Calculation of molecular electric polarizabilities and dipole moments. III. BH molecule

We use a previously proposed variation-perturbation method to calculate the electric polarizabilities and the electric dipole moment at equilibrium nuclear distance of the BH molecule. We obtain 3.56 × 10^?24 cm³ for the perpendicular polarizability α_xx and 3.22 × 10^?24 cm³ for the parallel polarizability α_zz. Our result for the electric dipole moment μ₀ is 1.734 debye units; there is no reliable experimental result to compare it with.     

Thermotropic ionic liquid crystals of pyridinium octylphosphate A N.M.R. and X-ray study

Abstract

A thermotropic ionic lamellar phase from non-stoichiometric pyridinium octyl-phosphates has been investigated by multinuclear N.M.R. and X-ray diffraction. At room temperature and above, this phase is formed for pyridine to octylphosphoric acid molar ratios from 0.2 to 0.8.²H and ¹³C relaxation experiments show that the pyridinium ion undergoes a very anisotropic motion with D_zz > D_xx ? D_yy, z and x being the perpendicular direction to the ring and the c ₂ symmetry axis, respectively. The order parameters given by the ²H quadrupolar splittings and the ¹³C chemical shift anisotropy (CSA) are S_zz = 0.13, S_yy = -0.08 and S_xx = -0.05, showing that the pyridinium ring is preferentially oriented parallel to the lamellar plane. The ³¹P CSA and the C1-P dipolar splitting yield S_zz = 0.33 and S_xx ? S_yy for the octylphosphate anion. The order parameters of alkyl C-H bonds have been obtained from the J resolved two-dimensional ¹³C N.M.R. spectra of oriented samples. Two limiting conformational models have been considered to calculate the S _CH. One of them is reasonably consistent with the structure derived from X-ray experiments and has been used to calculate the dipolar ³¹P relaxation. Taking into account the CSA contribution, the relaxation measurements performed at 36, 121 and 202 MHz show that the octylphosphate anion undergoes a quasi-axial reorientation about the long molecular axis x with D∥/D⊥ = 4 and D⊥ ? 10⁷ rad/s at 300 K.     

The sign of the Co quadrupole splitting in Co(III) compounds,and the magnitude of the Fe quadrupole moment

Using the partial quadrupole splitting values (PQS) for ligands such as Cl⁻, CN⁻ and NH₃ derived from Mössbauer spectra of Fe(II) low spin compounds, the sign of V_zz is obtained for (e²qQ)_Co in isoelectronic compounds of Co(III) low spin. The predicted signs of V_zz are: trans- [Co(NH₃)₄Cl₂]⁺, positive; [Co(NH₃)₅Cl]⁺⁺, positive, and [Co(NH₃)₅CN]⁺⁺, negative. A value of the ⁵⁷Fe quadrupole moment of 0.16 ± 0.03 has been determined by comparing the observed e²qQ values for the above Co(III) compounds with these calculated from PQS values for the corresponding hypothetical Fe(II) compounds.     

Formation of superoxide ions upon oxygen adsorption on magnesium-doped magnesium oxide: An EPR investigation

Oxygen adsorption on the surface of MgO, previously submitted to magnesium metal vapour, gives rise to the formation of at least five distinct species of adsorbed superoxide (O₂⁻) characterized by different g_zz values. Besides species previously observed on MgO, where O₂⁻ is generated by other methods, two new species are observed, having g_zz = 2.091 and g_zz = 2.064 respectively. The latter species is tightly bound to the surface and is assigned to O₂⁻ adsorbed on top of an array of five Mg²⁺ ions exposed because of the presence of an anion vacancy. The former is assigned to O₂⁻ weakly adsorbed on top of a five-coordinated Mg²⁺ ion at the (100) planar face of the oxide. This species differs from the other species present at the surface in that it is mobile and, as the temperature is raised from 77 to 298 K, exhibits rotation around the axis perpendicular to the surface. Spectra obtained by employing ¹⁷O-enriched O₂ indicate that both superoxide species are symmetrically adsorbed on the surface.     

