UHF Levels Research Articles

Can substitution accomplish intact polycationic stability in polyatomic molecules? Illustration with acetylene molecule

In this article, substitution is viewed as a parameter to stabilize intact polycationic species. Acetylene has been chosen for the study and ab-initio structure calculations for acetylene and di-halogen substituted acetylene have been performed using 6-311G** at UHF and DFT (B3LYP) and using 3-21G* at UHF level of theory. In substituted polycationic species the C-X bond length contracts and lead to quadruply charged cationic stability in C2X2 (where X = F, Cl, Br and I) molecule. Transition state calculations for C2I2 dication are also presented.

Electronic properties of the low-lying spin states of dimethylnitrosamine coordinated to Fe(III) heme models: An ab initio study

The unusual O-coordination mode of nitrosamines to Fe(III) heme models has been observed in the bis(dimethylnitrosamine)(meso-tetraphenylporphyrinate)iron(III) cation. For the first time, this latter as well as the simpler bis(dimethylnitrosamine)(porphinate)iron(III) heme model cations have been studied through ab initio methods. The sextet, quartet, and doublet spin states of both cations have been studied through single-point calculations based on the experimental (X-ray) geometry. Their energies, charges, and spin densities have been analyzed. The obtained results (at the UHF/cc-pVDZ and ROHF/cc-pVDZ levels) indicate that the peripheral benzene rings are of secondary importance for the coordination of dimethylnitrosamine to the Fe(III) porphyrin core. The obtained energy ordering is sextet < quartet < doublet, at all computational levels. The UHF, ROHF, and UMP2 results indicate an excess of alpha spin density around the Fe atom, a low covalency for the FeO bond and a substantial charge transfer to the Fe atom. Our best estimates [obtained at ROMP2 level with the mixed cc-pVDZ/cc-pVTZ-DK(Fe) basis set] for the energy differences (in eV) between the three spin states considered are 0.929 for the sextet-quartet gap and 0.812 for the quartet-doublet gap, which indicate that the spin crossover (at room temperature) is very unlikely. These results represent the substantial decrease in the uncorrelated values. The implications of spin contaminations at the UHF and UMP2 levels for subsequent geometry optimizations to be performed in the smaller cation have also been discussed. © 2013 Wiley Periodicals, Inc.

Calculations of hyperfine coupling constant of the TMPD molecule

The calculations of the hyperfine coupling constants (HFCC) of tetramethyl-p-Phenylenediamine (TMPD) are presented. Several DFT functionals are employed and the results are compared with the UHF and MP2 level of theory. Sensitivity of HFCC to the choice of the basis set is investigated. Impact of the different conformers of TMPD and the equivalence of methyl group hydrogens on the HFCC values is considered. The solvent effects are introduced via polarizable continuum model and compared with in vacuo calculations. Last but not least, molecular dynamics is employed to include the solvent molecules as well as the dynamics explicitly, accounting similar conditions comparing to the liquid phase EPR experiment.

Some considerations about Gaussian basis sets for electric property calculations

Recently, segmented contracted basis sets of double, triple, and quadruple zeta valence quality plus polarization functions (XZP, X = D, T, and Q, respectively) for the atoms from H to Ar were reported. In this work, with the objective of having a better description of polarizabilities, the QZP set was augmented with diffuse (s and p symmetries) and polarization (p, d, f, and g symmetries) functions that were chosen to maximize the mean dipole polarizability at the UHF and UMP2 levels, respectively. At the HF and B3LYP levels of theory, electric dipole moment and static polarizability for a sample of molecules were evaluated. Comparison with experimental data and results obtained with a similar size basis set, whose diffuse functions were optimized for the ground state energy of the anion, was done. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Linear-Polarized IR-Spectroscopic and Structural Elucidation of Glycyl-L-Phenylalanyl-Glycine Hydrochloride Monohydrate

As a part of our spectroscopic and structural elucidation of small peptides, their salts and metal complexes, the polarized IR-spectroscopic and structural elucidation of the protonated form of glycyl-(L)-phenylalanyl-glycine as hydrochloride monohydrate (H-Gly-Phe-Gly-OHxHClxH(2)O) is reported. The obtained structure of the protonated tripeptide is supported by quantum chemical ab initio calculations at UHF level of theory and 6-31++G, showing only one stable conformer with two intermolecular NH(3)(+)OH(2) and (C=O)OHOH(2) hydrogen bonds. The N(terminus) amide O=C-N-H amide fragment is flat trans-configured with a dihedral angle value of 179.0(7)degrees, while the corresponding group at C(terminus) is with transoide-configurated and value of the dihedral angle of 151.1(3)degrees, respectively. In addition data of (1)H- and (13)C magnetic resonance spectroscopy (NMR), thermogravimetry (TGA), differential scanning calorimetry (DSC) and high performed liquid chromatography (HPLC) with tandem mass spectrometry (HPLC-MS/MS) as presented.

