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Broadband Chirped-Pulse Fourier-Broadband Chirped-Pulse Fourier-Transform Microwave (CP-FTMW) Transform Microwave (CP-FTMW) Spectroscopic Investigation of the Spectroscopic Investigation of the Structures of Three Diethylsilane Structures of Three Diethylsilane
ConformersConformersAmanda L. SteberAmanda L. Steber, Daniel A. Obenchain, Rebecca A. Peebles, , Daniel A. Obenchain, Rebecca A. Peebles,
and Sean A. Peeblesand Sean A. PeeblesDepartment of Chemistry, Eastern Illinois University, 600 Lincoln Department of Chemistry, Eastern Illinois University, 600 Lincoln
Avenue, Charleston, IL 61920 Avenue, Charleston, IL 61920
Justin L. Neill, Matt T. Muckle, and Brooks H. PateJustin L. Neill, Matt T. Muckle, and Brooks H. PateDepartment of Chemistry, University of Virginia, Department of Chemistry, University of Virginia, Charlottesville, Charlottesville,
VA 22904VA 22904
Gamil A. GuirgisGamil A. GuirgisDepartment of Chemistry and Biochemistry, Department of Chemistry and Biochemistry,
The College of Charleston, Charleston, SC 29424The College of Charleston, Charleston, SC 29424
22
IntroductionIntroduction
Extend series of pentane analogs to include Extend series of pentane analogs to include Diethylsilane (CDiethylsilane (C22HH55))22SiHSiH22
Compare conformer stabilities, structural Compare conformer stabilities, structural parametersparameters
4 possible conformers4 possible conformers Gauche-GaucheGauche-Gauche Trans-GaucheTrans-Gauche Trans-TransTrans-Trans Gauche-Gauche’Gauche-Gauche’
33
Ab Initio StructuresAb Initio Structures
Gauche-gauche Trans-gauche
Gauche-gauche’
Trans-trans
+4.6 kJ/mol+1.3 kJ/mol
+1.2 kJ/mol0 kJ/mol
Gaussian 03: MP2/6-311+G(2df,2pd) level
Relative energies are zero-point energy corrected
44
Experimental TechniqueExperimental Technique
Chirped-Pulse Fourier-Transform Microwave Spectrometer at the University of Virginia
55
Experimental TechniqueExperimental Technique
Sample concentration: 0.2% diethylsilaneSample concentration: 0.2% diethylsilane He/Ne carrier gas He/Ne carrier gas 1,000,810 acquisitions1,000,810 acquisitions 10 FIDs per pulse 10 FIDs per pulse 3 nozzles3 nozzles
66
Analysis ProcedureAnalysis Procedure
Import data into Origin Import data into Origin Using ab initio structure, isotopic Using ab initio structure, isotopic
rotational constants were used to predict rotational constants were used to predict experimental lines within a few MHzexperimental lines within a few MHz
SPFITSPFIT11 and SPCAT and SPCAT11 were used to predict were used to predict and fit the experimental lines of the and fit the experimental lines of the spectrumspectrum
1Pickett, H. M. J. Mol. Spectrosc. 1991, 148, 371.
77
Broadband SpectrumBroadband Spectrum
212-101 TT212-101 TG
212-101 GG
88
Magnified region of broadband spectrum from 9275 MHz to 9425 MHz containing 13C lines for the trans-gauche conformer. The 111←000 rotational transitions for the four unique 13C substitutions are highlighted.
