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CHEM2017 - Physical Chem > Spectroscopy > Flashcards

Spectroscopy Flashcards

(61 cards)

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1
Q

What does the Born-Oppenheimer eq show us

A

E(tot)=E(el)+E(nuc)
which can be factorised to
E(tot)=E(el)=E(vib)+E(rot)

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2
Q

What is the change in wavelength range for electronic transitions?

A

UV/Vis region
change on wavelength 500-100nm

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3
Q

What is the change in wavelength range for vibrational transitions?

A

IR region
change of wavelength between 100-2nm

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4
Q

What is the change in wavelength for rotational transitions

A

Microwave region
change in wavelength between 10cm-1nm

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5
Q

In Boltzmann’s law what does g(i) represent?

A

degeneracy of the energy level i
(how many states with the same energy)

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6
Q

In Boltzmann’s law, what does change inE represent?

A

change in energy between ground state and energy level (i)

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7
Q

What does the Rydberg constant depend on?

A

depends on the mass of the nucleus.

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8
Q

What is Russel-Saunders coupling?

A

coupling between orbital angular momentum (L) and spins (S), determines energy levels
combination of L+S gives spin-orbit angular momentum, J

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9
Q

Why are atomic term symbols useful?

A

easy way of writing down J,L and S numbers
isoelectronic species will have same term symbol

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10
Q

how do we find multiplicity part of the term symbol

A

mS is either +1/2 or -1/2 for each electron
S= sum of mS for every electron being considered
multiplicity= 2S+1
can be singlet, doublet triplet …

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11
Q

What is the atomic term symbol for a closed shell atom?

A

S=0 as all electrons coupled up so multiplicity =1
L=0 as all orbitals filled
J=0
so 1S0

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12
Q

What is the atomic term symbol for alkali metal atoms?

A

singular valence electron so ms=1/2=S
multiplicity=2
only 1 electron being considered so L=0
J=1/2
so 2S1/2

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13
Q

What is the atomic term symbol for excited alkali metal atoms?

A

s-electron excited into p-orbital
S=1/2, multiplicity doesnt change =2
L=1 as now considering p-orbital
J=3/2 and 1/2
so 2P3/2 and 2P1/2

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14
Q

What are the selection rules involved with an excitation transition? What is one thing to watch out for?

A

change in J =0,+1,-1
change in S=0
need to watch for mL numbers when looking at p/d orbitals. may effect change in J

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15
Q

What is the Pauli exclusion principle?

A

no two electrons in atom/ion can share the same set of quantum numbers

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16
Q

What are Hunds rules used for? What are they?

A

used for finding ground state term symbols of atoms
1- the term with the largest S is lowest in energy
2-for a given S, term with the largest L is lowest in energy
3-for term with several levels, is the subshell is less than half full, the lowest J level is lowest in energy, if the shell is more than half full, the highest J level is lowest in energy

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17
Q

What is moment of Inertia calculation?

A

I =u x r^2
u= reduced mass
r= bond length

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18
Q

What is a rigid rotor

A

model of ideal rotation of diatomic molecule

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19
Q

What are the selection rules of a rotational translation?

A

molecules must have a permanent dipole moment
Transitions only occur if change in J= +-1
J=rotational quantum number

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20
Q

What is B?

A

rotational constant

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21
Q

What are pure rotational transitions?
How do you find energy spacing between levels?

A

they use rigid rotor approximation
E(J+1-E(J)=2B(J+1)
when J=0,1,2…
useful to find I and therefore bond length

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22
Q

What is centrifugal distortion?
How does this effect the rotational energy levels?

A

when molecules rotate, the centrifugal force pushes outward on molecules, increasing bond length.
centrifugal distortion constant=D
decreases spacing between higher levels as B and r are inversely proportional

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23
Q

What is the perturbation theory?

A

Given in eq. list
F(J)=BJ(J+1)-D[J(J+1)]^2
considers effect of distortion

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24
Q

Why can we get rotational frequency from vibrational spectrum?