Structure and dynamics of high-spin Ru 2+ in aqueous solution: Ab initio QM/MM molecular dynamics simulation

The structural and dynamical properties of high-spin Ru 2+ in aqueous solution have been theoretically studied using molecular dynamics (MD) simulations. The conventional MD simulation based on pair potentials gives the overestimated average first shell coordination number of 9, whereas the value of 5.9 was observed when the three-body corrected function was included. A combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to take into account the many-body effects on the hydration shell structure of Ru 2+. The most important region, the first hydration shell, was treated by ab initio quantum mechanics at UHF level using the SBKJC VDZ ECP basis set for Ru 2+ and the 6-31G ∗ basis sets for water. An exact coordination number of 6 for the first hydration shell was obtained from the QM/MM simulation. The QM/MM simulation predicts the average Ru 2+–O distance of 2.42 Å for the first hydration shell, whereas the values of 2.34 and 2.46 Å are resulted from the pair potentials without and with the three-body corrected simulations, respectively. Several other structural properties representing position and orientation of the solvate molecules were evaluated for describing the hydration shell structure of the Ru 2+ ion in dilute aqueous solution. A mean residence time of 7.1 ps was obtained for water ligands residing in the second hydration shell.

Origin of regioselectivity in the intramolecular [2+2] photoreaction of α, β-unsaturated furanones to a terminal alkene

The intramolecular [2+2] photoreaction of α,β-unsaturated furanone to a terminal alkene presents a remarkable change in regioselectivity, with variation of the chain length of the terminal alkene. Head-to-tail (ht) regioselectivity was exclusively produced through one-carbon atom connection (furanone-1 n), whereas it was completely inverted when connection of two-carbon atoms takes place (furanone-2 n). The origin of this remarkable change in regioselectivity was elucidated by means of B3LYP, UHF and PM5 levels of calculation. The transition state (TS) analysis was used to rationalize this change in regioselectivity. The results indicate that a TS on the lowest-lying triplet potential energy surface may play an important role in controlling the regioselectivity. The role of the biradical intermediates and, in particular, the relative energies of the closure TS and the retro-cleavage TS leading to the original furanones were also studied. We observed that the behaviour of the biradicals that are appropriate for ring closure do not follow the so-called ‘rule of five’.

Ab Initio Study of the Cyclooctatetraenyl Radical

Ab initio calculations have been carried out on the 1,3,5,7- and 1,2,4,7-tetraene configurations of the cyclooctatetraenyl radical at UHF, ROHF, MCSCF, ROCISD, QCISD, and CCSD(T) levels of theory with 6-311G(d,p) and cc-pVDZ basis sets. Although spin contamination is present, the ROCISD calculations support the energies obtained from less intensive, UHF-based coupled cluster calculations over the energies obtained from MCSCF analysis of the pi-electron orbitals. The 1,3,5,7-form is a local minimum at the coupled cluster levels, higher in energy than the resonance-stabilized 1,2,4,7-form by 10-13 kJ/mol, but bounded by a barrier of less than 0.5 kJ/mol. The isomerization surface connecting these two structures is described and results reported from integration of the vibrational Schrödinger equation on that surface. Excited vibrational states at energies just above the isomerization barrier are dominated by the character of the 1,3,5,7-tetraenyl radical, which suggests that chemistry involving this intermediate at typical combustion temperatures may branch at this juncture.

