J. Phys. Chem. 1991, 95, 1183-1 188 electron energies that are generally too low to effect extensive fragmentation, gives rise to ions of the formula Cr(CO),+ (x = 0, I , 2).38 Such extensive fragmentation at low electron impact energies is not surprising in light of the fact that the internal energy of the neutral Cr(C0)4 is quite high. We might expect, then, that the 248-nm MPI of the nascent, solvated Cr(C0)4 photoproduct in our cluster experiments would also give rise to a distribution of highly unsaturated daughter ions, S,Cr(CO),+ (x < 4). This expectation agrees well with our observations. On the other hand, two-photon ionization of internally excited (30-45 kcal/mol; vide supra) Cr(C0)5 at 350 nm should give rise to a nascent Cr(CO)5+ photoion with perhaps only 21-36 kcal/mol of internal energy. This is probably sufficient for dissociation of only one additional carbonyl ligand from the incipient parent ion. Again, this expectation agrees well with our experimental observations at 350 nm. Conclusions
We have examined the multiphoton dissociation and ionization dynamics of mixed van der Waals clusters containing molecules of Cr(C0)6 surrounded by several solvent molecules of methanol. We find that the multiphoton photophysics of these clusters is not analogous to that of naked Cr(C0)6 in the gas phase. That is, multiphoton excitation of these heteroclusters is not described by a dynamical scheme in which complete ligand stripping takes place initially, followed by photoionization of an atomic metal photoproduct. Instead, we find that a coordinatively unsaturated chromium carbonyl species is created initially within the cluster via single-photon dissociation. On the basis of the interpretation of our mass spectrometric results, we conclude that irradiation of these clusters at 248 nm gives rise to product clusters containing Cr(CO)4, while irradiation at wavelengths around 350 nm gives
1183
rise to product clusters containing Cr(COb,. This type of dynamical behavior is reminiscent of the single-photon dissociation dynamics of naked Cr(C0)6 in the gas phase. The primary photoproduct subsequently undergoes MPI to give nascent cluster ions of empirical formula S,Cr(CO),+ where x = 4 or 5 for excitation at 248 or 350 nm, respectively. Internally excited nascent ions may relax in one of at least three ways: subsequent ligand loss from the parent ion, ion-molecule reactions with surrounding solvent molecules within the cluster, and intracluster V-V energy transfer to the solvent bath. For the case of 248-nm irradiation, extensive fragmentation of the highly excited nascent parent appears to take place, and the observed photoion distribution is dominated by ions of empirical formula S,Cr(CO),+ ( x = 0, 1, 2). For the case of irradiation at wavelengths near 350 nm, the nascent parent ions, S,Cr(CO)5+, appear to undergo an H atom (or D atom) transfer reaction with an adjacent solvent molecule within the cluster, yielding S,Cr(CO),(H/D)+. In general, CD30D appears to be more efficient than C H 3 0 H in cooling the internal energy of an excited Cr(CO),+ ion via intracluster V-V energy transfer. Although we are unable to assess the extent of photoinduced evaporation of solvent monomers from the mixed clusters, we suspect that this may be an important mechanism for disposal of excess energy in the cluster ions due to subsequent photon absorption following ionization. Perhaps most importantly, our ability to photolytically prepare and spectroscopically probe van der Waals clusters containing specific coordinatively unsaturated organometallic species may allow us to elucidate the electronic structure and chemical reactivity of this important class of molecules.
Acknowledgment. We gratefully acknowledge the financial support of this work provided by the Office of Naval Research.
