2927
J. Phys. Chem. 1991, 95, 2927-2931 Multiplying both sides of (B2) by exp(t/rH) we obtain
,’exp(t/rH)
+ Q/TH)
Z(td) = exp(
exp(t/rH) = f ( t )exp(t/TH)
We can use the identity d(xy)/dr = i y d b exp(t/rH))/dt
+ x,’
which gives
eXP(f/TH)
which can then be integrated to determine the concentration y = [H(t)] at a given time t = 1’: I’
Y exp(t/TH)If=V =
y(t3 = eXp(-f’/TH)
f(t) exp(t/rH) dr +
L f d k texp( .
[
2)
x ‘ n t ) exp(
x
i) ] dt
exp(
5)
dt’ (B5)
The integral in (B3)can be approximated by %ti)
i=O
c
x‘hf)e X p ( t / r ~ )dt + c
):
.
exp(ti/TH)h
which when substituted into eq B3 gives m
(B3)
At t = 0 the value of the integral is zero, leaving y(t? = C. The boundary condition y(t? = 0 exists which implies that C = 0. The rate of product formation may be written
Assuming [DCI] remains constant over time we have k[DCI] = k’, and (B4)may then be written in a form analogous to (B2): 2 + z/7H k’y(f)
W ( t m ) I = exp(-tm/TH)x.Lf(fi) exP(ti/r~)hl
(B6)
i=o
where h is the width of the interval used in the summation. In this study the summation was performed using 0.5-ns increments. Similarly, the integral in (BS) can be approximated by
to give
and solved to give a t the time of detection, t = td: Z(fd) = eXp(-fd/Tp)xfdk’y(t‘) eXp(f’/rp) dt’ Substituting in (B3) we obtain an expression for the product concentration a t time tdr [P(td)] = Z(td):
where [P(t,)] can be either [D(t)ld or [Cl(t)]d and TD
or
T~
is either
TCI.
Matrix Isoiatlon and Mass Spectrometric Studies of the Vaporization of Alkali Metal Oxoseienium Salts: Characterization of Molecular M,SeO,, M2Se0,, and MSeO, Alan K. Brisdon, Robin A. Gomme, and J. Steven Ogden* Departmen t of Chemistry, The University, Southampton SO9 5NH, UK (Received: October 5. 1990)
Solid samples of ,alkali metal selenates (M2SeO4)and selenites (M2Se03)have been heated in vacuo, and the vaporization products studied by using a combination of mass spectrometricand matrix isolation IR techniques. Although some decomposition was observed, evidence was obtained for the existence of the ternary molecular salts M2Se04and M2Se03 (M = Na, K, Rb, Cs) and also for the novel Se(II1) species M W 2 . The molecular symmetries are identified as Dzd,C, and C, respectively, on the basis of vibrational selection rules, and the various 0-Se-0 bond angles in these species are estimated from selenium isotope shifts and relative band intensities.
Introduction Until relatively recently, our knowledge of alkali metal sulfates and selenates has largely been confined to the condensed phases, and the chemistry of these salts has been extensively documented. However within the past 20 years, it has been found that anhydrous sulfates can be sublimed in vacuo to yield molecular M2S04 species. These ternary molecules were originally identifd by mass spectrometry,’ and subsequent electron diffraction studies indicated that their structures are based on bisbidentate coordination, leading to Du symmetrya2 This model was later supported by matrix isolation
Corresponding experiments on the vaporization of alkali metal sulfites do not appear to have been carried out, but it has been shown that under matrix conditions, the cocondensation of T120 and SOz yields molecular TI2So3.’ The third known member of this series of molecular sulfur oxo anion salts is provided by the electron-transfer reaction between an alkali metal and SO2 in low temperature matrices. This reaction leads to ion pair molecules of general stoichiometry MS02.6 In contrast to these studies on sulfur oxo anion salt molecules, nothing is known about the existence of selenium analogues. Alkali metal selenates and selenites reportedly undergo complex thermal decompositions on heating,’** leading to a variety of solid products,
(1) See, e.&: Buchler, A.; Stauffer, J.; Klemperer, W. J . Chem. Phys.
1967, 46, 605.
( 2 ) Spiridonov, V. P.;Lutoshkin. B. 1. Vesr. Mark.Uniu. Ser. Khim. 1970,
No.6. 509. (3) Atkins, R. M.; Gingerich, K. A. Chem. Phys. LLrr. 1978, 53, 347. (4) Belyaeva. A. A.; Dvorkin. M. 1.: Shcherba, L. D. Opr. Specrrarc. 1971, 31, 309.
