Chemical Inequivalence Research Articles

Characterisation of the weak halogen bond in N2⋯ICF3 by pure rotational spectroscopy

Rotational spectra of the symmetric-top complexes 14N2⋯ICF3 and 15N2⋯ICF3 were observed and analysed to give rotational constants B0, centrifugal distortion constants DJ and DJK, and nuclear quadrupole coupling constants χaa(X). Significantly different values of χaa(14Ni) and χaa(14No) establish chemical inequivalence of the two 14N nuclei and an average zero-point oscillation angle of 19.8(5)° for the N2 subunit. A distance rN⋯I=3.443(1)Å only slightly shorter than the sum of the N and I van der Waals radii, a small intermolecular stretching force constant kσ=2.94Nm−1 and negligible charge redistribution on complex formation demonstrate the presence of a weak halogen bond.

Iridium(III) hydrido N-heterocyclic carbene-phosphine complexes as catalysts in magnetization transfer reactions.

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

Temperature Variation of Ultralow Frequency Modes and Mean Square Displacements in Solid Lasamide (Diuretic Drug) Studied by 35Cl-NQR, X-ray and DFT/QTAIM

The application of combined (35)Cl-NQR/X-ray/DFT/QTAIM methods to study the temperature variation of anisotropic displacement parameters and ultralow frequency modes of anharmonic torsional vibrations in the solid state is illustrated on the example of 2,4-dichloro-5-sulfamolybenzoic acid (lasamide, DSBA) which is a diuretic and an intermediate in the synthesis of furosemide and thus its common impurity. The crystallographic structure of lasamide is solved by X-ray diffraction and refined to a final R-factor of 3.06% at room temperature. Lasamide is found to crystallize in the triclinic space group P-1, with two equivalent molecules in the unit cell a = 7.5984(3) Å, b = 8.3158(3) Å, c = 8.6892(3) Å; α = 81.212(3)°, β = 73.799(3)°, γ = 67.599(3)°. Its molecules form symmetric dimers linked by two short and linear intermolecular hydrogen bonds O-H···O (O-H···O = 2.648 Å and ∠OHO = 171.5°), which are further linked by weaker and longer intermolecular hydrogen bonds N-H···O (N-H···O = 2.965 Å and ∠NHO = 166.4°). Two (35)Cl-NQR resonance frequencies, 36.899 and 37.129 MHz, revealed at room temperature are assigned to chlorine sites at the ortho and para positions, relative to the carboxyl functional group, respectively. The difference in C-Cl(1) and C-Cl(2) bond lengths only slightly affects the value of (35)Cl-NQR frequencies, which results mainly from chemical inequivalence of chlorine atoms but also involvement in different intermolecular interactions pattern. The smooth decrease in both (35)Cl-NQR frequencies with increasing temperature in the range of 77-300 K testifies to the averaging of EFG tensor at each chlorine site due to anharmonic torsional vibrations. Lasamide is thermally stable; no temperature-induced release of chlorine or decomposition of this compound is detected. The temperature dependence of ultralow frequency modes of anharmonic small-angle internal torsional vibrations averaging EFG tensor and mean square angle displacements at both chlorine sites is derived from the (35)Cl-NQR temperature dependence. The frequencies of torsional vibrations higher for the para site than the ortho site are in good agreement with those obtained from thermal parameters obtained from X-ray studies. The mean square angle displacements are in good agreement with those estimated from X-ray data with the use of the TLS model. The detailed DFT/QTAIM analysis suggests that the interplay between different hydrogen bonds in adjacent molecules forming dimers is responsible for the differences in flexibility of the carboxyl and sulphonamide substituents as well as both C-Cl(1) and C-Cl(2) bonds. Three ultralow wavenumber modes of internal vibrations in Raman and IR spectra obtained at the B3LYP/6-311++G(d,p) level close to those obtained within the TLS model suggest that internal and external modes of vibrations are not well separated.

