UV-VIS Absorption Spectroscopy UPDATED ACTUAL
Questions and CORRECT Answers
ultraviolet light wavelength spectrum 200-400 nm
visible light wavelength spectrum 400-780nm
when UV and VIS light are absorbed by molecules, that temporarily excite valence electrons in molecular orbitals to higher energy
energy is used to molecular orbitals
EX. look at the molecule of formaldehyde and label 1 sigma
sigma bonds, pi bonds, and lone pairs 1 pi
2 lone pairs
- peaks at 240 and 290 nm are due to _______________
specific excitations of formaldehyde's valence electrons to higher energy
molecular orbitals
each MO can hold up to ____ electrons 2
high energy orbitals are termed_____ and____ bonding antibonding (*) and weaken
between atoms
EX. Use up and down arrows to show each of the
electron "promotions" listed to the right ("from - to").
EX. Use previous slides information. Absorption of 250 250nm - pi to pi*
nm and 290 nm light in the UV spectrum is attributed to 290nm - n to pi*
the 2 promotions that require the
least energy. Which absorption peak corresponds to
which promotion?
250nm __________________
290nm___________________
what functional groups are associated with n to pi* lone pairs
what functional groups are associated with pi to pi* double bonds / triple bonds
EX. fill out the table
for molecules with extensive conjugation, the energy lower
required for pi to pi* excitation is (lower/higher) longer
- they absorb (shorter/longer) wavelengths of light
Questions and CORRECT Answers
ultraviolet light wavelength spectrum 200-400 nm
visible light wavelength spectrum 400-780nm
when UV and VIS light are absorbed by molecules, that temporarily excite valence electrons in molecular orbitals to higher energy
energy is used to molecular orbitals
EX. look at the molecule of formaldehyde and label 1 sigma
sigma bonds, pi bonds, and lone pairs 1 pi
2 lone pairs
- peaks at 240 and 290 nm are due to _______________
specific excitations of formaldehyde's valence electrons to higher energy
molecular orbitals
each MO can hold up to ____ electrons 2
high energy orbitals are termed_____ and____ bonding antibonding (*) and weaken
between atoms
EX. Use up and down arrows to show each of the
electron "promotions" listed to the right ("from - to").
EX. Use previous slides information. Absorption of 250 250nm - pi to pi*
nm and 290 nm light in the UV spectrum is attributed to 290nm - n to pi*
the 2 promotions that require the
least energy. Which absorption peak corresponds to
which promotion?
250nm __________________
290nm___________________
what functional groups are associated with n to pi* lone pairs
what functional groups are associated with pi to pi* double bonds / triple bonds
EX. fill out the table
for molecules with extensive conjugation, the energy lower
required for pi to pi* excitation is (lower/higher) longer
- they absorb (shorter/longer) wavelengths of light
