OBSERVATION OF THE A-X ELECTRONIC TRANSITION OF

C6-C10 PEROXY RADICALS

Neal D. Kline and Terry A. MillerLaser Spectroscopy Facility The Ohio State University

~~

Motivation for Study of Peroxy Radicals

• Key combustion intermediates

• Atmospheric chemistry: lead to formation and destruction of ozone

RO2

OH

R’CHO

RO

RH

RO2NO2

RONO2

HO2

CO

CO2

R’C(O)O2

PAN

ROOH

O2

NO2HO2

NONO

hn

OH NO2

NO NO2

hn

OH

O2

O2

NO2

NO

DIsomerisation

Multistep

hn

NO2

O2

1. Lightfoot et. al. Atmos. Envir. 26A, 1805.

1

C6-C10 Peroxy Radicals

• Gasoline mixtures contain a mixture of hydrocarbons: parrafins, olefins, and aromatics.

• Larger chain hydrocarbons(>C5) make up a significant portion of gasoline mixtures; heptane and 2,2,4-trimethyl pentane (isooctane) are standards for octane rating scale with ratings of 0 and 100, respectively.

Picture of refineries

Low-Lying Electronic Transitions of Peroxy Radicals

B−X transition: • Strong (c.f. O2: B3Σu

− ← X3Σg−)

• Located in UV: ca. 49000 cm-1 • Repulsive → little structural

information, poor selectivity

HOMO-1

HOMO

SOMO

O. J. Nielsen and T. J. Wallington, in Peroxyl Radicals, (John Wiley and Sons, New York, 1997), pp. 72-73.

~~

Jafri, J. A, and D. H. Phillips. J. Am. Chem. Soc. 112 (1990) 2586. RO–O

X~

A~

B~

Low-Lying Electronic Transitions of Peroxy Radicals

A−X transition: • Weak (c.f. O2: a1Δg ← X3Σg

−) • Located in NIR: ca. 7500 cm-1

• Bound → resolved vibrational structure

HOMO-1

HOMO

SOMO

Chung et al. J. Chem. Phys. 127 (2007) 044311.

~~

Jafri, J. A, and D. H. Phillips. J. Am. Chem. Soc. 112 (1990) 2586. RO–O

X~

A~

B~

Principles of CRDS

time

Inte

nsity

τabs

σ Nl+= cL )/(

R1 -( )

Principles of CRDS

τ0

cL )/(R1 -

=

A = L/cτabs - L/cτ0

L

l

R

time

Inte

nsity

A

Sirah dye laser570-705 nm

Nd:YAG: 532 nm

Raman cell (H2, 300 psi)

2nd Stokes:6000-9000 cm-1

Room Temperature Cavity Ringdown Setup

20 Hz~600 mJ/pulse

~70-80mJ/pulse

~1-2mJ/pulse

Photolysis:Excimer LaserArF, 193 nmKrF, 248 nm

HighlyReflective

Mirror(99.995 %)

HighlyReflective

Mirror(99.995 %)

Production of RadicalsCl O

O Cl

hv193 nm

2Cl + CH3(CH2)nCH3 CH2(CH2)nCH3 CH3CH(CH2)nCH3+

O2

OOCH2(CH2)nCH3 CH3CH(CH2)nCH3+OO

CH2C

CH3

H3C

CH3

CH

CH3

CH32Cl + CH2C

CH3

H3C

CH3

CH

CH3

CH2 CH2C

CH3

H3C

CH3

C

CH3

CH3

CH2C

CH2

H3C

CH3

CH

CH3

CH3

+

O2

OO

OO

OO

+

Wavenumber7200 7400 7600 7800 8000 8200 8400 8600 8800

pp

m/p

ass

-20

0

20

40

60

80

100

Hexyl PeroxyHeptyl Peroxy (Shifted -20 ppm)Octyl Peroxy (Shifted -30 ppm)Nonyl Peroxy (Shifted -30 ppm)DecylPeroxy (Scaled by 3, Shifted -45 ppm)

Origin Region

COO Bending

-OO Stretch

Wavenumber

7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200

pp

m/p

ass

20

30

40

50

60

70

Octyl Peroxy (Scaled by 1/3, Shifted +45 ppm)Isooctyl Peroxy

Assignment of Spectra

• To assign spectra for peroxy radicals, typically perform A-X origin calculations using G2 level of theory. Frequency calculations with DFT methods.

