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.     

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

Abstract

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.     

Determination of one-bond 14N?13C coupling constants in amines using 13C T1ρ measurements

¹³C T_1ρ values measured for isobutylamine, diethylamine, pyrrolidine, piperidine and triethylamine yield one-bond ¹⁴N? ¹³C coupling constants and ¹⁴N spin-lattice relaxation times. A decrease of ¹J(¹⁴N¹³C) was observed in sterically hindered secondary amines.     

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

Abstract

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.     

A 13C NMR investigation of some tetrakisisocyanocopper(I) tetrafluoroborate complexes: Chemical shift of,and coupling to,the isocyano carbon

The ¹³C n.m.r. spectra twelve isocyanides, RNC, and their Cu(I) complexes, Cu(RNC)₄BF₄, were recorded in the solvents CDCl₃ and DMSO. The resonances of the isocyano carbon, C⁰, could be observed for both free and coordinated isocyanides. In the spectra of the complexes this C⁰ resonance was present as a broad line. For some complexes the ¹⁴N? ¹³C⁰ coupling could only be detected after heating the sample or by adding small amounts (<7 mo1%) of Cu(CH₃CN)₄BF₄. For the remaining complexes neither J(¹⁴N? ¹³C⁰) nor J(¹⁴N? ¹³C¹), could be detected. No carbon-copper coupling could be detected. The line shape of C⁰ is discussed in terms of exchange of an isocyanide ligand and in terms of quadrupolar nitrogen and copper relaxation. In most complexes exchange seems to be dominant, whereas in other complexes quadrupolar nitrogen relaxation is also of importance. On coordination of the isocyanide, the C⁰ resonance shifts upfield (15 to 20 ppm) and J(¹⁴N? ¹³C⁰) shows a threefold increase. The upfield shift is associated with a larger excitation energy while the increase of J(¹⁴N? ¹³C⁰) is ascribed to a larger effective nuclear charge on C⁰. The chemical shifts of the α-carbon, the β-carbon and the solvent effect on all n.m.r. parameters in both free and coordinated isocyanides are also discussed.     

Chemical polarisation of 13C and 15N nuclei in the thermal decomposition of diazoaminobenzene (1,3-diphenyltriazene)

Chemical polarisation of ¹³C and ¹⁵N nuclei in the thermal decomposition of diazoaminobenzene has been investigated. Most of the polarised products with the exception of benzene result from cage recombination processes, and both the ¹³C and ¹⁵N true nuclear polarisation coefficients are rather high, up to 10000 and more. ¹³C CIDNP has been shown to provide a new and useful technique for the measurement of ¹³C spin-lattice relaxation time T₁. Free radical reactions of organic compounds and the corresponding CIDNP mechanisms can be studied very successfully using proton spectra. Since the hyperfine coupling constants with ¹³C and ¹⁵N nuclei can, however, be considerably larger, the nuclear relaxation times longer and the spectra generally much simpler and easier to interpret in detail, the study of heavier nuclei provides many advantages over ¹H-CIDNP, particularly in the case of σ-radicals.     

Rational Design of [13C,D14]Tert‐butylbenzene as a Scaffold Structure for Designing Long‐lived Hyperpolarized 13C Probes

Dynamic nuclear polarization (DNP) is a technique to polarize the nuclear spin population. As a result of the hyperpolarization, the NMR sensitivity of the nuclei in molecules can be dramatically enhanced. Recent application of the hyperpolarization technique has led to advances in biochemical and molecular studies. A major problem is the short lifetime of the polarized nuclear spin state. Generally, in solution, the polarized nuclear spin state decays to a thermal spin equilibrium, resulting in loss of the enhanced NMR signal. This decay is correlated directly with the spin‐lattice relaxation time T₁. Here we report [¹³C,D₁₄]tert‐butylbenzene as a new scaffold structure for designing hyperpolarized ¹³C probes. Thanks to the minimized spin‐lattice relaxation (T₁) pathways, its water‐soluble derivative showed a remarkably long ¹³C T₁ value and long retention of the hyperpolarized spin state.     

Nuclear magnetic triple resonance studies on formamide-14N

The magnitudes of the two spin-spin coupling constants J(N¹H …? ¹³C), and all relatives signs of the couplings J(¹H …? ¹H) and J(¹H …? ¹³C) of formamide-¹⁴N, were determined by triple resonance experiments of the types ¹H—{¹³C}—{¹⁴N} and ¹H—{¹H}—{¹⁴N}.     

