An electron diffraction study of the structure of 1,1,2-trichloroethane in the gas phase

An electron diffraction study of the structure of 1,1,2-trichloroethane in the gas phase

23 Journal of Molecular Structure, 21 (1974) 23-27 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands AN ELECTRON DIFF...

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23 Journal of Molecular Structure, 21 (1974) 23-27 @ Elsevier Scientific Publishing Company, Amsterdam

- Printed in The Netherlands

AN ELECTRON DIFFRACTION STUDY OF THE STRUCTURE 1,1,2-TRICHLOROETHANE IN THE GAS PHASE

OF

P. HUISMAN Department of Applied Physics, Technical Unioersity, Delft (The Netherlands) F. C. MIJLHOFF Gorlaeus Loboratoria der Rijksunicersireit, Postbus 75, Leidea (The Netherlands) (Received

19 July 1973)

ABSTRACT

The molecular structure of 1,1,2-trichloroethane has been determined by gas phase electron diffraction_ The molecule is asymmetric_ The geometrical parameters (ra structure) are: r(C-Cl) 1.776 A; r(C-H) 0.98 A; L(C-C-Cl) 107”; L(Cl-C-Cl) projected along the C-C bond 116”; dihedral angle (Cl-C-C-Cl) 75”. The parameters L(C-C-H) 102” and the projected (H-C-H) angle 136” are inaccurate. The structure is rather insensitive to the r(C-C) value, which is unusually long, 1.56 to 1.58 A.

INTRODUCTION

A study by BovCe shows that it is possible to determine intermolecular distances between hydrogen atoms from proton magnetic resonance relaxation measurements [I]. 1,1,2-Trichloroethane proved to be a compound ideally suited to test the method in its development. An electron diffraction determination of the structure was considered to be useful to complement the NMR study. No structure determination of this substance has been reported before.

EXPERIMENTAL

A commercial sample of CHC12-CH2Cl was used. The electron diffraction patterns were recorded at 50 and 25 cm camera distance, 50 “C nozzle temperature, using the Balzers’ KDG2 equipment at the Leiden laboratory. Kodak Electron

Image plates were used, developed in HRP developer mixed with antifog for intermediate speed and grain. For each distance four plates were selected for analysis. The optical densities were measured with a Joyce Loebl Autodensidator, modified to allow an oscillation of the plate of about 30”. The electron wavelength of the 60 keV electrons was calibrated by a TlCI powder pattern. The cell constant was taken from Witt [2] (a = 3.84145 A). The usual procedure [3] for data reduction was used to obtain the experimental intensities in the ranges 3.5 < s < 14.75 and 7.25 G s d 34.50. This resulted in a total number of 156 intensity data with an interval As = 0.25 A-‘.

STRUCTURE

ANALYSIS

Theoretical radial distribution functions (r.d.f_) were calculated for symmetric and asymmetric molecules. Since the latter compared more favourably with the experimental r.d.f. it was assumed that thismolecule was present exclusively (Fig. 1). The model was refined by least squares, fitting Z/B to 1 +&M(s)

Fig.

1. Molecular

model

of 1,1,2-trichloroethane.

where B is the empirical background. Only anharmonicity of the C-H bond distances ((I = 3.0 A) was taken into account. A diagonal weight matrix was used, the weights were taken proportional to 2s/s,.,_, for each camera distance. The data of each s range were treated as independent. Fig. 2 gives the levelled experimental intensities and final backgrounds_ The experimental radiai distribution curve (obtained from the sM(s) curves of the two camera distances blended together and supplemented with theoretical values at low s) and the difference curve are given in Fig. 3. All parameters were refined together, using the method of the damped least squares [4] in which the diagonal elements of the correlation matrix are increased by a suitable constant in order to suppress effects of near dependence and nonlinearity on irregular behaviour of the solution vector. The C-C bond length is masked by the three C-Cl bond distances. The structure obtained by varying all other parameters proved to be very insensitive to changes in r(C-C) between 1.56 and 1.60 A. The latter value was obtained, together with a corresponding vibrational amplitude of 0.080 A, when the C-C distance was free to vary. Constraining the C-C

distance

to 1.58 and 1.56 resulted in amplitudes

of 0.050

and 0.036

A,

25

HCl,C-CClH2

Fig. 2. 1,1,2-Trichloroethane.

