles φ1 and φ2 across the phenyl groups and ethylenic carbons (Cph-Ce), fitted to analytical functions q i (φ1, φ2) and added to the force field for calculation of the Coulombic interactions. In spite of the non-polar character of tS, the quantum mechanically calculated atomic charges in the linkage region are observed to vary more than 0-1 |e| with respect to the changes of the torsional angles φ1 and φ2. Compared to reference molecular dynamics simulations, carried out simultaneously with conventional ‘fixed’ charges, there is a clear effect on the solvation structure when ‘flexible’ atomic charges on tS are used. It is expected that the effect of geometry...

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.

The hyperfine structure of the 14NH2 radical is investigated by means of multireference single and double configuration interaction (MRCI) techniques. Particular attention is paid to the dependence of the coupling constants on the basis set, reference space, and configuration selection energy threshold. It is found that convergence can be obtained only if more than 83 reference configurations are included with an energy threshold of at least 10−7 hartree. With up to 126 reference configurations, an energy threshold smaller than 10−8 hartree and an uncontracted (13s8p2d/8s2p) basis set, the MRCI isotropic couplings (27.44 and −68.47 MHz for N and H, respectively) are in very good agreement with experimental data (27.9 and −67.2 MHz, respectively).

The interactions between an alkali metal atom (Li, Na, K) and acetylene/vinylidene are investigated by means of gradient corrected (‘PWP’) density functional theory. Particular emphasis is made on the determination of equilibrium geometries, energetics and atomic hyperfine structures of the MHCCH van der Waals addition complexes, and the MHCCH and MCCH2 charge-transfer complexes. The PWP optimized geometries are found to be similar to previous ab initio data, and the generated hyperfine structures are in excellent accord with experimental matrix isolation ESR data recorded at 4 K. In the charge transfer complex formation, the metal atoms are found to donate 0.6–0.8 electron from an excited p orbital to the antibonding CC π∗ orbital of the hydrocarbon fragment. The calculated trend in charge donation matches the order of increasing atomic s → p excitation energies, K < Li < Na. This is the first application of the present method to metal atom hyperfine structures, and the results are promising for future studies.

The reactivity of disulfide and diselenide derivatives towards F(-) and CN(-) nucleophiles has been investigated by means of B3PW91/6-311+G(2df,p) calculations. This theoretical survey shows that these processes, in contrast with the generally accepted view of disulfide and diselenide linkages, do not always lead to SS or SeSe bond cleavage. In fact, SS or SeSe bond fission is the most favorable process only when the substituents attached to the S or the Se atoms are not very electronegative. Highly electronegative substituents (X) strongly favor SX bond fission. This significant difference in the observed reactivity patterns is directly related to the change in the nature of the LUMO orbital of the disulfide or diselenide derivative as the electronegativity of the substituents increases. For weakly electronegative substituents, the LUMO is a σ-type SS (or SeSe) antibonding orbital, but as the electronegativity of the substituents increases the π-type SX antibonding orbital stabilizes and becomes the LUMO. The observed reactivity also changes with the nature of the nucleophile and with the S or Se atom that undergoes the nucleophilic attack in asymmetric disulfides and diselenides. The activation strain model provides interesting insights into these processes. There are significant similarities between the reactivity of disulfides and diselenides, although some dissimilarities are also observed, usually related to the different interaction energies between the fragments produced in the fragmentation process.

This chapter reviews the performance of density functional theory for the calculation of hyperfine coupling tensors and the development of computational methodology for biological radicals. Remarkable advances in density functional theory (DFT) have made it possible to study biological radicals as straightforwardly as closed-shell molecules. The conformational and charge dependence of the observed hyperfine coupling tensors can therefore be explored by employing reliable computational procedures. Although, hyperfine coupling tensors are highly dependent on the structure of the radical, temperature and environmental effects are typically smaller than 10%. As a consequence, hyperfine coupling tensor calculations on isolated biological radicals (at zero kelvin) closely reproduce the hyperfine coupling tensors of most radicals observed in condensed phases (at higher temperatures), and are appropriate for the purpose of theoretical assignment and interpretation. Often, precise identification of unknown radicals can be made by systematic comparison of observed experimental values with hyperfine coupling tensors calculated for all possible conformations of a radical and its protonated or deprotonated derivatives. The aim of this chapter is to review the ability of DFT methods to account for the hyperfine coupling tensors of a variety of biological radicals.

A computational investigation of the electronic properties of an experimentally prepared ADADA helix indicates that the helix is held together with four strong hydrogen bonds as well as many other weak interactions. Determination of the electronic energy changes, as well as thermodynamic parameters, suggests that helix formation is a favorable process, driven by the formation of the hydrogen bonds. For instance, the unsubstituted helix has an electronic binding energy of -85.8 kJ/mol. Furthermore, the strength of binding can be tuned to some extent by the careful selection of substituents. The hydrogen bonds are strengthened when the pyridine ring (H-bond acceptor) is substituted with an electron-donating group such as an amine, while electron-withdrawing groups on the thiazine ring (H-bond donor) are preferred. The most significant enhancement in binding is achieved when the helix is constructed from monomers that consist of contiguous hydrogen-bond acceptors or donors. This so-called AAAAA-DDDDD helix exhibits a binding energy almost 4-fold greater than that of the unsubstituted ADADA helix at -335.4 kJ/mol, a dramatic improvement over the ADADA helix.

