CHEM2210 Chapter Notes - Chapter 6: Fine Structure, Hydrogen Bond, Polarizability
Temperature Effects:
Decreasing the temperature allows the vibrational fine structure to emerge.
• Vibrational bands are embedded within the electronic bands represent
the transitions from v=n to v'=n. Generally, the v=0 to v'=0 transition is
the one with the lowest frequency.
• From there, increasing energy, the transitions can be from v=0 to v'=n,
where n=1,2,3... With a higher temperature, the vibrational transitions
become averaged in the spectrum due to the presence of vibrational
hot bands and Fermi Resonance, and with this, the vibrational fine
structure is lost at higher temperatures.
• The bands represent the jumping of the electron from the ground
electronic state/ground vibrational state to the excited electronic
state/ground vibrational state
Solvent Effects:
• The effect that the solvent plays on the absorption spectrum is also
very important.
• It is clear that polar solvents give rise to broad bands, non-polar
solvents show more resolution, though, completely removing the
solvent gives the best resolution.
- This is due to solvent-solute interaction. The solvent can interact
with the solute in its ground state or excited state through
intermolecular bonding.
E.g. A polar solvent like water has the ability of hydrogen bonding with the
solute if the solute has a hydrogen bonding component, or simply through
induced dipole-dipole interactions.
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Document Summary
Decreasing the temperature allows the vibrational fine structure to emerge: vibrational bands are embedded within the electronic bands represent the transitions from v=n to v"=n. Solvent effects: the effect that the solvent plays on the absorption spectrum is also very important. It is clear that polar solvents give rise to broad bands, non-polar solvents show more resolution, though, completely removing the solvent gives the best resolution. The solvent can interact with the solute in its ground state or excited state through intermolecular bonding. This is due to the solvent"s tendency to align its dipole moment with the dipole moment of the solute. The effects of peak broadening are most severe for polar solvent, less so for non-polar solvents, and absent when the solute is in vapour phase. Hot bands: knowing whether a transition will be allowed by symmetry is an essential component to interpreting the spectrum.
