VADIM L. STAKHURSKY*, LILY ZU†, JINJUN LIU, TERRY A. MILLER

Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University

120 W. 18th Avenue, Columbus OH 43210.

CONFORMATIONAL ANALYSIS

OF SECONDARY ALKOXIES

VIA HIGH-RESOLUTION B-X LIF SPECTROSCOPY~ ~

* Present address

Radiation Oncology, Duke University Clinic,

P.O. , NC, 27710

† Present address

Department of Chemistry

Beijing Normal University

Motivation

Alkoxy radicals (RO·) are important intermediates in the combustion of hydrocarbon fuels and play important role in the balance of ozone in the atmosphere.

Rotational structure of large number of primary alkoxy radicals (CnH2n+1O·, n=3-7) was analyzed in our lab.

Until recently good rotational analysis of secondary alkoxies is not achieved yet.

Experimental Setup of Moderate-Resolution LIF

CnH2n+1ONO+He

Moderate-Resolution LIF Spectrum of 2-Butoxy

ExcimerLaser(XeCl)

Pulse Amplifier

PMT

PD

Ar Laser 20WCW ringDye Laser

Computer

XeFPhotolysis

Laser

PD

I 2

+

5-10 mJ~120 MHz

DoublingCrystal

0.5mJ

Experimental Setup of High-Resolution LIF

High resolution LIF Spectra of 2-Butoxy

Band B

Band D

2-Butoxy Conformers (ab initio Studies)

G+T G-

Gaussian 03 UHF(6-31G) calculations for ground state

Energy (cm-1)

Torsion Angle

18060 -60

Prediction of Molecular Parameters

G+ T G-For each stable conformer rotational constants were calculated in the ground

electronic state (UHF, B3LYP, CISD) and in the excited electronic state (CIS). Different basis sets were tested for convergence (6-31 to 6-311++G**).

Quantum chemistry calculations were done to predict transition dipole moments and components of the spin-rotation tensor. The experimentally determined parameters of ethoxy were used as a reference at the beginning. After parameters of one of the 2-butoxy conformers were obtained, they were used as a reference to predict parameters of the other two.

Vibrational frequencies were calculated for all three stable geometries to identify vibrationally excited bands in the 2-butoxy spectrum.

Relative energies of the conformers and isomerization barriers were estimated to understand the population and dynamics of the conformers in the jet.

H = HRot + HSR

HRot = ANa2 + BNb

2 + CNc2

HSR = ½ (S + S)

ParametersRotation Spin-Rotation

A', B', C'A", B", C" aa

", bb", cc

"

½(ab" + ba

")

½(ac" + ca

")½(bc

" + cb

")

H = HRot

Ground state Excited state

Hamiltonian Model

Negligible effect on spectra

Simulation of Band A

and A, exp.

and A, exp.

G+, Hrot Only, c type

T, Hrot Only, c type

G-, Hrot Only, c type

Frequency/cm-1

T=1.2K

Asym. rotor,ab initio, c-type

Asym. rotor,fit, c-type

Asym. rotor &spin-rotation, fit, c-type

Asym. rotor &spin-rotation, a,b-type

Final fita:b:c=0.05:0.35:1

Band A, exp.

Rotational Analysis of Band A (G+ conformer)

Frequency/cm-1

T=1.2K

Red wing

Blue wing

Frequency/cm-1

Rotational Analysis of Band A (G+ conformer)

Selectivity of the Spectroscopic Method, Band A

ground state calculations by Gaussian 03, UHF (6-31+g*) excited state calculations by Gaussian 03, CIS(6-31g)

Experimental G+ conformer T conformer G- conformer

A’ 8.106(3) 8.280 7.768 6.496

B’ 3.495(3) 3.460 3.478 3.863

C’ 2.775(4) 2.700 2.653 3.199

A” 8.627(5) 8.685 7.923 6.905

B” 3.471(4) 3.447 3.553 3.857

C” 2.781(4) 2.715 2.706 3.254

εaa 0.8(5) 0.3 -1.5 0.2

ε bb -1.4(7) -1.2 -0.4 -1.1

ε cc -0.0(1) -0.12 -0.04 -0.22

½(εab+ ε ba) <0.2 0.1 1.3 -0.3

½(εbc+ εcb) 0.38 -0.11 0.73

½(εac+ εca) -0.06 0.22 -0.13

Unit: GHz

Rotational Analysis of Band a

G -, asym. rotor,ab-initio, c-type

Band a, exp.

