Accurate ab initio binding energies of alkaline earth metal clusters

The effects of basis set superposition error (BSSE) and core-correlation on the electronic binding energies of alkaline earth metal clusters Y(n) (Y = Be, Mg, Ca; n = 2-4) at the Moller-Plesset second-order perturbation theory (MP2) and the single and double coupled cluster method with perturbative triples correction (CCSD(T)) levels are examined using the correlation consistent basis sets cc-pVXZ and cc-pCVXZ (X = D, T, Q, 5). It is found that, while BSSE has a negligible effect for valence-electron-only-correlated calculations for most basis sets, its magnitude becomes more pronounced for all-electron-correlated calculations, including core electrons. By utilizing the negligible effect of BSSE on the binding energies for valence-electron-only-correlated calculations, in combination with the negligible core-correlation effect at the CCSD(T) level, accurate binding energies of these clusters up to pentamers (octamers in the case of the Be clusters) are estimated via the basis set extrapolation of ab initio CCSD(T) correlation energies of the monomer and cluster with only the cc-pVDZ and cc-pVTZ sets, using the basis set and correlation-dependent extrapolation formula recently devised. A comparison between the CCSD(T) and density functional theory (DFT) binding energies is made to identify the most appropriate DFT method for the study of these clusters.     

Environmental effects on the structure of metal ion-DOTA complexes: an ab initio study of radiopharmaceutical metals

Quantum chemical calculations were performed to study the differences between the important radiopharmaceutical metals yttrium (Y) and indium (In) bound by DOTA and modified DOTA molecules. Energies were calculated at the MP2/6-31+G(d)//HF/6-31G(d) levels, using effective core potentials on the Y and In ions. Although the minimum energy structures obtained are similar for both metal ion-DOTA complexes, changes in coordination and local environment significantly affect the geometries and energies of these complexes. Coordination by a single water molecule causes a change in the coordination number and a change in the position of the metal ion in In-DOTA, but Y-DOTA is hardly affected by water coordination. When one of the DOTA carboxylates is replaced by an amide, the resulting structures show a large variation between the Y and In ions. A six-residue model of the active site containing metal ion-DOTA showed that the Y-DOTA structure optimized to a structure similar to the crystal structure but that the water molecule in In-DOTA disrupts the salt bridge between Arg98B and a carboxylate side chain of DOTA. These observed differences could in part explain the differential binding constants for Y-DOTA and In-DOTA to the antibody 2D12.5.     

Towards accurate ab initio calculations on the vibrational modes of the alkaline earth metal hydrides

The fundamental modes of the alkaline earth metal hydrides (BeH2, MgH2, CaH2) and their dimers, HX(H)2XH, have been studied by vibrational configuration-interaction calculations based on very accurate potential energy surfaces. Comparison with experimental data obtained from matrix isolation and gas phase measurements is provided and the agreement was found to be excellent for the monomers but poor for the dimers. In addition, many fundamental bands are predicted which have not yet been detected experimentally.     

Investigation of exchange reactions between alkaline earth metal oxides and some transition metal halides

DTA and the simultaneous recording of electrical conductivity were applied for the investigation of exchange reactions between alkaline earth metal oxides and some lead, copper and nickel halides. Although the possibility of the gaseous phase is not excluded for the majority of the reactions investigated, it is either the appearance of a liquid phase or the polymorphous transformation of CaO that has the decisive effect on the interaction mechanism.     

Substituent effects in halogen bonding complexes between aromatic donors and acceptors: a comprehensive ab initio study

Substituent effects in halogen bonding complexes involving aromatic rings are investigated. We have analyzed how the interaction energy (the RI-MP2/aug-cc-pVDZ level of theory) is affected by the substitution in both halogen bond donor and acceptor aromatic moieties. In addition, we have used two different aromatic electron donor molecules pyridine and cyanobenzene, which allow us to study the effect of having the electron donor nitrogen atom forming part of the ring or outside the ring (-CN). Interestingly, the effect of the substituents on the interaction energies is similar in both cases. We have obtained the Hammett's plots for four combinations of aromatic donors and acceptors and in all cases we have obtained good regression plots (interaction energies vs. Hammett's σ parameter). We have also studied and compared bifurcated halogen bonds using both possible combinations, that is two donors and one acceptor and vice versa. In addition, we have analyzed the effect of the solvent on the interaction energies using COSMO. Finally, we have used Bader's theory of "atoms-in-molecules" to demonstrate that the electron density computed at the bond critical point that emerges upon complexation can be used as a measure of bond order in this noncovalent interaction.     

