Question

Howo to know π-π* transition using UV spectroscopy? I got cupric oxlate UV data, and lamda max is 250 nm.
Professor saids, it is a result of π-π* transition. But why 250 nm means π-π* transition ? How can I know it?

Answer 1

The UV spectroscopy involves absorption of electromagnetic radiation when energy contained in photon is absorbed by an electron resulting in a transition to an excited state. Typically its region extends from 200nm-400nm.

pi to pi* transition is allowed only in UV as pi electrons are loosely held and require lesser energy for excitation to higher energy level (pi*). In UV spectroscopy, pi electrons of a carbonyl group, conjugated unsaturated system absorbs the radiation in UV region. Also, conjugation of double bonds shifts the wavelength of maximum absorption to longer wavelengths.

Cupric oxalate compound contains a carbonyl group as well as it is a conjugated system. So, pi to pi* transition is identified only in UV spectroscopy and due to the presence of carbonyl group and conjugated system, it absorbs radiation at longer wavelength i.e 250nm.