Acetylene, , is a linear, centrosymmetric molecule with four atoms, point group . Its…
PHYSICS · Multi-part · Fermi University Physics · Mathematical Methods for Physics · Atomic, Molecular & Optical Physics · Molecular structure and spectra
Deriving the Spectrum, Ch Polyatomic molecules: normal modes and symmetry
Problem
Acetylene, HC≡CH, is a linear, centrosymmetric molecule with four atoms, point group D∞h. Its seven fundamentals fall into five symmetry species: ν1 and ν2 (both Σg+, the C-H and C-C symmetric stretches), ν3 (Σu+, the C-H antisymmetric stretch), ν4 (Πg, a doubly degenerate bend) and ν5 (Πu, a doubly degenerate bend).
((a)) State which of these five modes are infrared active, which are Raman active, and whether any mode is active in both.
((b)) For each infrared-active mode, predict whether its band is parallel or perpendicular.
((c)) Classify acetylene as a rotor by its principal moments and predict whether it has an allowed pure rotational spectrum, giving the reason.