Techniques & Approaches Flashcards
We need a strategy for detecting short-lived intermediates at room temperature - generating them at a high concentration & detect them
What are the various ways can we do this?
- Use photochemistry - initiate chemical reactions
a) cooling down to make intermediates very long-lived ≤ 77K - traps complex in solid glass (transparent) or matrix (noble gas)
b) cool down to make intermediates and reactive products long-lived - complex in inert solvent (noble gas)
c) Detect at room temperature using fast spectroscopy
What are two ways we can study intermediates
- Low temperature in a solid matrix
- Or a liquidifed noble gas to study intermediates
Describe the matrix isolation approach for intermediate monitoring?
- Uses inert gases (e.g. Ar) at ca. 12K or hydrocarbon glass (e.g. methyl cyclohexane) at 77K
- Using low temperatures and rigid matrix means that intermediates are long-lives and can be studied using conventional spectroscopy e.g. FTIT or UV/Vis
- UV or visble light are used to initiate reactions
How many IR spectrums might you take when analysing intermediates in a reaction using matrix isolation?
- Run the IR to get a starting spectrum of the products
- Run a second IR once the reaction has started to monitor the spectrum of photoproduction
- New band in the middle showing intermediate
For the following reaction:
Cr(CO)₆ + PPh₃ → Cr(CO)₅(PPh₃) + CO
The experiment of: trapping Cr(CO)₆ in Ar matrix at 12K and photolyse this, forming
How can this be measured?
- CO will produce a characteritic peak of its own the the IR
- The C4v product is predicted to have 3 IR bands
- The D3h product is predicted to have 2 IR bands
- e.g. on graph before photolysis shows 1 CO peak and after photolysis it shows 3 new CO peaks (C4V)
There is also large differences in the UV before and after photlysis for the following reaction:
Cr(CO)₆ + PPh₃ → Cr(CO)₅(PPh₃) + CO
What did it suggest
- Effect was not a generalised matrix effect
- suggested matrix coordination e.g. Cr(CO)₅…CH₄
A similar example of the following reaction undertaken in a matrix of isolation in Argon for:
What are two things you can comment on this reaction
- Solid Argon prevents reaction warming allowing diffusion
- Shows activation energy is around ~0°
Here is a similar reaction taken in a maxtrix isolation in Argon:
How can we characterise this reaction process?
Why is there no back reaction?
- Ni(CO)₃(N₂) - characterised by v(CO) and v(NN) IR bands
- N₂ only shows on IR once coordinated due to the formation of a dipole - sigma donation + pi backbonding
- There is no reverse reaction (30K) - Ea>0 (18e- compound) - importance of kinetic of reactivity not just intermediates
How can be study unstable species in solution?
- Using liquified noble gases - inert
- This is done at 20 atm pressure to increase the range they are liquid
Why are unstable species studied in noble gases?
- Need solvent to be transparent to IR
- IR absorbed by vibration of bond between atoms - Ar, Kr, and Xe only have one atom in the molecule
What is interesting about the following reaction
- The thermal back reaction can go by either Id or D pathway depending on the temperature (Id at low T)
- By using liquid Krypton, we can probe the mechanism, measuring its thermodynamic parameters and deduce what is happening
Intermediates are short-lived at room temperature
What is required to detect them?
- Rapid detect
- To characterise intermediates
- Allowing reaction kinetics to be obtained
If we have a fast reaction with the t½ (half-life) «_space;single scan of the FT-IR, we need a new approach…
- UV/Visible Flash Photolysis
UV/Visible Flash Photolysis uses a pump-probe technique
What does this entail?
- Use pump (flash) to generate intermediates
- Use probe to monitor reaction
- Detection can be in UV/visible/IR region
- Original apparatus - use UV/Vis detection
UV/Vis Flash Photolysis uses a point-by-point appraching
What does this entail?
- Select on λ₁ (specific wavelength) and use pump to generate intermediate
- Measure change in absorbance at λ₁ with regards to time
- Select λ₂ and repeat
- Repeat for many λ
- Plot change in absorance vs λ for a certain Δt after flash
- We get kinetic (left) and spectra (right) information from this approach
What are some benefits of the UV/Visible Flash Photolysis
- Works well for nanoseconds/miliseconds/microseconds → due to having a continous wave (cw) probe and can use electronics to get the timings right
- (CW - light on probe on all the time)
- Obtain kinetics
- Can now use multichannel detectors to obtain all wavelengths at once
Lazers can be very fast but sometimes reactions occur on the picosecond/femtosecond scale - might need another approach to get the timng right
What is a way to get to get around this?
