MOLECULAR STRUCTURE OF TRIFLUOROETHANOL
June 20, 1956
alkylnaphthalenes higher than amylnaphthalene. The values of McClellan and PimentelZ for naphthalene are included for convenience. These authors estimate t h a t their values are probably accurate to A0.5 cal. per degree mole. The values for the substituted naphthalenes are subject to additional uncertainty. We estimate the uncertainty [CONTRIBUTION FROM
THE
DEPARTMENT O F CHEMISTRY
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to range from 0.5 for the simpler derivatives a t the lower temperatures to 1.0 for the larger molecules a t the higher temperatures. Acknowledgment.-We wish to acknowledge the aid of Mr. Donald E. Petersen in assisting with some of the calculations. BERKELEY 4, CALIF.
AND THE P U R D U E
RESEARCHFOUNDATION, P U R D U E UNIVERSITY]
An Electron Diffraction Investigation of the Molecular Structure of Trifluoroethanol' BY R. L. LIVINGSTON AND G. VAUGHAN RECEIVED JANUARY 9, 1956 The molecular structure of trifluoroethanol has been investigated by electron diffraction using the visual corrtlation procedure. Th: structural parameters as determined by this investigation are as follows: C-F = 1.34 zk 0.02 A , , C-C = 1.52 =k 0.05A.. C-0 = 1.41 =k O . O 5 f i . , L F C F = 108.5 =t 1,5",and LCCO = 110 =t 4".
Introduction There has been considerable interest in the effect of halogen atoms on the carbon-carbon bond distances in simple organic halides. Early results2 gave a C-C distance of about 1.45 A. in CzFe and CFpCHBbut more recent work indicates that these distances are considerably longer, probably between 1.50 and 1.54 L4.3'4The determination of the structure of trifluoroethanol was undertaken to determine further the effect of fluorine atoms on the C-C distance and, in addition, to determine any effects on the C-0 bond length and the CCO angle.
VIS
Experimental
K3
The sample of trifluoroethanol used in this work was purchased from the Minnesota Mining and Manufacturing Company. A 60-1111. sample was rectified in a glass helices packed column which was equivalent to fifteen theoretical plates. Ten milliliters of the middle fraction, b.p. 77.7 =k 0 : l " (uncorrected), was collected for use in preparing the diffraction photographs. The infrared spectrum of this fraction showed no spurious features when compared with the spectrum from a sample with a known purity greater than 99%. The diffraction photographs were obtained in the usual manner5 using a camera designed and constructed by Professor H. J. Yearian of the Purdue Physics Department. Twelve satisfactory photographs of varying density were obtained from the sample described above, using a camera distance of 108.2 mm., an electron wave length of 0.05923 A. and Eastman Kodak 33 plates. Visual interpretation and measurements of the patterns were obtained out to a qvalue of approximately 95 from three of the best plates. Interpretation of the Diffraction Pattern.-The visual correlation method5j8and the radial distribution method73 were used in the interpretation of the diffraction pattern, The measurements of the patterns are summarized in Table I. The qo-values are based upon measurements of each feature by two independent observers. The qualitative (1) Contains material from the Ph.D. thesis of G. Vaughan, Purdue Research Foundation Fellow in Chemistry, 1951-1953. ( 2 ) A survey of electron diffraction results through 1949 is found in the tabulation by P. W. Allen a n d L. E . Sutton, Acla Cryst., 8 , 46 (1950). (3) J. L. Brandt a n d R. L. Livingston, THISJOURNAL,76, 2096 (1954). (4) J. L. Brandt, Ph.D. Thesis, Purdue University, 1952. (5) L. 0. Brockway, Rev. Modern Phys., 8 , 231 (1936). (0) L. Pauling and L. 0. Brockway, J . Chem. Phys., 2 , 867 (1934). (7) L. Pauling and L. 0. Brockway, THISJOURNAL,67, 2684 (1935). ( 8 ) P A . Shaffer, V. Schomaker and L.Pauling, J . Chenr. P h y $ . , 14, G.59 (194G).
EF3 Q3 Q2 K4 J2 K2
E3
G3 u2 L2
I
,
I 20
,
I
40
I
I
60
,
I 00
I
I I00
Fig. 1.-Radial distribution, visual intensity and theoretical intensity curves for trifluoroethanol.
Vol. 78
R. L. LIVINGSTON AND G. VAUGHAN
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TABLEI pc/qo VALUESFOR ACCEPTABLE MODELS OF TRIFLUOXOETHANOL Feat 11 re Max. M I D
1
1 2 2 3 3 4 4 5 5 G 6
PO
14 18 25 29 32 36 42 46 54 58 63 68
7
i2 i6
8
81 85 90 94
7 8 9 9 Wt. mean Av. dev.
78 50 89 54 88 45 08 18 66 92 57 09 86 65 79 72 43 52
Q2
Fa
FK:
Ks
KLs
Model KQr
EFd
EFa
K4
Wt.
