UV visible Spectroscopy

Question 1

what is UV vis spec used fo

Answer 1

used to obtain the absorbance spectra of a compound in solution or as a solid where the absorbance of light energy is measured

Question 2

what happens to electrons when they are excited in UV vis

Answer 2

they are excited from the ground state to the first singlet excited state of the compound

Question 3

what are the two stages of absorption

Answer 3

1. electron excitation where M + hv -> M*
   the lifetime of this excited stage is short, as the stage is terminated by any of several relaxation processes where the most commo is the conversion of excitation energy to heat.
2. M* -> M + Heat

relaxation can also = decomposition of M* to form new species (photochemical reaction) . relaxation can also include fluorescent or phosphorescent re-emission of radiation

the absorption of UV radiation = excitation of bonding electrons, so the wavelength peak can be associated with the type of binds that exist- useful for identifying functional groups in a molecule + determining quantitative amounts of compound containing groups

Question 4

what 4 types of molecular orbital transitions

Answer 4

sigma sigma , n -> sigma, pi -> pi, n -> pi

Question 5

which transitions occur in vaccum

Answer 5

sigma sigma * and n sigma* (UV region <200nm)

Question 6

what are the UV regions for n -> pi* and pi -> pi*

Answer 6

200nm - 380 nm
380 - 800nm

Question 7

describe the types of electrons in relation to formaldehyde

Answer 7

bonding electrons in signa, an pi, and non bonding electrons in lone pairs

sigma bonds exist between C - H and C-O

pi bond exists between C - O to form a double bond

non bonding electrons form at the O molecules (two pairs)

Question 8

write a note on sigma sigma star transitions

Answer 8

requires a lot of energy to occur in vacuum region

methane (CH4) has C-H bonds only, which only undergo sigma sigma * transitions

ethane (c2H6) because of the C-C bond is less strong than C - H bonds, less energy is needed for excitation so the absorption peaks at a longer wavelength

Question 9

write a note on n sigma * transitions

Answer 9

also need to be in vacuum

seen in saturated compounds with unshared electron pairs (nonbonding electrons). require less energy than sigma sigma * transitions and can be brought about with radiation in 150-250nm region. most peaks appear at <200nm

these solvents are suitable as dissolution solvent for compounds with chromophores in the 200-750nm

Question 10

n - pi* and pi-pi* transitions

Answer 10

most applications of absorption of organic compounds are based on these transitions because required energy to excite the electrons bring them into experimentally convenient spectral region

both transitions need unsaturated functional group to provide pi orbitals (chromophores)

n - pi* = shorter wavelengths with inc polarity of solvent (hexane to methanol to water)
pi-pi* = mostly shows the reverse trent