Handbook of Solvents - George Wypych - ChemTech - Ventech!

640 Mati Karelson
Figure 11.1.1. The origin of two types of solvatochromic shifts in the spectra of chromophoric compounds.
the ground state, the acetone molecule has a
significant dipole moment (2.7 D) arising
from the polarity of the C=O bond. Because
of the difference in the electronegativity of
bonded atoms, the electron distribution is
shifted towards the oxygen that could be
characterized by a negative partial charge
on this atom. However, in the excited state
the electron cloud is shifted from the oxygen
atom to the bond and, correspondingly,
Figure 11.1.2. The schematic representation of the the dipole moment of the molecule is sub-
change of the solvation energy of the ground (S0) and exstantially reduced. In result, the interaction
cited state (S1) of acetone moving from a non-polar sol- of the dipole of the solute (acetone) with the
vent (1) to a polar non-hydrogen bonded solvent (2) to a
surrounding medium in more polar solvents
polar hydrogen-bonded solvent (3).
is larger in the ground state as compared to
the excited state (Figure 11.1.2).
Thus, the energy of the ground state is lowered more by the electrostatic solvation than
the energy of the excited state. Consequently, the excitation energy increases and the respective
spectral maximum is shifted towards the blue end of spectrum (negative
solvatochromic shift). In the hydrogen-bonding solvents, the ground state of acetone is ad-