Page 347 - Instant notes

I6
ELECTRONIC SPECTROSCOPY

Key Notes
The separation of energy levels arising from different
configurations of electrons in atomic and molecular orbitals
usually corresponds to radiation in the visible and ultraviolet
regions of the electromagnetic spectrum (wavelengths between
700 and ~100 nm).
Nuclei are sufficiently more massive than electrons that an
electronic transition (electron rearrangement) occurs so fast that
the nuclei do not alter their relative positions during the
transition.
The excitation of an electron from the π bonding orbital of a C=C
bond to the π* antibonding orbital is called a π−π* transition. The
excitation of one of the lone pair electrons of the O atom in a
C=O bond to the π* antibonding orbital is called an n−π*
transition. The energy of π−π* and n−π* transitions shifts to
longer wavelength radiation as conjugation of the C=C and C=O
bonds increases. A charge transfer transition involves electron
movement between the d orbital of a metal atom and a ligand.
Fluorescence is the emission of radiation directly following
absorption of excitation radiation. It is usually shifted to
frequencies lower than the absorption because some vibrational
excitation is lost in molecular collisions. Phosphorescence is the
slow emission of radiation after absorption ceases and usually
emanates from a triplet state accessed by spin-forbidden
intersystem crossing from the initial excited singlet state.
A photoelectron spectrum is obtained by measuring the kinetic
energies of electrons emitted from a molecule following
absorption of high energy (ultraviolet or X-ray) monochromatic
radiation. The difference between the energy of the incident
photon and the kinetic energy yields the energy of the orbital
from which the electron was ejected.
Related topics Valence bond theory (H2) General features of
spectroscopy (I1)
Molecular orbital theory of
diatomic molecules I (H3)
Practical aspects of
spectroscopy (I2)
Molecular orbital theory of
diatomic molecules II (H4)
Photochemistry in the real
world (I7)