19F and 1H NMR study of five-membered ring triflones and related σ -adducts. Long range fluorine-proton spin spin coupling.

Fluorine and proton chemical shifts for various trifluoromethylsulfonyl thiophene, selenophene, furan and N-methylpyrrole derivatives are reported and compared with those for nitro analogs. The ¹⁹F chemical shift of the SO₂CF₃group is found to be insensitive to its ring position as well as to the nature of the heterocycle. It is also unaffected by the loss of aromaticity and the presence of a negative charge which result from a nucleophilic addition to the ring. In most triflones studied, long range fluorine-proton coupling constants are observed but the proton involved in these couplings depends upon the α or β position of the SO₂CF₃ group as well as the nature of the ring heteroatom.     

High resolution photoemission and x-ray absorption spectroscopy of a lepidocrocite-like TiO2 nanosheet on Pt(110) (1 × 2)

The electronic structure of TiO(2) nanosheets on the Pt(110)-(1 × 2) surface has been investigated by using high resolution photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The Ti 2p XAS spectra of the deposited TiO(2) films have been theoretically evaluated and, from the comparison with the experimental data, the assignment to a lepidocrocite-like structure is confirmed. Coexistence of TiO(2) islands with PtO(2) stripes for incomplete nanosheets is confirmed by high resolution photoemission data. The location of the valence and conduction band edges of the nanosheet has been experimentally determined allowing us to describe in details subtle electronic effects due to the interface with the substrate. The locations of the valence band maximum and the leading peak in the O 1s XAS spectrum indicate a band gap similar to anatase but with the Fermi level closer to mid-gap than found for bulk, n-type TiO(2).     

NMR study on solubility of benzyl alcohol and its transfer free energy from D_2O to cationic micelle

A new kind of surfactant, [CnH_(2n+1)OCH2CH(OH)CH2N(CH3)3]Cl (n=12, 14, 16) was synthesized. The solubility of benzyl alcohol in micellar solutions was determined by 1H NMR method. The results indicate that the length of alkyl chains of surfactant affects the solubility of ben-zyl alcohol in 2.5 × l0~(-2) mol/L micellar solutions. The solubility of benzyl alcohol per liter of micellar solution is 0.095 mole for n=12, 0.115 mole for n=14, 0.165 mole for n=16. The transfer free energy of benzyl alcohol from aqueous phase to micellar phase is -24.29 kJ/mol for n=12, -24.37 kJ/mol for n=14, -24.49 kJ/mol for n=16.     

Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II∕III) complex with two crystallographically inequivalent iron sites

The magnetic, electronic, and Mo?ssbauer spectral properties of [Fe(2)L(μ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mo?ssbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).     

Quantum study of electronically non-adiabatic collinear reactions. III. Influence of vibrational and electronic excitations on the Cs + HH → CsH + H reaction

A new hyperspherical close-coupling method, involving non-orthogonal diabatic electronic bases adapted to each arrangement, is developed to compute collisional transition probabilities for the collinear Cs + HH system, close to the threshold of the reactions Cs^*(7p) + HH(υ = 0) → CsH(υ = 0) + H. A simple model, taking into account the overall rotation of the linear CsHH complex, provides estimates of inelastic and reactive cross sections. A hyperspherical adiabatic analysis, based on adiabatic electronic potential energy surfaces, points out two prominent features in the dynamics of the CsHH system namely: (i) Approximate electrovibrational adiabaticity, which gives rise to large cross sections (≈ 5 Ų), for both the vibrationally adiabatic process Cs^*(7p) + HH(υ = 0) → CsH(υ = 0) + H and the vibrational-into-electronic energy conversion Cs(6s) + HH(υ = 6) → CsH(υ = 0) + H. (ii) Sharp resonances, which yield rapid oscillations on total cross sections, related to the rotational structure of an intermediate linear long-lived complex.     

High resolution spectral analysis of oxygen. II. Rotational spectra of a(1)Δ(g) O2 isotopologues

As part of a comprehensive review on molecular oxygen spectroscopy, we have measured rotational spectra of isotopic forms of molecular oxygen in its a(1)Δ(g) electronic state with high-resolution terahertz spectroscopy. The data are recorded in close proximity to predicted positions. Due to the high resolution and good signal-to-noise ratio, the fundamental hyperfine parameters eQq and C(I) are determinable for (17)O-substituted species for the first time. A refined nuclear spin orbit coupling constant, a = -211.9328(283) MHz, was determined, and is roughly two orders of magnitude more precise than values determined from near infrared spectroscopy or electron spin resonance studies. Vibrationally excited oxygen in the a(1)Δ(g) electronic state was also observable with small signal levels for many of the rotational transitions.     

