ri05: ftir studies of the photochemistry of deuterated formic acid in a parahydrogen matrix david t....
TRANSCRIPT
RI05: FTIR STUDIES OF THE PHOTOCHEMISTRY OF DEUTERATED FORMIC ACID IN A PARAHYDROGEN MATRIX
David T. Anderson
Department of Chemistry, University of Wyoming
Laramie, WY 82071-3838
RI05: 02:38 PM – 02:53 PMRI. MATRIX ISOLATION (AND DROPLETS)
Y
Reaction of atomic hydrogen with formic acid#
Kr matrix
0102030
T/K
H + HCOOH → trans-H2COOH
trans-H2COOH → trans-cis-HC(OH)2
#Q. Cao, S. Berski, Z. Latajka, M. Räsänen, and L. Khriachtchev, PCCP 16, 5993 (2014).
HCOOH/HBr/Kr (1/2/1000)
1. 193 nm photolysis at 4.3 K2. Anneal at 31 K, induce H-atom mobility3. Re-cool to 4.3 K and record FTIR scans
Solid parahydrogen (para-H2) matrix isolation
0102030
T/K
pH2 matrix
H + DCOOD → HD + trans-DOCO
1. 193 nm photolysis at 1.9 K, generate H-atoms2. H-atoms mobile, record repeated FTIR scans
J=0pH2
HCOOH/pH2 (1/10,000)
HCOOH + hn → CO + D2O → CO2 + D2
K. Kufeld, W. Wonderly, L. Paulson, S. Kettwich, and DTA, JPC Lett. 3, 342-347 (2012).W. Wonderly and DTA, Low Temp. Phys. 38, 853–859 (2012).
HDO + H
+ pH2
reaction withpH2 host
→ DCO + OD
H+DCOOD in pH2 at 1.9 K: Experimental protocol
atmosphere
vacuum
FTIR beam
radiationshield
opticalsubstrate
pH2
crystal
pre-cooledpH2 gas
dopantgas
UVbeam
M.E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237-4241 (1998).
Deposit crystal at <2.5 K(rapid vapor deposition)
Photolyze sample(193 nm, 85 mJ cm-2 sec-1)
Repeated FTIR scans(5 min acquisition times,at 0.03 cm-1 resolution)
Liquid helium bath cryostat
Infrared spectroscopy of DCOOD
n1 (O-D stretch)
K.L. Goh, P.P. Ong, H.H. Teo, and T.L. Tan Spectrochimica Acta A 59, 1773 (2000).
n3 (C=O stretch)
F. Madeja, A. Hecker, S. Ebbinghaus, M. Havernith Spectrochimica Acta A 59, 1773 (2003).
n4 (C-O stretch)
n5 (C-O bend)
T.L. Tam, K.L. Goh, P.P. Ong, H.H. TeoJ. Mol. Spectrosc. 195, 324 (1999).
Infrared spectroscopy of DCOOD in solid pH2
• Frequencies agree well with literature values
• Isotopic scrambling leads to some production of DCOOH and HCOOD
Fermi
193 nm photolysis of DCOOD/pH2
As-deposited, 1.9 K
193 nm photolysis, 7.2 min, 1.9 K(32% reduction in DCOOD)
9.5 hrs, dark, 1.9 K
Experimental conditions[DCOOD]=57, [DCOOH]=8,
[HCOOD]=2, [HCOOH]=0.5 ppm
2.5(1) mm thick
H + DCOOD reactions after photolysis
As-deposited, 1.9 K
193 nm photolysis, 7.2 min, 1.9 K(32% reduction in DCOOD)
9.5 hrs, dark, 1.9 K
HCO clusters
C=O stretch region
Identifying and assigning trans-DOCO
D
O O
CDaniel Forney, Marilyn E. Jacox, and Warren E. Thompson, “Infrared spectra of trans-HOCO, HCOOH+, and HCO2
- trapped in solid neon,” J. Chem. Phys. 119, 10814-10823 (2003).
