ALANSNELSON
250
Infrared Spectra of the Beryllium Halides by Alan Snelson I I T Research Institute, Chicago, Illinois
60616
(Received July 18, 1967)
The infrared spectra of BeC12, BeBrz, and Be12 have been observed in the spectral range 4000-200 cm-l using the matrix isolation technique. Two frequencies were recorded for the chloride and bromide and one for the iodide. Assuming a linear geometry for these molecules, the frequencies were assigned as follows: BeC12, v3 = 1135 and v2 = 250 cm-'; BeBr2, v 3 = 1010 and v2 = 220 cm-I, and Be12, v3 = 873. The unobserved and infrared inactive frequencies have been estimated for all the beryllium halides.
Introduction In a recent investigation' using the matrix isolation technique, the infrared spectra of beryllium fluoride and beryllium chloride were reported. Two frequencies were observed for the former in the spectral region 4000200 cm-' and were assigned to v3 and v2. For the latter, only one frequency v3 was observed. The entropy of BeF2 calculated using this frequency assignment agreed well with that obtained experimentally from thermodynamic data. A similar entropy calculation for BeC12in which the bending frequency (v2 = 240 cm-l) was estimated by assuming the ratios of the stretching to bending force constants to be the same in both BeFz and BeC1, was not satisfactory. Using the thermodynamic data for BeC12, v2 may be calculated at 150 cm-I, suggesting that the technique of transferring force constants ratios in similar molecules may be unsatisfactory for BeC12. Subsequent studies in this laboratory, however, have showri that the single-beam spectrometer (Perkin-Elmer 12B) used in the beryllium chloride investigation had rather poor resolution in the spectral region 250-200 cm-I, and that absorption bands in this region could have been missed. If this were the case for BeC12, then the thermodynamically derived entropy would be in error. To resolve these difficulties, a reexamination of the beryllium chloride spectrum was undertaken, using a Perkin-Elmer 621 spectrophotometer. I n addition, the spectra of beryllium bromide and iodide were also 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 a previous paper,' and only those details peculiar to the present experiments are given. Beryllium chloride was supplied by A. D. Mackay and was purified prior to use by sublimation in a stream of hydrogen chloride. Beryllium bromide and iodide were prepared by passing analytical reagent grade bromine and iodine vapor, respectively, over heated beryllium The Journal of Physical Chemistry
metal powder. The latter was supplied by K and K Laboratories with an indicated purity of 99%. The halides were sublimed once prior to use. Graphite effusion tubes were used to contain the halides, and in all cases the unsaturated vapor was super-heated a t about 700" to decrease the concentration of polymeric species being trapped in the matrix. Research grade neon and argon, supplied by Msltheson Co., were used to form the matrices. Deposition rates of halide and matrix were of the same order as in previous experiments. The spectra were recorded on a Perkin-Elmer 621 infrared spectrophotometer. The reported frequencies are believed accurate to 1 cm-'.
*
Results Beryllium Chloride (Figure 1A). Under the best 6000 for a neon matrix, conditions of isolation, M/H two strong absorption bands occurred at a and d, together with two very weak features at b and c. The latter were comparable in intensity to the bands at a 500). and d under poor conditions of isolation (M/H In addition, other features also appeared. The bands a t b and c are assigned to polymeric material. This may have been present in the beryllium chloride vapor ; the degree of superheating used in the experiment being insufficient to dissociate the polymeric material completely. Alternatively, polymeric material may have been formed due to incomplete isolation of the monomer during the trapping process in the matrix. The ratios of the intensities of the bands at a and d under various conditions of isolation remained constant to within 5% and these two features are assigned to monomeric beryllium chloride. Beryllium Bromide (Figure 1B). The spectrum of beryllium bromide in a neon matrix is analogous to that of beryllium chloride. Two strong and two weak absorption bands at a, d and b, c, respectively, appeared under the best isolation conditions. The latter increased markedly in intensity under poor conditions of isolation. The features at b and c are assigned to poly-
*
(1) A. Snelson, J . Phys. Chem., 70, 3208 (1906).
