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ALANSNELSON
A series of measurements of differential capacity a t different frequencies (100, 300, 1000, and 3000 cps) was made, and a dependence on the frequency was observed at potentials more positive than -0.7 v (us. nce), Consequently, the above calculation was also carried out by using the differential capacity values extrapolated to zero frequency, but the discrepancy was not diminished a t all. The reasonable explanation for this has not yet been given. However, the results in Figure 9 account qualitatively for the behavior of coalescence and repulsion of mercury droplets in K I solution. From the results that the critical potential rl.0in the symmetrical case was -50.5 mv (corresponding to E- = -128 mv), which is marked by the horizontal chain line, for the pure K F solution of 0.025 31 (in the absence of specific adsorption), it may be expected that no coalescence takes place a t any poten-
tial E in the K I solutions more concentrated than 5 X M and coalescence occurs in the K I solutions less M . This expectation is supdilute than 1 X ported by the experimental results as previously described. Thus, the qualitative interpretation of the coalescence of mercury droplets in the presence of specific adsorption of I- ions is possible in terms of the potential of the outer Helmholtz plane a t the mercurysolution interface. Acknowledgment. This investigation was supported in part by a Grant-in-Aid for Cooperate Research from the Ministry of Education. We are grateful to Professor Akira Watanabe of Kyoto Technical University for his valuable discussions and to Assistant Professor Tsuneo Ikegami of the Chemical Research Institute of n’on-Aqueous Solutions of Tohoku University for his help with assembling our devices.
Infrared Spectrum of AlF,, Al,F,, and AlF by Matrix Isolation
by Alan Snelson IIT Research Institute, Chicago, Illinois 60616 (Received February
1.A 1967)
The matrix isolation technique has been used to obtain the infrared spectrum of AIF3, AlzFs, and AIF. Orientation effects observed for A1F3 trapped in an argon matrix aided in the assignment of some absorption bands. The following are the three infrared-active frequencies of AIFI: v4(E’)270 cm-’, vg(E’) 965 cm-l, and v2(A2”) 300 cm-’. Six frequencies are attributed to the dimer A12Fa. Assuming a planar bridge structure, point group D2h, the frequencies are assigned as follows: vs(B1,) 995 cm-’, v9(Blu)340 cm-’, v13(Bzu) 660 cm-l, m(B3u) 805 cm-‘, v17(Bsu) 575 cm-’, v18(B3J 300 cm-l.
Introduction Mass spectroscopic investigationsly have shown aluminum trifluoride vaDor at about 1000°K to consist Of the ’pecies and A12F6,the latter being present to the extent Of Or 2%* The hightemperature gas-phase infrared spectrum of AlF3 has been reported by Ruchler3 and Margrave, et d 4 The matrix spectrum has been Observed by Linevskys and Snelson6but is not reported in the open literature. No T h e Journal of Physical Chemistry
data are available on the dimer species. The current investigation by matrix isolation was initiated to fill (1) R. F.Porter and E. E. Zeller, J . Chem. Phys., 33, 858 (1960). (2) A. Buchler, “Study of High Temperature Thermodynamics of Light Metal Compounds,” Progress Report KO.3, Arthur D. Little, Inc., Cambridge, Mass., 1962. (3) A. Buchler, ref 2, Progress Report N o . 2. (4) L. D. McCory, R . C. Paul, and J. L. Margrave, J . Phys. Chem., 67,1986 (1963).
INFRARED SPECTRUM OF AIF,, A12Fs, AND AlF
BY
MATRIX ISOLATION
-
this gap, and also supply comparison spectra for a projected study on the mixed species LiAlF,. The spectra obtained initially for AlF, were more complex than anticipated and the possibility that the species AlF might be present in the vapor was investigated.
