Organic Spectroscopy: An Introduction to UV-Vis and the Electromagnetic Spectrum

Comprehensive vocabulary flashcards covering the properties of electromagnetic radiation, UV-Vis spectroscopy principles, molecular orbital transitions, chromophores, and pharmaceutical applications.

Photon

A particle of electromagnetic radiation carrying energy defined by the equation $E = h\nu$.

Planck's Constant ($h$)

A fundamental physical constant equal to $6.626 \times 10^{-34}\,\text{J}\cdot\text{s}$ that relates the energy of a photon to its frequency.

Electromagnetic Radiation

Energy displaying both wave and particle properties, consisting of an electric vector ($E$) and a magnetic vector ($B$) propagating at right angles to each other.

Wavelength ($\lambda$)

The distance between equivalent points on two successive waves, measured in metres ($\text{m}$).

Frequency ($\nu$)

The number of wave cycles per second, expressed in Hertz ($\text{Hz}$) or $\text{s}^{-1}$.

Wavenumber ($\sigma$)

A unit commonly used in infrared spectroscopy defined as the reciprocal of wavelength ($\sigma = \frac{1}{\lambda}$), measured in $\text{cm}^{-1}$, which is directly proportional to energy.

Polychromatic Light

Light emitted by continuum sources (such as the Sun) that contains radiation of more than one wavelength.

Monochromatic Radiation

Electromagnetic radiation consisting of a single wavelength, whose energy depends solely on its frequency or wavelength rather than beam intensity.

Ground State

The normal electronic configuration of an atom or molecule where electrons occupy orbitals of the lowest energy.

Excited State

A higher-energy state achieved when an atom or molecule absorbs a photon with the precise energy required to promote an electron to a higher permitted energy level.

Ultraviolet (UV) Region

The region of the electromagnetic spectrum spanning wavelengths from $180\,\text{nm}$ to $400\,\text{nm}$.

Visible (VIS) Region

The region of the electromagnetic spectrum spanning wavelengths from $400\,\text{nm}$ to $780\,\text{nm}$.

Electronic Transitions in Molecular Orbitals

Energy promotions of electrons between bonding ($\sigma$, $\pi$), nonbonding ($n$), and antibonding ($\pi^*$, $\sigma^*$) molecular orbitals caused by absorption of specific photons.

$n \rightarrow \pi^*$ Electronic Transition

The promotion of a nonbonding electron (lone pair) into a $\pi$ antibonding molecular orbital; it requires a smaller energy difference ($\Delta E$) and thus absorbs at longer wavelengths.

$\pi \rightarrow \pi^*$ Electronic Transition

The promotion of an electron from a $\pi$ bonding molecular orbital into a $\pi$ antibonding molecular orbital; it requires a larger energy difference ($\Delta E$) and thus absorbs at shorter wavelengths.

Chromophore

The part of a molecule responsible for its ultraviolet or visible absorption spectrum, such as conjugated double bonds, carbonyl groups, or aromatic rings.

Conjugation (UV-Vis Effect)

The presence of alternating multiple and single bonds that raises the energy of the HOMO and lowers the energy of the LUMO, reducing the required transition energy and increasing $\lambda_{\max}$.

$\beta$-Carotene

An orange compound containing an extended system of conjugated $\pi$ bonds (a long chromophore) that shifts its absorption band into the visible region.

Spectrophotometer

An analytical instrument consisting of a light source, an entrance slit, a dispersion device, an exit slit, and a sample cuvette used to measure light absorbance across wavelengths.

Dissolution Testing

A quantitative pharmaceutical application of UV/Vis spectroscopy used to measure the release profile of drugs from formulations over time.