Band a, exp.

T, asym. rotor,ab-initio, a, b-type

G -, asym. rotor,ab-initio, a-b-type

T, asym. rotor,ab-initio, c-type

Rotational Analysis of Band a (G- Conformer)

Asym. rotor,Ab-initio

Asym. Rotor & spin-rotation, fit

Band a, exp.

Band a, exp.

Selectivity of the Spectroscopic Method, Band a

ground state calculations by Gaussian 03, UHF (6-31+g*) excited state calculations by Gaussian 03, CIS(6-31g)

Experimental G+ conformer T conformer G- conformer

A’ 6.427(14) 8.280 7.768 6.496

B’ 3.896(11) 3.460 3.478 3.863

C’ 3.188(11) 2.700 2.653 3.199

A” 6.83(2) 8.685 7.923 6.905

B” 3.903(17) 3.447 3.553 3.857

C” 3.216(11) 2.715 2.706 3.254

εaa 0.1(1) 0.3 -1.5 0.2

ε bb -0.93(7) -1.2 -0.4 -1.1

ε cc -0.23(6) -0.12 -0.04 -0.22

½(εab+ ε ba) -0.53(27) 0.1 1.3 -0.3

½(εbc+ εcb) 0.38 -0.11 0.73

½(εac+ εca) -0.06 0.22 -0.13

Unit: GHz

Simulation of Band B (T conformer)

Band B, exp.

asym. rotor & spin-rotation ab initio

A-X energy separation =55(10)cm-1 (from dispersed

fluorescence experiment).

Quasi-degenerate electronic states A and X render

Hamiltonian model used inadequate.

A-X energy separation =55(10)cm-1 (from dispersed

fluorescence experiment).

Quasi-degenerate electronic states A and X render

Hamiltonian model used inadequate.

Moderate-Resolution LIF Spectrum of 2-Butoxy

26600 26800 27000 27200 27400 27600 27800 28000

26740.7

26761.9

27069.1

27321.027679.8

21.2

559.1

610.7

a

DC

B

A

Frequency/cm-1

a

G- G+T

G- G+T

Conformer Frequency (cm-1)

(Band)

Assignment Obs. Freq. Separation

(cm-1)

Calc. Freq. Separation

(cm-1)

G+ 26761.9 (A)

Origin    

G+ 27321.0 (C)

C-O stretch 559.1 532

T 27069.1 (B)

Origin    

T 27679.8 (D)

C-O stretch 610.7 536

G- 26740.7 (a) Origin 738

Vibrational Assignment of Bands in 2-Butoxy

Band A, exp.

Convolution of two bands(Band A used, δ=5.95GHz)

Band C, exp.

Simulation of Band C (G+ conformer)

Conclusions and Future work

All three theoretically predicted conformers, G+, G- and T, are identified in the spectrum based on comparison of experimentally “extracted” rotational constants with ab-initio predictions, establishing spectroscopic “fingerprinting” of 2-butoxy conformers.

Quantum chemistry prediction of transition dipole moment and spin-rotation tensor elements significantly simplifies the analysis of fine structure of radicals.

All three theoretically predicted conformers, G+, G- and T, are identified in the spectrum based on comparison of experimentally “extracted” rotational constants with ab-initio predictions, establishing spectroscopic “fingerprinting” of 2-butoxy conformers.

Quantum chemistry prediction of transition dipole moment and spin-rotation tensor elements significantly simplifies the analysis of fine structure of radicals.

The effective Hamiltonian model should be refined to consider the effect of quasi-degeneracy of two lowest states on rotational structure of T conformer.

Other secondary alkoxies to be analyzed.

The effective Hamiltonian model should be refined to consider the effect of quasi-degeneracy of two lowest states on rotational structure of T conformer.

Other secondary alkoxies to be analyzed.

ACKNOWLEDGMENTS

Miller’s GoupMiller’s Goup

NSF$$$

NSF$$$

Thank you all!Thank

you all!Dr. Patrick DupréDr. Patrick Dupré