Question

In the ultraviolet (UV) range, pyrimidine has maximum absorptions at 240 nm due to π electron excitation and at 280 nm due to nonbonding electron excitation. The nonbonding electrons on nitrogen in pyrimidine undergo which of the following transitions when absorbing UV light?

1) π → π*
2) n → π*
3) π* → π
4) π* → n

Answer 1

The ultraviolet (UV) absorption at 280 nm in pyrimidine is due to the n → π* transition, where a nonbonding electron on nitrogen is excited to an antibonding pi-star (π*) orbital.

Step-by-step explanation:

In the ultraviolet (UV) range, molecules like pyrimidine can absorb light, resulting in electrons being excited to higher energy levels. In this case, the nonbonding electrons on nitrogen in pyrimidine absorb UV light and undergo the n → π* transition.

This transition specifically refers to the promotion of an electron from a nonbonding (n) orbital to an antibonding pi-star (π*) orbital when it absorbs energy from UV radiation.

Absorptions at 240 nm typically involve π → π* transitions where electrons in the pi (π) bonding orbital are excited to the corresponding antibonding (π*) orbital, whereas the 280 nm absorption involves the n → π* transition as previously mentioned.