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CHEM2302 Dina > Molecular spectroscopy and structure: Electronic spectra > Flashcards

Molecular spectroscopy and structure: Electronic spectra Flashcards

1
Q

How does molecular orbital theory construct molecular orbitals?

A

as a linear combination of atomic orbitals (LCAO) to predict the properties of the isolated molecule

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2
Q

Which categories can molecular orbitals be divided into?

A

valence and core

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3
Q

How are molecular orbitals classified?

A

by symmetry

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4
Q

What are the six categories of molecular orbital?

A
  • σ bonding orbitals (no nodal planes between nuclei)
  • σ* antibonding orbitals (one perpendicular nodal plane)
  • π bonding orbitals (one nodal plane in plane of nuclei)
  • π* antibonding orbitals (two nodal planes, one in plane and other perpendicular to plane of nuclei)
  • n non-bonding orbitals (no restriction on the number of nodal planes)
  • d-electrons
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5
Q

Explain σ bonding orbitals

A

σ bonding orbitals are formed when atomic orbitals overlap constructively along the line joining the centres of atoms. Electrons rarely play any part in electronic spectra but hold the molecule together and form its backbone. They are usually denoted as single bonds in chemical diagrams.

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6
Q

Explain σ* antibonding orbitals

A

σ* antibonding orbitals are formed when two atomic orbitals overlap destructively along the line joining the centres of two atoms. σ* orbitals usually lie higher in energy than σ orbitals. Typically the energy gap corresponds to high frequency UV photons.

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7
Q

Explain π bonding orbitals

A

π bonding orbitals are formed when p atomic orbitals overlap constructively above and below the line joining the centres of the atoms forming the bond. These correspond to ‘double bonds’ and delocalised aromatic bonding.

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8
Q

Explain π* antibonding orbitals

A

π* antibonding orbitals are formed when atomic orbitals overlap destructively above and below the line joining the centres of the atoms forming the bond. The energy separation between π and π* orbitals is much less than for σ and σ* orbitals and electronic transitions between these orbitals fall within the UV-visible region.

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9
Q

Explain n orbitals

A

These are orbitals in the valence shell that do not play any part in bonding. The electrons in these non-bonding orbitals are often called lone pairs. These are usually the least tightly bound electrons in a molecule and, if present, are responsible for most of the electronic transitions seen in the electronic spectrum. Usually they are excited into empty π* orbitals.

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10
Q

Explain d-electrons

A

When metal atoms with unfilled d-orbitals are present, excitation of these d-electrons may give rise to prominent colours.

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11
Q

What happens when π-orbitals become delocalised?

A

They form groups of orbitals with a lower separation between the highest energy occupied orbital (HOMO) and the lowest energy unoccupied molecular orbital (LUMO).

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12
Q

What are the molecular orbitals for methanal?

A

1s(O)

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13
Q

Which of these are the core orbitals?

A

1s(O)

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14
Q

What is the total spin angular momentum of a ground state ‘closed shell’ molecule?

A

All electrons are paired up in molecular orbitals such that their spins ‘cancel’ to give a total spin angular momentum, S = s1 + s2 = 0

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15
Q

What are states with S=0 known as?

A

singlets;

S0 ground state—>S1—>S2—>S3

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16
Q

What are states with S=1 known as?

A

triplets;

T1—>T2—>T3

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17
Q

Why does the labelling start from 0 for singlet states and from 1 for triplet states?

A

This reflects the fact that the electronic ground state is almost invariably a singlet because most molecules have closed shell ground state configurations.

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18
Q

Where do the names ‘singlet’ and triplet’ come from?

A

the spin multiplicity of the state

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19
Q

What is the relationship between a singlet and triplet state arising from the same electron configuration?

A

There is a strong tendency for the triplet state to be lower in energy than the singlet state.

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20
Q

What is a closed shell configuration?

A

This corresponds to a state with all molecular orbitals doubly occupied or empty (a singlet state).

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21
Q

What is the excited singlet state?

A

the pairing of electrons is preserved, but one is in the excited state

spins +1/2 and -1/2

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22
Q

What is the excited triplet state?

A

the electrons are not paired and the spin of the electrons change

spins +1/2 and +1/2

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23
Q

What is the multiplicity rule?

A

2s+1

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24
Q

How is electronic excitation usually brought about?

A

by the absorption of a photon of the correct frequency

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25
Q

What does the intensity of this process correspond to?

A

the rate; the number of photons being absorbed/molecules being excited

26
Q

What is the selection rule?

A

Transitions between electronic states as the result of the absorption or emission of a photon must occur with no change in spin multiplicity.

27
Q

How can the triplet state be populated directly from the ground state?

A
  • bombarding the molecule with electrons
  • collisions with other electronically excited molecules
  • chemical reactions that leave one of the products in an excited (possibly triplet) state
28
Q

How can the excited state ‘relax’ back to the inherently more stable ground state?

