Conjugation and UV-vis Spectroscopy Flashcards

These flashcards cover the vocabulary and core concepts from the lecture on conjugated systems, heats of hydrogenation, and UV-vis spectroscopy principles.

Conjugated (p) bonds

Double bonds that are more stable than isolated ones, a property observed through lower heats of hydrogenation.

Heats of hydrogenation ($\Delta H^{\circ}_{hydrog}$)

Experimental data used to measure alkene stability; for example, ethylene has a value of $-30.1\,kcal/mol$ and buta-1,3-diene has $-56.4\,kcal/mol$.

UV-vis spectroscopy

The study of the interaction of ultraviolet (UV) and visible (vis) light with pi-conjugated organic molecules, involving electronic transitions.

Electronic transitions

The process of exciting an electron from a bonding orbital to an antibonding orbital, specifically from the HOMO to the LUMO, upon absorbing light.

$$E = h\nu$$

A fundamental equation in spectroscopy where energy ($E$) equals Planck's constant ($h$) times frequency ($\nu$).

$$\nu = c/\lambda$$

The equation relating frequency ($\nu$) to the speed of light ($c$) and wavelength ($\lambda$).

$$E = hc/\lambda$$

The equation showing that the energy ($E$) of light is inversely proportional to its wavelength ($\lambda$).

HOMO

Acronym for Highest Occupied Molecular Orbital.

LUMO

Acronym for Lowest Unoccupied Molecular Orbital.

HOMO-LUMO gap

The energy difference ($\Delta E$) required to excite an electron; this gap decreases as conjugation increases.

$$-56.4\,kcal/mol$$

The specific heat of hydrogenation ($\Delta H^{\circ}_{hydrog}$) for buta-1,3-diene.

Visible region

The portion of the electromagnetic spectrum where molecules start absorbing light above approximately $400\,nm$, causing the compounds to appear colored.

$\beta$-carotene

A colored compound with an absorption maximum ($\lambda_{max}$) of $454\,nm$.

Hexa-1,3,5-triene

A conjugated system characterized by six p atomic orbitals and six molecular orbitals.

Relationship between energy and wavelength

Wavelength and energy are inversely proportional; longer wavelengths correspond to lower energy photons.