Infrared Spectra of UF,, WF,, MoF,, and SF, Complexes with Hydrogen Fluoride in Solid Argon Rodney D. Hunt,*,+ Lester Andrews, Chemistry Department, University of Virginia, Charlottesuille, Virginia 22901
and L. Mac Toth Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: July 20, 1990)
UF,, WF6, MoF6, and SF6 have been codeposited with hydrogen fluoride in excess argon at 12 K. The FTlR spectra of uF6 and HF revealed a strong UF6-HF complex absorption at 3848 cm-' along with a weak, broad band at 3903 cm-' due to the UF,-FH complex. Only one 1:2 complex with a UF6-H,F-HbF arrangement was detected at higher HF concentrations and sample annealing. Similarly, the HF interaction with tungsten hexafluoride formed two 1 :I complexes. However, the 3884-cm-' band due to the anti-hydrogen-bondedcomplex WF6-FH was considerably stronger than the WF6-HF absorption at 391 I cm-I. Increasing the HF concentration produced a single 1:2 complex with a WF6-FHb-FHa structure. The band positions and relative intensities for the MoF, complexes with HF and DF were very comparable to their WF, counterparts. This change in the dominant binding interaction for these HF-metal hexafluoride complexes was anticipated due to the significant difference in the fluoride affinities of UF6 and WF6 (46 and 69 kcal/mol, respectively). In sharp contrast to the metal hexafluorides, SF, and HF produced a weak triplet at 3905, 3903, and 3901 cm-' and a strong doublet at 3819 and 3818 cm-I, which are tentatively assigned to the SF6-HF and SF6-HaF-HbF structures, respectively. Finally, 1:3 and 1:4 complexes were observed with each hexafluoride, and the structures of these HF complexes are determined by the cyclic nature of the HF polymers.
Introduction
Hydrogen-hnding and Lewis acid-base interactions have been the focus of numerous theoretical and experimental investigations, since these binding interactions exert a large influence on the physical properties of a wide range of complexes. The competition 'Oak Ridge National Laboratory, Oak Ridge, T N 37831-6226.
0022-3654/91/2095-1183$02.50/0
between these intermolecular interactions is typified by the CI2 and CIF complexes with HF. Even though the most recent ab initio calculations' have shown that the hydrogen-bonded complexes CI2-HF and CIF-HF are slightly less stable than the ( I ) Rendell, A. P. L.; Bacskay, G. G.; Hush,N . S.J . Chem. Phys. 1987, 87, 535.
0 1991 American Chemical Society
1184 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
anti-hydrogen-bonded complexes C12-FH and FCI-FH, molecular beam electric resonance2*’studies have only observed the lower energy anti-hydrogen-bonded complexes. In contrast to the gas-phase studies, a recent matrix infrared stud9 has characterized complexes with both structural arrangements. The matrix isolation technique can trap both stable forms because the matrix cage rapidly quenches internal energy and prevents the rearrangement from the less stable form to the more stable form. A similar competition between hydrogen-bonding and Lewis acid-base interactions is expected to exist in the metal hexafluoride (UF,, WF,, and MoF,) complexes with H F due to the high fluoride affinities of the hexafluorides. The primary purposes of this matrix isolation infrared study are to characterize the different H F complexes with the hexafluorides and to examine the roles that the hexafluorides play in determining whether H F serves as a Bronsted acid or a Lewis base. In addition, this work will be useful in characterizing H F complexes that are prevalent in two important chemical systems: the hydrolysis of metal hexafluorides and MF,-doped H F chemical lasers. The final objective of this work is to examine physical properties of the hexafluorides such as the proton and fluoride affinities. Earlier studies5*,have shown a direct correlation between the frequency shift of the H F stretching mode in the H F complex and the base’s proton affinity if the basic sites are similar. A similar correlation between the frequency shift of H F stretching mode in the complex and the fluoride affinity of the hexafluoride should exist, and therefore an approximate value of fluoride affinity can be obtained from the H F frequency shift in the complex. Experimental Section The vacuum and cryogenic apparatus and the FTIR spectroscopic techniques for the H F experiments with the hexafluorides in solid argon at I2 K have been discussed in detail previously.’.