General Interest

1) High resolution NMR of water absorbed in single-wall carbon nanotubes

Sekhaneh, Wassef; Kotecha, Mrignayani; Dettlaff-Weglikowska, Urszula; Veeman, Wiebren S.

Abstract

1H MAS NMR study of water absorbed in open single-wall carbon nanotubes (SWCNTs) of average diameter 1.0 nm, synthesized with a Fe catalyst, was performed. Magic angle spinning significantly increases the spectral resolution and the spectrum shows that there are at least two distinct chemical shift regions for the water proton. From temperature dependent measurements, we assign the two chemical shift ranges to water absorbed inside and outside the nanotube. The spectra as a function of the water content show that the first water molecules that are absorbed by the dry material are absorbed inside the tubes.

CPL vol. 428 p 143

2) Characterization of violet emission from Rb optical pumping cells used in laser-polarized xenon NMR experiments

Indrajit Saha, Panayiotis Nikolaou, Nicholas Whiting, Boyd M. Goodson

Abstract
Visible emission from Rb optical pumping cells was characterized under a range of conditions relevant to the production of laserpolarized xenon (including temperature, partial pressures, and D1-resonant 795 nm laser power). Bright 421 nm (6P ! 5S) emission was consistent with energy-pooling processes of the type: Rb*(5P1/2) + Rb*(5P1/2,3/2)!Rb*(6P1/2,3/2) + Rb(5S1/2), with processes transiting through 5D states likely contributing at higher temperatures/lower N2 partial pressures. Under such conditions a number of Rb lines may be observed, indicating population of Rb states to P9D (31822 cm1). Such energies exceed those required for efficient production of laser-induced plasma.

CPL vol. 428 p 268

3) Double-quantum-filtered intermolecular single-quantum coherences in nuclear magnetic resonance spectroscopy and imaging

Zhong Chen, Xiaoqin Zhu, Bingwen Zheng, Shuhui Cai, Jianhui Zhong

Abstract
Based on the double-quantum-filtered technique, a three-pulse sequence was designed to effectively eliminate contamination of residual conventional single-quantum coherences (SQC) from intermolecular SQC signals in single-resonance spin-1/2 samples such as water. Analytical expressions were derived from the modified Bloch equations with dipolar fields. All experimental observations and numerical simulation results are in excellent agreement with the theoretical predictions. Compared to the CRAZED sequence, the sequence can effectively suppress residual conventional SQC signals even with great imperfection of radio-frequency pulse flip angles. For the first time, signals from pure intermolecular SQCs were selectively observed in forms of spectrum or image.

CPL vol 429 p 611

4) STARTMAS: A MAS-based method for acquiring isotropic NMR spectra of spin I = 3/2 nuclei in real time

Michael J. Thrippleton, Thomas J. Ball, Stefan Steuernagel, Sharon E. Ashbrook, Stephen Wimperis

Abstract
We describe a novel method, which we call STARTMAS, that allows acquisition of high-resolution or ‘isotropic’ spin I = 3/2 NMR spectra in solids in real time. Unlike the DOR technique of Samoson et al. [Mol. Phys. 65 (1988) 1013], STARTMAS is performed on a standard MAS probe and fast spinning rates are possible. The method consists of a multiple-pulse sequence interleaved with real-time data acquisition and exploits satellite (mI = ±3/2M±1/2) and double-quantum (±3/2M «1/2) transitions. We show that STARTMAS experiments can be used to obtain one-dimensional DOR-like spectra or provide an ‘ultrafast’ route to high-resolution two-dimensional spectra.

CPL vol 431 p 390

5) Improved double-quantum NMR correlation spectroscopy of dipolar-coupled quadrupolar spins

Mattias Ede´n a,*, Dan Zhou b, Jihong Yu

Abstract
We show that symmetry-based zero-quantum dipolar recoupling sequences sandwiched between central transition selective p/2-pulses may be used to excite double-quantum coherences (2QC) between the central transitions of homonuclear half-integer quadrupolar spins undergoing magic-angle spinning. The 2QC excitation is explored experimentally by 23Na and 27Al NMR on Na2SO3 and a-Al2O3 model compounds. When incorporated into 2Q–1Q 2D correlation protocols, the new recoupling method established all expected 27Al-27Al connectivities over 6A ˚ in the open-framework aluminophosphate AlPO–J19 [(NH4)2Al4(PO4)4HPO4 Æ H2O] and the layer mineral chlorite [Mg9Al6Si5O20(OH)16].