(5) David, S. J.; Auk, B. S. Inorg. Chem. 1984, 23, 1211. (6) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971. 55, 1003. (7) Giolito, I.; Ionashiro, M. Thermochim. Aera 1980, 38, 341. (8) Selivanova, N. M.; Schneider, V. A.; Streltsov, I. S. Rum. J . Inorg. Chcm. 1959, 4, 667. Selivanova, N. M.; Schneider, V. A,; Streltsov, I. S. Russ. J. Inorg. Chem. 1960, 5, 1101.
0022-3654191/2095-2927$02.50/0 0 1991 American Chemical Society
Brisdon et ai.
2928 The Journal of Physical Chemisrry, Vol. 95, No. 7, 1991
'"-T 1100 I
,
I
I
,
I
300
I
,
I
,
l
cm-l
Figure I. Nitrogen matrix IR spectra obtained from the vaporization of alkali metal selenates: (a) low-resolution spectrum from Na2Se04;(b) low-resolution spectrum from K2Se04;(c) low-resolution spectrum from Rb$e04; (d) low-resolution spectrum from Cs$3e04;(e) high-resolution spectrum from Na2Se04before diffusion; (f) after diffusion; (g) calculated spectrum.
including Se02, but there appears to have been no attempt to monitor the vapor species directly. The principal aim of this research was therefore to investigate the in vacuo vaporization of a range of alkali metal selenates and selenites and to characterize any molecular species produced. Matrix isolation IR spectroscopy was employed as the primary experimental technique, but supporting studies were also carried out using molecular beam mass spectrometry. In addition to these vaporization studies on the oxo anion salts, experiments were also carried out in which an alkali-metal atom vapor was cocondensed with molecular Se02.
Experimental Section The samples of alkali metal selenates and selenites used in this work were either obtained commercially (Cs2Se04 Pfaltz and Bauer, K2Se04 and Na2Se03 BDH, Na2Se04 HW), or were prepared by reacting the appropriate metal carbonate with either H2SeOd9(to yield M2Se04) or with solid S e 0 2 (Fluka), which leads to M2Se03formation on fusion. All solid samples were subsequently vaporized from small platinum boats contained in an alumina tube heated inductively via a tantalum susceptor. In the experiments involving deposition, samples of the alkali metal and Se02were heated resistively in separate Pyrex tubes mounted radially in close proximity. The principal features of our matrix isolation apparatus have been described previously.I0 In the present experiments, we employed a V-shaped double central window cooled to ca. 12 K as the condensing surface in order to enable continuous spectral monitoring during deposition. Both nitrogen and argon (BOC, 99.999%) were used as matrix gases, but spectral quality was generally better in nitrogen and could often be improved by controlled diffusion to ca. 30 K followed by recooling. Spray-on times were typically 60-90 min, and IR spectra were recorded by using a Perkin-Elmer PE983G instrument interfaced with a data station. Mass spectra were obtained by using the VG quadrupole models SXP400 and SXP600 operating in the cross-beam mode within the ionization energy range 20-70 eV. Results Matrix 1R spectra obtained from the vaporization of alkali metal selenates yielded absorptions that not only showed an obvious correlation from metal to metal but that could also be obtained from the corresponding selenites. It is therefore convenient to discuss the low-resolution spectra obtained from these systems together and to combine the frequency data in a single table. (9) Toul, F.; Dostal, K. Coll. Czech. Chem. Commun. 1951, 16, 531. ( 10) See, e.g.: Ogden, J. S.;Wyatt, R. S.J. Chem. Soc., Dolton Trans. 1987, 859.
. _ . II I I
3w I
I
I
I
I
I
> c m - l
Figure 2. Matrix IR spectra obtained from the vaporization of alkali metal selenites: (a) low-resolutionspectrum obtained from Na&03 (N2 matrix); (b) low-resolutionspectrum obtained from Cs2Se03(N2matrix); (c) calculated spectrum, assuming all OSeO angles lolo; (d) calculated spectrum, assuming angle parameters in Table 11; (e) high-resolution spectrum of A" band of Cs2Se03(Ar matrix); (f)calculated spectrum.
