Rotational spectroscopy of oxygen bearing radicals and radical complexes

　　

Yasuki Endo

2006/June/19

The University of Tokyo

Main interests

High-resolution spectroscopy of short livedreactive species

FTMW spectroscopy : Observe pure rotationaltransitions

LIF spectroscopy : Electronic transitions

Short lived species produced in a supersonic jet

TimingGenerator

MainOscillator

Stabilizer

Nozzle

Mix.

CavityMix. Monitor LocalOscillator

StabilizerMix.

SSB Mix.Amp.Amp.

Amp.

DC~3 MHz

4~40 GHz

FrequencyCounter Personal

ComputerSynthesizer

TransientDigitizer

PIN SW2PIN SW1

Cavity Control

Block diagram of the FTMW spectrometer

Frequency coverage： 4〜 40 GHz

FTMW spectrometer 4–40GHz

Production of short lived species

development of pulsedischarge nozze

spectrum of HC9NAp. J. 371 L45 (1991)

Pulse Discharge Nozzle

AnodeCathode

Pulse Valve

Pulsed electric discharge1.0–2.0 kV, 0.2 msec

Free radicals

Discharge samples containing appropriate parent molecules in Ar or Ne (0.2 – 0.5 %)to produce target species

Radical compexes

Carbon chain species studies by FTMW spectroscopy

C3H C4H* C5H C6HCCN* C3N* C5NCCO* C4O C6O C8O C3O C5O C7O C9O CCS* C3S C4S C5S HCCN*HC3N HC5N HC7N HC9NHCCO HC3O HC4O HCCS* HC3S HC4S* HC6S*NCCO NC3O*NCCS NC3S*HNC3 H2C3N H2C4N H2C3HCH3CO CH3OOFeCO Fe(CO)3 Fe(CO)4 MgCl

*studied by LIF spectroscopymainly motivated by radio astronomy

Spectroscopy of complexes

Ion complexesAr–D3

Ar, Kr–HCO+

Ar, Kr–HN2+

Radical complexesAr–OH Sumiyoshi et al. TE06Ne, Kr–OHAr–SHNe, Kr–SHH2O–OHAr–HO2

FTMW–mmW double resonance method

W. Jaeger and M. C. L. GerryJ. Chem. Phys. 102, 3587 (1995)

FTMW–mmW double resonance method

FTMW-Optical double resonance

M. Nakajima, Y. Sumiyoshi, and Y. EndoRev. Sci. Instrum. 73, 165 (2002)

Principle of the double resonance method

FID

N+1

mm-wave

K=1

N

N+1

K=0

π/2 pulse

N

N+1

N+1

K=1

K=0

FID

Population changeDestruction of macroscopic polarization

An examlpe of the double resonance spectraIn

ten

sity

(%

)

Frequency /MHz

100

80

60

40

20

0

78623 78624 78625

37ClOO111 - 000

J = 1.5 - 0.5 F = 3 - 2

Can observe transitions in the mm-wave region

Merits of the double resonance spectroscopy

Extend the observable frequency regionb-type transitions of near prolate topsvdW modes of complexes e. g. A-SH

Assignments of complicated fine and hyperfineStructures

fairly common to open-shell free radicals

Assignments of the speciesPDN system – mixture of various speciestwo or more transitions belong to one species

Oxygen bearing free radicals

Species with more than one ogygen atomsHOOH, HOO, FOO, O3

X-OO, CH3OO, HOOOH, HOOO, …(oxygen chain species

cf. carbon chain species)

Oxygen bearing radical complexesH2O–OH, Ar–HO2, HO2–H2O

important in atmospheric chemistry

ClOO Important in atmospheric chemistry

Catalytic process of ozone destruction in the region

ClO + ClO + M → ClO-OCl + MClO-OCl + h → ClOO + Cl ClOO + M → Cl + O2 + M

2 [Cl + O3 → ClO + O2]

Net: 2O3 → 3O2

polar region: ClOx cycle does not work efficiently

Halogen peroxide radicals

35ClOOiter. = 100

11688.5 11689.0

Frequency /MHz

79BrOOiter. = 20

202 - 101

J = 2.5 - 1.5 F = 4 - 3

9591.5 9592.1

Frequency /MHz

101 – 000

J = 1.5 - 0.5 F = 3 - 2

FTMW spectra of ClOO and BrOO

Discharge in a Cl2 (Br2) and O2 mixture

FTMW–mmW　→ Molecular structure is determined experimentally

ClOO

Fre

qu

en

cy /G

Hz

000

101

202

303

404

110

111

211

212

312

313

Ka = 0

Ka = 1

120

100

80

40

20

60

BrOO

Fre

qu

en

cy /G

Hz 120

100

80

40

20

60

000

101

202

303

404

505

606

Ka = 0

Observed transitions for ClOO and BrOO

re structure MRCI+Q/aVQZ　 (Reproduces Bobs within 1%)

O

OCl1.204Å

2.081Å

115.0°

O

OBr

1.209Å

117.1°

2.371Å

ClOO BrOO

O O

1.208Å

O2

Indicates van der Waals like nature of X O‥ 2

Determined molecular structures

Anomalously weak X-O bonds

X O bond becomes weaker as the size of X increases!‥

Bond lengths of XOO and XO r

XO

/Å

0.5

1.0

1.5

2.0

2.5

XOO

XO

H F Cl Br

F Cl Br

En

erg

y /k

ca

l/mo

l

Diss. Energy : D0 ( XOO → X+O2 )

