1. 1 Broadband Chirped-Pulse Fourier- Transform Microwave (CP-FTMW) Spectroscopic Investigation of the Structures of Three Diethylsilane Conformers Amanda L. Steber, Daniel A. Obenchain, Rebecca A. Peebles, and Sean A. Peebles Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, IL 61920 Justin L. Neill, Matt T. Muckle, and Brooks H. Pate Department of Chemistry, University of Virginia, Charlottesville, VA 22904 Gamil A. Guirgis Gamil A. Guirgis Department of Chemistry and Biochemistry, The College of Charleston, Charleston, SC 29424

2. 2 Introduction  Extend series of pentane analogs to include Diethylsilane (C 2 H 5 ) 2 SiH 2  Compare conformer stabilities, structural parameters  4 possible conformers  Gauche-Gauche  Trans-Gauche  Trans-Trans  Gauche-Gauche’

3. 3 Ab Initio Structures Gauche-gauche Trans-gauche Gauche-gauche’ Trans-trans +4.6 kJ/mol +1.3 kJ/mol +1.2 kJ/mol 0 kJ/mol Gaussian 03: MP2/6-311+G(2df,2pd) level Relative energies are zero-point energy corrected

4. 4 Experimental Technique Chirped-Pulse Fourier-Transform Microwave Spectrometer at the University of Virginia

5. 5 Experimental Technique  Sample concentration: 0.2% diethylsilane  He/Ne carrier gas  1,000,810 acquisitions  10 FIDs per pulse  3 nozzles

6. 6 Analysis Procedure  Import data into Origin  Using ab initio structure, isotopic rotational constants were used to predict experimental lines within a few MHz  SPFIT 1 and SPCAT 1 were used to predict and fit the experimental lines of the spectrum 1 Pickett, H. M. J. Mol. Spectrosc. 1991, 148, 371.

7. 7 Broadband Spectrum 2 12 -1 01 TT 2 12 -1 01 TG 2 12 -1 01 GG

8. 8 Magnified region of broadband spectrum from 9275 MHz to 9425 MHz containing 13 C lines for the trans- gauche conformer. The 1 11 ←0 00 rotational transitions for the four unique 13 C substitutions are highlighted.Spectrum 1 2 3 4

9. 9 Internal Rotation Effects 2 11 ←2 02 Trans-trans conformer Low resolution 2 11 ← 2 02 transition High resolution 2 11 ← 2 02 transition  Splittings consistent with recent results for pentane 1 1 Churchill, G.B.; Bohn, R.K. J. Phys. Chem. A. 2007, 111, 3513

10. 10 Ab Initio and Experimental Rotational Constants ParameterAb initio 28 Si 129 Si 30 Si 13 C-1 13 C-2 A (MHz)1345513297.8209(23)13241.9991(18)13187.9967(18)13286.2457(19)13156.7163(19) B (MHz)15241515.5923(25)1515.6052(13)1515.6204(13)1479.9248(14)1504.8349(14) C (MHz)14391431.8589(24)1431.2189(11)1430.5984(11)1399.8558(12)1420.6206(12) ParameterAb initio 28 Si 129 Si 30 Si 13 C-1 13 C-2 13 C-3 13 C-4 A (MHz)77987803.8028(13)7736.5198(19)7671.9870(18)7789.3691(20)7745.5637(20)7771.0500(25)7700.0470(22) B (MHz)18541826.1829(5)1826.0601(6)1825.9424(7)1780.4239(7)1815.7959(7)1806.0891(8)1791.9036(11) C (MHz)16401622.9829(5)1620.1475(5)1617.3892(6)1586.2386(6)1613.2609(6)1607.2661(8)1592.0327(10) ParameterAb initio 28 Si 129 Si 30 Si 13 C-1 13 C-2 A (MHz)49005010.0042(10)4959.5457(9)4911.2883(9)4954.1310(10)4990.4718(10) B (MHz)25012386.8613(7)2386.8651(9)2386.8756(9)2342.1570(10)2360.4120(10) C (MHz)20151959.8881(6)1952.0817(11)1944.5236(11)1923.6417(11)1944.9999(12) Trans-gauche (C 1 ) conformer Trans-trans (C 2v ) conformer Gauche-gauche (C 2 ) conformer 1 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137.

