Knowledge Bank

The infrared spectra $(3600to50cm-1)$ of the gaseous and solid states, and the Raman spectra (3600 to $10cm-1)$ of the gaseous, liquid and solid states of 2-methylpropanal (isobutyraldehyde). $(CH3)2CHCHO$, and the corresponding deuterium compound, $(CD3)2CDCHO$, have been recorded. Additionally, qualitative depolarization ratios have been obtained from the Raman spectra of the liquids. These data have been interpreted for the fluid phases on the basis that the gauche conformation (oxygen atom eclipsing a methyl group) is thermodynamically preferred over the high energy trans conformation (oxygen atom eclipsing the secondary hydrogen) and is the only rotamer present in the spectra of the annealed solids. From the relative intensities of the Raman lines of the liquid at $633cm-1$ (gauche) and $551cm-1$ (trans) as a function of temperature, the energy difference is found to be $440\pm 17cm-1(1.26\pm 0.05kcal/mol)$. A complete vibrational assignment is proposed which is based on infrared band contours, depolarization values, isotopic shifts and group frequencies. The vibrational frequencies determined from experiment will be compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and $6-31G\ast$ basis sets. Additionally, structural parameters previously provided will be compared to those determined from ab initlo methods.