Inorganic 5

Under normal (thermal) conditions energy must be distributed…

over all modes of excitation (translational, rotational, vibrational, electronic)

The Boltzmann distribution

n(2)/n(1) = e^(-ΔE/kT)

ΔE = difference in energy

n(2)/n(1) = fraction of molecules in ex st.

whats the issue w using very high temperatures in rxns

thermal degradation of reactants

what can Light provide

the energy needed to both overcome the barrier to a reaction and to drive reactions thermodynamically uphill

energy of a photon, E =

ℎν

h = planks constant

v = freq

λ =

c/ν

what does light promote

electronic excitation in a specific manner e.g. bond cleavage

What happens to molecules after excitation by visible light

visible light absorption changes pop of electronic ex sts of molecules

what is Stimulated absorption

photon excites a molecule from a low-energy state to a high energy state

what is Stimulated emission

photon induces a molecule to transfer from high energy to low energy state

what is Spontaneous emission

transition from high energy to low energy state independent of radiation

e.g. of spontaneous emission

• flourescence

• phosphorescence

what does Born Oppenheimer approximation tell us

that electronic motion is infinitely fast compared to nuclear motion. (e-s move faster than nuclei)

So we assume that the nuclei are static when the electron is transferred

Quantum mechanically what do we use the Franck-Condon factor to describe

transition intensities

what is The transition moment, TM, related to

the probably of a photon being absorbed

what can the total wf (Ψ) be factorised into

the electronic part of the wf, ψ, and the vib parts, θ

spin selection rule

ΔS = 0

what transitions are allowed according to spin selection rules

spin cant change, so singlet→ singlet or triplet → triplet allowed, singlet → triplet or vice versa isnt allowed

when can spin selection rule can be circumvented

s (particularly with heavy transition metals), spin-orbit coupling, means that wf of gr st cannot be factorized into a 1e- wf and Spin

Laporte’s rule

g → u and u → g allowed.

u → u and g → g forbidden.

is μ(w a hat) g or u

u

g × g =

g

u × u =

g

g × u =

u

what is on the horizontal axis of a spectrum

en of the vertical transition

what does the franck condon transition relate to

diff in en btwn gr st and ex st

WL and en

WL not vary linearly w en - reciprocal relationship

what is on the vertical axis of a spectrum

intensity of transition

how to quantify intensity of transition

beer lambert law to give abs coeff

intensity of transition relates to

transition moment, so how well selection rules are obeyed

what can happen to ex st once formed

• vr

• isc

• fluorescence

• phosphorescence

what is luminescence

the emission of light (photon)

what is Quantum Yield (φ):

The number of reactant molecules consumed for each photon of light absorbed.

what is quantum yield a measure of

the ‘efficiency’ of the photochemical reaction.

what does it mean if quant yield > 1

suggests that secondary reactions are occurring.

what does it mean if quant yield > 2

indicates a chain reaction.

what is often the cause of very low Quant Yield

• solvent cage - geminate recombination

• high radiative or non-radiative relaxation of the initial excited state may also cause this.

what orbitals do Ligands which can get involved in p-bonding have

orbitals with t2g symmetry, so these metals orbitals change from being non-bonding. This then changes Δ(o)

what happens to Δ(o) when hv pi-donor ligs

decreases

pi donor lig e.g.

I-, cl-, br-, F-, OH-

what happens to Δ(o) when hv pi-acceptor ligs

increases

pi-acceptor lig e.g.

CO, PR3

weak pi donor e.g.