SpectrumSpectrum1
2 34
99
Internal Rotation EffectsInternal Rotation Effects
211←202 211←202
Trans-trans conformerTrans-trans conformer
Low resolution 211 ← 202 transition
High resolution 211 ← 202 transition
Splittings consistent with recent results for pentaneSplittings consistent with recent results for pentane11
1 Churchill, G.B.; Bohn, R.K. J. Phys. Chem. A. 2007, 111, 3513
1010
Ab Initio and Experimental Rotational Constants Ab Initio and Experimental Rotational Constants
Parameter Ab initio 28Si1 29Si 30Si 13C-1 13C-2
A (MHz) 13455 13297.8209(23) 13241.9991(18) 13187.9967(18) 13286.2457(19) 13156.7163(19)
B (MHz) 1524 1515.5923(25) 1515.6052(13) 1515.6204(13) 1479.9248(14) 1504.8349(14)
C (MHz) 1439 1431.8589(24) 1431.2189(11) 1430.5984(11) 1399.8558(12) 1420.6206(12)
Parameter Ab initio 28Si1 29Si 30Si 13C-1 13C-2 13C-3 13C-4
A (MHz) 7798 7803.8028(13) 7736.5198(19) 7671.9870(18) 7789.3691(20) 7745.5637(20) 7771.0500(25) 7700.0470(22)
B (MHz) 1854 1826.1829(5) 1826.0601(6) 1825.9424(7) 1780.4239(7) 1815.7959(7) 1806.0891(8) 1791.9036(11)
C (MHz) 1640 1622.9829(5) 1620.1475(5) 1617.3892(6) 1586.2386(6) 1613.2609(6) 1607.2661(8) 1592.0327(10)
Parameter Ab initio 28Si1 29Si 30Si 13C-1 13C-2
A (MHz) 4900 5010.0042(10) 4959.5457(9) 4911.2883(9) 4954.1310(10) 4990.4718(10)
B (MHz) 2501 2386.8613(7) 2386.8651(9) 2386.8756(9) 2342.1570(10) 2360.4120(10)
C (MHz) 2015 1959.8881(6) 1952.0817(11) 1944.5236(11) 1923.6417(11) 1944.9999(12)
Trans-gauche (C1) conformer
Trans-trans (C2v) conformer
Gauche-gauche (C2) conformer
1 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137.
1111
Structural FitStructural Fit
Using Kraitchman’s equation: “rs structure” (for the heavy atom backbone) KRA1
EVAL1
Using the STRFITQ2 program: “r0 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.
ParameterParameterGG GG
(MP2)(MP2) GGGG (r (rss)) GGGG (r (r00))TGTG
(MP2)(MP2) TGTG (r (rss)*)* TGTG (r (r00))TTTT
(MP2)(MP2) TTTT (r (rss)) TTTT (r (r00))GGGG' '
(MP2)(MP2)
C1-C2C1-C2 1.528Å 1.557(3)Å 1.538(5)Å 1.528Å 1.538(5)Å 1.544(13)Å 1.528Å 1.538(5)Å 1.539(1)Å 1.527Å
C2-Si3C2-Si3 1.876Å 1.852(1)Å 1.877(2) Å 1.875Å 1.883(72)Å 1.875(10)Å 1.874Å 1.870(3)Å 1.878(4)Å 1.877Å
Si3-C4Si3-C4 --- --- --- 1.875Å 1.852(104)Å 1.878(4)Å --- --- --- 1.880Å
C4-C5C4-C5 --- --- --- 1.528Å 1.532(5)Å 1.541(2)Å --- --- --- 1.529Å
C1-C2-Si3C1-C2-Si3 112.7° 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-C4C2-Si3-C4 110.3° 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-C5Si3-C4-C5 --- --- --- 113.1° 114.0(81)° 113.4(5)° --- --- --- 114.3°
C1-C2-Si3-C1-C2-Si3-C4C4 55.5 ° 57.7 ° 57.5(2)° 178.7° 179.6 ° 180.6(13)° 180.0° 180.0° 180.0° 82.0°
C2-Si3-C4-C2-Si3-C4-C5C5 --- --- --- 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
1313
ComparisonComparison
Group 4 AnalogsGroup 4 Analogs PentanePentane DiethylsilaneDiethylsilane DiethylgermaneDiethylgermane
Group 6 AnalogsGroup 6 Analogs DiethyletherDiethylether DiethylsulfideDiethylsulfide
1414
Pentane vs. DiethylsilanePentane vs. Diethylsilane
ParameterParameter PentanePentane DiEtSi (DiEtSi (GGGG) r) r00 DiEtSi (DiEtSi (TGTG) r) r00 DiEtSi (DiEtSi (TTTT) r) r00
C-CC-C 1.531(2) Å1.531(2) Å 1.538(5) Å1.538(5) Å 1.544(13) / 1.541(2) Å1.544(13) / 1.541(2) Å 1.539(1) Å1.539(1) Å
C-X-CC-X-C 112.9(2)°112.9(2)° 111.6(4)°111.6(4)° 111.6(5)°111.6(5)° 111.7(5)°111.7(5)°
TTTT most stable for Pentane most stable for Pentane GGGG most stable in Diethylsilane (DiEtSi)? most stable in Diethylsilane (DiEtSi)? C-C bonds slightly larger for DiEtSiC-C bonds slightly larger for DiEtSi C-X-C angle is slightly smaller for DiEtSiC-X-C angle is slightly smaller for DiEtSi
Similar trend in Propane vs. DimethylsilaneSimilar 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.
