11/12. Spectroscopy: Light and Matter

(5.2) Arrange the following types of electromagnetic radiation from lowest to highest energy (top of the list) to highest energy (bottom of the list: visible, IR, vacuum UV, x-ray, microwave, UV, gamma

(Lowest)

Microwave

IR

Visible

UV

Vacuum UV

X-ray

Gamma

(Highest)

(1.3a) T/F? Refraction is the bending of light, as it encounters an opening or a barrier it is often used for wavelength selection.

False; diffraction

(1.4a) List the five things that can occur when light interacts with matter.

1. Diffraction

2. Refraction

3. Reflection

4. Polarization

5. Absorption

(3.3a) T/F? Atomic and molecular photon absorption events occur at energies equivalent to the difference in energy between ground and excited states for the species of interest.

True

(3.3c) T/F? Light that reaches a phase boundary experiences diffraction when it enters the new phase.

False; refraction

(5.1) What does it mean to say that the absorption of electromagnetic radiation by an atom or a molecule is “quantized”?

The magnitude of energy absorbed is equal to the difference in energy between the ground and excited state: 

E_{photon absorbed} = E_{excited state} - E_{ground state}

Different ground and excited state electronic, vibrational, and rotational transitions occur depending on the chemical structure of the molecule or the electronic arrangement of atoms.

(6.3) What limits the measurement of alkanes using a typical UV/Vis spectrophotometry instrument?

In general, the presence of solvent in a typical UV/Vis spectrophotometer limits the measurement of absorption at wavelengths lower than the absorption cut-off of the solvent.

Alkanes undergo σ to σ* transitions in the deep UV range (e.g. 80-150 nm). Measurements made in this absorption regino must be performed in the absence of a significant absorbing background (i.e. vacuum or in a GC carrier gas)

(6.5e) What is diffraction and what is it used for in molecular spectroscopy measurements?

Diffraction is the bending of light at a sharp barrier or narrow opening.

It is used for wavelength selection in monochromators (i.e. diffraction grating)

(6.5f) In absorption spectroscopy, what does it mean to say that we measure quantized transitions in a molecule?

Absorbed photons have an E equivalent to the ΔE between the excited and ground states in an atom. The difference in energy between energy levels and quantized. 
E_{photon absorbed} = E_{excited state} - E_{ground state}

What type of transmittance can be used for quantitative vs qualitative?

Quant: Electronic transmittance

• UV/Vis

• Atomic Spect (Abs/Emi)

Qual: Vibrational/Rotational transmittance

• FTIR

Example of each excitation phase?:

1) σ to σ*

2) n to σ*

3) π to π*

4) n to π*

1) Alkane

2) DCM

3) Benzene

4) ??

Diffraction

Bending of parallel radiation waves by passage through a narrow opening or around a sharp barrier.

a. Leads to constructive and destructive interference.

b. Used for wavelength selection: “diffraction grating”

Refraction

Bending of light by a phase barrier

• velocity of light changes from medium to medium

• frequency of light is independent of medium

• wavelength varies with medium

• dependent on refractive index of medium: refractive index (n) = speed of light in vacuum (c) / speed of light in medium (v)

Reflection

Bouncing / deflecting of light off a surface or medium

• increases with inc delta n

Absorption

Absorption of radiation raises the molecule to a higher internal E level and dE is equal to the energy of the absorbed radiation (hv)

• Types of internal E are “quantized”, existing at discrete levels

• Types of transitions (in inc E): rotational, vibrational, electronic

• Epho = Ee - Eg

What region of the electromagnetic spectrum do the transitions cover?

1) Rotational

2) Vibrational

3) Electronic

1) Microwave (qual)

2) IR (qual)

3) UV/Vis (quant)

Types of e’s (in order of dec excitation E)

1) Closed-shell e'- : do not contribute to absorption process

2) Covalent single-bond e- (σ) - excitable in VUV region

3) Paired non-bonding outershell e- aka valence e- (n), found on NOS and halogens

4) Electrons in pi orbitals aka delocalized e-; (double/triple bonds), most readily excitable and contribute most to UV/Vis spectra