Name: nmr signal boosting with inept pulses
Uploaded: 2024-12-05T18:32:26.000Z
Duration: 1 min 15 s
Channel: JoVE

Insensitive nuclei enhanced by polarization transfer, or INEPT, uses protons or other sensitive, abundant nuclei to strengthen NMR signals originating from low-abundance, low-sensitivity nuclei, like carbon-13 and nitrogen-15.
For example, the proton&#8211;carbon-13 pulse sequence involves a combination of 90-degree and 180-degree pulses applied to the high-sensitivity and low-sensitivity nuclei.
Coupling-constant-based delays between pulses allow precise relaxation through space or bond interactions and transfer polarization to low-sensitivity nuclei via spin coupling.
The pulses and delays together invert the net magnetization of protons coupled to carbon-13 in only one of its spin states, ultimately enhancing the carbon-13 signals more than NOE does.
INEPT&#39;s effectiveness is seen in the proton-coupled carbon-13 spectrum of pyridine. The three types of pyridine carbon nuclei are all split into doublets of some additional multiplicity. For example, C2 is a doublet of multiplets and is located farthest downfield at about 40 ppm.

insensitive nuclei enhanced by polarization transfer (inept)

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT)

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and enhance the signals of low-abundance nuclei, such as carbon-13 and nitrogen-15, in small molecules. The fundamental principle behind INEPT is the transfer of polarization from a more abundant and highly polarizable nucleus, typically hydrogen-1, to the low-abundance nucleus of interest. This process effectively boosts the NMR signal of the low-abundance nuclei, making them more easily detectable.
The INEPT technique involves a series of carefully timed radiofrequency (RF) pulses applied to the sample. These RF pulses excite the more abundant nuclei, causing them to align their spins in a specific manner. Through the process of spin coupling, the polarization of the more abundant nuclei is then transferred to the low-abundance nuclei. As a result, the NMR signal of these low-abundance nuclei is significantly enhanced, allowing for a more detailed analysis of the molecular structure.
Various pulse sequences have been developed to improve the observability of these low-abundance nuclei when coupled to a high-receptivity nucleus. The INEPT pulse sequence combines a spin echo and selective population inversion, which includes 90&#176; and 180&#176; pulses on the high-sensitivity (the I spins) and low-sensitivity nuclei (the S spins), respectively. Coupling-constant-based delays between pulses facilitate magnetization transfer through space or bond interactions. Additional RF pulses and delays invert the low-gamma magnetization, producing an observable NMR signal. The net result is an enhanced NMR signal from low-gamma nuclei that would otherwise be challenging to detect.
Various pulse sequences have been developed to improve the detection of low-abundance nuclei when coupled with high-receptivity nuclei such as protons. The INEPT sequence yields enhanced signals for insensitive nuclei, with half of the CH group producing negative peaks and the other half positive peaks. This pattern allows for discrimination between different types of carbon-hydrogen bonds within the molecule.
Decoupling methods can be employed to enhance sensitivity and resolution in distinguishing various carbon-hydrogen bonds, making INEPT an invaluable tool in the chemical and pharmaceutical industries.

Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) is an advanced Nuclear Magnetic Resonance (NMR) technique specifically designed to detect and ...

insensitive-nuclei-enhanced-by-polarization-transfer-inept

Education

JoVE Core

Analytical Chemistry

Unit 1

Advanced Nuclear Magnetic Resonance Spectroscopy

advanced-nuclear-magnetic-resonance-spectroscopy

KEY TERMS AND CONCEPTS

nmr signal boosting with inept pulses

INEPT is an NMR method that boosts weak signals from low-abundance nuclei. It is used to detect nuclei such as carbon-13 and nitrogen-15 in small molecules. The goal is to make these hard-to-see nuclei easier to observe in an NMR spectrum.

The technique works by transferring polarization from a more abundant nucleus, usually hydrogen-1, to the nucleus of interest. Hydrogen-1 has higher receptivity, so it can provide the starting signal boost. Spin coupling then helps move that polarization to the low-abundance nucleus.

INEPT uses a sequence of carefully timed radiofrequency, or RF, pulses. These pulses include 90° and 180° pulses applied to the high-sensitivity I spins and the low-sensitivity S spins. Delays based on the coupling constant help the magnetization move through bond interactions and produce an observable signal.

The pulse sequence also combines a spin echo with selective population inversion. Additional RF pulses and delays invert the low-gamma magnetization, which leads to signal enhancement. The result is a stronger NMR response from nuclei that would otherwise be difficult to detect.

INEPT can also help distinguish carbon-hydrogen bond types in a molecule. In the CH group, half of the signal gives negative peaks and the other half gives positive peaks. Decoupling methods can further improve sensitivity and resolution, which makes INEPT useful in chemical and pharmaceutical analysis.

NMR Signal Boosting with INEPT Pulses

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¹H NMR can show how fast molecular motion changes the signals we see. In cyclohexane, the chair form flips rapidly between two equivalent chair conformers at room temperature. This happens about 10^5 times per second, and the two shapes stay in equilibrium. During ring flipping, an axial proton becomes equatorial, and an equatorial proton becomes axial. These changes are too fast to detect on the NMR timescale. As a result, the NMR spectrometer cannot tell the axial and equatorial protons...