Application of Schrödinger equation to study the tunnelling dynamics of proton transfer in the hydrogen bond of 2,5-dinitrobenzoic acid: proton T1 T1rho, and deuteron T1 relaxation methods

Temperature measurements of proton T1 (24.7 MHz), deuteron (deuterated hydroxyl group) T1 (55.2 MHz), and proton T1(rho) (B1 = 9 G) spin-lattice relaxation times of 2,5-dinitrobenzoic acid have been performed. An analysis of present experimental data together with previously published proton T1 (55.2 MHz) data has revealed the following molecular motions: proton/deuteron transfer in the hydrogen bond and two-site hopping of the whole dimer. It is shown that the proton-transfer dynamics are characterized by two correlation times tau(ov) and tau(tu), describing two fundamentally different motional processes, namely, thermally activated jumps over the barrier and tunneling through the barrier. The temperature dependence of 1/tau(tu) is the solution of Schr?dinger's equation, which also yields the temperature T(tun), where begins the tunnel pathway for proton transfer. A new equation for the spectral density function of complex motion consisting of the three motions is derived. The third motion (two-site hopping of the whole dimer characterized by tau(lib) correlation time) is responsible for a proton T1(rho) minimum in high temperatures, just below the melting point. Such a minimum is not reached by T1 temperature dependencies. The minimum of T1(rho) assigned to the classical hopping of a hydrogen-bonded proton occurs in the same low-temperature regime in which the flattening of the temperature dependencies of T1 points to the dominance of incoherent tunneling. This experimental fact denies the known theories predicting the intermediate temperature regime where a smooth transition between classical and quantum tunneling dynamics is expected. The fit of the derived theoretical equations to the experimental data T1(rho) and T1 is satisfactory. The correlation times obtained for deuterons indicate deuteron-transfer dynamics much slower than proton-transfer dynamics. It is concluded that the classical proton transfer takes place over the whole temperature regime, while the incoherent tunneling occurs below 46.5 (hydrogen) or 87.2 K (deuterium) only.     

Preparation,structure and magnetic study of a new linear 1-D Cu(II)–Gd(III) coordination polymer

A carboxylate-bridged Cu(II)–Gd(III) complex, [GdCu(CH₂CH(CH₃COO)₄(H₂O)₄] _n (NO₃) _n , was prepared and characterized. Single crystal X-ray analysis reveals the complex as a carboxylate-bridged 1-D Cu(II)–Gd(III) coordination polymer. The magnetic measurement showed this complex exhibiting weak ferromagnetic behavior.     

Two spin systems based on [Ni(dmit)2]− building blocks exhibiting 1-D chain and 2-D sheet structures: crystal structures and magnetic properties

The crystal structures and magnetic properties for two ion-pair complexes, [MG][Ni(dmit)₂] (1) and [RhB][Ni(dmit)₂] (2) (MG⁺ = N-(4-((4-(dimethylamino)phenyl)(phenyl)methylene)cyclohexa-2,5-dienylidene)-N-methylmethanaminium, RhB⁺ = 9-(2-carboxyphenyl)-3,6-bis(diethylamino)xanthylium; dmit^2? = 1,3-dithiole-2-thione-4,5-dithiolate), have been investigated. Complex 1 crystallizes in the monoclinic space group P2₁/c, the stacking interactions between the GM⁺ and [Ni(dmit)₂]^? lead to the anion exhibiting a banana-shaped molecular geometry; the neighboring anions form a 1-D alternating chain along the crystallographic [1 1 0] direction via intermolecular lateral-to-lateral and head-to-head S ··· S contacts. The temperature dependence of magnetic susceptibility from 2–350 K does not display the magnetic character of a 1-D chain; the magnetic susceptibility data were fit to the Curie–Weiss equation to give C = 0.351(15) emu K mol^?1, θ = ?3.51(28) K, and χ ₀ = 8.00(3) × 10^?5 emu mol^?1. Complex 2 belongs to the triclinic space group P-1 with two crystallographic inequivalent [Ni(dmit)₂]^? anions, which exhibit perfect planar geometry. Two ethyl groups of RhB⁺ are disordered over two positions at 296 K, and one becomes ordered at 150 K. The [Ni(dmit)₂]^? anions form a 2-D molecular layer via intermolecular lateral-to-lateral and head-to-lateral S ··· S interactions with the anionic layer parallel to the crystallographic (0 1 ?1) plane. The temperature variation of magnetic susceptibility from 2 to 350 K shows that antiferromagnetic coupling dominates in the anionic layer, and the disorder–order changes of cationic structure are not coupled to magnetic property.     