trans-DOCO
Matrix shifts: Observed trends
mode gas theory* Ne pH2 Ar
t-HOCO n1 O-H stretch 3635.70 3641.0(5.3) 3628.0(-7.7) 3612.20(-23.5) 3602.9(-32.8)
n2 C=O stretch 1852.57 1862.0(9.4) 1848.0(-4.6) 1845.08(-7.5) 1843.6(-9.0)
n3 H-O-C bend 1212.7 1210.4 1210.27 1211.2
n4 C-O stretch 1052.0 1050.4 1064.6
t-DOCO n1 O-D stretch 2684.11 2685.1(1.0) 2678.1(-6.0) 2667.26(-16.9)
n2 C=O stretch 1851.64 1859.8(8.2) 1846.2(-5.4) 1843.26(-8.4) 1841.7(-9.9)
n3 D-O-C bend 1086.4 1082.6 1086.44 1092.6
n4 C-O stretch 902.6
Table 1. Vibrational frequencies (matrix shifts) in cm-1 for t-HOCO and t-DOCO.
*X. Huang, R.C. Fortenberry, Y. Wang, J.S. Francisco, T.D. Crawford, J.M. Bowman, T.J. Lee, “Dipole Surface and Infrared Intensities for the cis- and trans-HOCO and DOCO Radicals,” J. Phys. Chem A. 117, 6932-6939 (2013).
In situ photochemistry: HCOOH → productshn
-18 ppm
+18 ppm
+1 ppm +8 ppm
+17 ppm
+19 ppm +25 ppm
DCOOD + hn → CO + D2O
DCOOD + hn → DCO + OD
D + CO H + HDO
hn + pH2
DCOOD + hn → D2 + CO2
significant channel (30%)
minor channel (5%)
major channel (65%)
photo
• DCOOD decrease approximately matches total product increase
H-atom diffuses via “chemical” tunneling
H + H2 → H2 + H Ea = 10 kcal/mol= 3500 cm-1
T. Kumada, “Experimental determination of the mechanism of the tunneling diffusion of H-atoms in solid hydrogen: Physical exchange versus chemical reaction,” Phys. Rev. B
68, 052301 (2003).
DOCO
Diffusion limited tunneling reaction
H + DCOOD → HD + DOCO Ea = 9.5(3) kcal/mol
A.M. Lossack, D.M. Bartels, E. Roduner, Res. Chem. Intermed. 27, 475-483 (2001).
= 3330(100) cm-1
DCOOD
DCOODH H DCOODH+DCOOD
HD+DOCO
HD
Anomalous temperature dependence
photo 1 photo 2
No reactions!
Deuterium substitution supports reaction scheme
H+DCOOD → HD + DOCO H+HCOOH → H2 + HOCO
• reaction with C-atom of formic acid
Kinetics change abruptly with temperature!
Very small kinetic isotopic effect (KIE)
kHOCO = 5.39(5)x10-3 min-1 kDOCO = 3.44(6)x10-3 min-1
rate-determining step (rds) is not tunneling, i.e., diffusion limited
secondary KIE, no bond to the D-atom is broken in the rds
57.1DOCO
HOCO
k
kKIE
Low-temperature H-atom chemistry in solid pH2
Anomalous temperature effects1. H-atom mobility2. Chemical reactivity
H+DCOOD → HD+DOCO
Tunneling Reactions of H-atoms with Formic Acid and Carbon Monoxide in Solid Parahydrogen I: Anomalous Temperature Effects
Tunneling Reactions of H-atoms with Formic Acid and Carbon Monoxide in Solid Parahydrogen II: Deuterated Reaction Studies
Submitted to J. Phys. Chem. A, under revision (2014).
Summary
Assign IR features to trans-DOCO
Isotopic substitution reveals reactive partner is DCOOD and reacts at C-atom
Chemical kinetics change abruptly at ~2.7 K · transition to “controlled” chemistry · quantum solvent effects
Reaction of hydrogen atoms with formic acid leads to qualitatively different products in Kr at 31 K compared to pH2 at 1.9 K
The people who do the work and funding
William R. Wonderly2011 REU
Now UCSB Graduate Student
This research was sponsored in part by the Chemistry Division of the US National Science Foundation (CHE 08-48330).
Fredrick M. Mutunga2nd year
UW Graduate Student
Shelby E. Follett1st year
UW Graduate Student