Experimental Section The matrix isolation cryostat, molecular beam furnace, and experimental procedures used in this investigation were essentially the same as that described by the author in ii previous paper.’ Only those details peculiar to the present experiments will be given. The aluminum fluoride was reagent grade supplied by Baker and .Idamson. It was dehydrated prior to being loaded into Knudsen cells, which were made of either graphite or platinum. The sample was heated at about 575’ to observe the spectrum of the saturated vapor, depositions lasting from 1 to 2 hr. To observe the spectrum of the undersaturated vapor, superheating at 1400’ was used. The species hlF was formed by heating spectroscopically pure aluminum metal with A1F3 a t about 800’. A variety of Al:AlF3 ratios was tried, the only effect on the spectrum being a slight enhancement in the intensity of the AlF3 absorption bands in mixtures rich in the latter compound. Most spectra were obtained with hI/H >4000 for neon and argon and >2000 for krypton. Under these conditions, good isolation of the species resulted. Typical deposit:on rates of halide and matrix gas were 8-50 X 10-lo and 5-25 X mole/sec, respectively. Spectra were recorded on a Perkin-Elmer 621 spectrophotometer. Frequencies are believed accurate to f1 cm-l. Results The spectra of saturated aluminum trifluoride vapor in matrices of neon, argon, and krypton are shown in Figure la-c, under conditions of good isolation. There are two groups of strong absorption features at approximately 940 and 260 cni-‘ in all matrices, together with six relatively weak absorption bands at A, C, D, E, F, and G. The spectrum of aluminum trifluoride vapor obtained under poor conditions of isolation is shown in Figure Id, and the intensity of these six weak absorption features is increased relative to the strong absorptions a t 940 and 260 cm-I. The former are attributed to polymeric specier. The spectrum of the superheated vapor gives further support to this assignment since only the strong absorption features remained, in addition to implying that the species responsible for the weak absorption bands must be vaporizing from the sample, rather than being formed in the matrix because
I
I
3203
H
-
a)Nean Matrix
-0.8
- 0.4
M / H e 7000
G
F
C
E D
P
I
A
I
.
H
0
Y
c
I
1
dlNeon Matrix M/H E 1000
40.8
- 0.4 I
2 00
300
800
600
900
cm-1
Figure 1. Infrared spectrum of the vapors over .41F3.
of poor isolation. The absorption features at B, H, and I are assigned to monomeric aluminum trifluoride, and those a t A, C, D, E, F, and G t o the dimer. Because the intensities of the latter bands were rather weak, relative intensities measurements could not be used to add further support to this assignment.
Vibrational Assignments 1. AZF3. Aluminum trifluoride vapor has been studied by electron djff ractions and infrared spectroscopy3 at high temperatures. The molecule has a planar configuration of Dsh symmetry, analogous to boron trifluoride. The three infrared-active frequencies have been assigned as follows: v 2 ( A 8 ” ) 297 cni-l, v3(E’)935 cm-’, and vd(E’) 263 cni-’. In Figure 1, the absorption feature at B is assigned to v3. In all matrices, this band shows considerable splitting, some of which may be due to the degeneracy of the mode being removed by the matrix environment, in addition to structure resulting from different trapping sites in the matrix. The bands at H (Y 284 cm-l) and I ( V 236 cm-l) in the neon matrix are assigned to v2 and v4, respectively, by analogy with the reported gas-phase frequencies. I n the krypton matrix only one absorption band occurs in this region and with greater intensity than might be expected for either u2 or v4 singly. I t is assumed that matrix effects have resulted in these two frequencies (5) AI. J. Linevsky, “Spectroscopic Studies of the Vaporization of Refractory Afaterials,” General Electric Space Sciences Laboratory, Contract Report AF33(615)-1150, Nov 1964. (6) A. Snelson, Report No. IITRI-C6013-6, 1964. (7) A. Snelson, J . P h y s . Chem., 70, 3208 (1966). ( 8 ) P. A Akishin, N. G. Rambidi, and E. Z. Zaswin, Kristullograjiya, 4, 186 (1959).