Synthesis, structure, and 13C and 31P CP/MAS NMR of the tetraphenylantimony(V) di-iso-propyl phosphorodithioate complex [Sb(C6H5)4{S2P(O-iso-C3H7)2}] and its solvated form [Sb(C6H5)4{S2P(O-iso-C3H7)2}] · 1/2C6H6: An example of the monodentate coordination of dithio ligands

The crystalline tetraphenylantimony(V) O,O′-di-iso-propyl phosphorodithioate complex [Sb(C₆H₅)₄{S₂P(O-i-C₃H₇)₂}](I) and its solvated form [Sb(C₆H₅)₄{S₂P(O-i-C₃H₇)₂}] · 1/2C₆H₆(II) were synthesized. Solid compounds I and II were studied by MAS NMR (¹³C, ³¹P). The ³¹P NMR chemical shift anisotropy ³¹P δ_aniso = (δ _zz ? δ_iso) and asymmetry parameter η = (δ _yy ? δ _xx )/(δ _zz ? δ_iso) were calculated using χ ² plots constructed on the basis of the ³¹P MAS NMR data. The O,O′-di-iso-propyl phosphorodithioate ligands in both complexes are characterized by predominantly the axially symmetric ³¹P chemical shift tensor (for the case δ _zz < δ _xx ≈ δ _yy ) with close values of anisotropy parameters (δ_aniso and η), which reflects their identical S-monodentate structural function. X-ray crystallography showed that II has a trigonal-bipyramidal molecular structure with the uncommon monodentate coordination of the Dtph ligands through an S atom in an axial position of the trigonal bipyramid and the benzene molecule in the outer sphere.     

Single crystal NQR and NMR study of (NH4)2InBr6·H2O

The ⁸¹Br NQR tensors of (NH₄)₂InBr₅·H₂O have been studied on single crystals by combining a 4π Zeeman goniometer with Fourier transform NQR. AT T=293K the coupling constants are 136.27 e²φ_ZZQh^?(su81Br)/Mhz ≦ 146.14 and the asymmetry parameter 0.013 ≦ η (⁸¹Br) ≦ 0.133. The EFG axes φ_zz(⁸¹Br) are almost parallel to the bond directions InBr. The strong changes of φ_yy(¹¹⁵In) and φ_zz(¹¹⁵In) with temperature can be explained by assuming small changes of the In(OH₂) bond distance in the complex anion (InBr₅·H₂Cl^2?. The D₂O molecule within the deuterated complexes flips freely in the range studied (100 ≦ T/K ≦ 298). Small EFG's at the ¹⁴N site in the cation ND^?₄ are due to the EFG of the surrounding lattice charges.     

127I NQR spectra in substituted α-iodothiophenes

We have studied the complete¹²⁷I spectra and obtained the values of the quadrupole coupling constants e²Qq_zz and the asymmetry parameters in a series of substituted -iodothiophenes. Using an additive scheme, we have calculated e²Qq_zz and determined the substituent increments.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1028–1030, May, 1990.     

Properties of halogen atoms for representing intermolecular electrostatic interactions related to halogen bonding and their substituent effects

The electrostatic properties of halogen atoms are studied theoretically in relation to their ability of halogen bonding, which is an attractive intermolecular interaction of a covalently bonded halogen atom with a negatively charged atom of a neighboring molecule. The electric quadrupole (of electronic origin) with a positive zz component Θ_zz of a covalently bonded halogen atom, where the z axis is taken along the covalent bond involving the halogen atom, is mainly responsible for the attractive electrostatic interaction with a negatively charged atom. This positive Θ_zz is an intrinsic property of halogen atoms with the p_x²p_y²p_z configuration of the valence electronic shell, as shown by ab initio molecular orbital calculations for isolated halogen atoms with this electronic configuration, and increases in the order of F < Cl < Br < I, in parallel with the known general sequence of the strength of halogen bonding. For halogen‐containing aromatic compounds, the substituent effects on the electrostatic properties are also studied. It is shown that the magnitude of Θ_zz and the electric field originating from it are rather insensitive to the substituent effect, whereas the electric field originating from atomic partial charges has a large substituent effect. The latter electric field tends to partially cancel the former. The extent of this partial cancellation is reduced in the order of Cl < Br < I and is also reducible by proper substitution on or within the six‐membered ring of halobenzene. Perspectives on the development of potential function parameters applicable to halogen‐bonding systems are also briefly discussed. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Molecular Conformations of Shape Anisometrically Variant Mesogens in Liquid Crystalline Phase Studied by 13C NMR Spectroscopy

