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

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. 11974-11983, 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.

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.

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

Rotational Constants Butyl Methyl Ether (42 transitions) A 10259.5099( 34) MHz B 1445.63599( 70) MHz C 1356.30672( 82) MHz   Butyl Methyl Thioether (23 transitions) A 7039.0615( 50) MHz B 1075.12732( 90) MHz C 1003.74855( 53) MHz Pentyl Methyl Ether (59 transitions) A 6870.5134( 90) MHz B 933.44221( 72) MHz C 865.25603( 70) MHz Pentyl Methyl Thioether (39 transitions) A 6667.1604(29) MHz B 659.24520( 37) MHz C 632.44027( 41) MHz Fit achieved using PGopher by ColinWestern see http://pgopher.chm.bris.ac.uk/

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.

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

Butyl Methyl Thioether PBE0/6-31G Conformers

Butyl Methyl Thioether (23 transitions) A 7039.0615(50) MHz B 1075.12732( 90) MHz C 1003.74855( 53) MHz   PBE0/6-31G A 7068.8 MHz B 1074.4 MHz C 991.1 MHz

Pentyl Methyl Thioether PBE0/6-31G Conformers

Pentyl Methyl Thioether (39 transitions) A 6667.1604(29) MHz B 659.24520( 37) MHz C 632.44027( 41) MHz PBE0/6-31G A 6668.9 MHz B 665.2 MHz C 633.5 MHz

Butyl Methyl Thioether Pentyl Methyl Thioether Butyl Methyl Ether A / MHz 10259.51 Ia / amu Å2 49.25957 Paa / amu Å2 336.472 B / MHz 1445.636 Ib / amu Å2 349.5894 Pbb / amu Å2 36.14214 C / MHz 1356.307 Ic / amu Å2 372.6141 Pcc / amu Å2 13.11743 Butyl Methyl Thioether 7039.062 71.79636 450.8798 1075.127 470.0643 52.61184 1003.749 503.4916 19.18453 Pentyl Methyl Ether 6870.513 73.55768 525.9685 933.4422 541.4143 58.11182 865.256 584.0803 15.44586 Pentyl Methyl Thioether 6667.16 75.80124 744.9474 659.2452 766.6025 54.1462 632.4403 799.0937 21.65503 13.11 / 5 = 2.62 19.18 / 5 = 3.83 15.45 / 6 = 2.57 21.66/ 6 = 3.60

COC = 111o r(Cm-O) = 1.41Å D(COCC) = 180o D(OCCC) = 60o Butyl Methyl Ether A 10259.5099( 34) MHz B 1445.63599( 70) MHz C 1356.30672( 82) MHz V3 (internal rotation) = 781 +/- 34 cm-1   MP2/6-311++G(d,p) A 10315.61 MHz B 1452.99 MHz C 1361.69 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 7039.0615( 50) MHz B 1075.12732( 90) MHz C 1003.74855( 53) MHz PBE0/6-31G A 7068.8 MHz B 1074.4 MHz C 991.1 MHz

COC = 114o r(Cm-O) = 1.45Å D(COCC) = 180o D(OCCC) = 64o Pentyl Methyl Ether A 6870.5134( 90) MHz B 933.44221( 72) MHz C 865.25603( 70) MHz   MP2/6-311++G(d,p) A 6855.9 MHz B 939.0 MHz C 869.1 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 6667.1604(29) MHz B 659.24520( 37) MHz C 632.44027( 41) MHz PBE0/6-31G A 6668.9 MHz B 665.2 MHz C 633.5 MHz

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

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 53451-UR6 Stew Novick and Pete Pringle for helpful conversations