User:Melikajvn/Nuclear magnetic resonance spectroscopy

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Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical tool in chemistry, utilizing the behaviour of atomic nuclei in a magnetic field. NMR provides unique spectra that offers insights into molecular structures, connection and chemical environments of atoms. NMR signals are characterized by chemical shift, spin-spin coupling (J-coupling), and relaxation time. Chemical shift, expressed in parts per million (ppm), reveals the electronic environment of nuclei in a magnetic field and spin-spin coupling provides information about the neighbouring atoms. NMR can also be used to study the molecules that are non-soluble phases such as crystal and gel. This type are NMR spectroscopy is commonly known as solid-state nuclear magnetic resonance. In biochemistry,NMR is used to study the structures and dynamics of protein,nucleic acid, and carbohydrate.

NMR-Active Criteria
The key determinant of NMR activity in atomic nuclei is the nuclear spin quantum number (I ). This intrinsic quantum property, similar to an atom's "spin," characterizes the angular momentum of the nucleus. To be NMR-active, a nucleus must have a non-zero nuclear spin (I ≠ 0). It is this non-zero spin that enables nuclei to interact with external magnetic fields and show signals in NMR. Atoms with both an odd number of protons and an odd number of neutrons, or an odd sum of protons and neutrons, exhibit half-integer values for the nuclear spin quantum number (I = 1/2, 3/2, 3/5, and so on). These atoms are NMR-active because they possess non-zero nuclear spin. Conversely, atoms with an even number of both protons and neutrons, or an even sum of protons and neutrons, have a nuclear spin quantum number of zero (I = 0). These nuclei do not exhibit active spin and are therefore not NMR-active. NMR-active nuclei, particularly those with a spin quantum number of 1/2, are of great significance in NMR spectroscopy. Examples include 1H, 13C, 15N, and 31P.

Deuterated solvents
The vast majority of molecules in a solution are solvent molecules, and most regular solvents are hydrocarbons and so contain NMR-active hydrogen-1 nuclei. In order to avoid having the signals from solvent hydrogen atoms overwhelm the experiment and interfere in analysis of the dissolved analyte, deuterated solvents are used where 99+% of the protons are replaced with deuterium (hydrogen-2). The most widely used deuterated solvent is deuterochloroform (CDCl3), although other solvents may be used for various reasons, such as solubility of a sample, desire to control hydrogen bonding, or melting or boiling points. The chemical shifts of a molecule will change slightly between solvents, and therefore the solvent used will almost always be reported with chemical shifts. Proton NMR spectra are often calibrated against the known solvent residual proton peak as an internal standard instead of adding tetramethylsilane (TMS), which is conventionally defined as having a chemical shift of zero.

Acquisition of spectra
Upon excitation of the sample with a radio frequency (60–1000 MHz) pulse, a nuclear magnetic resonance response - a free induction decay (FID) - is obtained. It is a very weak signal, and requires sensitive radio receivers to pick up. A Fourier transform is carried out to extract the frequency-domain spectrum from the raw time-domain FID. A spectrum from a single FID has a low signal-to-noise ratio, but it improves readily with averaging of repeated acquisitions. Good 1H NMR spectra can be acquired with 16 repeats, which takes only minutes. However, for elements heavier than hydrogen, the relaxation time is rather long, e.g. around 8 seconds for 13C. Thus, acquisition of quantitative heavy-element spectra can be time-consuming, taking tens of minutes to hours.

Following the pulse, the nuclei are, on average, excited to a certain angle vs. the spectrometer magnetic field. The extent of excitation can be controlled with the pulse width, typically ca. 3-8 µs for the optimal 90° pulse. The pulse width can be determined by plotting the (signed) intensity as a function of pulse width. It follows a sine curve, and accordingly, changes sign at pulse widths corresponding to 180° and 360° pulses.

NMR can trace molecules and allow for graphical representation. H1 graphs demonstrate the placement of hydrogens, which sense the effect of electron-withdrawing groups (EWG) and electron-donating groups (EDG). These groups can pull observed peaks downfield (to the left) by deshielding or upfield (to the right) by shielding the electrons. Another benefit to NMR is the measurement of J coupling, which allows for the detection of zusammen (Z) or entgegen (E) conformation.

H1 NMR graphs can be affected by resonance. EDG groups and EWG groups move electrons around ring structures. EWG describes a molecule that pulls electron density towards the EWG groups, while EDG groups provide electron density to the molecule. The redistribution of electron density within the structure occurs through resonance. EDG's and EWG's are known as electrophilic aromatic directing groups. There are three positions a molecule can add in six-member rings, Ortho, Meta, and Para. Characterization of EWG is acknowledged, as it forces molecules to add in the Meta position on ring structures, while EDG groups allow molecules to add Ortho or Para. Additionally, EWG groups have more electronegative molecules attached, including NO2, CN, and CO. The molecule that hangs off pulls the electron's density away, causing the molecule to become partially positive. The EWG group on the ring can then only add in the Meta direction, as this positive charge is established in the Ortho and Para direction. As less density can be found on the Meta position, the molecules with fewer electrons will be deshielded and found downfield.

Molecules such as OCH3, NH3, and OH are EDG groups. EDG groups will push electron density into the ring. The groups surrounding the molecule are less electronegative; thus, a partial positive charge on the group directly attaches to the ring. The ring structure then becomes negatively charged in the Ortho and Para positions. The negative charge is distributed to the structure from resonance. Having the reverse effect and forcing molecules upfield as the electron shields the molecule.

As the electron density moves depending on the EDG and EWG group, it affects the NMR graph: E–Z notation has distinguishing features itself from a lone hydrogen interaction. When hydrogens or protons are in proximity, they may appear on either the Z configuration (same side, cis) or the E configuration (opposite sides, trans). As previously mentioned, a higher positive density in the molecule pushes it further downfield; a higher negative density will have the opposite effect. Z configurations typically manifest downstream since their protons engage more actively. The E configurations are slightly more upfield as their protons are not interacting. This distinction, though subtle, is observable in H1 NMR and J coupling. The J coupling will have a higher frequency for the E configuration, but a lower frequency for the Z configuration. The J coupling can provide quantitative data, acting as a secondary confirmation.

In H1 NMR, J coupling occurs when neighboring hydrogens interact with each other's spin. This causes multiplicity and can be calculated using:$$(\Delta peak \; hieght )(Frequency\;of\;the\;NMR\;spectrometer)=J\;Coupling$$Where the difference in peak height is derived using the larger peak’s minus the smaller peak’s multiplicity. $$(\Delta peak \; hieght ) = (Multiplicity \;of a\; Signal _L - Multiplicity\; of\; a\; Signal_S)$$