Microwave Spectrum of Ethyl Cyanide; ^-Structure

Microwave Spectrum of Ethyl Cyanide; ^-Structure, Nitrogen Quadrupole Coupling Constants and Rotation-Torsion-Vibration Interaction H. Mäder, H. M. Heise, and H. Dreizler Abt. Chemische Physik im Institut für Physikalische Chemie der Universität Kiel (Z. Naturforsch. 29a. 164— 183 [1973] ; received October 27, 1973) An investigation of the microwave spectra of ethyl cyanide CH 3CH 2CN and the isotopes C H 2D CH 2CN, CH 3CD2CN was carried out. The ground vibrational state of CH 3C H 2CN was re­ examined under high resolution to give three centrifugal distortion constants D j , D j k and D k ■ From the rotational constants of the ground vibrational state of CH 3CH 2CN, CH3CD2CN, CH 2D CH 2CN (symmetric) and CH 2D C H 2CN (asymmetric). C H 3C H 2,3CN and CD3CHDCN a r0-structure is derived. For the isotopes CH 3CD2CN, CH 2D C H 2CN (symmetric) and CH 2DCH2CN (asymmetric) the diagonal elements yaa , ybb and yCc of the quadrupole tensor with respect to the principal axes of inertia were deduced from the hyperfine structure of the rotational lines. The offdiagonal element yab for CH3CD2CN and C H 2D C H 2CN (symmetric) and the principal elements '/.zz > Xxx of the quadrupole coupling tensor were obtained from yaa, ybb of the two molecules and from the principal axis rotation angle about the c-axis produced by isotopic substitution. For the analysis of the rotational spectra in the first excited states of methyl torsion and the lowest frequency in plane bending vibration of normal ethyl cyanide a molecular model with two internal degrees of freedom is considered. From the rotational line splittings in both states the coefficients Vo and V6 of the Fourier expansion of the potential hindering the internal rotation of the methvl group are determined.

The microwave spectrum of ethyl cyanide (propionitrile) was previously studied by several authors. A molecular structure was proposed in an investigation by Lerner and D aily1. The emphasis of the microwave studies reported by Laurie 2 is on the dipole moment, 14N-quadrupole coupling and barrier to internal rotation of the methyl group. The determination of the quadrupole coupling constants was completed by Li and Harmony 3. Our microwave studies of ethyl cyanide were ex­ tended to further isotopic species of this molecule - CH 3CD2CN and C H ,D C H XN - in order to ob­ tain a more reliable restructure as well as the prin­ cipal elements of the quadrupole coupling tensor. A large part of this work is the investigation of the interaction of overall rotation and the two lowest frequency vibrations, which are the methvl-torsion and the CCN-in-plane bending vibration. The effects of this interaction have been observed in the rota­ tional spectra of the first excited torsional and vi­ brational state of the normal molecule, CH;JCH 2CN. Experimental The microwave measurements wTere carried out in the frequency range from 8 to 41 GHz with a con­ ventional 100 kHz Stark modulation microwave spectrograph 4‘ 5 employing phase stabilized BW O’s as radiation sources. The absorption cell was cooled

with methanol flowing through a cooling jacket. The measurements were performed at temperatures of approximately —60 C and sample pressure of several microns. Normal ethvl cyanide was obtained from Merck Company (Darmstadt) and was used without further purification. The isotopic ethyl cyanides, CH;iCDoCN and CHoDCHXN, were prepared from the cor­ responding deuterated ethyl iodides CH.jCDol (99 atom^> D, Roth, Karlsruhe) and CHoDCHoI (98 atom% D, Sharp & Dohme, München). The method of preparation was a modified Kolbe reaction6. Sodium cyanide was dispersed in a solution of the ethyl iodide in triethyleneglycol at room tempera­ ture. The mixture was then heated slowly to 110 °C and held at this temperature for 1 hour. A magnetic stirrer war used to keep the sodium cyanide well dispersed throughout the mixture. The final product was isolated by distillation in vacuo and dried over P ,0 5 . The purity of the sample was controlled gas chromatographically, the instrument used being a Beckman/GC-2 gas chromatograph (column, carbowrax 400). Impurities found were the isonitrile and ethyl iodide. The sample purity of the ethyl cyanide Avas ^ 99.5% so that additional purification was unnecessary. G round V ibrational State All rotational transitions were clearly identified by their Stark pattern or nuclear quadrupole hyper-