13C NMR on the S=1/2 antiferromagnetically coupled spin chain compound [PM·Cu(NO3)2·(H2O)2]n (PM=pyrimidine)

We present ¹³C NMR experiments on the S=1/2 antiferromagnetically coupled spin chain compound [PM·Cu(NO₃)₂·(H₂O)₂]_n (PM=pyrimidine) at temperatures 5–120 K. In the NMR spectra, we observe three pairs of double lines. From the angular dependence of the NMR shift, we assign the three line pairs to the three inequivalent C sites on the pyrimidine molecule C₄N₂H₄, with the splitting of the signal from each site into two lines caused by the hydrogen atom bound to C. We obtain the temperature dependence of the NMR shift, which exhibits a qualitatively different behavior for the three inequivalent C sites.     

Influence of Dab2 and Pro3 configuration of [Leu]-enkephalins on the interactions with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). Herein we describe the influence of Dab and Pro chirality of cyclic [Leu]enkephalins (X¹-c[Dab²-Pro³-βNal(2)⁴-Leu⁵], where X = Tyr or Phe) on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for all cyclic [Leu]enkephalins studied as well as for Nal and AcNalNH₂. Moreover, change of Dab residue configuration has a greater influence on changes of the binding constant of cyclic enkephalin with β-CD than change of Pro chirality has. Also, the replacement of Tyr¹ residue by Phe¹ substantially changes the peptide chain conformation. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Nal, Tyr or Phe or Leu residue, a side and/or bottom association complexes are formed.     

Influence of pH and temperature on the enantioselectivity of propan-2-ol-treated Candida rugosa lipase in the kinetic resolution of (±)-4-acetoxy-[2,2]-paracyclophane

The reaction temperature, pH of the aqueous medium and use of an aqueous-n-hexane reaction medium markedly influenced the hydrolysis rate and enantioselectivity of (±)-4-acetoxy-[2,2]-paracyclophane with propan-2-ol-treated Candida rugosa lipase. The results have been justified on the basis of a possible conformational change in the enzyme as a consequence of the displacement of its polypeptide lid.     

In-situ High Resolution NMR Investigation on Polymerization Mechanism. II. Comparison between BF3·Bu2O and CI-N2PF6 as Catalysts in Homopolymerization of Trioxane and Copolymerization with Ethylene Oxide

A comparison of BF₃ ·Bu₂O and Cl-N₂PF₆ as catalysts for cationic homopolymerization and copolymerization of trioxane has been made by employing high resolution nuclear magnetic resonance techniques. While no substantial difference was detected for the homopolymerization, two important differences were observed for the copolymerization with ethylene oxide; viz., 1) with Cl-NPF₆ there is a lower build-up of formaldehyde concentration; 2) with Cl[sbnd]N₂PF₆, a lesser amount of cyclic compounds containing ethylene oxide units is formed (e.g., 1,3-dioxolane). Both observations suggest that depolymerization occurs to a lesser extent with the cl-N₂PF₆ catalyst.     

Conformational study of tetramethyl-N,N′-bis-arylcyclodisilazanes by 13C NMR and x-ray diffraction analysis: III. The conformation of tetramethyl-N,N′-bis(O-methoxyphenyl)cyclodisilazane

The conformation of tetramethyl-N,N′-bis(o-methoxyphenyl)cyclodisilazane was investigated by ¹³C NMR in solution and by X-ray diffraction in the solid state. The conformation in solution compares to that of o-methoxyaniline. The phenyl and the cyclodisilazane ring forms a dihedral angle of 7.4° and there is a non-bonded Si ⋯ O distance of 2.73 Å in the crystal. The molecular geometry suggests an analogy to S ⋯ O non-bonded interactions often observed in organosulphur chemistry.     