On the spatial structure of methyl 6,7-endo, sin-dibrom-7-anti-(phenylsulfonyl) bicyclo[3.1.1]heptane-6-exo-carboxylate

In the 1 H and 13 C NMR spectra of methyl 6,7-endo,sin-dibromo-7-anti-(phenylsulfonyl)bicycle-[3.1.1]heptane-6-exo-carboxylate, H(1) and H(5) protons as well as C(1) and C(5) carbon atoms show their chemical inequivalence determined by the hindered rotation of sulfonyl and ester groups about simple C-S and C-C bonds due to the existence of donor-acceptor interaction between the carbonyl C atom and the oxygen atom of the SO2Ph group. This interaction is indicated by the single crystal X-ray diffraction study detecting the shortened intramolecular contacts (2.49 A with the sum of the C…O van der Waals radii of 3.00 A). Other features of the norpinane skeleton conformation and the spatial orientation of substituents in the single crystal are discussed. By 1 H NMR methods, the parameters of the dependence of molecular conformations on the temperature of DMSO-d6 solution and the activation free energy △Gc≠ = 80.1 kJ/mol of conformational transitions are determined. By a quantum chemical DFT calculation of the potential energy surface it is revealed that the hindered rotation of the ester group makes the main contribution to the barrier of conformational transitions.

Michael versus retro-Michael reaction in the regioselective synthesis of N-1 and N-3 uracil adducts

By controlling the temperature or reaction time in the base-catalysed Michael-type addition of 5-substituted uracil derivatives we were able to synthesise N-1 or N-3 uracil adducts using methyl acrylate and acrylonitrile as acceptors. The mechanism of this chemical inequivalence was established using 1H NMR spectroscopic studies. The investigations revealed that formation of the N-1 adduct was achievable under kinetically controlled conditions irrespective to the type of the base used (TEA, DBU). In turn, synthesis of the N-3 adducts proceeded from the initially formed N-1,N-3 diadduct via a retro-Michael reaction which dominates at elevated temperature or prolonged reaction time.

Using the Combined Approach of 35Cl NQR, 14N NQR and DFT Calculations To Study the 14N NQR Spectrum of Diazepam

The electronic structure of diazepam was analyzed by pulsed 14N nuclear quadrupole resonance (NQR) spectroscopy and density functional theory (DFT) calculations using the hybrid method B3PW91 with the 6-31G(d,p) basis set. Before searching for 14N NQR spectra, a 35Cl NQR line in diazepam was recorded to evaluate the degree of disorder in the crystal lattice and thus the chances to find much weaker NQR spectra of nitrogen-14 in this substance. In order to speed up the detection of weak NQR spectra, the spin-locked spin-echo multipulse sequence had to be applied. Despite the expected chemical inequivalence of the two nitrogen atoms in the diazepam molecule, only two resonance lines ν+ and ν− were then found for this compound at 77 K. Possible reasons for this anomaly are discussed. From the experimental frequencies, the quadrupole coupling constant, the asymmetry parameter and the principal components of the electric field gradient tensor were determined. Those parameters were compared with the results of theoretical DFT calculations showing a reasonable accuracy of the DFT model used.

Chemical inequivalence behavior of the protons attached to the same carbon in ethylene–propylene copolymer (EPC)

Six ethylene–propylene copolymer samples with different structural parameters were analyzed by 1H NMR, 13C NMR and 1H– 13C COSY in this paper. With the aid of the 1H– 13C COSY spectra of ethylene–propylene copolymer, the chemical inequivalence behavior of the protons attached to the same carbon in the ethylene–propylene copolymer could be observed. Four basic types of chemical bonds were divided, and two kinds of main conformations were proposed to explain this phenomenon. Based on the calculation of conformation distribution, it can be proved that the explanation of this phenomenon was reasonable.

Identification of symmetry, structure and defects of dopant Mn(II) ions in Zn(C 3H 3O 4) 2(H 2O) 2 by single crystal EPR technique

In order to understand the symmetry, structure and defects of Mn(II) impurity incorporated in diaquabis[malonato(1-)-κ 2O,O′] zinc(II), single crystal EPR studies have been carried out at X-band frequencies at room temperature. Angular variation in the three orthogonal planes shows the presence of two defects, having a relatively large and small zero-field splitting parameter ( D), with orthorhombic symmetry. The spin-Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations, are: Defect I: g xx = 1.959, g yy = 1.998, g zz = 2.011; A xx = −8.73, A yy = −8.55, A zz = −9.10 mT; D xx = 5.58, D yy = 1.33, D zz = −6.91 mT. Defect II: g xx = 2.015, g yy = 1.996, g zz = 2.004; A xx = −8.18, A yy = −8.00, A zz = −8.58 mT; D xx = 33.48, D yy = 6.92, D zz = −40.40 mT. The observed large zero-field tensor for Defect II is due to the steric effects caused by the two malonate rings. The location of the two interstitial defects has been determined from the X-ray data of the host lattice. The powder EPR spectrum also confirms the chemical inequivalence of the two defects. The optical absorption spectrum shows the characteristic of Mn(II) ions in distorted octahedral symmetry.

