Homework Problems 5 Flashcards
What are the requirements for a vibrational mode in a molecule to show IR absorption?
change in permanent dipole moment of chemical bond during molecular vibration
What are the requirements for a vibrational mode to be raman active?
change in polarizability of the electron cloud
Under what circumstances will vibrational modes be both Raman and IR active?
for nonsymmetrical molecules
Under what circumstances will vibrational modes be raman active but not IR, and vice versa?
for molecules with a center of symmetry, the rule of mutual exclusion applies. For a symmetrical molecule, raman-active vibrations will be IR inactive, and IR-active will be Raman inactive
Consider the molecule chloroacetonitrile (ClCH2CN). How many vibrational modes should this molecule have?
3N-6, so 12
Chloroacetonitrile shows a strong Raman band at 2200cm-1 due to the C–N stretching mode. the corresponding IR absorption is very weak or absent. By comparing spectra in this region, what can you conclude about the C–N stretching mode?
the appearance of a particular vibrational mode in the raman spectrum but not in the IR implies that the bond either does not possess a permanent dipole moment or has a center of symmetry.
Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum
in fluorescence emission, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength. In excitation, the emission is measured at one wavelength while the excitation wavelengths are scanned
Which more closely resembles an absorption spectrum: emission or excitation spectrum?
excitation because the emission intensity is usually proportional to the absorbance of the molecule
the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity
fluorescence
single to singlet transition
fluorescence
the process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from an excited triplet state to a lower state of a different spin multiplicity
phosphorescence
triplet to singlet transition
phosphorescence
observed when an excited species emits radiation of the same frequency used to cause the excitation
resonance fluorescnece
in which the spins of the electrons of an atom or molecule are all paired so there is no net spin angular momentum
singlet state
one in which the spins of the electrons of an atom or molecule are unpaired so that their spin angular moments add to give a net non-zero moment
triplet state
the process by which a molecule loses its excess vibrational energy without emitting radiation
vibrational relaxation
the intermolecular process in which a molecule crosses to a lower electronic state with emitting radiation
internal conversion
a radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute
external conversion
a process in which a molecule in one spin state changes to another spin state with nearly the same total energy
intersystem crossing
single to triplet transition
intersystem crossing
occurs when a molecule changes from a higher electronic state to an upper vibrational level of a lower electronic state in which the vibrational energy is great enough to rupture the bond
predissociation
occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break
dissociation
the fraction of excited molecules undergoing the process of interest. example: ___ of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce
quantum yield
a process by which radiation is produced as a result of a chemical reaction
chemiluminescence
What is a property a compound can have that will lead to greater fluorescence quantum yield
Structural rigidity
