共查询到20条相似文献,搜索用时 15 毫秒
1.
2.
Manganese(II) enhanced spin-lattice relaxation rates (1/T1p) of 1H and 13C nuclei in quinoline-2-carboxylic acid, 8-methoxyquinoline-2-carboxylic acid, 8-hydroxyquinoline-2-carboxylic acid, 8-aminoquinoline-2-carboxylic acid, and 6-(hydroxymethyl)pyridine-2-carboxylic acid were measured in aqueous solution at various temperatures. Relative metal-nucleus distances were calculated from the r?6 dependence of 1/T1p. The results indicate that the Mn2+ ion in the 8-methoxyquinaldic acid chelate is coordinated to the carboxyl oxygen atom and the nitrogen atom but not t the methoxyl oxygen atom. 相似文献
3.
Infrared measurements of the dicroic ratio of poly-(methyl methacrylate) absorption bands provide a valuable method for determining the orientation as well as the relaxation of chains during stretching. Different strain rates and temperatures of stretching were used. Orientation relaxation was determined and a master curve was obtained at a reference temperature TO = 135°C. The master curve shows that orientation relaxation behaves similarly to mechanical relaxation. 相似文献
4.
The presence of small amounts of CO2 in poly(vinyl chloride) (PVC) results in increased main-chain molecular motions of the polymer as measured by the carbon rotating-frame relaxation rate. This effect increases with increasing gas concentration. Since molecular motions of the polymer and the diffusion coefficient of the gas are related, the latter must be concentration dependent. Main-chain motions of PVC also increase upon exposure to CO2 followed by degassing. This result is interpreted in terms of the effect of the penetrant gas on the interchain packing in amorphous PVC. These results cannot be reconciled with the dual-sorption-mobility model, which claims that gas molecules preferentially occupy preexisting sorption sites in a conditioned polymer with no perturbation of the polymer matrix. 相似文献
5.
Ng S Sathasivam RV Lo KM Xie Y Schaefer HF 《The journal of physical chemistry. A》2005,109(51):12059-12063
The Woessner approach is applied to the 13C relaxation data for tetraphenyltin (1) and tetra(p-tolyl)tin (2) in CDCl3 solution over the temperature range 5-42 degrees C to obtain correlation times for rotational motions and hence the activation barriers. Quantum mechanical computations were carried out to obtain the rotational energy barriers for comparison. For 2 the relaxation data indicate (1) slower ring rotation than in 1, (2) highly hindered internal rotation of the methyl group. IR and chemical shift data support the hypothesis of hyperconjugation of the methyl correlated with interaction between the pi-electrons and the 5d orbitals of tin in the (p-tolyl)Sn moiety to account for the hindrances to the rotations of the ring and the methyl. The activation barrier for the tolyl group rotation is found to be much higher than that for the phenyl rotation. However, the Woessner approach yields an anomalously high barrier for the methyl rotation. An explanation based on correlated rotations of the tolyl ring and the methyl is offered. 相似文献
6.
The binding of manganese(II) with nucleosides — adenosine (A), guanosine (G), cytidine (C), and uridine (U) — in an alkaline D2O solution has been investigated by the13C NMR method. It has been established that the structure of the paramagnetic Mn(II)—nucleoside complexes differs substantially in neutral and in alkaline media. The broadening of the resonance lines (C-2′, C-3′ > C-1′, C-4′ > C-5′) shows the localization of the Mn(II) in the C-2′ and C-3′ hydroxyls of the ribose in an alkaline medium. It has been shown for the case of U that the degree of complex-formation depends on the pH of the solution. It is assumed that the nucleoside forms intramolecular complexes (I) with Mn(OH)2. 相似文献
7.
