Quantitative Cross-polarization Research Articles

"Hidden" CO2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO2 Species.

Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times (T1). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO2 signals, which contributed to an empirical model of CO2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T1 values confirm the presence of CO2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13C-labeled CO2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO2 species, showing that 45% of chemisorbed CO2 versus 55% of physisorbed CO2 is formed from the overall confined CO2 in amine-modified hybrid silicas. A total of six distinct CO2 environments were identified from which three physisorbed CO2 were discriminated, coined here as “gas, liquid, and solid-like” CO2 species. The complex nature of physisorbed CO2 in the presence and absence of chemisorbed CO2 species is revealed, shedding light on what fractions of weakly interacting CO2 are affected upon pore functionalization. This work extends the current knowledge on CO2 sorption mechanisms providing new clues toward CO2 sorbent optimization.

Measurement of component content by a quantitative solid-state NMR method: Targeting systems with coexistence of 13C-19F and 13C-1H polymers

As an essential fluoropolymer, polytetrafluoroethylene (PTFE) is commonly mixed with 13C-1H engineering polymers to obtain favorable physical properties, such as high mechanical strength, excellent wear resistance and high chemical stability. It has been noted that, the relative content of PTFE determines the physical and electrochemical properties of PTFE contained material. However, a proper way to quantify the component content of such PTFE mixture/blend or other 13C-1H/13C-19F systems is still desired. In this work, a quantitative cross polarization (CP) method named dQCPz/dQCPzRC is proposed, which aims to quantify the component content of 13C-1H/13C-19F mixture or immiscible blend. Upon the tests for alanine/PTFE mixture, polystyrene/PTFE mixture and poly(vinyl alcohol)/PTFE blend, the performances of three methods are compared, including cross polarization (CP), direct polarization (DP) and dQCPz/dQCPzRC. It is demonstrated that, CP is not quantitative, whereas dQCPz/dQCPzRC provides an accuracy equivalent to the DP method. Moreover, dQCPz/dQCPzRC consumes notably shorter experimental time than that of DP. Those favorable performances imply that dQCPz/dQCPzRC method has potential to become a routine tool for measuring the component content of 13C-1H/13C-19F systems for industrial and scientific research.

Quantitative Cross-Polarization Optical Coherence Tomography Detection of Infiltrative Tumor Margin in a Rat Glioma Model: a Pilot Study

Quantitative Cross-Polarization Optical Coherence Tomography Detection of Infiltrative Tumor Margin in a Rat Glioma Model: a Pilot Study

Quantitative structural characterization of POSS and octavinyl-POSS nanocomposites by solid state NMR

The ratio between two different 29Si atoms in chloromethylphenyl isobutyl Polyhedral Oligomeric Silsesquioxane (POSS) was determined based on the quantitative cross polarization (QCP) (Shu et al., Chem. Phys. Lett. 462 (2008) 125) in solid-state NMR. For a 29Si/1H spin system, cross polarization and depolarization together with the reciprocity relation were performed with optimized experimental conditions. It saves considerable experimental time compared to the 29Si direct polarization experiment. The same method was further applied to octavinyl-POSS nanocomposites containing perfluoropolyether (PFPE) for deriving directly and accurately the average numbers of reacted vinyl groups, which may not be obtained by combining FTIR and solution 1H NMR. In principle, the aforementioned method proves to be valuable in quantitative characterization of silicon related structures in bulk materials.

A novel scheme for quantitative characterization of the structures of mesoporous silica by solid-state 29Si NMR

Based on the quantitative cross-polarization (QCP) and direct excitation (DE) solid-state 29Si NMR techniques, a novel scheme for quantitative characterization of silica structures is developed. The QCP technique is used to determine the ratio between geminal silanol (Q2) and single silanol (Q3), while the DE technique with a small number of scans is used to determine the ratio of Q3 and siloxane (Q4). As a result, the proportions of Q2, Q3 and Q4 groups in mesoporous SBA-15 silica are determined and the scheme saves considerable experimental time.

Quantitative cross-polarization at magic-angle spinning frequency of about 20 kHz

We study the QUantitative Cross-Polarization (QU-CP) method proposed by Hou et al. (Chem. Phys. Lett. 421 (2006) 356) under the moderate MAS speed of 23 kHz, re-examining its two building blocks, namely, the CP polarization transfer from 1H to 13C, and the thermal equalization of the 13C magnetizations. We show that the nuclear-integrated cross-polarization (NI-CP) scheme is conveniently used for 1H– 13C polarization transfer, because of its simplicity, robustness to rf-mismatch, and compatibility with fast sample spinning. In the mixing part, in addition to dipolar-assisted rotational-resonance (DARR) recoupling, we examine the Phase-Alternated Recoupling Irradiation Schemes (PARIS and PARIS xy ), and Second-order Hamiltonian among Analogous Nuclei Generated by Hetero-nuclear Assistance Irradiation (SHANGHAI) sequences, and show that SHANGHAI gives the best performances in equalizing the 13C magnetizations.

Towards uniform enhancement in solid-state cross polarization magnetic angle spinning NMR: A scheme incorporating cross polarization with rotational resonance

A recently proposed experimental scheme for achieving uniform cross polarization enhancement of low-gamma nuclear species in solids under magic angle spinning, termed quantitative cross polarization (QUCP) [Hou et al., Chem. Phys. Lett. 421, 356 (2006)], is described, supported with comprehensive theoretical analysis, numerical simulation, and experimental investigation with both uniformly labeled and naturally abundant solids. This method combines cross polarization with dipolar-assisted rotational resonance (DARR) [Takegoshi et al., Chem. Phys. Lett. 344, 631 (2001)] broadband homonuclear recoupling technique to achieve quantitative CP spectra under fast magic angle spinning. In addition to the correct and systematical interpretation on the phenomenon we reported in the previous Letter, a number of general guidelines for performing QUCP experiments are presented in this work. It is firmly established that while the enhancement factor in QUCP depends on the CP contact time, uniform enhancement can nevertheless be realized for all types of carbon group. For natural abundance samples, the polarization transfer rate is generally slower than that in labeled samples, but quasi-equilibrium among dilute spins in the mixing period can always be reached and uniform enhancement can be achieved albeit the DARR irradiation time needed can be much longer. For labeled samples, the time gain of QUCP experiment is almost the same as that of conventional CP. For natural abundance samples, it is generally much better than single-pulse experiment. Various representative systems, including uniformly (13)C-labeled DL-alanine and (13)C, (15)N labeled L-tyrosine, as well as naturally abundant alanine, tyrosine, and monoethyl fumarate, are used to verify the validity of our theoretical analysis and numerical simulation and to demonstrate the utility and advantages of the present approach.

Quantitative cross-polarization NMR spectroscopy in uniformly 13C-labeled solids

A novel QUantitative Cross Polarization (QUCP) scheme is proposed, which can be applicable for quantitative measurement of cross-polarization magic angle spinning (CP/MAS) spectroscopy in solid-state NMR. In this QUCP experiment, the combination of CP and broadband homonuclear recoupling technique (DARR) ‘homogenizes’ the non-uniform CP-prepared polarizations of dilute spins so that quantitative CP/MAS spectra can be obtained. Not only are all magnetizations enhanced uniformly by QUCP scheme, but also is the experimental time reduced greatly. In addition, the uniform enhancement is independent of the experimental parameters of cross-polarization. The present scheme is applicable for systems containing carbonyl, aromatic, and aliphatic carbons or some of these groups.