Spectroscopy

Question 1

Name the limitations of the Beer-Lambert law

Answer 1

◾ for dilute solutions only
◾ chemical deviations
◾ instrumental deviations
      - mismatched cells
      - scattered light
      - polychromatic light

Question 2

How do absorption and emission spectroscopy differ?

Answer 2

▶ ABSORPTION: based on absorption of radiation by the analytes; ground state ➡ excited state

▶ EMISSION: emission of radiation by molecules; excited state ➡ ground state (energy provided by heat/chemical reaction)

Question 3

What is a chromaphore?

Answer 3

A functional group that absorbs UV/Vis radiation

Question 4

Name the factors that affect the maximum wavelength of a molecule

Answer 4

◾ Strength of bonds - double bonds absorb at higher wavelengths than single bonds
◾ Conjugation - increase leads to higher max wavelength (smaller energy gap)
◾ Solvent - stablises ground/excited state
◾ Substituents - electron withdrawing/donating affects energy gaps

Question 5

Name the considerations to take into account when choosing a solvent

Answer 5

◾ should be transparent in the wavelength of interest
◾ analyte should be soluble in the solvent
◾ the solvent will affect the wavelength of max absorbance
◾ more polar solvents will reduce the fine structure in the spectra

Question 6

Describe the process that leads to molecular absorption in the UV region

Answer 6

◾ absorption of EM radiation causes excitation
◾ ground state ➡ excited state
◾ UV - corresponds to excitation of valence electrons from HOMO to LUMO
◾ energy of radiation should match energy difference between states

Question 7

Describe the process that leads to absorption in the IR region

Answer 7

◾ radiation corresponds to streching and bending vibrational frequencies
◾ radiation absorbed by bonds with the same streching and bending frequency
◾ absorption is quantised
◾ a single bond can absorb at more than one frequency

Question 8

Define isosbetic point

Answer 8

A point in UV spectra of 2 or more molecules where all spectra intersect; resulting from conversion of one species to another

(identifies if 2 principle species are present)

Question 9

Why does the spectra for the same analyte differ when it is in an apolar or polar solvent and when it is in the gas phase?

Answer 9

⏩ ENERGY TRANSISTIONS
◾ gas: see fine structure - rotational, vibrational and electronic transitions
◾ apolar: vibrational and electronic transitions (no free rotation)
◾ polar: analyte interacts more with solvent - evening out of energy transistions

Question 10

Name the trends for vibrational frequencies of molecules (IR)

Answer 10

◾Absorption frequency decreases with an increase in the mass of an atom
◾ Absorption frequency decreases with decreasing bond strength