Mesogens that vary in shape anisometry have been investigated by ¹³C solid-state NMR in the liquid crystalline phase to inspect the conformations. The molecules examined comprise of (i) rod-like mesogen with three-phenyl ring core and terminal hexyloxy chains, (ii) three-ring core linked to the fourth phenyl ring via a spacer, and (iii) trimesic acid connected to three side arms core units through a spacer. The order parameter (S_zz) values for the phenyl rings of the rod-like mesogen are 0.65–0.68, while the mesogen with a three-ring core linked to a phenyl ring via spacer showed dissimilarity. Consequently, for the core unit phenyl rings, S_zz is ~0.70, and the terminal phenyl ring showed a low value of 0.12. For the trimesic acid based mesogen, the S_zz value for the side arm phenyl rings is ~0.53, and for the central phenyl ring, a very low value of 0.11 is witnessed. By considering the ordering of the rod-like mesogen as a yardstick and employing the ratios of S_zz values of the phenyl rings, the average conformations of other mesogens are arrived. Accordingly, for the trimesic acid based mesogen, a tripod-like conformation instead of λ shape is proposed in the liquid crystalline phase.     

Lithium and sodium cation binding of cyclopentadienyl anions: Electronic effects of cyclopentadienyl substitution

The Li⁺ and Na⁺ binding of substituted cyclopentadienyl (Cp) anions were investigated using computational techniques. The ring centroid-metal distances and the binding energies of the Cp-metal complexes correlate very well with the ∑σ_m of the substituted Cp ring. These properties also correlate well with the Cp Θ_zz values. The trend in the correlations is the more electron-rich the Cp (negative ∑σ_m and Θ_zz values values), the shorter the Cp-metal bond and the stronger the binding energy. The NBO metal charges correlate, though not very well in either case, with the Cp Θ_zz and ∑σ_m values. However, there is a substantial increase in correlation when the sum of the absolute value of the Hammett σ_m (∑|σ_m|) is employed. The significantly improved correlation when the ∑|σ_m| values are employed leads us to propose a model for substituted Cp charge transfer upon Li⁺ or Na⁺ complexation, and it also informs us that the Hammett substituent constant σ_m contains information about substituent polarizabilities, at least in the case of Li⁺- and Na⁺-substituted Cp anions.     

127I NQR spectra in several iodonium compounds

Interionic coordination interactions and their relationship to the behaviour of NQR spectral parameters, e²Qq_zz, η and ϑν/ϑE_z have been studied for several unsubstituted iodonium salts. The quadrupole coupling constants, e²Qq_zz, ha found to be linearly related to the asymmetry parameter, η.     

Electron–nuclear motions in a series of meta-substituted iodobenzene derivatives as probed by <Superscript>127</Superscript>I NQR

A relation between the quadrupole coupling constant e ² Qq _zz and asymmetry parameter η has been found for a series of meta-substituted iodobenzene derivatives. The relationships e ² Qq _zz(η) for electrondonor and electron-acceptor substituents have the same form but different numerical values of parameters. The average time of full electron transfer via tunneling has been evaluated.     

