1. Joseph A. Fournier, Robert K. Bohn, John A. Montgomery, Jr. University of Connecticut, Storrs, CT Microwave Spectroscopy and Structures of Perfluorohexane and 1H-heptafluoropropane

2. M. D’Amore, et. al., J. Am. Chem. Soc. 128(2006)1099. C. Sperati, H. Starkweather, Jr. Fortcher. Hochpolym. Forsch. 2(1961)465

3. } d ρ θ τ 2345 φ r T. Shimanouchi, S. Mizushima. J. Chem. Phys. 23(1955)707. M. Iwasaki. J. Polym. Sci. A-1 (1963)1099. Helical Perspective Molecular Perspective

4. Helical vs. Dihedral Angle Helical Perspective (X-ray crystallographers) Molecular Perspective ρ – distance from each atom to the helix axis r – CC bond length θ – angle of rotation about the helix axis φ – CCC bond angle d – pitch; translation along the helix axis τ – CCCC dihedral angle Helix parameters as functions of molecular parameters: cosθ = ½[ - cosφ + cosτ - cosφ cosτ - 1] d 2 = r 2 (1 - cosτ)(1 - cosφ)/(3 + cosφ - cosτ + cosφ cosτ) ρ 2 = 2r 2 (1 + cosφ)/(3 + cosφ - cosτ + cosφ cosτ) 2 Molecular parameters as functions of helical parameters: r 2 = d 2 + 4ρ 2 sin 2 (θ/2) cos(φ/2) = (1 - d 2 /r 2 ) 1/2 sin(θ/2) tan(τ/2) = (d/r) tan(θ/2)

5. b a c Top view, Helical Twist, C 2 symmetry Perfluoropentane, C 5 F 12 J.A. Fournier, R.K. Bohn, J.A. Montgomery, Jr., M. Onda. J. Phys. Chem. A 114 (2010) 1118.

6. Pentane (C 5 H 12 ), all bonds trans (180 O ), C 2v symmetry C 5 F 12, bonds twisted from trans by 17 O, C 2 symmetry

7. Perfluorohexane, C 6 F 14 Hexane (C 6 H 14 ), C 2h C 6 F 14, C 2, ~17 o twist a b c

8. Observed Spectroscopic Constants A/MHz824.9001(9) B/MHz202.2195(8) C/MHz198.3355(10) D J /kHz0.0691(4) P aa /uÅ 2 2217.304 P bb /uÅ 2 330.798 P cc /uÅ 2 281.857 Kappa-0.9876 No. Lines46 Std. Dev./kHz1.8

9. Scaling the Calculated Model Scale the principal coordinates of each atom from the PBE0/VTZ calculations by the factor (P ii obs / P ii PBE0) 1/2 Scaling Factors: 0.99996(a), 1.00167(b), 1.00288(c). Calculate structural parameters from new coordinates. Obs'dPBE0/VTZ Scaled PBE0/VTZ A/MHz824.900828.6824.90 B/MHz202.220202.3202.22 C/MHz198.336198.4198.34 P aa /uÅ 2 2217.3042217.52217.30 P bb /uÅ 2 330.798329.7330.80 P cc /uÅ 2 281.857280.2281.86

10. Scaled Structure PBE0/VTZ Scaled PBE0/VTZ C 1 -C 2 1.5551.556 C 2 -C 3 1.5581.559 C 3 -C 4 1.561 / C 1 C 2 C 3 114.0113.9 / C 3 C 4 C 5 113.0112.9 C1C2C3C4C1C2C3C4 16.7 C2C3C4C5C2C3C4C5 18.0

11. 1H-heptafluoropropane HCCC Trans HCCC Gauche J.A. Fournier, R.K. Bohn. Dalton Trans. 39(2010) 4575.

12. Perfluoropropane, C 3 F 8 PBE0/VTZ computations predict a non-helical, C 2v structure. The computed spectroscopic constants are in excellent agreement with the observed. J.A. Fournier, R.K. Bohn, J.A. Montgomery, Jr., M. Onda. J. Phys. Chem. A 114 (2010) 1118.

13. Observed Spectroscopic Constants GaucheTrans A/MHz1995.4656(7)1752.4998(10) B/MHz1120.2799(4)1184.3437(19) C/MHz982.7300(5)1137.0414(20) Δ J /kHz0.0691(4)0.0938(20) Δ JK /kHz0.0315(11)0.763(10) Δ K /kHz0.0378(14)-0.758(8) δ J /kHz0.01578(19)0.0092(18) δ K /kHz-0.238(3)-1.12(8) Kappa-0.728360-0.846284 No. Lines7138 a-types812 b-types1826 c-types450 Std. Dev./kHz1.11.7

14. Dipole Moments Compound Predicted Dipole/DDihedral C 4 F 10 0.0314 o C 5 F 12 0.0817 o C 6 F 14 0.0617-18 o 0.817 o 0.714 o Trans 1H-HFP0.47o7o 0.23o3o 0.061o1o

15. Scaling the Calculated Model Scale the principal coordinates of each atom from the PBE0/VTZ calculations by the factor (P ii obs / P ii PBE0) 1/2 Scaling Factors: 1.00033(a), 1.00268(b), 1.00122(c) for gauche form. Calculate structural parameters from new coordinates. Obs'dPBE0/VTZ Scaled PBE0/VTZ Obs'dPBE0/VTZ Scaled PBE0/VTZ A/MHz1995.46562004.01995.461752.49981760.81752.50 B/MHz1120.27991121.41120.281184.34371184.51184.34 C/MHz982.7300984.8982.721137.04141138.61137.04 P aa /uÅ 2 356.060355.8356.06291.405291.8291.40 P bb /uÅ 2 158.205157.4158.21153.064152.1153.06 P cc /uÅ 2 95.05994.895.06135.312134.9135.31 Gauche Trans

16. Scaled Structures Gauche Trans PBE0/VTZScaled PBE0/VTZScaled C 1 -C 2 1.5361.5381.535 C 2 -C 3 1.5451.546 1.548 / C 1 C 2 C 3 114.0113.9117.0116.9 HC 1 C 2 C 3 60.860.9180 F4C1C2C3F4C1C2C3 179.1 60.1 F9C3C2C1F9C3C2C1 170.9 180

17. Summary The microwave spectra of the C 2 structure of perfluorohexane and the gauche and trans conformers of 1H-heptafluoropropane have been observed and assigned. The dihedral angles of C 6 F 14 are ~17-18 o away from trans. No c-type transitions were observed for the trans form, indicating an ab-plane of symmetry or an insufficient twist. Structural analysis was performed by scaling the calculated models to reproduce the observed spectroscopic constants.

18. Acknowledgments At UConn: Prof. Harvey Michels, James Dombrowski, Congtin Phan. At Wesleyan : Prof. Stewart Novick, Prof. Pete Pringle, Dr. Andrea Minei, Dan Frohman. Measurements were performed on the pulse-jet Fourier Transform Microwave Spectrometer of the Southern New England Microwave Consortium at Wesleyan University.