Ab Initio UHF Calculations Research Articles

Single-crystal EPR and DFT study of a VIAl–O−–VIAl center in jeremejevite: electronic structure and 27Al hyperfine constants

Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e β e = −0.881(1) mT, A 2/g e β e = −0.951(1) mT, and A 3/g e β e = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.

Liu–Parr–Nagy analysis on density functional theory calculations of spin densities in first-row atoms and diatomic hydrides

The Liu–Parr–Nagy (LPN) analysis was applied to the spin density at a nucleus ρ s(0), for the first row atoms from carbon to fluorine and the diatomic hydrides, CH, NH, and OH. The LPN ρ s(0) values for these radicals, were obtained with ab initio UHF and DFT calculations. In the DFT calculations, several fundamental exchange- and correlation-functionals were used with their combined or non-combined forms. Furthermore, various basis sets were also used. It can be shown that the LPN analysis presents the clear information to understand the differences of the ρ s(0) values among the employed functional forms, and among basis sets.

Modelling of Octahedral Manganese II Complexes with Inorganic Ligands: A Problem with Spin-States

Quantum mechanical ab initio UHF, MP2, MC-SCF and DFT calculations with moderate Gaussian basis sets were performed for MnX6, X = H2O, F-, CN-, manganese octahedral complexes. The correct spin-state of the complexes was obtained only when the counter ions neutralizing the entire complexes were used in the modelling at the B3LYP level of theory.

A new class of ferromagnetically-coupled mixed valence vanadium(IV/V) polyoxometalates.

Reaction of [V(VI)OCl(2)(thf)(2)] with a bidentate nitrogen-donor ligand (L: phen=1,10-phenanthroline, 5-mephen=5-methyl-1,10-phenanthroline, bipy=2,2'-bipyridine, 5,5'-me(2)bipy=5,5'-dimethyl-2,2'-bipy) in methyl alcohol, in the presence of triethylamine, leads to the formation of hexameric [V(2) (IV)V(4) (V)] oxo-alkoxo-vanadates of the general formula [V(6)O(12)(mu(2)-OCH(3))(4)(L)(4)].x H(2)O [L=phen (1.4 H(2)O), 5-mephen (2.6 H(2)O), bipy (3.4 H(2)O), 5,5'-me(2)bipy (4.H(2)O)]. X-ray structure analysis of 1.2 H(2)O and 4.8 CH(3)OH revealed a pair of V(3)O(13)N(4) trimeric units sharing two corners, with a centrosymmetric planar V(6)-core. In addition, a fully oxidized V(V) species [V(V) (4)O(8)(OCH(3))(2)(mu(3)-OCH(3))(2)(5,5'-me(2)bipy)(2)].3 CH(3)OH (5.3 CH(3)OH) was isolated from the reaction mixture used for the synthesis of 4.H(2)O. The crystal structure of 5.3 CH(3)OH revealed a dicubane-like framework with two missing vertices. Electron paramagnetic resonance (EPR) and variable temperature magnetic susceptibility studies for the hexamers 1.4 H(2)O and 3.4 H(2)O showed the complete localization of the single 3d electrons on the V(IV) ions and unusual ferromagnetic interaction between the two paramagnetic vanadium(IV) ions separated by a distance of about 5.1 A. Furthermore, intermolecular antiferromagnetic interactions through pi-contacts of phenyl rings were observed for these species below 8 K. The ferromagnetic exchange coupling observed in the hexanuclear compounds 1.4 H(2)O and 3.4 H(2)O is also discussed using ab initio UHF calculations on a model compound. The value of the exchange coupling constant (3.7 cm(-1)) for this model compound, calculated using the broken symmetry approach, is in good agreement, both in sign and magnitude, with the experimental J values (6.00 cm(-1) for 1.4 H(2)O and 8.54 cm(-1) for 3.4 H(2)O).

The electronic structure and stabilities of the symmetric gas phase anion clusters of hydrogen fluoride

Ab initio UHF, MP2 and MP4 calculations were used to study the geometries, the electronic structure and the thermochemical stabilities of the gas phase anionic clusters of hydrogen fluoride, (HF) n −, for n=2–6. Basis sets up to the aug-cc-pVDZ set were employed. These were augmented with a 6-term even tempered (sp) cavity set. The thermochemical stabilities of these complexes against vertical ionization as well as dissociation to various products were determined. These clusters involve a single shell of two to six HF molecules complexed in a symmetrical head-to-head orientation. They show higher thermochemical stabilities than similar clusters of water and ammonia and have stabilities similar to those of the neutral hydrogen bonded clusters of hydrogen fluoride. As with water, the smallest stable anionic cluster of hydrogen fluoride has two monomers which is considerably smaller than those observed for ammonia. The integrated electron densities as well as the natural bond orbital analysis reveals that a significant portion of the excess electron density is localized in the cavity region interior to the hydrogen atoms of the inner shell. The stability of the clusters increased with the number of monomers. Cavity orbitals had a nominal effect on the properties of the complexes and effected the prediction of the geometrical features of the complexes more than their stability.