Q3
0.966 0.961 0.962 0.964 0.963 0 , 9 6 4 0.963 0.960 0.964 0.962 1.038 1.045 1.044 1.042 1.041 1.038 1.045 1.038 1.049 1.031 0.981 0,981 0 . 9 8 1 0 , 9 8 1 0,981 0.981 0.981 0.980 0.982 0.981 1.027 1,016 1 , 0 1 9 1.022 1,019 1.022 1.018 1.018 1.018 1 . 0 2 4 1 . 0 2 7 1.011 1.014 1.021 1.014 1.022 1.022 1 . 0 2 1 1.016 1.023 1.003 0.993 0.996 1.000 0.996 1.000 1.003 0.999 0.998 0.996 0.996 0,983 0,984 0.987 0 , 9 8 7 0.988 0.987 0.985 0.989 0.989 1.014 1.025 1,016 1 , 0 1 3 1,013 1.012 1.013 1.010 1.008 1.019 1.017 0 . 9 9 8 1.004 1.001 1.018 1.028 1 . 0 0 3 1.010 1.015 1 014 1.038 1 , 0 1 5 1,020 1.029 1.022 1.032 1.005 1.015 1 . 0 1 7 1,024 1.011 1 . 0 0 5 0.982 0 , 9 9 4 1.014 1.002 1.014 0.983 1.000 0.960 0.983 0.973 0 976 0.979 0.980 0.988 0.974 0.986 0.951 0.990 0 997 0.989 0.992 0.994 0.994 0.996 0.984 0.992 0.984 0.996 1.013 1 , 0 1 6 1.015 1.014 1.015 1.014 1.016 1.016 1.015 1.011 1.021 1 , 0 1 4 1.016 1.018 1.016 1.018 1.013 1 . 0 1 1 1.019 1.013 1.013 1.009 1 . 0 0 8 1 , 0 1 4 1 . 0 1 1 1.012 1.009 1.012 1 . 0 1 3 1 012 0.997 1 , 0 0 0 1.001 1.004 1.000 1 . 0 0 1 1 . 0 0 5 1.004 1.001 0.999 1.001 1.004 1.005 1 , 0 0 7 1 , 0 0 3 1.005 1.010 1.008 1.007 1.001 1.012 1 , 0 0 3 1 , 0 0 6 1.010 1.007 1.010 1.002 1.005 1.002 1.009 f 0 . 0 1 2 rtO.010 1 I ~ 0 . 0 1 0rtO.009 3 ~ 0 . 0 0 9rtO.009 rtO.010 ~ t 0 . 0 0 75 0 . 0 1 4 1 0 . 0 0 8
appearance of the visual curve in Fig. 1 was drawn according to the interpretations of the patterns given by three independent observers, who were in close agreement on interpretations of all features. The shapes of the features in the interval q = 0 to q = 22 were copied from the most acceptable model, as is customary, in order to give a satisfactory radial distribution curve. The visual appearance of the fourth maximum is very similar to features which appear on the diffraction pattern of carbon dioxide. Before the indicated shape was assigned to this maximum, a careful examination was made of carbon dioxide patterns which had been previously obtained in this Laboratory. The qualitative shape of the features for carbon dioxide is known with certainty as a result of independent spectroscopic investigation~.~~'0 A careful comparison of the doublet in the interval q = 5.5 to q = 70 with a similar doublet occurring in the diffraction pattern of 1,1,l-trifluoroethane led the observers to conclude that the sixth minimum was real. T h e patterns of this latter compound were particularly useful for the compzrison since both sectored and unsectored photographs of this compound were a ~ a i l a b l e . ~ The radial distribution function, Fig. 1, for trifluoroethanol was calculated using the equation*
where I(g)"is the intensity read from the visual curve, Fig. 1. The value of b was determined by setting exp ( - b g 2 ) = 0.1 a t q = 95. The curve shows four major peaks, each of which is generated by more than one interatomic distance. T h e peak a t 1.37 A . corresponds to the C-F, C-C and (2-0 distances, that a t 2.19-2.34 b. to the F . , F , C..Q, C.,F, C..H and O..H distances and those a t 2.83 and 3.53 A. to the O..F and H..F distances. Due to the complex nature of the peaks, no attempt was made to resolve the distances; it is to be noted, however, that the positions of the lines corresponding to the interatomic distances determined from the acceptable models agree favorably with the peaks observed in the radial distribution curve. A complete determination of the structure of the trifluoroethanol molecule involves the evaluation of ten parameters if the usual assumptions of symmetry are made for the CFa group and only rigid models are considered. Preliminary investigation showed that varying the vibration factors from 0 to m for all terms for interatomic distances involving H atoms had a negligible effect on the theoretical intensity curves; these factors mere accordingly set equal to infinity for all of these distances. !