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Cyclohexane Ring Flipping in Variable-Temperature NMR

Cyclohexane ring flipping can be tracked with variable-temperature NMR. The experiment helps distinguish the axial and equatorial protons in a deuterium-substituted cyclohexane sample. Deuterium is used to replace eleven of the twelve protons. This reduces possible peak splitting from spin-spin coupling between neighboring protons. The remaining proton shifts between the axial and equatorial positions as the chair form of cyclohexane changes. At room temperature, ring flipping is very fast, so...

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&sup1;h nmr of labile protons:  deuterium (&sup2;h) substitution

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Using D2O to Identify OH Protons

Deuterium substitution can help identify labile protons in ¹H NMR spectra. Labile protons are hydrogens that can be replaced by another hydrogen isotope. In this lesson, deuterium is used to simplify the spectrum of ethanol. Deuterium, or ²H, is one of the three isotopes of hydrogen. Its nucleus contains one proton and one neutron. When D₂O is added to a pure dry ethanol solution, the labile proton on the –OH group is replaced by deuterium. A proton NMR spectrum does not show signals from...

nuclear overhauser enhancement (noe)

Using NOE to Map Nearby Nuclei

Nuclear Overhauser enhancement, or NOE, shows how irradiation of one spin-active nucleus can change the signal of nearby nuclei. These nuclei do not have to be chemically bonded or connected through J-coupling. The effect comes from through-space interactions between nuclear spins. NOE gets weaker as the distance between nuclei increases. It is generally not observed beyond about 4 angstroms. Dipole-dipole interactions between neighboring spin-active nuclei transfer nuclear spin polarization,...

INEPT is an NMR method that boosts weak signals from low-abundance nuclei. It is used to detect nuclei such as carbon-13 and nitrogen-15 in small molecules. The goal is to make these hard-to-see nuclei easier to observe in an NMR spectrum. The technique works by transferring polarization from a more abundant nucleus, usually hydrogen-1, to the nucleus of interest. Hydrogen-1 has higher receptivity, so it can provide the starting signal boost. Spin coupling then helps move that polarization to...

double resonance techniques: overview

Spin Decoupling in NMR Spectroscopy

Spin decoupling is a key use of double resonance in nuclear magnetic resonance (NMR) spectroscopy. It uses two radiofrequency pulses or frequencies at the same time to act on two different nuclear spins. This makes the spectrum simpler and easier to read. In spin decoupling, one type of nucleus is selectively suppressed while the signal from another nucleus is observed. The radiofrequency pulse sequence removes the coupling with one nuclide and can greatly improve resolution. As a result, the...

two-dimensional (2d) nmr: overview

2D NMR Spectra and Signal Mapping

Two-dimensional (2D) NMR helps make sense of crowded NMR data from large and complex molecules, such as natural products. A 1D NMR spectrum can have overlapping signals and complicated splitting patterns. 2D NMR adds a second frequency axis, which helps show the coupling information between nucleus A and nucleus B in a molecule. 2D spectra are built in four steps. First is the preparation period, when nucleus A is excited with a radiofrequency pulse. Next is the evolution, or wait, period t1.

2d nmr: overview of homonuclear correlation techniques

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2D NMR Correlation Paths in Molecules

2D NMR homonuclear correlation techniques show how nuclei of the same type connect within a molecule. In nuclear magnetic resonance (NMR) spectroscopy, these methods reveal scalar couplings, which are interactions between nearby nuclei through bonds. That information helps identify which atoms are linked and how a molecule is put together. COSY is the main homonuclear correlation experiment. COSY90 is the standard 2D COSY method and uses a second 90-degree pulse to improve sensitivity and...

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Reading Proton Coupling with COSY Maps

COSY is a 2-dimensional NMR method that shows which protons are coupled to each other. It is used to spot geminal and vicinal coupling in a sample. This makes it helpful for reading proton relationships in complex spectra. The ethyl acetate example shows how COSY is built from the 1D proton NMR spectrum. The same chemical shift scale appears on both the vertical and horizontal axes. The peaks along the diagonal are called diagonal peaks, and they match the three peaks seen in the 1D proton...

2d nmr: overview of heteronuclear correlation techniques

2d-nmr-overview-of-heteronuclear-correlation-techniques

2D NMR Correlation Peaks and Bond Links

Heteronuclear correlation spectroscopy is a 2D NMR method that links different nuclei, usually a proton and an X-nucleus such as carbon-13 or nitrogen-15. It helps scientists study the structure and composition of complex chemical compounds. In a heteronuclear correlation spectrum, one axis shows X-nucleus chemical shifts and the other axis shows the proton spectrum. Cross-peaks mark which protons are connected to which X-nuclei. The main heteronuclear correlation experiments are HETCOR, HSQC,...

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HSQC NMR Maps Carbon-Proton Connections

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key terms and concepts

Learn how INEPT uses timed RF pulses and polarization transfer to strengthen weak NMR signals from carbon-13, nitrogen-15, and other low-abundance nuclei.

NMR Signal Boosting with INEPT Pulses | JoVE K12

analytical chemistry

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NMR Signal Boosting with INEPT Pulses

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