13C NMR of tunnelling methyl groups

The dipolar interactions between the protons and the central ¹³C nucleus of a ¹³CH₃ group are used to study rotational tunnelling and incoherent dynamics of such groups in molecular solids. Single-crystal ¹³C NMR spectra are derived for arbitrary values of the tunnel frequency ν_t. Similarities to ESR and ²H NMR are pointed out. The method is applied to three different materials. In the hydroquinone/acetonitrile clathrate, the unique features in the ¹³C NMR spectra which arise from tunnelling with a tunnel frequency that is much larger than the dipolar coupling between the methyl protons and the ¹³C nucleus are demonstrated, and the effects of incoherent dynamics are studied. The broadening of the ¹³C resonances is related to the width of the quasi-elastic line in neutron scattering. Selective magnetization transfer experiments for studying slow incoherent dynamics are proposed. For the strongly hindered methyl groups of L-alanine, an upper limit for ν_t is derived from the ¹³C NMR spectrum. In aspirin? (acetylsalicylic acid), incoherent reorientations dominate the spectra down to the lowest temperatures studied; their rate apparently increases with decreasing temperature below 25 K.     

Measurement of Dipolar Couplings for Methylene and Methyl Sites in Weakly Oriented Macromolecules and Their Use in Structure Determination

A simple and effective method is described for simultaneously measuring dipolar couplings for methine, methylene, and methyl groups in weakly oriented macromolecules. The method is aJ-modulated 3D version of the well-known [¹H-¹³C] CT-HSQC experiment, from which theJand dipolar information are most accurately extracted by using time-domain fitting in the third, constant-time dimension. For CH₂-sites, the method generally yields only the sum of the two individual¹³C-¹H couplings. Structure calculations are carried out by minimizing the deviation between the measured sum, and the sum predicted for each methylene on the basis of the structure. For rapidly spinning methyl groups the dipolar contribution to the splitting of the outer¹³C quartet components can be used directly to constrain the orientation of the C-CH₃bond. Measured sidechain dipolar couplings are in good agreement with an ensemble of NMR structures calculated without use of these couplings.     

High-Temperature Phase Transition in N(CH3)4CdCl3 Studied Using Static NMR and MAS NMR

To clarify the nature of microscopic structural changes of N(CH₃)₄CdCl₃ at high temperatures, the nuclear magnetic resonance (NMR) spectra of the protons and carbons in N(CH₃)₄CdCl₃ were measured. NMR studies of the ¹H and ¹³C spin–lattice relaxation time, T _1ρ, in the rotating frame were also performed. No changes in the T _1ρ of ¹H and ¹³C associated with the N(CH₃)₄ groups were observed at the high-temperature transition from phase I to phase I′. However, the ¹⁴N NMR spectra reflected changes in the structural geometry during the transition to phase I′, indicating that this transition is driven by N(CH₃)₄.     

3D 1H-13C-14N correlation solid-state NMR spectrum

Nitrogen-14 (spin I = 1) has always been a nucleus difficult to observe in solid-state NMR and until recently its observation was restricted to one-dimensional (1D) spectra. We present here the first 3D ¹H–¹³C–¹⁴N NMR correlation spectrum. This spectrum was acquired on a test sample l-histidine·HCl·H₂O using a recently developed technique, which consists in indirectly observing ¹⁴N nuclei via dipolar recoupling with an HMQC-type experiment.     

Structural and 14N EFG Information on Solid Imidazole by 13C CP/MAS NMR Data

Residual dipolar coupling between carbons and ¹⁴N nuclei in the ¹³C CPMAS NMR spectrum of solid imidazole is studied. Calculations of expected splittings with a previously reported equation leads to the complete assignment of the solid state carbon chemical shifts. Additionally, information is provided on the location of ¹⁴N electric field gradient axes at the N-H site.     

C Chemical Shift and C–N Dipolar Coupling Tensors Determined by C Rotary Resonance Solid-State NMR

This work explores the utility of simple rotary resonance experiments for the determination of the magnitude and orientation of ¹³C chemical shift tensors relative to one or more ¹³C–¹⁴N internuclear axes from ¹³C magic-angle-spinning NMR experiments. The experiment relies on simultaneous recoupling of the anisotropic ¹³C chemical shift and ¹³C–¹⁴N dipole–dipole coupling interactions using 2D rotary resonance NMR with RF irradiation on the ¹³C spins only. The method is demonstrated by experiments and numerical simulations for the ¹³C^α spins in powder samples of -alanine and glycine with ¹³C in natural abundance. To investigate the potential of the experiment for determination of relative/absolute tensor orientations and backbone dihedral angles in peptides, the influence from long-range dipolar coupling to sequential ¹⁴N spins in a peptide chain (¹⁴N_i–¹³C^α_i–¹⁴N_i+1 and ¹⁴N_i+1–¹³C′_i–¹⁴N_i three-spin systems) as well as residual quadrupolar–dipolar coupling cross-terms is analyzed numerically.     