Prediction of reactive sites for the electropolymerization of p‐benzenesulfonic acid derivatives: Ab initio and experimental study

AbstractA theoretical study of the electronic structure of some p‐benzenesulfonic acid derivatives is carried out to explain the electrooxidation of these compounds, in particular the allyl ether p‐benzenesulfonic acid, sodium salt electrooxidation, and electrode surface modification observed during electrochemical studies. The effects of the alkyl side chain and of the aromatic ring during electrooxidation are considered using the UHF and B3LYP levels of theory. The changes in monomer structure after removal of an electron are explained by the nodal structure of the highest occupied molecular orbital (HOMO), α and β single occupied molecular orbitals (SOMO) and by their energies. Apart from frontier molecular orbital theory, charge reaction control, which is expected to occur at higher oxidation potentials, is also discussed. The calculations indicate that the unpaired electron spin distribution and atomic charge variation in the radical cation is very sensitive to the nature of the alkyl side chain. In fact, this distribution reflects the electronic structure of these monomers related to their oxidability. It is shown that the removal of an electron is carried out by two competing mechanisms. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Electronic, Structural, and Hyperfine Interaction Investigations on Rydberg Molecules: NH4, OH3, and FH2

The geometrical structures of NH4 and OH3 were optimized at the MP2 level with the aug-cc-pvdz, aug-cc-pvtz, and aug-cc-pvqz basis set plus s-type diffuse functions. The adiabatic ionization potential, barrier height, and dissociation energy of NH4 and OH3 were calculated with the above basis set series and were extrapolated to the infinite basis set limit. The theoretical ionization potential of NH4 was in very good agreement with the experimental value. The N−H bond lengths of NH4 and NH4+ at the infinite basis set limit were obtained by parabolic interpolation around the equilibrium point. FH2 was optimized at the UHF, B3LYP, and MP2 levels. However, only a dissociated ground state was found. The potential energy curves for dissociation of the above molecules were calculated with MP2. The relative stabilities of these molecules and their isotopic counterparts are discussed. Theoretical hyperfine calculations were performed in the gas phase as well as in a neon matrix for NH4 and OH3 with a single-refer...

Theoretical Study of Ligand Superhyperfine Structure. Application to Cu(II) Complexes

A theoretical and computational study of the nitrogen superhyperfine structure in Cu(II) complexes is reported. The determination of hybridization parameters for nitrogen donor orbitals from the data is examined. For most Cu(II) complexes the results deviate substantially from pure “sp2” or “sp3” hybridization. Semiempirical INDO/S calculations for five Cu(II) complexes were carried out at the UHF and ROHF level. The results suggest that the small anisotropy in the nitrogen hyperfine parameters is caused by spin polarization of the nitrogen valence shell orbitals. A simple, approximate way for the determination of the π-spin density from experimental data is outlined. A density functional study using various basis sets and functionals is reported for the same five complexes. Hybrid functionals, such as B3LYP and PWP1, give better predictions than functionals based on the generalized gradient approximation like BP or BLYP. Provided that at least a polarized triple-ζ basis is used, the hybrid functionals B3...

Theoretical Study of the Electronic Spectrum and ESR of the CH2OH Radical

The structure of the CH2OH radical was optimized at the UHF, UB3LYP, and MP2 levels. The calculated ionization potential was very close to the experimental one. The transition dipole moments from the ground state to the first four lowest excited states were calculated with and without Cs symmetry using the multireference single and double configuration interaction method (MRSD). The first four lowest excited states were also optimized with the single excitation configuration interaction method (CIS). The adiabatic excitation energies and transition dipole moments for vertical emission were calculated at the same level. None of these calculations lead to the assignment of the first excited Rydberg p state as 2A‘ ‘(3pz) as suggested in a recent paper (Chem. Phys. Lett. 318, 393, 2000). Some results of multireference perturbation theory are also presented. The isotropic hyperfine parameters calculated with MRSD were in good agreement with the experimental values.

An ab initio approach to the structure and EPR parameters of formaldiminoxy radical

The structures and hyperfine coupling constants of formaldiminoxyl are investigated using ab initio methods. Geometries have been obtained at the UHF, UMP2 and UQCISD levels in combination with different basis sets of 6-31G and 6-311G type. The effect of the CNO bond angle on the hyperfine parameters of the radical is discussed. The isotropic hyperfine constants, calculated using UQCISD and UQCISD(T) methods with a variety of basis sets, including D95(d,p), IGLO-III, 6-311 + G(d,p), 6-311 + G(2df,p), DZP + D of Chipman, [742|52] of van Duijneveldt, are generally in a good agreement with the experimental data. The analysis of direct, spin polarization and correlation contributions to the hyperfine constants is made using ROHF, UHF, UCCD, UQCISD, UQCISD(T), UQCISD(TQ) models combined with [421|21] and [742|52] basis sets.

