A. Naito

High resolution solid state 13C NMR spectra of carbons bonded to nitrogen in a sample spinning at the magic angle

High resolution solid state 13C NMR spectra of amino acids and nitrile compounds have been obtained by a combination of cross‐polarization, dipolar decoupling, and magic angle spinning techniques (CP–MAS). The resonances of Cα and cyano carbons in the observed 13C spectra show asymmetric doublet patterns. This problem has been treated by using the adiabatic approximation since the 14N quadrupole interaction is much larger than the spinning frequency. Theoretical spectra have been simulated for these carbons and they show very good agreement with experimental observations.

Chemical shielding tensor and 13C–14N dipolar splitting in single crystals of L‐alanine

Chemical shielding tensors have been determined for three chemically distinct carbons in a single crystal of L‐alanine using proton‐enhanced 13C NMR. The results indicate that the most shielded direction is perpendicular to the sp2 plane for the carboxyl carbon and the C3 axis for the methyl carbon. 13C–14N dipolar splittings have been observed for the Ca carbon, causing further complication in spectral analysis. Since the 14N quadrupole coupling constant is of comparable order with the 14N Zeeman interaction, 14N quadrupole interaction is effective in the 13C–14N dipolar splittings. The 14N quadrupole effect is observed and demonstrated by calculation and the sign of the quadrupole coupling constant e2Qq has been determined to be positive. The chemical shielding tensor for the Ca carbon is determined by considering the 13C–14N dipolar interaction and the 14N quadrupole interaction. It is shown that the 14N quadrupole interaction causes the asymmetric doublet pattern in the 13C NMR signal for the Ca carbo...

14N quadrupole effects in CP-MAS 13C NMR spectra of organic compounds in the solid state

Abstract The high-resolution solid-state 13 C NMR spectra of 2,6-dimethyl-3-nitroaniline, glycylglycine, glycyl- l -alanine, and trimethylimidazole, were obtained by a combination of cross-polarization and magic-angle spinning techniques (CP-MAS). The 13 C NMR lines of the carbon atoms bonded to nitrogen in these compounds showed a characteristic line broadening or asymmetric doublet patterns. The theoretical lineshapes of the 13 C NMR lines arising from the carbon atoms bonded to nitrogen were calculated using the adiabatic approximation since the rate of change of the Zeeman-quadrupole Hamiltonian of nitrogen nucleus is very slow when compared to the speed of the spinning sample holder in the MAS experiment. In the calculation, an asymmetric quadrupole coupling tensor of nitrogen nucleus was considered. It was found that the calculated lineshapes are in good agreement with the experimentally observed lineshapes, and that the 14 N quadrupole coupling tensor was responsible for the lineshapes of the carbon atoms bonded to nitrogen. The sign of the quadrupole coupling constants of the nitro and amino nitrogens in 2,6-dimethyl-3-nitroaniline were determined to be negative from an analysis of the lineshapes. In the case of trimethylimidazole, it was found that the 13 C NMR signals of the C 4 and C 5 carbon atoms could also be assigned from a comparison of the theoretical and experimental lineshapes. This resolution of the C 4 and C 5 NMR resonances was possible because of the freezing-out of the tautomerism involving the NH bonds.

Spin–spin and spin‐lattice contributions to the rotating frame relaxation of 13C in L‐alanine

The spin‐lattice relaxation times in both the Zeeman (TC1) and rotating (TC1ρ) frames were determined for three chemically distinct carbon atoms (13C) in polycrystalline L‐alanine by combining crosspolarization and magic angle spinning techniques together with proton decoupling. The spin‐lattice and spin–spin contributions to the experimentally measured TC*1ρ could be separated by an experiment in which the 13C spin‐locking field was varied. The spin‐lattice contributions (TC1ρ), which contain motional information, were determined to be 21.7, 23.4, and 138 ms for the Cα, CH3, and COO− carbons, respectively. The spin–spin contribution (TDCH) was found to be exponential, namely, (TDCH)−1 ∝exp(−2πνeCτD) in the low 13C spin‐locking field. Therefore, the assumption of a Lorentzian correlation function for the proton dipolar fluctuations is adequate for L‐alanine. Furthermore, the proton dipolar correlation times τD were found to be the same (31±1 μs) for all three carbons in L‐alanine. The spin‐lattice relaxat...

