Calculations of magnetic resonance parameters for the protonated nitroxide radical

Electron and magnetic resonance parameters of the protonated H₂NO radical have been calculated by the INDO and CNDO/SP methods for different models. Calculated changes of magnetic resonance parameters on protonation are consistent with experiment. The most appropriate structure has been found to be one in which the proton is in the plane of the radical with r(O?.H⁺) = 1.05 A.Calculated signns of the proton spin density for the models concerned are opposite to those of the spin density on the proton of a ligand involved in the hydrogen bonding for analogous models of hydrogen bond systems formed by the nitroxide radical.In the case of the protonated radical, taking into account the interaction with a solvent molecule leads to more reasonable results for large O?.H⁺ distances.     

Refined ab initio 6-31G split-valence basis set optimization of the molecular structures of biphenyl in twisted,planar, and perpendicular conformations

Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6-31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO-3G results. The torsional angle Φ_min obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED r_g-values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO-3G and 6-31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE⁹⁰) is 6.76 kJ/mol in close agreement with the experimental-31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D_2h-symmetry the calculated torsional energy barrier ΔE⁰ is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho-H atoms lead to a torsional angle Φ_min = 44.74° together with a dihedral angle towards ortho-H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE⁰ = 13.51 and ΔE⁹⁰ = 6.91 kJ/mol.     

Applications of the model potential method to transition metal compounds

Model potential parameters and basis sets, presented previously for the transition metal atoms Sc through Hg, are tested in calculations of the transition metal compounds (CuF, CuCl, Cu₂, TiCl₄, ZrCl₄, CoF₆^3?, CoF₆^2?, AgH, AuH, CrF₆, ScO, ZrO, Cr₂, Mo₂). Calculated values of the bond distances, vibrational frequencies, and some transition energies (for Cu₂ and CoF₆^2?) are compared with those given by all-electron calculations with basis sets of high quality. Singlet-triplet splittings in Cu₂ and correlation energies in CrF₆^n? (n = 0, 1, and 2) are also examined. The satisfactory results obtained by these calculations strongly support the contention that the model potential method is a reliable and economical alternative to the ab initio Hartree-Fock-Roothaan method.     

Vibrational analysis of Nb2Br10 and Ta2Br10: Harmonic force field and mean amplitudes

Normal coordinate analyses for Nb₂Br₁₀ and Ta₂Br₁₀ are performed. The final force constants are given in terms of symmetry coordinates Assignments of vibrational frequencies are given along with the corresponding potential energy distributions. Calculated man amplitudes of vibration for the bonded distances are reported.     

Strong Antiferromagnetic Coupling of Spins in the (MDABCO+)(C60.−) Salt with 3D Close Packing of the C60.− Radical Anions (MDABCO+: N‐Methyldiazabicyclooctanium Cation)

A new salt, (MDABCO⁺)(C₆₀^.?) ( 1 ; MDABCO⁺=N‐methyldiazabicyclooctanium cation), was obtained as single crystals. The crystal structure of 1 determined at 250 and 100 K showed 3D close packing of fullerenes with eight fullerene neighbors for each C₆₀^.?. These neighbors are located at 10.01–10.11 Å center‐to‐center distances (250 K) and van der Waals interfullerene C???C contacts are formed with four fullerene neighbors arranged in the bc plane. Fullerene ordering observed below 160 K is accompanied by the appearance of one and a half independent C₆₀^.? and trebling of the unit cell along the b axis. Fullerenes are packed closer to each other at 100 K. As a result, fullerenes are located in the three‐dimensional packing at 9.91–10.12 Å center‐to‐center distances and 18 short interfullerene C???C contacts are formed for each C₆₀^.?. Although they are closed packed, fullerenes are not dimerized down to 1.9 K. Magnetic data indicate strong antiferromagnetic coupling of spins in the 70–300 K range with a Weiss temperature of Θ=?118 K. Magnetic susceptibility shows a round maximum at 46 K. Such behavior can be described well by the Heisenberg model for square two‐dimensional antiferromagnetic coupling of spins with an exchange interaction of J/k_B=?25.3 K. This magnetic coupling is one of the strongest observed for C₆₀^.? salts.     

