Ab Initio UHF Research Articles

XRD, FT-IR, FT-Raman spectra and ab initio HF vibrational analysis of bis (5-amino-3-carboxy-1H-1,2,4-triazol-4-ium) selenate dihydrate

The selenic hydrogen bonded salt Bis(5-amino-3-carboxy-1H-1,2,4-triazol-4-ium) selenate dihydrate (BACTSe) has been synthesized and characterized by XRD, FT-IR and FT-Raman techniques. It crystallizes in the C2/c space group and displays a three-dimensional framework stabilized mainly via intermolecular OH⋯O, NH ⋯O and OH⋯N hydrogen bonds. The quantum chemical study performed with ab initio UHF method using 6-311G(d,p) base set, has permitted the determination of the structural parameters, the total energy, the HOMO and LUMO energies, the thermodynamic parameters and the NBO charges. Moreover, normal mode frequencies and corresponding vibrational assignments have been examined.

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.

Effect of charge on the interaction of two C 60 molecules from MNDO and ab initio UHF methods: stability and HOMO–LUMO gaps

We have investigated the interaction between two C 60 molecules with overall charge on the system n between +1 and −4 from infinite separation to a distance of 8.5 Å, using the MNDO and ab initio unrestricted Hartree–Fock methods. We have determined the dependence of the interaction energy, charge distribution, |HOMO–LUMO| gap and charge transfer, on distance. The system is found to be most stable at an interaction distance of ∼14 Å, for system charge between −1 and −4 a.u. Charge is shared equally between the two buckyballs for even total charge n and distributed unevenly between the molecules for odd n. The smallest values of |HOMO–LUMO| are found, in general, for the systems with net negative charge and an odd number of electrons.

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).

Theoretical study of the H-abstraction reaction of the CH 3O radical with formaldehyde

The purpose of the present Letter is to investigate the mechanism of the bimolecular abstraction reaction of the methoxy radical (CH 3O) with formaldehyde and to compute gas-phase thermal rate constants using conventional transition state theory including tunneling corrections. The potential information needed to calculate the rate constants is obtained from ab initio electronic structure calculations. We report the ab initio UHF, MP2, MP4, CASSCF, CASPT2 and MRCI results obtained by using ANO basis sets as well as HF-DFT (B3LYP) results obtained by using the 6-311++G ** basis set. The theoretical rate constants obtained from the MRSDCI potential are in good agreement with the experimental values.

Ab initiostudy ofMF2(M=Mn,Fe,Co,Ni)rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the groundstate structural, electronic, and magnetic properties of the rutile-type compounds MF2 ( M5Mn, Fe, Co, and Ni!. All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the @001#, @111#, and @100 #~ or @010#! directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed. Several transition-metal ~TM! oxides and fluorides crystallize in the rutile-type structure, which is the simplest and most common MX 2 structure, where each M atom is octahedrally coordinated by the X ligands. Slight distortions of the MF6 structural units are present due to electrostatic and/or Jahn-Teller effects. The physical properties of the TM compounds with rutile-type structure vary considerably: from ionic or covalent insulators to metals, from diamagnetic to strong ferromagnetic or antiferromagnetic semimetallic or insulating systems. This feature is a challenge to modern theoretical methods. In this context a series of compounds whose physical properties exhibits a smooth variation from one member to the other provide a suitable test of theory. This is precisely the case of the MF2 ( M5Mn, Fe, Co, and Ni! family of ionic compounds in which the interplay between structural parameters, chemical bonding, and magnetic coupling may be expected to display such a smooth variation. Two different ab initio periodic approaches are commonly used in solid-state physics to study TM compounds: the Hartree-Fock, 1,2 and the density-functional ~in its local or gradient corrected variants 3 ! schemes. In the latter both the exchange and correlation parts of the electron-electron interaction are taken into account in an approximate way. In the former the electron exchange part is treated exactly while electron correlation is neglected. When the spin-unrestricted form of the Hartree-Fock approach ~UHF! is used in the study of insulating magnetic systems, the correct sign and a reasonably good description of the magnitude of the magnetic coupling constants usually results. The ab initio UHF

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.

Theoretical Study on Magnetic Interactions of Mn-π Conjugated System

Magnetic interaction of Mn-π conjugated system, especially Mn(II)2-py-rimidine was investigated by the ab initio UHF, post-UHF, and DFT approximations. The calculated effective exchange integrals (Jab ) are negative, even though the m-phenylene type bridge is employed. This indicates that the superexchange effect plays a predominant role to determine the sign of Jab in Mn-π conjugated systems.