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H. Mäder et al. • Microwave Spectrum of Ethyl Cyanide

165

Table 1. Rotational transitions a (MHz) for the ground vibrational state of normal ethyl cyanide observed frequencies

unsplit frequencies

calculated frequencies15

Av H F S observed c

Av H F S calculated11

I’unspl — J'calc

0.14 - 0.78 1.64 - 0.25 1.34 0.35 0.73 - 1.15 - 0 .0 6 0.33 - 0.64 0.07 0.07 - 0 .9 5 - 0.95 1.70 0.20 - 0.84 - 0.21 - 0.21 0.21 - 0.83 1.18 - 0.47 - 0.15 0.52 - 0.53 - 0.53 1.16 0.93 - 1.16 - 0.01 0.32 - 1.16 - 0.68 1.36 0.25 - 0.84 0.82 0.05 - 1.10 1.48 0.09 - 0.21 0.09 0.08 - 0.21 0.08 - 0.87 1.09 0.24 - 0.80 0.81 - 0.15 0.30 - 0.15 - 0.86 1.27 1.01 - 1.16

0.17 - 0.83 1.65 - 0.27 1.34 0.35 0.73 — 1.14 - 0 .0 6 0.32 - 0.63 0.07 0.00 - 0 .8 3 - 0.99 1.66 0.19 - 0.83 - 0.21 - 0 .1 9 0.21 — 0.83 1.14 — 0.45 - 0.15 0.51 - 0.51 — 0.55 1.15 0.92 - 1.15 - 0.02 0.32 - 1.14 - 0.67 1.34 0.24 — 0.82 0.83 0.04 - 1.07 1.49 0.08 - 0.21 0.10 0.09 - 0.21 0.06 - 0.86 1.09 0.24 - 0.83 0.83 - 0.10 0.31 - 0.25 - 0.80 1.25 1.01 - 1.19

- 0.02

o

° O o

1

isition -K+— J'K ~ 'K +' F — F '

C H . j C H 2C N .

l o i— lio

Ooo— 111

lo i — 2o2

U o—2 n

111 —2i2

2o2— 2 n

lo i — 2l 2

220 — ^21

2o2— 3os

2 n — 3i2

2l2 — 3i3

221 — ^22

3o3 — 3l2

330 — 431 331 — ^32 321 — 422

1- 2 1-1 1- 0 2 —2 1-1 2 - 1 1- 2 0 - 1 1- 2 1- 1 1- 0 2-3 1- 2 0 - 1 2 —2 1- 1 2-3 1- 2 2—2 1- 1 2-3 1- 2 0- 1 2—2 3-3 2-2 1- 1 3-2 2 - 1 2-3 1- 2 2-3 1- 2 0- 1 2—2 1- 1 3-4 2-3 1- 2 3-4 3- 3 2 —2 3-4 2-3 1- 2 3-4 2-3 1- 2 3-3 2-2 3-4 2-3 1- 2 4-4 3-3 2-2 4-3 3-2 3-4 2-3

8 949.40 8 948.48 8 950.90 23427.89 23429.48 23428.49 23428.87 23426.99 31898.23 31898.62 31 897.65 17 891.07 17 891.07 17 890.05 17 890.05 17 892.70 18 377.91 18 376.87 18 377.50 18 377.50 17419.77 17418.73 17420.74 17 419.09 23 914.69 23 915.36 23914.31 23 914.31 23 916.00 23 915.77 23 913.68 40 368.58 40368.91 40 367.43 40 367.91 40 369.95 26 878.48 28 877.39 26 879.05 26 817.85 26 816.70 26 819.28 27 561.70 27 561.40 27 561.70 26 124.76 26 124.47 26 124.76 26123.81 26125.77 26 848.93 26 847.89 26 849.50 24 658.58 24 659.03 24 658.58 24 657.87 24 660.00 24 659.74 24 657.51