Volume 7 1 , Nzimber 10 September 1967
ALANSNELSON
3204
becoming coincident, rather than postulating that v4 has suffered a particularly large matrix shift resulting in it lying below 200 cm-', the long wavelength limit of the spectrometer. If the latter were the case, then v4(neon) - v4(krypton) > 56 cm-l. This is a considerably larger matrix shift than observed for v3(neon) - v3(krypton) 'v 20 cm-l. I n the alkaline earth fluoride^,^ for which data are available, the maximum difference between matrix shifts in neon and krypton for two frequencies of the same molecule is 12 cm-'. This suggests the above difference of >36 cm-' to be larger than usual, giving support to the initial assumption that the v4 and vz bands are accidently degenerate in the krypton matrix. The absorption bands in the aluminum trifluoride spectrum in an argon matrix a t H ( l ) and I are assigned to v2 and v4, respectively. An assignment for H(2), resulting presumably from matrix splitting of either v2 or v4, is not possible owing to the proximity of the three absorption maxima. However, an assignment for this peak may be inferred from a consideration of the orientation effects demonstrated in the spectra shown in Figure 2. In Figure 2a, the matrix surface is perpendicular to the direction of propagation of the incident radiation, while in Figure 2b, it is at an angle of about 40'. The ratios log (lo/l)b/log ( l O / I ) *for corresponding bands in the two spectra are: 1.15 (l), 1.19 (m), 1.92 (n), 2.02 (o), and 1.63 (p). The cross-sectional area of the analyzing beam in the spectrometer is slightly less than that of the matrix surface when the two are perpendicular to each other and rotation through a small angle results in an increase in the amount of material in the incident beam. This, in part, is responsible for all of the above ratios being >1, but is not the only factor involved since, if it were, the relative increase would be the same for all of the bands. The lack of constancy in the ratios is attributed to the aluminum fluoride molecules having certain preferred orientations in the matrix. The absorption band intensity for a given vibrating mode is proportional to [ @ , U / ~ Q ) A where ' ] ~ , ( b ~ / d Qis) the change in dipole moment with respect to the normal coordinate Q , and E is the electric field strength of the incident radiation, both p and E being vector quantities. The important feature in the present context is the scalar product; radiation with the electric vector perpendicular to the transition moment will not be absorbed. If the planes of all of the A1F3molecules are lying parallel to the window surface, a parallel beam of incident radiation perpendicular to the plane of the window would not interact with the v2 out-of-plane bending mode of AlF, since the electric vector of the radiation would be at right angles to the normal coordinate of The Journal of Physical Chemistry
t
(a) Matrix Surface 9Ooto Analyzing Beam in Spectrometer
Y
\
0
H
m
3
cm-1 Figure 2. Infrared spectrum of AIFI in an argon matrix.
this mode, and the vibration would have zero absorption intensity. The two degenerate E modes, however, will be active since their normal coordinates are parallel to the electric vector of the radiation. Rotation of the plane of the window to make an angle of less than 90' with the analyzing beam would result in v2 becoming active, since the above scalar product will not be zero. I n the experimental arrangement used in this work, the analyzing beam in the spectrometer is slightly convergent and polarizing effects occur in the spectrometer which make quantitative deductions from the observed ratios impossible. Qualitat,ively, the experimental results can be accounted for if it is assumed that trapping sites for AlF3 are preferred in which the plane of the molecule is more nearly aligned with the matrix surface t.han perpendicular to it. This is consistent with the relative increases in intensity being smallest for the bands a t (1, m) and p, the peaks assigned to the E modes v3 and v4, respectively, and greatest for the band a t n, the out-of-plane bending v2 mode. I n addition, the similarity in the relative increase for the ratios n = 1.92 and o = 2.02 suggests that both these bands may be assigned to the v2 bending mode. Further support for the above interpretation has been obtained in matrix studies on the planar molecules LizFt and BFs in this laboratory, for which intensity increases were found to be much greater for the out-of-plane bending modes than the in-plane stretching modes on rotating the plane of the matrix from an angle of 90' to about 40' with respect to the direction of propagation of the
INFRARED SPECTRUM OF AlF3, A12Fa,AND AlF BY MATRIXISOLATION
analyzing radiation. Weltnerg has also reported orientation effects in neon and argon matrices from electron spin resonance studies on NO2and C U ( N O ~ ) ~ . I n Table I the observed matrix frequencies of AIFB are listed together with their assignment to the symmetry species of the DSh point group. The estimated gas-phase frequencies in the last column were obtained as described in an earlier paper' from the values observed in the neon matrix. For the v3 mode, which exhibits considerable splitting, the neon matrix frequency was taken as 950 cm-l. Both the observed gas-phase values for v2 and v4 are in good agreement with the estimated frequencies reported here, which are probably accurate to *10 cm-'. The estimated and observed gas-phase frequencies for v3 differ by 30 cm-l which is larger than the estimated error of the former value. It is well known that the precise location of band centers in high-temperature infrared gas-phase studies is difficult, and since the frequency shifts in the three matrices show a regular trend toward the red, in the order neon < argon < krypton, it is believed that the estimated frequency, v2 965 cm-l, is a consistent assignment for this mode.