A

Diatomic molecules stretch slightly when rotated, so can deduce strength of bond from this.
vibrational frequency eq. in equation list
(can find force constant from we)

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25
What is Hooke's law?
Force=-Force constant x displacement
26
What is v?
vibrational quantum number
27
What is we (w=omega)
we = vibrational constant (cm-1) (wavenumber)
28
What is zero point energy? Is there a zero point in vibrational energy?
zero-point energy is energy of the ground state zero point in vibrational energy when v=0, we=1/2 shows even at 0K, bonds will be vibrating
29
what are the selection rules for a harmonic oscillator?
molecule must change dipole moment during vibration, transitions occur for only change in V=+-1
30
Why are molecules NOT harmonic oscillators?
we can break their bonds by pulling them apart infinitely, we can define a dissociation energy for breaking a bond, as we stretch a bond it gets weaker (restoring force is reduced, think like a spring)
31
What is xe? give units
anharmonicity constant, dimensionless value
32
What is D0?
dissociation energy, difference in energy between v0 and vmax.
33
What are the selection rules for anharmonic vibrational transitions?
molecules still must change dipole moment during vibration change in v= +-1, +-2, +-3 showing overtones and fundamental transitions
34
What is the fundamental transition? How can we find the change in E?
transition between v1 and v0 plug each into Schrödinger equation and find difference between them
35
What are overtones?
transitions between v0 and v2 (where change in v is greater than 1)
36
What is vibration-rotation spectra?
With each vibrational transition we observe rotational transitions as there a rotational energy levels associated with all vibrational levels.
37
Which transitions make up P-Branch and R-Branch?
when change in J is positive, these transitions give rise to R-Branch When change in J is negative, these transitions give rise to P-branch
38
What is the Q-Branch?
for when some molecules have a transition where change in J=0 so neither R or P branch. Lies in-between P and R branch
39
Whats the separation between R(0) and P(1)
4B as there's a Q-branch in between them
40
IsB0=B1=B a good approximation?
no as the spacing between each branch is not 2B, this is due to rotational-vibrational coupling and centrifugal distortion The spacing between R-branch lines become closer and P-Branch lines become further apart in energy.
41
What is Alpha?
vibration-rotation constant
42
What is Be?
equilibrium rotational constant
43
How can we find Be and Alpha?
if we know any two Bv values, can use simultaneous equations to derive both Be and alpha
44
How are electronic terms classified?
according to their overall angular momentum (LAMBDA), along internuclear axis
45
What symmetry does sigma/sigma-star and pi/pi0star MOs have? whats the values of lambda?
sigma orbitals have cylindrical symmetry, lambda=0 pi orbitals either have lambda value=+-1 depending on phase
46
What is LAMBDA?
LAMBDA= sum of all lambdas
47
How do we assign parity labels?
only relevant with heteronuclear diatomics as homonuclear wont have centre of inversion does the heteronuclear have symmetry with respect to the centre of inversion? (same phases opposite?)
48
How do we assign symmetry labels?
only really considered in SIGMA states, positive assigned to symmetric orbital electrons, negative assigned to asymmetric orbital electrons find product = +- given only get SIGMA- from configs with electrons in pi orbitals
49
how do you find OMEGA
OMEGA= |SIGMA+LAMBDA| replaces atomic J
50
What are the selection rules concerned with changes in angular momentum in diatomic and nuclear molecules
Change in LAMBDA=0,+-1 Change in Spin mult=0 Change in OMEGA=0,+-1 only g to u/ u to g transitions allowed only SIGMA(neg)to SIGMA(neg) or SIGMA(pos) to SIGMA(pos) allowed
51
Why do we get vibrational fine structure in electronic spectra
electronic transition may cause simultaneous change in vibrational state of molecule
52
What does the Franck-Condon principle take into account? what assumptions are made?
takes into account that nuclei are so much larger than electrons so electronic transitions happens so quickly the nuclei cant respond assume that while electronic transition occurs, nuclei stay frozen/unchanged
53
What are the selection rules of allowed vibrational changes in electronic spectra
there are no selection rules governing vibrational changes accompanying electronic transition. instead probability of transition (v'<~v'') governed by Franck-Condon factors
54
What is the dissociation limit?
when the energy levels converge at top of parabola. no longer a diatomic molecule
55
What is a vertical transition with electronic transitions?
vertical line between potential energy curves, shows that internuclear distance doesn't change but potential energy does
56
What is the Birge-Sponer extrapolation? What assumption made?
Area under a plot of transition wavenumer against vibrational quantum number is equal to dissociation energy,D0 of molecule Area=D0=1/2 x b x h assumption that differences approach 0 linearly (to form triangle)
57
How does the Franck-Condon principle explain intensities of vibrational transitions?
probability of a vibrational transition depends on overlap of vibrational wave functions which are governed by nature of electronic states involved
58
Draw molecular orbital diagram for HF molecule
check notes
59
Draw molecular orbital diagram for O2+
check notes
60
Draw molecular orbital diagram for NO
check notes
61
What happens to the value of the Rydberg constant as mass of nucleus becomes infinitely heavy?
the reduced mass becomes equal to the mass of electron due to the electron mass on denominator becoming insignificant. so Rydberg constant reaches maximum value

CHEM2017 - Physical Chem (5 decks)

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