The electronic structures of the Rydberg and related molecules belonging to the series [Formula: see text], NH4, and [Formula: see text] and H3O+, H3O, and H3O− have been investigated by analysis of the electron densities generated by the Hartree–Fock method (HF), the quadratic configuration interaction method including single and double excitations (QCISD), the coupled-cluster method including double excitations (CCD), and the density functional method with the Becke–Perdew gradient correction functionals (DFT-BP). Adding an electron to the stable cation core ([Formula: see text] and H3O+) results in an increase in the bond length, a larger increase in the distance of the bond critical point (BCP) from the nitrogen atom than from the hydrogen atom, a decrease in the electron density at the bond critical point, and an increase of the Laplacian of the electron density at the BCP. These trends are consistent with a decrease in the electronegativity of the central atom and a weakening of the bond as electrons are added to diffuse Rydberg orbitals. There is little effect, however, when a second electron is added to form the double Rydberg molecules ([Formula: see text] and H3O−). For NH4, the preferred geometry corresponds to a high atomic energy associated with the hydrogen basin, and to a low kinetic energy and a high potential energy in the basin. The high atomic energy associated with the hydrogen basin is attributed to the dominant contribution of the electron–nuclear attraction energy. Similar effects are observed in the Jahn–Teller distorted molecules: [Formula: see text] and [Formula: see text]. The DFT-BP method yields results in qualitative agreement with the results of the QCISD and CCD methods; however, it underestimates the bond strengths of the Rydberg molecules relative to high-level ab initio methods.

The linear combination of Gaussian-type orbitals–density functional theory (LCGTO–DFT) approach is used to study geometries and hyperfine structures of a set of neutral radicals. Each of the title molecules is investigated by means of local density approximation calculations, and using the Becke–Perdew and Perdew–Wang–Perdew corrections to the exchange and correlation functionals. The effects of local vs. non-local potentials and of various basis sets are investigated. Total densities and unpaired spin densities are compared. The isotropic couplings are found to be very dependent on the type of exchange functional used, whereas the anisotropic couplings are relatively insensitive to the choice of basis set and functional. In most cases, the Perdew–Wang exchange corrections provide isotropic couplings in satisfactory agreement with experiment.

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.

The electron densities of the 3 Σ - ground state of diazasilene (SiNN) derived from density functional methods and conventional ab initio (UHF and UMP2) methods are compared and analyzed. The spin contamination is investigated at the HF, MP2, MP3, MP4, and DFT levels. The optimized geometries and vibrational frequencies obtained by using Becke-Perdew gradient corrections to the LSDA exchange and correlation functionals are also reported. The failure of the Hartree-Fock method, and other methods in which the Hartree-Fock single determinant wave function is used as the reference configuration, to predict a bound structure for SiNN is attributed to the antibonding character of the spin density along the Si-N bond

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

The proton transfer process from a guanine to its complementary cytosine in a B-form d[GG] radical cation is investigated theoretically. The d[GG] radical cation is optimized by the ONIOM + PCM method. In this two-layer ONIOM method, the high layer consists of the π-stacked complementary base pairs with a hole charge, which is treated at the B97D/6-31G(d) level. The low layer includes the sugar-phosphate backbone and sodium ions, which is treated with the Amber99SB/Parmbsc0 force field. Our calculations reveal that the stabilization of the deprotonated state in d[GG] oligomers is related to the location of sodium ions. When sodium ions are located near phosphate groups, the proton prefers bonding with the guanine, and 80% of the hole charge is delocalized on the guanine residues. When sodium ions are placed in the major groove, the deprotonated state is favorable, and 70% of the hole charge localizes on the corresponding guanine-cytosine pair. According to the natural bond orbital analysis, the N-H···N hydrogen bond between the guanine-cytosine pair provides an important contribution to the stabilization of the deprotonated state. Stabilization of this hydrogen bond is very sensitive to the d[GG] oligomer configuration. In summary, the proton transfer process in a guanine-cytosine pair of poly(G)-poly(C) radical cations is largely affected by the arrangement of counterions.

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

The kinetic and thermodynamic CH 3 /CD 3 isotope effects (κ H /κ D and K H /K D ) in the forward and reverse directions of ten nonidentity methyl transfer reactions, X - +CH 3 Y→XCH 3 +Y - , and the kinetic isotope effects of five identity methyl transfer reactions have been computed ab initio using a 6-31++G(d,p) basis set. The kinetic isotope effects is smaller in the exoergic direction in five of the nonidentity reactions and larger in the exoergic direction in the other five. Over the entire data set there is no correlation between (κ H /κ D ) XY and either ΔG or ΔG

The small and subtle effects of electron correlation in the Rydberg-like 3{sup 1}D and 3{sup 3}D states of the helium-like ions are studied using very accurate explicitly correlated wave functions. In particular, the effects of electron correlation on one-electron densities, probability distributions for the interelectronic distance, and their moments are examined with the help of density and Coulomb holes. Statistical electron correlation is examined with radial and angular correlation coefficients. The different methods of analysis provide complementary insight.

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

Geometries for the molecules in the series C 2 H n F 6-n (n=0-6) and for the radicals produced by homolytic cleavage of the C-C and C-H bonds have been optimized at the MP2/6-31G(d,p) level. A few calculations at the MP4/6-311G-(d,p)//MP2/6-31G(d,p) level have been included to show that higher order correlation effects do not change the observed trends. The preferred conformations of the substituted ethyl radicals indicate that the gauche effect operates in the radicals as well as in the parent molecules