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′.

Density functional and ab initio study of the ketene and methylketene radical cations

The present study for the ketene and methylketene radical cations involves geometry, conformation, and hyperfine structure. The geometry and conformation (for the methylketene cation) investigations were based on the B3LYP/6-311G(d,p) and MP2/6-311G(d,p) calculations, and the isotropic proton hyperfine coupling constants (a(H)) were calculated using various density functional theory (DFT) methods and ab initio UHF, UMP2, and multireference single and double excitation configuration interaction (MRSDCI) methods with large basis sets. The ketene cation is predicted to have a C2v geometry. The barriers to methyl group rotation in the methylketene cation is predicted to be very low (<1 kcal/mol) and the cis (staggered) conformation is the most stable. On the rotation, the sum of the B3LYP/6-311G(d,p) a(H) values on the three β protons in the methylketene cation is almost a constant approximately equal to three times the experimental β coupling value. All the three ab initio methods predict too small a(H) values for the β protons. The B3LYP (also B3P86 and B3PW91) calculations predict a(H) values for the α and β protons in the two radicals in excellent agreement with experiment and the DFT methods do not underestimate the β couplings. The UMP2(FULL)/6-311G(d,p) hyperfine structure calculations predict a(H) values for the α protons in excellent agreement with experiment and the UMP2 hfcc results are overall better than the MRSDCI results. As an ab initio method for hyperfine structure calculations, the UMP2 method is suggested.

Monte Carlo and molecular orbital study of solvent effect on the electronic structure and hyperfine coupling constants of the (CH 3) 2NO radical: the effect of electron transfer between the solute and solvent molecules

Ab initio UHF MIDI-4 calculations have been carried out for the dimethyl nitric oxide (DMNO) radical in the H 2O and CHCl 3 solutions. The configurations of the solvent molecules were selected from the snapshots in the Monte Carlo (MC) simulation for the solution, and the electronic structure and the hyperfine coupling constant ( hfcc) of DMNO were obtained by averaging the results of 300 configurations. The point charge approximation in which all solvent molecules were represented by point charges indicated that the electrostatic interaction polarizes the N–O bond and increases the spin density on the N atom. When some of the solvent molecules near DMNO were treated as a supermolecule together with DMNO and the remaining solvent molecules were approximated by point charges, the electron transfer occurs between DMNO and H 2O molecules in two directions, from DMNO to H 2O around the N–O group and from H 2O to DMNO around the CH 3 groups. These electron transfers polarize the N–O bond in the same direction as the electrostatic interaction and increase the hfcc of the N atom. The contribution of the electron transfer accounts for 40% of the solvent effect on hfcc of DMNO.

Theoretical Studies of Multiple Metal–Metal Bonds between Divalent Molybdenum Ions in Dimers, Tetramers, and Clusters

Abstract The multiple metal–metal bonds between divalent molybdenum (Mo(II)) ions in dimers and tetramers were investigated theoretically. The orbital energy gaps, occupation numbers of natural orbitals (NO) and effective bond orders for the naked Mo(II)2 dimer (1) were calculated by ab initio UHF MO and density functional (DFT) methods. The effective exchange integrals (Jab) of 1 were calculated by approximately spin-projected UHF and DFT methods, and the complete active space (CAS) CI by the use of UHF NO. The calculated Jab values were discussed in relation to the temperature-dependent paramagnetism observed for the edge-sharing and face-sharing bioctahedral molybdenum complexes. The CAS CI calculations showed that the Heisenberg model is not appropriate for 1 under the condition that the Mo–Mo distance is shorter than 2.5 Å. The UHF and DFT MO calculations were also performed for the linear naked Mo(II)4 tetramer (2) to elucidate possible electronic structures of the d–d conjugated systems. The continuous changes from closed-shell (diamagnetic) electronic structures to antiferromagnetic states were investigated by elongating the Mo–Mo distances. Similarities and differences between Mo(II)2 (1) and naked Cr(II)2 (3) or between Mo(II)4 (2) and Cr(II)4 (4) were examined and compared, since the nature of direct exchange couplings between divalent chromium ions in 3 and 4 was already studied extensively in relation to the electron correlation effect. The metal-insulator transitions induced by elongation of the Mo(II)–Mo(II) distance in Mo(II) clusters were discussed from the viewpoint of electron correlation and size effects in mesoscopic systems.