- Use pulsed pump and probe pulses
- Change time between pump and probe by making the probe beam travel further
What are some important features of he lazers in ns-IR flash photolysis?
- Uses IR laser - orginally IR diodines and now Quantum Cascade Lazers
- These give out tunable CW IR light
- Each lazer covers 150 cm⁻¹ - available to cover whole IR region
How do you do ns-IR Flash Photolysis
- Tune IR diode lazer to one IR frequency, λ₁
- Pulse UV lazer and measure cange in IR intensity at λ₁
- Tune IR lazer to λ₂ and repeat measurement
- Build up IR spectra “point-by-point”
List some advantages of UV/Vis for Detection methods for flash photolysis
- Good CW sources e.g. Xe lamp
- Sensitive detectors
- Very good kinetics
- Commercially available nanosecond and picosecond systems
List some advantages of IR detection methods for flash photolysis
- Narrow absorptions
- Very good specificity
- Obtain structural information especially good for metal carbonyls
- Moderate Kinetics
List some disadvantages of UV/Vis for detection methods for flash photolysis
- Broad absorptions
- Difficult to obtain kinetics in multi-component systems
- Little structural information for large molecules in solution
List some disadvantages for IR detection methods for flash photolysis
- Detectors much less sensitive than those in UV/Vis
- Powerful IR sources (lasers) only cover part of the IR range
The following two molecules are isomers of [CpFe(CO)₂]₂
How would the photochemistry (IR) of them differ?
- The cis- and trans-isomers in solution (mainly trans in non-polar solvent)
- Cis isomer = 3 IR v(CO) bands → 2 v(CO) terminal and 1 bridging
- Trans isomer = 2 IR v(CO) bands → 1 terminal and 1 bridging
We can use matrix isolation to study the photochemistry of [CpFe(CO)₂]₂
How do we do so?
- C = central, T = terminal
- UV photolysis caused depletion of the parent bands and the production of free CO in the matrix (CO loss)
- Production of only one band in the IR - 1812, within the bridging region for 3x bridging CO
How can we confirm the suggested result of this bridging species
- UV flash photolysis at RT - leads to depletion of parent and the production of new bands at 510nm (matches matrix isolation result)
- Visible photolysis - leads to only depletion of the parent
What do these UV kinetic graphs show about the photochemistry of [CpFe(CO)₂]₂
- Two processes going on, within a very small timeframe (while the matrix isolation only indicated one process occuring)
- Differences between the photolysis between UV and Visible
- Second order process
What can we deduct from these two graphs then about the photolysis of [CpFe(CO)₂]₂
- UV photolysis - 2 processes - “fast” + “slow”
- Fast process decays via second order kinetics in ca 20 ms
- Visible photolysis produced only fast kinetcis and parent reformation (2nd order) within 20 ms
- Kinetics at 510 nm shows this intermediate does not decay on the 20 ms timescale - look at slower timescale
The photolysis of [CpFe(CO)₂]₂ was undertaken again under CO rather than Ar via 1st order kinetics over 1-2 mn to reform parent
What does this show
The UV-vis results suggest two process - fast radical formation and recombination and slow CO loss process
A flash photolysis in CO was taken for both UV and visible
The UV photolysis saw depletion of parent and production of three new IR bands at 2005, 1938 and 1824 cm⁻¹
The Visible photolysis produces only one transient and production of bands at 2005 and 1938 cm⁻¹
What can we draw from this?
- 2005 and 1939 cm⁻¹ decay rapidly at the same rate (2nd order) - due to the same species - kinetic match UV/Vis flash photolysis
- 1824 cm⁻¹ band decays slowly under CO - kinetic match UV/Vis flash photolysis - band position matched matrix results
What is Raman Spectroscopy?
- Is an analystic technique used to study the excited state of the vibrational, rotational, and other low-frequency modes of moleules
- Light interacts with a material and undergoes inelastic scattering, which results in a shift in wavelength, which provides information about molecular structure
What is the Stoke shift in Raman?
The molecule gain energy, and the scattered photon has lower energy than the incident photon
What is the Anti-Stokes shift in Raman?
