B3PW91 Methods Research Articles

Unimolecular Reactions of CH2BrCH2Br, CH2BrCH2Cl, and CH2BrCD2Cl: Identification of the Cl−Br Interchange Reaction

The recombination reactions of CH(2)Br and CH(2)Cl radicals have been used to generate vibrationally excited CH(2)BrCH(2)Br and CH(2)BrCH(2)Cl molecules with 91 kcal mol(-1) of energy in a room-temperature bath gas. The experimental unimolecular rate constants for elimination of HBr and HCl were compared to calculated statistical rate constants to assign threshold energies of 58 kcal mol(-1) for HBr elimination from C(2)H(4)Br(2) and 58 and 60 kcal mol(-1), respectively, for HBr and HCl elimination from C(2)H(4)BrCl. The Br-Cl interchange reaction was demonstrated and characterized by studying the CH(2)BrCD(2)Cl system generated by the recombination of CH(2)Br and CD(2)Cl radicals. The interchange reaction was identified from the elimination of HBr and DCl from CH(2)ClCD(2)Br. The interchange reaction rate is much faster than the rates of either DBr or HCl elimination from CH(2)BrCD(2)Cl, and a threshold energy of congruent with43 kcal mol(-1) was assigned to the interchange reaction. The statistical rate constants were calculated from models of the transition states that were obtained from density functional theory using the B3PW91 method with the 6-31G(d',p') basis set. The model for HBr elimination was tested versus published thermal and chemical activation data for C(2)H(5)Br. A comparison of Br-Cl interchange with the Cl-F and Br-F interchange reactions in 1,2-haloalkanes is presented.

Experimental and Theoretical Studies of the Homogeneous, Unimolecular Gas-Phase Elimination Kinetics of Trimethyl Orthovalerate and Trimethyl Orthochloroacetate

The rates of gas-phase elimination of trimethyl orthovalerate and trimethyl orthochloroacetate have been determined in a static system, and the reaction Pyrex vessels have been deactivated with the product of decomposition of allyl bromide. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 313-410 degrees C and 40-140 Torr, respectively. The rate coefficients for the homogeneous reaction are given by the following Arrhenius expressions: for trimethyl orthovalerate: log k (s(-1)) = [(14.00 +/- 0.28) - (196.3 +/- 1.7) (kJ/mol)] (2.303RT)(-1), (r = 0.9999); and for trimethyl orthochloroacetate: log k (s(-1)) = [(13.54 +/- 0.21) - (209.3 +/- 1.9)(kJ/mol)](2.303RT)(-1), (r = 0.9998). The theoretical calculations of the kinetic and thermodynamic parameters were carried out by using B3LYP, B3PW91, MPW1PW91, and PBEPBE methods. The theoretical results show reasonably good agreement with the experimental energy and enthalpy of activation values when using the B3PW91/6-31++G** method for trimethyl orthovalerate and PBEPBE /6-31++G** for trimethyl orthochloroacetate. These calculations suggest a molecular concerted nonsynchronous mechanism where C-OCH(3) bond polarization, in the sense C(delta+)...(delta-)OCH(3), is the rate-determining step. The increase in electron density of the oxygen atom at OCH(3) eases the abstraction of the hydrogen of the adjacent C-H bond in a four-membered cyclic structure to give methanol and the corresponding unsaturated ketal. The electron-donor substituent enhances decomposition rates by stabilizing the positive charge developing in the transition state at the carbon bearing the three methoxy groups, whereas the electron-withdrawing substituent destabilizes this charge, thus retarding the reaction.

Structures and energies of the radicals and anions generated from chlorpyrifos

The radicals and anions generated from chlorpyrifos by removing a hydrogen atom have been investigated using the hybrid density functional B3PW91 method. The results show that all the radicals have been classified as three groups and their stability order is methylene (radical 1, 3, 5, and 7) > methyl (radical 9, 11 and 13) > ring (15); the anions have the relative energetic order: methyl > methylene > ring. Moreover, some decomposition reactions are also reported. The large HOMO-LUMO gaps indicate that both radicals and anions are predicted to be high-kinetic stable molecules. We also find that radicals 9, 11 and 13 have the highest AEAs and anions 2, 4 and 6 have higher VDEs. Additionally, natural population analysis charges show that there is the lowest Deltaq (0.14) for the C7 and C9 atoms. We hope that our theoretical results may provide a reference for further experiment and practical application.

