11

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