Composite dipolar recoupling: anisotropy compensated coherence transfer in solid-state nuclear magnetic resonance

The efficiency of dipole-dipole coupling driven coherence transfer experiments in solid-state nuclear magnetic resonance (NMR) spectroscopy of powder samples is limited by dispersion of the orientation of the internuclear vectors relative to the external magnetic field. Here we introduce general design principles and resulting pulse sequences that approach full polarization transfer efficiency for all crystallite orientations in a powder in magic-angle-spinning experiments. The methods compensate for the defocusing of coherence due to orientation dependent dipolar coupling interactions and inhomogeneous radio-frequency fields. The compensation scheme is very simple to implement as a scaffold (comb) of compensating pulses in which the pulse sequence to be improved may be inserted. The degree of compensation can be adjusted and should be balanced as a compromise between efficiency and length of the overall pulse sequence. We show by numerical and experimental data that the presented compensation protocol significantly improves the efficiency of known dipolar recoupling solid-state NMR experiments.     

Detection of multiple-quantum coherences with projective nuclear magnetic resonance measurement

It is shown that in nuclear magnetic resonance, multiple-quantum (MQ) coherences can be detected "instantly" by exploiting the principle of quantum-mechanical projective measurement. Therefore, the mixing period, which involves collective multispin dynamics and converts MQ coherences into observable single-quantum coherence (magnetization), is not necessary. The experimental examples are given for two finite clusters: benzene in liquid crystal and liquid crystal 4'-n-pentyl-4-cyanobiphenyl, and for solid adamantane with an infinite network of dipolar couplings.     

Selaginellin A and B, two novel natural pigments isolated from Selaginella tamariscina

Two new unusual natural pigments were first isolated from the whole herbs of Selaginella tamariscina. The structure of selaginellin A (1) was established as (R,S)-4-[(4'-hydroxy-3-((4-hydroxyphenyl)ethynyl)biphenyl-2-yl)(4-hydroxyphenyl)methylene]-2,5-cyclohexadien-1-one and selaginellin B (2) as (R,S)-4-[(4'-methoxy-4-(methyl)-3-((4-methoxyphenyl)ethynyl)biphenyl-2-yl)(4-methoxyphenyl)methylene]-2,5-cyclohexadien-1-one, along with four known biflavonoids, amentoflavone (3), hinokiflavone (4), heveaflavone (5), and 7'-O-methylamentoflavone (6). Their chemical structures were elucidated by spectral analysis of electrospray ionization mass spectroscopy (ESI-MS), one-dimensional nuclear magnetic resonance spectroscopy (1D-NMR) and two-dimensional-nuclear magnetic resonance spectroscopy (2D-NMR) including (1)H-NMR, (13)C-NMR, distortionless enhancement by polarization transfer (DEPT) and heteronuclear multiple bond coherence (HMBC), and single-crystal X-ray diffraction techniques.     

High-resolution magic-angle spinning (13)C spectroscopy of brain tissue at natural abundance

High-resolution magic-angle spinning (MAS) (1)H and (13)C magnetic resonance spectroscopy (MRS) has recently been applied to study the metabolism in intact biological tissue samples. Because of the low natural abundance and the low gyromagnetic ratio of the (13)C nuclei, signal enhancement techniques such as cross-polarization (CP) and distortionless enhancement by polarization transfer (DEPT) are often employed in MAS (13)C MRS to improve the detection sensitivity. In this study, several sensitivity enhancement techniques commonly used in liquid- and solid-state NMR, including CP, DEPT and nuclear Overhauser enhancement (NOE), were combined with MAS to acquire high-resolution (13)C spectra on intact rat brain tissue at natural abundance, and were compared for their performances. The results showed that different signal enhancement techniques are sensitive to different classes of molecules/metabolites, depending on their molecular weights and mobility. DEPT was found to enhance the signals of low-molecular weight metabolites exclusively, while the signals of lipids, which often are associated with membranes and have relatively lower mobility, were highly sensitive to CP enhancement.     