Two- and Three-Dimensional H/C PISEMA Experiments and Their Application to Backbone and Side Chain Sites of Amino Acids and Peptides

Two-dimensional ¹H/¹³C polarization inversion spin exchange at the magic angle experiments were applied to single crystal samples of amino acids to demonstrate their potential utility on oriented samples of peptides and proteins. High resolution is achieved and structural information obtained on backbone and side chain sites from these spectra. A triple-resonance experiment that correlates the ¹H–¹³C_α dipolar coupling frequency with the chemical shift frequencies of the α-carbon, as well as the directly bonded amide ¹⁵N site, is also demonstrated. In this experiment the large ¹H–¹³C_α heteronuclear dipolar interaction provides an independent frequency dimension that significantly improves the resolution among overlapping ¹³C resonances of oriented polypeptides, while simultaneously providing measurements of the ¹³C_α chemical shift, ¹H–¹³C dipolar coupling, and ¹⁵N chemical shift frequencies and angular restraints for backbone structure determination.     

Inner shell electron energy loss spectra of the methyl amines and ammonia

Electron energy loss Spectroscopy has been used to obtain the inner shell excitation spectra of the methyl amines CH₃NH₂, (CH₃)₂NH and (CH₃)₃N for both the N 1s and C 1s regions. A spectrum of the N 1s region of NH₃ is also presented at higher resolution than previously published data. The C ls spectra are all very similar and the discrete portions may be assigned to Rydberg transitions. However, features attributable to a σ^* shape resonance are observed just above the N 1s and C ls ionization edges. The NH₃ spectrum is ascribed to Rydberg transitions. The N 1s spectra of the methyl amines, however, become increasingly dominated by a σ* resonance in the continuum with increased methylation. The features in the inner-shell spectra are compared with the reported valence-shell optical absorption spectra and support the Rydberg assignment. The inner-shell spectra of (CH₃)₃N and NH₃ are also compared with previously published inner shell electron energy loss spectra of NF₃ and the third row phosphorus analogues PF₃,P(CH₃)₃andPH₃.     

Crystal structure,NMR spectroscopy,electrical properties of catena-poly[(bis-glycinium-k 2 O:O)-di-μ-thiocyanate- k 2 N:S; k 2 S:N -cadmium (II)]

The crystal structure, the ¹¹¹Cd and ¹³C NMR spectroscopy, and the complex impedance have been carried out on a new polymeric hybrid compound: [Cd(NH₃CH₂COO)₂(SCN)₂] _n . Crystal structure shows that in the title compound the cadmium atoms have a 2N2S2O-hexa-coordination sphere, exhibiting pseudo-octahedral geometry. The cadmium atoms are bridged by two thiocyanate ions generating 1D polymeric chains. ¹¹¹Cd and ¹³C MAS NMR spectroscopy show multiplets that result from ¹¹¹Cd, ¹⁴N and ¹³C, ¹⁴N spin–spin coupling, respectively. The AC impedance measurements were performed as a function of both frequency and temperature. The AC and DC electrical conduction have been studied. The activation energy associated with the bulk resistance determined from equivalent circuit was found close to that of the activation energy obtained from DC conductivity. The conduction mechanisms are attributed to the correlated barrier hopping model.     

Submillimeter spectrum of methyl bromide (CH3Br)

Methyl bromide is a ubiquitous component of the atmosphere, but has yet to be remotely detected in the upper atmosphere. Due to the strong ozone depletion capability of the activated bromine species, the total atmospheric bromine load needs to be carefully monitored. Combined analysis of precise measurements and cataloging of the rotational spectrum of methyl bromide may enable its concentration to be monitored with future remote sensing instrumentation. In an effort to extend and improve previous work for this molecule, the spectrum of CH₃Br has been measured at JPL. Using an isotopically enriched ¹³CH₃Br (90%) sample, spectra have been recorded from 750 to 1200 GHz. Quantum number assignments cover the CH₃⁷⁹Br, CH₃⁸¹Br, ¹³CH₃⁷⁹Br and ¹³CH₃⁸¹Br isotopologues with J < 66 and K < 17 for the ground and ν₃ vibrational states. The dataset for the ¹²C isotopologues is more precise than previous THz measurements resulting in reductions of rotational and distortion parameter uncertainties by factors of 2-15. Parameters of the ν₃ state of the ¹²C isotopologues are improved by 2-105. The spectra of the ¹³C isotopologues are the first reported beyond J = 2.     