Fulvalenediyl-bridged heterobimetallic complexes consisting of sandwich and half-sandwich compounds with early-late transition metals

A straightforward synthesis methodology for the preparation of heterobimetallic [(η⁵-C₅H₅)(η⁵-C₅H₄-C₅Me₄)M] (3a, M = Fe; 3b, M = Ru) and [(η⁵-C₅H₅)((μ-η⁵:η⁵-C₅H₄-C₅Me₄)TiCl₃)M] (4a, M = Fe; 4b, M = Ru) in which early and late transition metals are connected by a fulvalenediyl bridge is reported.The structures of molecules 3b and 4a in the solid state are discussed. Most noteworthy in 4a is the exo arrangement of the iron and titanium atoms coordinated by the fulvalenediyl unit which itself is twisted with a dihedral angle between the joined cyclopentadienyl rings of 19.33(9)°. Electrochemical, UV/Vis/NIR spectroscopic and spectroelectrochemical experiments on 4a and Cp∗TiCl₃, for comparison, provide evidence for some transfer of electronic information between the conjoined ferrocene and half-sandwich titanocene trichloride subunits of 4a. Evidence comes from systematic potential shifts and the presence of a fairly intense Fe → Ti charge-transfer absorption band that vanishes upon oxidation and reduction of 4a.     

Heterobimetallic diethanolaminate complexes of titanium and zirconium with alkaline earth metals

Ti(OPr ⁱ )₄ or Zr(OPr ⁱ )₄ · Pr ⁱ OH react with hydrocarbon-insoluble complexes M{(OCH₂CH₂)NH(CH₂CH₂OH)}₂ (M = Mg, Ca, Sr, Ba) in a 2:1 molar ratio to yield hydrocarbon-soluble heterobimetallic diethanolaminate isopropoxide complexes [M{(OCH₂CH₂)₂NH}₂{M(OPr ⁱ )₃}₂] (M = Mg, Ca, Sr, Ba; M = Ti, Zr), which have been characterized by elemental analyses, molecular weight measurements and spectroscopic [i.r., n.m.r. (¹H and ¹³C)] studies.     

Vibrational circular dichroism and ab initio structure elucidation of an aromatic foldamer

Ab initio calculations together with vibrational circular dichroism (VCD) are validated as very accurate tools for studying conformations and estimating conformational energies and helical handedness preferences of an entire, large (112 atoms), abiotic foldamer.     

Asymmetric transition states of allylation reaction: an ab initio molecular orbital study

An ab initio MO study of the allylation of α-methoxypropanal by allylboronic acid has been carried out. The calculated most stable transition state arrangement was found to be similar to that proposed by Cornforth. The Felkin–Anh orientation was found to be less stable than the Cornforth-like arrangement at the B3LYP/6-31G* level of theory in the six-membered cyclic transition state.     

Photophysics of intramolecularly hydrogen-bonded aromatic systems: ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

Excited state reaction paths and the corresponding energy profiles of salicylic acid have been determined with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. At crucial points of the potential energy hypersurfaces, single-point energy calculations have been performed with the CASPT2 method (second-order perturbation theory based on the complete active space self-consistent field reference). Hydrogen transfer along the intramolecular hydrogen bond as well as torsion and pyramidization of the carboxy group have been identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer represents a local minimum of the S(1) energy surface, which is separated by a very low barrier from a reaction path leading to a low-lying S(1)-S(0) conical intersection via torsion and pyramidization of the carboxy group. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the carboxy group, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the two known minima of the S(0) potential energy surface (rotamer I, rotamer II). A novel structure, 7-oxa-bicyclo[4.2.0]octa-1(6),2,4-triene-8,8-diol, has been identified as a possible transient intermediate in the photophysics of salicylic acid.     

De novo prediction of the ground state structure of transition metal complexes using semiempirical and ab initio quantum mechanics. Coordination isomerism

The ground state coordination isomers for 30 different trigonal bipyramidal transition metal complexes have been predicted using different levels of quantum mechanics: semiempirical (PM3(tm)), ab initio (MP2//HF), pure (BPW91) and hybrid (B3PW91) density functional theory (DFT) methods. For species where these methods failed to reproduce crystallographic data, hybrid quantum mechanics/molecular mechanics (QM/MM) methods were used to study more exact experimental models. Literature deficiencies regarding ground state multiplicity of these species were supplemented by spin predictions using previously tested PM3(tm) methods. Geometry optimization calculations were performed for each possible coordination isomer. The predicted ground state minima provided by the different methods are compared to each other and with crystallographic data. Pure DFT functionals outperformed hybrid functionals and MP2//HF. The very rapid PM3(tm) parameterization method provided accurate predictions in comparison to other levels of theory. An integrated MM/PM3(tm)/DFT de novo scheme accurately reproduced crystallographic data for species where the individual methods failed.     