8 949.26

8 949.28

23428.14

23428.15

31 898.29

31 898.41

17 891.00

17 891.06

18 377.71

18 377.67

17 419.56

17 419.63

23 914.84

23 914.76

40 368.59

40368.76

26 878.23

26878.20

26 817.80

26817.86

27 561.61

27 561.64

26124.68

26124.65

26 848.69

26848.53

24 658.73

24 658.54

35814.10L 35 814.10L 35 866.11

35814.41 35 813.98 35 866.02

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- 0.01

- 0.12

- 0.06

0.04

- 0.07

0.08

- 0.17

0.03

- 0.06

- 0.03

0.03

0.16

0.19

- 0.31 0.12 0.08

166

H. Mäder et al. • Microwave Spectrum of Ethyl Cyanide

Transition J k -k +— J ' k -'k + F - F ' 322 — 423

3l2 — 4l3 3i3 —4i4 3o3— 4o4 4l3 — 322

4o4— 4l3

5i4 —423

4i4 — Ö05 5o5—5l4 5l5 — Ö06 6o6— 6l5

4 - 5 3 - 4 2- 3

5 4 3 5 4 3 6 5 4

observed frequencies

unsplit frequencies

calculated frequencies0

Av H F S observed c

Av H F S calculated*1

35 792.24 35 791.74 25 792.24

35 792.07

35 791.91

0.17 - 0.33 0.17

0.12 - 0.33 0.24

36 739.67L 34824.07 35 722.20 31388.43

36 739.67 34 824.02 35 722.26 31 388.55

25676.18

25 675.96

21270.96

21 270.84

23 710.84 26 988.67

23 710.76 26988.58

33 629.96 28 622.29

33 629.99 28 622.51

- 4 - 3 - 2 —■5 - 4 - 3 - 5 —4 - 3

31388.11 31 389.04 31388.11 25 676.07 25 676.41 25 676.07 21270.81 21271.28 21270.81

6- 6 5—5

26988.59 26988.85

7- 7 6- 6

28 622.20 28 622.47

- 0.32 0.61 - 0.32 - 0.11 0.23 - 0.11 - 0.15 0.32 - 0.15

- 0.22 0.61 - 0.43 - 0.08 0.23 - 0.16 - 0.13 0.32 - 0.20

- 0.06 0.18

- 0.07 0.19

- 0.09 0.18

- 0.07 0.17

I’unspl *'ealc 0.16

0.00 0.05 - 0.06 - 0.12

0.22

0.12

0.08 0.09 - 0.03 - 0.22

a A ll lines between 18 GHz and 41 GHz, except those labelled with L (taken from Ref. 2), were re-examined by H. Lutz. The transition / = 1 01—212 had not been reported by earlier workers 1-3. b Calculated with the rotational constants from Table 2 and Equation (1). c Hyperfine component shifts Z lrH F S = ^observed- v un split • d Calculated with the quadrupole coupling constants from Reference 3.

fine structure. Due to the larger a-component of the electric dipole moment

(]//„! = 3.78 D, |fth | = 1.38 D, |ju\= 4.02 D ) 2 the //a-transitions are about 8 times more intense than the //^-lines of comparable line strength. We investigated only rotational transitions up to the rotational quantum number J = 6 for which no line splitting due to internal rotation of the methyl group was resolvable in the ground vibrational state. The measured rotational transitions of the normal ethyl cyanide are listed in Table 1. The measured frequencies are compared with the spectrum of a centrifugal distorted asymmetric rotor. As the experimental data are not sufficient to determine all parameters in Watson’s form ula7, we cor­ rected the asymmetric rigid rotor energy levels EjK-K +(d, B, C) as follows:

Equation (1) approximates Watson’s formula for the near symmetric top. The molecular constants in ( 1 ), resulting from a least squares fitting procedure including all measured lines, are given in Table 2. Table 2. Rotational constants (MHz) and moments of iner­ tia a (amu A 2) for the ground vibrational state of normal ethyl cyanide CH 3CH 2CN. A B C Dj D jk Dk

27 663.751 (± 0.074)b 4 714.157 (± 0.016) 4 235.135 (± 0.011) 0.0031 (± 0.0003) - 0.041 (± 0.001) 0.546 (± 0.019)

la h Ic

18.268 52 107.203 89 119.329 37

a Conversion factor 5.05376 x 105 MHz amu Ä 2. b Standard errors obtained from the least squares fit in­ cluding all unsplit line frequencies of Table 1.