Relative AbundancesRelative Abundances
1515
Pentane % Abundances1 Pentane % Abundances2 DiEtSi % Abundances*
GGGG 7.37.3 1313 1616
TGTG 50.150.1 3636 6868
TTTT 43.643.6 5151 88
• Assume no relaxation of conformers in beam; Assume no relaxation of conformers in beam; TT = 298 K = 298 K• From intensities of transitions From intensities of transitions
DiEtSi % Abundances
Excluding Low Vibrations Including Low Vibrations
GGGG 7474 3535
TGTG 2525 6363
TTTT 0.010.01 0.020.02
• Calculated from ab initio values Calculated from ab initio values
*Calculated from: RTG
ttxx ttxxexx degen.N
N /
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.
DiethylgermaneDiethylgermane
Parameter Parameter Ab initio for Ab initio for 7474GeGe 7070GeGe 7272GeGe 7373GeGe 7474GeGe 7676GeGe
A A (MHz)(MHz) 3482.33482.33678.7129(273678.7129(27
))3651.8299(273651.8299(27
))3638.8025(63638.8025(6
))3626.1061(163626.1061(16
))3601.4660(453601.4660(45
))
B B (MHz)(MHz) 2373.22373.22310.6410(122310.6410(12
))2310.6467(112310.6467(11
))2310.6506(82310.6506(8
)) 2310.6543(8)2310.6543(8)2310.6577(192310.6577(19
))
C C (MHz)(MHz) 1672.31672.3 1684.3017(9)1684.3017(9) 1678.6286(8)1678.6286(8)1675.8610(61675.8610(6
)) 1673.1529(7)1673.1529(7)1667.8745(121667.8745(12
))
GGGG conformer assigned conformer assigned 7373Ge coupling constants (MHz): Ge coupling constants (MHz): aaaa
= 6.7389(121), = 6.7389(121), bbbb = -1.3056(47), = -1.3056(47), cccc = =
-5.4332(168) -5.4332(168) Recent broadband scan should Recent broadband scan should
facilitate identification of additional facilitate identification of additional conformersconformers
8665 8666 8667 8668Frequency / MHz
212←101
4←
55←
6
7←
6
6←
6
4←
4
5←
4
8665 8666 8667 8668Frequency / MHz
8665 8666 8667 8668Frequency / MHz
8665 8666 8667 8668Frequency / MHz
8665 8666 8667 8668Frequency / MHz
212←101
4←
55←
6
7←
6
6←
6
4←
4
5←
4
1717
DiEtS and DiEtE vs. DiEtSiDiEtS and DiEtE vs. DiEtSi
Diethylsulfide
Parameter GG TG TT
ΔE (kJ/mol) HF/6-31G* 3 3.49 1.73 0
ΔE (kJ/mol) MP2/6-311G** 3 0 1.38 2.4
Diethylsilane
Parameter GG TG TT
ΔE (kJ/mol) MP2/6-311+G(2df,2pd) 0 1.2 1.3
Diethylether
Parameter GG TG TT
ΔE (kJ/mol) HF/4-21G 1 8.4 4.2 0
ΔE (kJ/mol) MP2/6-31G** 2 --- 5.4 0
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
ConclusionsConclusions
GGGG or or TGTG is most stable and abundant is most stable and abundant conformer?conformer?
