Probing aerogels by multiple quantum filtered (131)Xe NMR spectroscopy.

At the interface between solid surfaces and cavities filled with gaseous or liquid xenon, the nuclear magnetization of (131)Xe (S = (3)/(2)) is subject to quadrupolar interactions which may lead to higher rank single-quantum coherences that can be described by tensor elements T(2,)(+/-)(1) and T(3,)(+/-)(1). This can be demonstrated by multiple-quantum filtered (MQF) NMR experiments. In gaseous xenon on Pyrex surfaces, the primary source of such coherences was shown to be coherent evolution induced by a nonvanishing average quadrupolar coupling. In this contribution, MQF NMR is applied to aerogels filled with liquid xenon to demonstrate the potential of this technique for material sciences. Xenon in the liquid phase provides a sufficient spin density to obtain reasonable signal-to-noise ratios. Coherent evolution and relaxation both contribute to the creation of higher rank coherences depending on the presence or absence of water molecules on the surface. These two processes can be distinguished experimentally and provide complementary information about the surface of the host material.     

Determination of coordination modes and estimation of the 31P-31P distances in heterogeneous catalyst by solid state double quantum filtered 31P NMR spectroscopy

To overcome the separation difficulty of the palladium-based homogeneous catalyst, the palladium complex can be anchored on various supports such as silica. However, it is difficult to determine the amounts of the two coordination modes of the Pd nucleus, that is, Pd coordinates with one phosphorus atom and Pd coordinates with two phosphorus atoms. Here a (31)P double-quantum filtered (DQ-filtered) method in solid-state NMR is introduced for the palladium-based heterogenous catalyst system. With the DQ-filtered method, we can not only determine the amounts of the two different kinds of palladium coordination modes, we can also estimate the interatomic distance of two (31)P nuclei bonded to a palladium nucleus. With the help of this method, we can quickly estimate interatomic distances in our designed system and accurately re-design the palladium system to accommodate either one (31)P or two (31)P.     

The two-dimensional NMR spectroscopy of macromolecules

Two-dimensional (2D) NMR is a versatile technique which exists in many versions. Two broad classes of 2D techniques are (1) correlated spectroscopy and (2) J-resolved spectroscopy. The first of these may be divided into two further subdivisions: COSY, which permits correlations of resonances via J-coupling, and NOESY, which allows direct measurement of intenuclear (usually interproton) distances by the nuclear Overhauser effect. COSY greatly facilitates the interpretation of complex spectra and spectral interpretation in terms of stereochemical sequences is placed on a firmer foundation. NOESY provides direct information concerning the local conformations of polymers in solution. By use of J-resolved 2D NMR, we can separate J-couplings and chemical shifts on different axes and thus achieve a degree of resolution of both these parameters far beyond what is attainable in the 1D spectrum. Finally, we may combine correlated and J-resolved spectroscopy and achieve the benefits of both in 3D NMR, in which the 2D cross peaks exhibit J-coupling fine structure. These techniques are illustrated for a variety of polymers including poly(methyl methacrylate), poly(vinyl fluoride), poly-y-benzyl L-glutamate, and poly(propylene oxide).     

Principles and features of single-scan two-dimensional NMR spectroscopy

Two-dimensional nuclear magnetic resonance (2D NMR) provides one of the foremost contemporary tools available for the elucidation of molecular structure, function, and dynamics. Execution of a 2D NMR experiment generally involves scanning a series of time-domain signals S(t(2)), as a function of a t(1) time variable which undergoes parametric incrementation throughout independent experiments. Very recently, we proposed and demonstrated a general approach whereby this serial mode of data acquisition is parallelized, enabling the acquisition of complete bidimensional NMR data sets via the recording of a single transient. The present paper discusses in more detail various conceptual and experimental aspects of this novel 2D NMR methodology. The basic principles of the approach are reviewed, various homo- and heteronuclear NMR applications are illustrated, and the main features and artifacts affecting the method are derived. Extensions to higher-dimensional experiments are also briefly noted.     

Application of the computer program GENOA and two-dimensional NMR spectroscopy to structure elucidation

The effectiveness and utility of the combined use of the interactive computer program GENOA and two-dimensional double quantum coherence ¹³C NMR spectroscopy to solve structure elucidation problems are demonstrated with three examples. The substructural information inferred from the ¹H NMR and ¹³C NMR spectra was insufficient to identify the structures of the example compounds. However, with the additional substructural information available from the two-dimensional NMR experiment (which identifies directly bonded carbon atoms), the problems were solved in an extremely efficient manner.     

