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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”