Search

We demonstrate that individual H-C-C-H torsional angles in uniformly labelled organic solids can be estimated by selective excitation of (13)C double-quantum coherences under magic-angle spinning at rotational resonance. By adapting a straightforward one-dimensional experiment described earlier [T. Karlsson, M. Eden, H. Luhman, M.H. Levitt, J. Magn. Reson. 145 (2000) 95-107], a double-quantum filtered spectrum selective for Calpha and Cbeta of uniformly labelled L-[(13)C,(15)N]valine is obtained with 25% efficiency. The evolution of Calpha-Cbeta double-quantum coherence under the influence of the dipolar fields of bonded protons is monitored to provide a value of the Halpha-Calpha-Cbeta-Hbeta torsional angle that is consistent with the crystal structure. In addition, double-quantum filtration selective for C6 and C1' of uniformly labelled [(13)C,(15)N]uridine is achieved with 12% efficiency for a (13)C-(13)C distance of 2.5A, yielding a reliable estimate of the C6-H and C1'-H projection angle defining the relative orientations of the nucleoside pyrimidine and ribose rings. This procedure will be useful, in favourable cases, for structural analysis of fully labelled small molecules such as receptor ligands that are not readily synthesised with labels placed selectively at structurally diagnostic sites.

In this publication, we introduce a version of the rotational resonance width experiment with a homogeneously broadened matching condition. The increase in the bandwidth is achieved by the reduction of the proton decoupling power during mixing, which results in the reduction of zero-quantum relaxation, and broadens the rotational resonance condition. We show that one can achieve recoupling of the carbonyl-aliphatic side chain dipolar interactions band selectively, while avoiding the recoupling of strongly interacting C′–Cα and C′–Cβ spin pairs. The attenuation of the multi-spin effects in the presence of short zero-quantum relaxation enables a two-spin approximation to be employed for the analysis of the experimental data. The systematic error introduced by this approximation is estimated by comparing the results with a three-spin simulation. The experiment is demonstrated in [U–13C,15N]N-acetyl- l -Val- l -Leu dipeptide, where 11 distances, ranging from 2.5 to 6 A, were measured.

The dependence of the quartic centrifugal distortion coefficients of a molecule on the vibrational quantum number vk is of type H ˜ 24 in the expansion of the effective Hamiltonian. It gives information on the quartic potential constants of the type k ′ k k l m and is therefore additional to that provided by the anharmonicity constants xkl, which depend only on the quartic potential constants of the type k ′ k k l l . The present work describes the calculation of these H ˜ 24 terms from a given potential by perturbation theory, using a computer program which calculates the coefficients in the successive contact transformations. Results are presented, in terms of directional centrifugal distortion constants, for the SO2, ClO2, O3, and H2O molecules. Good agreement with observed coefficients is obtained for SO2 and ClO2. For O3, the strong rotational resonance between the (100) and (001) states complicates the comparison, and the transformation to the effective Hamiltonian used empirically requires further study. For H2O, poor agreement is obtained for the (010) level because of quasilinear behaviour. The effects of the resonance between (100) and (001) are less severe than for O3, and the agreement with the observed is somewhat better.

The absolute orientations of the three 13 C chemical shielding tensors in the phosphoenolpyruvate (PEP) moiety in a PEP-model compound with known crystal structure are reported. The study uses a fully 13 C-enriched polycrystalline sample of triammonium phosphoenolpyruvate monohydrate, (NH 4 ) 3 (PEP)⋅H 2 O, and 13 C MAS NMR experiments fulfilling various different 13 C rotational-resonance conditions. The absolute 13 C chemical shielding tensor orientations are derived by iterative fitting, employing numerically exact simulations, of various rotational-resonance 13 C MAS NMR lineshapes of the three- 13 C-spin system in fully 13 C-enriched (NH 4 ) 3 (PEP)⋅H 2 O. The implications of the results of this study for future, biochemically oriented solid-state NMR studies on the PEP moiety are outlined.

It is demonstrated that internuclear distances can be evaluated from rotational-resonance (RR) experiments in uniformly (13)C-labelled compounds. The errors in the obtained distances are less than 10% without the need to know any parameters of the spin system except the isotropic chemical shifts of all spins. We describe the multi-spin system with a simple fictitious spin-1/2 model. The influence of the couplings to the passive spins (J and dipolar coupling) is described by an empirical constant offset from the rotational-resonance condition. Using simulated data for a three-spin system, we show that the two-spin model describes the rotational-resonance transfer curves well as long as none of the passive spins is close to a rotational-resonance condition with one of the active spins. The usability of the two-spin model is demonstrated experimentally using a sample of acetylcholine perchlorate with labelling schemes of various levels of complexity. Doubly-, triply-, and fully labelled compounds lead to strongly varying RR polarization-transfer curves but the evaluated distances using the two-spin model are identical within the expected error limits and coincide with the distance from the X-ray structure. Rotational-resonance distance measurements in fully labelled compounds allow, in particular, the measurement of weak couplings in the presence of strong couplings.

