David J. Ruben

A two-dimensional nuclear overhauser experiment with pure absorption phase in four quadrants☆

Abstract A method is described for obtaining pure absorption phase spectra in four quadrants in a two-dimensional nuclear magnetic resonance spin exchange experiment. It is shown that phase correction results in a substantial increase in resolution and discrimination while maintaining a signal-to-noise ratio comparable to that of the usual magnitude spectrum. Experimental results are presented for the application of the method to a biological macromolecule, the bovine pancreatic trypsin inhibitor.

Natural abundance nitrogen-15 NMR by enhanced heteronuclear spectroscopy

The detection of NMR spectra of less sensitive nuclei coupled to protons may be significantly improved by a 2-dimensional Fourier transform technique involving a double transfer of polarization. The method is adequate to obtain natural abundance 15N spectra in small sample vols. with a com. spectrometer. [on SciFinder (R)]

13C chemical shift and 13C-15N dipolar tensors for the peptide bond: [1-13C]glycyl[15N]glycine·HCl·H2O

Abstract The 13 C chemical shift and 13 C- 15 N dipolar tensors have been determined for single crystals of the dipeptide [1- 13 C]glycyl[ 15 N]glycine·HCl·H 2 O. The 13 C chemical shift tensor is axially asymmetric with σ 11 , σ 22 , and σ 33 values of −115.6, −48.6, and 40.6 ppm, respectively, from benzene. The σ 22 component lies approximately along the carbonyl bond and the most shielded component, σ 33 , is perpendicular to the plane defined by the amide oxygen, carbon, and nitrogen. Dipolar splittings in NMR spectra of the doubly labeled material also allow determination of a C-N bond length, yielding a value of 1.39 A which is in good agreement with previous X-ray crystallographic studies.

Uniform chemical shift scaling: Application to 2D resolved NMR spectra of rotating powdered samples

Chemical shift scaling experiments which reduce the number and intensity of sidebands in high field magic angle sample spinning (MASS) spectra are described. They rely on the fact that the scaling pulse sequence reduces the effective size of the shift anisotropy, so that moderate speed spinning results in spectra devoid of sidebands of appreciable amplitude. In principle, the technique allows a realization of the full signal strength available in high field MASS spectra. In addition a number of other interesting features have emerged from the experiments. First, the problem of decoupling in the presence of a scaling sequence has been solved by arranging the rf field strengths to mismatch the Hartmann–Hahn condition. Second, it is shown that the scaling cycles used in the experiments are uniform over a wide frequency range with respect to both the scaling factor and the linewidth. Third, under certain conditions, the frequencies for scaling and spinning are about the same size and consequently new features...

High resolution measurements of 14N, D quadrupole coupling in CH3CN and CD3CN

A molecular beam maser spectrometer was used to measure hyperfine structure on the J=1→ 0 rotational transitions in CH3CN and CD3CN. Measured nitrogen quadrupole coupling strengths are 1eqaaQ(N)=−4224.4 ± 1.5 kHz for CH3CN and eqaaQ(N)=−4229.3 ± 1.5 kHz for CD3CN. The deuterium quadrupole coupling strength along the C–D bond direction is eqzzQ(D)=167.5 ± 4.0 kHz Nitrogen quadrupole coupling is discussed in relation to the electronic structure. Spin‐rotation interaction strengths are reported. A convenient method for calculating matrix elements for hyperfine interactions in an arbitrary coupling scheme is presented.

Separation of dipolar and quadrupolar splittings in single crystal nitrogen-14 NMR

A new 2-dimensional Fourier technique is proposed to unravel overlapping resonances in single crystal 14N spectra due to proton-N dipolar couplings. One frequency domain is reserved for the proton decoupled quadrupolar spectrum, while the orthogonal frequency dimension shows the dipolar splittings alone, the quadrupolar interaction being eliminated by refocusing. [on SciFinder (R)]

Carbon‐13 chemical shielding in ammonium hydrogen oxalate hemihydrate

We have measured the two 13C chemical shift tensors in single crystals of ammonium hydrogen oxalate hemihydrate, NH4HC2O4⋅ (1/2)H2O, using solid state double resonance techniques. One of the two types of tensors in the unit cell, the one belonging to the ionic carboxyl group, is oriented with its least shield component σ11 within 6° of the C–C bond, while the other tensor, belonging to the protonated carboxyl group, has its σ11 twisted 24° off this bond. Both tensors have σ33 perpendicular to the oxalate plane. Protonation of the carboxyl group does not affect the shifts of the σ11 and σ33 tensor elements, but it does move the σ22 component in the direction of increased shielding. Implications of the orientation of these tensors to crystal and molecular structure determination are discussed.

Molecular beam measurement of hyperfine structure in fluoroform

Abstract The J = 1 → 0 transition in CF 3 H and CF 3 D was observed using a molecular beam maser spectrometer. Typical resonance linewidths were 6 kHz (F.W.H.M.). Hyperfine structure due to deuterium quadrupole coupling, spinrotation and spin-spin interactions was resolved. The strength of the deuterium quadrupole coupling along the bond axis is eqQ = 170.8 ± 2.0 kHz. Fluorine and hydrogen spin-rotation interaction strengths were obtained.

Deuterium quadrupole coupling in formic acid

The 101 → 000 and 313 → 312 transitions of HCOOD were measured using a beam maser spectrometer with 4 kHz resolution. The measurements of three completely resolved components for each of these transitions allowed an unambiguous assignment of the spectrum. The quadrupole coupling strength in the O–D bond direction is eqzzQ = 272 ± 3 kHz with an asymmetry η = −0.075 ± 0.010. Analysis of previous data on DCOOH yields a deuterium quadrupole coupling strength along the D–C bond direction of eqzzQ = 166 kHz.

Laser-molecular beam measurement of hyperfine structure in the I2 spectrum

Abstract Very high resolution measurements of hyperfine structure on the P(13) and R(15), 43−0, 3 gp 0 + u ← 1 Σ g + transitions in iodine 127 were made using laser molecular beam spectroscopy. The observed linewidth was 300 kHz (fwhm) giving a resolution of 5 × 10−10 The observed spectrum was fitted to obtain a quadrupole coupling strength difference of ΔeQq = 1906 ± 2 MHz and a spin rotation interaction strength difference of ΔCI = 181 ± 7 kHz between the upper and lower levels of the P(13) transition. For the R (15) transition ΔeQq = 1905 ± 2 MHz and ΔCI = 167 ± 5 kHz.