Natural abundance C REDOR coupled to a singly N-labeled nucleus: simultaneous determination of interatomic distances in crystalline ammonium [N] -glutamate monohydrate

REDOR technique was applied to natural abundance ¹³C nuclei coupled to a singly labeled ¹⁵N nucleus to determine the ¹³C, ¹⁵N interatomic distances simultaneously in crystalline ammonium [¹⁵N] -glutamate monohydrate (1). Consequently, the interatomic C–N distances between ¹⁵N and ¹³C=O, ¹³C_α, ¹³C_β, ¹³C_γ, and ¹³C_δ carbon nuclei for 1 were determined with a precision of ±0.15 Å, after the experimental conditions such as the location of samples in the rotor, length of π pulse etc. were carefully optimized. ¹³C-REDOR factors for three spin system, (ΔS/S₀)_CN1N2, and the sum of two isolated 2-spin system, (ΔS/S₀)^*=(ΔS/S₀)_CN1+(ΔS/S₀)_CN2, were further evaluated by the REDOR measurements on isotopically diluted 1 in a controlled manner. Subsequently, the intra- and intermolecular C–N distances were separated by searching the minima in the contour map of root mean square deviation (RMSD) between the theoretically and experimentally obtained (ΔS/S₀)^* values against two interatomic distances, r_C–N1 and r_C–N2. When the intramolecular C–N distance (r_C–N1) of the particular carbon nucleus is substantially shorter than the intermolecular one (r_C–N2), C–N distances within a single molecule were obtained with an accuracy of ±0.06 Å as in the cases of C=O, C_α and C_β carbon nuclei. C–N distances between the molecule in question and the nearest neighboring molecules can be also obtained, although accuracy was lower. On the contrary, it was difficult to determine the interatomic distances in the same molecule when the intermolecular dipolar contribution is larger than the intramolecular one as in the case of C_δ carbon nucleus.     

The crystal steueture of phenoxatellurine,C12H8OTe

The crystal structure of phenoxatellurine, C₁₂H₈OTe, was determined by X-ray diffracto-meter methods. The crystals are orthorhorhorhombic, P2₁ 2₁ 2₁, a = 6.036(1), b = 8.160(1), c = 20.717(5) Å at t = 22°. The positions of all atoms, including hydrogen, were found. The central ring is folded along the Te-O axis (138°). Average bond distances are Te-C = 2.098, C-O = 1.397, C-C = 1.382 Å. The phenyl rings are planar with a dihedral angle of 145°; C-Te-C = 89.4(3)°, C-O-C = 121.2(5)°. The structure is compared to those of phenoxathionine and pheuothiazine.     

The crystal structure of phenoxatellurine dinitrate,C12H8O7N2Te

The crystal structure of phenoxatellurine dinitrate, C₁₂H₈O₇N₂Te, has been determined by x-ray diffractometer methods. The crystals are monoclinic, C2/c, a = 12.916(3), b = 14.050(5), c = 7.532(2) Å; β = 96.65(3)° at t = 22°. The molecule is nearly planar (175°), with the Te and O atoms of the central ring in special positions on the twofold symmetry axis. The bond distances for the central ring are: Te-C = 2.068(4) Å, C-O = 1.366(5), C-C = 1.388(6) with C-Te-C = 93.5(2)° and C-O-C = 128.2(5). The bond distances and angles in the phenyl rings do not differ significantly from the normally accepted values of 1.40 Å and 120°. The two nitrate groups are close to Te and are related by the twofold axis of symmetry. The independent distances and angles are: Te-O1 = 2.201(3), O1-N = 1.325(5), N-02 = 1.229(6), N-O3 = 1.204(6) Å, O2-N-O3 = 126.3(4), O1-N-02 = 117.1(4), O1-N-03 = 116.6(4)°. All hydrogen atoms were located at or near their calculated positions. The final R value for 1679 independent reflections was 0.031. The planarity of the molecule is discussed qualitatively in terms of simple molecular orbital theory.     