Reactivity of iso-diiodomethane and iso-iodoform, isomers of CH2I2 and CHI3, toward the double bond of a variety of cycloalkenes

The metastable CH2I-I and CHI2-I isomers formed by UV photolysis of CH2I2 and CHI3 transfer methylene and iodomethylene groups, respectively, to a variety of cycloalkenes, leading to their cyclopropanation. More than a 100-fold increase of the reaction rate with increasing solvent polarity suggests a dipolar transition state. The fastest second-order rates observed were in CH3CN. However, CH2Cl2 will be the more appropriate reaction medium because the isomer thermal stability is greater in CH2Cl2 than in the more polar CH3CN.     

Synthesis and structural characterization of mono- and bisfunctional o-carboranes

Functionalized o-carboranes are interesting ligands for transition metals. Reaction of LiC2B10H11 with Me2NCH2CH2Cl in toluene afforded 1-Me2NCH2CH2-1,2-C2B10H11 (1). Treatment of 1 with 1 equiv. of n-BuLi gave [(Me2NCH2CH2)C2B10H10]Li ([1]Li), which was a very useful synthon for the production of bisfunctional o-carboranes. Reaction of [1]Li with RCH2CH2Cl afforded 1-Me2NCH2CH2-2-RCH2CH2-1,2-C2B10H10 (R = Me2N (2), MeO (3)). 1 and 2 were also prepared from the reaction of Li2C2B10H10 with excess Me2NCH2CH2Cl. Treatment of [1]Li with excess MeI or allyl bromide gave the ionic salts, [1-Me3NCH2CH2-2-Me-1,2-C2B10H10][I] (4) and [1-Me2N(CH2=CHCH2)CH2CH2-2-(CH2=CHCH2)-1,2-C2B10H10][Br] (6), respectively. Interaction of [1]Li with 1 equiv. of allyl bromide afforded 1-Me2NCH2CH2-2-(CH2=CHCH2)-1,2-C2B10H10 (5). Treatment of [1]Li with excess dimethylfulvene afforded 1-Me2NCH2CH2-2-C5H5CMe2-1,2-C2B10H10 (7). Interaction of [1]Li with excess ethylene oxide afforded an unexpected product 1-HOCH2CH2-2-(CH2=CH)-1,2-C2B10H10 (8). 1 and 3 were conveniently converted into the corresponding deborated compounds, 7-Me2NHCH2CH2-7,8-C2B9H11 (9) and 7-Me2NHCH2CH2-8-MeOCH2CH2-7,8-C2B9H10 (10), respectively, in MeOH-MeOK solution. All of these compounds were characterized by various spectroscopic techniques and elemental analyses. The solid-state structures of 4 and 6-10 were confirmed by single-crystal X-ray analyses.     

Solvent-dependent dynamics of the MQ*-->ReII excited-state electron transfer in [Re(MQ+)(CO)3(dmb)]2+

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) < ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) < CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.     

An experimental study of decoupling sequences for multiple-quantum and high-resolution MAS experiments in solid-state NMR

Recently, a sequence for heteronuclear dipolar decoupling in solid-state NMR, namely SWf-TPPM, was introduced by us. Under magic-angle spinning (MAS), the decoupling efficiency of the sequence was unaffected over a range of values for various experimental parameters such as the pulse length, pulse phase, and 1H resonance offset. We here demonstrate its use in multiple-quantum (MQ) and high-resolution (HR) MAS experiments. This sequence further improves the MQMAS spectra compared to the earlier reported decoupling sequences with improved immunity to any missets of the pulse length, pulse phase and decoupler offset. In contrast, for HRMAS, the simple CW scheme is as efficient as any of the decoupling schemes that were studied.     

Probing proton-proton proximities in the solid state: high-resolution two-dimensional 1H-1H double-quantum CRAMPS NMR spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.     