Homo- and hetero-nuclear chromium(III) complexes with natural ligands. Part 1. Spectroscopic and mass spectra studies on ternary [M–L1–L2] systems

Seven new ternary mono- and poly-nuclear Cr^III complexes with natural ligands: glycine, glutaminic, nicotinic and asparginic acids, cysteine and glutathione, have been isolated and physicochemically characterized. Four of them have been tested and appear to be non-toxic. The complexes have been analysed using spectroscopic (diffuse reflectance U.v–vis., i.r., f.i.r.), magnetic methods, and (some) by FAB mass spectra. Spectral analyses with the digital filter and band deconvolution methods have been also presented.     

Crystal structure and magnetic behavior of Cu3(O2C16H23)6·1.2 C6H12. An unexpected structure and an example of spin frustration

A copper(II) carboxylate has been obtained from the reaction of basic copper carbonate, CuCO₃·Cu(OH)₂, with 2,4,6-triisopropylbenzoic acid. It contains a trinuclear Cu₃⁶⁺ core with three-fold symmetry, with each pair of copper atoms bridged by two carboxylate ligands. The Cu···Cu separations are 3.131(3) Å, precluding any direct bonding. Magnetic susceptibility data from 1.8 to 380 K at 1 000 G indicate a doublet ground state, (g ≈ 2.1) between 10–50 K (μ_eff = 1.80 μ_B). Above 50 K, the magnetic moment increases to 2.69 μ_B at 380 K, as expected for an equilateral triangular system of Cu^II with antiferromagnetic interactions between spin carriers.     

Theoretical study on the structures and properties of (Br2AlN3) n (n = 1–4) clusters

To look for the single-source precursors, the structures and properties of (Br₂AlN₃) _n (n = 1–4) clusters are studied at the B3LYP/6-311+G* level. The optimized (Br₂AlN₃) _n (n = 2–4) clusters all possess cyclic structures containing Al-N_α-Al linkages. The relationships between the geometrical parameters and the oligomerization degree n are discussed. The gas-phase structures of the trimers prefer to exist in the boat-twisting conformation. As for the tetramer, the most stable isomers have the S ₄ symmetry structure. The IR spectra are obtained and assigned by the vibrational analysis. The thermodynamic properties are linearly related with the oligomerization degree n as well as with the temperature. Meanwhile, the thermodynamic analysis of the gas-phase reaction suggests that the oligomerization be exothermic and favorable under high temperature.     

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid: 1H, 31P,and 13C{1H} NMR and some comments on corresponding [AX]3 and AXX′2 spin systems and spectra

Abstract

The fully deprotonated anion of 1,3,5-benzene-triphosphonic acid 1,3,5-C₆H₃[PO₃^2?]₃ gives rise to deceptively simple ¹H and ³¹P NMR spectra due to a corresponding [AX]₃ spin system. The ¹³C{¹H} NMR spectrum reveals two ¹³C isotopomers which are identified via AXX′₂ systems. Results from analysis and iteration are described.     

Structure and magnetic properties of an unprecedented syn-anti μ-nitrito-1κO:2κO' bridged Mn(III)-salen complex and its isoelectronic and isostructural formate analogue

The preparation, crystal structures and magnetic properties of two new isoelectronic and isomorphous formate- and nitrite-bridged 1D chains of Mn(III)-salen complexes, [Mn(salen)(HCOO)](n) (1) and [Mn(salen)(NO(2))](n) (2), where salen is the dianion of N,N'-bis(salicylidene)-1,2-diaminoethane, are presented. The structures show that the salen ligand coordinates to the four equatorial sites of the metal ion and the formate or nitrite ions coordinate to the axial positions to bridge the Mn(III)-salen units through a syn-antiμ-1κO:2κO' coordination mode. Such a bridging mode is unprecedented in Mn(III) for formate and in any transition metal ion for nitrite. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 indicate the presence of ferromagnetic exchange interactions with J values of 0.0607 cm(-1) (for 1) and 0.0883 cm(-1) (for 2). The ac measurements indicate negligible frequency dependence for 1 whereas compound 2 exhibits a decrease of χ(ac)' and a concomitant increase of χ(ac)' on elevating frequency around 2 K. This finding is an indication of slow magnetization reversal characteristic of single-chain magnets or spin-glasses. The μ-nitrito-1κO:2κO' bridge seems to be a potentially superior magnetic coupler to the formate bridge for the construction of single-molecule/-chain magnets as its coupling constant is greater and the χ(ac)' and χ(ac)' show frequency dependence.