Figure la-d shows typical nitrogen matrix IR spectra (1 100-300 cm-l) obtained from samples of the selenates M3eO4 ( M = Na-Cs). Although these four survey spectra show significant differences in relative band intensities, there is a large degree of consistency regarding band positions, and it is evident that at least some of the bands are correlated. Many of these features lie in the spectral regions characteristic of Se-0 stretching and bending modes, and by comparison of relative intensities in the stretching region, at least six different species may be distinguished. These, however, are not all new. The absorptions a t ca. 967,928, and 372 cm-I arise from matrix-isolated Se02,11while the band a t 662 cm-' is identified as the bending mode in C02. The feature a t ca. 1000 cm-' has previously been reported in cocondensation reactions between alkali metals and oxygen and has been attributed to adducts of the type M(O2)".I2 The remaining absorptions, however, seem to be unique to these systems, and we believe that they arise from ternary oxo anion species. Nitrogen matrix spectra obtained from NalSe04 (Figure la) typically show an intense doublet at ca. 894/881 cm-I, which has obvious counterparts in the other three spectra. This doublet may also be shown to correlate with two absorptions in the bending region at 438 and 383 cm-I, and this species is denoted A. Although species A is present in the Cs2Se04system (Figure Id), it appears only as a minor component, and the spectrum is now dominated by two bands at ca. 780 and 695 cm-I. These are correlated with the weaker features at ca. 798,417, and 395 cm-' and are denoted B. The third species C is identified by the rather broad feature centered at ca. 750 cm?, and in this series of spectra appears most prominently in Figure IC. Attempts to obtain mass spectrometric data from N a w , were not successful owing to the high decomposition pressure, but the three remaining systems all showed ion peaks corresponding to the species M2Se03+,indicating that ternary salt molecules are present in these high-temperature vapors. These mass spectra (1 1) Cesaro, S.N.; Spoliti, M.;Hinchcliffe, A. J.; Ogden, J. S.J. Chem. Phys. 1971,55, 5834. (12) Smardzewski, R. R.; Andrews, L. J. Chem. Phys. 1972,57, 1327; J . Chem. Phys. 1973,58, 2258.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2929
Vaporization of Alkali Metal Oxoselenium Salts
TABLE I: Vibration Frequencies (cm-I) Observed" for Alkali Metal Oxoselenium Species Iwhted in Nitrogen Matrices
1
I/ Y I
1100
'
I
'
I
'
I
'
I
I
cm-'
300
Y
Figure 3. Nitrogen matrix IR spectra obtained from cocondensation of
K atoms with Se02: (a) low-resolution spectrum; (b) high-resolution
spectrum; (c) calculated spectrum.
were however dominated by signals due to 02+, Se02+,and Se+, and elemental selenium was observed to condense on the off-axis walls of the glass furnace during sample vaporization in both these studies and in the matrix experiments. Vaporization of the corresponding selenites showed many of the features present in Figure 1 and also a few minor differences. In particular, the Na2Se03system yielded two relatively intense shoulders in the Se-0 stretching region at ca. 815 and 806 cm-l. These are assigned to a fourth species D, and a typical spectrum is shown in Figure 2a. In contrast, the vaporization of Cs2Se03 (Figure 2b) yields essentially only B. Finally, a series of experiments was carried out in which Se02 was codeposited with sodium or potassium vapor into a nitrogen matrix. These experiments were prompted by the indication, from M(02), formation, that alkali-metal atoms are produced in the vaporization of the ternary salts, and the possibility that at least some of the observed bands might be due to reaction between alkali-metal atoms and Se02to form species analogous to M+SOy. Figure 3a shows a typical spectrum obtained when potassium atoms are codeposited with Se02. As anticipated, prominent absorptions are found at 967,928, and 372 cm-l due to unreacted Se02, but the bands assigned to species D at a.815 and 806 cm-' are now essentially the only other features in the spectrum. A new bending mode at ca. 395 cm-' could also be associated with D as a result of these experiments, and bands due to this fourth species were subsequently also found in the rubidium and cesium systems. In fact, closer inspection of the spectrum obtained from Rb$e04 (Figure IC) shows the presence of D as a weak shoulder on the highest frequency absorption of B. These experiments have therefore identified at least four new sets of absorptions, and Table I summarizes the frequencies observed in nitrogen matrices for the different cation systems. This tabulation shows that for some of these bands, notably the bending modes, there is a significant cation dependence thus supporting their assignment to ternary species. However, it was also noted that small variations in frequency of up to ca. 3 cm-' sometimes occurred unpredictably between experiments on the same system. Shifts of this magnitude are outside normal experimental error, and we believe that they may be caused by varying amounts of oxygen evolved during sample vaporization, resulting in a somewhat variable environment for the trapped species. For this reason, the bands listed in Table I are quoted with somewhat larger error limits than might otherwise be expected. However this phenomenon did not prevent useful high-resolution data from being obtained, and prior to such studies, it was found that controlled diffusion in nitrogen to ca. 30 K generally produced the best spectra. The one exception to this guideline was one particular argon matrix experiment in which the intense feature at ca. 780 cm-l (B) arising from Cs2Se03showed (Figure 2e) fine structure that was better resolved than in nitrogen. In general, however, argon matrices yielded somewhat broader features shifted
Na
K
Rb
894 881 438 383 803 783 691 412
894 881 445 389 801 784 690 426 377 815 806 395
894 881 440 385 800 783 693 422 385 814 806
815 806 382
Cs 894 880 438 377) 798 780 695 417 395} 812 807 399
species
assgt Estr B2 str
.
