Knowledge Bank

The quantum defect calculated when the origin of the sharply structured system of intense absorption bands lying to the blue of the broad $\pi \ast \leftarrow \pi$ transition of trans-butene-2 is fitted to the Rydberg equation with an assumed ionization potential of 9,11 eV is found to be comparable with that obtained for transitions to ns Rydberg states of ethylene: t-butene-1.16; ethylene-1.09. The zero point energy shift between the systems origins in t-butene and perdeutero-t-butene is also comparable with that observed between light and heavy ethylene: $-230cm-1vs-180cm-1$. From these we conclude that the $1670\backslash AA$ transition of t-butene-2 is the transition to the 4s Rydberg state of this molecule. A comparison between the vibrational frequency intervals observed in t-butene-2 and perdeutero-t-butene-2 affords an approximate description of the motions responsible for the stronger vibronic bands. A further comparison between the frequency intervals observed in the 4s Rydberg state and those reported for the ground state of the light compound indicates that the carbon-carbon double bond and the $sp2$ carbon-hydrogen bonds are weaker in the 4s Rydberg state than in the ground state but that the $sp2$ $carbon-sp3$ carbon bonds are stronger in the Rydberg state. The vibrational structure of the $1670\backslash AA$ transition was observed to differ from that observed in the $2000\backslash AA$ transition, suggesting that the latter is not the missing first member of the Rydberg series.