CPL vol 431 p 397

6) 27Al double rotation two-dimensional spin diffusion NMR: Complete unambiguous assignment of aluminium sites in 9Al2O3 Æ 2B2O3

I. Hung, A.P. Howes, T. Anupold, A. Samoson, D. Massiot, M.E. Smith, S.P. Brown, R. Dupree

Abstract
High-resolution two-dimensional 27Al spin diffusion NMR spectra of 9Al2O3 Æ 2B2O3 recorded under double rotation for different mixing times are presented. The two crystallographically distinct pentahedral Al sites are distinguished by the presence or absence of correlation peaks with neighboring tetrahedral sites.

CPL vol. 432 p 152

7) Solid-state 17O NMR in carbohydrates

T.H. Sefzik, J.B. Houseknecht, T.M. Clark, S. Prasad, T.L. Lowary, Z. Gan, P.J. Grandinetti

Abstract
Solid-state 17O magic-angle spinning nuclear magnetic resonance measurements at 19.5 Tesla were performed on 17O-enriched methyl a-D-galactopyranoside (4-17O), methyl b-D-glucopyranoside (2-17O), methyl a-D-glucopyranoside (4-17O), methyl a-D-glucopyranoside (6-17O), and a-D-glucopyranosyl (1 !6) a-D-glucopyranoside (6-17O). The 17O quadrupolar coupling constants and asymmetry parameters measured can be predicted with a model based entirely on the first-coordination sphere around oxygen. For the hydroxyl sites observed in the methyl glucosides, the quadrupolar coupling parameters are nearly identical, within 10% as predicted, given their nearly identical first-coordination sphere structures.

CPL 434 p 312

8) Fast adiabatic pulses for solid-state NMR of paramagnetic systems

Gwendal Kervern, Guido Pintacuda, Lyndon Emsley

Abstract
We present a simple framework for determining the parameters for efficient adiabatic pulses in solids with very large shift anisotropies under very fast MAS. We introduce the notion of the effective adiabatic quality factor to describe the spin dynamics. We then demonstrate the use of short, high-power adiabatic pulses (SHAPs) for population inversion in paramagnetic solids, and show its application to various systems with differing proton anisotropies in iron, ytterbium, and terbium complexes, from 400 to more than 1000 ppm. We then show how these pulses yield improvements in refocusing schemes and TEDOR NMR experiments on paramagnetic solids.

CPL vol 435 p 157

9) Proton-detected 14N MAS NMR using homonuclear decoupled rotary resonance

Zhehong Gan, Jean Paul Amoureux, Julien Trebosc

Abstract
A robust sensitivity-enhanced 1H/14N MAS HMQC experiment is described for proton-detected 14N NMR of solids. The sensitivity enhancement is achieved by using dipolar recoupling for coherence transfer with a so-called n = 2 rotary resonance. Rotary resonance occurs when a cw rf field matches certain ratios with the sample spinning frequency, n = x1/xr. The theory of rotary resonance for chemical shift anisotropy, heteronuclear and homonuclear dipolar interactions is presented in the irreducible representation. It is shown that the n = 2 rotary resonance decouples the homonuclear dipolar interactions while recoupling the heteronuclear dipolar interaction for proton-detected 14N NMR. The dipolar recoupling, T 02 lengthening, and 1H/14N HMQC experiment under the n = 2 rotary resonance are demonstrated.

CPL vol 435 p 163

10) High-resolution two-dimensional NMR spectra of half-integer-spin quadrupolar nuclei from one-dimensional projections

Thomas Vosegaard a,*, Dominique Massiot

Abstract
We present a technique providing high-resolution spectra of quadrupolar nuclei with half-integer nuclear spin displaying significant second-order linebroadening. The technique, dubbed chemical shift – quadrupolar projection-reconstruction of one-dimensional spectra (CQ-PRODI) exploits the different magnetic field dependence of the chemical shift and second-order quadrupolar effect. One-dimensional spectra recorded at different magnetic field strengths are represented as projections along different directions in the two-dimensional spectrum that correlates the chemical shift and the second-order quadrupolar lineshape. We present CQ-PRODI spectra for 27Al in 9Al2O3 Æ 2B2O3 and for 71Ga in b-Ga2O3 which display resolution of all sites in the chemical shift dimension.

