The Overtone Spectrum of HOONO: A Twisted Tale Juliane L. Fry, Andrew Mollner, Paul Wennberg, Mitchio Okumura California Institute of Technology Anne B. McCoy Joseph S. Francisco Ohio State UniversityPurdue University Air Resources Board, NASA, NSF, Dreyfus

The Weekend Ozone Effect SAT SUN Mean Residual Ozone Day of Week H. Tran, CARB Report, 2000 OZONE IN LA DURING THE WEEK SUNDAY IN PASADENA: A BAD OZONE DAY NO x : down 50% Ozone: up 25-30%

2 1 Overtone Spectrum of HOONO Nizkorodov and Wennberg (June, 2001) ACTION SPECTRUM: HOONO + IR  OH + NO 2 OH detected by LIF Room Temperature 5 Torr OH Product LIF signal

cis-perp ν 1 = 3554 cm -1 A = MHz B = 7415 MHz C = 5559 MHz Conformers of Peroxynitrous Acid (ab initio results) cis-cis ν 1 = 3298 cm -1 A = MHz B = 8081 MHz C = 5859 MHz trans-perp ν 1 = 3574 cm -1 A = MHz B = 4902 MHz C = 4550 MHz  E  1-2 kcal BDE = 19.6 kcal  E = +3.5 kcal

HO + NO 2 cis-cis 0 kcal/mol trans-perp HOONO -16 kcal/mol perp- perp Energetics of HOONO Vibrational Predissociation HO + NO 2 0 kcal/mol cis-cis HOONO kcal/mol cis- perp

Temperature Dependence of 2 OH Action Spectrum Two Isomers Seen At Different T Isomerization k  10 3 s  1 300K k  2  10 2 s  1 228K E act = 33 kJ 293 K 213 K t-p c-c trans-perp isomer is not present in room temperature spectrum

HO + NO 2 cis-cis HOONO -19 kcal/mol 0 kcal/mol trans-perp HOONO -16 kcal/mol 0 kcal/mol HO + NO 2 N 0 cc N 0 tp Isomerization of thermalized tp-HOONO   1ms 300K 30-50% perp- perp Master Equation Modeling (based on Golden, Barker, Lohr)

trans-perp isomer is not present –spectrum is distinguishable –isomerizes too quickly at room T cis-perp isomer predictions –B3LYP: minimum with kcal barrier –CCSD(T): calculations predict little or no barrier –cis-perp isomer is not stable?  Spectrum arises from cis-cis isomer only

Anomalous intensity pattern? 2 ν OH D0D0 F(PD) changing across spectrum

1.Modeled Quantum Yields HOONO (2 1 )  OH + NO 2 - RRKM model (Multi-Well)  expect large changes with T 233 K 298 K 2. Experimental Overtone Action Spectra - no change seen in relative band intensities from T = 193 K to 300 K   1 for all bands Is the intensity pattern due to quantum-yield effects? D 0 =19.6 kcal

Cavity Ringdown Spectroscopy Low pressure discharge flow reactor OH + NO 2 + M Ringdown cell H + NO 2 → NO + OH 5-40 Torr He/Ar/N 2 5x10 15 NO K / N 2 Mirrors R=99.98% Discharge flow reactor

Cavity Ringdown Spectrum of the 1 Fundamental of HOONO t-HONO c-HONO cc-HOONO v OH 0  1 1.

Direct Absorption spectrum: Cis-cis HOONO cavity ringdown spectrum wavenumber, cm -1 HONO HNO 3 HOONO Action Spectrum 6365 cm -1 origin

cis-HOONO HOON-torsion potential V e /cm -1 CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ Fully relaxed except  (OONO)=0º  (HOON), degrees cis-perp

cis-perp is not technically bound, but... are these torsional levels effectively cis-perp on average? 1Dimensional Torsional levels on CCSD(T)/pVTZ potential

Excitation of OH torsion leads to blue-shifted OH stretch Cis-cis HOONO Internal H-bonding OH ~ 3300cm -1 2 OH ~ 6400cm -1 Cis-perp HOONO Free OH OH ~ 3550cm -1, stronger 2 OH ~ 6900cm -1  hot cis-cis not torsionally excited

SUMMARY CRD ABSORPTION SPECTRUM OF 2 OH –Only origin band is observed (6365 cm -1 ) –Significantly different from photodissociation spectrum  Quantum yield effects are important CCSD(T)/pVTZ TORSIONAL V el (  HOON ) –cis-perp HOONO is a shelf (not bound) –OH stretch becomes free as OH rotates  Torsionally hot molecules

Two factors influence the action spectrum 1.Quantum yield effects hv  D 0 action spectrum I = I abs ( )   diss ( ) hot molecules 2. Torsional excitation to form cis-perp isomer torsional motion breaks the H-bond OH stretch and torsion strongly coupled HOONO 2 1 HOONO* OH (X( 2  )) + NO 2 OH (A( 2  )) LIF detection of OH fragments D0D0  cis-cis OH ~3300cm -1 weaker cis-perp OH ~3500cm- 1

Anne McCoy relaxing at a Telluride BBQ

“No problem!”

1. Where is the origin? FREE OH Overtone HONO 2 trans-perp cis-perp? 6970 cm -1 H-Bonded OH Overtone? cis-cis? 6365 cm -1

Spectroscopy of HOONO Matrix Isolation IR –YP Lee et al, National Taiwan, Overtone Action Spectroscopy –Nizkorodov and Wennberg, Caltech 2002, 2004 –Mathews and Sinha, UCSD 2004, 2005 Fundamental OH Cavity Ringdown Spectroscopy –Bean et al, Caltech, 2003 Free Jet Action Spectroscopy –Lester and co-workers, UPenn, 2003, 2005