Solid state structure and solution behaviour of organoselenium(II) compounds containing 2-{E(CH2CH2)2NCH2}C6H4 groups (E = O, NMe)

Cleavage of the Se-Se bond in [2-{O(CH(2)CH(2))(2)NCH(2)}C(6)H(4)](2)Se(2) (1) and [2-{MeN(CH(2)CH(2))(2)NCH(2)}C(6)H(4)](2)Se(2) (2) by treatment with SO(2)Cl(2), bromine or iodine (1 : 1 molar ratio) yielded [2-{O(CH(2)CH(2))(2)NCH(2)}C(6)H(4)]SeX [X = Cl (3), Br (4), I (5)] and [2-{MeN(CH(2)CH(2))(2)NCH(2)}C(6)H(4)]SeI (6). The compounds were characterized in solution by NMR spectroscopy (1H, 13C, 15N, 77Se, 2D experiments). The solid-state molecular structures of 1-3, 4.HBr, 5 and 6 were established by single crystal X-ray diffraction. In all cases T-shaped coordination geometries, i.e. (C,N)SeSe (1, 2), (C,N)SeX (3, 5, 6; X = halogen) or CSeBr(2) (4.HBr), were found. Supramolecular associations in crystals based on hydrogen contacts are discussed.     

n-Alkyl fluorenyl phases in chromatography. II. Dynamic behavior and high-performance liquid chromatography applications.

The dynamic behavior of two n-hexyl fluorenyl phases (fluorene-6A [3a(Tn)(Qm)y], fluorene-6B [3b(Tn)(Qm)y]) and three n-decyl fluorenyl phases (fluorene-10A [4a(Tn)(Qm)y], fluorene-10B [4b(Tn)(Qm)y], and fluorene-10C [4c(M1)(Qm)y]) is investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy using the dipolar filter technique with both 13C and 1H detection. These results are compared with those from other dynamic measurements, like the relaxation times in the rotating frame (T1pH) and the variation of the contact time (T(CH)). Additionally, another type of a fluorenyl phase [5a(Tn)(Qm)y], which has an aromatic moiety connected to the silica gel by amido couplings, was also investigated by the dipolar filter method. The solid-state NMR dynamic measurements indicate an increased mobility of the n-alkyl fluorenyl phases compared to the amido coupled fluorenyl phase. The lower the ligand density of the studied n-alkyl fluorenyl phases, the higher their mobility. The separation behavior of the respective phases in high-performance liquid chromatography was investigated with samples containing polycyclic aromatic hydrocarbons and nitro explosives. Depending on the amount of the chemically bound aromatic moiety and the length of their n-alkyl spacer groups, pi-pi interactions with the solute molecules are involved in the separation process and cause it to proceed at a different rate. Therefore, n-alkyl fluorenyl phases can be classified as mixed-mode phases.     

SESAME-HSQC for simultaneous measurement of NH and CH scalar and residual dipolar couplings

We present a novel pulse sequence, SESAME-HSQC, for the simultaneous measurement of several NH and CH scalar and residual dipolar couplings in double labeled proteins. The proposed Spin-statE Selective All Multiplicity Edited (SESAME)-HSQC combines gradient selected and sensitivity enhanced (15)N- and constant-time (13)C-HSQC experiments with the recently introduced spin-state selective method (Nolis et al., J. Magn. Reson. 180 (2006) 39-50) for measuring couplings simultaneously at amide and aliphatic regions. Excellent resolution and high sensitivity is warranted by removing all coupling interactions during the indirectly detected t(1) period, and by employing pulsed field gradients for coherence selection and utilizing coherence order selective spin-state selection. The scalar and residual dipolar couplings can be readily measured from a two-dimensional (15)N/(13)C-HSQC spectrum without additional spectral crowding. SESAME-HSQC can be used for epitope mapping by observing chemical shift changes in both amide and aliphatic regions. Simultaneously, potential conversion in protein conformation can be probed by analyzing changes in residual dipolar couplings induced by ligand binding. The pulse sequence is experimentally verified with a sample of (15)N/(13)C enriched human ubiquitin. The internuclear vector directions determined from the residual dipolar couplings are found to be in excellent correlation with those predicted from ubiquitin's refined solution structure.     

