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

Question

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

Quizplus · Accepted Answer

11ecba2d_12ba_81bf_a3bf_3b977a1746c9_MD0008_00 The electromagnetic spectrum consists of waves of varying wavelengths, energy, and frequency. The spectrum spans from radio waves (long wavelength and low frequency and energy) to gamma rays (short wavelength and high frequency and energy). Ultraviolet (UV) light, which corresponds to wavelengths between 200 nm and 400 nm, is used in UV spectroscopy. In UV spectroscopy, a compound in solution is irradiated with UV light; the amount of UV light absorbed by the compound is measured at each wavelength and a spectrum is generated by plotting absorbance as a function of wavelength. Absorption of UV light causes an electron transition, or excitation, from the ground state to a higher energy level. In the ground state, π electrons from double bonds are in the π bonding molecular orbital, and nonbonding electrons are in the n nonbonding molecular orbital. Upon interactions with UV light of sufficient energy, these electrons are excited to the Lowest Unoccupied Molecular Orbital (LUMO), called the π* antibonding orbital. The lone electron pairs on nitrogen in pyrimidine are nonbonding (n) electrons. Therefore, when absorbing UV light, these electrons are excited to the antibonding (π*) orbital and undergo a n → π* transition. (Choice A) A π → π* electron transition describes excitation of a π electron from the bonding to the antibonding molecular orbital rather than a nonbonding electron to an antibonding molecular orbital. (Choices C and D) The π* → π and π* → n electron transitions describe an excited π electron and excited nonbonding electron, respectively, falling from the antibonding orbital to its ground state (π and n). Educational objective: Ultraviolet (UV) light is electromagnetic radiation that corresponds to wavelengths between 200 nm and 400 nm, and UV absorption causes an electron transition from the ground state to a higher energy level. In the ground state, π electrons from double bonds are in the π bonding molecular orbital, and nonbonding electrons are in the n nonbonding molecular orbital. These electrons can be excited to the π* antibonding orbital.

In the Ultraviolet (UV) Range, Pyrimidine Has Maximum Absorptions at 240