A
  • fluorescence
  • phosphorescence
  • vibrational relaxation
  • internal conversion
  • intersystem crossing
  • physical quenching
  • chemical reaction
29
Q

Explain the process of fluorescence

A

emission of a photon with no change in spin (allowed by the spin selection rule); radiative process, ΔS=0

30
Q

What is the approximate timescale of fluorescence?

A

10^-5 - 10^-9 s

31
Q

Explain the process of phosphorescence

A

emission of a photon with a change in spin (forbidden by the spin selection rule); radiative process, ΔS=/=0

32
Q

What is the approximate timescale of phosphorescence?

A

10^-3 - 10^-2 s

33
Q

Why is phosphorescence slower than fluorescence?

A

because phosphorescence is a forbidden process

34
Q

Explain the process of vibrational relaxation

A
  • change in vibrational level but no change in electronic state
  • non-radiative process
  • involves the transfer of vibrational energy from one species, R, to another, M, during a collision
  • the energy can go into rotational, vibrational or translational modes of M
  • these are so closely spaced that they form an effective ‘classical heat bath’
35
Q

Why is VR quite efficient?

A

because there is almost always a state with the correct energy to which M can be excited by absorbing energy from R

36
Q

Which processes does VR compete with?

A

with luminescence (fluorescence and phosphorescence); the process that occurs depends on the relative rates of VR and spontaneous emission

37
Q

What is the timescale of VR?

A

10^-12 s; VR usually takes place before any other process has time to occur

38
Q

Explain the process of internal conversion

A

non-radiative change in electronic state with no change in spin; ΔS=0

39
Q

Explain the conservation of energy of internal conversion

A
  • electronic energy is converted into vibrational energy on the ground electronic surface
  • this then vibrationally relaxes, ultimately producing heat in the system
40
Q

What is the timescale of internal conversion?

A

slow, increasing exponentially with the energy gap between the surfaces

41
Q

Explain the process of intersystem crossing

A

non-radiative change in electronic state with a change in spin; ΔS=/=0

42
Q

Explain the conservation of energy of intersystem crossing

A
  • electronic energy in the upper state is converted into vibrational energy on the lower state
  • this rapidly decays to the lowest vibrational level of the lower electronic state by vibrational relaxation
43
Q

What is the timescale of intersystem crossing?

A

generally slow; depends strongly on the precise nature of the molecule; depends exponentially on the energy gap between the electronic states

44
Q

Explain the process of physical quenching

A

loss of electronic excitation via collision with another molecule

45
Q

Explain the conservation of energy of physical quenching

A

electronic excitation becomes (mostly) translational motion in the quenching species (ie. heat~)

46
Q

What is the timescale of physical quenching?

A

depends on pressure (for a gas)

47
Q

Explain the process of chemical reaction

A

with another species; isomerisation, dissociation

48
Q

What is the timescale of chemical reaction?

A

depends on the reactant and physical conditions

49
Q

What order process is fluorescence?

A

first order

50
Q

What does pseudounimolecular mean?

A

the chemical reaction involves two or more reactants, but the rate of reactions depends only upon the concentration of one of the reactants

51
Q

What is the quantum yield of fluorescence?

A

the ratio of photons emitted through fluorescence to the total number of photons absorbed

52
Q

What is a Jablonski diagram?

A
  • an energy diagram, arranged with energy on a vertical axis
  • the rest of the diagram is arranged into columns
  • each column represents a specific spin multiplicity for a particular species
  • within each column, horizontal lines represent eigenstates for that particular molecule
  • bold horizontal lines are representations of the limits of electronic energy states
53
Q

When do vibrational levels not have to be accounted for?

A

for systems with efficient vibrational relaxation (liquids and dense gases)

54
Q

What is an energy state diagram?

A

a simplified diagram with all singlet states in one column and all triplet states in another

55
Q

Why is the rate of change of concentration of S1 equal to zero?

A

because S1 is considered a ‘reactive intermediate’ and so we assume that after a short period of initiation, its concentration approaches a constant value (steady state approximation, SSA)

56
Q

What is the intercept of a Stern-Volmer plot?

A

1/I(abs)

57
Q

What is the gradient of a Stern-Volmer plot?

A

k(Q)/[ k(f)I(abs) ]

58
Q

What is the assumption of a Stern-Volmer plot?

A
  • the only significant intersystem crossing is from S1 to T1

- the presence of the quencher does not directly affect the relaxation of the triplet, which is only by phosphorescence

59
Q

What is the primary quantum yield?

A

Φ(i) = number of excited molecules, R*, undergoing process i / total number of photons absorbed by R

60
Q

What is the assumption of the primary quantum yield?

A
  • the initial R* state is formed only by absorption of a photon and not by any secondary process
  • all R* ultimately relax from the excited state R* to some lower energy states (either directly or indirectly to the ground state of R or to some product P)
61
Q

What is the sum of all quantum yields for all pathways leaving the state occupied by R*?

A

1

62
Q

What does the primary quantum yield give an indication of?

A

the efficiency of each process and therefore an indication as to the relative likelihood that a given process will ‘win’ for a given excited molecule

CHEM2302 Dina (9 decks)

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