1. Time Resolved Infrared Spectroscopy
Asmita Shrestha & Moumita Bhattacharya
University of Utah

2. Principles of Time Resolved IR Spectroscopy (TRIR)
Overview:
TRIR combines UV-flash photolysis and fast infrared detection for determining excited states and reaction intermediates, which are often inaccessible to conventional spectroscopy.1 It is possible to monitor processes within a span of 10-6 s.
Siebert and co workers pioneered the development of TRIR in studying the photo-dissociation of CO-myoglobin.2
Fig 1. Diagram of TRIR instrument, OPA: optical parametric amplifier, DFG: difference frequency generator, MCT: detector. A tunable subpicosecond mid IR pulses can be generated using a solid state Ti: Sapphire laser based system at high repetition rate (1KHz).1
1 . Dalton Trans., 2003, 3996; 2. Biophys. Struct. Mech.,1980,6, 139

3. TRIR Spectrum
In TRIR, the transmission at one particular frequency is monitored following UV-vis excitation and then repeated over the spectral region of interest. In a TRIR spectrum, the change in IR intensity(delta mOD) is plotted against frequency at any given time delay after excitation1.
Fig 2. A TRIR spectrum. The positive ∆mOD suggests the appearance of a product while negative ∆mOD indicates the disappearance of a reactant.
1 . Dalton Trans., 2003, 3996

4. Applications of TRIR
Probing excited states in photochemical reactions: TRIR is useful to study the structure of excited states of molecules such as metal carbonyls and cyanides. Due to the high oscillator strengths of v(CO) and v(CN) vibrations, their sensitivity towards the presence of electron density distribution in the molecule can be detected using TRIR.
Investigating mechanism and finding intermediates: TRIR can be used to monitor reactions and identify short-lived intermediates in gas phase, solution phase, supercritical fluids, zeolites and TiO2.1
1 . Dalton Trans., 2003, 3996

5. Problem 1
2,5- Dihydrofuran (L) can bind to the metal through the oxygen atom (1O) or through the double bond (1C). The following reaction, W(CO)5(CyH) + L → W(CO)5L gives only 1C. By monitoring the reaction via TRIR, 1O is also detected after 20μs (refer to the given data). Organometallics,2000, 19, 2237.
Correlate the dotted and solid lines to the starting material and 1O and 1C isomers. Justify your answer on the basis of their relative stretching frequencies.
Which one is thermodynamic product. Explain on the basis of HSAB principle.
Correlate the dotted and solid lines to the starting material and 1O and 1C isomers. Justify your answer on the basis of their relative stretching frequencies.
Which one is the thermodynamic product? Explain on the basis of HSAB principle.

6. Problem 1:
3. What is the order of the reaction? Is all 1O converted to 1C?
4. The following table suggests that kobs is independent of [W(CO)6] and [2,5-DHF], is the isomerization inter or intramolecular?
[W(CO)6],molL-1 (X103 )
[2,5-DHF], molL-1
T(oC)
kobs (s-1)
0.2
0.022 20
1.09
59.6
46.94
0.5
0.042
29.3
2.84
59.4
40.55 1C 1O

7. Problem 2 Which functional group is being probed?
There are two proposed reaction pathways for alkyl diazo ester compounds upon UV irradiation. Which one is operative? (Hint: Stretching frequency for singlet carbene is expected to appear at 1606 cm-1)
Draw the resonance structures of MDP and the product . Why are the stretching frequencies different?
J. Am. Chem. Soc., 2010, 132, 2126

8. Solution 1 1. W(CO)5(CyH) 1O 1C
Both CO and alkene are strong pi acceptors. In 1O, W is involved in pi back bonding with CO only. While in 1C, W is involved in pi-backbonding with both CO and alkene. So the electron donation in the anti-bonding orbital of CO in 1C is less resulting in a stronger CO bond and a greater νCO.

9. Solution 1
2. 1C is the thermodynamic product because W and C(alkene) have soft-soft acid-base interaction making it stable.
3. It is overall a 2nd order reaction. Some 1O remains in the solution because the time dependent absorbance at 1934cm-1 does not return to its baseline.
4. As the kobs is independent of the respective concentrations of the starting materials over variable temperature range, the isomerization is intramolecular.

10. Solution 2
The TRIR spectrum shows the appearance and disappearance of peaks in the region characteristic of CO stretching frequency.
Pathway B is operative since the TRIR spectrum does not show any change in
the stretching frequency region corresponding to the singlet carbene intermediate.
3. In MDP, the positive charge on the N makes N more electron withdrawing, and pulls electron density out of bonding orbital of CO resulting in a weaker CO bond and less vCO. In the product, the positive charge is on C, which is less electronegative hence it pulls out less electron density from bonding orbital of CO.

11. Contributed by:
Asmita Shrestha & Moumita Bhattacharya (Undergraduate Students)
University of Utah
2014