This document describes the principles and applications of time-resolved infrared spectroscopy (TRIR). TRIR combines ultraviolet flash photolysis with fast infrared detection to monitor excited states and short-lived reaction intermediates. It provides TRIR spectra of a reaction between a metal carbonyl and an organic ligand, showing the appearance and disappearance of product and reactant peaks. The document also presents two example problems demonstrating how TRIR can be used to determine reaction mechanisms and identify isomers.

1. Organic Pedagogical Electronic
Network
Time Resolved Infrared
Spectroscopy
Asmita Shrestha & Moumita Bhattacharya
University of Utah

2. Principles of Time Resolved IR Spectroscopy (TRIR)
http://en.wikipedia.org/wiki/Time-resolved_spectroscopy, http://www.stfc.ac.uk/
1 . Dalton Trans., 2003, 3996; 2. Biophys. Struct. Mech.,1980,6, 139
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

3. TRIR Spectrum
1 . Dalton Trans., 2003, 3996
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.

4. Applications of TRIR
1. 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.
2. 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.
1. 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.
2. Which one is thermodynamic
product. Explain on the basis of
HSAB principle.
1. 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.
2. 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?
1O
1C
[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

7. Problem 2
J. Am. Chem. Soc., 2010, 132, 2126
1. Which functional group is being probed?
2. 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)
3. Draw the resonance structures of MDP
and the product . Why are the stretching
frequencies different?

8. Solution 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.
1.

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
1. The TRIR spectrum shows the appearance and disappearance of peaks in the
region characteristic of CO stretching frequency.
2. 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. This work is licensed under a
Creative Commons Attribution-
ShareAlike 4.0 International
License.
Contributed by:
Asmita Shrestha & Moumita Bhattacharya (Undergraduate Students)
University of Utah
2014