C, N and H magnetic relaxation and anisotropic molecular reorientation of nitrobenzene

Magnetic relaxation rates of ¹³C and ¹⁴N have been measured for neat nitrobenzene and several 50% (v/v) solutions thereof. Using known values of the ¹⁴N quandrupole coupling constant and asymmetry parameter and the ¹³CH bond distances we have determined the three reorientational correlation times and the orientation of the ¹⁴N quadrupole coupling tensor. The correlation times are compared with those previously obtained for nitrobenzene from a combination of depolarized Rayleigh scattering ad ¹³C relaxation data. Knowledge of the principal components of the rotational diffusion tensor permits determination of the quadrupole coupling constant for the ortho deuterons in nitrobenzene-d₅ and the unambiguous assignment to molecular axes of previously determined components of the chemical shift tensor for ¹⁵N in nitrobenzene ¹⁵N. In addition, the implications of our assignment of the ¹⁴N quadrupole coupling tensor components for earlier studies of the alignment of nitrobenzene by electric fields and in a liquid crystal are discussed.     

The dynamics of the CH3CN molecule in the isotropic phase and in a nematic solution

NMR lineshape studies of acelonitrile in the isotropic and the liquid-crystalline nematic phase of PCH have been performed. The scalar relaxation of the second kind due to the presence of the ¹⁴N quadrupolar nucleus has been confirmed as the most important relaxation mechanism for this molecule in both the isotropic and the anisotropic phase. It has been found largely responsible for the selective broadening on ¹³C and ¹H transitions. A minor contribution arising from intramolecular dipolar relaxation mechanism has also been investigated. Linewidth analysis of the NMR spectra allowed us to determine the quadrupolar relaxation time T_IN of the ¹⁴N nucleus. This is connected to the correlation time for rotational diffusion perpendicular to the molecular symmetry axis. A possible explanation of a residual selective broadeining which effects the ¹³C and ¹H NMR transitions and is not taken into account by this mechanism, is also given.     

High-resolution ODMR- and ODNQR spectroscopy of acridine-d9 in its lowest excited triplet state

High-resolution ODMR- and ODNQR spectroscopy have been performed on acridine-d₉ in its photoexcited triplet state. The nuclear quadrupole constants e²qQ/h = −4.36(3) MHz and η = 0.22(1) have been measured for the ¹⁴N nucleus, where the sign of e²qQ/h is determined relative to the signs of the fine structure parameters, D and E. The ¹⁴N ODNQR spectrum, with linewidths as low as 60 kHz, exhibits a satellite structure attributable to the ²D nucleus located at the meso-position in the molecule. For this nucleus |e²qQ/h| = 180(30) kHz may be estimated. The intensity behaviour of the ODNQR transitions as a function of the NMR rf power level leads to a coarse determination of the in-plane components of the ¹⁴N hyperfine tensor with |A_yy^N| 0.2 MHz and |A _yy^N| 4.7 MHz. Owing to accidental overlap of two transitions in the 2E-ODMR spectrum of acridine, the ODNQR technique had to be extended to a five-pulse sequence.     

自然丰度氮-15核磁共振的研究(Ⅰ)——哌啶衍生物中4-位取代基对15N化学位移的远程相互作用及其与13C化学位移的关系

测定了8个4-位取代的2,2,6,6-四甲基哌啶的自然丰度¹⁵N核磁共振谱及¹³C谱,发现4-位取代基对¹⁵N化学位移有一定的远程相互作用,¹³C化学位移与4-位卤素取代基的原子电负性有近似的线性关系。     

Muon—nuclear quadrupolar level crossing resonance in solid nitrogen. Evidence for N2μ ion formation

Both integral and time-differential muon level crossing resonance techniques were used to identify the source of the diamagnetic μ⁺ SR signal in crystalline nitrogen. When the muon Zeeman splitting and the ¹⁴N nuclear quadrupole splitting are equal, the longitudinal field μ⁺ spin relaxation rate is increased due to dipolar ¹⁴N—μ⁺ coupling. At T = 37 K the resonance occurs at a magnetic field of 172 ± 2 G with a fwhm of 10 ± 2 G. The resonance parameters unambiguously confirm the formation of the N₂μ⁺ ion in solid nitrogen.     

