JEOL Resources

Introduction of a method to analyze 3D structures using homonuclear couplings_NM210004E

Mon, 29 Nov 2021 10:14:39 GMT

Structural analysis by NMR can provide not only a planar molecular structure but also three-dimensional structural information. In this Note, we describe a method for obtaining information on dihedral angles by using ¹H-¹H coupling constants (J_HH values). For example, hydrogen atoms attached to a cyclohexane ring are either located in axial or equatorial positions in respect to the cyclohexane ring (Fig. 1). The dihedral angles between vicinal protons are known to be ∠H_ax-C-C-H_ax ≈ 180°, ∠H_ax-C-H_eq ≈ 60°, and ∠H_eq-C-C-H_eq ≈ 60°. If we look at the Karplus curve shown in Fig. 2, we can see that ³J_HH of around 4 Hz can be expected in the case of the dihedral angle of 60°, while ³J_HH of around 13 Hz corresponds to the dihedral angle of 180°. In reality, ³J_HH values depend on substituents attached to the cyclohexane ring in substituted cyclohexanes, so the analysis is not straightforward, but the basic trend of having a larger J-value for a 180° dihedral angle compared to a 60° dihedral angle remains unchanged. Therefore, from the value of ³J_HH of the methylene protons, it is possible to differentiate between the dihedral angle of 60° or 180°.

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Separation of 13C spectra of polyurethane soft and hard segments by ROSY_NM200013E

Tue, 09 Mar 2021 16:22:11 GMT

Separation of ¹³C spectra of polyurethane soft and hard segments by ROSY

The ROSY (Relaxation Ordered SpectroscopY) is a method in which the ¹³C CPMAS spectrum of a mixture is classified by a longitudinal relaxation time of ¹H, and the ¹³C CPMAS spectrum is displayed separately for each substance. In solution NMR, each peak in the ¹H spectrum has its own longitudinal relaxation time. In solid-state NMR, however, spin diffusion occurs due to the dipolor interaction between ¹H, and all ¹H have the same longitudinal relaxation in the domain within a certain distance. The ¹³C spectrum can be separated for each domain by using this difference in relaxation time of ¹H. The longitudinal relaxation time (T₁^H) obtained by the saturation recovery method as shown in Fig.1a is usually used to separate the ¹³C spectrum of the mixture. The size of the domain that can be separated by this method is about 100 nm. To separate domains smaller than this, a measurement using the relaxation time at rotational flame (T_1ρ^H) obtained by the spinlock method as shown in Fig.1b is effective. The domain size that can be separated by T_1ρ^H is about several nm, and it is possible to determine the phase separation structure of block copolymers and the molecular compatibility.

Analysing complex mixtures using Pure shift DOSY_NM190018E

Tue, 15 Sep 2020 10:43:42 GMT

Diffusion-ordered spectroscopy (DOSY) is a powerful NMR method for the analysis of mixtures. In DOSY, signals in the NMR spectrum are resolved according to the measured diffusion coefficient for each signal, yielding a 2D spectrum which has chemical shift along the x-axis and diffusion coefficient along the y-axis.

Observation of NOE by HSQC-NOESY_NM200005E

Mon, 14 Sep 2020 16:53:22 GMT

NOE (Nuclear Overhauser Effect) correlations comprise important information to estimate internuclear distance and determine structure. However, NOE correlation peaks are very weak compared with diagonal peaks in 2D NOESY. For this reason, it is difficult to observe NOE correlation peaks in the vicinity of much larger diagonal peaks.

On Accurate Measurements of Diffusion Coefficients by PGSE NMR Methods

Tue, 28 Jul 2020 10:02:16 GMT

In this article, we will show the accurate measurements of the diffusion coefficients (D) by using room-temperature ionic liquids (RTIL, IL) as examples.

Need for high resolution 2D spectra

Tue, 28 Jul 2020 09:59:38 GMT

¹³C NMR spectra provide wide range chemical shift, and it suggests that can easily distinguish each signals. But carbon resolution of 2D spectra such as HSQC and HMBC is worse than 1D ¹³C spectra due to small data points. In order to analyze a compound with close ¹³C chemical shifts, a high resolution 2D spectrum is required frequently. In this document, some improvements to distinguish each signals on ¹³C axis of 2D hetero nuclear experiments are presented.