CPL vol 437 p 120

11) Gas-Phase NMR Measurements, Absolute Shielding Scales, and Magnetic Dipole Moments
of 29Si and 73Ge Nuclei

W. Makulski, K. Jackowski, A. Antusek, and M. Jaszunski

Abstarct
New gas-phase NMR measurements of the shielding constants of 29Si, 73Ge, and 1H nuclei in SiH4 and GeH4 are reported. The results, extrapolated to zero density, provide accurate isolated molecule values, best suited for comparison with theoretical calculations. Using the recent ab initio results for these molecules and the measured chemical shifts, we determine the absolute shielding scales for 29Si and 73Ge. This allows us to provide new values of the nuclear magnetic dipole moments for these two nuclei; in addition, we examine the dipole moments of 13C and 119Sn.

JPCA vol 110 p 11462

12) An ab Initio Study of 15N-11B Spin-Spin Coupling Constants for Borazine and Selected
Derivatives

Janet E. Del Bene, Jose Elguero, Ibon Alkorta, Manuel Yanez, and Otilia Mo

Abstract
Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond 11B-15N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B-N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B-N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B-N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B-N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B-N coupling constants for borazine are similar to F and Li substitution effects on 13C-13C coupling constants for benzene.

JPCA vol 110 p 9959

13) Proton NMR measurements of the local magnetic field in the paramagnetic metal
and antiferromagnetic insulator phases of lamda-(BETS)2FeCl4

Guoqing Wu, P. Ranin, W. G. Clark, S. E. Brown, L. Balicas, and L. K. Montgomery

Abstract
Measurements of the 1H-NMR spectrum of a small 4g single crystal of the organic conductor BETS2FeCl4 are reported with an applied magnetic field B0=9 T parallel to the a axis in the ac plane over a temperature
T range 2.0–180 K. They provide the distribution of the static local magnetic field at the proton sites in the paramagnetic metal
PM and antiferromagnetic insulator AFI phases, along with the changes that occur at the PM-AFI phase transition. The spectra have six main peaks that are significantly broadened and shifted at low T. The origin of these features is attributed to the large dipolar field from the
3d Fe3+ ion moments
spin Sd=5/2. Their amplitude and T dependence are modeled using a modified Brillouin function that includes a mean field approximation for the total exchange interaction
J0 between one Fe3+ ion and its two nearest neighbors. A good fit is obtained using J0=−1.7 K. At temperatures below the PM-AFI transition temperature TMI=3.5 K, an extra peak appears on the high frequency side of the spectrum and the details of the spectrum become smeared. Also, the rms linewidth and the frequency shift of the spectral distribution are discontinuous, consistent with the transition being first order. These measurements verify that the dominant local magnetic field contribution is from the Fe3+ ions and indicate that there is a significant change in the static local magnetic field distribution at the proton sites on traversing the PM to AFI phase transition.

PRB vol 74 p 064428

14) Structural, Solid-State NMR and Theoretical Studies of the Inverse- Coordination of Lithium Chloride Using Group 13 Phosphide Hosts

Melinda J. Duer, Felipe Garc1a, Jonathan M. Goodman, Jçrg P. Hehn, Richard A. Kowenicki, Vesal Naseri, Mary McPartlin, Matthew L. Stead, Robin Stein, and Dominic S. Wright

Abstract: The reaction of MeAlCl2 with ’PhPLi2’ in THF gives [{MeAl-(PPh)3Li4·3THF}4ACHTUNGTRENUNG(m4-Cl)]
Li+ (1). The GaIII and InIII analogues, [{MeEACHTUNGTRENUNG(PPh)3Li4·3THF}4ACHTUNGTRENUNG(m4-Cl)]
Li+·(THF)3 (E=Ga (2), In (3)), are obtained by the in situ reactions of MeECl2 with PhPLi2 in THF. For all of the complexes, the cage anions have an unusual cubic arrangement that is similar to a zeolite, and contain large voids (ca. 17 8). The location of the Li+ counterions in 1–3 and their coordination environment appears to subtly reflect variations in packing and lattice energy. Whereas in 1 highly mobile, loosely coordinated Li+ counterions are present, 2 and 3 contain less mobile THF-solvated counterions within the cavities. X-ray crystallographic and solid-state NMR studies are reported on 1–3, together with model DFT calculations on the selectivity of halide coordination.

CEJ vol 13 p 1251