3205
on the dimer AlzFs but comparison with the other aluminum ha1ide~ll-l~suggests a planar molecule with Dzh symmetry. This configuration results in eight infrared-active frequencies belonging to the symmetry species Blu(vs, v9, VIO), BZu(v13, v14), and Bau(VM, V17, vI8). The frequency numbering is that given by Bell and Longuet-Hjggins.14 In the present investigation, only six absorption bands were recorded; their frequencies are listed in Table 11. One of the unobserved frequencies, vl0, an out-of-plane ring-bending mode, almost certainly lies below the long-wavelength limit of the spectrometer (200 cm-l). The same may be true for the second missing frequency, though the possibility that it lies above 200 cm-l and is unobserved owing to either low-absorption intensity or coincidence with other bands cannot be excluded. Table 11: Infrared Frequencies of A12Fe Estd gas-
Symmetry species
Figure designation
phase -0bsd Neon
freq, cm-1-
freq,
Argon
Krypton
em-'
A
979
973
971
995
F D
327 586 790 568 290
324 578 785) 581) 362 278
340 600
C E G
326 580 779) 7821 563 281
BI"
B*"
Table I : Infrared Frequencies of AlFa Figure designation
-0bsd Neon
freq, cm-1Argon Krypton
Estd gasphase freq, cm-1
B 3u
805 575 300
960 945
94 1
942
938
930
925
927
921
951
B
944
965
940 937 284
267
€I 276 256 I
244
300
244
270
256 247
252
The estimated gas-phase frequencies were used to calculate the force constants kl = 4.91 X lo5 dynes/ cm, k S / l 2 = 1.85 X lo4 dynes/cm, and lcA/12 = 3.24 X lo4 dynes/cm, using the formulas and nomenclature given by Herzberg.lO From this value of k l , the AI' symmetric stretching frequency was calculated a t 660 cm-l. 2. AlzFe. S o structural information is available
The vibrational assignment for the observed six infrared-active frequencies was made by comparison with the known spectra of other bridged XzY6niolecules, and also to give the best agreement between the observed and computed values using the equations given by Bell and Longuet-Higgins. In the latter calculations the outer AI-F bond distance was taken as 1.63 A, the same as in A1F3.* The bridge AI-F bond distance was chosen so that the ratio of hl-F(outer)/AI-F (bridge) bond lengths was equal to that in AI2Cl6.l6 The F-A1-F outer bond angle e2 was reduced slightly from the AlF3 value of 60' to allow for repulsion from (9) P. H . Kasai, W. Weltner, and E. B. Whipple, J . Chem. Phys., 42, 1120 (1965). (10) G. Hersberg, "Molecular Spectra and Molecular Structure," D. Van Norstrand Co., Inc., New York, N . Y., 1960. (11) H . Gerding and E. Smit, Z . Physik. Chem., 50B, 171 (1941). (12) E. J. Rosenbaum, J . Chem. Phys., 8, 643 (1940). (13) W . Klemperer, ibid., 24, 353 (1956). R.P. Bell and H . C. Longuet-Higgins, Proc. Roy. Soc. (London), A183, 357 (1945). (14)
(15) K. J. Palmer and N. Elliot, J . Am. Chem. Soc., 60, 1852 (1938).
Volume 7 1 , d$rumber10 September 1Y67
ALANSNELSON
3206
the bridge fluorine atoms. The magnitude of 201, corresponding to the F-Al-F angle in the ring, was chosen so that the two bridge fluorine atoms were just touching at their van der Waals radii. This contrasts with the geometry of Al2Cl6lsin which there appears to be considerable overlap of the bridge halogens. However, it does allay objections to putting the F-F and the A1-A1 ring interaction constants equal to zero, data being unavailable for their computation. The terminal A1-F bending (d2) and stretching (f2) force constants were assigned by comparison with AIF1. The remaining force constants were chosen to give the best fit between the observed and calculated frequencies. The frequency assignments and the results of the calculat,ion are given in Table 111. Agreement between the observed and calculated frequencies is about as good as can be expected on the basis of the above as-
Table 111: Force Constants (dynes/cm) and Frequencies (cm-1) of A12F6 fl
= 2.0
x
106 .A = 4 . 9 0
x
106
dl = 4 . 5 X lo4 dz = 2 . 0 X lo*
a = 1.80A
b
=
1.63 A
g1
=
0
g2 = 0
ds = 0
dc = 2 . 8 5 X lo4
0, = 49'
e2
Calcd
=
58' Obsd
939 602
191 874 326 980 349 970 259 645 232 336 860 550 315
995 340
600 805 575 300
Neon Matrix M/H I :6000
- 0.8 Argon Matrix
- 0.4
M I H " 5000
~
5
Krypton Matrix M/H = 3000
400
500
6 00
Figure 3. Infrared spect,ra of the vapors Over a mixture Of 2A1 and *lF3.
