Structure of bromochlorofluoromethane by electron diffraction

Journal of Molecular Structure. 52 (1979) 63+9 d Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

STRUCTURE OF BROMOCHLOROFLUOROMETHANE DIFFRACTION

BY ELECTRON

E. JEAN JACOB The Bowman-Oddy Laboratories, Ohio 43606 (U.S.A.)

Department

of Chemistry,

University

of Toledo,

Toledo,

(Received 7 August 1978)

ABSTRACT Structure parameters for bromochlorofluoromethane, HCBrClF, were determined from gas phase electron diffraction intensities. Little useful information concerning the position of the hydrogen atom was obtained. The main carbon-halogen parameters, in an ‘a basis are: r(CF) = 1.348(5) A, r(CC1) = 1.753(5) A, r(CBr) = 1.927(6) A, LFCC~ = 109.6(6)“, LFCBr = 109.2(6)“, and LClCBr = 111.5(4)“. If the hydrogen were to be located equiangularly from the three CX bonds, an angle LHCX = 108.8” would result. Vibrational amplitudes based on published force fields were calculated and compared with the diffraction results. INTRODUCTION

Of the 34 possible halomethanes (excluding iodides) all but eleven have been the subject of direct structure determination of some kind. Since the early investigations of the 1930’s through 1950’s chemists have subjected several of these compounds to increasingly intensive diffraction and spectroscopic investigation as experiment and theory have advanced. Meanwhile, little effort has been directed to reducing the list of unstudied derivatives. Each of these latter contains at least one bromine together with one or more chlorine or fluorine atoms. The present investigation is the first report of the structure of bromochlorofluoromethane (HCBrCIF). It is in fact the first structure reported for any of the four bromochlorofluoro-derivatives. As one of the simplest chiral molecules, HCBrClF has been the subject of recent spectroscopic interest in connection with circular differential Raman experiments [l].A primary motivation for this work was to provide reliable geometric parameters for use in a normal coordinate analysis in support of this work 12, 33. EXPERIMENTAL

Diffraction photographs were made using the Oregon State University apparatus with 44.2-kV incident electrons, a calibrated ?-sector, and 8 X 10

64

in. Kodak Projector slide plates (medium). The sample reservoir was kept at -75 to -65”C, while the nozzle temperature was 25°C. The estimated vapor pressure of HCBrClF at the reservoir temperatures is l-3 torr. With a beam current of 0.40 PA, exposures of 1.25-2.00 min at the 750-mm and 2.53.5 min at the 300-mm camera distance were used. Incident electron wavelengths were calculated from accelerating voltages, these latter having been calibrated against the s-scale established by CO* diffraction patterns assuming r,(CO) = 1.1646 A and r,(O . . . 0) = 2.3244 8. The sample (racemic) was synthesized and purified in the manner described by Diem and Burow for their spectroscopic investigations [ 21. The sample was transferred under dry N, to the sample reservoir for the diffraction experiment. It was stored in vacua, refrigerated, and protected from light. All diffraction photographs were made within 48 hours of opening the original sealed ampoule in order to reduce the possibility of decomposition. The most likely impurities are Brz from photolytic decomposition, together with halogenated ethanes. No evidence of such species was seen in the data upon analysis. ANALYSIS

OF THE DATA

Data from four plates at the 750-mm and five plates at the 300-mm camera distance were used in the analysis. Usual procedures were followed for tracing the plates and for converting optical densities to total intensities. Intensities were interpolated to intervals of s = 0.25 A-’ and levelled by multiplying by s4. Data from the two camera geometries spannings values of 2.00-12.25 A-’ and 6.00-31.75 A-’ were used. For each plate the contribution to the total intensity from molecular interference terms was obtained by subtracting a smooth background [4-6] from the total intensities *. The difference curves, multiplied by an additional factor of s, are comparable to model curves calculated according to the right-hand side of eqn. (1). $1,

(s) =

k & AiAjrij”

exp (-lij2s2/2)

cos IQi -

qj I

sin rijs

(1)

In the above expression elastic electron scattering amplitudes multiplied by s2 and phase shifts are denoted by A and 7) respectively. Values for these quantities were obtained by interpolations of published values to 44 kV [6]. The exponential term in eqn. (1) represents the harmonic approximation for the vibrational damping envelope used in this analysis. Anharmonicity was also ignored in the argument of the sine term of eqn (1). Least-squares analyses were carried out simultaneously on average intensity curves for the two camera distances. A diagonal weight matrix with unit weights for residuals of $1,,,(s) was employed. In Fig. 1 the two experimental curves are compared with the theoretical curve for the final structure model. Radial distribution curves, though not used directly in structure refinements, *Levelled total intensities and background curves used to calculate molecular parameters of HCBrClF are available from BLLD as Sup. Pub. No. SUP 26111 (7 pages).