* All spectra were recorded with a Nicolet 7199 Fourier transform infrared spectrometer between 4000 and 400 cm-’ at I-cm-’ resolution. Samples of UF6, WF,, MoF, (Oak Ridge National Laboratory), SF,, and H F (Matheson) were condensed at 77 K and evacuated before each use to remove any extraneous volatile components. DF was prepared by reacting F2 (Matheson) with D2 (Air Products) at low pressures in a passivated stainless steel vacuum system. The H F and hexafluoride samples were diluted between 100/1 and 400/1 mole ratios with argon (Air Products) and codeposited on a Csl window at rates of 12-1 5 mmol/h for 3.5-5.5 h. After the sample codeposition was completed, the matrices containing UF, and MoF, were then photolyzed for 1 h at 12 K with a mercury arc. Then, each matrix was annealed at 22-23 K for IO min and then recooled to 12 K in order to promote further aggregation of HF. Finally, each matrix was warmed to 28-29 K for an additional 10 min and then cooled back to 12 K . IR spectra were taken before, during, and after sample preparation, photolysis, and annealings. Samples were also prepared and examined for each reagent separately. Results Fourier transform matrix infrared spectra of H F and HF/DF mixtures with each hexafluoride will be described in turn. While the majority of the new product absorptions will involve the H F fundamental stretch, hexafluoride submolecular modes were also perturbed by HF. Uranium Hexafluoride. Three experiments were conducted with UF6 and H F at different dilutions of argon at 12 K. After ~~
~~~
(2) Baiocchi, F.A.; Dixon, T. A.; Klemperer, W. J . Cfiem. Pfiys. 1982, 77, 1632. (3) Novick, S. E.; Janda, K. C.; Klemperer, W. J . Cfiem. Pfiys. 1976.65,
Hunt et al. 1.6
1.4
1.2
1.o
I
I
1::
”U
0.4
0.2
0
I
I
I
I
I
I
I
WAVENUMBER
Infrared spectra in the 4000-3650-cm-’ region of hydrogen fluoride-uranium hexafluoride samples in solid argon: (a) after codeposition of 27 mmol of Ar/UF6 = 200/1 and 27 mmol of Ar/HF = 200/1 at 12 K in 3.5 h; (b) after annealing to 23 K and recooling to 12 K; (c) after warming to 29 K and recooling to 12 K. Figure 1.
the codeposition of Ar/UF6 = 200/1 and Ar/HF = 200/1 samples was completed, the spectrum in Figure la shows the H F monomer, nonrotating H F monomer, H F dimer, and H F trimer bands (labeled HF, Q,D, and T, respectively),9,’0an absorption due to water (labeled W),” and a band due to the N2-HF complex (labeled N).I2 The strong band at 3848 cm-’ ( A = absorbance = 0.66, labeled us) with satellite bands at 3871, 3854, and 3852 cm-’ was the major product absorption. In addition, a weak doublet at 3907 and 3903 cm-’ ( A = 0.06, labeled au,) and weak bands at 3764 and 3717 cm-’ (labeled Ysb and us,, respectively) were observed. The intensities of the au, bands were comparable to the weak 3896-cm-’ band, which has been previously assigned to the Y, mode of the (HF)2.9 Finally, no perturbed UF, modes were observed. Photolysis of the H F and UF, matrix failed to produce any new bands that could be attributed to H F complexes with UF5. The only photolysis product was isolated UF,, which has absorptions at 584 and 561 cm-’.” The spectrum in Figure Ib exhibits the changes due to annealing the matrix to 23 K and then recooling to 12 K. The H F dimer and trimer bands, as well as the us absorption, increased by approximately 25%, while the av, band remained relatively unchanged. A new absorption due to the N2-(HF),I2 complex appeared at 3788 cm-’ (labeled M), and a doublet at 3427 and 3404 cm-I, which has been assigned to the cyclic H F tetramer,I0 was observed. While the u, and uSb absorptions grew markedly, a new weak absorption at 3688 cm-’ (labeled T’) appeared. The spectrum in Figure I C displays the effects of warming the matrix to 29 K and then recooling to 12 K. This second annealing revealed a new product absorption at 3405 cm-’ (labeled C’). The other H F and UF, experiments produced similar results, and the relative intensity of the us and au, bands was not affected by the different concentrations of H F and UF, in the matrix. Two additional UF, experiments were performed with DF/HF (60% DF) mixtures and UF, samples. The principal absorptions in the H F experiments were observed again. The strong us DF
5115. (4) Hunt, R. D.; Andrews, L. J . Pfiys. Cfiem. 1988, 92, 3769. ( 5 ) Andrews, L.; Davis, S. R.; Johnson, G. L. J . Pfiys. Cfiem. 1986, 90, 4273. (6) Lascola, R.; Withnall, R.; Andrews, L. J . Pfiys. Cfiem. 1988, 92, 2145. (7) Andrews, L.; Johnson, G.L. J . Cfiem. Pfiys. 1982, 76, 2875. (8) Johnson, G. L.; Andrews, L. J . Am. Cfiem. SOC.1983, 105. 163.