A (.M S e 0 3
B (M2Se0,)
( A'str izztd A" str
D(MSeOZ)
( :'End A" bend Ai str BI str AI bend
Low-resolution data: frequency accuracy h2 cm-'. by up to ca. 5 cm-l from their nitrogen counterparts, and controlled diffusion produced little change other than a gradual decrease in band intensity. The main reason for carrying out high-resolution studies was to confirm the presence of selenium isotope structure on these new bands and to use the isotope shifts quantitatively to gain some indication of molecular structure. It is convenient to discuss the interpretation of these isotope patterns in the context of the general spectral assignment presented below. Spectral Interpretation and Discussion The mass spectrometric and low-resolution IR studies both indicate that the new species A-D may be identified as molecular selenium oxo anion salts, and a preliminary assignment of species A and B is afforded by comparing their IR absorptions with those found in solid-phase spectra of alkali metal selenates and selenites. For selenates, the IR-active T, stretch in the "free" ion is reported" at 875 cm-I, while for selenites, peaks at 807 and 737 cm-' have been assigned as the AI and E modes, respectively, of the pyramidal (C3J S e O P moiety.I4 On the basis of these relative band positions in the solid materials, we therefore provisionally identify the species labeled A as molecular selenates and the species B as selenites. Species A. By analogy with the previous studies on alkali metal sulfate m ~ l e c u l e salkali , ~ ~ metal selenate vapor species are expected to adopt a D2d structure in which the central SeO, unit is coordinated via oxygen bridges to the two cations. For this molecular shape, it may be shown that the Se-0 stretching modes transform as AI + B2 E. The B2 and E modes derive from the T2 mode of the free ion and are expected to occur close together in frequency and to have an approximate intensity ratio of 1:2 in the IR. The AI stretch is not IR-active, but Raman studies on the SeOP ion in solution13have located this mode a t 833 cm-I. As indicated in Table I, all four alkali metals yield a characteristic doublet at ca. 894/881 cm-I, which is associated with A, and on the above basis, we assign the more intense 894-cm-' component as the E Se-0 stretching mode in molecular M2Se04 and the 881 cm-' as the B2. Figure le,f show these bands in the sodium system under high resolution, before and after diffusion. Comparison of these two spectra shows that whereas the 881-cm-I feature remains essentially unchanged, the 894-cm-' band becomes considerably less complex as a result of this process and ultimately is seen to consist of several sharp components, which correspond closely to the isotope pattern found for elemental selenium. This behavior is consistent with the above spectral assignment, since it is a relatively common feature of nitrogen matrix environments that they induce a small splitting in degenerate modes.15J6 If one assumes that the Se-0 stretching modes may be separated from the remaining vibrations in the molecule, then it may
+
Landolt-Bornstein, Pysikalisch-Chemische Tabellen 1951, No. 2. Siebert, H. 2.Anorg. Allg. Chem. 1955, 275, 225. Brisdon, A. K.; Ogden, J. S.J . Mol. Srrucr. 1987, 157, 141. Beattie, I. R.; Ogden, J. S.;Price,D.D.J . Chem. Soc.,b l r o n Trans. 1982, 505. (1 3) (14) (15) (16)
2930
Brisdon et al.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Figure 4. Possible structures for molecular selenates and selenites: (a) DU model for M2S&4, with definitions of parameters; (b) C, bismonodentate model for MzSe03;(c) C, monobidentate model for M$e03, with definitions of parameters; (d) C, bisbidentate model for M2Se03.