Electrostatically Driven Geometry Changes Accompanying Charge Separation in Supposedly Rigid Bichromophoric Systems

Calculations on the structures of a variety of charge-separated (CS) bichromophoric systems in the gas phase were carried out at the UHF, CIS, and UB3LYP theoretical levels, using the 3-21G, 6-31G(d), and 6-31+G(d) basis sets. The results indicate that the relatively computationally inexpensive UHF/3-21G method gives a reliable description of the molecular structure and charge distribution in these states. It is predicted that the CS states of most of the bichromophoric systems studied will undergo strong structural distortions due to the Coulombic forces operating between the two charged chromophores. These structural distortions are largely associated with changes in the geometries of the charged chromophores, although the putative “rigid” saturated hydrocarbon bridges connecting the chromophores can also undergo some distortion. In the case of 8b, the distorted CS geometry predicted by these calculations is consistent with results obtained from recent experimental charge recombination studies on the CS state of the similar system, 8a. Geometry changes accompanying charge separation or charge recombination processes, on the scale predicted here, might influence the interpretation of rate data based on semiclassical ET theories.

Transannular Effects in Dicobalta−Superphane Complexes on the Mixed-Valence Class II/Class III Interface: Distinguishing between Spin and Charge Delocalization by Electrochemistry, Spectroscopy, and ab Initio Calculations

The oxidative electron-transfer properties of several superphane complexes consisting of cyclopentadienylcobalt cyclobutadiene moieties linked by either three (compounds 1 and 4) or five (compounds 2 and 5) bridging methylene groups have been studied by experimental and theoretical methods. In both cases two separate one-electron oxidations are found. The mixed-valent monocations of the (−CH2−)5-bridged complexes are valence-trapped with very weak interactions between metal centers. The (−CH2−)3 complexes, however, have strong interactions between the two molecular halves. The intervalence transfer (IT) band of 1+ has characteristics of both class II (localized) and class III (delocalized) behavior, but the IR spectra of carboxy-labeled 4+ clearly establish trapped valence for the monocations of the propano-bridged systems. Photoelectron spectra and ab initio calculations at the UHF level show that, in the ground electronic state, 1+ has a half-filled orbital (i.e., electron spin) that is essentially localized in one Co dxy, orbital, but that the charges on the two metals are unequal owing to inductive electronic effects which give unequal electron flow from the ligands to the two metal centers. Calculations and IR (carbonyl) spectral shifts suggest about a 70:30 charge ratio between the two metal centers in 1+ and 4+, whereas both spin and charge localization is virtually complete in the pentano-bridged complexes 2+ and 5+. The intervalence transition in 1+ is proposed to proceed through a “hole”-transfer process mediated by a π-cyclobutadiene MO, ultimately involving a through-bond transannular mechanism.

Theoretical Studies on the Reaction Path Dynamics and Variational Transition-State Theory Rate Constants of the Hydrogen-Abstraction Reactions of the NH(X3Σ-) Radical with Methane and Ethane

The ground-state imidogen radical, NH(X{sup 3}{Sigma}{sup {minus}}), has received a great deal of attention in recent years because it is an important chain carrier in the combustion of energetic materials. The hydrogen-abstraction reactions of the radical NH(X{sup 3}{Sigma}{sup {minus}}) with methane and ethane have been studied by using ab initio molecular orbital theory and the canonical variational transition-state theory. The geometries of the reactants, transition states, and products were optimized at the UHF, UMP2, UMP4(sdq), and UQCISD levels of theory, and the forward and reverse reaction potential barriers were calculated accurately at the UQCISD/6-311+G(3df,2p) and Gaussian 2 levels. The reaction paths were calculated by the intrinsic reaction coordinate theory at the UMP2/6-311G{sup **} level. The changes of the geometries and generalized normal-mode vibrational frequencies along the IRC were discussed. The energy profile along the IRC was further improved by the Gaussian 2 method. The forward and reverse reaction rate constants for the temperature range from 300 to 2000 K were evaluated by the conventional transition-state theory and the canonical variational transition-state theory with a small curvature tunneling correction. The theoretical rate constants of the forward and reverse reactions are all in good agreement with the experimental ones in the measuredmore » temperature range.« less