Ozone-water 1:1 complexes O3-H2O: an ab initio study

Ab initio MO calculations have been carried out for the ozone-water 1:1 complexes in order to elucidate the structures and electronic state of the complexes. The QCISD calculations indicated that three structures are obtained as stable forms of O(3)-H(2)O. The most stable structure of O(3)-H(2)O has C(s) symmetry where the central oxygen of O(3) and all atoms of H(2)O are located on the molecular C(s) plane. The dipole of H(2)O orients toward the central oxygen atom of O(3) (i.e., dipole orientation form). The other two forms are cis and trans forms of O(3)-H(2)O where all atoms are located on the molecular plane, and a hydrogen of H(2)O binds to one of the terminal oxygen atoms of O(3) by a hydrogen bond. The binding energies of O(3) to H(2)O for dipole, cis, and trans forms are calculated to be 2.39, 2.27, and 2.30 kcal/mol, respectively, at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311++G((d,p) level. The dipole orientation form is more stable in energy than the cis and trans forms. Rotational constants for the dipole orientation form are calculated to be A = 11.897, B = 4.177, and C = 3.318 GHz which are in good agreement with the experimental values (A = 11.961, B = 4.174, and C = 3.265 GHz). The electronic states of O(3)-H(2)O were discussed on the basis of theoretical results.     

Cation–π complexes formed between cyclooctatetraene and alkaline earth metals: Predicted and recorded NMR features

The complexes formed by cyclooctatetraene (COT) and alkaline earth metals were theoretically studied but without convincible experimental evidence. To validate very strong cation–π interaction in the complexes, the NMR properties are experimentally recorded and theoretically calculated in this study. The good agreement between the recorded ¹³C NMR data and all the calculated results, including NMR chemical shift, interaction distances and strength, and atomic charge in DMSO at different theoretical levels by PCM method, strongly indicates that the complexes between COT and alkaline earth metal atoms do exist via strong cation–π interaction.     

CopperII-mediated aromatic ortho-hydroxylation: a hybrid DFT and AB initio exploration

Mechanistic pathways for the aromatic hydroxylation by [CuII(L1)(TMAO)(O)](-) (L1=hippuric acid, TMAO=trimethylamine N-oxide), derived from the O--N bond homolysis of its [CuII(L1)(TMAO)2] precursor, were explored by using hybrid density functional theory (B3LYP) and highly correlated ab initio methods (QCISD and CCSD). Published experimental studies suggest that the catalytic reaction is triggered by a terminal copper-oxo species, and a detailed study of electronic structures, bonding, and energetics of the corresponding electromers is presented. Two pathways, a stepwise and a concerted reaction, were considered for the hydroxylation process. The results reveal a clear preference for the concerted pathway, in which the terminal oxygen atom directly attacks the carbon atom of the benzene ring, leading to the ortho-selectively hydroxylated product. Solvent effects were probed by using the PCM and CPCM solvation models, and the PCM model was found to perform better in the present case. Excellent agreement between the experimental and computational results was found, in particular also for changes in reactivity with derivatives of L1.     

Lithium hydroxide phase transition under high pressure: an ab initio molecular dynamics study.

The high-pressure phase transition in the deuterated lithium hydroxide crystalline state has been studied by Car-Parrinello molecular dynamics simulations, in the constant-pressure, constant-temperature ensemble. The recently developed metadynamics approach has been applied to encourage the system to transform into different phases in an affordable simulation time. A previously not completely characterized high-pressure phase has been obtained. The structural and spectroscopic properties have been studied and compared with the neutron scattering, infrared and Raman measurements. It has been found that the calculated structure differs slightly from the experimental hypothesis, and that the presence of strong hydrogen bonds is the source of the red shift and of the characteristic features of the OD-stretching bands in both IR and Raman spectra.     