High sensitivity and quantitative 13C measurements using "Q-POMMIE"

Tue, 28 Jul 2020 09:54:20 GMT

POMMIE (Phase Osacillations to Maximize Editing) is a ¹³C experiment that, like the more familiar DEPT experiment, utilizes polarization transfer to enhance the intensities of the ¹³C signals. However, unlike DEPT, POMMIE edits the spectrum by varying pulse phase rather than adjusting pulse flip angle.

Sample tube grade affects resolution

Mon, 29 Jun 2020 09:15:49 GMT

Some low-grade, inexpensive NMR sample tubes have large warpage, low wall thickness uniformity, and large distortion, which may adversely affect the resolution. The effect of low-grade sample tubes, such as disposable ones, on the resolution is small in low-field NMR, but it may be noticeable in high-field NMR. In addition, some disposable sample tubes are thicker or thinner than the nominal value and will not fit in the sample holder.

Good resolution cannot be expected with low grade sample tubes in high-field NMR

To examine the effect of disposable sample tubes on resolution, data measured on 400 MHz and 600 MHz spectrometers were compared. The sample solution was prepared in advance and dispensed into 10 disposable sample tubes. The gradient shim was performed with two iterations of magnetic field correction.
The resolution of one out of ten samples was poor on the 400 MHz spectrometer (shown in Figure A). On the other hand, when these ten samples were measured with the 600 MHz spectrometer, four of the ten samples showed good resolution, but five did not have very good resolution and one had poor resolution (shown in Figure B). The distortion of the static magnetic field resulting from the distortion of a sample tube is proportional to the magnetic field strength. Note that as the magnetic field strength increases, the effect of sample tube distortion on the resolution increases.

Resolution of 10 identical samples
Results of gradient shimming on (A) 400 MHz and (B) 600 MHz NMR.

If good resolution is not achieved, it may be possible to improve the resolution by increasing the number of iterations of the magnetic field correction or by adding the magnetic field correction with auto shim or FID shim after gradient shimming, but this may not be possible. In other words, using a highly accurate sample tube depending on the magnetic field strength of the NMR will lead to stable and high resolution data. If the sample tube is not suitable for the magnetic field strength, it cannot be said that the resolution will definitely increase.
If a sample is measured with a 400 MHz NMR and the resolution is good, but not good when the same sample is measured with a higher field NMR, it may be due to the grade of the sample tube as mentioned above. However, a low-grade sample tube does not necessarily impair the resolution. After sample preparation, the grade of the sample tube tends to be unknown. If the sample tube grade is suspected to be the cause of the poor resolution, it is recommended to consider changing the sample tube.

Resolution when using a scratched sample tube

Even if the sample tube is highly accurate, scratches can cause localized distortion of the magnetic susceptibility, resulting in poor resolution. The spectrum in the center of the figure below is the data obtained by measuring with a scratched sample tube. Gradient shim conditions are the same as those without scratches (left in the figure below), but resolution did not improve. If you use the Gradient Shim Tool for shimming, you can see the magnetic field map at the time of magnetic field correction (lower part of the figure below). In this figure, you can see that the magnetic field map is distorted where there are scratches. Even if the number of iterations was increased until the magnetic field correction results converged, the resolution did not improve (right figure below). It is strongly recommended to replace the sample tube if it becomes scratched. In order to always achieve good resolution, it is important to be careful not to mix low-precision or scratched sample tubes with high-precision ones.

Effect of scratches on resolution
Spectra shown in the upper row and the magnetic field maps in the bottom row. Left; No scratch, 2 iterations. Center; After measuring the data on the left, the sample tube was scratched. The number of magnetic field-corrected iterations is two, which is the same as without a scratch. Right; The same sample as the central data. The number of magnetic field-corrected iterations was judged by automatic convergence to increase the number of iterations.

NOAH-NMR Supersequences with Nested Acquisition for Small Molecules

Fri, 12 Jun 2020 12:48:15 GMT

NOAH (NMR by Ordered Acquisition using ¹H-detection)[1] is a group of nested NMR experiments combining several conventional two-dimensional (2D) NMR pulse sequences, such as COSY, HSQC and HMBC, into one supersequence. Therefore, two or more 2D NMR data can be obtained from a single NOAH experiment. By using a single relaxation delay, the NOAH method significantly reduces the total data collection time and increases the throughput of an NMR instrument in structure elucidation of small organic molecules.

High Resolution 2D spectra

Fri, 12 Jun 2020 12:46:00 GMT