The N and C solid state NMR study of intramolecular hydrogen bond in some Schiff bases

In present work set of eight Schiff bases derived from substituted salicylaldehydes and aliphatic and aromatic amines has been studied in the solid state by ¹⁵N and ¹³C CPMAS NMR methods. ¹⁵N CPMAS NMR measurement is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers. In the solid state, four of the compounds examined were shown by ¹⁵N CPMAS NMR to exist as OH tautomeric forms, and the remaining four as the corresponding NH forms with different stage of proton transfer process, from oxygen to nitrogen site.

This was confirmed by ¹³C CPMAS. The results reported were compared with those obtained for only two compounds in CDCl₃ solutions (solubility problems).     

基于稳定同位素的5种蔬菜产地真实性判别

为保护蔬菜产地真实性,以上海市场5种常见蔬菜(生菜、茼蒿、辣椒、番茄和黄瓜)为研究对象,应用单因素方差分析上海和其他产地蔬菜的 δ13 C、δ15 N、δ2 H和 δ18 O值差异,并通过主成分分析(PCA)和偏最小二乘判别分析(PLS-DA)建立上海地产蔬菜真实性判别模型.结果表明,上海地产5种蔬菜总的δ15 N、δ...     

N, C and Sn NMR and other spectroscopic studies of 8-quinolinol, its O- and N-methyl derivatives, and chelate di- and tri-organotin(IV) complexes

The nature of the 8-quinolinato ligand in various forms has been examined by ¹⁵N, ¹³C and ¹¹⁹Sn NMR spectroscopy, with evidence also from electronic spectroscopy. These forms include 8-quinolinol (HQ), 8-quinolinate, the 8-hydroxyquinolinium ion, O- and N-methyl derivatives, 8-methoxyquinoline (MeQ), the zwitterionic N-methylquinolinium-8-olate and the N-methylquinolinium ion, and the chelating ligand in organotin(IV) complexes. The ¹⁵N shift from MeQ to HQ affords a measure of the intramolecular hydrogen bonding in HQ. The ¹⁵N shifts and ²J(¹⁵N¹H) couplings afford criteria of chelation, and the O- and N-methyl compounds provide useful reference points for its assessment. Evidence for chelation is demonstrated in three groups of compounds, [SnR₂Q₂] (R = Me, Et, Buⁿ, Octⁿ or Ph), [SnR₃Q] (R = Me, Et, Buⁿ or Ph) and [SnR₂ClQ] (R = Me, Et, Buⁿ or Octⁿ), the ¹⁵N and ¹¹⁹Sn shielding increasing from the [SnR₃Q] to the [SnR₂Q₂] compounds.     

Calculation of some N and C nuclear shielding parameters for some ureas and thioureas

Molecular orbital calculations using CNDO/S parameters are employed to determine bond-orders, charge-densities and the shielding of the ¹⁵N and ¹³C nuclei of a series of ureas and thioureas. For the sterically uncrowded molecules a linear relationship is noticed between the ¹⁵N chemical shifts and C---N bond-orders. When steric crowding occurs this simple relationship is no longer applicable.     

Phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D2O system as determined by 2H and 14N NMR

The high resolution phase diagram of the tetramethylammonium heptadecafluorononanoate (TMAHFN)/D₂O system has been mapped out using ²H and ¹⁴NNMR spectroscopy. The ¹⁴N quadruple splittings are more than an order of magnitude larger than corresponding ²H splittings, while the line widths are only two to three times larger. Thus, ¹⁴NNMR offers an order of magnitude improvement over ²H NMR in the resolution of the spectra from coexisting phases. The ²H spectra of samples in biphasic regions are often complicated by chemical exchange of D₂O molecules between coexisting phases, particularly at low TMAHFN concentrations. Analysis of the ²H line shapes of a TMAHFN/D₂O sample with a weight fraction of TMAHFN of 0.230 obtained at various times following cooling of the sample into the isotropic/nematic biphasic region shows that the mean diameter for the dispersed nematic droplet grows from about 7 to about 26 μm over a period of 2 h. At a mean droplet size of 7 μm the exchange of TMA⁺ ions between the coexisting phases is slow on the NMR time-scale and exchange effects are not observed in ¹⁴N spectra. The TMAHFN/D₂O phase diagram exhibits the generic form of those of the CsPFO/water and APFO/D₂O systems, which are the only other systems composed of stable discotic micelles for which high resolution phase diagrams are currently available, but the nematic phase is displaced to smaller TMAHFN concentrations. Specifically, a discotic nematic phase N_D⁺, intermediate between an isotropic micellar phase I and a lamellar phase L, exists for weight fractions of TMAHFN between 0.149 (φ = 0.105) and 0.420 (φa = 0.325) and temperatures between 277.3 and 327.6 K.     

Sensitized charge carrier injection into organic crystals studied by isotope effects in weak magnetic fields

The magnetic field (H ≈ 50 Oe) dependence of the rhodamine sensitized triplet exciton density in anthracene crystals is influenced by isotopic substitution. This confirms the hyperfine interaction as mechanism explaining the change of the spin multiplicity in the initially formed singlet state of the radical pair. The isotope effect occurs in the sensitizing dye (¹⁴N/¹⁵N) rather than at the molecular site of the injected charge within the crystal. This can be understood in terms of the high hopping frequency of the charge carriers as compared to the time constant of the hyperfine induced singlet-triplet transition. Since the dye molecules adsorb in an oriented fashion, the angular dependence of the magnetic field modulation of the triplet exciton density can be interpreted without assuming any additional interactions.     

Selectivity of methylation of some new [1,2,3]triazolo[4,5-d]pyridazines and structure elucidation by H–N NMR spectroscopy

Alkylation of some selected [1,2,3]triazolo[4,5-d]pyridazines having five or more nitrogen atoms capable for alkylation was investigated. Pyridyl derivatives substituted also on the [1,2,3]triazole ring gave quaternary pyridinium salts, whereas in the case of the analogues compounds unsubstituted at the triazole moiety, the alkylation of the triazole ring was also observed. Unambiguous structure elucidation was provided by ¹H–¹⁵N HMBC experiments which also allowed the assignment of the ¹⁵N NMR shifts.     

C, N and Sn NMR spectral evidence for tin five-coordination in triorganotin(IV) oxinates

The ¹³C and ¹¹⁹Sn NMR spectra of tri(1-butyl)tin(IV) and triphenyltin(IV) oxinates and 1-naphthoxides in neat liquid and deuteriochloroform, pentadeuteriopyridine and hexamethylphosphortriamide solutions, and the ¹⁵N NMR spectra of both the oxinates and 8-methoxyquinoline in deuteriochloroform have been recorded. From the comparison of chemical shifts δ(¹³C), δ(¹⁵N) and δ(¹¹⁹Sn) and coupling constants ⁿJ(¹¹⁹Sn¹³C) of the compounds it is concluded that the triorganotin(IV) oxinates, both as the neat liquid and in solution, form complexes containing five-coordinate tin atoms. In the neat liquid and in deuteriochloroform (a non-coordinating solvent) oxinates form chelate complexes with a cis-trigonal bipyramid arrangement. In coordinating solvents (pentadeuteriopyridine, hexamethylphosphortriamide) these are equilibria involving the formation of small amounts of oxinate complexes with one solvent molecule. These complexes have trans-trigonal bipyramid geometry with butyl or phenyl groups in equatorial plane and the monodentate oxinate group and a solvent molecule in axial positions.     

