CHAPTER 15

Absorption of energy

The process by which organic molecules absorb energy from different types of light on the electromagnetic spectrum to determine their structure.

Nuclear Magnetic Resonance (NMR)

A technique that provides information about the bonding of nuclei through nuclear magnetic spin transitions at radio frequencies.

NMR active nuclei

Nuclei that have an odd atomic number or an odd mass number, allowing them to be analyzed by NMR.

Precession

The characteristic frequency at which a nucleus aligned with a magnetic field spins, similar to a spinning top.

Chemical shift

The location in ppm of H-1 nuclei in an NMR spectrum, defined by its frequency shift from tetramethylsilane (TMS).

Equivalent Hydrogens

Protons in the same sp3 chemical environment that have the same chemical shift, bonded to the same hybridized carbon with free bond rotation.

Integration values

The calculated area under the curve of each peak in the NMR spectrum, indicating the number of equivalent protons.

Shielding

The effect that decreases a nucleus's exposure to the NMR magnet, resulting in lower chemical shift values.

Deshielding

The effect that increases a nucleus's exposure to the applied magnetic field, leading to higher chemical shift values.

Electronegativity effect

The influence of nearby electronegative atoms on protons, causing deshielding and higher chemical shift values.

Diamagnetic anisotropy

The phenomenon where electrons in a conjugated pi system circulate in a magnetic field, creating their own magnetic field and affecting chemical shifts.

n+1 Rule

A principle used in NMR where neighboring protons split a peak into n+1 lines, with n being the number of neighboring protons.

Homotopic protons

Protons that have identical chemical shifts and simple splitting patterns, tested by deuterium replacement.

Enantiotopic protons

Protons that have identical chemical shifts but can produce a chiral center upon deuterium replacement, unless in a chiral environment.

Diastereotopic protons

Protons that do not have the same chemical shift and do not exhibit simple splitting patterns, often found next to a chiral center.

Index of Hydrogen Deficiency (IHD)

A calculation that indicates the number of double bonds or rings in a molecule, aiding in functional group identification.

NMR spectrum analysis

The process of interpreting NMR spectra by identifying peaks, chemical shifts, integration values, and splitting patterns to deduce molecular structure.