1. NMR is the most important spectroscopic tool in the elucidation of the structuresof organic molecules.
2. Spectroscopy is possible because molecules exist in various energetic forms, those at lower energy being convertible into states of higher energy by absorption of discrete quanta of electromagnetic radiation.
3. NMR is possible because certain nuclei, especially 1H and 13C, when exposed to a strong magnetic field, align with it (α) or against it (β).The α-to- β transition can be effected by radiofrequency radiation, leading to resonance and a spectrum with characteristic absorptions. The higher the external field strength, the higher the resonance frequency. For example, a magnetic field of 7.05 T causes hydrogen to absorb at 300 MHz, a magnetic field of 14.1 T causes it to absorb at 600 MHz.
4. High-resolution NMR allows for the differentiation of hydrogen and carbon nuclei in different chemical environments. Their characteristic positions in the spectrum are measured as the chemical shift, δ, in ppm from an internal standard, tetramethylsilane.
5. The chemical shift is highly dependent on the presence (causing shielding) or absence (causing deshielding) of electron density. Shielding results in relatively high-field peaks [to the right, toward (CH3)4Si], deshielding in low-field ones. Therefore, electron-donor substituents shield, and electron-withdrawing components deshield. The protons on the heteroatoms of alcohols, thiols, and amines show variable chemical shifts and often appear as broad peaks because of hydrogen bonding and exchange.
6. Chemically equivalent hydrogens and carbons have the same chemical shift. Equivalence is best stablished by the application of symmetry operations, such as those using mirror planes and rotations.
7. The number of hydrogens giving rise to a peak is measured by integration.
8. The number of hydrogen neighbors of a nucleus is given by the spin – spin splitting pattern of its NMR resonance, following the N + 1 rule. Equivalent hydrogens show no mutual spin – spin splitting.
9. When the chemical-shift difference between coupled hydrogens is comparable to their coupling constant, non-first-order spectra with complicated patterns are observed.
10. When the constants for coupling to nonequivalent types of neighboring hydrogens are different, the N +1 rule is applied sequentially.
11. Carbon NMR utilizes the low-abundance 13C isotope. Carbon – carbon coupling is not observed in ordinary 13C spectra. Carbon – hydrogen coupling can be removed by proton decoupling, thereby simplifying most 13C spectra to a collection of single peaks.
12. DEPT 13C NMR allows the assignment of absorptions to CH3, CH2, CH, and quaternary carbons, respectively.
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