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$2600$

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$$physical $$chemistry $$course$!!!!$

Absorption and emission of electronic energy:

a$)$ You find two bottles in the lab that have lost their labels. You know that the two

compounds are $2,3-$dimethyl$-1,3-$butadiene and $2,5-$dimethyl$-1,3,5-$hexatriene. You

find that UV absorbance maxima are at $226nm$ and $252nm.$ Which spectrum belongs

to which compound? How do you know this?

b$)$ The $(n$ to ${}^{**})$ transition of $N=0$ has a maximum at $660nm,$ while the $($ to ${}^{**})$ transition in $C=C$ is at $180nm.$

Why would the transition for the $N=O$ molecule be at a much longer wavelength? You may wish to use simple

HOMO and LUMO energy diagrams for clarity.

c$)$ Diamond and graphite are allotropic forms of carbon. Consider the molecular structures shown and the types

of bonds. Use them to explain why graphite $($LEFT$)$ is black while diamonds $($RIGHT$)$ are colourless.

Segment of

a sheet of

graphite