• Larger straight chain peroxy radicals are computationally expensive to calculate at the G2 level of theory.

• Assignments were deduced from spectral/structural relationships that have been determined from prior studies, and previous experience with assigning peroxy radical spectra

~ ~

Spectral/Structural Relationships of Peroxy Radicals

Peroxy Radical

Origin(cm-1)

Methyl 7383

Ethyl 7362

1-Propyl 7332

1-Butyl 7355

Isobutyl 7306

Primary Peroxy Radicals Secondary Peroxy RadicalsPeroxy Radical

Origin(cm-1)

2-Propyl 7567

2-Butyl 7560

2-Pentyl 7564

3-Pentyl 75723-Methyl-2-

butyl 7577

Tertiary Peroxy Radicals

Peroxy Radical

Origin(cm-1)

t-Butyl 7755

1. Sharp, E. N.; Rupper, P.; and Miller, T. A. Phys. Chem. Chem. Phys., 2008, 10, 3955.

Assignments of Straight Chain Peroxy Radicals

Peroxy Radical

Origin COO Bend

-OO Stretch

Hexyl Peroxy 7588 8053 8500Heptyl Peroxy 7591 - 8515Octyl

Peroxy 7596 - 8513Nonyl Peroxy 7597 - 8513

Decyl Peroxy 7573 - 8505

Wavenumber7200 7400 7600 7800 8000 8200 8400 8600 8800

pp

m/p

ass

-20

0

20

40

60

80

100

Hexyl PeroxyHeptyl Peroxy (Shifted -20 ppm)Octyl Peroxy (Shifted -30 ppm)Nonyl Peroxy (Shifted -30 ppm)DecylPeroxy (Scaled by 3, Shifted -45 ppm)

Assignment of Isooctyl Peroxy

Wavenumber

7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200

pp

m/p

ass

10

20

30

40

50

60

5 microsec delay500 microsec delay1 millisec delay

CH2C

CH3

H3C

CH3

C

CH3

CH3

OO

A A’

B’ BC’

Band Assignment Frequency

AOrigin secondary

isomer 7503

A’Origin tertiary

isomer 7804B’ COO bend 8230B -OO Stretch 8460C' -OO Stretch 8734

1. Glover, B. and Miller, T.A. J. Phys. Chem. A., 2005, 109, 11191.

Conclusions

Conclusions:

A-X spectra of C6-C10 straight chain peroxy radicals and isooctyl peroxy have been obtained and analyzed.

Assignments were made by examining the spectral/structural relationships of peroxy radicals.

Acknowledgments

• Prof. Terry Miller• Miller group:

• Funding: -US Department of Energy (DOE)

-Dr. Dmitry Melnik-Dr. Mourad Roudjane-Rabi Chhantyal-Pun-Terrance Codd-Meng Huang

Assignment of Isooctyl Peroxy

Wavenumber

7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 9200

pp

m/p

ass

10

20

30

40

50

60

5 microsec delay500 microsec delay1 millisec delay

CH2C

CH3

H3C

CH3

C

CH3

CH3

OO

Peroxy Radical

Origin COO Bend

-OO Stretch

tert-isooctyl Peroxy 7804 8230 8734Heptyl Peroxy 7503 - 8460Octyl

Peroxy 7596 - 8513Nonyl Peroxy 7597 - 8513Decyl

Peroxy 7573 - 8505

A A’

B’ BC’

Production of Radicals

• Production method will produce mixture of secondary and primary peroxy radical isomers.

• Secondary peroxy radicals will be produced preferentially.

Spectral/Structural Relationships of Peroxy Radicals

Peroxy Radical

OriginT1… Conformers

Origin G1G2… Conformers

Origin G1T2…,G1’G2...Conformers

Methyl 7383 - -

Ethyl 7362 7592 -

1-Propyl 7332 7508 7569

1-Butyl 7355 7525 7591

Isobutyl 7306 7480 7536

Primary Peroxy Radicals

Secondary Peroxy RadicalsPeroxy Radical

OriginG1…

Conformers

OriginT1…

Conformers

2-Propyl 7567 7701

2-Butyl 7560 7605

2-Pentyl 7564 7613

3-Pentyl 7572 76433-Methyl-2-

butyl 7577 7606

Tertiary Peroxy RadicalsPeroxy Radical

Origin

t-Butyl 7755

1. Sharp, E. N.; Rupper, P.; and Miller, T. A. Phys. Chem. Chem. Phys., 2008, 10, 3955.