Infrared, Raman and temperature-dependent NMR spectra, vibrational assignments, normal coordinate analysis, and DFT calculations of benzoxazoline-2-thione

The tautomerism of benzoxazoline-2-thione (BOT; thio- keto) and 2-mercaptobenzoxazole (MBO; thio- enol) has been thoroughly investigated by means of Raman (3600–100 cm −1), infrared (4000–200 cm −1), 1H and 13C NMR spectra and X-ray powder diffraction (XRD). In addition, temperature-dependent 1H NMR spectra from −90 to +90 °C were acquired. To complement experimental results with theoretical predictions, we performed Density Functional Theory (DFT) calculations utilizing B3LYP, B3PW91 and SVWN methods at 6-31G(d) basis set. Both computational and spectral results were in favor of thio- keto BOT structure with no evidence for the existence of thio- enol (MBO) tautomer which firmly eliminates the possibility of an existing equilibrium between keto and enol forms. Moreover the dimerization percentage of thio- keto benzoxazoline-2-thione (BOT) and benzothiazoline-2-thione (BTT) were found to be 11.9% and 2.5%, respectively which favors strong hydrogen bonding interactions in BOT. Aided by normal coordinate analysis, force constants in internal coordinates and potential energy distributions (PEDs), a complete vibrational assignment for all fundamentals was obtained. The results are compared with the sulfur analogue, benzothiazoline-2-thione (BTT) whenever appropriate.

Natural bond orbital analysis of some S-nitrosothiols biological molecules

Theoretical study of several S-nitrosothiols biological molecules has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform natural bond orbital (NBO) analysis. It is noted that the weakness in the SN sigma bond is due to nO1σ delocalization and is responsible for the longer SN bond length in S-nitrosothiols. It is also noted that decreased occupancy of the localized σSN orbital in the idealized Lewis structure or increased occupancy of σ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital of σSN bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of substituent groups, and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intramolecular and intermolecular interaction. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Theoretical Study for High-Energy-Density Compounds Derived from Cyclophosphazene. IV. DFT Studies on 1,1-Diamino-3,3,5,5,7,7-hexaazidocyclotetraphosphazene and Its Isomers

In the present study, a theoretical study of 1,1-diaminohexaazidocyclotetraphophazene (DAHA) and its isomers has been performed, using quantum computational density functional theory (B3LYP and B3PW91 methods) with 6-31G* and 6-31G** basis sets implemented in Gaussian 03 program suite. Molecular structure and bonding, vibrational frequencies, Milliken population analysis, and natural bond orbit (NBO) have been studied. The heats of formation from atomization energies have also been calculated based on the optimized geometry. The obtained heats of formation data are compared with their homologous cyclophosphazene in order to demonstrate the accuracy of the methods, which indicate that the studied compounds might be potentially used as high energetic materials. In addition, the relative stability of five isomers have been deduced based on the total energy and the gap of frontier orbital energies.

THEORETICAL STUDIES ON HEATS OF FORMATION OF PYRIDINE N-OXIDES USING DENSITY FUNCTIONAL THEORY AND COMPLETE BASIS METHOD

The heats of formation (HOFs) for 11 pyridine N-oxide compounds are calculated by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86, PBE1PBE) methods with 6-31G** basis set and ab initio CBS-4M method. It is demonstrated that the B3PW91 method is accurate to compute the reliable HOFs for pyridine N-oxide compounds. It is also noted that the HOF is the smallest for the pyridine N-oxide which has the substituent group on the para-position, such as 4-NC–c- C 5 H 4 NO , 4- H 2 NOC – C 5 H 4 N – O , and 4- HO 2 C –c- C 5 H 4 NO . In addition, we think that the HOF of 2- HO 2 C –c- C 5 H 4 NO is much larger than that of 3- HO 2 C –c- C 5 H 4 NO and 4- HO 2 C –c- C 5 H 4 NO , which may be the result of intramolecular hydrogen bond formation and further measurements are needed to reexamine the HOFs for 2- HO 2 C –c- C 5 H 4 NO , 3- HO 2 C –c- C 5 H 4 NO , and 4- HO 2 C –c- C 5 H 4 NO .

