trans-Stilbene

trans-Stilbene is the E geometric isomer of 1,2-diphenylethene: two phenyl groups attach to opposite carbons of a central carbon–carbon double bond, on opposite sides of that double bond. Its molecular formula is ${\mathrm{C}}_{14}{\mathrm{H}}_{12}$, and its CAS identifier is103-30-0.^[ref]

The formula can be counted directly from ${\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{CH}=\mathrm{CH}-{\mathrm{C}}_6{\mathrm{H}}_5$: two phenyl groups contribute twelve carbons and ten hydrogens, and the central pair contributes two carbons and two hydrogens. cis-Stilbene has the same atom counts but a different arrangement around the double bond. Cis-trans isomerism therefore concerns structure, not a difference in molar mass or elemental composition.

This makes a useful distinction in spectroscopy: a molecular formula alone does not specify every vibrational feature or optical response. Structural assignments require appropriate comparison data and a stated sample environment. In an original mixture example, equal amounts of cis and trans material still have the same total carbon-to-hydrogen ratio as either pure isomer; elemental composition alone cannot reveal that mixture.

A primary subsurface Raman spectroscopy experiment used trans-stilbene powder beneath a1-mm layer of poly(methyl methacrylate) powder. Collecting scattered light from regions laterally displaced from the excitation spot improved the unprocessed contrast of the buried trans-stilbene layer by a reported factor19.^[ref] The compound served as a chemically identifiable buried layer, illustrating how collection geometry can change which material dominates a measured spectrum. That reported contrast gain is a property of the experiment, not a universal material constant of trans-stilbene.