(9) Andrews, L.; Johnson, G. L. J . Pfiys. Cfiem. 1984,88, 425. (IO) Hunt, R. D.; Andrews, L. J . Cfiem. Phys. 1985,82, 4442. ( I I ) Andrews, L.; Johnson, G. L. J . Cfiem. Pfiys. 1983, 79, 3670. (12) Andrews, L.; Davis, S. R. J . Cfiem. Pfiys. 1985,83, 4983. (13) Paine, R. T.: McDowell, R. S.; Asprey, L. 8.; Jones, L. H. J . Cfiem. Phys. 1976, 64, 3081.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
IR Spectra of UF6, WF6, MoF,, and SF6 Complexes
1185
TABLE I: Absorptions (cm-') Produced on Codeposition of Hexafluoride and Hydrogen Fluoride or Deuterium Fluoride with Excess Argon at 12 K
UF6 + HF
UF6 + DF
WF6 + HF 391 1
3907 3903
2865
387 I 3854 3848 3764
2831 2826 2822 2762
3892 3887 3884
2126 271 1 2522
287 1 285 1 2848
+ HF
391 1 3883 3818
MoF,
+ DF
SF6 + HF
2871
+ DF
SF,
3905 3903 3901
2866
assign1
group ident 2nd
"S
US
us
2848 2844
1st
aus ays
2nd
US US "S
3819 3818 3768
3717 3688 3405
WF6 + DF MoF,
2163
3173 3760
2166 2751
3693 3410
2712 2524
65 I
65 1
639
636
2797
4th
usb "sb
3rd
avsb aysb
T' C'
4th 5th 6th
VIC
MF6 fund.
"sa
3693 3408 713 679 677 662
2712 2524 773 679 677 659
271 1 2523 172 642
3690 3407 172 642
u2c YIC
auZc 523
absorption at 2822 cm-l ( A = 0.42) with satellites at 2826 and 2837 cm-' dominated the weak au, band at 2865 cm-l ( A = 0.02). Annealing the matrices to 23 K and then to 28 K did not change the intensities of the us and au, absorptions. However, these annealing produced several secondary product absorptions (labeled usat Vsb, T', and c'). The frequencies of these absorptions are given in Table I. Tungsten Hexafluoride. Similar experiments were conducted with tungsten hexafluoride and hydrogen fluoride in argon. The frequencies of these product absorptions are also listed in Table I. Deposition of the least concentrated samples produced a strong au, 3884-cm-' absorption ( A = 0.32) with weaker components at 3892 and 3887 cm-I. Other product absorptions were observed in 391 1 cm-l ( A = 0.03, labeled us) and 3768 cm-l ( A = 0.02, labeled au,,,). In sharp contrast to the UF6 and H F experiments, several displaced WF6 absorptions were observed at 773,679,677, and 662 cm-' (labeled u I c , u2c, uZc, and aut, respectively). The initial warm-up to 23 K produced new T' and C' absorptions at 3693 and 3408 cm-I, respectively. The second annealing to 28 K reduced all product absorptions with the exception of the 3408-cm-I absorption. Figure 2 is representative of the spectra observed in the WF, experiments conducted with HF/DF mixtures. After the samples were deposited, the spectrum in Figure 2a displays a strong au, doublet at 2851 and 2848 cm-' ( A = 0.46), a very weak us absorption at 2871 cm-' ( A = 0.01), and a secondary product absorption at 2763 cm-l ( A = 0.06, labeled au,,). It should be noted that the intensity of the au, absorption with respect to us absorption was higher in the DF experiments than in their H F counterparts. I n addition, the new WF, modes that were observed in the H F experiments were again present. However, the auZcmode was red-shifted by 3 cm-I. The sample annealing that are shown in Figure 2b,c produced a larger increase in the us band than in the au, band. Also, the annealing produced a new secondary product band at 2712 cm-' and another at 2524 cm-' (not shown); the latter absorption again exhibited the largest increase in intensity. Molybdenum Hexafluoride. A parallel study was performed with molybdenum hexafluoride and hydrogen fluoride. The results of these experiments were similar to those results for WF, and HF. A strong au, doublet at 3883 and 3878 cm- ( A = 0.70), a weak us absorption at 391 1 cm-l ( A = 0.10), and a broad auSbband at 3760 cm-' appeared after the most concentrated samples (Ar/MoF, = 100/1 and Ar/HF = 150/1) weredeposited. Only two perturbed MoF, modes were observed at 651 and 639 cm-I (labeled u; and au2c, respectively). The very strong u3 mode prevented the detection of the u l Cmode. Photolysis of the MoF6 and H F sample failed to produce any new product absorptions. The secondary H F products at 3693 and 341 1 cm-I appeared on sample anncaling. The experiments using DF and MoF, produced