Rotational spectrum of acetonitrile CH3CN and CH3CN in a doubly excited degenerate vibrational state

The rotational spectrum of the acetonitrile molecules CH₃C¹⁴N and CH₃C¹⁵N in the degenerate vibrational state v₈ = 2 has been studied for the transitions J = 0 → 1, 2 → 3, 3 → 4, 4 → 5, 5 → 6, 6 → 7, and 7 → 8.     

Anisotropies in carbon-13 chemical shift tensor,H-C-H bond angles and the signs of the coupling constants in 13CH3I, 13CH3Br, 13CH3Cl

A pulsed fast Fourier transform N.M.R. spectrometer is used to investigate the temperature dependence of the ¹³C chemical shifts in the nematic liquid crystal solutions of the following halides: ¹³CH₃I, ¹³CH₃Br, ¹³CH₃Cl. The H-C-H bond angles are accurately measured in this investigation from proton magnetic resonance studies. The signs of the coupling constants D _13CH , D _HH, J _13CH and the ordering parameter have been assigned to be all positive. The ¹³C magnetic shielding anisotropy is found to become more positive with the increasing electronegativity of the halogen substituents. The values of the anisotropies are: Δσ = σ _∥ - σ _⊥ = - 101 ± 15 p.p.m. (¹³CH₃I), - 10 ± 5 p.p.m. (¹³CH₃Br), 22 ± 5 p.p.m. (¹³CH₃Cl). Results for Δσ obtained from the slope of σ_n versus S ₃₃ agree with those obtained from the nematic-isotropic phase subtraction method.     

Recoupled Polarization-Transfer Methods for Solid-State H–C Heteronuclear Correlation in the Limit of Fast MAS

An in-depth account of the effects of homonuclear couplings and multiple heteronuclear couplings is given for a recently published technique for ¹H–¹³C dipolar correlation in solids under very fast MAS, where the heteronuclear dipolar coupling is recoupled by means of REDOR π-pulse trains. The method bears similarities to well-known solution-state NMR techniques, which form the framework of a heteronuclear multiple-quantum experiment. The so-called recoupled polarization-transfer (REPT) technique is versatile in that rotor-synchronized ¹H–¹³C shift correlation spectra can be recorded. In addition, weak heteronuclear dipolar coupling constants can be extracted by means of spinning sideband analysis in the indirect dimension of the experiment. These sidebands are generated by rotor encoding of the reconversion Hamiltonian. We present generalized variants of the initially described heteronuclear multiple-quantum correlation (HMQC) experiment, which are better suited for certain applications. Using these techniques, measurements on model compounds with ¹³C in natural abundance, as well as simulations, confirm the very weak effect of ¹H–¹H homonuclear couplings on the spectra recorded with spinning frequencies of 25–30 kHz. The effect of remote heteronuclear couplings on the spinning-sideband patterns of CH_n groups is discussed, and ¹³C spectral editing of rigid organic solids is shown to be practicable with these techniques.     

Solid State C NMR of 30-Crown-10 Ether and 30-Crown-10.4H2O

The ¹³C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state ¹³C NMR spectrum of the 30-crown-10.4H₂O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable ¹³C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic ¹³C shieldings for simpler model compounds. Results of dipolar dephased ¹³C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H₂O, indicating that it also is relatively rigid in the solid.     

An experimental approach to the analysis of electric field gradients in ionic crystals

The spectrum of a partially oriented sample of ethanol has been analysed by making use of the simpler spectra obtained from the species CD₃CH₂OH and CH₃CD₂OH, together with ¹H?{²H} double resonance. With p-ethoxy-benzylidene-p-n-butylaniline (EBBA) as the nematic solvent the dipolar couplings of CH₃ and CH₂ protons with the OH proton can be observed, and their magnitudes are compared with values calculated assuming different models for C-O-H internal rotation. Information on the quadrupole coupling constant tensor elements for CD₃ and CD₂ deuterium nuclei is obtained.     