Rapid Acquisition of 14N Solid‐State NMR Spectra with Broadband Cross Polarization

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR‐active isotopes, ¹⁴N and ¹⁵N, solid‐state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first‐order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality ¹⁴N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen‐containing systems. We demonstrate that it is possible to dramatically enhance ¹⁴N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., ¹H). The BR oadband A diabatic IN version C ross‐ P olarization ( BRAIN–CP ) pulse sequence is combined with other elements for efficient acquisition of ultra‐wideline SSNMR spectra, including W ideband U niform‐ R ate S mooth‐ T runcation ( WURST ) pulses for broadband refocusing, C arr– P urcell M eiboom– G ill ( CPMG ) echo trains for T₂‐driven S/N enhancement, and frequency‐stepped acquisitions. The feasibility of utilizing the BRAIN–CP/WURST–CPMG sequence is tested for ¹⁴N, with special consideration given to (i) spin‐locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN–CP experiments are shown to provide increases in signal‐to‐noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where ¹⁴N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.     

High-Resolution Solid-State NMR Characterization of Morphology in Annealed Polylactic Acid

Single-pulse ¹³C NMR spectra and spin-lattice relaxation times T₁(¹H), detected indirectly via ¹³C carbons, and T₁(¹³C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the ¹³C NMR lines and proton spin-lattice relaxation times T₁(¹H). The spin-lattice relaxation times T₁(¹³C) related to the respective carbons of the α-polymorph of PLA are also discussed in the study.     

Chain behavior of an amphoteric cellulose derivative: O-carboxymethyl-O-2-hydroxy-3-(trimethylammonio) propylcellulose

The ¹³C NMR spin-lattice relaxation times (T₁) of anhydroglucose units vary with the number of substituents, and the T₁ values of unsubstituted anhydroglucose units of O-carboxymethylcellulose are longer than those of amylose. Those results indicate that in water, the rotational motions of anhydroglucose units of cellulose derivative are quite important local motions contributing to the ¹³C NMR spin-lattice relaxation, and within a cellulose chain, anhydroglucose units rotate with different degrees of freedom depending on their environment. Moreover, the ¹³C NMR spin-lattice relaxation data indicate that the mobilities of ionic substituents are dependent on substitution positions as well as their ionic interaction. © 1995 John Wiley & Sons, Inc.     

Thermotropic ionic liquid crystals of pyridinium octylphosphate A N.M.R. and X-ray study

Abstract

A thermotropic ionic lamellar phase from non-stoichiometric pyridinium octyl-phosphates has been investigated by multinuclear N.M.R. and X-ray diffraction. At room temperature and above, this phase is formed for pyridine to octylphosphoric acid molar ratios from 0.2 to 0.8.²H and ¹³C relaxation experiments show that the pyridinium ion undergoes a very anisotropic motion with D_zz > D_xx ? D_yy, z and x being the perpendicular direction to the ring and the c ₂ symmetry axis, respectively. The order parameters given by the ²H quadrupolar splittings and the ¹³C chemical shift anisotropy (CSA) are S_zz = 0.13, S_yy = -0.08 and S_xx = -0.05, showing that the pyridinium ring is preferentially oriented parallel to the lamellar plane. The ³¹P CSA and the C1-P dipolar splitting yield S_zz = 0.33 and S_xx ? S_yy for the octylphosphate anion. The order parameters of alkyl C-H bonds have been obtained from the J resolved two-dimensional ¹³C N.M.R. spectra of oriented samples. Two limiting conformational models have been considered to calculate the S _CH. One of them is reasonably consistent with the structure derived from X-ray experiments and has been used to calculate the dipolar ³¹P relaxation. Taking into account the CSA contribution, the relaxation measurements performed at 36, 121 and 202 MHz show that the octylphosphate anion undergoes a quasi-axial reorientation about the long molecular axis x with D∥/D⊥ = 4 and D⊥ ? 10⁷ rad/s at 300 K.     

Ion-solvent interaction in nonaqueous solutions. II. Spin-lattice relaxation times of23Na and133Cs

The spin-lattice relaxation time (T ₁) of ²³ Na was measured in solutions of NaClO ₄ and (or) NaBr in formamide,N-methylformamide,N,N-dimethylformamide (DMF), MeCN, Me₂CO, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO), and ¹³³ Cs in a solution of CsCl in formamide. The values of (1/T ₁) ₀ obtained by extrapolation are discussed in terms of current theories of quadrupolar magnetic relaxation of ionic nuclei. A correlation was found between (1/T ₁) ₀ for ²³ Na and Gutmann's donor numbers.For Part I, see ref. 1.     

