Molecular structure and conformational equilibria of gaseous tetrakis(chloromethyl)methane, C(CH2Cl)4, as determined by electron diffraction

Journal of Molecular Structure, 140 (1986) 167-169 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Short communication

MOLECULAR STRUCTURE AND CONFORMATIONAL OF GASEOUS TETRAKIS(CHLOROMETHYL)METHANE, AS DETERMINED BY ELECTRON DIFFRACTION

EQUILIBRIA C(CH,Cl),,

MAY A. BRAATHEN and R. STPLEVIK Department Trondheim

of Chemistry, (Norway)

University

of Trondheim,

AVH, Rosenborg,

N-7000

(Received 1 July 1985)

Gaseous C(CH,Cl), was studied earlier by ED [1] at 105°C. Two conformers, D and S, were detected (Fig. 1). The two conformers were present in equal amounts at that temperature. The high-energy conformers with parallel C-Cl bonds have been described in our previous study [ 11. We decided to study the conformational composition at higher temperatures. Unfortunately, the compound dissociated at temperatures higher than ca. 210°C. The results obtained at 195”C, based on essentially the same experimental conditions and data reductions as described previously [ 1] , are reported here. Electron diffraction photographs were made in the Oslo apparatus [ 2 ] . The force field of ref. 1 with torsional force constants 0.27(D) and 0.97(S) mdyn a (rad)-2 were used in the calculation of vibrational quantities. Mean amplitudes of vibration (u) and K values were calculated at 195°C and included [1] in the least-squares refinements. Several u-values were also refined, and their values were essentially equal to the calculated values. The average deviation between the two sets was less than 8%, while the average relative uncertainty of the refined u-values was 12%. The values are consistent with those reported in ref. 1. Results from the least squares refinements and standard deviations (a) corrected for correlation in the experimental data are given in Table 1. The uncertainty in the electron wavelength is also included in the u-values. Intensities in the s-range 1.50-30.50 A-’ was used in the final refinements with equal weights. Results for C(CH2C1)4 at 105°C [1] and 195” C, as well as average values corresponding to 15O”C, together with the ED results [3] for C(CH2Br)4 at 14O”C, have been included for comparison between the two molecules. In these molecules the carbon-atom frameworks of both conformers show small deviations from tetrahedral arrangement. The CCX angles are greater than llO”---111” which is normal for C-CH2X groups. Also, in the molecules (ClH,C),C(CH,), and (C1H2C)$CH3 the CCC1 angles are greater than normal, 0022-2860/86/$03.50

o 1986 Elsevier Science Publishers B.V.

168

x,

H;

Fig. 1. Conformers D and S of C(CH,X),. S has S, symmetry.

Conformer D has D,,-J symmetry and conformer

TABLE 1 Structure parameters for C(CH,X),

compounds.

Standard deviations are in parentheses (0)

Temperature (“C)

105

195

150

140

Halogen, X=

Cl

Cl

Cl

Br

Bond lengths, rs-type in a C-H 1.132(9) c-c 1.541(4) c-x 1.794(3) Bond angles, La-type in degrees ccx 113.5(4) CCH 108.0(10) ClC2C3(D) 107.1(9) ClC2C3(S) 111.9(9) XCH 108.1(10) HCH 110.9(10) ClC2C4(D) 111.2(9) ClC2C4(S) 108.3(9)

1.128(13) 1.548(4) 1.792(2)

113,.8(3) 108.9(10) 106.5(9) 112.5(9) 107.3(5) 109.5(10) 110.0(9) 108.0(9)

1.130(8) 1.545(3) 1.793(2)

113.7(3) 108.5(7) 106.8(6) 112.2(6) 107.8(5) 110.2(7) 110.6(6) 108.2(6)

1.090(50) 1.554(9) 1.951(8)

114.2(8) 111.7(32) 108.7(g) 110.3(9) 105.8(16) 109.5(20) 109.9(9) 109.1(9)

Torsional deviation parameter in S conformer 4.1(8) 2.9(7) *%J

3.5(5)

0.1(8)

Mole fraction of S conformer 0.50(l)

0.52(2)

0.42(2)

0.53(2)

with values 114.3” [4] and 113.9” [5], respectively. However, in these molecules the C-C bonds have normal lengths. The C-C bond lengths are greater than normal, while the C-X bond lengths are normal. For C(CH,C1)4 the torsion angles of conformer S show small deviations from exactly staggered values, but for C(CH,Br), the deviations are not significant.

169

----I

A_-

I

A

-_

2

___---

3

Fig. 2. Radial distribution was 0.002 AZ.

-__

4

5

curves for C(CH,Cl),

6

d)

at 195°C.

The artificial

damping

constant

The peak in the RD curve marked with X*.*X (AA) corresponds to the contribution from the internuclear distances X, .*.X3 and X4*.*X5 in conformer D. The peak marked with X* *lX (AG) corresponds to the contribution from the internuclear distances XI --*X4, X1 -*-X5, X3-**X4 and X3*.* X5 in the conformer S. Both conformers contribute at the peaks marked X*.*X (GG), X..*C (g) and X*** C (a). Thus the relative amounts of the conformers were easily determined. Using the expression R In K = AS - AE/T with K = x(D)/x(S), and assuming that the conformational difference in energy AE = E(D) -E(S) aswell as the entropy difference AS = S(O) - S(S) are temperature-independent quantities, their values were calculated from the data at the two temperatures. The values with estimated error limits are AE = -0.5 * 0.5 kcal mol-’ and AS = -1.2 f 0.9 cal mol-’ K. Molecular-mechanics calculations on C(CH,X)4 molecules with X = F, Cl, Br will be presented in a separate paper. ACKNOWLEDGEMENTS

We are grateful to cand. real. Al Almenningen for his help in recording the electron diffraction data. REFERENCES 1 2 3 4 5

R. 0. S. R. R.

St@levik, Acta Bastiansen, 0. Rustad and R. Stdlevik, Acta Stdlevik, Acta

Chem. Stand., Ser. A, 28 (1974) 327. Hassel and E. Risberg, Acta Chem. Stand., 9 (1955) 232. Stdlevik, Acta Chem. Stand., Ser. A, 30 (1976) 209. Chem. Stand. Ser. A, 28 (1974) 455. Chem. Stand., Ser. A, 28 (1974) 612.