微波等离子体化学汽相沉积法沉积CNx膜的研究

对用微波等离子体化学汽相沉积法沉积在Si基片上的CN_x膜分别进行Raman散射、X射线光电子能谱、X射线衍射和扫描电子显微镜等技术的分析与测试. Raman散射的研究结果表明在CH₄与N₂的流量比低于1∶8时，CN_x膜的散射谱中以非晶石墨峰的形式出现.当流量比为1∶8时，则表现为较尖锐的C≡N键(2190cm^-1)的特征峰；从X射线光电子能谱的分析结果可以看出C，N成键的方式主要是C≡N键和C—N 关键词：     

Evidence for an anisotropic contribution to 3 J FF from the 19F N.M.R. spectrum of hexafluorocyclopropane dissolved in a nematic phase

Analysis of the fluorine N.M.R. spectrum of a sample of hexafluorocyclopropane dissolved in p-ethoxybenzylidene-p-n-butylaniline (EBBA) has given the magnitudes and relative signs of three anisotropic and three isotropic coupling constants. The anisotropic couplings ² T _FF and ³ T _FF(cis) appear to be entirely dipolar in origin, whereas the value of ³ T _FF(trans) can be considered as 86 per cent dipolar, the remaining 14 per cent being attributed to an anisotropy in the electron-coupled spin-spin interaction.     

Determination of Reorientation Angles from Two-Dimensional Exchange Spectra of Powders in Radiospectroscopy

A method for transforming a 2D exchange spectrum into an orientation representation is proposed which is based on finding the reorientation probability as a function of angles. A procedure of evaluation and analysis of two-dimensional magnetic spectra of powders is described and a method for finding the asymmetry parameter from 2D exchange spectra is proposed. The capabilities of the method are illustrated for a model ³⁵Cl 2D nutation exchange NQR spectrum of chloral hydrate CCl₃CH(OH)₂ and an experimental ¹³C 2D exchange NMR spectrum of dimethylsulphone CH₃–SO₂–CH₃. The asymmetry parameter of the chemical shift tensor for this compound is determined.     

Separating Structure and Dynamics in CSA/DD Cross-Correlated Relaxation: A Case Study on Trehalose and Ubiquitin

We present two new sensitivity enhanced gradient NMR experiments for measuring interference effects between chemical shift anisotropy (CSA) and dipolar coupling interactions in a scalar coupled two-spin system in both the laboratory and rotating frames. We apply these methods for quantitative measurement of longitudinal and transverse cross-correlation rates involving interference of ¹³C CSA and ¹³C–¹H dipolar coupling in a disaccharide, α,α- -trehalose, at natural abundance of ¹³C as well as interference of amide ¹⁵N CSA and ¹⁵N–¹H dipolar coupling in uniformly ¹⁵N-labeled ubiquitin. We demonstrate that the standard heteronuclear T₁, T₂, and steady-state NOE autocorrelation experiments augmented by cross-correlation measurements provide sufficient experimental data to quantitatively separate the structural and dynamic contributions to these relaxation rates when the simplifying assumptions of isotropic overall tumbling and an axially symmetric chemical shift tensor are valid.     

Theoretical study on the reactions of CH3NHNH2 with ground state O(3P) atom and excited state O(1D) atom

The reaction mechanisms of methylhydrazine (CH₃NHNH₂) with O(³P) and O(¹D) atoms have been explored theoretically at the MPW1K/6-311+G(d,p), MP2/6-311+G(d,p), MCG3-MPWPW91 (single-point), and CCSD(T)/cc-pVTZ (single-point) levels. The triplet potential energy surface for the reaction of CH₃NHNH₂ with O(³P) includes seven stable isomers and eight transition states. When the O(³P) atom approaches CH₃NHNH₂, the heavy atoms, namely N and C atoms, are the favourable combining points. O(³P) atom attacking the middle-N atom in CH₃NHNH₂ results in the formation of an energy-rich isomer (CH₃NHONH₂) followed by migration of O(³P) atom from middle-N atom to middle-H atom leading to the product P6 (CH₃NNH₂+OH), which is one of the most favourable routes. The estimated major product CH₃NNH₂ is consistent with the experimental measurements. Reaction of O(¹D) + CH₃NHNH₂ presents different features as compared with O(³P) + CH₃NHNH₂. O(¹D) atom will first insert into C–H2, N1–H4, and N2–H5 bonds barrierlessly to form the three adducts, respectively. There are two most favourable paths for O(¹D) + CH₃NHNH₂. One is that the C–N bond cleavage accompanied by a concerted H shift from O atom to N atom (mid-N) leads to the product P_I (CH₂O + NH₂NH₂), and the other is that the N–N bond rupture along with a concerted H shift from O to N (end-N) forms P_IV (CH₃NH₂ + HNO). The similarities and discrepancies between two reactions are discussed.