gl, the interaction constant, since this is the only mode
in which this constant is of importance. 3. AIF. The electronic spectrum of A1-F is well characterized,16 the vo + v1 vibrational transition of the ground state having a frequency of 793 cm-l. I n Figure 3 the spectrum of A1-F in neon, argon, and krypton matrices is shown. The position and frequency of the band a t A in the three matrices permits an immediate assignment to the monofluoride. Comparison of the matrix frequencies of this band (Table IV), with those of the band at C in the A1F3 spectra (Figure l), clearly indicates that different species are responsible for these bands in the two sets of spectra. This eliminates the possibility that the band at C (Figure 1) may be attributed to A1F formed owing to the presence of a metallic impurity in the AIFBsample.
Table IV : Infrared Frequencies of Aluminum Monofluoride and Polymers Molecular species
A1F
Point group
C,
Symmetry species
2'
Polymers
sumptions. The quotient fz/fi = 2.45 for the nonbridge to bridge Bl-F stretching constants is of the same order as in diborane (2.4) and AIzC16(1.9). The values of the bending constants dp, d ~ and , dd appear reasonable by comparison with those derived for BzH1e,14 but di is an order of magnitude larger. I n deriving the frequencies corresponding to the A, mode, only two of the possible four roots of the quartic equation were real. This may be due in part to assuming a zero value for The Journal of Physical Chemistry
900
800
cm-1
Figure
Y F r e q , cm-1-
desig-
nation
Neon
Argon
A B1 Bz Ba B4
785 439 405 420 455
776 432 404 415 453
Kvpton
773 426 400
The appearance of the A1F absorption band a t A in the neon, argon, and krypton matrices shows the same type of structure as monomeric lithium fluoride in the same mat rice^.'^^'^ This may be attributed to the (16) S. M.Naude and
T.J. Hugo,
Can. J . Phys., 35, 64 (1957).
INFRARED SPECTRUM OF AlF,, A12Fe, AND A1F BY MATRIXISOLATION
similar dimensions of the two molecules; for AlF, re = 1.65 A and for LiF, re = 1.56 A, allowing the same matrix trapping sites to be occupied in both cases. There is, however, a marked difference in frequency shifts for the two molecules; vgas - vneon matrix = 8 and 32 cm-' for AlF and LiF, respectively. In matrix studies, the largest frequency shifts are usually associated with molecules having large dipole moments resulting in strong dipole-induced dipole interactions with the surrounding rare gas atoms. Lithium fluoride ( p = 6.3 D.) is a molecule of this type, compared to AIF(p = 1.53 D.).19 Although repulsive and dispersive effects betlweenthe trapped molecule and the matrix also result in frequency shifts, for LiF and AIF they are likely to be of the same order of magnitude since the molecular sizes are similar. Hence it is probable that the major factor responsible for the large difference in the frequency shifts of these two molecules is the dipole-induced dipole interaction with the matrix. The absorption bands at approximately 950 cm-l are assigned to monomeric A1F3, a small amount vaporizing from the sample, together with the AI-F. The bands at B are assigned to polymeric or agglomerate material since their intensity relative to the monomeric band a t
3207
A increased markedly under poor conditions of isolation. Under the best conditions of isolation, it was possible to eliminate the peaks at B3 and B4 and a t the same time reduce the intensities of B1 and Bz,the maximum decreases occurring for Bz. This behavior implies that none of these bands are attributable to the same chemical species. Mass spectroscopic studies on the AIFa AI system show only the presence of the vapor-phase species AIF and A1F3. This is fairly strong evidence against the existence of polymers of the type (AlF),, but the possibility that a species exists whose fragmentation pattern does not lead to its detection cannot be excluded. In the present case it is believed that the polymers are being formed during the deposition of the matrix. However, the available evidence does not permit an assignment of these bands to specific molecular species.
+
Acknowledgment. The author gratefully acknowledges the support of the Army Office of Research and Air Force Office of Research in funding this research. (17) A. Snelson and K. S. Pitzer, J . Phys. Chem., 67, 882 (1963). (18) R.L. Redington, J . Chem. Phys., 44, 1238 (1966). (19) D.R.Lide, {bid., 42, 1013 (1965).
Volume 7 1 , Number 10 September 1967
