Knowledge Bank

The microwave spectrum of trans, trans-2, 4-hexadienal can be assigned to the species with anti-periplanar conformations about the C-C single bonds (1). Hexadienal is a nearly symmetric top ($kappa=-0.997$, calc’d) whose a-type R-branch pileups form narrow (5 MHz FWHM) bands whose K components are unresolved but the bands from each vibrational species are resolved from each other. The spectrum reveals a complex array of vibrational satellite bands extending upward in frequency from the ground state band (B+C = 1592.5 MHz). Assuming the torsional modes about the two central C-C single bonds are the molecule’s lowest frequency vibrational modes, that torsional potential functions from acrolein and butadiene are transferable to hexadienal, and that the operator for B+C is represented by the geometrically calculated B+C as a function of the two torsional angles, much of the vibrational satellite structure of the spectrum can be simulated. This model describes the microwave spectrum with one glaring deficiency: the intense, broad absorption bands displaced to slightly higher frequencies (B+C = 1604.0 MHz) from the ground state and the discrete vibrationally excited species are not explained. Various models for this intense band series will be discussed.