We describe a new method for exciting triple-quantum coherences in 13C-labelled powder samples under MAS. The proposed method combines selective double-quantum excitation with rotational resonance and frequency-selective composite pulses. The spin dynamics of this new method are described theoretically. Numerical calculations of the spin dynamics are compared to experimental results on fully 13C-labelled L-alanine. The observed triple-quantum filtering efficiency is around 10% for the most intense spectral peak. The method is also demonstrated on other fully 13C-labelled compounds, including a uniformly 13C-labelled amino acid.

A new 13 C – 13 C recoupling mechanism which occurs under magic-angle spinning (MAS) is presented. The mechanism can basically be attributed to rotational resonance (R 2 ), but the conventional R 2 condition is modified by a recoupled 13 C – 1 H dipolar interaction. The 13 C – 1 H recoupling is attained by 1 H rf irradiation fulfilling a rotary-resonance condition. The present method does not have the drawbacks associated with rf irradiation on 13 C and is applicable for band-selective recoupling between carbonyl/aromatic carbons and aliphatic carbons. The 13 C – 13 C recoupling mechanism under 13 C – 1 H recoupling is theoretically explained and is experimentally demonstrated using N -acetyl[1,2- 13 C] dl -valine and uniformly 13 C, 15 N-labeled glycylisoleucine.

A recent analysis of rotational resonance lineshapes (M. Helmle et al., J. Magn. Reson. 140, 379-403, 1999) predicted the existence of coherence transfer signals, which are generated by mechanically induced coherence transfer during the detection process. These signals correspond to the generation of observable coherences at spin sites that have no magnetization at the beginning of the observation interval but which acquire coherence while the detection is underway. The coherence transfer signals disappear for powder samples in conventional magic-angle-spinning solid-state NMR experiments. In this Communication, we report the successful detection of coherence transfer signals in rotor-synchronized experiments performed on a single crystal of [1,2-(13)C(2)]glycine. Copyright 2000 Academic Press.

We present a new technique for double-quantum excitation in magic-angle-spinning solid-state NMR. The method involves (i) preparation of nonequilibrium longitudinal magnetization; (ii) mechanical excitation of zero-quantum coherence by spinning the sample at rotational resonance, and (iii) phase-coherent conversion of the zero-quantum coherence into double-quantum coherence by frequency-selective spin inversion. The double-quantum coherence is converted into observable magnetization by reversing the excitation process, followed by a pi/2 pulse. The method is technically simple, does not require strong RF fields, and is feasible at high spinning frequencies. In [(13)C(2),(15)N]-glycine, with an internuclear (13)C-(13)C distance of 0.153 nm, we achieve a double-quantum filtering efficiency of approximately 56%. In [11, 20-(13)C(2)]-all-E-retinal, with an internuclear (13)C-(13)C distance of 0.296 nm, we obtain approximately 45% double-quantum filtering efficiency.

Rotational resonance phenomena are investigated in the solid-state magic-angle spinning NMR of all-E-[11,20- 13 C 2 ]-retinal at a magnetic field of 4.7 T. We find good agreement between experiments and numerical simulations for the rotational resonance spectral peakshapes and for the rotor-driven magnetization exchange. The internuclear distance between the 13 C -labelled C11 and C20 sites is determined to be 0.301±0.008 nm (from rotational resonance spectra) and 0.300±0.010 nm (from rotor-driven magnetization exchange), in agreement with the X-ray crystallographic distance of 0.296 nm. We show rotational resonance spectra which display perturbations from intermolecular homonuclear spin–spin interactions.

A protocol is presented for the determination of internuclear distances using rotational-resonance magnetization-exchange NMR in systems with inhomogeneously broadened lines. Non-linear least-square fitting procedures are used to obtain the distance, the inhomogeneous broadening, the zero-quantum relaxation time, and error estimates for these parameters. We apply this procedure to a biological system of unknown structure.

