CH05 - Molecular Spectroscopy in UV-Vis

Question 1

Vacuum UV is absorbed by _______.

Answer 1

O₂, water vapor and other gasses present in the air.

(Remember: CO2 causes hothouse effect by absorbing UV and vibrating)

Question 2

Vacuum UV wavelength range is?

Answer 2

100Å-200nm (overlaps with X-ray)

Question 3

UV wavelength range is?

Answer 3

200nm-400nm

Question 4

Vis wavelength range is?

Answer 4

400nm-800nm

Question 5

What type of bonds do sigma bonds form?

Answer 5

single (from superposition of s orbitals or from “axial” overlapping of p orbitals)

Question 6

What type of bonds do pi bonds form?

Answer 6

double or triple (from superposition of p orbitals which are perpendicular to the inter-nuclear axis).

Question 7

Are sigma bond absorption bands (sigma to sigma*) visible in UV\Vis?

Answer 7

No, absorption energies (frequencies) are higher than those of UV\Vis.

Question 8

Are pi bond absorption bands (pi to pi*) visible in UV\Vis?

Answer 8

Yes

Question 9

what kind of bonds do n electrons form?

Answer 9

non-bonding. Occurs in atoms right of C on the table (atoms with non-bonding pairs such as those on oxygen).

Question 10

When an non-hybridized p orbital is full, it will form a _____ molecular orbital.

Answer 10

“n” (nonbonding) molecular orbital. Fully populated and non-hybridized p orbitals do not tend to participate in bond formation.

Question 11

The energetic position of _______ molecular orbitals is the same as that of the original p orbitals.

Answer 11

“n” molecular orbitals

Question 12

Which transitions between MOs absorb energy in UV\Vis?

Answer 12

π→π^*
n→π^*
n→σ^*

Question 13

What is the Franck–Condon principle?

Answer 13

During an electronic transition, a change from one vibrational energy level to another will be more likely to happen if the two vibrational wave functions overlap more significantly.

Question 14

Simple molecules in the gas phase often show the _______ levels
superimposed on the electronic transitions. Those are absent from the _____ phase due to______.

Answer 14

Vibrational,
Liquid,
Interaction between solvent and solute molecules

Question 15

Sample holders in UV Vis spectrum are made from ______ or ______.

Answer 15

Quartz and fused silica (glass). Glass absorbs IR but does not absorb UV\Vis.

Question 16

A PMT detector has high _____ and high________ ratio.

Answer 16

Sensitivity, signal to noise

Question 17

A semiconductor has a ________ valence band, an ________ conduction band at 0K and an energy gap________ than 2.5 eV.

Answer 17

Full, Empty, Smaller

Question 18

An insulator has a________ valence band, an ________ conduction band at 0K and an energy gap________ than 2.5 eV.

Answer 18

Full, Empty, Greater

Question 19

An conductor has an ________ valence band.

Answer 19

Half-empty (thus conducts electricity even at 0K)

Question 20

_________ can be used as detectors for electromagnetic radiation.

Answer 20

Semiconductors (photons with sufficient energy can excite valence electrons to conduction band, thus inciting electric current).

Question 21

How does a photo-diode detector work?

Answer 21

1. PN junction is hed in reverse-bias (n to + and p to - of voltage source).
2. A depletion zone is formed in the junction.
3. A photon (with E>Egap) strikes the junction, forming hole-electron pairs in the depletion zone.
4. Holes move toward “-“ terminal, electrons move toward “+” terminal, forming a current proportinal to the intensity of incident light.

Question 22

When a diode array is used as a sensor, _____ wavelengths can be measured simultaneously.

Answer 22

Multiple

Question 23

When a diode array is used as a sensor, the _____ is placed after the sample.

Answer 23

Dispersion device (prism or grating).

Question 24

Describe optical diagram of a diode array spectrophotometer?

Answer 24

Source → Sample → Entrance Slit → dispersion device → diode array sensor

Question 25

Advantages\disadvantages of photo-diode detector?

Answer 25

Advantage:
1. Fast - suitable for scanning fast reactions to determine
kinetics or measurements on flowing material (as in chromatography).
2. High SNR is achievable through multiple measurements - for applications involving low light levels.
Disadvantage:
1. Low resolution - suitable for molecular spectroscopy but not atomic spectroscopy.

Question 26

What is a photomultiplier tube (PMT)?

Answer 26

Detector used in the UV/Vis spectral region that acts like a series of phototubes. The cathode and anode are biased by 400 - 2500 mV and separated by a series of intermediate dynodes, each progressively more positively biased, which provides an electron multiplication cascade.

Question 27

What is a chromophore?

Answer 27

A group of atoms, with their associated electrons, in a molecule that absorbs light in certain wavelengths. (e.g. an Alkyne group)

Question 28

What is an auxchrome?

Answer 28

Substituent groups with nonbonding electrons that are attached to the basic chromophore structure and change the position and/or intensity of the chromophore’s absorption band; typical examples include methyl, hydroxyl, alkoxyl, halogen, and amino groups.