In situ 13C DEPT-MRI as a tool to spatially resolve chemical conversion and selectivity of a heterogeneous catalytic reaction occurring in a fixed-bed reactor

The distortionless enhancement by polarisation transfer (DEPT) nuclear magnetic resonance (NMR) technique, combined with magnetic resonance imaging (MRI), has been used to provide the first in situ spatially-resolved and quantitative measurement of chemical conversion and selectivity within a fixed-bed reactor using natural abundance 13C NMR.     

Can Sn(OCH2CH2NMe2)2 behave as a stannylene? Equatorial-axial isomerism in the tin(II)-iron(0) complex (Me2NCH2CH2O)2Sn-Fe(CO)4

Equatorial-axial isomerism of the tin(II)-iron(0) complex (Me2NCH2CH2O)2Sn-Fe(CO)4 (), which indicates that the free Sn(OCH2CH2NMe2)2 () ligand can behave as a stannylene, has been revealed and studied by NMR and IR spectroscopy in solution as well as by Raman spectroscopy and X-ray diffraction analysis in the solid-state.     

Synthesis and characterization of six-coordinate "3 + 2" mixed-ligand oxorhenium complexes with the o-diphenylphosphinophenolato ligand and tridentate coligands of different N and S donor atom combinations

A series of octahedral six-coordinate oxorhenium(V) mixed ligand complexes containing the common [ReO(L)]2+ fragment (L = o-OC6H4P(C6H5)2] have been synthesized and characterized. Hence, it was shown that the [ReO(L)]2+ moiety can accommodate a variety of tridentate ligands containing a central amine group amenable to deprotonation and different combinations of lateral groups, such as ethylamine, substituted ethylamine, ethylthiol, and ethylthioether arms. In particular, by reaction of equimolar amounts of the pertinent HLn ligands with the [(n-C4H9)4N][ReOCl3(L)] precursor in refluxing acetonitrile/methanol or dichloromethane/methanol mixtures, the following series of [ReO(Ln)(L)]+/0 oxorhenium(V) complexes has been generated: ReO[[N(CH2CH2NH2)2][o-OC6H4P(C6H5)2]]Cl (1); ReO[[C2H5)2NCH2CH2NCH2CH2S][o-OC6H4P5)2]] (2); ReO[[(CH2)4NCH2CH2NCH2CH2S][o-OC6H4P(C6H4P(C6H5)2]] (3); and ReO[[C2H5SCH2CH2NCH2CH2S][o-OC6H4P(C6H5)2]] (4). The complexes are closed-shell 18-electron oxorhenium species, which adopt octahedral geometries both in solution and in the solid state, as established by conventional physicochemical techniques including multinuclear NMR and single-crystal X-ray diffraction analyses.     

15N solid-NMR and X-ray diffraction studies of N-confused porphyrins

Using (15)N high-resolution solid-state NMR and X-ray diffraction, the structure of N-confused porphyrin (NCP) in the solid state was studied. A 1D (15)N magic angle spinning (MAS) experiment and a 2D dipolar assisted rotational resonance (DARR) (15)N-(15)N spin exchange experiment of N-confused tetratolylporphyrin (Tol) crystallized from CH(2)Cl(2)/hexane indicate that Tol is the inner 3H-type tautomer and has two magnetically different molecules in the unit cell. Further, a FSLG-2 & 4macr; 2 (1)H-(15)N dipolar recoupling NMR measurement indicates no fast ring flipping motion which is consistent with the planar structure in the X-ray analysis. The planarity of Tol is ascribed to crystal packing enforced by pi-pi stacking and CH-pi interactions.     

Multinuclear solid-state high-resolution and 13C -{27Al} double-resonance magic-angle spinning NMR studies on aluminum alkoxides