Effect of gold atom contact in conjugated system of one dimensional octane dithiolate based molecular wire: A theoretical charge density study

To understand the effect of Au and thiol atoms in octane molecule, a structural and charge density analysis has been carried out by high level ab initio quantum chemical calculations using MP2 and B3PW91 methods with the basis sets 6-311G(d,p) and LANL2DZ. The optimized geometries, specifically, the geometry obtained from both levels reveal the effect of S- and Au-atoms in octane molecule. An introduction of sulfur atom in octane molecule lengthen its backbone C–C bond distances, and further adding of Au-atom at the terminals of octane dithiolate stabilizes these distances. The bond densities of the C–C bonds of octane are ∼1.6 eÅ −3, these values are decreased significantly and the charges are largely depleted, when thiol and Au-atoms added in the octane molecule. The presence of negative Laplacian ∇ 2 ρ( r) at bond critical points of C–C and C–H bonds, indicate, the charges are concentrated in these bonds, confirm that these bonds exhibit an open shell type interaction. The moderate values of density and the negative Laplacian of S–C bonds confirm the covalent character. The positive ∇ 2 ρ( r) value of Au–S bonds, characterize, the bonding interaction is a closed shell interaction. The combined observed low value of electron density and the positive Laplacian of Au–S bond comprises, the gold and S interaction is not a covalent interaction, but it is a very weak coordination bond interaction. The small positive value of total energy density in Au–S bond indicates, the charges in these bonds are highly depleted and this is further confirmed by the Laplacian of bond characterization.

Theoretical studies on bond dissociation energies for some pyridine N-oxide biological compounds by density functional theory and CBS-4M method

Density functional methods (B3LYP, B3PW91, B3P86, MPWB1K, MPW1B95, TPSS and PBE1PBE) and complete basis method are employed to investigate the bond dissociation energies (BDEs) of the terminal N O bond for 12 pyridine N-oxides compounds. It is demonstrated that the B3PW91/6-31G ∗∗ is the most accurate method to compute the reliable BDEs for the compounds studied. In order to test whether the non-local BLYP method suggested by Ref. [Y.R. Luo, Handbook of Bond Dissociation Energies in Organic Compounds, CRC Press, Boca Raton, 2003] is general for our study and whether B3PW91 method is sensitive to the basis set, the BDEs for 12 pyridine N-oxides compounds are also calculated by using BLYP/6-31+G ∗ and B3PW91 method with 6-31+G ∗, 6-31+G ∗∗, 6-31G ∗, 6-311+G(2df,2p) and cc-pVTZ basis sets for comparison. The obtained results show B3PW91 method with a moderate or a larger basis set, such as 6-31G ∗ and 6-31+G ∗∗, may be more suitable to calculate the BDEs of the N O bond for pyridine N-oxides compounds. In addition, pyridine N-oxide substituted with CONH 2 is stabilized relative to that substituted by CO 2H, which showed that the carboxyamide derivatives have an increased double bond character in the N O bond possibly due to the negative charge on the oxygen atom in the N O bond being partially distributed to the CONH 2 group.

A theoretical study on the metal cation-π complexes of Zn2+ and Cd2+ with benzene and cyclohexene

Transition metal cation-π complexes of Zn2+ and Cd2+ with benzene and cyclohexene have been studied with density functional B3PW91 method. Natural bond orbital (NBO) analyses on the direct cation-π complexes (with direct π-ion contact) indicate that the interaction between the π orbital of the π systems and the empty valance s orbital of the cation (πPI → s M2+ interactions) dominates the cation-π interactions. Energetical results indicate that formation of metal cation-π complexes in the gas phase is always energetically favourable and these complexes can thus be prepared and observed in experiments in the gas phase. However, in aqueous solution the desolvation penalty of Zn2+ and Cd2+ is so high that it is energetically unfavourable to form metal cation-π complexes from hydrated Zn2+ and Cd2+ at both 0 K and room temperature. It would thus be predicted that metal cation-π complexes of Zn2+ and Cd2+ are not likely to be observed in aqueous solution.

Ionic Hydrogen-Bond Networks and Ion Solvation. 1. An Efficient Monte Carlo/Quantum Mechanical Method for Structural Search and Energy Computations: Ammonium/Water

Hydrogen-bond networks about solvated ions can form many alternative structures, requiring extensive conformational searches with accurate but affordable energy computation. For this purpose we are combining Monte Carlo searches with a computationally efficient density-functional-based tight-binding (DFTB+) method. The approach is tested for the stepwise hydration energies of the ammonium ion in NH(4)(+)(H(2)O)(n) clusters (n = 1-8), for which experimental data are available. For each cluster size n, we perform Monte Carlo searches, where for each conformation we calculate the minimized energy using the DFTB+ method (and, for comparison, using MM3 or OPLS-AA force field). The Monte Carlo/DFTB+ search identifies the lowest energy structure that can be reoptimized with other quantum mechanical methods (here with HF, B3LYP, B3PW91, MP2, DFT, and CBS-Q, CBS-QB3, and CBS-APNO methods). Calculated geometries and charge densities for the clusters are also presented. The results show that the binding energies calculated by the DFTB+ method reproduce the values measured experimentally and predicted by highly correlated, but significantly more computationally intensive, ab initio quantum chemical methods. The encouraging results suggest that the Monte Carlo/DFTB+ approach is a computationally efficient quantum chemical method for relatively large solvated systems, as demonstrated here for cluster ions.