4-Ethoxycarbonyl-4′,5,5′-trimethyltetrathiafulvalene and its radical cation: Langmuir–Blodgett film studies, EPR spectra and the X-ray crystal structure of (Me3TTF-CO2Et)2·TCNQ complex

Langmuir–Blodgett films of the title trimethyltetrathiafulvalene derivative 10 have been assembled without the need for added fatty acid: room temperature, in-plane, dc conductivity values of the films before and after doping with iodine vapour were σrt = 10–5 S cm–1 and 10–1 S cm–1, respectively. Over the temperature range 300–100 K the films exhibited a linear current versus voltage relationship, with an activation energy for conductivity Ea = 0.11 eV. Cyclic voltammetric data were obtained for LB films of 10 and the best response was recorded for a 2-layer film; electroactivity gradually disappeared after a few cycles, which is consistent with film desorption from the electrode. UV–vis and IR spectra of the LB films are consistent with a conjugative intramolecular charge-transfer interaction between the triMe-TTF (donor) and ester (acceptor) moieties of 10 (see structure 10′). This increases the amphiphilic nature of 10 and is believed to play an important role in stabilising the LB film structure. Iodine doping of the LB films leads to the formation of radical cations 10·+, and polarised spectra show that the molecules are aligned at a high angle to the substrate surface. Simultaneous electrochemistry and EPR (SEEPR) studies provide insight into the electronic structure of the radical cation species 10·+ in solution; the ethoxycarbonyl group strongly polarises the spin density, and these data are supported by ab initio UHF and B3LYP calculations. The single crystal X-ray structure of the complex (10)2·TCNQ (TCNQ = tetracyano-p-quinodimethane) reveals a mixed ⋯ADDADD⋯ stacking motif with a low degree of charge transfer between the D and A moieties. Analysis of the bond lengths in the donor moiety confirms a contribution from the zwitterionic structure 10′.

Theoretical Studies on Magnetic Interactions in Prussian Blue Analogs and Active Controls of Spin States by External Fields

The symmetry rules for the d-orbital interactions are considered for simple prediction of the sign of effective exchange integrals between transition metal ions in Prussian blue analogs. Ab initio UHF and DFT calculations were carried out for binuclear transition-metal cyanides in order to elucidate variations of the through-bond exchange integrals with combinations of transition metal ions having different d-electron configurations. It was shown that the sign of effective exchange integrals predicted from the symmetry rules and the ab initio calculations are consistent with the experiments but their magnitudes calculated for tetracentric systems MCNM' (M=Cr(III), M'=V(II), Mn(II), etc.) by the use of triple zeta basis sets are larger than the observed values for Prussian blue analogs. The calculated results clearly indicated that the direct exchange interactions between M and M' are negligible, whereas the through-bond superexchange and spin polarization interactions are crucial for the high-Tc ferri (or ferro) magnetism in Prussian blue analogs. The modifications of electronic states in antiferromagnetic or paramagnetic precursors are also discussed in relation to our previous theoretical proposals for molecular spinics and recent experiments to obtain the switching type molecular magnets controlled by photochemical, electrochemical and other techniques.

Theoretical Studies of Direct Exchange Couplings Between Transition Metal Ions II. Tetranuclear Transition Metal Systems

Abstract The model clusters for the tetranucler transition metal complexes synthesized by Mashima et al. were investigated theoretically. The ab-initio UHF calculations were performed for the Ni-Cr(?)-Cr(II)-Ni and Pd-Mo(?)-Mo(?)-Pd systems to study the nature of the intermetallic bonds. The bond orders and the effective exchange integrals were calculated changing the intermetallic bond distances. It is shown that the tetranuclear dσ-bonds are formed in the cases: Ni(I) and Pd(I).