The rotational line frequencies of 1,1-dideuteroethyl cyanide, CH 3CD 2CN, are listed in Table 3. The measurements are compared with an asymmetric Ecn = Ejk-s . (A ,B ,C )- D j P ( J + l )2 rigid rotor pattern obtained from a least squares fit - D j k J ( J + 1 ) ( P * ) - D k (Pz>) ( 1) including the rotational transitions up to ] = 2. The rotational constants and moments of inertia are with A , B , C rotational constants A ^.B ^>C given in Table 4. D j , D j r , D r centrifugal distortion constants In the case of 2-monodeutero-ethyl cyanide, ( Pzn) expectation value of Pzn (n = 2, 4) in the asymmetric rigid CH 2DCH 2 CN, two rotational spectra were found corresponding to the position of the deuteron with rotor eigenstate £/k_ k + (A, B, C ).

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167

H. Mäder et al. • Microwave Spectrum of Ethyl Cyanide Table 3. Rotational transitions (MHz) for the ground vibrational state of CH 3CD2CN. Transition JK-K+ — J 'K - 'K +' F - F ' ©

7

o o O

lot — lio

Ooo— I n

lo i — 2o2

lio —2 n

1- 2 1- 1 1- 0 2-2 1- 1 2- 1 1-0 1-2 0 - 1 1- 2 1- 1 1-0 2-3 1- 2 0- 1 2-2 1- 1 2-3 1- 2 0- 1 2-2 1 1 2-3 1- 2 0- 1 2-2 1- 1 3-3 2-2 1- 1 3-2 2- 1 2-3 1- 2 2-3 1- 2 0 - 1 2-2 1- 1 3-4 2-3 1- 2 3-4 2-3 1- 2 3-3 2-2 3-4 2-3 1- 2

-

111 —2l2

2o2— 2 n

lo i — 2i2

2l2 —3l3

220 — 321

2o2— 3o3

3-3 2 n —3i2

2321-

2 4 3 2

2 4 3 2

2 4 3 2

3-3

3o3—3l2

-

observed frequencies 8 688.27 8 687.24 8689.81 17 854.86 17 856.51 17 855.47 17 854.86 17 855.90 17 853.92 26030.71 26031.12 26030.07 17 364.93 17 364.93 17 363.88 17 363.88 17 366.57 17 888.73 17 887.67 17 889.90 17 888.35 17 888.35 16 863.99 16 862.89 16 864.99 16863.33 16 863.99 18 378.64 18 379.30 18 378.25 18 378.25 18 380.00 18 379.78 18 377.55 34206.47 34206.84 34205.28 34205.80 34 207.87 25 288.71 25288.41 25288.71 26 109.23 26108.10 26109.86 26 109.23 26108.10 26019.22 26019.22 26019.22 26018.12 26020.77 26825.63 26825.34 26825.63 26 824.90 26 826.32 19 185.08 19185.54 19185.08

unsplit frequencies

calculated frequencies a

8 688.10

8688.06

17 855.12

17 855.15

26030.77

26030.81

17 364.86

17 364.94

17 888.53

17 888.52

16863.77

16 863.72

18378.78

18 378.74

34206.49

34206.47

25288.61

25288.66

26108.98

26108.90

26019.24

26019.47

26825.54

26825.71

19185.23

19184.98

Av H F S observed

Av H F S I’unspl calculated13 — I’ealc

0.17 - 0.86 1.71 - 0 .2 6 1.39 0.35 - 0.26 0.78 - 1.20 - 0.06 0.35 - 0.70 0.07 0.07 - 0.98 - 0.98 1.71 0.20 - 0 .8 6 1.37 - 0.18 - 0 .1 8 0.22 - 0 .8 8 1.22 - 0.44 0.22 - 0.14 0.52 - 0.53 - 0.53 1.22 1.00 - 1.23 - 0.02 0.35 - 1.21 - 0.69 1.38 0.10 - 0.20 0.10 0.25 - 0.88 0.88 0.25 - 0.88 - 0.02 - 0 .0 2 - 0 .0 2 - 1.12 1.53 0.09 - 0 .2 0 0.09 - 0.64 0.78 - 0 .1 5 0.31 - 0.15

