Knowledge Bank

Internal rotation in tetrolaldehyde, $CH3.C\equiv C.CHO$, has been studied in the ground state and in the electronically-excited singlet state of the $\pi ast\leftarrow n$ transition. The infrared vapor spectra are consistent with relatively unhindered rotation in the ground state. The electronic bands $(\nu o=26611.85cm-1)$ are perpendicular in type and have the structure associated with an electric dipole moment charge occurring in the frame rather than in the methyl top. Features of the electronic band contour are consistent with a low barrier to internal rotation, but the details are not well reproduced unless the barrier is considered to change slightly, on the order of $12-18cm-1$, between the ground and excited electronic states. Tetrolaldehyde is a near-prolate asymmetric top $(k\simeq -0.996)$. Contour calculations were however based on symmetric top formulas, and the effect of this approximation will be discussed.