Conformational stabilities are particularly Conformational stabilities are particularly sensitive to level of calculation sensitive to level of calculation
GG’GG’ consistently not seen in any diethyl consistently not seen in any diethyl compoundcompound
1818
Future WorkFuture Work
Further comparison of Diethylsilane Further comparison of Diethylsilane with other similar compoundswith other similar compounds DiethyldifluorosilaneDiethyldifluorosilane11
3,3-Difluoropentane3,3-Difluoropentane
Finish assignment of Diethylgermane Finish assignment of Diethylgermane Compare to DiEtSi to determine Compare to DiEtSi to determine
systematic changes due to systematic changes due to substitutionsubstitution
19191 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137.
222020-1-11010
222121-1-11010
440404-3-31313
222121-1-11111
3,3-difluoropentane (0.1% in He/Ne)3,3-difluoropentane (0.1% in He/Ne)Center frequency = 12807.0 MHzCenter frequency = 12807.0 MHzChirp = 50-150 MHzChirp = 50-150 MHz150 gas pulses150 gas pulsesTrans-gaucheTrans-gauche conformer conformer
Future WorkFuture Work
Further comparison of Diethylsilane Further comparison of Diethylsilane with other similar compoundswith other similar compounds DiethyldifluorosilaneDiethyldifluorosilane11
3,3-Difluoropentane3,3-Difluoropentane
Finish assignment of Diethylgermane Finish assignment of Diethylgermane Compare to DiEtSi to determine Compare to DiEtSi to determine
systematic changes due to systematic changes due to substitutionsubstitution
20201 Peebles, S. A.; Serafin, M. M.; Peebles, R. A.; Guirgis, G. A.; Stidham, H. D. J. Phys. Chem. A, 2009,113, 3137.
2121
AcknowledgementsAcknowledgements
NSFNSF Professor Robert Bohn Professor Robert Bohn The Peebles’s groupsThe Peebles’s groups
• To calculate populations:To calculate populations:
II = intensity; = intensity; NN = concentration; = concentration; = absorption coefficient= absorption coefficient
• From Townes and Schawlow:From Townes and Schawlow:
ffvv = fraction of molecules in ground vibrational state = = fraction of molecules in ground vibrational state =
WWvv = MP2 vibrational energy (used MP2 zero point vibrational energy) = MP2 vibrational energy (used MP2 zero point vibrational energy)
nn = vibrational frequency (product only over = vibrational frequency (product only over < 1000 cm < 1000 cm-1-1))
ddnn = degeneracy of the n = degeneracy of the nthth vibrational mode vibrational mode
WWJKaKcJKaKc = energy of lower rotational level = energy of lower rotational level
||ijij||22 = = bb22 x line strength ( x line strength (SS) (used only ) (used only bb-types, -types, S S equal for trans. compared)equal for trans. compared)
= rotational transition frequency= rotational transition frequency
= line width at half maximum= line width at half maximum
• To calculate percentages (assume To calculate percentages (assume gggg’ negligible):’ negligible):
1max,
2max,
2
1
2
1
I
I
N
N
221
2
1,ij1111,v
122
2
2,ij2222,v
1max,
2max,
1,JKaKc
2,JKaKc
kTW
kTW
eCBAf
eCBAf
nnv
n
1d
kTh
kTW
ee
%1001
xx%
tt
tg
tt
gg
tt
xx
NN
NN
NN