Theoretical Calculation of Effective Exchange Integrals for One-and Two-Dimensional Poly(Phenylenemethylene) Systems. Possibilities of Organic Ferro-and Ferri-Magnetic Solids

Abstract Ab initio post UHF, DFT, CASSCF and semiempirical INDO calculations were carried out for one-and two-dimensional poly(phenylenmethylene) in order to elucidate variations of the effective exchange integrals with molecular connectivity, cluster size and hole (electron) doping. It was shown that the ferromagnetic effective exchange integrals by UCCSD(T) and DFT/4–31G are about 400 cm−1 for m-phenylene bis(methylene). The nigh-spin ground states of hole-doped polycarbenes were also studied, indicating a possibility of obtaining the high Tc organic ferrimagnets induced by the one electron transfer between polycarbene and spacer (electron donor or acceptor).

An ab initio molecular orbital study of methyl-substituted ethylene radical cations. Substitution pattern and rotational barrier

The radical cations of ethylene ( 1) and of all its methyl derivatives were investigated by ab initio UHF calculations with the 3–21G basis set. The planar (or almost planar) form was calculated to be the most stable for all the radical cations except the tetramethylethylene radical cation, the optimized structure of which is twisted about the olefinic CC bond by 11°. The 90°-twisted geometries were also optimized; even for the symmetrically substituted ethylene radical cations, the 90°-twisted structure, of a lower symmetry in which the positive charge and the unpaired electron are separated on different olefinic carbon atoms, is energetically favorable because of the conical intersection between the ground doublet (D 0) and the first excited doublet (D 1) states at the symmetrical 90°-twisted form. It was found that the rotational barriers of the symmetrical derivatives are similar to that of 1 .+, while those of the unsymmetrical derivatives are lower than 1 .+. This relationship between the substitution pattern and the rotational barrier found for the methyl-substituted ethylene radical cations is expected also to hold for other olefin radical cations.

Heisenberg model for radical reactions. Part 3. Direct exchange coupling between transition metal ions and triplet methylene

Ab initio UHF calculations have been carried out for naked transition metal-methylene ions. The low-spin open-shell states are found to be the ground states of these ions, in accord with the GVB-CI results. The low-spin transition metal-methylene ions are regarded as direct exchange-coupled systems formed from high-spin transition metal ions and the triplet monocentric diradical. The direct exchange couplings between these species can be described on the basis of the Heisenberg model. The CASSCF calculations using UHF natural orbitals (UNOs) have been carried out for low-spin states, showing that UNOs and their occupation numbers are good trials. However, they provide poor binding energies for the transition metal-carbon double bonds and indicate the necessity for dynamical correlation corrections. The approximate spin-projected UHF Møller-Plesset and UHF coupled-cluster methods provide reasonable binding energies for the transition metal-methylene ions, which are close to those of modified coupled-pair functional (CPF) and average CPF methods.

Heisenberg model for radical reactions. Part 3. Direct exchange coupling between transition metal ions and triplet methylene

Ab initio UHF calculations have been carried out for naked transition metal—methylene ions. The low-spin open-shell states are found to be the ground states of these ions, in accord with the GVB-CI results. The low-spin transition metal—methylene ions are regarded as direct exchange-coupled systems formed from high-spin transition metal ions and the triplet monocentric diradical. The direct exchange couplings between these species can be described on the basis of the Heisenberg model. The CASSCF calculations using UHF natural orbitals (UNOs) have been carried out for low-spin states, showing that UNOs and their occupation numbers are good trials. However, they provide poor binding energies for the transition metal—carbon double bonds and indicate the necessity for dynamical correlation corrections. The approximate spin-projected UHF Møller—Plesset and UHF coupled-cluster methods provide reasonable binding energies for the transition metal methylene ions, which are close to those of modified coupled-pair functional (CPF) and average CPF methods.

A theoretical study of the H+SO 2 reaction

Ab initio UHF and MP2 calculations have been used to characterize the relative stabilities, equilibrium geometries, and harmonic frequencies of the intermediates involved in the H+SO 2 reaction. Calculations for the entrance channel potentials at the MP2/DZP level of theory indicate pure repulsion for planar attack ( 2A′ surface) by the H atom on the S atom of SO 2 and an 11 kcal/mol barrier to the nonplanar ( 2A″ surface) attack. Much larger barriers are predicted for the formation of the HOSO adducts on this surface. Implications to experiments involving the reaction of SO 2 with translationally hot H atoms are discussed.

