Knowledge Bank

The microwave spectrum of isopropyl carboxaldehyde has been investigated and for the ground vibrational state of the more stable gauche conformer the rotational constants were found to be $A=7494.39\pm 0.21,B=4107.45\pm 0.01$, and $C=2980.74\pm 0.01$ MHz and for the high energy trans $A=7706.75\pm 0.35,B=3736.52\pm 0.02$, and $C=2815.15\pm 0.01$ MHz. The respective dipole moment components for the trans and gauche conformers were determined to be: $|\mu a|=2.82,|\mu c|=0.46$ and $|\mu t|=2.86D$ and $|\mu a|=2.43,|\mu b|=0.79$ and $|\mu c|=0.83$ with $|\mu t|=2.69D$. From the far infrared spectrum of the gas the asymmetric torsion for the gauche conformer was observed as a series of Q-branches starting at $90.3cm-1$ and from the Raman spectrum of the gas, a ``double jump'' transition has been observed for the trans conformer at $110cm-1$. From these data the asymmetric potential function has been calculated and the following potential constants have been evaluated: $V1=-181\pm 21,V2=-298\pm 19,V3=392\pm 4$, and $V6=-7\pm 1cm-1$. Additionally, variable temperature studies of the Raman spectrum of the gas and liquid have been carried out and the enthalpy differences were determined to be $248\pm 35cm-1$ and $440\pm 17cm-1$, respectively. Evidence will be presented which indicates the two methyl rotors are potentially coupled for the gauche conformer and the barrier to internal rotation of the methyl tops has been calculated to be $1060cm-1$ (3031 cal/mol). These data, in addition to a discussion of the vibrational assignment, will be discussed in relation to other similar molecules.