65

EXPERIMENTFIL

THEORETICAL

20

10

30 s

Fig. 1. Molecular intensities for HCBrClF. Difference curves are experimental minus theoretical.

were constructed in a manner previously described [ 71. For the curves shown in Fig. 2, molecular intensities were modified by multiplying both experimental and theoretical curves by Zc,Z,,/A c1A Br. The damping factor used was exp (-0.0025 s2).

THEORETICAL

0

DIFFERENCE

--

-

1

2

3

4A

Fig. 2. Radial distribution curves for HCBrClF. Vertical lines marking the contribution to n$iZj/r,y. The difference curve is experimental minus theoretical. rG are in length proportional

66 RESULTS

Bromochlorofluoromethane has no elements of symmetry. While nine independent coordinates are required to describe the relative positions of the nuclei, there are ten distances in the rij “spectrum”. Thus imposition of geometrical consistency among the internuclear distances is but a minor constraint, and angles derived will be close to true eg or 0, values. Several conclusions follow from examination of the radial distribution curves in Fig. 2. First, there are certainly insufficient resolved “features” in the f(r) curve to allow precise determination of all nine parameters. Second, relative contributions from the H . . . X distances are so small that valence angles involving the CH bond will be ill-determined even if the r(H . . . X) are well separated from other distances. Finally, the large error signal in the region of the CH peak may seriously affect the determination of r(CH) and amplitudes of vibration for both the CF and CH bonds. Results of least-squares analyses support these conclusions. The six parameters relating the non-hydrogen atoms are reasonably well determined by the data, as shown in Table 1. A very large uncertainty (kO.05 A) is associated with r(CH). Furthermore the CH amplitude could not be refined to any sensible value, and was constrained to a typical value, 0.0775 A. Nor could any parameters depending on r(H . . . X) distances be refined. The most promising one in terms of scattering power TABLE

1

Structure parameter for HCBrClFa*b

r&W r(CF)

r( Ccl) r( CBr) L FCC1 LFCBr LClCBr LHCX

1.13(5) 1.348(5) 1.753(5) 1.927(6)

1.350 1.755 1.929

109.6(6) 109.2(6) 111.5(4) [lOS.S]d

r(F . . . Cl) r(F . . . Br) r(C1. . . Br)

2.544( 7) 2.691(S) 3.044(4)

R, index of fitC

0.092

2.546 2.693 3.046

aDistances are reported in angstroms, angles in degrees. aValues in parentheses are estimated uncertainties, 20, in the right-most digit quoted for the respective parameters. Estimates of systematic errors and correlation among the observations are included. =R = t z r+i* /z wiZiz(obsd)] If2; Zj = siZ, (si) as defined by eqn. (1); Ai = Zi(Obsd) - Zi(CdC). &Value which places hydrogen equi-angularly from the three carbon-halogen bonds. See text. eDerived from ra and the diffraction amplitudes in Table 3 using the afiproximate expression rg = r, + l’/ra.

67

and resolution, namely r(H . . . Cl) (See Fig. 2), was sequentially fixed at values corresponding to a range of 106-112” for LHCCl, but the index of fit, R, (see Table 1) and the other parameters alike were insensitive to these changes in r(H . . . Cl). Within broad.1imit.s the hydrogen atom can be placed at will relative to the heavy atom grouping and still be consistent with the diffraction data. The HCX angle reported in Table 1 is the one which locates the CH bond equi-angularly from the three CX bonds, assuming the interhalogen angles to be those of Table 1. DISCUSSION

It is difficult to find other compounds which provide any useful comparisons for HCBrClF. Although it has, by virtue of its asymmetry, at least one parameter in common with each of the other halomethanes, by the same token only the other (unstudied) bromochlorofluoromethanes have more than superficial similarity to its heavy atom skeleton. Reliable comparisons are also hampered by the fact that presently available structure information on other halomethanes is in many cases imprecise and based on questionable assumptions for some of the parameters. It is moderately well established that for a given X, e.g. F, Cl, Br, r(CX) is longest in CHJX and shortest in CF3X among the halomethanes, with the range of r(CX) being greatest for X = F. Values between the respective extremes are expected for each of the CX bonds in HCBrClF. As seen from Table 2, this expectation is realized for X = F and Cl. Although the pattern of r(CBr) seems anomalous, the relatively large error estimates on the table entries can accommodate values which are in t.he expected order. In support of a lower value of r(CBr) in CF,Br than the one given, we note that the ra derived from diffraction data alone, and from which the r, in Table 2 was derived, is 0.010 A larger than the rAv derived TABLE 2 Comparison of carbon-halogen