The vibration-rotation spectrum of the ν4 + ν8 − ν8 band for CH3NC has been observed and measured with an interferometric spectrometer with a spectral resolution of 0.004 cm−1; 175 transitions were assigned to the P( J, k, l) branches spanning J = 5-29 and k = 3-8. Spectroscopic constants for the ν4 + ν8 state were obtained through a least-squares fit by explicitly taking account of the l-resonance. The validity of the additivity rule of molecular constants is discussed. Small anomalies detected in the K = 3 and 4 subbands of the ν4 + ν8 − ν8 band were ascribed to a rotational resonance (Δl = ∓1, Δk = ±2) between the ν4 + ν8 and 4ν−28 states.

The first completely resolved high-resolution infrared vibration-rotation spectrum of methyl isocyanide (CH3NC) has been obtained. The ν4 band was measured between 900 and 990 cm−1 with a resolution of 0.006 cm−1; 971 transitions were assigned to the P and R branches of the ν4 fundamental band spanning K = 0 to K = 9 and J = 0 to J = 65. Reliable and highly accurate sets of spectroscopic constants for both of the ν4 and ground states were generated. Small anomalies were found in a group of lines in the low-J region of the K = 4 subband and near J = 61 of the K = 3 subband. A perturbation analysis was performed based on symmetry considerations. A rotational resonance ( Δl = ⊣ 1 , Δk = ±2) between the ν4 and 3ν8±1 (kl

The two low-frequency perpendicular infrared active fundamentals, ν 14 and ν 13 , of the heaviest stable isotopic benzene molecule, 13 C 6 D 6 , with band origins at 808.5478 and 1295.7546 cm −1 , respectively, were measured with a high-resolution Fourier transform spectrometer, in an effort to obtain further accurate data needed for a determination of the structural parameters and of a force field for the benzene molecule. Subbands with K Δ K ranging from −60 to +60 have been assigned in the ν 14 band. The r R K branches for K = 36 to 42 are strongly affected by a second-order rotational resonance with the B 2u state ν 10 , which is inactive in both the infrared and the Raman spectrum. In the ν 13 band, subbands with K Δ K = −62 to + 59 have been observed. No perturbations by extraneous states have been detected in this band. Rotational and centrifugal constants for the ground vibrational state, B 0 = 0.1489154 cm −1 , D J 0 = 2.190 × 10 −8 cm −1 , and D JK 0 = −3.651 × 10 −8 cm −1 , were determined from 1478 combination differences obtained from the two bands. This value for B 0 was combined with the previously obtained ground state B 0 -values for 13 C 6 H 6 and 12 C 6 D 6 , with the newly revised highly accurate B 0 determined by Junttila et al. for the main isotopomer 12 C 6 H 6 , and with the B 0 recently obtained by Ruoff et al. for 12 C 6 H 3 D 3 (1,3,5), in a determination of improved structural parameters for the benzene molecule, r CC = 1.3902 ± 0.0002 A , and r CH = 1.0862 ± 0.0015 A , using approximate corrections for zero-point vibrations. Accurate spectroscopic constants are reported for the upper states ν 14 and ν 13 of 13 C 6 D 6 , and tentative values are also obtained for the constants of the perturbing B 2u state ν 10 .

Temperature behavior of the rotational resonance ( J = 0 → 1, F = 3/2→5/2) transition of CH 3 Cl 35 under various pressures was investigated over a range of 193 ≤ T ≤ 366K using a source-modulated spectrometer to ascertain how the central resonance shifted as the perturbing gas pressure within the absorption cell was varied. Frequency shifts were determined for measured pressures of 1 ≤ P ≤ 30mTorr for the temperature range specified. Experimental results are compared with recent theoretical calculations.

Infrared-microwave triple resonance was applied to microwave spectroscopy of propynal, HCCCHO, in the v 2 = 1 vibrational state. Eleven microwave transitions were newly observed by using a Zeeman-tuned 3.51 μm HeXe laser as the infrared radiation source. The spectrum was analyzed including the 11 transitions previously observed by double resonance. A least-squares fit of the 13 low- J transitions revealed that the rotational constants in the K −1 = 1 states were slightly different from those of the K −1 = 0 states. A higher-order rotational resonance was proposed for explanation of the effect.

The Coriolis interaction between ν 2 + ν 3 and ν 3 + ν 5 of 12 CH 3 F and the essential l (2,- 1) rotational resonance in ν 3 + ν 5 , have been treated rigorously up to the fourth order of the Hamiltonian. Eighteen spectroscopic constants are necessary to reproduce 820 experimental data with a standard deviation of 0.0013 cm −1 . Parameters of the anharmonic force field have been derived.

We show that self-broadening of the ammonia inversion lines with J = K can be explained by taking account of the rotational resonance effect.