A density functional study of N NQR parameters of imidazole derivatives and extrapolation to PfHRP2–Fe-PPIX complex

In this paper, density functional theory (DFT) was used to calculate ¹⁴N nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, for a series of imidazole derivatives: imidazole, 5-methylimidazole and histidine. These calculations were carried out with the PW91P86 method via the Gaussian 98 package. A systematic theoretical investigation of the different environmental effects on (χ, η) values of amino ¹⁴N1 and imino ¹⁴N2 of imidazole ring of these compounds, reveals that the local surrounding of nitrogen atoms play an important role in determining their χ and η values. Our calculations in solution show that adding explicit solvent molecules to the polarizable continuum model (PCM) has a strong effect on (χ, η) values, thereby indicating that for long-range effects, PCM, is not sufficient to describe the whole solvent effects. We also evaluate the influence of [Fe³⁺ (S = 1/2)] on the (χ, η) values of proximal and remote nitrogens of an axial ligand and compare with those of free ligands. The results show that Fe³⁺ has a strong effect on the (χ, η) values of proximal nitrogen unlike remote nitrogen. Finally, our results predict (χ = 1.56 MHz, η = 0.690) for proximal nitrogen and (χ = 2.75 MHz, η = 0.169) for remote nitrogen in PfHRP2–Fe³⁺-PPIX complex.     

Effects of solar and artificial UV irradiation on pigments and assimilation of N ammonium and N nitrate by macroalgae

Three macroalgal species, Fucus vesiculosus, Laminaria saccharina and Ulva lactuca were exposed to natural daylight in UV-transparent and UV-non-transparent Plexiglas vessels at different water depths. Uptake of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ was measured at different light intensities. In general, uptake rates in August surpassed those in May 1987 and Ulva assimilated significantly more nitrogen than Fucus and Laminaria. A negative influence of ambient solar UV radiation on nitrogen uptake was dound in Fucus and Ulva. Additionally, the impact of UV of different wave bands, using cut-off filters (WG 295, WG 305 and WG 320) and special UV-A and UV-B lamps, on pigments and ¹⁵N ammonium assimilation by several macroalgae was studied under controlled laboratory conditions. UV-B irradiation of shorter wave bands led to a reduction in the main pigments of Fucus except antheraxanthin and those of Ulva except lutein and violaxanthin. Uptake of ¹⁵N-ammonium by Fucus and Ulva was reduced after UV-B radiation whereas an increase was observed after UV-A irradiance in Ulva. Ammonia utilization by Halidrys was more damaged by UV-A than UV-B. The impact of UV radiation on the labelling of free amino acids and the pattern of pool sizes varied with the species and the UV wave bands. A marked reduction in ¹⁵N incorporation into the amino acids of Plocamium was found after UV-B exposure. The ¹⁵N labelling of the amino acids of Halidrys and Rhodomela was less affected. Results are discussed with reference to photoinhibitory effects on the enzymes of the nitrogen metabolism.     

Intermediate phases in the dodecyltrimethylammonium chloride/water system

Two intermediate phases have been found in the concentration range between the hexagonal and concentrated cubic phases in the binary system dodecyltrimethylammonium chloride (C₁₂TACl)/water. This region in the phase diagram was studied by means of ²H NMR of specifically deuteriated surfactant as well as by ¹⁴N NMR. Below 35°C, an intermediate phase with non-cylindrical aggregates is formed in the concentration range 80 to 84 wt% surfactant, X-ray data from this phase can be indexed to a centred rectangular lattice. In addition, there is a uniaxial phase with a reduced quadrupole splitting. The aggregates comprising the centred rectangular phase were analysed by means of bandshape analysis of the NMR spectra and by small angle X-ray scattering.     