be shownI6 that the secular equations for the stretching modes simplify to AA,
ABI
= cf, +fn + 2f,d)(l/MO)
= cf, - f r s - 2frd)[(l/MO)
(1)
+ 4(1/MSe)
cos’
(2)
AE = Vr -fn)[(l/Mo) + 2 ( 1 / M d sin2 61
(3)
and that the IR intensity ratio based on the bond dipole mode1’6*17 is given by IE = tan2 6
IB*
[
(Msc
+ 2M0 sin2 6 )
(M&+ 4Mo cos2 6)
]
(4)
The force constant and angle parameters are defined in Figure 4. From the measured selenium isotope splittings, eqs 2 and 3 above yield estimates for the angle 20 of 118’ and 112’, respectively, while eq 4 also yields 112’. The estimated uncertainties in each case are fca. 5’, and in the subsequent analysis, we used a mean value of 115’. By combining our matrix IR frequencies with the solution Raman value of 833 cm-I for the AI mode, initial estimates of the force constant parameters were obtained and subsequently refined in a more complete vibrational analysis that included the two bending modes. This required an arbitrary assignment of the bands at ca. 438 and 383 cm-’ as E and B2 modes, respectively, but a reversal of this assignment had a negligible effect on the appearance of the stretching region. Table I1 compares the observed and calculated frequencies for molecular Na2Se04, and the calculated isotope patterns in the Se-0 stretching region are reproduced in Figure lg. As indicated above, a mean value of 115’ was used for 26 in these calculations, and although the error limits are somewhat large, this value is higher than that found in sulfates (ca. 109°)2 and significantly higher than the 104-105’ range reported for molecular chromates, molybdates, and Selenium isotope structure was also obtained for the species A bands found in other alkali-metal systems, and a similar vibrational analysis resulted in almost identical molecular parameters. Species B . Absorptions belonging to this species have been provisionally assigned to the molecular salts M W 3 . As indicated earlier, for the isolated C3, anion, the Se-0 stretching modes transform as AI E, and these modes lie at 807 and 737 cm-I, respectively. However, coordination to two cations via oxygen must inevitably split the degeneracy, and three possible structures for molecular M2Se03are shown in Figure 4. One is based solely on monodentate coordination, while the other two involve bidentate
+
(17) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrutions; McGraw-Hill: New York. 1955. (18) Ugarov, V. V.;Ezhov, Yu.S.;Rambidi, N. G. Z h . Srrukr. Khim.
1913, 14, 358.
TABLE II: Observed and Calculated Isotopic Frequencies (em-’) for Molecular N a B O , ( D u ) ,Cs@e@ (C#),and KSeOI (C,) obsd freq calcd freq assgt 898.9 Na2Se04(in N2)’ 897.4 897.4 896.0 896.0 893.2 893.2 890.4 890.6 886.9 885.4 885.5 883.8 883.9 881.0 881.0 878.2 878.3 785.9 Cs2Se03(in Ar)b 784.9 783.8 783.9 782.0 782.0 780.2 780.3 8 17.8 KSe02 (in N2)C 816.9 8 16.0 815.8 814.5 814.5 8 13.0 813.2 810.4 810.4 809.2 809.3 808.3 808.3 806.3 806.3 804.4 804.6
’Assuming DU symmetry, with 28 = 115’ and force constant parameters/, = S.696,fd = O.OOZ,f, = -0.150,mdyn/A andf, = 1.501, f v = 1.866,and f w = 0.04 mdyn A/rad2. bAssuming C, symmetry, with a = I O I O , fl = 1 1 5 O , and force constant parameters Fd = 4.33,F, = 4.73,Fd = 0.74,and F,, = 0.1 mdyn/A. CAssumingC, symmetry, with 26 = 102’ and force constant parameters K, = 5.14,K,,= 0.204 mdyn/A and fe = 2.0 mdyn A/rad2. binding, and the highest symmetry attainable in each case is C,. For a selenite ion in these environments, the AI and E Se-0 stretches of the free ion reduce to A’ and A’ + A’’. The vibrational problem is thus similar to that encountered in an earlier description of the anion modes in molecular carbonates.I9 Here it was found that in molecular K2C03,cation coordination resulted in two strong C-0 bonds and one weaker one, and a structure similar to that shown in Figure 4c was proposed. For this type of structure, the antisymmetric component of the split degeneracy lies at higher frequency than the symmetric component. If we now consider the spectrum of B obtained from Cs2Se03 (Figure 2b), it is most unlikely that the two bands at 798 and 780 cm-I have the same symmetry. They are separated by less than 20 cm-I and have somewhat different intensities, indicating that they have not been subject to vibrational coupling. C, symmetry is therefore indicated (rather than C l ) ,and as the components of the split E mode are expected to have similar intensities, we assign the three S e - 0 stretches at 798 A’, 780 A”, and 695 cm-l A’ and consider the structure in Figure 4b to be unlikely. To confirm these assignments, attempts were made to model both the relative band intensities and the isotope shifts observed on the 780-cm-l band, using the “stretch only” approximation. The 0-Se-0 bond angles in the “free” selenite ion have been reported to be ca. 101’ in MgSe03.6H20,20and initial attempts to model the observed spectrum employed this single angle parameter, two principal force constants F, and Fd, and a single interaction constant (F, = Fd). The same bond dipole derivative was used for the two types of Seo bond, and the resulting spectral simulation for the three fundamentals is shown in Figure 2c. The predicted intensity pattern is moderately satisfactory, but the highest frequency band is rather too intense. However, the calculated selenium isotope splittings are almost identical ,with those found experimentally, and this comparison is shown in Figure (19)Ogden, J. S.;Williams, S.J. J. Chrm. Soc., Dalton Trans. 