Theoretical investigation of the ethene-ethene radical cation addition reaction

The addition reaction of an ethene radical cation and a neutral ethene molecule, as previously observed in low-temperature matrix isolation ESR measurements, has been studied using a number of computational methods, ranging from the semiempirical AM1 and PM3 methods to ab initio UHF, MP2, and MP4 calculations using different basis sets, with and without spin projection included. An intermediate addition complex is observed as a local minimum on the potential energy surface of the reaction

Long-range spin density propagation in saturated hydrocarbons: 3-[n]staffyl radicals

The bridgehead radicals derived from the first three [n]staffanes (n=1-3), oligomers of [1.1.1]propellane, have been generated from the corresponding bromides, and their solution EPR spectra have been recorded. Remarkably long-range hyperfine coupling has been found to e, ζ, and even ι hydrogens, in qualitative agreement with ab initio UHF calculations. The coupling to the bridgehead hydrogen is attenuated by a factor of about 25 per added bicyclo[1.1.1]pentane cage

Ab initio study of 1,4-dithiin and its cation radical and dication

The structures of 1,4-dithiin in the neutral, radical cation, and dication states have been investigated by ab initio RHF and UHF calculations with the MIDI-4 basis set. The MIDI-4 calculations predict a folded form for the neutral state, and planar forms for the cation and dication states. These structural changes caused by one- or two-electron oxidation processes are influenced by two electronic factors: one is the weak π conjugation between the sulfur and carbon atoms and the other is the σ interaction between two sulfur atoms.

Bond alternation and unpaired spin distributions in the radical anions of cyclooctatetraene and monosubstituted derivatives. An ab initio study

Ab initio UHF, ROHF, and CI calculations have been performed on the radical anion of 1,3,5,7-cyclooctatetraene (COT •− ) with the 3-21G and 6-31G* basis sets. At the UHF level, a D 4h structure with alternating bond angles was spuriously found to be lower in energy than a D 4h structure with alternating bond lengths. Calculations on methyl-, fluoro-, and cyano-COT •− have also been performed, and the computed hyperfine coupling constants for these derivatives of COT •− are reported. These three substituents are calculated to have little effect on the degree of bond alternation in COT •− , but fluoro and cyano are each found to reduce the barrier to bond equalization by more than 50%

An Ab Initio Molecular Orbital Study on Twisting in Silyl-Substituted Ethylene Cation Radicals

Abstract Twisting in two silyl-substituted ethylene cation radicals, the cation radicals of vinylsilane (1) and vinylidenebissilane (2), were investigated by ab initio UHF calculations with the 3-21G and 3-21G(*) basis sets. Both 1+· and 2+· were found to have twisted equilibrium structures, the twist angle being 29° and 42°, respectively. The planarization barriers evaluated for 1+· and 2+· are about 1 and 7 kcal mol−1, respectively. The planar and 90°-twisted forms of 2+· have comparable energies. The hyperconjugation of the C-Si bonds is reflected in the calculated molecular structures.

Ab initio study of the SiH 5 species

Ab initio ROHF, UHF and UMP4 calculations are reported for the SiH, species. None of the SiH 5 structures considered appeared to be stable with respect to SiH 4+H or SiH 3+H 2. The frequency analysis made at the ROHF/6–31G∗//ROHF/6–31G∗ level revealed that the D 3h Structure of SiH 5 is the transition state for the substitution reaction H .+ SiH 4 → SiH 3H .+H (assuming the inversion model) and that its higher energy C 4v form is the third-order saddle point. Our predictions concerning the D 3h structure of SiH 5 are in accord with very recent theoretical results based on both the valence bond curve-crossing diagram model and other ab initio calculations.

Structure of the azetidine radical cation and the neutral azetidin-1-yl radical. An ab initio and electron paramagnetic resonance study

The electronic structures of the azetidine radical cation and the neutral azetidin-1-yl radical have been investigated by EPR spectroscopy and theoretical ab initio UHF, MP2 and SDCl calculations. The radical cation was assigned a structure with a planar ring according to the theoretical and experimental results. Ab initio UHF calculations predicted the neutral azetidin-1-yl radical to have a puckered ring structure, with puckering angles 12.0 and 15.4° on the STO-3G and 6-31G* levels, respectively. This result was supported by the observed nonequivalent values of the experimental β-proton hyperfine coupling constants, suggesting that the neutral radical has a puckered structure at low temperature in a solid halocarbon matrix. Calculated SDCl values for the isotropic and anisotropic hyperfine coupling constants of the azetidine cation and the azetidin-1-yl radical were found to be in qualitative agreement with the experimental data.