A. Spyros Photis Dais R. H. Marchessault 《Journal of Polymer Science.Polymer Physics》1995,33(3):367-378
Variable-temperature 13C NMR spin-lattice relaxation times (T1) and Nuclear Overhauser Enhancements (NOE) at two magnetic fields have been used to study the dynamics of the amorphous part of a semicrystalline sample (33% of crystallinity) of poly(β-hydroxyoctanoate) (PHO). The interpretation of the relaxation data of the backbone carbons was made by employing a number of motional models. Among these, the DLM model offered the best interpretation of the relaxation data in terms of conformational transitions and librational motions of the backbone C? H vectors, and proved to be superior to unimodal distribution functions. The interpretation of temperature- and frequency-dependent T1 and NOE data of the carbon nuclei in the n-pentyl side chain was made by employing a newly developed composite spectral density function for multiple internal C? C bond rotations of restricted amplitude and chain segmental motion. The temperature dependence of the linewidths of the various protonated carbon resonances of PHO has been discussed in terms of the semicrystalline character of this polymer. © 1995 John Wiley & Sons, Inc. 相似文献
8.
The pK(a) values of ionisation of a set of phenols ortho, meta and para substituted are studied by spectrophotometry and (13)C NMR spectroscopy. A dual substituent analysis of equilibrium and NMR results, according to the Swain and Lupton procedure, is presented. The results of this analysis allow the assignment of the contribution of field and resonance contributions, both on equilibrium constants and chemical shifts. 相似文献
9.
Oxidized human [2Fe-2S] ferredoxin has a notably slow electronic relaxation rate, which precludes the observation of signals from nuclei near the iron-sulfur cluster by conventional 2D or 3D methods that utilize proton detection. We have demonstrated the utility of (13)C[(13)C]CT-COSY in identifying connectivity information from fast relaxing carbon nuclei near the paramagnetic center, including those from residues that ligate the metal center. 相似文献
10.
The stability of an inclusion complex of quinuclidine with alpha-cyclodextrin in solution was investigated by NMR measurements of the translational diffusion coefficient. A 1:1 stoichiometry model yielded an association constant of 35 +/- 3 M(-1). The guest molecules exchange rapidly between the host cavity and the bulk solution. The reorientational dynamics of the guest and host molecules was studied using carbon-13 NMR relaxation at two magnetic fields. The relaxation of the host nuclei showed very little dependence on the guest-host concentration ratio, while the 13C spins in quinuclidine were sensitive to the solution composition. Using mole-fraction data, it was possible to extract the relaxation parameters for the bound and free form of quinuclidine. Relaxation rates of the guest molecule, free in solution, were best described by an axially symmetric model, while the data of the complex species were analyzed using the Lipari-Szabo method. Applying the axially symmetric model to the complexed quinuclidine indicated that the anisotropy of its reorientation in the bound form was increased. 相似文献
11.
12.
Carbon-13 NMR data are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, RC6H4Sn(CH3)3, where R = para-N(CH3)2, para-OCH3, para-OC2H5, para-CH3, meta-CH3, -H, para-F, meta-OCH3, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH3)3. In the para-derivatives, correlation coefficients with Hammet σ-constants of greater than ca. 0.9 are obtained with the tin-carbon couplings to methyl, C1 and C4 carbons, and with the carbon-13 chemical shifts δ(C(1)). In the meta-derivatives, the couplings |J(Sn-CH3)|:, |:J(Sn-C(1))|:, |:J(Sn-C(3))|: and |:J(Sn-C(6))|:, and the shifts δ(C(1)) and δ(C(5)) correlate well with Hammett σ. In the para-derivatives, sensitivity to change in substituent falls off C(4) > C(3, 5) > C(1) > C(2, 6) > CH3 as registered by the δ(C), while in the meta-derivatives δ(C) changes decrease C(3) > C(2), C(4) > C(1) > C(5), C(6) > CH3. The magnitudes of the tin coupling constants decrease C(1) > CH3 > C(3, 5) > C(2, 6) > C(4) in the para-derivatives, while in the meta-series the order is C(1) > CH3 > C(3), C(5) > C(2) > C(6) > C(4). The two sets of one-bond |:J(Sn-CH3)|: and |:J(Sn-C(1)|: values correspond closely to the 0.25/0.33 ratio of coefficients in the LCAO approach, and are interpreted in terms of s-electron redistributions at the tin atom with change in substituent. 相似文献
13.
