Answer: Types of molecular spectroscopy: rotational (microwave), vibrational (infrared), Raman, electronic (UV-visible), nuclear magnetic resonance (NMR) and electron spin resonance (ESR). NMR spectroscopy studies the absorption of radio-frequency radiation by nuclei (such as \(^1\)H) placed in a strong magnetic field.
Types of molecular spectroscopy (classified by the region of electromagnetic radiation and the type of molecular energy change):
- Rotational (microwave) spectroscopy: transitions between rotational energy levels of polar molecules; microwave region.
- Vibrational (infrared) spectroscopy: transitions between vibrational levels; used to identify functional groups.
- Raman spectroscopy: study of the inelastically scattered light; gives vibrational/rotational information, including for non-polar molecules.
- Electronic (UV-visible) spectroscopy: transitions of electrons between electronic energy levels (e.g. \(\pi\rightarrow\pi^*\), \(n\rightarrow\pi^*\)).
- Nuclear magnetic resonance (NMR) spectroscopy: transitions between nuclear spin states in a magnetic field; radio-frequency region.
- Electron spin resonance (ESR) spectroscopy: transitions of unpaired electron spins in a magnetic field; microwave region.
NMR spectroscopy
Principle: Nuclei with an odd number of protons or neutrons (e.g. \(^1\)H, \(^{13}\)C) have a nuclear spin and behave like tiny magnets. In the absence of a field their spin states have the same energy. When placed in a strong external magnetic field, the nuclei (for \(^1\)H, spin \(\tfrac12\)) align either with the field (lower energy) or against it (higher energy). When radio-frequency radiation whose energy exactly equals this energy gap is applied, the nuclei absorb it and flip from the lower to the higher spin state. This absorption is called nuclear magnetic resonance: \(\Delta E=h\nu\), and \(\nu\) depends on the applied field strength.
Chemical shift: Electrons around a nucleus produce a small opposing field, so the nucleus is shielded. Protons in different chemical environments are shielded to different extents and therefore resonate at slightly different positions. This shift is measured relative to a reference, tetramethylsilane (TMS), and expressed in ppm:
\[\delta=\frac{\nu_{\text{sample}}-\nu_{\text{TMS}}}{\nu_{\text{spectrometer}}}\times10^6\ \text{ppm}\]
TMS is chosen because its 12 equivalent protons give a single sharp signal at \(\delta=0\), it is inert and volatile. Protons near electronegative atoms are deshielded and appear at higher \(\delta\) values.
Instrumentation (outline): a powerful magnet, a radio-frequency source, a sample tube (sample dissolved in CDCl\(_3\) or CCl\(_4\)) spinning between the magnet poles, an RF detector and a recorder. Either the field or the frequency is varied until resonance occurs and the absorption signal is recorded.
Information from an NMR spectrum:
- Number of signals: number of different kinds of protons.
- Position of signals (chemical shift): the chemical environment of each kind of proton.
- Area under each signal (integration): relative number of protons of each kind.
- Splitting of signals (spin–spin coupling, \(n+1\) rule): number of neighbouring protons.
Applications: determination of the structure of organic compounds, distinguishing isomers, checking purity, study of reaction kinetics and molecular dynamics, and MRI in medical diagnosis. It is non-destructive.