523
WF, 1.4
-
1.2
-
1.0
-
0.4
-
0.2
4-
usc
+ HFlDF
T
D
1:; 0
--
ID
1186 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 SF, t HF
I
I
I
4000
3850
3800
I 3850
I
I
I
1
3800
3750
3700
3650
WAVENUMBER
spectra in the 4000-3650-cm-' region of H F and SF, samples in solid argon: (a) after codeposition of 38.2 mmol of Ar/SF6 = 100/1 and 39.5 mmol of Ar/HF = 200/1 at 12 K in 4.8 h; (b) after warming to 28 K and recooling to 12 K . Figure 3. FTlR
band positions, as well as the other DF counterparts absorptions, are given in Table I.
Discussion The new product absorptions will be identified, and vibrational assignments will be made for these HF-hexafluoride complexes. In addition, the bonding trends and physical properties of the hexafluorides will be examined. Idenfifcarion. The product bands that are listed in Table I were not observed in the argon matrix sample of the hexafluorides or hydrogen fluoride alone. However, these absorptions were produced when the reagents were mixed during the condensation and annealing processes. Six groups of H F product absorptions can be identified on the basis of concentration, annealing behavior, band position, and fluoride affinity. After the codeposition was completed, the absorptions in the first group (labeled av,) were strong in the WF, and MoF, experiments and weak in the UF6 studies. The frequencies of these absorptions are slightly redshifted (WF, and MoF,) or blue-shifted (UF,) from the H F stretching mode in the anti-hydrogen-bonded FCI-FH complex4 at 3896 cm-I. Except for U F , the absorptions in the second group (labeled u,) remained weak throughout the experiments, regardless of concentration, and their band positions were close to the H F absorption in the hydrogen-bonded F2-HF complex4at 391 5 cm-l. In the case of the UF6, the band was strong after deposition, and its frequency was similar to the H F mode observed in the Br2-HF complex4 at 3851 cm-'. The intensities of the absorptions in the third (labeled av,b) and fourth groups (labeled v,, and vSb)were dependent on sample concentration and annealing. These absorptions were strong on codeposition of the concentrated samples. In contrast, these bands in the most dilute experiments were not strong until the first sample annealing. The frequencies of these bands were between the H F dimer band at 3826 cm-' and the H F trimcr absorption at 3702 cm-I. The absorptions in the fifth and sixth groups (labeled T' and C', respectively) are apparent only after sample annealing. The bands in the fifth group were slightly red-shifted from the (HF), band while the frequencies on the final group were between the cyclic H F tetramer bandsi0 at 3427 and 3404 cm-I. The au, and us groups of H F product absorptions were produced primarily by sample codeposition, which indicates the presence of two major product species for the metal hexafluorides. These groups of bands maintained constant relative intensities over the wide range of sample concentrations. The au, absorptions are better assigned to the 1:l complex with an anti-hydrogen-bonded