Determination of the 14N quadrupole coupling tensor and the 13C chemical shielding tensors in a single crystal of l‐serine monohydrate

The 14N quadrupole coupling tensor for the NH3+ nitrogen nucleus in a single crystal of l‐serine monohydrate was completely determined. The quadrupole coupling constant, and the asymmetry parameter, were evaluated to be e2Qq/h=1.069 MHz and η=0.214 at room temperature by measuring the angular variation of the proton‐enhanced 14N NMR spectra about the three experimental axes. It was found that the direction of the largest principal value of the 14N quadrupole coupling tensor is at an angle of 3.5° to the direction of the C–N bond. The sign of the 14N quadrupole coupling constant was determined to be positive by analyzing the 13C–14N dipolar splitting pattern in the 13C NMR signal of the Cα carbon. The 13C chemical shielding tensors for three chemically different carbon nuclei, namely COO−, Cα, and Cβ were determined and assigned to the molecule in the unit cell by inspecting the 13C–14N dipolar interaction rather than relying only on the local symmetry. The most shielded direction of the COO− carbon is per...

Spin-lattice relaxation of 13C in solid amino acids using the CP-MAS technique

Abstract It is shown by a simple application of relaxation theory that the 13C magnetization decays nonexponentially, in principle, in the CP-MAS experiment because of the distribution of the spin-lattice relaxation times; however, the deviation from the exponential decay is quite small. The transient Overhauser effect also contributes appreciably to the nonexponential decay of the 13C magnetization when the protons are not saturated during the 13C T1 measurements and the correlation time of the group rotational motion satisfies the condition, ω2τc2 ≦ 1. It is shown by both experiment and theory that the transient Overhauser effect in the solid state is much smaller than that expected for the liquid state. The 13C spin-lattice relaxation times of l -alanine, deutero- l -alanine, glycine, and l -serine were determined for the individual carbon atoms. The experimentally obtained 13C T1 values agree well with calculated ones, showing that the CH3 group rotation provides the main source of the relaxation in alanine, while the NH3+ group motion plays an important role for the relaxation in glycine and serene.

Determination of the 14N quadrupole coupling tensors in a single crystal of l-histidine hydrochloride monohydrate by NMR spectroscopy

Abstract The 14N quadrupole coupling tensors of the NH3+, N2, and N3 nitrogen nuclei in a single crystal of l-histidine hydrochloride monohydrate were determined using proton enhanced 14N NMR with high-power proton decoupling. The signs of the 14N quadrupole coupling constant, e 2 Qg h , for the three chemically distinct nitrogen nuclei could be determined by analyzing the lineshapes of the 13C CP-MAS NMR signals of the C2, C4, and C6 carbon nuclei which are directly bonding to one of those nitrogen nuclei. The 14N quadrupole coupling constants including their signs and the asymmetry parameters, η, were evaluated to be e 2 Qg h = 1.147 MHz and η = 0.189 for the NH3+ nitrogen nucleus, e 2 Qg h = 1.465 MHz and η = 0.268 for the N2 nitrogen nucleus, and e 2 Qg h = −1.287 MHz and η = 0.946 for the N3 nitrogen nucleus. The direction of the largest principal axis of the 14N quadrupole coupling tensor for the NH3+ nitrogen nucleus is almost parallel to the NlC2 bond direction, while that for the N2 nitrogen nucleus makes an angle of 33° with the N2C4 bond direction in the C4N2C5 plane, and for the N3 nitrogen nucleus it is perpendicular to the C5N3C6 plane. Application of the Townes and Dailey model shows the orbital populations of the pμ and the NH orbitals are larger than those of the NC orbital of the N2 and N3 nitrogen nuclei. The difference between the 14N quadrupole coupling tensors of N2 and N3, can be attributed to the difference in the hydrogen bonding at those two sites.

