Kanmi Mao

Indirectly detected through-bond chemical shift correlation NMR spectroscopy in solids under fast MAS: studies of organic-inorganic hybrid materials.

Indirectly detected, through-bond NMR correlation spectra between (13)C and (1)H nuclei are reported for the first time in solid state. The capabilities of the new method are demonstrated using naturally abundant organic-inorganic mesoporous hybrid materials. The time performance is significantly better, almost by a factor of 10, than in the corresponding (13)C detected experiment. The proposed scheme represents a new analytical tool for studying other solid-state systems and the basis for the development of more advanced 2D and 3D correlation methods.

Study of Intermolecular Interactions in the Corrole Matrix by Solid‐State NMR under 100 kHz MAS and Theoretical Calculations

Recent progress in solid-state (SS)NMR spectroscopic methods based on fast magic angle spinning (MAS) has enabled new opportunities for the structural study of small quantities (< 5 mg) of natural abundance samples. Utilizing throughspace and through-bond polarization transfer, indirect detection of low-g nuclei, and suitable homoand heteronuclear decoupling, oneand two-dimensional (1D and 2D) spectra of such samples can be measured with excellent sensitivity and resolution. However, determination of the short-range intermolecular order often remains elusive. Such analyses can be well-served by studying heteronuclear correlations that take advantage of the large chemical shift range of most low-g nuclei (for example, C or N). Indeed, heteronuclear correlation (HETCOR) NMR spectroscopy and measurements of internuclear distances, often in concert with theoretical calculations, have provided structural details of complex hydrogen-bonded systems in chemistry and biology, blended materials, and host–guest pairs. Still, intermolecular polarization transfers to low-g nuclei are often hampered by low sensitivity. A promising solution to this challenge is offered by homonuclear H–H 2D correlation methods, such as double-quantum (DQ)MAS or spin-diffusion (NOESYlike) experiments, provided that sufficient resolution is achieved in both dimensions. One of the possible approaches is the use of H CRAMPS decoupling in concert with fast MAS to boost resolution in these experiments. The recent development of ultrafast MAS (at 100 kHz and more) provides access to appropriate H resolution without RF decoupling. Herein, we report the first application of H 2D SSNMR measurements under MAS at 100 kHz, which are used in combination with indirectly detected H{C} and H{N} HETCOR experiments and theoretical calculations to scrutinize the interactions within a host–guest (HG) system consisting of 5,10,15-tris(pentafluorophenyl)corrole 1, and toluene (Scheme 1).

Directly and indirectly detected through-bond heteronuclear correlation solid-state NMR spectroscopy under fast MAS

Two-dimensional through-bond (1)H{(13)C} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are presented. Remarkable efficiency of polarization transfer can be achieved at MAS rates exceeding 40 kHz, which is instrumental in these measurements. Schemes utilizing direct and indirect detection of heteronuclei are compared in terms of resolution and sensitivity. A simple procedure for optimization of (1)H homonuclear decoupling sequences under these conditions is proposed. The capabilities of these techniques were confirmed on two naturally abundant solids, tripeptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (f-MLF-OH) and brown coal.

Indirectly detected heteronuclear correlation solid-state NMR spectroscopy of naturally abundant 15N nuclei.

Two-dimensional indirectly detected through-space and through-bond (1)H{(15)N} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are evaluated. Remarkable efficiency of polarization transfer can be achieved at a MAS rate of 40 kHz by both cross-polarization and INEPT, which makes these methods applicable for routine characterizations of natural abundance solids. The first measurement of 2D (1)H{(15)N} HETCOR spectrum of natural abundance surface species is also reported.

Conformations of silica-bound (pentafluorophenyl)propyl groups determined by solid-state NMR spectroscopy and theoretical calculations.

The conformations of (pentafluorophenyl)propyl groups (-CH(2)-CH(2)-CH(2)-C(6)F(5), abbreviated as PFP), covalently bound to the surface of mesoporous silica nanoparticles (MSNs), were determined by solid-state NMR spectroscopy and further refined by theoretical modeling. Two types of PFP groups were described, including molecules in the prone position with the perfluorinated aromatic rings located above the siloxane bridges (PFP-p) and the PFP groups denoted as upright (PFP-u), whose aromatic rings do not interact with the silica surface. Two-dimensional (2D) (13)C-(1)H, (13)C-(19)F and (19)F-(29)Si heteronuclear correlation (HETCOR) spectra were obtained with high sensitivity on natural abundance samples using fast magic angle spinning (MAS), indirect detection of low-gamma nuclei and signal enhancement by Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence. 2D double-quantum (DQ) (19)F MAS NMR spectra and spin-echo measurements provided additional information about the structure and mobility of the pentafluorophenyl rings. Optimization of the PFP geometry, as well as calculations of the interaction energies and (19)F chemical shifts, proved very useful in refining the structural features of PFP-p and PFP-u functional groups on the silica surface. The prospects of using the PFP-functionalized surface to modify its properties (e.g., the interaction with solvents, especially water) and design new types of the heterogeneous catalytic system are discussed.

Homonuclear dipolar decoupling under fast MAS: Resolution patterns and simple optimization strategy

A simple method is shown for optimization of (1)H homonuclear dipolar decoupling at MAS rates exceeding 10 kHz. By monitoring the intensity of a spin-echo under the decoupling conditions, it is possible to optimize the amplitude of the RF magnetic field, the cycle time of the decoupling sequence and the resonance offset within minutes. As a result, the decoupling efficiency can be quickly and reliably fine-tuned without using a reference sample. The utility of this method has been confirmed by studying the resolution patterns for the supercycled PMLG scheme, which were found to be in excellent agreement with earlier theoretical predictions and verified in high-resolution 2D (1)H-(1)H experiments.

Molecular ordering of mixed surfactants in mesoporous silicas: a solid-state NMR study.

The use of mixed surfactants in the synthesis of mesoporous silica nanoparticles (MSNs) is of importance in the context of adjusting pore structures, sizes and morphologies. In the present study, the arrangement of molecules in micelles produced from a mixture of two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) was detailed by solid-state NMR spectroscopy. Proximities of methyl protons in the trimethylammonium headgroup of CTAB and protons in the pyridinium headgroup of CPB were observed under fast magic angle spinning (MAS) by (1)H-(1)H double quantum (DQ) MAS NMR and NOESY. This result suggested that CTAB and CPB co-exist in the pores without forming significant monocomponent domain structures. (1)H-(29)Si heteronuclear correlation (HETCOR) NMR showed that protons in the headgroups of CTAB are in closer proximity to the silica surface than those in the CPB headgroups. The structural information obtained in this investigation leads to better understanding of the mechanisms of self-assembly and their role in determining the structure and morphology of mesoporous materials.

Spectral editing in 13C solid-state NMR at high magnetic field using fast MAS and spin-echo dephasing

A simple method is proposed for separating NMR resonances from protonated and non-protonated aromatic carbons in solids under fast magic angle spinning (MAS). The approach uses a MAS-synchronized spin-echo to exploit the differences in rotational recoupling of the dipolar interactions while fully refocusing the isotropic chemical shifts. This strategy extends the relevant time scale of spin evolution to milliseconds and circumvents the limitation of the traditional dipolar dephasing method, which in fast rotating solids is disrupted by rotational refocusing. The proposed approach can be used for quantitative measurement of carbon aromaticities in complex solids with poorly resolved spectra, as demonstrated for model compounds.