11. 11 Structural Fit   Using Kraitchman’s equation: “r s structure” (for the heavy atom backbone)   KRA 1   EVAL 1   Using the STRFITQ 2 program: “r 0 structure”   H parameters fixed to ab initio values during fit 1 Kraitchman coordinates and propagated errors in parameters calculated using the KRA and EVAL code, Kisiel, Z. PROSPE–Programs for Rotational Spectroscopy; http://info.ifpan.edu.pl/~kisiel/prospe.htm, accessed July 2006. 2 Schwendeman, R. H. In Critical Evaluation of Chemical and Physical Structural Information; Lide, D. R., Paul, M. A., Eds.; National Academy of Sciences: Washington, DC, 1974. The STRFITQ program used in this work is the University of Michigan modified version of Schwendeman's original code.

12. Parameter GG (MP2) GG (r s ) GG (r 0 ) TG (MP2) TG (r s )* TG (r 0 ) TT (MP2) TT (r s ) TT (r 0 ) GG' (MP2) C1-C21.528Å1.557(3)Å1.538(5)Å1.528Å1.538(5)Å1.544(13)Å1.528Å1.538(5)Å1.539(1)Å1.527Å C2-Si31.876Å1.852(1)Å1.877(2) Å1.875Å1.883(72)Å1.875(10)Å1.874Å1.870(3)Å1.878(4)Å1.877Å Si3-C4--- 1.875Å1.852(104)Å1.878(4)Å--- 1.880Å C4-C5--- 1.528Å1.532(5)Å1.541(2)Å--- 1.529Å C1-C2-Si3112.7°113.5(2)°113.3(5)°113.0°113.9(75)°112.9(2)°113.1°113.4(3)°113.2(8)°114.8° C2-Si3-C4110.3°110.9(10)°111.6(4)°111.7°112.1(58)°111.6(5)°112.5°111.9(2)°111.7(5)°112.6° Si3-C4-C5--- 113.1°114.0(81)°113.4(5)°--- 114.3° C1-C2-Si3- C4 55.5 °57.7 °57.5(2)°178.7°179.6 °180.6(13)°180.0° 82.0° C2-Si3-C4- C5 --- 58.7°60.9°61.9(19)°--- 54.7° * Coordinates that are close to zero for the Si atom in this conformer lead to increased uncertainty in the determination of any structural parameters that involve the Si atom

13. 13 Comparison  Group 4 Analogs  Pentane  Diethylsilane  Diethylgermane  Group 6 Analogs  Diethylether  Diethylsulfide

14. 14 Pentane vs. Diethylsilane ParameterPentane DiEtSi (GG) r 0 DiEtSi (TG) r 0 DiEtSi (TT) r 0 C-C 1.531(2) Å 1.538(5) Å 1.544(13) / 1.541(2) Å 1.539(1) Å C-X-C112.9(2)°111.6(4)°111.6(5)°111.7(5)°  TT most stable for Pentane  GG most stable in Diethylsilane (DiEtSi)?  C-C bonds slightly larger for DiEtSi  C-X-C angle is slightly smaller for DiEtSi  Similar trend in Propane vs. Dimethylsilane 112.4(2)° 1 110.56° 2 1 Lide, D.R. J. Chem. Phys. 1960, 33, 1514. 2 Pierce, L. J. Chem. Phys. 1961, 34, 498.