Structures, vibrational frequencies, and electron affinities of SF 5O n/SF 5O n− ( n = 1–3)

The molecular structures, vibrational frequencies, and electron affinities of the SF 5O n /SF 5O n − ( n = 1–3) species have been examined with four hybrid density functional theory (DFT) methods. The basis set used in this work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. The SF 5O n ( n = 1–3) species should be potential greenhouse gases. The anion SF 5O 2 − with C s symmetry has a 3A″ electronic state, and the neutral SF 5O 3 with 2A″ electronic state has C s symmetry. The anions SF 5O 2 − and SF 5O 3 − should be regarded as SF 5 −·O 2 and SF 5O −·O 2 complexes, respectively. Three different types of the neutral–anion energy separation presented in this work are the adiabatic electron affinity (EA ad), the vertical electron affinity (EA vert), and the vertical detachment energy (VDE). The EA ad values predicted by the B3PW91 method are 5.22 (SF 5O), 4.38 (SF 5O 2), and 3.61 eV (SF 5O 3). Compared with the experimental vibrational frequencies, the BHLYP method overestimates the frequencies, and the other three methods underestimate the frequencies. The bond dissociation energies D e (SF 5O n → SF 5O n − m + O m) for the neutrals SF 5O n and D e (SF 5O n − → SF 5O n − m − + O m and SF 5O n − → SF 5O n − m + O m −) for the anions SF 5O n − are reported.

Natural bond orbital (NBO) population analysis of para-substituted S-Nitroso-thiophenols

Theoretical study of several para-substituted S-Nitroso-thiophenols has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6-31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform NBO analysis. It is noted that the weakness in the S N sigma bond is due to n O 1 → σ S 3 N 2 ∗ delocalisation and is responsible for the longer S N bond length in para-substituted S-Nitroso-thiophenols. It is also noted that decreased occupancy of the localized σ SN orbital in the idealized Lewis structure, or increased occupancy of σ SN ∗ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital (NHO) of σ SN bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of subsitutent groups and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intra- and intermolecular interaction.

Density functional theory investigation of electrophilic addition reaction of bromine to tricyclo[4.2.2.22,5]dodeca-1,5-diene1

The geometry and the electronic structure of tricyclo[4.2.2.2(2,5)]dodeca-1,5-diene (TCDD) molecule were investigated by DFT/B3LYP and /B3PW91 methods using the 6-311G(d,p) and 6-311++G(d,p) basis sets. The double bonds of TCDD molecule are syn-pyramidalized. The structure of pi-orbitals and their mutual interactions for TCDD molecule were investigated. Potential energy surface (PES) of the TCDD-Br(2) system was studied by B3LYP/6-311++G(d,p) method and the configurations [molecular charge-transfer (CT) complex, transition states (TS1 and TS2), intermediate (INT) and product (P)] corresponding to the stationary points (minima or saddle points) were determined. Initially, a molecular CT-complex forms between Br(2) and TCDD. With a barrier of 22.336 kcal mol(-1) the CT-complex can be activated to an intermediate (INT) with energy 15.154 kcal mol(-1) higher than that of the CT-complex. The intermediate (INT) then transforms easily (barrier 5.442 kcal mol(-1)) into the final, N-type product. The total bromination is slightly exothermic. Accompanying the breaking of Br-Br bond, C1-Br, C5-Br and C2-C6 bonds are formed, and C1 = C2 and C5 = C6 double bonds transform into single bonds. The direction of the reaction is determined by the direction of intramolecular skeletal rearrangement that is realized by the formation of C2-C6 bond.

A comparison of C–C rotational barrier in [2]staffane, [2]tetrahedrane and ethane

Analysis of internal rotation about C–C single bond in [2]staffane, [2]tetrahedrane, and ethane has been carried out using HF, B3LYP, B3PW91, MPW1PW91, MP2, QCISD and QCISD(T) methods combined with 6-31G ∗∗, 6-311++G ∗∗ and AUG-cc-PVTZ basis sets. The calculated rotational barriers Δ E and the trend in them depend on basis set and method. Both hyperconjugation interaction, which stabilizes the staggered conformation more than eclipsed one, and steric effect, which is negligible in ethane and is comparable with hyperconjugation interaction in [2]tetrahedrane and [2]staffane, cause the hindered rotation. In addition, the Δ E values are in good agreement with electron charge densities calculated at central C–C bond critical point.