Component activities in the system thorium nitrate-nitric acid-water at 25°C

The equilibrium composition of the vapor above thorium nitrate-nitric acid-water mixtures has been studied as a function of the concentrations of thorium nitrate and nitric acid using a transpiration technique. At 25°C, the thorium nitrate concentrations m _T ranged from 0.1 to 2.5 molal and the nitric acid concentrations m _N from 0.3 to 25 modal. The vapor pressure of the nitric acid was found to increase with increasing thorium nitrate concentration for a constant molality of nitric acid in aqueous solution. At constant m _T , the nitric acid vapor pressure was particularly enhanced at low nitric acid concentrations. The water vapor pressures decreased regularly with increasing concentrations of both nitric acid and thorium nitrate. The experimental data were fitted to Scatchard's ion-component model, and to empirical multiparameter functions. From the fitting parameters, and available literature data for the nitric acid-water and thorium nitrate-water systems at 25°C, expressions were calculated for the variation of water and thorium nitrate activities, as functions of the nitric acid and thorium nitrate concentrations, using the Gibbs-Duhem equation. Calculated values for the thorium nitrate activities were strongly dependent on the form of the function originally used to fit the vapor pressure data.Issued as AECL-7461.     

Weak hydrogen bonds from Cp donors to CC acceptors

In the crystal structure of 1, pairs of molecules are connected by mutual C(Cp)–H?CC interactions which have a geometry suggestive of weak hydrogen bonding (1[(η⁵-C₅H₅)Fe(CC−C₇H₇–2,4,6)(CO)₂]). The shorter of the H?C distances is only 2.59 Å, which is a distance that is typically observed for C–H?CC hydrogen bonds of stronger donors such as chloroform or alkyne groups.     

The crystal structure of bisphenoxatelluronium dinitrate,C24H16O9N2Te2

Bisphenoxatelluronium dinitrate is monoelinie, P2₁/c: a = 11.638(4), b = 28.266(8), c = 8.546(3) å, β = 119.73(2)°, z = 4 at t = 22°. All atoms including hydrogen were located. The two ring systems, I and II, are folded along their Te-O axes, 147° and 163°, respectively. The average ring bond distances are: Te-C = 2.091, C-C = 1.377, C-O = 1.370 Å. Each Te is bonded to one NO₃ group, Te1-ON1 = 2.485(5), Te2-ON4 = 2.393(4) Å, and an oxygen bridge connects the ring systems, Te1-OB = 1.966(4), Te2-OB = 2.001(4) Å, Te1-OB-Te2 = 125.0(2)°. The bond distances and angles of the structure are compared to those of related compounds.     

The cyclic vanadylacetate (C24H20P)[V4O8(C2H3O2)4(NO3)]

In the title complex, tetraphenylphosphonium μ₄‐nitrato‐κ⁴O‐cyclo‐tetrakis(μ‐acetato‐O:O′)tetra‐μ‐oxo‐tetrakis[oxovana‐dium(V)], the anion lies about a twofold axis and consists of the cyclic [V₄O₈] unit coordinated by four acetato ligands with interatomic V?V distances of 3.269 (1) and 3.273 (1) Å. The double‐bonded O atom [N=O 1.102 (6) and N—O 1.268 (4) Å] of the nitrato ligand links the four V atoms with V—O bond distances of 2.613 (2) and 2.813 (2) Å. The negative charge of the complex is balanced by tetraphenyl­phosphonium cations occupying the Na positions in the NaCl‐type ionic packing of the structure.     