INFRARED SPECTRA OF
O,\
THE
251
BERYLLIUM HALIDES
o"m = 7000
7I
A
0.0
cm-1
quencies to v2. The one frequency observed for beryllium iodide is assigned to v3. In Table I, the observed matrix frequencies are listed, together with estimated gas-phase values obtained using the simple procedure indicated previously.' I n the beryllium iodide spectrum, the main maximum of the absorption band in the argon matrix lies at a higher frequency than the comparable maximum in the neon matrix. This contrasts
0. -
Table I : Frequencies of BeC12, BeBr2, and BeIz --BeCIs--
-BeBrn
,b
C
7AO
560
4hO
8AO
9dO
cm-'
!hd
Neon Argon Estimated gas phase frequencies Q
:::f--ji\.....:... M / H = 7000
w 0 -
0.0
800
900
1
I
1000
cm-'
800
900
1000
Figure 1. Infrared spectra of beryllium halides: A, beryllium chloride in a neon matrix; B, beryllium bromide in neon and argon matrices; C, beryllium iodide in neon and argon matrices.
meric material, and those a t a and d to monomeric beryllium bromide. The spectrum of beryllium bromide vapors in an argon matrix is similar to that in neon except that, the absorption band at d in the latter matrix does not appear. This is presumably due to a larger frequency shift in the argon matrix resulting in this band lying below 200 cm-'. Beryllium Iodide (Figure 1C). The spectrum of beryllium iodide was recorded only under conditions of good isolation. In both the neon and argon matrices, an absorption feature with several maxima occurred a t about S70 cm-I. These maxima are believed to be due to matrix effects, and the band is assigned to monomeric beryllium iodide.
Discussion using the Recent studies by Klemperer, et electric quadrupole deflection of molecular beams to detect dipole moments, indicate that all the beryllium halides are linear symmetric triatomic molecules. This type of molecule has two infrared-active vibrations, assigned to va, the asymmetric stretching mode, and Of the four absorption bands v2, the bending mode. attributed to monomeric beryllium chloride and bromide from the experimental data, the two higher frequencies are assigned to va, and the two lower fre-
238 250
Be12
VI
Y2
Y8
va
1122 1108" 1135
207
993 985 1010
878,872 877,867 873
Y2
220
Seeref 1.
with the behavior of the other alkaline earth halides for which matrix spectra are available, the frequencies shifting further to the red in the order neon < argon < krypton. Beryllium iodide appears to be undergoing a blue shift, and the estimated "gas-phase frequency" in Table I is simply taken as the average of the four matrix frequencies. In Table 11, the vibrational frequencies and force constants of all the beryllium halides are listed. In Table I1 : Valence Force Constants of the Beryllium Halides -Cm-1-VI
BeFz BeCl2 BeBrz Be12 a
(680)b (390) (230) (160)
-106
k.
VP
345 250 220 (175)
1555 1135 1010 873
The values were estimated.
5.15 3.28 2.53 1.96
dynes/omk/V
0.12 0.07 0.06 0.04a
kI/(k/Z')
43 47 42 44a
* Calculated values.
the last column, the ratios of the stretching to bending force constant for the fluoride, chloride, and bromide are seen to be of similar magnitude. This gives some confidence to the estimated value of the bending force constant calculated for beryllium iodide using the mean of the observed values for the other halides, kl/(k6/Zz)=
44. The values of vl were calculated using the simple valence force field approximation; however, a better estimate of these frequencies might be obtained if the (2) L. Wharton, R. A. Berg, and W. Klemperer, J . Chem. Phys., 39,2023(1963). (3) A. Buchler, J. L. Stauffer, W. Klemperer, and L. Wharton, ibid.,39,2299 (1963). (4) A. Buohler, J. L. Stauffer, and W. Klemperer, ibid., 40, 3471 (1964).
