1. 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

2. 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 (C n H 2n+1 O·, n=3-7) was analyzed in our lab.  Until recently good rotational analysis of secondary alkoxies is not achieved yet.

3. Experimental Setup of Moderate-Resolution LIF C n H 2n+1 ONO+He

4. Moderate-Resolution LIF Spectrum of 2-Butoxy

5. Experimental Setup of High-Resolution LIF

6. High resolution LIF Spectra of 2-Butoxy Band D

7. 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

8. Prediction of Molecular Parameters G+TG-  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.

9. H = H Rot + H SR H Rot = AN a 2 + BN b 2 + CN c 2 H SR = ½   (   S  + S    )  Parameters RotationSpin-Rotation A', B', C' A", B", C"  aa ",  bb ",  cc " ½(  ab " +  ba " ) ½(  ac " +  ca " ) ½(  bc " +  cb " )     H = H Rot Ground stateExcited state Hamiltonian Model Negligible effect on spectra

10. Simulation of Band A  and A, exp. G+, H rot Only, c type T, H rot Only, c type G-, H rot Only, c type Frequency/cm -1 T=1.2K

11. 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 fit a:b:c=0.05:0.35:1 Band A, exp. Rotational Analysis of Band A (G+ conformer) Frequency/cm -1 T=1.2K

12. Red wing Blue wing Frequency/cm -1 Rotational Analysis of Band A (G+ conformer)

13. 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) ExperimentalG+ conformerT conformerG- conformer A’8.106(3)8.2807.7686.496 B’3.495(3)3.4603.4783.863 C’2.775(4)2.7002.6533.199 A”8.627(5)8.6857.9236.905 B”3.471(4)3.4473.5533.857 C”2.781(4)2.7152.7063.254 ε aa 0.8(5)0.3-1.50.2 ε bb -1.4(7)-1.2-0.4-1.1 ε cc -0.0(1)-0.12-0.04-0.22 ½(ε ab + ε ba )<0.20.11.3-0.3 ½(ε bc + ε cb )0.38-0.110.73 ½(ε ac + ε ca )-0.060.22-0.13 Unit: GHz

14. Rotational Analysis of Band a G -, asym. rotor, ab-initio, c-type 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

15. Rotational Analysis of Band a (G- Conformer) Asym. rotor, Ab-initio Asym. Rotor & spin-rotation, fit Band a, exp.

16. 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) ExperimentalG+ conformerT conformerG- conformer A’6.427(14)8.2807.7686.496 B’3.896(11)3.4603.4783.863 C’3.188(11)2.7002.6533.199 A”6.83(2)8.6857.9236.905 B”3.903(17)3.4473.5533.857 C”3.216(11)2.7152.7063.254 ε aa 0.1(1)0.3-1.50.2 ε bb -0.93(7)-1.2-0.4-1.1 ε cc -0.23(6)-0.12-0.04-0.22 ½(ε ab + ε ba )-0.53(27)0.11.3-0.3 ½(ε bc + ε cb )0.38-0.110.73 ½(ε ac + ε ca )-0.060.22-0.13 Unit: GHz

17. 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. 

18. Moderate-Resolution LIF Spectrum of 2-Butoxy a G- G+ T G- G+ T

19. ConformerFrequency (cm -1 ) (Band) AssignmentObs. Freq. Separation (cm -1 ) Calc. Freq. Separation (cm -1 ) G+26761.9 (A)Origin G+27321.0 (C)C-O stretch559.1532 T27069.1 (B)Origin T27679.8 (D)C-O stretch610.7536 G-26740.7 (a)Origin738 Vibrational Assignment of Bands in 2-Butoxy

20. Band A, exp. Convolution of two bands (Band A used, δ=5.95GHz) Band C, exp. Simulation of Band C (G+ conformer)

21. 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.

22. ACKNOWLEDGMENTS Miller’s Goup NSF $$$ NSF $$$ Thank you all! Thank you all! Dr. Patrick Dupré