Powder‐XRD and 14N magic angle‐spinning solid‐state NMR spectroscopy of some metal nitrides

Some metal nitrides (TiN, ZrN, InN, GaN, Ca₃N₂, Mg₃N₂, and Ge₃N₄) have been studied by powder X‐ray diffraction (XRD) and ¹⁴N magic angle‐spinning (MAS) solid‐state NMR spectroscopy. For Ca₃N₂, Mg₃N₂, and Ge₃N₄, no ¹⁴N NMR signal was observed. Low speed (ν_r = 2 kHz for TiN, ZrN, and GaN; ν_r = 1 kHz for InN) and ‘high speed’ (ν_r = 15 kHz for TiN; ν_r = 5 kHz for ZrN; ν_r = 10 kHz for InN and GaN) MAS NMR experiments were performed. For TiN, ZrN, InN, and GaN, powder‐XRD was used to identify the phases present in each sample. The number of peaks observed for each sample in their ¹⁴N MAS solid‐state NMR spectrum matches perfectly well with the number of nitrogen‐containing phases identified by powder‐XRD. The ¹⁴N MAS solid‐state NMR spectra are symmetric and dominated by the quadrupolar interaction. The envelopes of the spinning sidebands manifold are Lorentzian, and it is concluded that there is a distribution of the quadrupolar coupling constants Q_cc's arising from structural defects in the compounds studied. Copyright © 2015 John Wiley & Sons, Ltd.     

Multinuclear magnetic resonance study of organomercury fulminates. The structure of mercury fulminate in solution

¹H, ¹³C, ¹⁴N and ¹⁹⁹Hg NMR data were obtained for organomercury fulminates, RHgCNO (R ? Me, Ph, CNO). The relatively slow relaxation processes effective for the quadrupolar nitrogen muclei of these compounds allow the detection of ¹⁹⁹Hg? ¹⁴N scalar coupling constants. This feature is ascribed to the presence of low electric field gradients around the ¹⁴N nucleus, owing to the presence of the mercury atom. In mercury fulminate, the ¹⁹⁹Hg nucleus appears to be coupled with two equivalent ¹⁴N nuclei, thus indicating the existence of a covalent structure Hg(CNO)₂ in solution.     

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

SCF MO calculations at the 6-31G^** level of approximation are reported for ²H and ¹⁴N electric field gradients in HCN?HCN, HCN?HF, and CH₃CN?HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)₂. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.     

Spin densities,ligand exchange and 13C relaxation in aniline-Ni II complexes

The electron spin distribution in aniline and alkylaniline-Ni(AA)₂ complexes is deduced from ¹H, ¹³C and ¹⁴N contact shifts. The nitrogen hybridization state is given by the experimental values of a_NH^H/a^N compared to the results of INDO calculations. The ¹³C relaxation times in complexed 4-ethyl aniline indicates an N-Ni distance of 2 A and an electron relaxation time T₁ of the order of 10^?10 s.     

Nuclear magnetic relaxation of α-13C nuclei of helical poly(γ-hexyl-L-glutamate) and poly(γ-benzyl-L-glutamate)

Spin-lattice relaxation times (T₁), spin-spin relaxation times (T₂), and nuclear Overhauser enhancements (NOE), at 75.5 MHz are reported for α-¹³C nuclei of poly (γ-benzyl-L -glutamate) in deuterated dimethylformamide at 60°C and of poly(γ-hexyl-L -glutamate) in cyclohexanone at 48 and 79°C. It is shown that for molecular weights above 10⁵, the polypeptides cannot be considered as essentially rigid helices with internal librational motions; additional backbone flexing motions contribute to the relaxation behavior.     

13C and 15N spin–lattice relaxation and chemical shift studies of pentane-2,4-diamine

¹³C and ¹⁵N NMR chemical shift and spin–lattice relaxation data have been measured for both meso- and racemic-pentane-2,4-diamine. At high pH (12), relaxation is consistent with hindered rotation of the NH₂ group due, in part, to the formation of intramolecular hydrogen bonds. At low pH (2), relaxation is consistent with relatively unhindered rotation of the NH₃⁺ group. Rotational jump rates and barriers are reported, determined from the NT₁ ratios between ¹⁵N and ¹³C nuclei. In all cases, the ratios for the racemic diastereomer are higher than those of the meso compounds; this is interpreted in terms of conformationally more stable intramolecular hydrogen bond formation in the meso compound. Chemical shifts for the diastereomeric amines show that ¹⁵N shifts move downfield on protonation along with methyl and methylene carbons, while the methine carbon resonances move upfield.     

14N Nuclear magnetic resonance spectroscopy of N-chloroamines

¹⁴N-Chemical shifts δ(¹⁴N) of eight noncyclic N-chloroamines, 1-3 (Rn₃₋NCl_n;n = 1,2,3); and of two cyclic N-chloroamines 4 [δ(CH₂)_nNCl;n = 2, 5], are reported. The existence of a linear relationship between δ(¹⁴N) and δ(¹³C) (if the nitrogen atom is replaced by the CH-fragment) shows that changes in the magnetic shielding of the ¹⁴N nuclei are not controlled by the presence of the lone electron pair.