Discussion
A band shifts from 320 nm in hexane to 305 nm in water. Which transition is it?
Answer: A
A shift to shorter wavelength in the more polar solvent is the signature of an n to pi star band, whose ground-state lone pair is stabilised by hydrogen bonding. A pi to pi star band would move the other way. The sigma transitions lie far outside the measurable range. This single experiment distinguishes the two transitions that both fall in the accessible region, which is why a change of solvent is often the quickest structural question to ask. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.
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