An Investigation of Chlorine Ligands in Transition-Metal Complexes via 35Cl Solid-State NMR and Density Functional Theory Calculations

35Cl solid state NMR (SSNMR), in tandem with 35Cl NQR and density functional theory calculations, was used to characterize chlorine ligands in a series of transition-metal complexes exhibiting structural motifs common to organometallic catalysts. The differentiation of the various chlorine environments was possible, and insight into the origins of the 35Cl electric field gradient tensor parameters was provided. The applicability of 35Cl SSNMR to the study of surface supported transition-metal complexes was demonstrated, validating the use of this technique in the characterization of heterogeneous catalysts

Homogeneous broadenings in 2D solid-state NMR of half-integer quadrupolar nuclei

The question of the homogeneous broadening that occurs in 2D solid-state NMR experiments is examined. This homogeneous broadening is mathematically introduced in a simple way, versus the irreversible decay rates related to the coherences that are involved during t1 and t2. We give the pulse sequences and coherence transfer pathways that are used to measure these decay rates. On AlPO4 berlinite, we have measured the 27Al echo-type relaxation times of the central and satellite transitions on 1Q levels, so that of coherences that are situated on 2Q, 3Q, and 5Q levels. We compare the broadenings that can be deduced from these relaxation times to those directly observed on the isotropic projection of berlinite with multiple-quantum magic-angle spinning (MAS), or satellite-transition MAS. We show that the choice of the high-resolution method, should be done according to the spin value and the corresponding homogeneous broadening

Resolution enhancement in 1D solid-state NMR spectra of spin-9/2 quadrupolar nuclei

NMR is an insensitive spectroscopy, which often requires numerous accumulations, especially for 2D high-resolution methods (MQMAS and STMAS) for quadrupolar nuclei in solids. This may be a very important limitation for the case of insensitive nuclei, where a 1D spectrum with better resolution than the central-transition is then highly desirable. This problem has been addressed for the case of spin-5/2 nuclei by the Double-Quantum Filtered Satellite Transition Spectroscopy: DQF-SATRAS-ST1. We extend this concept to the spin-9/2 nuclei with the SATRAS-ST2 method. This method allows the observation of 1D spectra with a much better resolution than that observed in the isotropic projection of 2D MQ/ST1-MAS spectra. This enhanced resolution results from the much smaller homogeneous broadening that occurs on the SATRAS-ST2 method as compared to MQ/ST1-MAS spectra. The main interest in this method is for well-crystallized samples

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC

We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R3) irradiation, or on two continuous rotor-synchronized modulations (SFAM1 and SFAM2), or on four symmetry-based sequences (R 211, SR 412, R 1235, R 2059), or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR 412 and SFAM (SFAM1 or SFAM2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM1 and SFAM2 will be the most efficient methods

Residual entropy and spin gap in a one-dimensional frustrated antiferromagnet

We show that the low-energy sector of the S=1/2, antiferromagnetic Heisenberg model on a one dimensional lattice of coupled tetrahedra consists of 2N replica of the spectrum of the dimerized Heisenberg chain, where N is the number of tetrahedra. This provides a proof of the following properties: (i) there is a residual ground-state entropy per spin equal to (1/4)ln 2; (ii) there is a singlet-triplet gap as long as the coupling between the tetrahedra is smaller than the internal one. These properties are compared to available results on the pyrochlore lattice

Residual entropy and spin gap in a one-dimensional frustrated antiferromagnet

We show that the low-energy sector of the S=1/2, antiferromagnetic Heisenberg model on a one dimensional lattice of coupled tetrahedra consists of 2N replica of the spectrum of the dimerized Heisenberg chain, where N is the number of tetrahedra. This provides a proof of the following properties: (i) there is a residual ground-state entropy per spin equal to (1/4)ln 2; (ii) there is a singlet-triplet gap as long as the coupling between the tetrahedra is smaller than the internal one. These properties are compared to available results on the pyrochlore lattice

Comparison of high-resolution solid-state NMR MQMAS and STMAS methods for half-integer quadrupolar nuclei

International audienceSeveral different amplitude-modulated two-dimensional high-resolution methods, based on MQMAS and STMAS, are compared. They include 3QMAS. 5QMAS, DQ-STMAS, and DQF-STMAS experiments. A new method, called t(1)-split-STMAS, is also proposed for spin-3/2 nuclei. The comparison is performed in terms of isotropic resolution and spectral-width, efficiency, and sensitivity to magic-angle offset and spinning speed fluctuations

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

International audienceA 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 = ω1/ωr. 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, T2′ lengthening, and 1H/14N HMQC experiment under the n = 2 rotary resonance are demonstrated

Last-year new methods in SS-NMR

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