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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.

Study of Chemical Exchange by 2D NMR

The NMR signal of a spin reflects its local magnetic environment. If a spin due to chemical exchange samples two magnetically different states then its NMR signal would reflect both states. Its appearance on a NMR spectrum would be determined by the dynamics of the exchange event. In the case of chemical exchange that is slow on the NMR time scale, it is possible to observe two distinct signals for the same spin, one signal for each state under exchange. Presence of chemical exchange is often demonstrated with the exchange experiment (2D NOESY experiment). A exchange peak (cross peak) of the same sign can be observed between the two autopeaks (diagonal peaks) that represent the two states under exchange. Because the same results can be interpreted in a different way based on NOE, further evidence is desirable.

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Corona - Glow Discharge (DART Ion Source)

February 22, 2020
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