Reasons for the line broadening in the epr spectra of copper ions in Li2-2x Zn2+x (MoO4)3 lithium-zinc molybdate crystals

An EPR study of Li_2?2x Zn_2+x (MoO₄)₃ crystals activated by copper ions shows that they occupy the M2 site, one of the three possible sites of both lithium and zinc. In the EPR spectra of Cu²⁺ copper a broadening of HFS lines and a nonequidistant splitting between them, which are unusual for the orientation H‖g _zz , A _zz , are observed. In this work possible reasons for such a broadening of HFS lines from copper ions are analyzed: a distortion of the oxygen octahedron due to the introduction of copper ions, second order perturbation theory corrections, superposition of HFS from ⁶³Cu and ⁶⁵Cu isotopes, and the effect of the charge redistribution in the oxygen octahedron because the cation vacancy providing charge compensation can be located at different distances from the copper ion. It is shown that the first three reasons do not explain the features observed in the EPR spectra. In the case of cation vacancies located in the M3 site and remote at different distances from the copper ion, the charge redistribution in the oxygen octahedron of copper should occur along with the dispersion of HFS parameters and the g-factor. The studies performed in X and Q bands confirm this assumption. The width of HFS lines from copper ions in the EPR spectra measured in the Q band is three times more compared to that measured in the X band.     

Nucleus Independent Chemical Shift (NICS) at Small Distances from the Molecular Plane: The Effect of Electron Density

When close to the molecular plane, the behavior of nucleus independent chemical shift (NICS) as a function of the distance from the molecular plane deviates from its behavior at larger distances. By using a dense grid of NICS-probes (BQs) it is shown that, when close to the molecular plane, maximal (absolute) NICS values are obtained above the atoms. These maxima move towards the center as the grid is elevated until the (absolute) maximum NICS is obtained at the center and stay there when the grid is further elevated. It is shown that this behavior is a result of the current density, which is influenced by the electron density, according to the Biot-Savart law, which, in turn, causes the induced magnetic field measured by the NICS. It is thus concluded that if magnetic aromaticity is studied, the NICS calculations should be carried out at a large enough distance so that only the π-ring current affects the NICS. At distances ≥2 Å, NICS(r)_π,zz=A+B*C^r. Using non-linear correlation for obtaining A, B and C and extrapolate to NICS(1)_π,zz and NICS(1.7)_π,zz is recommended as measures for aromaticity.     

Polymeric gold(I) diisobutyl dithiophosphate, [Au2{S2P(O-iso-C4H9)2}2]n: Synthesis,supramolecular self-organization (a role of aurophilic interaction), 13C and 31P MAS NMR spectroscopy,and thermal behavior

A new polymeric gold(I) diisobutyl dithiophosphate (Dtph), [Au₂{S₂P(O-iso-C₄H₉)₂}₂] _n (I), was preparatively obtained and characterized by ¹³C and ³¹P MAS NMR spectroscopy and X-ray diffraction (CIF file CCDC no. 977818). Diagrams of the χ² statistic were constructed from the complete ³¹P MAS NMR spectra and used to calculate the ³¹P chemical shift anisotropy (δ _aniso = δ _zz ? δ _iso ) and the asymmetry parameter η = (δ _yy ? δ _xx )/(δ _zz ? δ _iso ). The main structural unit of complex I is the noncentrosymmetric dinuclear molecule [Au₂{S₂P(O-iso-C₄H₉)₂}₂], in which the gold atoms are linked by two bridging ligands Dtph. The central cyclic structural fragment of the dimer [Au₂S₄P₂] is additionally stabilized by the intramolecular aurophilic interaction Au?Au. Further supramolecular self-organization of the complex involves intermolecular aurophilic bonds Au?Au that serve to unite adjacent dinuclear molecules [Au₂{S₂P(O-iso-C₄H₉)₂}₂] with different spatial orientations into the polymer chains ([Au₂{S₂P(O-iso-C₄H₉)₂}₂]) _n . The thermal behavior of complex I was examined by synchronous thermal analysis under argon. The character of the thermolysis of the complex to reduced metallic gold as a final product was determined.     

