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

danderso@uwyo.edu

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