Hunt et al. structure (MF,-FH) while the U, bands can be assigned to another 1 : 1 complex with the hydrogen-bonded form (MF6-ff F). The principal justification for these assignments in the UF6 and WF6 studies is based on the fluoride affinities of UF6 (46 kcal/mol)14 and WF, (69 kcal/m01)'~ This large difference in fluoride affinities should have a pronounced effect on the relative intensities (populations) and band positions of these two major complexes. Since WF, should form a more stable anti-hydrogen-bonded complex than UF6, a stronger au, mode for WF6 should be observed displaced more to the red from its UF6 counterpart. Conversely, the WF6-HF complex should be weaker than the UF,-HF complex, so a less intense v, mode for WF6 should be detected red-shifted from the H F Q branch9 at 3919 cm-' less than the UF, analogue. These observed relationships along with a comparison of the 1:1 complexes in this work and with diatomic halogens4 and H F support these structural identifications. In the MoF6 study, the assignments of the major H F complexes have been based primarily on the striking similarity of the MoF, and WF, spectra, which indicates the proton and fluoride affinities of MoF6 and WF, are comparable. In the case of SF,, only one 1:1 product was detected, and this complex has been tentatively assigned to a hydrogen-bonded structure, SF6-HF. The F2-HF complex whose structure was identified on the basis of ab initio calculation^'^^^^ serves as the most useful model for the assignment of this SF, complex. The ausb, us,, and v,b absorptions displayed a higher order concentration dependence on H F which is characteristic of a 1:2 complex. The two structural arrangements of 1.2 complexes that were characterized in the earlier diatomic halogen and H F study4 served as useful models in the identification of the bands in both groups. The avSbabsorptions that were observed in WF, and MoF, studies can be attributed to a 1:2 complex with an MF6-FHb-FH, structure. Similarly, the uSaand u,b bands that were seen in UF6 and sF6experiments can be assigned to a different 1:2 complex with an MF6-HaF-HbF structure. It is not surprising that the binding interaction that was dominating in the 1:l complexes determined the structure of the 1:2 complexes. The T' and C' product bands exhibited an even higher order concentration dependence on HF, and these absorptions are apparently due to 1 :3 and 1:4 complexes. The H F molecules in these complexes form a ring configuration that was slightly perturbed by the hexafluorides. If the H F molecules formed an open chain, additional H F product absorptions would have been observed. Due to the weak interaction of the hexafluorides with the cyclic H F polymers, the orientation of the MF, to these cyclic polymers could not be determined from the infrared spectra. In conjunction with these H F product absorptions, new product bands (labeled vc and avC)were assigned to several HF-perturbed WF6, MoF,, and SF, modes which were normally Raman active only. These absorptions were identified by their close proximity to their gas-phase fundamentals1*and displayed the same H F concentration dependence as the av, and v, bands. Similar H F activation of usually infrared-inactive fundamentals has been observed for H2, O2,I9N2,12C02,20C2H2,2Iand (CN)22complexed to HF. In these complexes, the infrared activation of these MF6 modes is due to the electrical asymmetry induced by the H F ligand. The most unusual aspect of the perturbed hexafluoride modes is the presence of two distinctly different perturbed u2 modes for WF6 and MoF,. The uzc mode has been assigned to the hydro(14) Beauchamp, J. L. J . Chem. Phys. 1976, 64, 929. (15) George, P. M.; Beauchamp, J. L. Chem. Phys. 1979, 36, 345. (16) Sapse, A. M. J . Chem. Phys. 1983, 78, 5733. (17) Reed, A. E.; Weinhold, F.; Curtiss, L. A,; Pochatko, D. J. J . Chem. Phys. 1986,84, 5687. (18) Bosworth, Y . M.; Clark, R. J . H.; Rippon, D. M . J . Mol. Spectrosc. 1973, 46, 240. (19) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1987,86, 3781. (20) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6353. (21) Andrews, L.; Johnson, G.L.; Kelsall. B. J. J . Phys. Chem. 1982,86, 3374. (22) Hunt, R. D.;Andrews, L. J . Phys. Chem. 1987, 91, 5594.
