Homework Problems 5

Question 1

What are the requirements for a vibrational mode in a molecule to show IR absorption?

Answer 1

change in permanent dipole moment of chemical bond during molecular vibration

Question 2

What are the requirements for a vibrational mode to be raman active?

Answer 2

change in polarizability of the electron cloud

Question 3

Under what circumstances will vibrational modes be both Raman and IR active?

Answer 3

for nonsymmetrical molecules

Question 4

Under what circumstances will vibrational modes be raman active but not IR, and vice versa?

Answer 4

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

Question 5

Consider the molecule chloroacetonitrile (ClCH2CN). How many vibrational modes should this molecule have?

Answer 5

3N-6, so 12

Question 6

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?

Answer 6

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.

Question 7

Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum

Answer 7

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

Question 8

Which more closely resembles an absorption spectrum: emission or excitation spectrum?

Answer 8

excitation because the emission intensity is usually proportional to the absorbance of the molecule

Question 9

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

Answer 9

fluorescence

Question 10

single to singlet transition

Answer 10

fluorescence

Question 11

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

Answer 11

phosphorescence

Question 12

triplet to singlet transition

Answer 12

phosphorescence

Question 13

observed when an excited species emits radiation of the same frequency used to cause the excitation

Answer 13

resonance fluorescnece

Question 14

in which the spins of the electrons of an atom or molecule are all paired so there is no net spin angular momentum

Answer 14

singlet state

Question 15

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

Answer 15

triplet state

Question 16

the process by which a molecule loses its excess vibrational energy without emitting radiation

Answer 16

vibrational relaxation

Question 17

the intermolecular process in which a molecule crosses to a lower electronic state with emitting radiation

Answer 17

internal conversion

Question 18

a radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute

Answer 18

external conversion

Question 19

a process in which a molecule in one spin state changes to another spin state with nearly the same total energy

Answer 19

intersystem crossing

Question 20

single to triplet transition

Answer 20

intersystem crossing

Question 21

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

Answer 21

predissociation

Question 22

occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break

Answer 22

dissociation

Question 23

the fraction of excited molecules undergoing the process of interest. example: ___ of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce

Answer 23

quantum yield

Question 24

a process by which radiation is produced as a result of a chemical reaction

Answer 24

chemiluminescence

Question 25

What is a property a compound can have that will lead to greater fluorescence quantum yield

Answer 25

Structural rigidity