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A COMPARISON OF THE MOLECULAR STRUCTURES OF C4H9OCH3, C4H9SCH3, C5H11OCH3, AND C5H11SCH3 USING MICROWAVE SPECTROSCOPY BRITTANY E. LONG, Chemistry Department,

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Presentation on theme: "A COMPARISON OF THE MOLECULAR STRUCTURES OF C4H9OCH3, C4H9SCH3, C5H11OCH3, AND C5H11SCH3 USING MICROWAVE SPECTROSCOPY BRITTANY E. LONG, Chemistry Department,"— Presentation transcript:

1 A COMPARISON OF THE MOLECULAR STRUCTURES OF C4H9OCH3, C4H9SCH3, C5H11OCH3, AND C5H11SCH3 USING MICROWAVE SPECTROSCOPY BRITTANY E. LONG, Chemistry Department, Trinity University, San Antonio, TX, USA; JUAN BETANCUR, Natural and Social Science, Purchase College SUNY, Purchase, NY, USA; YOON JEONG CHOI, Department of Chemistry, Wesleyan University, Middletown, CT, USA; S. A. COOKE, Natural and Social Science, Purchase College SUNY, Purchase, NY, USA; G. S. GRUBBS II, Department of Chemistry, Missouri University of Science and Technology, Rolla, MO, USA; JONATHAN OGULNICK and TARA HOLMES, Natural and Social Science, Purchase College SUNY, Purchase, NY, USA

2 Motivation American Chemical Society Petroleum Research Fund 53451-UR6
This project concerns the experimental characterizations of the potential energy surfaces of derivatives of long chain hydrocarbons. Predictive thermochemistry Gruzman and co-workers have demonstrated that correct conformational energy ordering has a significant impact on the calculated enthalpy function and Gibbs energy function for the C4 to C8 alkanes. Karton and Martin further highlight the “challenging” nature of correct energy ordering, via quantum chemical calculations, even for isomers and conformers of “simple” hydrocarbons. Engaging students out of the Organic Chemistry sequence D. Gruzman, A. Karton and J. M. L. Martin, "Performance of Ab Initio and Density Functional Methods for Conformational Equilibria of CnH2n+2 Alkane Isomers (n = 4−8)," Journal of Physical Chemistry A, vol. 113, pp , 2009. A. Karton and J. M. L. Martin, "Explicitly correlated benchmark calculations on C8H8 isomer energy separations: how accurate are DFT, double-hybrid, and composite ab initio procedures?," Molecular Physics, vol. 110, 2012.

3

4 Experiments G. S. Grubbs II, R. A. Powoski, D. Jojola and S. A. Cooke. J. Phys. Chem. A 114(2010) 8009. Experiment covers 7 GHz to 18 GHz. Picture credit: C. T. Dewberry.

5 Spectral Handling (Talk FC09 last year)
Unphased Phased/ Absorption DISPA Magnitude spectrum Absorption spectrum – Voigt profile

6 Rotational Constants Butyl Methyl Ether (42 transitions) A ( 34) MHz B ( 70) MHz C ( 82) MHz Butyl Methyl Thioether (23 transitions) A ( 50) MHz B ( 90) MHz C ( 53) MHz Pentyl Methyl Ether (59 transitions) A ( 90) MHz B ( 72) MHz C ( 70) MHz Pentyl Methyl Thioether (39 transitions) A (29) MHz B ( 37) MHz C ( 41) MHz Fit achieved using PGopher by ColinWestern see

7 Quantum Chemical Calculations
OMEGA from OpenEye Scientific was used to generate a large number (20 – 200+) of starting geometries OpenBabel file conversion Gaussian was used to optimize all starting geometries using the PBE0/6-31G level of theory.

8 Quantum Chemical Calculations
Optimized geometries OpenBabel file conversion Shell scripts written to extract conformer energies and rotational constants

9 Butyl Methyl Thioether
PBE0/6-31G Conformers

10 Butyl Methyl Thioether (23 transitions)
A (50) MHz B ( 90) MHz C ( 53) MHz PBE0/6-31G A MHz B MHz C MHz

11 Pentyl Methyl Thioether
PBE0/6-31G Conformers

12 Pentyl Methyl Thioether (39 transitions)
A (29) MHz B ( 37) MHz C ( 41) MHz PBE0/6-31G A MHz B MHz C MHz

13 Butyl Methyl Thioether Pentyl Methyl Thioether
Butyl Methyl Ether A / MHz Ia / amu Å2 Paa / amu Å2 B / MHz Ib / amu Å2 Pbb / amu Å2 C / MHz Ic / amu Å2 Pcc / amu Å2 Butyl Methyl Thioether Pentyl Methyl Ether Pentyl Methyl Thioether 13.11 / 5 = 2.62 19.18 / 5 = 3.83 15.45 / 6 = 2.57 21.66/ 6 = 3.60

14 COC = 111o r(Cm-O) = 1.41Å D(COCC) = 180o D(OCCC) = 60o
Butyl Methyl Ether A ( 34) MHz B ( 70) MHz C ( 82) MHz V3 (internal rotation) = 781 +/- 34 cm-1 MP2/ G(d,p) A MHz B MHz C MHz D(COCC) = 180o D(OCCC) = 60o D(CCCC) = 180o D(CSCC) = 180o D(SCCC) = 180o D(CCCC) = 65o CSC = 100o r(Cm-S) = 1.87Å Butyl Methyl Thioether A ( 50) MHz B ( 90) MHz C ( 53) MHz PBE0/6-31G A MHz B MHz C MHz

15 COC = 114o r(Cm-O) = 1.45Å D(COCC) = 180o D(OCCC) = 64o
Pentyl Methyl Ether A ( 90) MHz B ( 72) MHz C ( 70) MHz MP2/ G(d,p) A MHz B MHz C MHz D(COCC) = 180o D(OCCC) = 64o D(CCCC) = 180o D(CCCCm) = 180o D(CSCC) = 180o D(SCCC) = 180o D(CCCC) = 180o D(CCCCm) = 65o CSC = 100o r(Cm-S) = 1.87Å Pentyl Methyl Thioether A (29) MHz B ( 37) MHz C ( 41) MHz PBE0/6-31G A MHz B MHz C MHz

16 CH3-O/S Internal Rotation?
Butyl Methyl Ether V3 (internal rotation) = 781 +/- 34 cm-1 Calculated 700 cm-1 Too little data to make any bold statements! Butyl Methyl Thioether V3 (internal rotation) > 400 cm-1 Calculated 450 cm-1

17 Conclusions The thioether and ether functional groups cause differences in the geometries of the alkyl tails of the butyl and pentyl species. The CSC angles in the thioethers are more acute than the COC angles in the ethers. The manifest effects of internal rotation of the O-CH3 and S-CH3 tops are very similar but more work is needed here to say anything substantive. Acknowledgements American Chemical Society Petroleum Research Fund UR6 Stew Novick and Pete Pringle for helpful conversations

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