Study of Neutral Fe(III) Complexes of Pyridoxal-N-Substituted Thiosemicarbazone with Desolvation-Induced Spin-State Transformation above Room Temperature

The preparation and characterization of two new neutral ferric complexes with desolvation-induced discontinuous spin-state transformation above room temperature are reported. The compounds, [Fe(Hthpy)(thpy)].CH3OH.3H2O (1) and [Fe(Hmthpy)(mthpy)].2H2O (2), are low-spin (LS) at room temperature and below, whereas their nonsolvated forms are high-spin (HS), exhibiting zero-field splitting. In these complexes, Hthpy, Hmthpy, and thpy, mthpy are the deprotonated forms of pyridoxal thiosemicarbazone and pyridoxal methylthiosemicarbazone, respectively; each is an O,N,S-tridentate ligand. The molecular structures have been determined at 100(1) K using single-crystal X-ray diffraction techniques and resulted in a triclinic system (space group P1) and monoclinic unit cell (space group P21/c) for 1 and 2, respectively. Structures were refined to the final error indices, where RF = 0.0560 for 1 and RF = 0.0522 for 2. The chemical inequivalence of the ligands was clearly established, for the "extra" hydrogen atom on the monodeprotonated ligands (Hthpy, Hmthpy) was found to be bound to the nitrogen of the pyridine ring. The ligands are all of the thiol form; the doubly deprotonated chelates (thpy, mthpy) have C-S bond lengths slightly longer than those of the singly deprotonated forms. There is a three-dimensional network of hydrogen bonds in both compounds. The discontinuous spin-state transformation is accompanied with liberation of solvate molecules. This is evidenced also from DSC analysis. Heat capacity data for the LS and HS phases are tabulated at selected temperatures, the values of the enthalpy and entropy changes connected with the change of spin state were reckoned at DeltaH = 12.5 +/- 0.3 kJ mol-1 and DeltaS = 33.3 +/- 0.8 J mol-1 K-1, respectively, for 1 and DeltaH = 6.5 +/- 0.3 kJ mol-1 and DeltaS = 17.6 +/- 0.8 J mol-1 K-1, respectively, for 2.

NMR paramagnetic relaxation enhancement: test of the controlling influence of zfs rhombicity for S = 1.

Prior theoretical work has predicted that the NMR paramagnetic relaxation enhancement (NMR-PRE) produced by electron spin S = 1 ions is highly sensitive to orthorhombic terms in the static zero field splitting (zfs) tensor. Zfs orthorhombicity (which implies chemical inequivalence of the three principal directions of the zfs-principal axis system and is described by the zfs E-parameter) is predicted to suppress the NMR-PRE profoundly relative to the reference cylindrical zfs-limit situation. This expectation was tested experimentally by a comparison of the zfs-limit NMR-PRE produced by [Ni(II)(en)(3)](2+) (en = ethylenediamine), a trigonal complex which lacks zfs-rhombicity, with the zfs-limit NMR-PRE produced by two orthorhombic complexes, [Ni(II)(en)(2)(H(2)O)(2)](2+) and [Ni(II)(en)(H(2)O)(4)](2+). As predicted, the zfs-limit NMR-PRE produced by the orthorhombic complexes in the proton resonance of a dioxane probe species in the solvent was strongly suppressed (by factors of approximately 5 and 7, respectively) relative to the comparable measurement on the trigonal complex. The suppression of the NMR-PRE due to the orthorhombic zfs terms is counteracted by an applied Zeeman field, leading to a predicted rise in the NMR-PRE with increasing Zeeman field strength; this rise occurs when the Zeeman energy is comparable to the orthorhombic zfs splitting, 2E. This second prediction of theory was likewise confirmed: the expected rhombicity-induced magnetic field dependence in the NMR-PRE was observed for the orthorhombic complexes but not for the trigonal complex.