The molecule loses energy (if it was already in an excited vibrational state), and the scattered photon has higher energy than the incident photon
(smaller because fewer molecules in excited state)
What is the process behind Raman Spectroscopy?
- Raman involves shining a lazer light and collect data at 90°
- Then the distance between the light that gets emitted leaving the sample in the higher vibration level vs the light that comes straight out - results in a raman shift
- Raman leaves the molecule in a vibrationally excited state (this is a different mechanism to IR however)
What is the selection rule for Raman Spectroscopy?
What are the main molecules we use Raman for?
- Requires a change in polarisation if the vibration is to be Raman Active
- (this is about how easy it is to distribute electron density within a molecule)
- Mainly use it for O₂ and N₂ molecules)
What does the intensity of Raman Spectra depend on?
- The intensity of Raman spectra depends on vₒ
- As vₒ approaches the frequency allowed for UV/vis transition in metal centre, the modes that are vibronically active in the electron transition become enhanced
What are the benefits of Raman?
- Can help assign electronic transitions
- Only those transitions near the site of an electronic transition become enhanced
How can LMCT be measured in a Raman spectrum?
- Example: [Fe(Scys)₄] centres - if the UV/vis transition results from S→Fe LMCT and the Raman experiment is performed at the energy of this LMCT transition, the the Fe-S bond stretches should be enhanced in the RR spectrum
- We can use the Resonance Raman to work out if bond is LMCT or d-d (if is LMCT we will see Raman metal-sulfur vibrations and d-d is weak/no Raman m-s vibrations)
Why is resonance enhancement needed for Raman?
- The Raman effect is weak
- If we have the wavelength of light correspond to a UV vis absorption, you get a huge enhancement (up to 1m time)
- The time-resolved raman needed this resonance enhancement
What is the major issue with Raman?
A major problem with many samples which have been photolysed by pulsed lazers is that they may luminesce strongly in the same spectral region as the Raman scattered photons
Why would you used a One-Colour experment for Time-resolved resonance Raman?
Due to the advantages of the single-colour technique is simplicity and it can be used to record the Raman spectra of a transient species which have sufficenet lifetimes
You can use a One-Colour Experiement to obtain Raman Spectrum of a Transient species
How does it work?
(A transient species is a short-lived, highly reactive intermediate species that exists only temporarily during a reaction)
- You can do a simple experiment using one lazer pluse, which excites the molecules which initates the photoreaction AND can obtain a Raman Spectrum
- There is a low power lazer - which obtain Raman spectrum of parent
- And then, a short pulse high power lazer which generates intermediate and resulting Raman spectrum (contains signals of parent and intermediate)
What is the difference between a One- and Two-colour experiment
- In a two-colour experiment you can vary the time between pump and probe
In a One-Colour Experiment, at very high irradience, most of the incident photons will encounter photolysed sample and the signal is dominated by the transient species which are created
The irradience level at which this occurs will depend on a number of factors…
- The sample concentration
- The absorption coefficient of the unphotolysed sample
- The lifetime of the transient species
- The relative Raman scattering probabilities (cross-sections) of the starting material and the transient
Why was time-resolved resonance Raman used for this following Ru complex?
- To understand if in the excited state if the electron is localised on one of the ligands or is it delocalised across all 3
- When comparing the spectrum to another chemical reduced species, it was used to determine what happening to the electronic excited states
What does this time-resolved resonance Raman tell us about the electronics of this [Ru(bipy)₃]³⁺
(Comparision of [Ru(bipy)₃]²⁺ (top) to excited spectrum of bipy radical (bottom))
- Due to the two spectrums being similar you can draw the conclusion that its probably bipy ⁻ not bipy⅓⁻, as the shifts would be different
- Band positions therefore show the metal-to-ligand charge transfer is localised
What are the Advantages of Raman Detection methods for flash photolysis?
- Moderately narrow absorptions
- Very good specificity
- Obtain structural information especially good for probing fingerprint vibtations
- Moderate Kinetics
What are the disadvantages for Raman Detection Methods for Flash Photolysis
- Detectors as the UV/visible but signals are much weakers
- Weak signals can be masked if irradiation causes fluorescence which swamps the Raman photons
- Specific marker bands more difficult to assign compared to v(CO) for metal carbonyls
- Not commerically available for nanosecond and picosecond systems