Synthesis and Structure of K[Cd(O2N‐C6H4‐NNN‐C6H4‐NO2)3], an Anionic 1, 3‐Bis(4‐nitrophenyl)triazenido Complex of Cadmium

The reaction of cadmium acetate in methanol with 1, 3‐bis(4‐nitrophenyl)triazene in THF in the presence of KOH yields K[Cd(O₂NC₆H₄NNNC₆H₄NO₂)₃] in form of hexagonal prismatic, red crystals with the trigonal space group R3¯ and a = 12.229(2), c = 48.988(10) Å and Z = 6. In the anionic cadmium complexes, which are located along the threefold axis, the Cd atoms are coordinated in a trigonal prismatic arrangement by the atoms N(1) and N(3) of three triazenido ligands. The potassium cations are coordinated icosahedrally by oxygen atoms of each one nitro group of six neighbouring anionic complexes. The Cd‐N distances are 2.376(4) and 2.350(4) Å, and the K‐O distances are in the range of 2.833(6) to 3.365(6) Å.     

On the Mechanism of the α-Alkynone Cyclization: A Theoretical Study

The acetylene → carbene rearrangement of three model a-alkynones, namely propynal ( 1a ), 2-butynal ( 1b ) and butynone ( 1c ) is studied by ab initio double-zeta, double-zeta plus polarization and double-zeta plus self-consistent electron pairs calculations. Transition states are located by force-gradient geometry optimization. Calculated minimum energy reaction paths reveal substituent effects on the activation parameters, which indicate that a H-atom or an alkyl group competes favorably with an acyl group in the [1,2]-shift from a-alkynones 1 to the corresponding acylvinylidenes 5 .     

Exploring the Molecular Structure of Imidazolium–Silica‐Based Nanoparticle Networks by Combining Solid‐State NMR Spectroscopy and First‐Principles Calculations

A DFT‐based molecular model for imidazolium–silica‐based nanoparticle networks (INNs) is presented. The INNs were synthesized and characterized by using small‐angle X‐ray scattering (SAXS), NMR spectroscopy, and theoretical ab initio calculations. ¹¹B and ³¹P HETCOR CP MAS experiments were recorded. Calculated ¹⁹F NMR spectroscopy results, combined with the calculated anion–imidazolium (IM) distances, predicted the IM chain density in the INN, which was also confirmed from thermogravimetric analysis/mass spectrometry results. The presence of water molecules trapped between the nanoparticles is also suggested. First considerations on possible π–π stacking between the IM rings are presented. The predicted electronic properties confirm the photoluminescence emissions in the correct spectral domain.     

Theoretical study in [C2H4–Tl]+ and [C2H2–Tl]+ complexes

We studied the attraction between [C₂H_n] and Tl(I) in the hypothetical [C₂H_n–Tl]⁺ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C–Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree–Fock (HF), second‐order Møller–Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance R_e (C–Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C₂H_n]–Tl⁺ interaction may be related mainly to charge‐induced dipole and dispersion terms, both involving the individual properties of the olefinic π‐system and thallium ion. However, the charge‐induced dipole term (R^?4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Ab initio calculations of small hydrides including electron correlation

Calculations based on the independent-electron-pair approximation (IEPA) and the direct determination of approximate natural orbitals for the different pair correlation functions, including intra- and interpair correlation effects, are performed for the BH ground state at several internuclear distances r. The dependence of the different pair correlation contributions on r is investigated. The contributions involving the K-shell orbitals of B are practically independent of r. Calculated equilibrium distances and force constants including correlation effects are in better agreement with experiment than are the corresponding Hartree-Fock values.

---

Zusammenfassung Rechnungen, die auf der Näherung der unabhängigen Elektronenpaare (Independent-electron-pair approximation IEPA) und der direkten Bestimmung genäherter natürlicher Orbitale beruhen und die sowohl Intra- als auch Interpaar-Korrelationseffekte erfassen, wurden für das BH-Molekül bei verschiedenen interatomaren Abständen r durchgeführt. Die Abhängigkeit der verschiedenen Paar-Korrelationsbeiträge von r wird untersucht. Die Beiträge, an denen die K-Schale des B-Atoms beteiligt ist, erweisen sich als praktisch unabhängig von r. Die Berücksichtigung der Korrelationseffekte führt zu einer Verbesserung der berechneten Werte von Gleichgewichtsabstand und Kraftkonstante in Richtung auf die experimentellen Werte.

