Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Nuclear magnetic resonance decoupling

Nuclear magnetic resonance decoupling (NMR decoupling for short) is a special method used in nuclear magnetic resonance (NMR) spectroscopy where a sample to be analyzed is irradiated at a certain frequency or frequency range to eliminate fully or partially the effect of coupling between certain nuclei. NMR coupling refers to the effect of nuclei on each other in atoms within a couple of bonds distance of each other in molecules. This effect causes NMR signals in a spectrum to be split into multiple peaks which are up to several hertz frequency from each other. Decoupling fully or partially eliminates splitting of the signal between the nuclei irradiated and other nuclei such as the nuclei being analyzed in a certain spectrum. NMR spectroscopy and sometimes decoupling can help determine structures of chemical compounds.

Explanation
NMR spectroscopy of a sample produces an NMR spectrum, which is essentially a graph of signal intensity on the vertical axis vs. chemical shift for a certain isotope on the horizontal axis. The signal intensity is dependent on the number of exactly equivalent nuclei in the sample at that chemical shift. NMR spectra are taken to analyze one isotope of nuclei at a time. Only certain types of isotopes of certain elements show up in NMR spectra. Only these isotopes cause NMR coupling. Nuclei of atoms having the same equivalent positions within a molecule also do not couple with each other. 1H (proton) NMR spectroscopy and 13C NMR spectroscopy analyze 1H and 13C nuclei, respectively, and are the most common types (most common analyte isotopes which show signals) of NMR spectroscopy.

Homonuclear decoupling is when the nuclei being radio frequency (r f) irradiated are the same isotope as the nuclei being observed (analyzed) in the spectrum. Heteronuclear decoupling is when the nuclei being r f irradiated are of a different isotope than the nuclei being observed in the spectrum. For a given isotope, the entire range for all nuclei of that isotope can be irradiated in broad band decoupling, or only a select range for certain nuclei of that isotope can be irradiated.

Practically all naturally occurring hydrogen ( H ) atoms have 1H nuclei, which show up in 1H NMR spectra. These 1H nuclei are often coupled with nearby non-equivalent 1H atomic nuclei within the same molecule. H atoms are most commonly bonded to carbon ( C ) atoms in organic compounds. About 99% of naturally occurring C atoms have 12C nuclei, which neither show up in NMR spectroscopy nor couple with other nuclei which do show signals. About 1% of naturally occurring C atoms have 13C nuclei, which do show signals in 13C NMR spectroscopy and do couple with other active nuclei such as 1H. Since the percentage of 13C is so low in natural isotopic abundance samples, the 13C coupling effects on other carbons and on 1H are negligible, and for all practical purposes splitting of 1H signals due to coupling with natural isotopic abundance carbon does not show up in 1H NMR spectra.

Similarly for all practical purposes, 13C signal splitting due to coupling with nearby natural isotopic abundance carbons is negligible in 13C NMR spectra. However, practically all hydrogen bonded to carbon atoms is 1H in natural isotopic abundance samples, including any 13C nuclei bonded to H atoms. In a 13C spectrum with no decoupling at all, each of the 13C signals is split according to how many H atoms that C atom is next to. In order to simplify the spectrum, 13C NMR spectroscopy is most often run fully proton decoupled, meaning 1H nuclei in the sample are broadly irradiated to fully decouple them from the 13C nuclei being analyzed. This full proton decoupling eliminates all coupling with H atoms and thus splitting due to H atoms in natural isotopic abundance compounds. Since coupling between other carbons in natural isotopic abundance samples is negligible, signals in fully proton decoupled 13C spectra in hydrocarbons and most signals from other organic compounds are single peaks. This way, the number of equivalent sets of carbon atoms in a chemical structure can be counted by counting singlet peaks, which in 13C spectra tend to be very narrow (thin). Other information about the carbon atoms can usually be determined from the chemical shift, such as whether the atom is part of a carbonyl group or an aromatic ring, etc. Such full proton decoupling can also help increase the intensity of 13C signals.

There can also be off-resonance decoupling of 1H from 13C nuclei in 13C NMR spectroscopy, where weaker r f irradiation results in what can be thought of as partial decoupling. In such an off-resonance decoupled spectrum, only 1H atoms bonded to a carbon atom will split its 13C signal. The coupling constant, indicating a small frequency difference between split signal peaks, would be smaller than in an undecoupled spectrum. Looking at a compound's off-resonance proton-decoupled 13C spectrum can show how many hydrogens are bonded to the carbon atoms to further help elucidate the chemical structure. For most organic compounds, carbons bonded to 3 hydrogens (methyls) would appear as quartets (4-peak signals), carbons bonded to 2 equivalent hydrogens would appear as triplets (3-peak signals), carbons bonded to 1 hydrogen would be doublets (2-peak signals), and carbons not bonded directly to any hydrogens would be singlets (1-peak signals).

Another method in 1H NMR is specific proton decoupling, where a rather narrow 1H frequency range is irradiated to decouple 1H nuclei (protons) from that frequency range with other nuclei, including other protons. This can be done within one 1H spectrum to help determine which protons in the 1H spectrum are next to (and thus coupled with) the irradiated protons in the molecule. This specific decoupling method is also used to help elucidate chemical structure, sometimes along with 13C NMR, mass spectrometry, infrared spectroscopy, and other methods.