H2O

no pi-effect lig e.g

NH3

how does pi donor ability affect Δ(o)

• strong pi - donor = small Δ(o)

• sigma donor = medium Δ(o)

• stron pi-acceptor = large Δ(o)

d-d transition features

• oft found at ~25,000 cm-1

• hv abs coeff of ~100Lmol-1cm-1

• La Porte forbidden as in complex w centre of symmetry its a g-g transition

For a d1 metal complex where there are no ligands…

treat atom as having spherical symmetry - all d orbs hv same en

For an octahedral d1 metal complex, when the ligand field increases (i.e. d orbs not all same en)

there are 2 configs possible

as lig dield increases, what happens to gap btwn t2g^1 and eg^1

increases so higher en light is needed to form ex st

in the spectrum of [Cr(NH3)6] 2+, there two transitions from the t2g 2eg^1 ← t2g³ - why?

any of the 3 e-s in t2g set can migrate to either of 2 eg orbs - although they appear same at first, repulsion btwn diff e-s means they are diff ens

discuss: transition from dz2 ← dxy Qualitatively

dz² ← dxy transition - from xy plane to z plane - already got d(xz) and d(yz) that hv e- density in z plane - more e- density in z axis than to start - more e- e- repulsion

discuss: transition from dz² ← dzx

keep e- density in z plane - diff e- arrangement so still diff en but e- e- repulsion so roughly same as kept e- density in z plane

term symbols have general form

^(2S+1) L

spin of e-

½

for a d² complex, what are the lowest energy terms

³F and ³P

what is the Racah parameter, B

a measure of e- e- repulsion

what is LMCT

ligand-metal charge transfer

LMCT vs d-d

LMCT far more intense than d-d transitions

why are LMCT far more intense than d-d transitions

because they are allowed under Laporte’s rule

what do CT transitions oft show

solvatochromatism bc is large change in dipole moment w transition

what is MLCT

metal-lig charge transfer

what do MLCT transitions involve

electrons moving from mainly metal-derived orbitals to those located on a ligand

when are MLCT transitions oft observed

when complex has low lying pi* orbital

diff btwn prompt photochem rxn and delay photochem rxn

length of time from photon absorption to resulting photochem

what is 1 fs

1 × 10^-15 s

M(CO)5 fragments are….

v Lewis acidic

what is a delayed process.

if an excited state has a long lifetime, then it can be considered to ‘equilibrate’ and remains for long enough to undergo chemistry

In general what does charge transfer excitation (MLCT or LMCT) cause

a radial redistribution of electrons between the metal and ligands

what does an MLCT excitation correspond to

metal oxidation (lig red)

what does an LMCT excitation correspond to

metal red (lig ox)

whta is photoredox behaviour

light promoted e- transfer rxns

ex st vs gr state redox

ex st is better red and ox agent

why is ex st easier to oxidise

• gr st -high ionisation en - lot of en to remove e- so hard to ox

• ex st - bc e- in higher orbitall, IE reduced so ox easier

why is ex st easier to red

• gr st low e- affinity so doesnt like gaining e- so hard to red

• ex st ‘hole’ in bonding manifold so EA increases and ex st easier to red

The electron affinity of A is greater than the electron affinity of A* by

the energy needed to excite A to A*

a more -ve potential means

better red agent

a more +ve potential means

better ox agent

what is Photosensitisation

the use of molecules which absorb light to promote light driven reactions of molecules which do not themselves absorb

what are LAS

absorb light and direct its use to subsequent chem

2 types of Photosensitisation

• LAS - light absorption sensitisers

• LES - light emission sensitisers

what do LES do

act to emit light en. exploit chem rn produced in a rxn by molecules which cannot emit light themselves

whta is deltaG for a rxn to be thermodyamically fav

-ve

peak irradiance in solar spectrum

at ca 500 nm

leading method of creating device which utilises as much of the Sun’s radiation as possible

through Grätzel, or dyesensitised solar cell (DSSC)

Grätzel, or dyesensitised solar cell (DSSC) oft based on

semi-conductor e.g TiO2

what is used to absorb visible light in Grätzel, or dyesensitised solar cell (DSSC)

sensitiser e.g. Ru complex

components of the Grätzel cell

3 components

• TiO2 semiconductor

• photosensitiser (PS) e.g. Ru complex

• reductant

overall affect of the Grätzel cell

to convert absorbed light into en

what is an attractive way to make dihydrogen

light-promoted splitting of water

light-promoted splitting of water rxn:

H2O → H2 + ½O2

deltaG +ve

ex st of [Ry(bpy)3]2+ lies at…

2.12 ev