Everything you wanted to know about phase and reference frequency in one- and two-dimensional NMR spectroscopy

The fundamental concept of phase discussed in this tutorial aimed at providing students with an explanation of the delays and processing parameters they may find in nuclear magnetic resonance (NMR) pulse programs. We consider the phase of radio-frequency pulses, receiver, and magnetization and how all these parameters are related to phases and offsets of signals in spectra. The impact of the off-resonance effect on the phase of the magnetization is discussed before presenting an overview of how adjustment of the time reference of the free induction decay avoids first-order correction of the phase of spectra. The main objective of this tutorial is to show how the relative phase of a pulse and the receiver can be used to change the reference frequency along direct and indirect dimensions of NMR experiments. Unusual of phase incrementation with non-90° angles will be illustrated on one- and two-dimensional NMR spectra.     

The study of longifolene by two-dimensional NMR spectroscopy

The complete assignment of the ¹³C NMR spectrum of longifolene was achieved from double quantum coherence measurements, while combined evaluation of a ¹H? ¹³C heteronuclear chemical shift correlation diagram and a homonuclear ¹H J-resolved diagram provided all proton chemical shifts. Conformational information on the seven-membered ring of the tricyclic sesquiterpene was obtained from proton chemical shift considerations.     

Organometallic complexes in supported ionic-liquid phase (SILP) catalysts: a PHIP NMR spectroscopy study

para-Hydrogen induced polarization (PHIP) NMR spectroscopy emerges as an efficient and robust method for on-line monitoring of gas-phase hydrogenation reactions. Here we report detailed investigations of supported ionic liquid phase (SILP) catalysts in a continuous gas-phase hydrogenation of propene with PHIP NMR spectroscopy. A relocation of the rhodium complex in the thin layer of ionic liquid in the SILP catalyst at the initial stage of the propene hydrogenation is demonstrated. PHIP NMR spectroscopy can provide profound insight into the evolution of SILP catalysts during hydrogenation reactions.     

Cryogenic NMR spectroscopy of endohedral hydrogen-fullerene complexes

We have observed 1H NMR spectra of hydrogen molecules trapped inside modified fullerene cages under cryogenic conditions. Experiments on static samples were performed at sample temperatures down to 4.3 K, while magic-angle-spinning (MAS) experiments were performed at temperatures down to 20 K at spinning frequencies of 15 kHz. Both types of NMR spectra show a large increase in the intramolecular 1H-1H dipolar coupling at temperatures below 50 K, revealing thermal selection of a small number of spatial rotational states. The static and MAS spectra were compared to estimate the degree of sample heating in high-speed cryogenic MAS-NMR experiments. The cryogenic MAS-NMR data show that the site resolution of magic-angle-spinning NMR may be combined with the high signal strength of cryogenic operation and that cryogenic phenomena may be studied with chemical site selectivity.     

Study of conformation of cryptahemispherands by two-dimensional NMR spectroscopy

Two-dimensional NMR spectroscopy (NOESY, COSY-DQF) has been used in a study of nitrogen-containing cryptahemispherands and their complexes with alkali metals. On the basis of experimentally established interproton distances, the preferred conformations of these molecules have been established. It has been shown that the important features of structure determining the molecular conformation are (a) the orientation of the N-CH₂(Ar) bond relative to the plane of the crown ether and (b) the length of the methylene bridge (spacer). The spatial and torsional angles have been determined. It has been established that as the length of the bridge is increased, the structure of these molecules approaches a betaine structure, with mutually parallel positions of the phenyl rings. Upon complexation, their orientation changes to mutually perpendicular, and this leads to enlargement of the inner cavity of the molecule and a corresponding rearrangement of its dimensions toward the ionic radius of the alkali metal.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 406–411, March, 1993.     

Characterization of noninnocent metal complexes using solid-state NMR spectroscopy: o-dioxolene vanadium complexes

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 to -700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy tensor in reasonable agreement with the experimental results, but surprisingly the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron-donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron-withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because (51)V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox-active complexes that exhibit coordination chemistry similar to that of the vanadium catechol complexes.     

Coherence transfer via longitudinal spin order generalized pulse pair filtering for pure phase two-dimensional NMR spectroscopy

A generalized pulse pair has been suggested in which the longitudinal spin order is retained and the transverse components cancelled by random variation of the interval between pulses, in successive applications of the two-dimensional NMR algorithm. This method leads to pure phases and has been exploited to provide a simpler scheme for two-spin filtering and for pure phase spectroscopy in multiple-quantum-filtered two-dimensional NMR experiments.     

Multinuclear and two-dimensional NMR spectroscopy of monoazo derivatives of chromotropic acid

By the use of one-dimensional (¹H,¹³C, and¹⁵N) and two-dimensional NMR spectra (H,H-COSY, H,C-COSY, and NOESY), it has been established that the phenylazo derivative of chromotropic acid in DMSO has a quinonehydrazone structure; under normal conditions, there is no azo-quinonehydrazone tautomerism. Spectral evidence has been found for the distribution of intramolecular hydrogen bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2530–2535, November, 1989.     