1981,456. (20) Weiss, R.; Wendling, J.-P.; Grandjean, D. Acta Crysrullogr. 1966. 20, 563.
Vaporization of Alkali Metal Oxoselenium Salts 2e,f. Since these splittings relate to the (single) A" mode, it may be shown that they are determined only by the value of the unique 0-Se-0 angle (Y (see Figure 4c), and this angle was therefore held constant at 101' throughout subsequent refinements. Additional flexibility was then introduced into the model by allowing F,, and FIdto vary independently and also by varying the bond dipole derivatives and angle parameter 0. All these factors were found to affect the relative intensities of the two A' modes, and a unique solution was not possible. However, significant improvements regarding the modeling of band intensities could be achieved, and a typical spectrum is shown in Figure 2d. This semiquantitative agreement with the observed spectrum thus supports our assignment of B, but in view of the approximations inherent in this simple model, no real significance should be placed on the precise values of the parameters used to generate the computed spectrum. However, despite this caveat, we belive that cation coordination in these selenites results in one Se-0 bond that is weaker than the other two, as indicated by the relative values of the principal force constants. On this basis, we reject structure b in Figure 4 and prefer c over d, since this could in principle result in lower cation-cation repulsion. It is interesting to note, however, that coordination type d would be an obvious intermediate, and perhaps even the final geometry in a cocondensation reaction such as has been proposed between T120 and SOZ5. Species C. The somewhat broad feature centered at 750 cm-I did not yield any resolvable selenium isotope structure, and since the measured band width was typically ca. 5-10 cm-I, it is possible that it may arise from polymeric material. Apart from the observation that it lies close in frequency to the E mode found in solid selenites, no further conclusions could be drawn regarding its origin. Species D. The three absorptions at ca. 815,806, and 395 cm-' were most prominent in experiments involving alkali metal/Se02 mndensation, and Table I lists the bands observed for the various alkali metals. These bands are assigned to Se-O stretching and bending modes, and although no direct evidence was found for cation motion, the above frequencies were cation dependent. Under high resolution, the two higher frequency bands showed partially resolved fine structure, and a typical spectrum obtained from the potassium system is shown in Figure 3b. This fine structure is similar to that expected for selenium isotopes in natural abundance, and taking into account the circumstances of their formation, we assign these bands to the SeOz unit in the species KSe02. In the analogous studies involving the species MS02,6 sulfur isotope data indicated a bent structure for the SO2 unit with a bond angle 0 - S 4 of ca. 110': the alkali metal was then assumed to complete a four-membered ring to give overall C,, symmetry. The same shape has also been proposed for the species MPOZ, MAsO2, and MSb02, where corresponding bond angles of ca.114, 115, and 106' have been estimated.2' For the majority of C, triatomics, bond angles >90° lead to a situation where the antisymmetric (B,)stretch is more intense than the symmetric (A,) stretch, and this has been found to be the case in the M X 0 2species above. We therefore assign the bands a t 806 and 815 cm-' in KSeOZas the BI and A, stretches and identify the low-frequency feature at 395 cm-l as the (A,) 0-Se-O bend. As was found earlier in the case of molecular M2Se04species, the specific assignment of these two stretching modes in principle leads directly to three independent estimates of the 0 - S e - 0 bond angle, if one makes the usual assumptions regarding the decoupling of high-frequency modes. The first estimate comes directly from the intensity ratio of the B,and A, bands, where it may be shown17 that (21) Ogden. J . S.;Williams, S. J. J . Chem. Soc.,Dalton Tram. 1982,825.
