Indirect detection of nitrogen-14 in solid-state NMR spectroscopy.

NMR spectra of (14)N (spin I=1) are obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring "spy" nucleus with S=1/2, such as (13)C or (1)H, to single- or double-quantum transitions of (14)N nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter are also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolar coupling constant C(Q) (typically a few megahertz), on the asymmetry parameter eta(Q) of the (14)N nucleus, and on the orientation of the internuclear vector r(IS) between the I ((14)N) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional averaging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in (13)C or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.     

High resolution NMR of N in solids

¹⁵N NMR spectra, obtained in the solid state by transfer of polarization from protons, are reported for (NH₄₂SO₄, NH₄NO₅, and glycine.     

Nitrogen-14 NMR spectroscopy using residual dipolar splittings in solids

It is shown that nuclear magnetic resonance (NMR) spectra of nitrogen-14 (spin I = 1) can be obtained by indirect detection in powders spinning at the magic angle (MAS). The method relies on the transfer of coherence from a neighboring nucleus with S = 1/2, such as carbon-13, to single- or double-quantum transitions of nitrogen-14 nuclei. The transfer of coherence occurs through second-order quadrupole-dipole cross terms, also known as residual dipolar splittings. The two-dimensional NMR spectra reveal powder patterns determined by the second-order quadrupolar interactions of nitrogen-14. Analysis of the spectra yields the quadrupolar coupling constant, CQ, and asymmetry parameter, etaQ, of nitrogen-14. These parameters can be related to the structure of nitrogen-containing solids.     

High-Resolution 14N Solid-State NMR Spectroscopy

Even without expensive isotope enrichment, it is possible to obtain nitrogen NMR parameters in the solid state. The isotropic chemical shifts in hexagonal and cubic boron nitride, and for the hexagonal modification also the quadrupole coupling, can thus be obtained for the first time. The recorded ¹⁴N MAS NMR spectrum (28.809 MHz) of hexagonal boron nitride is shown on the right.     

Homonuclear J-couplings and rotationally induced sideband enhancements in NMR spectra of rotating solids

We demonstrate direct detection of ¹³C-¹³C J-couplings in magic angle sample spinning NMR spectra despite the presence of the much larger homogeneous broadening caused by the homonuclear dipolar couplings. Carbon-carbon J-couplings were observed in doubly ¹³C-enriched samples of sodium acetate, glycine and glucose. The resolved J-coupling permits carbon-carbon connectivities to be established with standard two-dimensional techniques. Interesting spectral features are observed when the rotational sidebands of the coupled spins overlap: when a sideband from a dipolar-coupled pair approaches the centerband of its partner, a significant enhancement in sideband intensity is observed as well as small shifts in the resonance frequency.     

Measuring amide nitrogen quadrupolar coupling by high-resolution 14N/13C NMR correlation under magic-angle spinning

The measurement of amide nitrogen 14N quadrupolar coupling by two-dimensional 14N/13C correlation experiment is presented with a natural abundant polypeptide. Directly bonded 14N/13C pairs are correlated through J and residual dipolar coupling under magic-angle spinning using a HMQC-type pulse sequence. The 14N quadrupolar coupling is measured from the isotropic second-order quadrupolar shift obtained by comparing the 14N peak positions with the 15N chemical shifts. The high spectral resolution and sensitivity through 13C detection make this method applicable to many organic, inorganic, and biological molecules for the measurement and the use of 14N quadrupolar coupling as a probe for molecular structure and dynamics.     

14N NMR studies on some N-sulphinylamines

High precision ¹⁴N nuclear screening data are presented for some alkyl- and aryl-N-sulphinylamines. For the alkyl compounds the β effect is seen to produce a decrease in screening and the opposite influence from the γ effect is also observed. An exception to the general tendency for the β effect is noted for (CH₃)₃CNSO. CNDO/S parameterized nitrogen screening calculations show that for this molecule the distorted cis form, with all of the methyl carbons rotated out of the CNSO plane, is the most probable structure. A similar effect is observed upon substituting the bulky tert-butyl group in the 2 position of the aryl compounds studied. Nitrogen screening calculations again indicate the presence of the distorted cis form.     

14N NMR of amminecobalt(III) compounds

Directly detected ammine ¹⁴N NMR chemical shifts of 20 amminecobalt(III) compounds are reported. The coordination shifts, δ_CS = δ_coord ? δ_free, are in all cases negative and range from ?4.4 ppm for the trans ammine ligand in [Co(NH₃)₅(CH₃)]²⁺ to ?73.6 ppm for the trans ammine ligand in [Co(NH₃)₅(F)]²⁺. Among the ligands studied, the NO₂^? ligand is unique in that it exerts a significant cis influence. The regularity in trans or cis influences upon the ammine nitrogen chemical shifts provides a basis for assignments in cases where this cannot be deduced from intensity ratios. Copyright © 2003 John Wiley & Sons, Ltd.     