Levelled experimental

intensities

and backgrounds.

the latter with significantly worse fit to the experimental data. For presentation of the structure in Table 1 we selected the results obtained with r(C-C) = 1.58 A. They differ from those found with r(C-C) = 1.56 A mainly in the C-C-H valency angle (lOSo) and the Hz-C-H, projected valency angle (126”), i.e. the relative positions of the H atoms, which cannot be but poorly determined.

Fig. 3. 1,1,2-Trichloroethane. Experimental damping factor is 0.0015 A’.

radial distribution

function and difference curve. The

26 TABLE

I

1 ,I ,?-TRICHLOROETHANE

Geometrical parameters (in I% and degrees) and root mean square amplitudes. standard deviations are given in parentheses. Geometrical

Their estimated

parameters

1 2

r(C-C)

1.58=(I) 1.776( I )

6 7

<(C-C-H)

lOl(25)

r(C-Cl)

<(Cl

I I7.0(0.5)

3

r(C-H) < (C-C-Cl, <(C-C-CI3)

O-977(5) 106.4(g) 107.9(1.3)

8

<(Hz-C-H

x)b <(Cl ,-c-c-C12)

136(8) 75 (I)

4 5

)

9

,-C-c12)b

Root mean square amplitudes 10

c-c

16 17

0.072(

C-Cl

0.050(l0) 0.058(5)

H - - Cl

II

Cl--Cl

O.OSOC5)

12 13

C-H

0.042(9)

18

H----Cl

0.150’(-)

c..ct

O-087(5)

19

Cl - - . . Cl

0.149=(11)

14

C.-H H--H

0.060”(-) 0.100”( -)

20

Cl - - - . Cl

0.979d(5)

I5

Generalized il Assumed. TABLE

I .46 x IO- 3, I56 data. b Projected valency angles.

100)

R factor

’ Synclinal.

d Antiperiplanar.

2

I,I.2-TRICHLOROETHANE Strong correlations I-10 4-5 4-7 5-7

-0.82 -0.87 -0.96 to.81

(>0.6).

The numbers correspond 6-8 6-18 8-16

to those in Table

1.

f0.69 +0.81 -0.70

-

DISCUSSION

By the present study it is shown that in the gas phase 1,1,2-trichloroethane exists in the asymmetrical conformation, the dihedral angle between the synclinal C-C-Cl planes being 75”. The C-Cl bond distance is 1.776 A, the C-C-Cl bond angle on the C, atom (106.4”) seems to be somewhat smaller than that on the C, atom (107.9”). The C-H bond distance, which is well resolved, is remarkably short, only 0.977 A. In hydrocarbons a value between 1.08 and 1.12 is usually found. The C-C bond distance is longer than for ethane (rs = 1.533) [S], but its exact value escapes accurate determination. It compares reasonably well with the value found in hexachloroethane by Almenningen, Andersen and Traetteberg [6] (1.564 A, e.s.d. 0.014 A). The same can be said of the C-Cl bond distances (1.769 A, e.s.d. 0.003 A), especially since we find an e.s.d. of 0.003 A when the C-C

27 distance is constrained to 1.56 A. The value they report for the C-C-Cl bond angle (1 lO.O”, e.s.d. 0.5”) is a few degrees larger than that found in our study. ACKNOWLEDGEMENTS

This study was carried out under the auspicies of the F.O.M.R.E. with financial support of the Netherlands Organisation for Pure and Applied Research (Z.W.O.). The authors wish to thank Dr. W. BovCe for initiating this investigation and for his stimulating interest in the results. REFERENCES I 2 3 4 5 6

W. Bow%, in progress, private communication. W. Witt, 2. Naturforsch., 19a (1964) 1363. H. J. Geise and F. C. Mijlhoff, Lob. Rep., Leiden, ED 001 and ED 002. J. Pliva, V. Spirko and S. Toman, J. Mol. Specfrosc.. 15 (1965) 502. K. Kuchitsu, f_ Chern. Phys., 49 (1968) 4456. A. Almenningen, B. Andersen and M. Traetteberg, Acfa Chern. Stand., 18 (1964)

603.