---

Résumé Un calcul basé sur l'approximation des paires d'électrons indépendantes (Independent-electron-pair approximation IEPA) et la détermination directe des orbitales naturelles approchées a été fait pour l'état fondamental de la molécule de BH; les contributions provenant de la corrélation intra- et interpaire ont été calculées pour plusieurs distances internucléaires r, et la dépendance de ces contributions en fonction de r a été étudiée.On montre que les contributions dans lesquelles le niveau K intervient sont pratiquement independantes de r. Le fait de tenir compte explicitement de la corrélation améliore les valeurs calculées de la distance d'équilibre et de la constante de force qui se rapprochent des valeurs expérimentales.

     

Understanding of bonding and mechanical characteristics of cementitious mineral tobermorite from first principles

This paper reports density functional theory study of the structural and mechanical properties of tobermorite mineral (9 Å phase) as one of the main component of cementitious materials in a concrete chemistry. Calculated bulk modulus and elastic constants reflect a relatively high resistance of the tobermorite structure with respect to external isostatic compression. Moreover, the elastic constants proved the anisotropic character of the tobermorite structure. The directions parallel to the axb plane are more resistant to the compression than the perpendicular direction. The largest contribution to this resistance comes from the “dreierketten” silicate chains. The bonding analysis linked macroscopic mechanical properties and the atomic structure of the tobermorite. It was found that polar covalent Si? O bonds are stiffer than iono‐covalent Ca? O bonds. The SiO₄ tetrahedra are resistant with respect to the compression and the effect of external pressure is reflected by the large mutual tilting of these tetrahedra as it is shown by changes of the Si? O? Si bridging angles. Polyhedra with the seven‐fold coordinated Ca²⁺ cations undergo large structural changes. Especially, axial Ca? O bonds perpendicular to the axb plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the axb plane, are mainly responsible for the high resistance of the tobermorite structure to external pressure. The main mechanism of a dissipation of energy entered to the structure through the compression is proceeded by the tilting of the tetrahedra of the silicate chains and by large shortening of the axial Ca? O distances. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Molecular and Crystal Structure of Magnolol—C18H18O2

Magnolol, 2, 2′-dihydroxy-5, 5′-diallylbiphenyl (C₁₈H₁₈O₂) was isolated from the heartwood of Taiwan sassafras, Sassafras randaiense (Hay.) Rehd. (Lauraceae) and characterized by single crystal X-ray diffraction. It crystallized in monoclinic P 2₁/C. The cell parameters are a=10.905(3), b=8.834(4), c=16.103(9) Å, β=106.76°(3), z=4. The structure was solved by direct method. The dihedral angle between two benzene rings is about 45′ which seems to be the best arrangement for intra- and inter-molecular H-bondings. O … O distances are ～2.6 Å which indicate very strong H-bonding. The solid structure can be discribed as a helix chain of molecules connected through H-bonds parallel to b-axis.     

Steric effects upon transition states of radical addition polymerizations

Transition states (TSs) of radical addition homopolymerization reactions of methyl acrylate, methyl methacrylate, dimethyl itaconate, and N-methyl itaconimide were examined with two-unit radical models using MOPAC (PM3 UHF) semiempirical method. Calculated activation energies (E_as) show good correlations with experimental values. Calculated activation entropies (−ΔS^‡s) are found to be well proportional to E_as. The entropy terms play an important role as well as E_a in radical additions. E_a depends on the angle (θ_rs) between reaction points of radical and of monomer at TS. The bond length between reaction points at TS is constant regardless of monomers studied. The geometries and thermodynamical properties calculated here for TS indicate the importance of steric effects caused by substituted group(s) rather than electronic perturbation energies reported previously. © 1996 John Wiley & Sons, Inc.