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

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 (JHH 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 ∠Hax-C-C-Hax ≈ 180°, ∠Hax-C-Heq ≈ 60°, and ∠Heq-C-C-Heq ≈ 60°. If we look at the Karplus curve shown in Fig. 2, we can see that 3JHH of around 4 Hz can be expected in the case of the dihedral angle of 60°, while 3JHH of around 13 Hz corresponds to the dihedral angle of 180°. In reality, 3JHH 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 3JHH of the methylene protons, it is possible to differentiate between the dihedral angle of 60° or 180°.

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

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 (T1H) 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 (T1ρH) obtained by the spinlock method as shown in Fig.1b is effective. The domain size that can be separated by T1ρH is about several nm, and it is possible to determine the phase separation structure of block copolymers and the molecular compatibility.

Need for high resolution 2D spectra

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.

Sample tube grade affects resolution

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.

NOAH-NMR Supersequences with Nested Acquisition for Small Molecules

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

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.

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Corona - Glow Discharge (DART Ion Source)

January 28, 2022
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