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2931
_ -- tan2 4 (Msc + 2M0 sin2 4) 181 [A,
(M& + 2Mo cos2 4)
The other two estimates are based on the magnitudes of the selenium isotope shifts on the A, and B, stretches, for which the simplified secular equations are
+ 2(1 / M s e ) sin2 4) In these expressions, the 0-Se-0 bond angle is 24, and the A B , = (Kr - KrrI((1 / M o l
parameters K, and K , represent the principal and interaction SeO stretching constants. From our (low resolution) spectra of KSe02, we estimate the intensity ratio of the B, and A, modes to be ca. 2.2, which yields a bond angle of 103', while comparison of the observed selenium isotope shifts with those calculated separately for the A, and B1 modes indicates marginally lower values of 101' and 102', respectively. Although these three estimates are remarkably similar, we believe that uncertainties in the precision of our intensity and frequency data will produce attendant errors of ca. f5' on these angles. Figure 3c reproduces the spectral simulation for KSe02 based on a bond angle of 102', while Table I1 summarizes the numerical data. The isolation of MSe02 species during the course of this work provides useful complementary data to recent PES studies2* on the gas-phase species generated in a discharge ion source containing Se vapor and N20. In these studies, it was shown that electron loss resulted in the formation of (neutral) Se02, and the precursor was deduced to be SeO;, with an estimated A, stretch at 810 f 80 cm-I in the ground state. Although the error limits are somewhat large, the agreement with our matrix value is clearly very satisfactory. Conclusians
These experiments have shown first that the vaporization of alkali metal selenates and selenites results in the formation of novel ternary species in addition to decomposition products such as selenium dioxide. As might have been anticipated, the IR spectra obtained for molecular selenates are consistent with the bisbidentate structure similar to those found for molecular sulfates, while in molecular selenites, cation coordination to the central pyramidal unit may involve both monodentate and bidentate binding. Second, it is suggested that the cocondensation reaction between alkali metals and selenium dioxide under matrix isolation conditions leads to the formation of a new dioxoselenium(II1) species. For all three new species, various estimates of bond angles are proposed based on a combination of intensity and isotope data.
Acknowledgment. We gratefully acknowledge the financial support of the SERC and of AEA Technology, Winfrith, for this work and also thank Dr. S. Dickinson for experimental support. Registry No. Na2Se04,13410-01-0; K2Se04, 7790-59-2; Rb2Se04, 7446-17-5; Cs2SeO4, 10326-29-1; Na,SeO,, 10102-18-8; K2Se03, 10431-47-7; Rb,SeO,, 15123-97-4; Cs,SeO,, 15586-47-7; NaSe02, 13 1973-12-1; KSeO,, 13 1973-1 3-2; RbSeO,, 13 1973-14-3; CsSe02, 13 1973-1 5-4; NaJ6Se04, 13 1793-43-6; NaJ7Se04, 5 1780- 13-3; Na2"Se04, 13 1793-44-7; Na280Se04, 13 1793-45-8; Nain2Se04,131793-46-9; CsJ6Se0,, 131793-47-0; CsJ7Se0,, 131793-48-1; CsJ8Se0,, 13179349-2; CsZn0SeO,, 131793-50-5; Cs,82Se0,, 131793-51-6; K7%e02, 131793-52-7; K77Se02,131793-53-8; K78Se02,131793-54-9; K8%e02, 13 1793-55-0; KB2Se02,131793-56-1; SeO,, 7446-08-4; Na, 7440-23-5; K, 7440-09-7; 76Se,13981-32-3; "Se, 14681-72-2; '?Se, 14833-16-0 %e, 14681-54-0; 82Se, 14687-58-2. (22) Snodgrass, J. T.; Coe, J. V.; McHugh, K. M.;Friedhoff, C. Bowen, K.H.J . Phys. Chem. 1989, 93, 1249.
D.;