Question 29

What is a Batochromic Shift?

Answer 29

When wavelength of maximum absorption is shifted to a longer wavelength (“Red Shift”).

Question 30

What is a Hypsochromic Shift?

Answer 30

When wavelength of maximum absorption is shifted to a shorter wavelength (“Blue Shift”).

Question 31

What causes Batochromic\Hypsochromic shifts ?

Answer 31

Interaction of solute molecules with solvent.

Question 32

What causes changes in absorption intensity (Hyperchromism\Hypochromism) for a given wavelength?

Answer 32

Changes in molecular structure (e.g. as a result of protonation\deprotonation of phenolphthalein)

Question 33

In a Batochromic shift, dipole-dipole interactions and hydrogen bonds in a ____ solvent lowers energy of ____ orbitals more than that of π orbitals.

Answer 33

Polar, π^*

Question 34

For a given molecule, the π→π^* bandgap in a ____ solvent is smaller than the π→π^* bandgap in a ____ solvent.

Answer 34

Polar, Nonpolar

Question 35

What electronic transition is affected by a Batochromic Shift?

Answer 35

π→π^*

Question 36

What electronic transition is affected by a Hypsochromic Shift?

Answer 36

n→π^*

Question 37

What causes a Hypsochromic Shift?

Answer 37

Compared to π^* orbitals, “n” molecular orbitals are affected more strongly by interactions in polar solvent ⇒ n orbitals are lowered even more than π^* ⇒ (n→π^*) gap increases ⇒ wavelength shortens

Question 38

____ transitions also undergo Hypsochromic Shifts, but are invisible since they are in the vacuum range.

Answer 38

n→σ^*

Question 39

When solvent forms hydrogen bonds with the molecules, the energy change in “n” orbitals ≈ ____

Answer 39

Energy of hydrogen bonds that were formed.

Question 40

Increase in polarity of solvent ____ wavelength of n→π^* transition.

Answer 40

Decreases (because interactions with polar solvent decreases energy of n orbital electrons, increasing n-pi gap. Hypsochromic shift)

Question 41

In cyclical Ketones, positions of carbons are marked α,β,γ… ____

Answer 41

Along the conjugated diene chain (alpha is the nearest to carbonyl carbon, delta is the furthest)

Question 42

What is an exocyclic double bond?

Answer 42

A double bond attached on one end to an atom which is shared between two rings.

Question 43

λ_max in UV\Vis is different for two ____ of the same compound.

Answer 43

Isomers. Thus, UV-Vis absorption can be used to differentiate between isomers.

Question 44

What is a Singlet State (singlet energy level)?

Answer 44

In quantum mechanics, a singlet state usually refers to a system in which all electron spins are paired.

Question 45

What is a Triplet State (triplet energy level)?

Answer 45

A triplet state is a system with two unpaired electrons.

Question 46

In Fluorescence, energy of photon is ____ and photon wavelength is ____ than that of the absorbed radiation.

Answer 46

Smaller, longer (this process of fluorescence is called “Stokes Fluorescence\Stokes Shift”

Question 47

Describe the process of Fluorescence.

Answer 47

1. UV radiation excites an electron from ground singlet state S0 to first excited singlet state S1.
2. The electron relaxes in a radiationless process to the lowest vibrational state in S1. This is called an Internal Conversion (IC).
3. From here, there are 2 options:
   * The electron returns to ground state S0 while emitting a photon this process is called Fluorescence.
   * The electron can relax to the ground state in a radiationless process as a result of intermolecular collision. This process is called “Quenching”.

Question 48

Describe the process of Phosphorescence.

Answer 48

1. UV radiation excites an electron from ground singlet state S0 to first excited singlet state S1.
2. Inter System Conversion (ISC) to first excited triplet state T1 (radiation-less).
3. From here, there are 2 options:
   * The electron returns to ground state S0 while emitting a photon this process is called Phosphorescence.
   * The electron can relax to the ground state in a radiationless process. This process is called “Quenching”.

Question 49

In fluorescence, the lifetime of the electrons in the excited state is ____.

Answer 49

Very short (10-20ns)

Remember: Fluorescent paint shines only when lit by UV.

Question 50

In phosphorescence, the lifetime of the electrons in the excited state is ____ in fluorescence.

Answer 50

Longer (ms to a few seconds)

As in “glow in the dark” materials.

Question 51

Wavelengths emitted in ____ are longer than those emitted in ____ (choose fluorescence/phosphorescence also explain why).

Answer 51

Phosphorescence, Fluorescence
(Because energy of T1 state is lower than that of S1)

Question 52

Reason for difference in lifetime of excited electrons in Fluorescence and Phosphorescence?

Answer 52

The process of phosphorescence involves a forbidden transition (S1->T1).

Question 53

A rigid molecular structure increases ____.

Answer 53

F (intensity of fluorescence). Rigid molecules are less susceptible to quenching and radiation less relaxation.

Question 54

Fluorescence occurs in molecules with ____ transitions.

Answer 54

π→π^*
(aromatic compounds and polycyclic aromatic compounds)