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R. H. HAUGE,J. W. HASTIE,AND J. L. MARGRAVE
Ultraviolet Absorption Spectra of Gaseous SnF, and PbF,
by R. H. Hauge, J. W. Hastie, and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas 77001
(Received April 86, 1968)
Absorption spectra in the ultraviolet region have been assigned to gaseous SnFz and PbFz. A vibrational analysis of the SnFz bands gives values of 180 and 120 cm-I for the bending frequencies in the ground and excited electronic states, respectively, with the O,O,O-O,O,O transition at 40741 cm-". The bending frequencies of PbFz are estimated to be 145 and 105 cm-1 by extrapolation from o!her group IVa difluorides. An absorption continuum assigned to molecular PbFz has a maximum at 2435 A, and the O,O,O-O,O,O transition is estimated to be 40,560 j: 100 cm-l.
Introduction Spectra attributed to gaseous tin and lead difluoride molecules have not previously been reported. From studies of the spectra of carbon,l silicon,2 and germanium3 diff uorides, one would expect absorption spectra for SnFz and PbFz to exist somewhere above 2200 8. It has been shown from mass spectroscopic studies4 that molten SnFz and PbFz vaporize partly as molecular SnFz and PbFz. In the case of SnFz there is some dimerization so that the vapor at 343" is approximately 80% SnFz and 20% (SnF& with a detectable amount of trimer. For PbFz there is considerable disproportionation, and at 715" the vapor composition is approximately 66% PbF4, 25% PbFz, 2% PbF, and 7% Pb.
Experimental Section The experimental apparatus was the same as that previously used for obtaining absorption spectra for GeFZa3Anhydrous samples of SnFz and PbF2, contained in nickel boats (purity >99%), were vaporized at successively higher temperatures. For SnFz, absorption spectra first appeared at 340" and became quite intense at a temperature of 400". At the lower temperatures some weako regular banded structure could be seen in the 2425-A region. These bands have been measured and analyzed by analogy with the other group IVa difluoride spectra, and the vibrational quantum number assignment is given in Table I. Figure 1 shows a microphotometer trace of the observed absorption bands with their vibrational assignment. At the higher temperatures and concentrations, the spectra appear continuous. For PbF,, absorption spectra first appear at approximately 640". At this temperature a continuum absorption is observedowith an intensity maximum at approximately 2435 A. As the temperature is raised, absorption spectra due to PbF become increasingly intense. Also the absorption continuum increases in intensity and width. Unfortunately, no band structure The Journal of Physical Chemistry
was observed, but it is most likely that this continuum is due to the t'riatomic PbFz species. Table I : Deslandres Table for SnFz Spectra" IJ',
v", om-11
7
om - 1
0
0 1 2
3 4
41,101 (2432.3), 124 41,225 (2425 0), 117 41,342 (2418. l ) , 122 41,464 (2411.0)
177
182
I
5
6
173
187
7 a
40,804 (2450.0), 120 40,924 (2442.8), 119 41,043 (2435.7), 126 41,169 (2428,3), 108 41,277 (2421.9), 120 41,397 (2414.9)
2
163
172
186
40,641 (2459.8), 111 40,752 (2453. l ) , 105 40,857 (2446.8)
Values in parentheses are band centers in Etngstroms; all
other values are in cm-1.
Discussion The absorption spectra assigned here to SnFz and PbFz were actually first observed by Jenkins and RoChester6 and Rochester6 but were attributed to the (1) B. A. Thrush and J. J. Zwolenik, Trans. Faraday SOC.,59, 582 (1963). (2) V. M. Khanna, G. Besenbruch, and J. L. Margrave, J. Chern. Phys., 46, 2310 (1967). (3) R. Hauge, V. M.Khanna, and J. L. Margrave, t o be published, J. Mol. Spectrosc., in press. (4) K. Zmbov, J. W. Hastie, and J. L. Margrave, Trans. Faraday Soc., 64, 861 (1968). (5) F. A. Jenkins and G. D. Rochester, Phys. Rev., 52, 1135 (1937). (6) G. D. Rochester, Proc. Roy. SOC.,A167, 567 (1938).
ULTRAVIOLET A.BSORPTION SPECTRA OF GASEOUS SnFz AND PbFz
c a 0 n m
I
2443
I
1
2415
I
I
I
I 2378
Figure 1. Microphotometer recording of SnFz absorption spectrum.
respective monofluorides. The current assignment of these spectra to the difluorides is more reasonable, both because they occur at lower temperatures in the absence of known monofuoride spectra, particularly for the case of SnF2, and because they are similar to the known difluoride spectra of other group IVa elements. The vibrational assignment for the measured bands of SnF2 is based on its close analogy to GeF2 at similar concentrations. From the analysis of Table I, the best measure of the bending frequency of the ground electronic state is 180 cm-1 and for the excited electronic state is 120 cm-l, with the usual uncertainty of f 5 cIy1-l. The O,O,O-O,O,O transition is predicted by extrapolation from the measured bands to occur at 40,741 cm-I. It is of note that both Ge and Sn difluoride band spectra are accompanied by some continuous absorption, and in fact P b difluoride shows no discrete band spectra. It was not possible to determine whether the continuous absorption is due to some dissociative pro-
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cess or is in fact a pseudocontinuum caused by absorption from higher vibrational levels resulting in considerable band overlapping. Some dissociation might be expected from a comparison of the thermochemical measurements of the energy necessary to remove a fluorine atom from the difluoride with the energy of maximum continuous absorption. The energy at which absorption maxima occur, if due to dissociation, must indicate an upper limit to the energy of dissociation. For GeF2, SnF2, and PbFz, these energies are 5.62, 5.13, and 5.09 eV, respectively, whereas the corresponding known thermochemical dissociation energies4 (for the F-MF bond in MF2) are 4.56 f 0.25, 4.95 i 0.4, and4.51 f 0.3 eV. However, it also seems possible that the absorption spectrum is a pseudocontinuum caused by the high temperatures and the relatively low bending frequencies. The fact that CF2 and SiF2 do not show any absorption continua at low temperatures seems t o argue against the suggestion of photodissociative phenomena occurring for the other group IVa difluorides. The bending frequency (vZ) of PbF2 for the ground and excited electronic state can be estimated by extrapolating from the measured v2 of the other group IVa difluorides. A plot of the respective v2 against measured7psand estimated reciprocal AF-F internuclear distances gives VZ” = 145 cm-l and v2’ w 105 f 10 cm-’. By analogy to separations of O,O,O-O,O,O bands and continuum maxima for Ge and Sn difluorides, the O,O,O-O,O,O band position for PbFz is estimated to be 40,560 f 100 cm-I.
Acknowledgments. This work was supported by the United States Atomic Energy Commission and by the Robert A. Welch Foundation. (7) V. M. Rao, R. F. Curl, P. L. Timms,and J. L. Margrave, J. Chem. Phys., 43, 2557 (1965). (8) F. X. Powell and D. R. Lide, Jr., ibid., 45, 1007 (1906).
Volume 79,Number 10 October 1968