Formation of 2-hexene by cationic dimerization of propene: an ab initio and density functional theory study

The formation of a 2-hexene radical cation from a propene radical cation and a neutral propene molecule is investigated by means of ab initio UHF and spin projected MP2 calculations, as well as the SVWN and B3LYP levels of density functional theory. A stable addition complex, with loose CC bonds, is found. To proceed from the addition complex to the product, a locally planar transition state must be passed, with a migrating hydrogen located half-way between the donating and the accepting carbon atoms. At the highest computational levels considered, PMP2/6-31G(d,p)//MP2/3-21G and B3LYP/6-31G(d,p), this transition state lies approximately 11 and 13 kcal/mol, respectively, above the addition complex. The high barrier is believed to be one reason why radical cation oligomerization of propene has not been detected experimentally, in contrast to the case of ethene, where the corresponding barrier is only a few kcal/mol.

Casci and Casscf Studies of Dinuclear Transition Metal Systems with Quadruple Metal-Metal Bonds (M=Cr(Ii), Mo(Ii))

The complete active space (CAS) configuration interaction (CI) and CASSCF calculations for naked Cr(II)-Cr(II) (1) and Mo(II)-Mo(II) (2) dimers were carried out in order to examine the reliability of the previous MO-theoretical results obtained by the ab initio UHF and DFT methods. From the CASCI and CASSCF calculations, it was concluded that the formal bond orders (one τ, two π, and one Δ bonds) are reduced to the effective ones due to the strong correlation and also that the valence-bond (VB) like model such as the Heisenberg model works well in 1. The effective exchange integrals (Jab) calculated for the system were compatible with the experimental results available. On the other hand, the σ and π bonds in 2 were strong, showing that the MO picture is reliable. The Δ bond of 2 was calculated to be rather weak, although it is stronger than that of 1. The CASCI and CASSCF results support the previous UHF and DFT results for 1 and 2.

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 Study and Comparison with Experiments for Atacamite, Cu2Cl(Oh)3

The dependency of the magnetic susceptibility on temperature has elucidated that the mineral atacamite Cu2Cl(OH)3 has the antiferromagnetic property. In order to explain this behavior and the spin arrangement derived from experimental studies, the ab initio UHF MO and UNO CASCI calculations were carried out for the tetranuclear cluster of copper(II), Cu4Cl(OH)3, which is the constitution in the atacamite crystal structure. The results of the CASCI calculations appreciate the relative stabilities among various spin arrangements for the tetranuclear cluster, being consistent with the experiments.

Theoretical Approaches to Direct Exchange Couplings between Divalent Chromium Ions in Naked Dimers, Tetramers, and Clusters

Direct exchange interactions between divalent chromium ions in naked dimers, tetramers, and clusters were investigated theoretically. The orbital energy gaps, occupation numbers, and bond orders for the naked Cr(II)−Cr(II) dimer (1) were calculated by ab initio UHF MO and density functional (DFT) methods. All these calculations indicated that the orbital energy gaps are very small for 1, and therefore the occupation numbers of the bonding σ, π, and δ molecular orbitals and formal quadruple bond orders were largely reduced because of strong electron correlation effects. The UHF and DFT MO calculations were also performed for the linear naked Cr(II) tetramer to elucidate possible electronic structures of the d−d conjugated systems. The energy differences among singlet, triplet, and quintet states for 1 were calculated from the total energies obtained by the CASCI and CASSCF methods based on the UHF and DFT natural orbitals (UNO). The reliability of the Heisenberg model Hamiltonian was elucidated from variations of these calculated effective exchange integrals with the interatomic distance. The calculated Jab values are compared with the experimental results for several binuclear Cr(II) complexes with different interatomic distances. The Heisenberg spin Hamiltonian was used to discuss the spin alignments in large d−d conjugated Cr(II) clusters and a possibility of the macroscopic quantum spin tunneling in mesoscopic magnetic systems.

Spin-orbit effect on the magnetic shielding constant using the ab initio UHF method: tin tetrahalides

The 119Sn NMR chemical shifts of SnX 4(X=H, Cl, Br and I) and SnBr 4− n I n ( n = 1, 2, 3) are calculated by the ab initio UHF method including the spin-orbit (SO) interaction combined with the finite perturbation method. The calculated Sn chemical shifts are in good agreement with experiment when the SO interaction is included. As the halogen ligand becomes heavier, the SO effect increases and the chemical shift moves to a higher field. The normal halogen dependence by the substitution from Cl to I in tin tetrahalides is reproduced only when the SO interaction is included. The origin of the SO effect is ascribed to the Fermi contact term.