14N and15N NMR spectra of 2-substituted 5-nitrofurans

The¹⁴N and¹⁵N NMR spectra of a number of 2-substituted 5-nitrofurans were studied. On the basis of experimental data it was concluded that there is considerable π-electron density on the nitrogen atom of the nitro group. A linear relationship between the chemical shifts in the¹⁴N NMR spectra and the frequencies of the asymmetrical deformation vibrations of the nitro group in the IR spectra was found for the series of investigated 5-nitrofurans. The observed¹⁵N-H spin-spin coupling constants showed that in the 5-nitrofuran molecule transmission of spin information through the ring oxygen atom to the H₂ nucleus is appreciably greater than through the carbon atom to the H₄ nucleus. It was established by measurement of the¹⁵N NMR spectra that the crude adduct formed in the nitration of furfural diacetate with acetyl nitrate is a mixture of trans and cis isomers of 5-nitro-2-acetoxy-2,5-dihydrofurfural diacetate in a ratio of 7∶1. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 741–743, June, 1980.     

Probing platinum azido complexes by 14N and 15N NMR spectroscopy

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) azido complexes is extremely broad (W(1/2)≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic相似文献   

14N NMR studies of thallium nitrate solutions in liquid ammonia

¹⁴N chemical shifts and linewidths were determined for NO ₃ ^– and NH₃ in liquid ammonia solutions of thallium nitrate at concentrations between 0.07 and 10 M. The concentration dependences of the¹⁴NO ₃ ^– shift and linewidth are consistent with the presence of C_2v ion pairs at a 2:1 mole ratio of NH₃ to TINO₃ and C_3v ion pairs at mole ratios of 3:1 or higher. Previous studies had indicated the formation of ion pairs at low concentrations. The small value of the¹⁴NO ₃ ^– linewidth below 1 M suggests that these are contact ion pairs. Studies of the¹⁴NH₃ linewidth as a function of thallium salt concentration indicate slow solvent exchange at very high concentrations.¹⁴NH₃ exhibits a downfield shift upon incorporation into the solvation sphere of the Tl⁺NO ₃ ^– ion pair, in constrast to upfield shifts reported for complexation with transition metal cations.     

14N Solid‐State NMR Spectroscopy of Amino Acids

¹⁴N ultra‐wideline solid‐state NMR (SSNMR) spectra were obtained for 16 naturally occurring amino acids and four related derivatives by using the WURST–CPMG (wideband, uniform rate, and smooth truncation Carr–Purcell–Meiboom–Gill) pulse sequence and frequency‐stepped techniques. The ¹⁴N quadrupolar parameters were measured for the sp³ nitrogen moieties (quadrupolar coupling constant, C_Q, values ranged from 0.8 to 1.5 MHz). With the aid of plane‐wave DFT calculations of the ¹⁴N electric‐field gradient tensor parameters and orientations, the moieties were grouped into three categories according to the values of the quadrupolar asymmetry parameter, η_Q: low (≤0.3), intermediate (0.31–0.7), and high (≥0.71). For RNH₃⁺ moieties, greater variation in N?H bond lengths was observed for systems with intermediate η_Q values than for those with low η_Q values (this variation arose from different intermolecular hydrogen‐bonding arrangements). Strategies for increasing the efficiency of ¹⁴N SSNMR spectroscopy experiments were discussed, including the use of sample deuteration, high‐power ¹H decoupling, processing strategies, high magnetic fields, and broadband cross‐polarization (BRAIN‐CP). The temperature‐dependent rotations of the NH₃ groups and their influence on ¹⁴N transverse relaxation rates were examined. Finally, ¹⁴N SSNMR spectroscopy was used to differentiate two polymorphs of l ‐histidine through their quadrupolar parameters and transverse relaxation time constants. The strategies outlined herein permitted the rapid acquisition of directly detected ¹⁴N SSNMR spectra that to date was not matched by other proposed methods.     

14N NMR spectroscopy of nitrofuroxans-qualitative and quantitative method for their determination

For the first time, the¹⁴N NMR spectra of nitrofuroxans were investigated, and it was shown that the¹⁴N signals are narrow; this allows this method to be utilized analytically. Taking the example of nitrochlorofuroxan, the unusual rearrangement of the 4-nitro isomer to the 3-nitro isomer was found.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1020–1023, May, 1990.