0.17 - 0.86 1.72 - 0.27 1.37 0.34 - 0.16 0.76 - 1.21 - 0.07 0.35 - 0.70 0.07 0.00 - 0.86 - 1.04 1.73 0.20 - 0.86 1.37 - 0.25 - 0 .1 6 0.22 - 0.86 1.21 - 0.44 0.16 - 0.15 0.51 - 0.51 - 0.60 1.21 0.96 - 1.21 - 0.03 0.35 - 1.21 - 0.68 1.37 0.10 - 0.22 0.07 0.25 - 0.86 0.86 0.25 - 0 .8 7 0.04 0.00 - 0.17 - 1.11 1.56 0.09 - 0 .2 2 0.10 - 0 .6 7 0.80 - 0 .1 0 0.30 - 0.24

a Calculated with the rotational constants from Table 4 and asymmetric rigid rotor formula. b Hyperfine component shifts calculated with the quadrupole coupling constants from Table 11.

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0.04

0.03

- 0.04

- 0.08

0.01

0.05

0.04

0.02

- 0.05

0.08

- 0.23

- 0 .1 7

0.25

H. Mäder et al. • Microwave Spectrum of Ethyl Cyanide

168

respect to the symmetry plane of the molecule. The molecule may be called C H 2D C H 2C N (symmetric) or C H o D C H X N (asymmetric) depending on wheth­ er the deuteron is positioned in or out of plane.

Table 9. r0-structural parameters of ethyl cyanide.

rc-c

rc-cx rc-N rc-H b

Table 4. Rotational constants (MHz) with standard errors a and moments of inertia (amu Ä2) for the ground vibrational state of CHoCDoCN. A

B C

21942.980 (±0.027) 4 600.231 (±0.015) 4087.830 (±0.014)

la

h

Ic

23.031 32 109.858 83 123.629 40

a Obtained from least squares analysis including all unsplit line frequencies of Table 3 up to J = 2.

The rotational frequencies and molecular con­ stants of both isomeric forms are given in Tables 5, 6 , 7 and 8 . The rotational constants and the fre­ quencies calculated from these constants are derived from a least squares analysis of rotational lines up to J = 2 , analogous to the case of the CH3CD2CN isotope. Table 6. Rotational constants (MHz) with standard errors a and moments of inertia (amu Ä2) for the ground vibrational state of CHoDCHXN (sym.). A B C

27 650.897 (±0.049) 4425.142 (±0.027) 4 000.821 (±0.019)

la

h

Ic

18.277 02 114.205 60 126.31807

a Obtained from least squares analysis including all unsplit line frequencies of Table 5 up to 7 = 2. Table 8. Rotational constants (MHz) with standard errors a and moments of inertia (amu Ä2) for the ground vibrational state of CHODCHXN (asym.). A B C

25 022.652 (±0.041) 4583.476 (±0.022) 4110.264 (±0.016)

la

h Ic

20.196 74 110.260 42 122.954 63

a Obtained from a least squares analysis including all unsplit line frequencies of Table 7 up to J = 2.

r 0-Structure

Lerner and D aily1 derived six restructure parameters from six rotational constants, which are the B and C constants of normal ethyl cyanide and of the CH3CH213CN and CD.jCHDCN isotopes. This experimental information appeared to us be insuf­ ficient to obtain reliable values for the proposed structural parameters. Therefore, we decided to re­ vise the restructure taking into account all available experimental data. The structure fitting calculations

TC —H b

CCN y, c -*■ x) ; c Zl