Frederick T. Greenaway James C. Dabrowiak Robert Grulich Stanley T. Crooke 《Magnetic resonance in chemistry : MRC》1980,13(4):270-273
Using the carbon assignments of bleomycin A2 as well as those of a series of model compounds, the 13C NMR assignments of the antitumor antibiotic tallysomycin (TLM) have been made. With the aid of the carbon assignments the metal binding properties of the new antibiotic toward Zn(II) have been investigated. 相似文献
14.
15.
16.
The transverse magnetic relaxation of 13Cα nuclei has been studied in concentrated solutions of polystyrene. The magnetic relaxation rate was measured as a function of molecular weight at several temperatures (313,318, and 323 K) and at several concentrations (0.53, 0.43, and 0.34 g/cm3). The spin-system response of these nuclei in natural abundance exhibits a characteristic evolution from pseudosolid properties to liquidlike one, induced by decreasing the molecular weight of polymer molecules. This evolution is analogous to that already observed in protons attached to polyisobutylene or polydimethylsiloxane chains; it is assumed to be induced by an increase of the disentanglement rate of polymer chains. The spin-system response may be considered as reflecting single-chain magnetic properties, because of the low concentration of 13CCα nuclei, although all chains are in dynamic interaction with one another. The NMR disentanglement transition is interpreted in terms of a two-step motional averaging effect involving submolecules. A numerical analysis of NMR properties is given using a model of polymer chain relaxation based on a multiple-mode relaxation process, characterized by (i)a terminal relaxation time τv1 depending upon M3, the molecular weight, and approximately proportional to the polymer concentration C (like the reptation time); (ii)a relaxation-time spectrum analogous to a Rouse spectrum; (iii)a terminal relaxation time τv1 = 2.5 × 10?2s for M = 2.5 × 105, C = 0.53 g/cm3 in carbon tetrachloride at 313 K. 相似文献
17.
The complete labeling pattern of ergosterol isolated from Saccharomyces cerevisiae grown with [1-13C] acetate is detailed and the results of this study have led to a reassignment of carbons 5, 8, 12, 16, 22 and 23 in the 13C n.m.r. spectrum of ergosterol. 相似文献
18.
《Journal of Inorganic and Nuclear Chemistry》1980,42(9):1253-1255
The reaction of bis(disulfurdinitride)nickel(II), Ni(S2N2H)2, with various aldeleydes and amines yields a condensation product in which the protons of the two planar ring S2N2 ligands have been replaced by an organobridging ligand. Some of these complexes contain two chiral carbon atoms and the separation of the diasteroisomers of one complex is described. 13C NMR spectra are reported for these complexes and the assignments are in agreement with proton NMR data. The IR and electronic spectra of the complexes are also reported. 相似文献
19.
Carbon-13 nuclear spin relaxation in 1,8-bis(dimethylamino)naphthalene (DMAN) was investigated in a dimethylformamide- d 7 solution. In addition, the chemical shielding tensors were measured in the crystalline powder. Detailed analysis of (13)C longitudinal relaxation in this molecule yielded its rotational diffusion tensor. Comparison to the protonated form of DMAN, DMANH(+), leads to conclusions concerning interaction of the latter with its counterion. 相似文献
20.
《Magnetic resonance in chemistry : MRC》2018,56(4):276-284
Multinuclear solid‐state NMR and powder X‐ray diffraction data collected for phosphonate materials Zr(O3PC6H4PO3) · 3.6H2O and Sn(O3PC6H4PO3)0.85(O3POH)0.30 · 3.09H2O have resulted in the layered structure, where the phosphonic acids cross‐link the layers. The main structural motif (the 111 connectivity in the PO3 group) has been established by determination of chemical shift anisotropy parameters for phosphorus nuclei in the phosphonate groups. An analysis of the variable‐temperature 31P T1 measurements and the shapes of the phosphorus resonances in the 31P static NMR spectra have resulted in the dipolar mechanism of the phosphorus spin‐lattice relaxation, where the rotating phenylene rings reorient dipolar vectors P…H as a driving force of the relaxation process. It has been found that water protons do not affect the 31P T1 times. The activation energy of the phenylene rotation in both compounds has been determined as low as 12.5 kJ/mol. The interpretation of the phosphorus relaxation data has been independently confirmed by the measurements of 1H T1 times for protons of the phenylene rings. 相似文献