0

4

8

12

Cl+OCl+O22 ClOOClOO

Observation of the equilibrium constantDetermine dissociation energy experimentally

　 D0 = 4.69±0.10 kcal mol-1

Detection of the HO3 radical

O + HO2 OH + O2

H + O3 OH + O2 : reaction intermediate

MRMP2: trans is more stable （ O. Setouchi et al.,JPC 104, 3204 (2000))

Most MO calculations

Production of HOOO

H2O + O2 / Ar HOOOdischarge

O2 : 20%H2O : 0.15%

Stagnation press. 6 atm.Large amount of O2 is required to produce HO3

Observed spectra

Double resonance spectra

observe b-type transitions to determine its structure

Energy level diagram and observed transitions

FTMW

Double resonance

8 a-type transitions5 b-type transitions

Observe similar transitions for DOOO

Determined molecular constants

HOOO DOOO

A B C A B C

cis 66,824 10,986 9,435 58,304 10,778 9,096

trans 70,676 10,103 8,839 67,765 9,502 8,333

exp. 70,778 9,987 8,750 67,857 9,449 8,299

cis, trans: ab initio calculations(MRSDCI / aug-cc-pVTZ)

Molecular structure is concluded to be of trans form

Molecular structure

Planer trans formFairly long O-O bond: weakly bound adduct of OH + O2

structure similar to FOOK. Suma et al. Science 308, 1885 (2005)

Another molecule with O-O-O bonds

HOH water, very well known

HOOH hydrogen peroxydealso well known

HOOOH no gas phase datamatrix IR, NMR

HOOOOH

HOOO vs. HOOOHopen shell radicalclosed shell molecule

Production of HOOOH

H2O2 + O2 / Ar HOOOHdischarge

O2 : 10%H2O2 / H2O : passed through a reservoir

Similar conditions to produce HOOO

Observed FTMW spectrum of HOOOH

Only one line in 4-40 GHz

No fine and hyperfine structure

It is impossible to confirm this line is due to HO3H

Double resonancespectroscopy

Energy level diagram and observed transitions of HOOOH

Determined rotational constants of HOOOH

cis, trans: ab initio calculationsCCSD(T) / cc-pVQZ

The determined constants agree with those of trans.

Trans structure : C2 symmetry … spin statistics

Determined molecular structure of HOOOH

O–O bond length: slightly shorter than that of HOOH(1.464 Å)

Large amplitude motions

Molecule with 2 C1 tops

Barriers : c.a. 2000 cm-1

Almost no splittings

K. Suma et al. TH08

Chemistry of oxygen chain molecules

Halogen peroxides : very weak X–O bondHO–OO : similar to XO2 radicals

ab initio calculation: multi ref. naturequite difficult to reproduce their structure

HOOOH : OO bond length shorter than H2O2

single ref. ab initio calculation

HOOOOH?XOOO, XOOOH …

Oxygen bearing radicals in atmospheric chemistry

OH, HO2: playing important roles in atmospheric chemistry

Spctroscopic studies of oxygen bearing radical complexes

OH: OH–H2OOH–CO (HOCO)HO–O2 (HOOO)Rg–OH analysis of large amplitude

motions

HO2: Ar–HO2

H2O–HO2

O2: O2–H2O FTMW spectraestimation of abundance

Sizable contributions in atmospheric chemistry?

Rotational spectroscopy of Ar–HO2

Prototype of HO2 bearing complexes

CH3OH + O2 / Ar Ar–HO2disch.

Both a-typeand b-typetransitions

Determined molecular structure

Agrees with that of ab intio calculationsFairly large binding energy: c.a. 270 cm-1

Very small induction effects on fine and hyperfine coupling constants

Spectroscopic study of HO2–H2O

Recombination reaction of HO2

HO2 + HO2 H2O2 + O2

is enhanced if H2O exists(explained by the contribution of the water complex)

A large number of ab initio calculationsNo direct spectroscopic detection in the gas phase

Observed sptctra of H2O–HO2

Production schemeH2O + O2 / Ar H2O–HO2

(unlike the case of Ar–HO2)

Two series of spectra wth different nuclear spins

Predicted molecular structure

5 membered ring with two hydrogen bondsLarge amplitude motions

Tunneling motions in H2O–HO2

Tunneling motions: the groupe G4

The group G4

Character tableG4: E (12) E* (12)*A+ 1 1 1 1A– 1 1 –1 –1B– 1 –1 –1 1B+ 1 –1 1 –1

A+ IH2 = 0

B+ or B– IH2 = 1

A–with different nuclear spins

G4

Observed sptctra of H2O–HO2

Two series of spectra wth different nuclear spins

A+ state B+/B- state

Observed rotational transitions

Each transitons has A+ and B+/B- components

Determined molecular structure of H2O–HO2

O1–H3 bond: 1.795 A farily shortcf. 2.019 A for water dimer

Conclusions of the study of HO2–H2O

First direct spectroscopic detectionEvidences for the large amplitude motions

need more sophisticated analysis

Large binding energy :9.4 kcal/mol by ab initio calculations supported by the observed centrifugal constants

Provide spectroscopic data for in situ detection

K. Suma et al. Science, 311, 1278 (2006)RA03

Members of the LaboratoryK. Suma Y. Sumiyoshi

Acknowledgement

Dr. Y. Sumiyoshi

Graduate studentsK. Suma (got PhD degree, HO3, H2O3 etc) K. KatohH. ToyoshimaC. MotoyoshiW. FunatoH. Yoshikawa

Support: Grant-in-aid for priority research field“Radical chain reactions”