15. Relative Abundances 15 Pentane % Abundances 1 Pentane % Abundances 2 DiEtSi % Abundances* GG7.31316 TG50.13668 TT43.6518 Assume no relaxation of conformers in beam; T = 298 KAssume no relaxation of conformers in beam; T = 298 K From intensities of transitionsFrom intensities of transitions DiEtSi % Abundances Excluding Low VibrationsIncluding Low Vibrations GG7435 TG2563 TT0.010.02 Calculated from ab initio valuesCalculated from ab initio values *Calculated from: 1 Bonham, R.A.; Bartell, L.S.; Kohl, D.A. J. Am. Chem. Soc., 1959, 81, 4765. 2 Churchill, G.B.; Bohn, R.K. J. Phys. Chem. A. 2007, 111, 3513 Experiment: Medvedev, I., et al J. Mol. Spectrosc. 2004, 228, 314. Townes, C. H.; Schawlow, A. L. Microwave Spectroscopy, Dover Publications Inc., New York, 1975. Ab initio: Salam, A.; Deleuze, M. S. J. Chem. Phys. 2002, 116, 1296. Churchill, G. B.; Milot, R. L.; Bohn, R. K. J. Mol. Struct. 2007, 837, 86.

16. Diethylgermane Parameter Ab initio for 74 Ge 70 Ge 72 Ge 73 Ge 74 Ge 76 Ge A (MHz) 3482.33678.7129(27)3651.8299(27)3638.8025(6)3626.1061(16)3601.4660(45) B (MHz) 2373.22310.6410(12)2310.6467(11)2310.6506(8)2310.6543(8)2310.6577(19) C (MHz) 1672.31684.3017(9)1678.6286(8)1675.8610(6)1673.1529(7)1667.8745(12)  GG conformer assigned  73 Ge coupling constants (MHz):  aa = 6.7389(121),  bb = -1.3056(47),  cc = -5.4332(168)  Recent broadband scan should facilitate identification of additional conformers

17. 17 DiEtS and DiEtE vs. DiEtSi Diethylsulfide ParameterGGTGTT ΔE (kJ/mol) HF/6-31G* 3 3.491.730 ΔE (kJ/mol) MP2/6-311G** 3 01.382.4 Diethylsilane ParameterGGTGTT ΔE (kJ/mol) MP2/6-311+G(2df,2pd)01.21.3 Diethylether ParameterGGTGTT ΔE (kJ/mol) HF/4-21G 1 8.44.20 ΔE (kJ/mol) MP2/6-31G** 2 ---5.40 1 Kuze, N. et al. J. Mol. Struct. 1993, 301, 81. 2 Medvedev, I. et al. J. Mol. Spectrosc. 2004, 228, 314. 3 Plusquellic, D.F. et al. J. Chem. Phys. 2001, 115, 3057

18. Conclusions  GG or TG is most stable and abundant conformer?  Conformational stabilities are particularly sensitive to level of calculation  GG’ consistently not seen in any diethyl compound 18

19. Future Work  Further comparison of Diethylsilane with other similar compounds  Diethyldifluorosilane 1  3,3-Difluoropentane  Finish assignment of Diethylgermane  Compare to DiEtSi to determine systematic changes due to substitution 19 1 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137. 2 20 -1 10 2 21 -1 10 4 04 -3 13 2 21 -1 11 3,3-difluoropentane (0.1% in He/Ne) Center frequency = 12807.0 MHz Chirp = 50-150 MHz 150 gas pulses Trans-gauche conformer

20. Future Work  Further comparison of Diethylsilane with other similar compounds  Diethyldifluorosilane 1  3,3-Difluoropentane  Finish assignment of Diethylgermane  Compare to DiEtSi to determine systematic changes due to substitution 20 1 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137.

21. 21 Acknowledgements  NSF  Professor Robert Bohn  The Peebles’s groups

22. To calculate populations:To calculate populations: I = intensity; N = concentration;  = absorption coefficient From Townes and Schawlow:From Townes and Schawlow: f v = fraction of molecules in ground vibrational state = W v = MP2 vibrational energy (used MP2 zero point vibrational energy)  n = vibrational frequency (product only over  < 1000 cm -1 ) d n = degeneracy of the n th vibrational mode W JKaKc = energy of lower rotational level |  ij | 2 =  b 2 x line strength (S) (used only b-types, S equal for trans. compared) = rotational transition frequency = rotational transition frequency  = line width at half maximum To calculate percentages (assume gg’ negligible):To calculate percentages (assume gg’ negligible):