Knowledge Bank

The long $observed1$ closely spaced absorption bands superimposed on the strong peaks to the blue of the diffuse bands identified as belonging to the $\pi \ast \leftarrow \pi$ transition of 2-methylpropene have been shown, by temperature variations, to be sequence bands. The number of sequence bands observed, the absence of an observed excited state vibrational frequency interval that, when subtracted from any of the ground state vibrational frequencies gives the observed frequency difference, and the extremely low temperature necessary to appreciably depopulate the excited ground state vibrational levels suggest that the ground state fundamental responsible for these sequence bands is of very low frequency---most likely the methyl torsion(s). If we assume that the sequency bands arise from the methyl torsions, the ratio of the 2--1 to the 1--0 transitions plus the absolute value of the $1-0$ transition frequency interval can be used (in conjunction with solutions of the Mathieu equation for a three-fold barrier) to roughly estimate the torsional barrier and the effective rotational constant of the methyl group in this excited state. These values are: $Vs=100cm-1,F4=6.68cm-1$. The corresponding ground state values are: $V3=773cm-1,F4=5.80cm-1,2$. The assignment of this electronic state itself is unknown.