Advanced Nuclear Magnetic Resonance Spectroscopy

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...

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...

The OH proton in alcohols can shift and split differently in a proton NMR spectrum. Its position usually falls around 2 to 5 ppm, but the exact value depends on the alcohol and the sample conditions. This proton is labile, which means it can exchange rapidly with other molecules.

Hydrogen bonding to oxygen or nitrogen changes the electron density around the proton. That change affects the chemical shift seen in the proton NMR spectrum. The exchange rate also affects spin-spin coupling between...

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, 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...

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 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 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...

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...

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,...

HSQC NMR maps the direct links between hydrogen atoms and the carbons they are attached to. It is a 2D NMR method, so it shows the data on two axes instead of one. The experiment is similar to heteronuclear correlation spectroscopy, or HETCOR, but HSQC is more sensitive.

In an HSQC spectrum, the proton chemical shift appears on the horizontal F2 axis. The 13C chemical shift appears on the vertical F1 axis. The spectrum also includes the matching proton and 13C spectra. The contour plot does...