bonds in selected halomethanes

r(CX) X=F CH,X HCBrClF CF,X

1.385(3)/O 1.350(5)/g 1.323(5)/ED

Cl

Br

1.781(2)/s 1.755(5)/g 1.751(4)/g

1.939(1)/s 1.929(6)/g 1.935(7)/g

aThe type of structure parameter is indicated after the solidus (/) of each table entry. 0, s, and g are the conventional subscripts for distance parameters. ED denotes an electron diffraction value whose meaning was not specified in the primary reference. bPrimary references for structures represented are as follows. CH,F: F. A. Andersen, B. Bak and S. Brodersen, J. Chem. Phys., 24 (1956) 989; CH,Cl, CH,Br: R. H. Scbwendeman and J. D. Kelly, J. Chem. Phys., 42 (1965) 1132; HCBrCIF: this study; CF,: L. E. Sutton, Tables of Interatomic Distances and Configuration in Molecules and Ions, The Chemical Society, London, Special Pub. No. 11, 1958, p. M105; CF,Cl, CF,Br: ref. 8.

68

from joint diffraction/microwave analysis [ 81. The (ra-r,‘) implied by the harmonic force field for this bond was only 0.0017 A. No other bond in the set CF,Br, CF$I, CFJ showed such a large discrepancy between the two values for Ar. Angles between the CX bonds in HCBrClF are unexceptional. In the mean (110.1”) they arevery similar to the 110.9” and 110.8” reported for the trihalomethanes CHC13 [9] and CHBr, [lo] respectively. Agreement between diffraction and spectroscopic vibrational amplitudes for halocarbons has proved to be erratic more often than not, for reasons not yet understood. The present study is no exception. Amplitudes for the CX bonds in HCBrClF are well within the rather large ranges of reported diffraction amplitudes in other molecules containing tetracoordinate carbons. Amplitudes calculated using the GVFF and the Urey-Bradley potential constants reported by Diem and Burow for HCBrClF are given in Table 3 for comparison with the diffraction values. All the diffraction amplitudes are larger than their spectroscopic counterparts. Some systematic error in the amplitudes involving fluorine was expected in view of the recently reported error in the elastic electron scattering amplitudes for fluorine [ 111. TABLE 3 Comparison between diffraction and spectroscopic

CH CF cc1 CBr F . . . Cl F Br Cl’.‘.‘. Br H...F H . . . Cl H . . . Br

amplitudes of vibration

Diffractions

GVFF”

Urey-Bradleye

[ 0.07751 0.054( 5) 0.054(B) 0.058(B) 0.069( 7) 0.081(8) 0.078(4) [O.llO] [0.114] [O.llO]

0.0775 0.0480 0.0516 0.0556 0.0633 0.0686 0.0727 0.100 0.106 0.115

0.0765 0.0466 0.0514 0.0511 0.0619 0.0685 0.0722 0.094 0.107 0.106

*Present study. Bracketed values were not refined. Values in parentheses represent estimated uncertainties, 20, in the right-most digit quoted. bRef. 2. CRef. 3. The force field based on harmonic frequencies for v , was used for calculating the above amplitudes.

ACKNOWLEDGEMENTS

D. F. Burow is gratefully acknowledged for his gift of the sample of HCBrClF. Financial support for this work was provided by a University of Toledo Faculty Research Award to the author and by the National Science Foundation under Grant GP-27763X to Oregon State University.

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REFERENCES 1 2 3 4 5 6

7 8 9 10 11

M. Diem, J. L. Fry and D. F. Burow, personal communication. M. Diem and D. F. Burow, J. Chem. Phys., 64 (1976) 5179. M. Diem and D. F. Burow, J. Phys. Chem., 81 (1977) 476. L. Hedberg, Abstracts, Fifth Austin Symposium on Gas Phase Molecular Structure, Austin, Texas, March 1974, p. 37. Inelastic scattering factors used in background calculations were taken from D. T. Cromer, J. Chem. Phys., 50 (1969) 4857, except those for hydrogen, which were taken from C. Tavard, D. Nicholas and M. Rouault, J. Chim. Phys., 64 (1967) 540. Elastic electron scattering amplitudes and phase shifts were interpolated to 44 kV from values tabulated by L. Schiifer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 55 (1971) 3055. A. Almenningen, 0. Bastiansen, A. Haaland and H. M. Seip, Angew. Chem. Int. Ed. Engi., 4 (1965) 819. V. Typke, M. Dakkouri and H. Oberhammer, J. Mol. Struct., 44 (1978) 85. P. N. Wolfe, J. Chem. Phys., 25 (1956) 976. Q, Williams, J. T. Cox and W. Gordy, J. Chem. Phys., 20 (1952) 1524. H. L. Sellers, L. Schiifer and R. A. Bonham,. J. Mol. Struct., 49 (1978) 125.