Volume 72,Number 1 January 1088
ALANSNELSON
252 interaction constants klz were known. Hoare5 have shown that the expression t
- r dA-B)
(~A-B
k12
= 3kl
r
Linnet and
t
A-B
is obeyed fairly well for triatomic molecules, rtA--B and TdA-B being the bond lengths between the atoms A and B in the triatomic and diatomic species, respectively. The bond lengths in all the beryllium dihalides were determined by Akishin, el u Z . , ~ but for the monohalides only the value for BeF has been observed experimentally.’ It is thus necessary to estimate bond lengths for the remaining diatomic beryllium halides. For BeCl, the relationship given by Hershbach and Laurie,* relating the internuclear distance, r , to the vibrational force constant, k, may be used. From spectroscopic datag on BeCl, k = 3.01 X lo5 dynes cm-l. Substituting into the expression r = 2.02 0.53 log k, a value of 1’ = 1.77 A is obtained. A similar calculation for BeF with k = 5.75 X lo5 dynes cm-l and r = 1.73 - 0.47 log k, gives r = 1.37 A, in good agreement with the experimental value of 1.361 A, and suggests that the value of r calculated for BeCl is probably good to about 1%. Estimation of the bond lengths for BeBr and Be1 cannot be made using the same approach since no spectroscopic data are available for these species. Instead, use is made of a relationship which appears to hold for some gaseous diatomic halides. The ratios of the bond distances MFIMC1, hlF/MBr, and MF/MI in certain groups of halides are quite constant. For M = Na, I(, Rb, and Cs, the above ratios fall in the range 0.811 f 0.006, 0.767 f 0.004, and 0.711 f 0.004, respectively. The same ratios for M = Li and A1 are 0.774 f 0,001, 0.721 i 0,001, and 0.654 i 0,001. Unfortunately, the only other diatomic halides for which accurate bond distances are available are those of hydrogen. The ratios are, 0.719, 0.649, and 0.572, which do not fall into either of the other two groups. The ratios for the alkali metals, Na, K, Rb, and Cs are constant to approximately f 1%,while those for Li and A1 are good to about 0.5%. The ratio of the bond lengths of BeF/ BeCl is 0.769 which, within the accuracy of the estimated bond length for BeC1, suggests that the diatomic beryllium halides are similar to the lithium and aluminum species and enables the bond lengths of BeBr and Be1 to be calculated at 1.89 and 2.08 A, respectively, with an estimated error of *3%. In Table 111, the above data on the beryllium halides are summarized. The frequencies of the diatomic
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The Journal of Physical Chemistry
Table I11 : Molecular Constants of the Beryllium Halides Bond
BeF BeCl BeBr
Be1
7 , A, diatomio
r , A, triatomic
1.361 1.77 1.89 2.12
1.43 1.77 1.92 2.08
(atri
- adi),
0.07 0.00 0.03 -0.04
V,
A
diatomic
1247 835 (760) (680)
species in parentheses were calculated using the estimated bond lengths and the formulas of Herschbach. They are believed to be accurate to *50 cm-’. The difference in bond lengths rtri - rdiis significant for the BeF bonds, but the values of 0.00, 0.03, and -8.04 A for BeCI, BeBr, and Be1 can probably best be taken to indicate that within the estimated error of the bond lengths, the differences are close ta zero and do not justify making a calculation of klz. The value of klz calculated for BeFz is 0.76 X lo6 dynes cm-l resulting in v1 = 770 cm-l. Using this frequency assignment, the calculated entropy of BeFz is 68.1 eu at 880”K, agreeing with the observed value of 69.0 2.0 eulO at the same temperature. A similar calculation for the entropy of BeClz gives 66.6 eu a t 500“ compared to observed values of 69.2 f 2.0 or 68.2 & 2 eu.10 It is clear that the spectroscopic data favor the lower value, and that the agreement between the calculated and observed values, though not as good as with BeFz, is satisfactory. Finally, it must be stressed that in view of the uncertainties in the methods of estimating bond lengths and frequencies for the beryllium halides, the values of the infrared inactive symmetrical stretching frequencies vp, reported here for the triatomic species must be regarded as somewhat speculative until more accurate data are available.
Ackuowledgment. The author gratefully acknowledges the support of the Air Force Office of Scientific Research in funding this study. (5) J. W. Linnet and M. F. Hoare, Trans. Faraday SOC.,45, 844 (1949). (6) P. A. Akishin, V. P. Spiridonov, and G. A. Sobolev, Dokl. Akad. Naulc SSSR, 118,1134 (1958). (7) G. Herzberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic hlolecules,” D. Van Nostrand Co., Inc., New York, N. Y.,1950. (8) D. R. Herschbaoh and V. M. Laurie, J. Chem. Phya., 35, 458 (1961). (9) M. M. Novikov and L. N. Tunitskii, Opt. Spectry., 8,396 (1960). (10) D. L. Hildenbrand, L. P. Theard, E. Murad, and F. J u , Aeronutronic Final Report No. U-3068,1965.
