E. WICKE
358
Vol. 56
THE DISSOCIATION ENERGY OF FLUORINE BY E. WICKE Institute of Physical Chemistry, University of Gottingen, Gottingen, Western Germany Received JanuarU 1, 1961
Chemical bonds of abnormal strength are formed by combination of fluorine with nearly all other elements. Most of these bonds are of an ionic character because of the extreme density of the electron distribution in fluorine. Ionic binding forces must be unimportant for bonds between the fluorine atoms. Thus a relatively small dissociation energy for the fluorine molecule might be expected. Whereas direct spectroscopic methods proved to be unsuccessful, combination of the spect,roscopically determined dissociation energies for C1F and C11 (60.3 kcal./mole and 57.2 kcal./mole, resp.) with the heat of formation of C1F ohtained from calorimetric measurements, yields 33.4 kcal./mole and 40.2 kcal./mole for the dissociation energy of the fluorine molecule. The first value results by making use of a heat of formation of CIF of 15 kcal./mole, the second by inserting 11.6 kcal./mole. The results of accurate thermal conductivity measurements on gaseous fluorine by means of the hot wire method using pure nickel tubes and filaments showed no dissociation influence on the heat transport and its pressure dependence up to 500". Sensitivity and reproducibility were considered to be sufficient to rule out data smaller than 40 kcal./mole. Probably the energy of dissociation of fluorine has to be assumed to be somewhat higher than 40 kcal./mole but not to be beyond 45 kea]./ mol&
1. Introduction Among all halogen molecules fluorine is until now the only one to which the well known spectroscopic methods of determining the dissociation energy are not applicable. Discrete absorption from the ground level of the molecule hitherto could not be observed, probably the excited state referred to is merely a repulsive one without potential minimum.' Making use of the continuous absorp tion of fluorine and the other halogens, von Wartenberg some 20 years ago extrapolated the dissociation energy of fluorine t o D(Fz) = 63 kcal./mole.2 Meanwhile a series of indirect determinations as well as estimations has been carried out, which yield an appreciably lower value, even lower than the dissociation energy of chlorine. The mentioned indirect methods are bmed on spectroscopic determinations of the dissociation energy of diatomic fluorides. If these are gaseous, for instance C1F and HF, then the relation holds 0.5D(F2) = D(XF)
- &r(XF)- 0.5D(Xz)
(1)
with Qf = thermochemical heat of formation of the fluoride considered. As far as metal fluorides are concerned, such as RbF, CsF, TlF, the equation may be written 0.5D(F2)
D(XF)
+ L(XF) - L(X) - &f(XF)(2)
wherein L(XF) and L ( X ) are the heats of vaporization of the fluoride and the metal, respectively. The statement which can be made hitherto by evaluating these equations is summarized in Table I. Disregarding ClF, among these values, the spectroscopic dissociation energies of the fluorides and their heats of vaporization are especially uncertain. According t o equations (1) and (2) these insecurities enter with double amount into the values of the dissociation energy of fluorine. Nevertheless regarding the conformity of the figures in the last column of Table I the dissociation energy of this molecule should apparently lie between 40 and 50 kcal./mole. Another possibility to estimate this value is offered by half-empirical relations between the dissociation energy and the vibrational frequency as well as the internuclear distance of diatomic (1) R. S. Mulliken, Rev. Jlod. Phys., 4, 1 (1932). (2) H. von Wartenberg, G. Sprenger and .I, Taylor, Z. physik. Chem., Bodensten-FeBtband, 61 (1931).
TABLEI VALUESOF D(Fz) FROM EQNS.( 1 ) AND (2) Energies in kcal./mole at 18"; uncertain values bracket,ed XF
D(XF)
Qr(XF)
D(Xz)
11.64 64.5 133.2 131.7 76.8
58.0 104.2
C1F
60.53 HF (140)5 RbF' (125.5) CsF6 (130) TIE"' GllO
.,.
L(XF)
L(X)
....
.... . ...
....
(52.2) (45.7) (29)
... ...
D(Fz)
40 (47) 18.9 (51.2) 18.8 (50.4) (39.7) (