Ab Initio Study of the Most Stable C4H5 Isomers

Ab initio calculations have been carried out on the lowest energy isomers of C4H5 to predict spectroscopic properties and relative stabilities. Geometry optimizations were carried out on stationary point conformations of seven configurational isomers at UHF, B3LYP, MP2, CISD, QCISD, MCSCF(7,7), and MCSCF(9,9) levels of theory. Single-point energies were evaluated at the MP4, CCSD(T), MCSCF(11,11), and multireference CISD levels. Disparities as large as 70 kJ mol-1 are found between relative energies predicted by single- and multireference methods for the same isomer. Comparison to experimental values suggests that the multireference methods inadequately model the relative correlation energy. Zero-point corrected relative energies (in kJ mol-1) obtained at the QCISD level with a 6-311G(d,p) basis set are the following: 2-butyn-1-yl (0); 1-butyn-3-yl (10); 1,2-butadien-4-yl (13); cyclobuten-3-yl (17); 1,3-butadien-2-yl (50); 1,3-butadien-1-yl (59); and 1-butyn-4-yl (64). Relative energies calculated by multireference methods are higher, and decrease slowly as the active space size increases. Relative energies and hyperfine constants obtained at the QCISD level are in agreement with available experimental data and empirical estimates. Some of these isomers are candidates for relocalization, a phenomenon that results in predicted multiple minima and unusually flat vibrational potential energy surfaces for the isoelectronic C3H3O isomers. Of the present series of molecules, only 2-butyn-1-yl exhibits an especially flat bending potential along the appropriate isomerization coordinate. Predicted vibrational transition frequencies and intensities, dipole moment components, Fermi contact hyperfine constants, and conformational potential energy curves are presented.

Density Functional Theory Investigation of the Electronic Structure and Spin Density Distribution in Peroxyl Radicals

The electronic structure and spin density distribution of peroxyl radicals are investigated by density functional theory (DFT) at the B3LYP level. Results found for superoxide anion and tert-butyl peroxyl radicals at a variety of basis sets suggest that 6-31G is the most appropriate basis set for calculation of hyperfine coupling constants (hfcc's) of carbon-based peroxyl radicals. Calculation of parallel 17O hfcc's [A∥(17O)] for a series of substituted methyl peroxyl radicals with the 6-31G basis set yielded calculated values with a maximum deviation of 2.2% from experiment. Spin density distributions estimated from experiment A∥(17O) are compared to theoretical estimates from Mulliken orbital population analysis. Electronegative substitution at the carbon alpha to the peroxyl group results in an increase of terminal oxygen hyperfine coupling and spin density, shortening of C−O, and lengthening of O−O. In cases involving significant steric hindrance, however, C−O bond shortening was prevented. A∥(17O) values for the terminal peroxyl oxygen atom correlate well with Taft σ* substitutent parameters for the R group in the peroxyl radicals (ROO•). Thiyl peroxyl radicals are reinvestigated using B3LYP for comparison to previous theoretical work at UHF level. This resulted in confirmation that the effect of the addition of an electron pair donor (hydroxide ion) to CH3SOO• is to alter the spin density distribution in the peroxyl group. Structural models of lipid peroxyl radicals show that vinyl peroxyl radicals may be distinguished from saturated, allylic, and ester-based peroxyl radicals on the basis of hyperfine coupling constants.

Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Geometry optimizations at the UHF/6-31G* and UMP2/6-31G* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS + · 1 and TS + · 2 , were obtained which have C 1 and C 2 symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS + · 1 is the true transition state connecting N + · and Q + · , while TS + · 2 is a second order saddle point.

Relocalization in Floppy Free Radicals: Ab Initio Calculations of the C3H3O Isomers

Several isomers with molecular formula C3H3O may exhibit unusual properties by virtue of a configurational isomerization pathway that changes the localization of an unpaired electron. A series of ab initio calculations was undertaken at the UHF, B3LYP, MCSCF, and QCISD levels with the 6-311G(d,p) basis set to ascertain the geometries, relative energies, isomerization barriers, and relevant spectroscopic properties of the lowest energy structural and configurational C3H3O isomers. The most stable structures, with QCISD relative energies (kJ mol-l) of the stable configurations given in parentheses, are H2C2HCO (0, 8, 8), H3C3O (47), H2C3HO (87, 100), and HC2HCHO (94, 95, 101, 107). The H2C2HCO and H2C3HO structures are found to have distinct isomers connected by relocalization pathways. The H3C3O structure possesses two favorable canonical geometries, but the single optimized structure is an average of these. Potential surfaces have been calculated along two vibrational bending coordinates for H2C2HCO and H2C3HO and along a single bending coordinate for H3C3O. Extremely large amplitude bending motions are predicted for these three lowest energy structures, and at typical combustion temperatures it is likely that these molecules exist effectively as admixtures of two or more canonical structures.