Syntheses,crystal structures and thermoanalyses of alkaline earth metal complexes derived from dinitropyridone

Six new alkaline-earth metal compounds derived from dinitropyridone ligands (3,5-dinitropyrid-2-one, 2HDNP; 3,5-dinitropyrid-4-one, 4HDNP and 3,5-dinitropyrid-4-one-N-hydroxide, 4HDNPO) were synthesized and characterized by elemental analysis, FT-IR and partly by powder XRD, TG-DSC and X-ray single-crystal diffraction analysis. The structural determination revealed that one molecule of both magnesium salts (Mg(2DNP)₂ ·?8H₂O, (1), and Mg(4DNP)₂ ·?6H₂O (4)) comprise one cation [Mg(H₂O)₆]²⁺ and two anions displaying centro-symmetry with the Mg atom located at the center. Two anions (and crystalline water molecules) are joined by hydrogen bonds. The barium salt Ba(4DNP)₂ ·?4H₂O (5), is a centro-symmetric dimer with each Ba(II) being coordinated by one monodentate ligand anion, two bidentate ligand anions (different coordination pattern) and five water molecules. Another barium salt, Ba(4DNPO)₂ ·?6H₂O (6), is a coordination polymer, the ten-coordinate (BaO₁₀) barium environment comprising four water molecules, a pair of 4DNPOs via the pyridine-N-oxide oxygen, and one 4DNPOs from an adjacent metal atom offering chelating nitro group oxygen, bridging adjacent bariums. Abundant intermolecular hydrogen bonds link the molecules of each complex into multi-dimensional chains. The X-ray powder diffraction analysis confirmed the phase homogeneity of the polycrystalline samples. The TG-DSC results revealed that Mg(2DNP)₂ ·?8H₂O and Ba(4DNP)₂ ·?6H₂O each has three main weight-loss stages. The first step is the loss of all water molecules and the last step is the loss of the nitro groups and/or decomposition of the pyridine rings with the release of heat.     

Interactions of Li, Ca, and Al with aromatic carbon materials: an ab initio study

The interactions of benzene (C6H6), naphthalene (C10H8), and perinaphthene (C13H9) with metal atoms (Li, Ca, and Al) were studied using second-order M?ller-Plesset perturbation theory. By analyzing the frontier molecular orbitals, geometric structures, binding energies, and charge transfers, it was found that these metal atoms can bond strongly with C13H9, but can only bond weakly with C6H6 and C10H8. The bonding nature between a metal atom and C13H9 at their ground state depends significantly on the valence orbital of the metal atom and the pi-bonding distribution of the aromatic hydrocarbons. The spindly shaped 3p valence orbital of an Al atom results in the deviation of the adsorption site to the edge of C13H9, whereas the ball-shaped 2s/4s valence orbitals of a Li and a Ca atom facilitate their overlap with the second lowest unoccupied molecular orbital of C13H9. Further, Hartree-Fock and density-functional theory methods were demonstrated generally to be unreliable in describing the interactions of metal atoms with these pi systems.     

Vibrational frequencies and infrared intensities of acetonitrile coordinated with metal cations: an ab initio study

Vibrational frequencies and infrared intensities have been calculated at the 6-31G and 6-31G^** levels for acetonitrile and for the complexes of acetonitrile with Li⁺ and Na⁺ cations. The changes in the infrared characteristics from an isolated acetonitrile to acetonitrile coordinated with metal cations (Li⁺ and Na⁺) have been evaluated. The ab initio calculations predict an essential increase of the intensities of the stretching CN, C-C and deformation CH₃, CCN vibrations in the complexes of acetonitrile with Li⁺ and Na⁺ cations.     

Interaction of the early 3d transition metals Sc, Ti, V, and Cr with N2: an ab initio study

The interaction of the early 3d transition elements M=Sc, Ti, V, and Cr with N2(X 1Sigmag+) has been studied by coupled-cluster and multiconfigurational techniques in conjunction with quantitative basis sets. We investigated both triatomic (MN2) and tetratomic (M2N2) species but focused mainly on high-spin linear and T-shaped triatomics. The lowest bound states of ScN2(4B1),TiN2(5Delta), and VN2(6Sigma+) correlate to the first excited state of the M atom, with M-N2 binding energies (De) of 24, 14, and 8 kcal/mol, respectively. In CrN2, the first bound state (7) product operator correlates to the sixth excited state of the Cr atom (7P) with De = 27 kcal/mol. The M-N2-M bond strength of high-spin linear tetratomics is twice as large the binding energy of the corresponding M-N2 linear triatomics, M = Sc, Ti, V, and Cr.