Multiple quantum filtered NMR studies of the interaction between collagen and water in the tendon

We studied the physical processes and the chemical reactions involved in magnetization transfer between water and large proteins, such as collagen, in bovine Achilles tendon. Since the NMR spectrum for such proteins is broadened by very large dipolar interactions, the NMR peaks of the various functional groups on the protein cannot be separated from one another on the basis of their different chemical shifts. A further complication in observing the protein spectrum is the intense narrow peak of the abundant water. Thus, magnetization transfer (MT) within the protein or between water and the protein cannot rely on differences in the chemical shifts, as is commonly possible in liquids. We present a method that separates the protein spectrum from that of the water spectrum on the basis of their different intramolecular dipolar interactions, enabling exclusive excitation of either the protein or water. As a result, the protein spectrum as well as the effect of spin diffusion within the protein can be measured. In addition, the MT rates from the protein to water and vice versa can be measured. Two types of mechanisms were considered for the MT: chemical exchange- and dipolar interaction-related processes (such as NOE). They were distinguished by examining the effects of the following experimental conditions: (a) temperature; (b) pH; (c) ratio of D(2)O to H(2)O in the bathing liquid; (d) interaction of the protein with small molecules other than water, such as DMSO and methanol. Our results lead us to the conclusion that the MT is dominated below the freezing point by the dipolar interaction between the protein and water, while an exchange of protons between the protein and the water molecules is the most significant process above the freezing point. On the basis of the fact that the spin temperature is established for the protein on a time scale much shorter than that of the MT, we could measure protein spectra that are distinguished by the contributions made to them by the various functional groups; i.e., contributions of methylenes were distinguished from those of methyls.     

Poly(vinylpyrrolidone): Configurational assignments by one- and two-dimensional NMR spectroscopy

The configurational assignment of poly(vinylpyrrolidone) (PVP) prepared by peroxide-initiated solution polymerization was studied by the combination of one- and two-dimensional NMR spectroscopy. The broad and overlapping ¹H-NMR and ¹³C{¹H}-NMR spectra of PVP were assigned to the configurational triad, pentad (CH, ²CH₂, ³CH₂, and ⁴CH₂ regions), and tetrad (β-CH₂ region) sequences. The configurational assignments of the various carbon resonances were confirmed with the help of two-dimensional experiments such as heteronuclear single quantum correlation (HSQC), heteronuclear single quantum correlation–total correlation spectroscopy (2-D HSQC–TOCSY). The various geminal and vicinal couplings within the configurational sequences were assigned with the help of total correlation spectroscopy (TOCSY low mixing time). The propagation pathway was studied using the ¹³C{¹H}-NMR (carbonyl carbon) and ¹⁵N{¹H}-NMR spectra. The ¹⁵N{¹H} resonance signals were assigned to pentad-level configurational sequences. The results obtained by the analysis of the area under the resonance signals confirmed that poly(vinylpyrrolidone) obeys Bernoullian statistics. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3922–3928, 1999     

Decoupling two-dimensional NMR spectroscopy in both dimensions: pure shift NOESY and COSY

An increase in the resolving power in 2D NMR spectra is obtained by collapsing 2D signals with multiplet structure into 2D singlets. This resolution gain is achieved by combining 2D experiments with pure shift techniques and covariance processing (see picture). The method should be of value in both manual and automated structure determination.     

SPEEDY: spin-echo enhanced diffusion filtered spectroscopy. A new tool for high resolution MAS NMR

The pulsed field gradient diffusion edited experiment, bipolar LED, has been combined with the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence for the analysis of solid phase resin samples. Spin-echo enhanced diffusion filtered spectroscopy (SPEEDY), when optimized, filters both the compounds that demonstrate fast diffusion rates as well as the compounds that demonstrate fast T(2) relaxation rates. Using this technique, compounds that are not covalently attached to the resin are not observed and contributions from the resin matrix are greatly attenuated. The interpretation of the resulting spectrum is more readily accessible. This technique lessens the importance of completely removing reaction residues or the wash solvents simply for analytical evaluation. The utility of the combined filtering scheme was demonstrated by the implementation into a NOESY sequence.     

Assignment of proton NMR spectra and conformational analysis by two-dimensional heteronuclear relayed correlation NMR spectroscopy

The method of two-dimensional heteronuclear relayed correlation spectroscopy was used to establish the assignment of the severely overcrowded part of the proton spectrum of menthol by relating it to the previously assigned carbon spectrum. Extrapolation of the signal-to-noise ratio obtained with overnight data accumulation on a 10 mM solution suggests that this experiment should be feasible on as little as 10 mg of a moderate-sized organic compound.