A combination of 27Al magic-angle spinning (MAS)/multiple quantum (MQ)-MAS, 13C-1H CPMAS, and 13C-{27Al} transfer of population in double-resonance (TRAPDOR) nuclear magnetic resonance (NMR) were used for the structural elucidation of the aluminum alkoxides aluminum ethoxide, aluminum isopropoxide, and aluminum tertiarybutoxide. Aluminum alkoxides exist as oligomers with aluminum in different coordinations. High-resolution 27Al MAS NMR experiments with high-spinning speed distinguished the aluminum atoms in different environments. The 27Al MAS NMR spectrum gave well-resolved powder patterns with different coordinations. Z-filter MQ-MAS was performed to obtain the number and types of aluminum environments in the oligomeric structure. 13C-1H CPMAS chemical shifts resolved the different carbon species (-CH3, =CH2, =CH-, and =C=) in the structures. 13C-{27Al} TRAPDOR experiments were employed to obtain relative Al-C dipolar interactions and to distinguish between terminal and bridging alkoxides in the crystallographic structures. The complete characterization of selected aluminum alkoxides using advanced NMR methods has evidenced the tetrameric structure for aluminum isopropoxide and the dimeric structure for aluminum tertiary-butoxide, as reported in the literature, and proposed a polymeric structure for aluminum ethoxide.     

Accurate heteronuclear distance measurements at all magic-angle spinning frequencies in solid-state NMR spectroscopy

Heteronuclear dipolar coupling is indispensable in revealing vital information related to the molecular structure and dynamics, as well as intermolecular interactions in various solid materials. Although numerous approaches have been developed to selectively reintroduce heteronuclear dipolar coupling under MAS, most of them lack universality and can only be applied to limited spin systems. Herein, we introduce a new and robust technique dubbed phase modulated rotary resonance (PMRR) for reintroducing heteronuclear dipolar couplings while suppressing all other interactions under a broad range of MAS conditions. The standard PMRR requires the radiofrequency (RF) field strength of only twice the MAS frequency, can efficiently recouple the dipolar couplings with a large scaling factor of 0.50, and is robust to experimental imperfections. Moreover, the adjustable window modification of PMRR, dubbed wPMRR, can improve its performance remarkably, making it well suited for the accurate determination of dipolar couplings in various spin systems. The robust performance of such pulse sequences has been verified theoretically and experimentally via model compounds, at different MAS frequencies. The application of the PMRR technique was demonstrated on the H-ZSM-5 zeolite, where the interaction between the Brønsted acidic hydroxyl groups of H-ZSM-5 and the absorbed trimethylphosphine oxide (TMPO) were probed, revealing the detailed configuration of super acid sites.

A new and robust technique dubbed phase modulated rotary resonance (PMRR) was proposed for the accurate determination of heteronuclear dipolar coupling under a broad range of MAS conditions in solid-state NMR spectroscopy.     

Perturbation of conjugation in internally solvated allylic lithium compounds: variation of ligand structure. NMR and X-ray crystallography

Several allyic lithium compounds were prepared with different potential ligands tethered at C2. These are with CH3OCH2CH2NCH3CH2-, 5 and 1-TMS 6, with (CH3)2NCH2CH2NCH3CH2-, 1-TMS 7, and with ((CH3)2NCH2CH2)2NCH2-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point determinations. Compounds 6, 7, and 9 are monomeric in THF-d8 or diethyl ether-d10 solution and exhibit one bond 13C1, 6Li scalar coupling at low temperature. Taken together the crystallographic and NMR data indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8 undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond 13C, 6Li coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C-Li exchange with Delta H++ values of 6.7, 12, and 13 kcal x mol(-1), respectively. Averaging of the diastereotopic N(CH3)2 13C resonances of 7 is indicative of fast transfer of coordinated ligand between faces of the allyl plane Delta H++ = 5.3 kcal x mol(-1) combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees of delocalization.     

对DEPT实验技术的一点改进

将DEPT -1 3 5实验的 1 3 5° -脉冲分成一个 90°脉冲和一个 4 5°脉冲 ,通过相位循环 ,设计了两个新的DEPT实验分别称为DEPT-90 -4 5和DEPT -90 +4 5。DEPT -90 -4 5谱中只有 -CH3的吸收峰。DEPT -90 +4 5谱中没有 >CH -的吸收峰 ,其 -CH3为正信号 ,-CH2 -为负信号。实验结果显示 ,对比DEPT -90谱 ,这两个新的DEPT谱中完全没有所选基团之外的残余干扰信号 ,而且DEPT -90 -4 5谱比DEPT-90 +4 5谱更好。