Electron Paramagnetic Resonance and DTA Investigation of Cr3+ in Tris(guanidinium) Hexafluoroaluminate Single Crystals

Abstract Electron Paramagnetic Resonance (EPR) studies of Cr 3+ in single crystals of tris(guanidinium) hexafluoroaluminate, [C(NH2)3]3 AlF6 , have been carried out in the X-band region. A temperature dependent study of the zero-field splitting parameter D in the range 77-398 K shows the presence of a phase transition, which is supported by Differential Thermal Analysis. In addition, 19F superhyperfine struc-ture has been observed in the 9.3% naturally abundant 53Cr isotope hyperfine structure. D shows a large decrease with increasing temperature. The phase transition brings about a chemical inequivalence in the two chemically equivalent but magnetically inequivalent room temperature (CrF6)3- species. Compar-ison is made with the alums AlCl3 • 6H20, as well as other guanidinium aluminum salts.

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO4.3H2O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO−3 radical and two pairs of lines (A1, A2) and (B1, B2) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO−3 indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T1 and T2) of SO−3 have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO−4 radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO−3 and chemical inequivalence of two types of SO−4 radicals. The principal g-values of the SO−4 radical were evaluated to be: g1 = 2.007, g2 = 2.011, g3 = 2.014 for species A and g1 = 2.008, g2 = 2.012, g3 = 2.015 for species B. The low microsymmetry of the SO2−4 ion in the lattice seems to promote the radiation damage.

Solid-state distortions of linear mono-, bi-, and trimetallic bis(tri- n-butylphosphine) nickel and palladium complexes having 1,4-tetrafluorophenylene bridges as observed via 31P CP/MAS NMR spectroscopy

31P CP/MAS NMR spectra were recorded for a series of eight linear mono-, bi-, and trimetallic bis(tri- n-butylphosphine) nickel and palladium complexes having 1,4-tetrafluorophenylene bridges. A 2 type spectra were obtained with Ni(PBu 3) 2(1,4-C 6F 4H) 2 ( I), Pd(PBu 3) 2(1,4-C 6F 2) ( V) and [Pd(PBu 3) 2(1,4-C 6F 4H)] 2(1,4-C 6F 4) ( VII). AB type spectra were obtained with Ni(PBu 3) 2(1,4-C 6F 4H)Br ( II), [Ni(PBu 3) 2(1,4-C 6F 4)Br 2 ( III) and [Pd(PBu 3) 2Cl] 2(1,4-C 6F 4) ( VI). The 2 J PP scalar coupling constants fell in the region of 300 to 450 Hz as expected for P-metal-P pairs in a trans configuration. The spectra of [Ni(PBu 3) 2] 3(1,4-C 6F 4) 2Br 2 ( IV) and [Pd(PBu 3) 2] 3(1,4-C 6F 4) 2Cl 2 ( VIII) consisted of overlapping A 2 and AB spins systems. Large upfield and downfield shifts were measured in the solid samples relative to the corresponding 31P NMR spectra in CDCl 3 solution. Correlating the 31P CP/MAS NMR data with reported X-ray crystallographic measurements revealed that both the chemical inequivalence and the solid versus solution resonance frequency shifts were the result of ring current effects brought about by distortions from square-planar geometry at the metal centre in the solid state.

Cyclodextrin—chiral substrate interactions : High field nuclear magnetic resonance and molecular graphics studies

The inclusion compounds of (+)-, (−)- and (±)-menthol have been studied by 500-MHz nuclear magnetic resonance (NMR) spectroscopy in deuterium oxide and by molecular graphics techniques. The molecular graphics modelling suggests a slightly reduced degree of Van der Waals interactions in the case of (−)-menthol coupled with greater chemical inequivalence of the CH 3 groups of the isopropyl function. These results have been confirmed by 1H NMR studies which show the CH 3 protons to be sensitive to the external environment.

ESR study of Cd(BrO3)2⋅2H2O single crystals γ irradiated at 300 and 77 K

In Cd(BrO3)2⋅2H2O single crystals γ irradiated at 300 K, BrO2 and O−3 centers are observed. A correlation between the ESR and optical absorption studies has revealed that the optical absorption band for BrO2 lies around 470 nm. After γ irradiation at 77 K, a complex defect center identified as H⋅⋅⋅BrO3⋅⋅⋅HO could be observed. The chemical inequivalence and the effect of thermal or UV light bleaching on the populations of these centers are discussed.