1. PHOTODISSOCIATION OF FORMIC ACID ISOLATED IN SOLID PARAHYDROGEN Y
David T. Anderson and Leif O. Paulson
Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA.
This research sponsored in part by the Chemistry Division of the National Science Foundation (CHE ).
FE02 MATRIX/CONDENSED PHASE
2015 McPherson Lab
8:47 am Friday, June 24, 2011
66th International Symposium on Molecular Spectroscopy

2. Photochemistry of the simplest carboxylic acid
DH°(kcal/mol)
HCOOH + hn → CO + H2O
→ CO2 + H
→ HCO + OH
→ HCOO + H
→ H + COOH
CO(n→p*)
S1←S0
s(193nm)=0.8x10-19 cm2
Ephoton(193nm)=148 kcal/mol
Formic acid (FA)
(trans)
H. Su et al., J. Chem. Phys. 113, 1891 (2000).

3. Photochemistry of FA in gas phase
IR emissions
HCO(2A´)+OH(2P)
H2O+CO
H2+CO2
t-HCOOH
[CO]/[CO2] ≈ 11
H. Su et al., J. Chem. Phys. 113, 1891 (2000).

4. Photochemistry perturbed by matrix environmentCO
CO2 Ar Xe
Ar/Xe
HCOOH + hn → CO---H2O
→ CO2---H2
→ HCO + OH
→ HCOO + H
→ H + COOH
Ubiquitous cage effect – only
detect vdw clusters
External heavy atom effect
J. Lundell and M. Räsänen, J. Mol. Struct. 436, 349 (1997)

5. In situ photochemistry in solid parahydrogen (pH2)
Negligible “cage effect”
Nonexistent heavy atom effect
Promotes rapid internal conversion
Concentration of “caged” CO-H2O clusters
Branching ratio among open channels
Detection of radical channels
Reaction of nascent photoproducts with pH2 host

6. Photochemical experimental setupUV
beam
atmosphere
dopant
gas
vacuum
pH2
crystal
FTIR
beam
optical
substrate
radiation
shield
pH2
gas

7. FA 193 nm photolysis products in solid pH2
monomer
P=3.5 mW
20 Hz
15 min
[FA]i = 18 ppm
monomer
vdw
clusters
radicals

8. Assigning cluster peaks – control experiments

9. In situ photokinetics: FA → productshn
In situ photokinetics: FA → products
kdecay ≈ kformation
Secondary
photolysis?

10. Determine branching ratio in solid pH2
Photolysis
Time (min)
D(FA)
(ppm) CO
CO2
HCO
HOCO 5
-1.0
1.1
0.067
0.024
0.022 15
-2.7
3.2
0.21
0.025
0.034 30
-6.9
6.3
---
[CO]:[CO2]
gas 11:1
Ar :1
pH :1
Radical channels are minor and don’t scale
with photolysis time!

11. Wait a minute - HOCO photokinetics not right?

12. Photochemistry more complex than direct photolysis
HCOOH + hn → CO + H2O
→ CO2 + H2
→ HCO + OH
→ HCOO + H
→ H + COOH
91%
<1%
→ H2O + H
+H2
reactions with
pH2 host
HCO + hn → H + CO
HOCO + hn → H + CO2
2nd photolysis
H· + HCOOH → H2 + HOCO
H· + CO2 → HOCO
H· + CO → HCO
Tunneling
reactions

13. Growth of HOCO AFTER UV photolysis!

14. HOCO growth displays first-order kinetics
t1/2=100 min

15. Spectral assignment of trans-HOCO

16. Identifying tunneling reaction mechanism
rate = k[H·][FA]
mobile
H· + HCOOH → H2 + HOCO
Kinetics don’t match up – may be due to IR spectroscopy of FA in solid pH2*
*Leif O. Paulson and DTA, JPC A 113, 1770 (2009).

17. H+HCOOH hydrogen abstraction reaction
Linear transition state
EA=2840 cm-1
reaction must be slightly exothermic ?
H+HCOOH
H2+HOCO
A.M. Lossack et al., Res. Chem. Intermed. 27, 475 (2001)

18. Changes in HCO concentration with time
changes in HCO consistent with H atom production

19. Conclusions and future directions
Formic acid 193 nm photochemistry in solid pH2 very different from rare gas matrices – negligible cage effect and reactions with host
Can study low temperature H-atom abstraction reactions (new synthetic strategy to form radicals)
Need to record IR spectra right after photolysis is complete with “high” time resolution (< 1 min)
Study the photochemistry of other systems (e.g., CH3OH)

20. Acknowledgements
The authors wish to acknowledge the contributions of Kylie A. Kufeld in the laboratory.
Leif O. Paulson, PhD
This research was sponsored in part by the Chemistry Division of the US National Science Foundation (CHE ).