Studies on adducts of rhodium(II) tetraacetate and rhodium(II) tetratrifluoroacetate with some amines in CDCl3 solution using H, C and N NMR

¹H, ¹³C and ¹⁵N NMR spectroscopy has been applied for investigation of amine adducts with rhodium(II) tetraacetate dimer and rhodium(II) tetratrifluoroacetate dimer in CDCl₃ solution. Subsequent formation of two adducts, 1:1 and 2:1, was proved by NMR and VIS titration experiments, and by NMR measurements at reduced temperatures, from 233 to 273 K. The adduct formation shift, defined as Δδ=δ_adduct−δ_ligand and characterizing complexation reaction, varies from ca. 0 to +1.6 ppm for ¹H, from ca. −10 to +6 ppm for ¹³C and from −4.4 to −39 ppm for ¹⁵N NMR. Formation of N–Rh bond slows the inversiof on the nitrogen atom and generates, in the case of N-methyl-(1-phenylethyl)-amine, a nitrogenous chiral center in the molecule. VIS spectra of amine-dirhodium salt mixture contain two bands in the 532–597 nm spectral range, assigned to 1:1- and 2:1-adducts.     

Crystal structure of Schiff base derivative of 2,2′-dihydroxybiphenyl-3-carbaldehyde with n-butylamine

Crystals of the Schiff base derivative of 2,2′-dihydroxybiphenyl-3-carbaldehyde with n-butylamine were examined using X-ray diffraction, FT-IR and CPMAS spectroscopy. Their space group is with a=8.377(2), b=12.214(2), c=14.774(3) Å, =76.62(3)°, β=81.34(3)°, γ=86.62(3)° and Z=4. The unit cell contains two symmetry-independent zwitterions. The hydrogen atom of the protonated N atom of the Schiff base is linked to the oxygen atom of the carbonyl group at position 2, which in turn is linked to the hydroxyl group by a short hydrogen bond [molecule A: NO=2.614(3), OO=2.520(3) Å; molecule B: NO=2.594(4), OO=2.526(3) Å]. The OHO⁻H⁺N bifurcated intramolecular hydrogen bonds are crystallographically asymmetric. The results of the FT-IR, ¹H, ¹³C, ¹⁵N NMR and CPMAS study of the crystals are in agreement with the X-ray data. Instead of a continuous absorption, only a broad band is found indicating relatively low proton polarizability in the two types of the cooperative relatively short intramolecular hydrogen bonds. The ¹⁵N NMR chemical shift indicates the protonation of the Schiff base.     

An ESR on formation of iminoxy free radicals—solvent effect on hyperfine splitting constants

The solvent effect on hyperfine interaction in three different types of iminoxy radicals obtained by oxidation of di-2-pyridiketoxime (1), diethyl hydroxyimino(2-fluorophenyl)methanephosphonate (2) and isonitrosoacetophenone (3) have been analyzed. Linear correlations of hyperfine constants of ¹⁴N (both iminoxyl and pyridyl nitrogens of the iminoxyl derived from 1), ³¹P (isomer Z of the iminoxyl derived from 2) and ¹⁹F (both Z and E isomer of iminoxyls derived from 2 on E_T (30) solvent parameters have been found. Opposite directions of the dependencies for different nuclei of the same radical have been established. Anisotropic spectra of the radical derived from 1 and iminoxy radicals derived from -furildioxime have been obtained by γ-irradiation of solid oximes.     

13C N.M.R. and 14N N.Q.R. in ferroelectric liquid crystals Polar versus quadrupolar ordering

¹³C nuclear magnetic resonance and ¹⁴N nuclear quadrupole resonance spectra of ferroelectric smectic C*liquid crystals and their non-chiral analogues allow for a microscopic determination of the polar and quadrupolar (or bipolar) biasing of rotation around the long molecular axis as well as for a determination of the anisotropy in the fluctuations of this axis. The results show that the microscopic origin of the biquadratic coupling between the polarization and the tilt, which has been recently introduced into the extended Landau model of the S_A-S*_C transition, is the quadrupolar (or bipolar) rotational bias induced by the anisotropy in the fluctuations of the long molecular axis. The tilt induced anisotropy in the fluctuations is practically identical in chiral and non-chiral smectic C phases.