Development of eclipsed and staggered forms in some hydrogen bonded complexes

AbstractIntermolecular hydrogen bonding in X3CH···NH3 (X = H, F, Cl, and Br) complexes has been studied by B3LYP, B3PW91, MP2, MP3, MP4, and CCSD methods using 6‐311++G(d,p) and AUG‐cc‐PVTZ basis sets. These complexes could exist in both eclipsed (EC) and staggered (ST) forms. The differences between binding energies of EC and ST forms are negligible and all EC and ST shapes correspond to minimum stationary states. The order of stabilities of them is in an agreement with the results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. On the basis of low differences between binding energies, ST forms are more stable than EC forms in all complexes with the exception of Br3CH···NH3, which behaves just opposite. Although the differences between binding energies are negligible, they are consistent with the results of AIM analysis. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Using the Combined Approach of 35Cl NQR, 14N NQR and DFT Calculations To Study the 14N NQR Spectrum of Diazepam

The electronic structure of diazepam was analyzed by pulsed 14N nuclear quadrupole resonance (NQR) spectroscopy and density functional theory (DFT) calculations using the hybrid method B3PW91 with the 6-31G(d,p) basis set. Before searching for 14N NQR spectra, a 35Cl NQR line in diazepam was recorded to evaluate the degree of disorder in the crystal lattice and thus the chances to find much weaker NQR spectra of nitrogen-14 in this substance. In order to speed up the detection of weak NQR spectra, the spin-locked spin-echo multipulse sequence had to be applied. Despite the expected chemical inequivalence of the two nitrogen atoms in the diazepam molecule, only two resonance lines ν+ and ν− were then found for this compound at 77 K. Possible reasons for this anomaly are discussed. From the experimental frequencies, the quadrupole coupling constant, the asymmetry parameter and the principal components of the electric field gradient tensor were determined. Those parameters were compared with the results of theoretical DFT calculations showing a reasonable accuracy of the DFT model used.

Structures, Vibrational Spectra, and Relative Energetics of CH3COIO3 Isomers

Abstract The structures, vibrational spectra, relative energetics, and enthalpies of formation of CH3COIO3 isomers have been investigated with B3LYP, B3P86 and B3PW91 methods in conjugation with the 6‐31+G(d), 6‐311+G(d,p) and 6‐311++G(3df,3pd) basis sets. The CH3COOIO2 structure was found to be the most stable form among the isomers with an estimated enthalpy of formation of −314.6 kJ·mol−1. The enthalpies of formation for CH3COOOOI, CH3COOOIO and CH3COIO3 are −180.7, −184.9 and −50.6 kJ·mol−1, respectively. The implication of the formation of CH3COIO3 isomers from the atmospheric cross‐reactions of the acetylperoxy (CH3COO2) and iodine monoxide (IO) radicals was examined and the possible dissociation products of the most likely CH3COIO3 isomers were determined.

Static electric polarizability and hyperpolarizability of sodium clusters: The case of the sodium tetramer

We have calculated the static polarizability and second hyperpolarizability of sodium tetramer using conventional ab initio and density functional theory based methods. Our best values for the mean and the anisotropy of the dipole polarizability are α¯= 548.81 and Δα=489.14 e$^{2}$a$_{0}^{2}$E$_{h}^{-1}$. The mean second hyperpolarizability is very large at γ¯=5531 × 10$^{3}$ e$^{4}$a$_{0}^{4}$E$_{h}^{-3}$. The differential-per-atom polarizability is negative but the differential-per-atom hyperpolarizability is positive. Density functional theory methods offer a rather fuzzy picture of the second hyperpolarizability of the molecule. Nevertheless, we have found that the widely used B3LYP and B3PW91 methods perform rather reliably for both the polarizability and the hyperpolarizability of Na$_{4}$.

Theoretical study for high-energy-density compounds from cyclophosphazene III. A quantum chemistry study: High nitrogen-contented energetic compound of 1,1,3,3,5,5,7,7-octaazido-cyclo-tetraphosphazene: N 4P 4(N 3) 8

Molecular structure, vibrational frequencies and infrared intensities of 1,1,3,3,5,5,7,7-octaazido-cyclo-tetraphosphazene have been studied employing HF, B3LYP and B3PW91 methods using 6-31G ∗ basis set. The study showed that this molecule has non-planar structure and there is no imaginary frequency. Furthermore, there exist four sets of special P N bonds in the P 4N 28 ring; the P N bonds and the azide groups conjoint to them have special characters. The NBO population analysis was used to help us understand the interactions between donor orbit and acceptor orbit in the nitrogen phosphorus systems. Furthermore, three methods with the same basis set are further employed to calculate the heats of formation for the compound.