Two-dimensional nuclear magnetic resonance studies on a single crystal of l-alanine. Separation of the local dipolar fields; and 2D exchange spectroscopy of the 14N relaxation processes

Two types of 2D NMR techniques, namely, separated local field 2D NMR (SLF 2D NMR), and 2D exchange NMR spectroscopy were applied to a single crystal of l‐alanine at room temperature. In the SLF 2D NMR experiments, we found that the 13C–1H dipolar field at the Cα carbon nucleus could be separated not only from the chemical shift interaction, but also from the 13Cα–14N dipolar field. The angular variation of the 13Cα–1H dipolar splitting was measured when the static magnetic field was rotated about three orthogonal axes (a, b, and c axes). The 13Cα‐1H dipolar coupling tensor was determined and the Cα–H bond length was evaluated to be 1.073 A. In the 2D exchange NMR experiment for Cα carbon nucleus, the off‐diagonal cross peaks due to the single quantum and the double quantum transitions for the spin‐lattice relaxation processes of the adjacent 14N nucleus were observed. The single quantum transition rate constant was evaluated to be 0.8 s−1, and the double quantum transition rate constant was estimated to b...

Intermolecular hydrogen-bonding effects on the 13C NMR shielding tensor of the carbonyl carbon nucleus in a single crystal of dimedone

Abstract High-resolution 13C NMR techniques have been employed to study the effects of inter-molecular hydrogen bonding on the chemical-shielding tensor of the carbonyl carbon in a single crystal of dimedone. The principal values, in ppm, relative to an external reference of TMS were found to be σ11 = 284.1, σ22 = 254.9, and σ33 = 79.0. The σ33 axis is almost perpendicular to the plane spanned by the Csp2−C=O moiety, and the σ11 axis is tilted by 22° from the C=O bond direction in that plane. The principal value of the chemical-shielding tensor which is directed along the CO bond (σ11 for dimedone) shows a strong downfield shift (∼50 ppm), as compared to that of acetophenone (σ22 = 231.5); this is undoubtedly because of the intermolecular hydrogen bonding in the former compound. The chemical-shielding tensors for the other seven chemically distinct carbon nuclei in the molecule have also been determined and a comparison is made with the chemical-shift tensors in analogous compounds.

Ligand ENDOR studies of Cu(II)-doped l-alanine single crystals. The nuclear quadrupole coupling tensor and the hyperfine coupling tensor in ligand molecules

Abstract The electron-nuclear double-resonance technique was applied to Cu(II)-doped l -alanine single crystals at 4.2 K. Analysis of ENDOR spectra allows us to determine not only the hyperfine coupling tensor, but also the nuclear quadrupole coupling tensor and the relative sign between them for nitrogen in such a paramagnetic system. The hyperfine coupling tensor; the nuclear quadrupole coupling constant, e 2 Qq ; and the asymmetry parameter, η, for nitrogen in l -alanine ligand closer to Cu(II) were determined, and these values are found to be different from those determined by NQR in pure polycrystalline l -alanine. Simple theoretical calculation for the quadrupole coupling constant was performed by using the Townes and Dailey approximation for this system. The results indicate that the electronic state of nitrogen in l -alanine ligand has sp 2 hybrid orbitals rather than spa hybrid orbitals. Four proton ENDOR signals were assigned to be three amino protons and one α proton and the hyperfine coupling tensors were determined. It was found that the two amino protons and one a proton belong to the l -alanine ligand closer to Cu(II) and a fourth proton belongs to the distant l -alanine ligand.