Microwave spectroscopic study on the molecular structure and the quadrupole coupling constants of thionyl chloride

Microwave spectra of thionyl chloride, SO³⁵Cl₂ and SO³⁵Cl³⁷Cl, in the frequency range 8–25 GHz have been analyzed. The rotational constants have been obtained from the low J transition frequencies. The r_S coordinates of Cl atoms and the r_o structure have been evaluated with some assumptions: r(S-O) = 1.435 ± 0.011± Å, r(S—Cl) = 2.072 0.005 Å, ∠ OSCl = 108.00 ± 0.06°, ∠ ClSCl = 97.15 ± 0.30°. Nuclear quadrupole coupling constants have been obtained for the SO³⁵Cl₂, species: x_aa = ?25.02 ± 0.04 MHz, x(_bb = ?0.25 ± 0.04 MHz, X_cc = 25.27 ± 0.08 MHz, and X_zz = ?96.75 MHz. The values obtained are compared with those of other workers.     

Calculation of the rotational magnetic moments of the ammonia and water molecules

We calculate the rotational magnetic moments and the corresponding g-factors for water and ammonia. Our results are g_xx = 0.397, g_yy = 0.769 and _zz = 0.345 for water and g_xx = g_yy = 0.69 and g_zz = 0.51 for ammonia. The experimental values are g_xx = 0.585, g_yy = 0.742 and g_zz = 0.666 for water and g_xx = g_yy = 0.56 and g_zz = 0.484 for ammonia.     

Aromaticity Effects on the Profiles of the Lowest Triplet‐State Potential‐Energy Surfaces for Rotation about the CC Bonds of Olefins with Five‐Membered Ring Substituents: An Example of the Impact of Baird’s Rule

A density functional theory study on olefins with five‐membered monocyclic 4n and 4n+2 π‐electron substituents (C₄H₃X; X=CH⁺, SiH⁺, BH, AlH, CH₂, SiH₂, O, S, NH, and CH^?) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet‐state (T₁) potential‐energy surface (PES) for twisting about olefinic C?C bonds. It exploited both Hückel’s rule on aromaticity in the closed‐shell singlet ground state (S₀) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH₂?CH(C₄H₃X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C₄H₃X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S₀‐antiaromatic/T₁‐aromatic (C₅H₄⁺) to strongly S₀‐aromatic/T₁‐ antiaromatic (C₅H₄^?). Our hypothesis is that the shapes of the T₁ PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T₁ [ΔE(T₁)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T₁) and the (anti)aromaticity changes of the C₄H₃X groups, as measured by the zz‐tensor component of the nucleus‐independent chemical shift ΔNICS(T₁;1)_zz, are found both for sets A and B separately (linear fits; r²=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r²=0.851). For sets A and B combined, strong correlations are also found between ΔE(T₁) and the degree of S₀ (anti)aromaticity as determined by NICS(S₀,1)_zz (sigmoidal fit; r²=0.963), as well as between the T₁ energies of the planar olefins and NICS(S₀,1)_zz (linear fit; r²=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T₁ PESs suitable for adiabatic Z/E photoisomerization.     

Paramagnetic absorption of pentavalent molybdenum in polycrystalline tin dioxide

Part of molybdenum introduced in SnO₂ by various methods is found to be stabilized in the pentavalent state at substitutional positions. The ESR spectra of Mo⁵⁺ ions are characterized by the following parameters: g_xx = 1.891; g_yy = 1.835 and g_zz = 1.923, A_xx = 24 G, A_yy = 30 G and A_zz = 70 G. There are, in addition, two sets of superhyperfine structures due to the two tins located along the crystallographic c axis (290 G) and to the four tins lying in a diagonal plane of the unit cell containing four oxygens (51 G), consistent with a 3d_x²-y² + λ 3d_z2 ground state. The variations of the Mo⁵⁺ ESR spectrum upon heating the samples in various atmospheres (O₂, H₂) suggest that molybdenum enters the SnO₂ lattice also as Mo⁴⁺.     

Triplet State Aromaticity: NICS Criterion,Hyperconjugation, and Charge Effects

Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet‐state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)_zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet‐state aromaticity. In addition, calculations suggest that five‐membered cyclic species with “hyperconjugative” aromaticity (and antiaromaticity) in the S₀ state will become antiaromatic (and aromatic) in the T₁ state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r²=0.708) is identified between the NICS(1)_zz values and spin distributions.