IR Spectra O f uF6, WF6, MOF6, and SF, COmpleXeS gen-bonded 1:l complex, while the a u j mode has been attributed to the anti-hydrogen-bonded 1:I complex. The stronger auq bands exhibited much larger perturbations from the gas-phase fundamentals’* than the weaker counterparts. The results were quite reasonable, since the favored WF6-FH complex exhibited a much stronger binding interaction than the WF6-HF complex. In addition, these assignments are supported by an HCI study with WF, and M O F , . ~ ~While the frequencies of the u2c modes were not affected by the HCI substitution, the frequencies of the au? modes exhibited smaller red shifts than their H F counterparts. Also, the intensities of the au; and u q absorptions, as well as the au, and us absorptions, were comparable. These observations were expected due to the destabilizing effect of the larger covalent radius of chlorinc in HCI. In addition, the auCmodes play a critical role determining the structure of the anti-hydrogen-bonded complexes. The relatively small perturbations on the au; modes indicate the fluorine on the H F interacts directly to a fluorine on the hexafluoride and not to the metal center. A direct interaction between the fluorine and the metal center would have a much larger effect on the hexafluoride submolecule modes since the symmetry of the metal hexafluorides would be lowered in the anti-hydrogen-bonded complexes. Finally, the larger perturbation by DF than by H F of the auzCmode is probably due to the smaller librational amplitude of DF. Other HF s t ~ d i e s ’that ~ , ~have ~ observed the greater perturbing effect of DF in relatively weak hydrogen-bonded complexes have employed a similar rationalization. With the anti-hydrogen-bonded complexes, the time average position of electron density which is directly related to average position of the hydrogen or the deuterium determines the average binding interactions of these complexes. Therefore, the MF6-FD complexes should be slightly more stable than their H F counterparts. Assignment. The usually strong, sharp product absorptions (labeled nu,) had similar DF counterparts with HF/DF frequency ratios of 1.362-1.365, which is characteristic of the fundamental vibration4 of the H F ligand. These absorptions are slightly redshifted (12-41 cm-’) from the 3919-cm-’ H F f ~ n d a m e n t a l ~in. ’ ~ solid argon due to the Lewis acid-base interaction between the fluorine and the HF. Similarly, the sharp bands (labeled us) exhibited DF absorptions with HF/DF frequency ratios of 1.362-1.364. These bands that are assigned to the MF6-HF complex are red-shifted (8-71 cm-I) from the H F matrix fundamental due to the hydrogen-bonding interaction. The wider range of us frequencies demonstrates that the H F stretching fundamental, which is largely a hydrogen motion, is more sensitive to hydrogen bonding than Lewis acid-base interaction. The aubbabsorptions are assigned to the MF6-FH-FH complex. The H F nature of these vibrations is confirmed by the HF/DF frequency ratio of 1.364. The ausbabsorptions are substantially red-shifted (53-66 cm-I) from the corresponding V,b band at 3826 cm-’ for the H F dimer6 due to the Lewis acid-base interaction between WF6 (or MoF6) and the H F dimer. In addition, a comparison of the H F frequency shifts for au, and auSbclearly indicates that the Lewis acid-base interaction is stronger with (HF), than with HF. This stronger perturbation is expected since the kydrogen-bonding interaction in the (HF), increases the basicity of the hydrogen-bonded H F with respect to isolated HF. The usU and u,b bands that have HF/DF frequency ratios of 1.363-1.365 are assigned to the fundamental vibrations of the H F molecules in the MF,-HF-HF complex. These assignments seem quite feasible when these bands are compared to the corresponding bands in the N2(HF),I2 and F 2 ( H F ) t complexes. The use mode for the SF6 complex is probably obscured by the 3881-cm-’ band due to the N2-HF ~ o m p l e x . ~ . The T’ and C’ bands which have appropriate HF/DF frequency ratios for cyclic H F polymers1° have been attributed to cyclic H F trimers and tetramers which have been slightly perturbed by the hexafluorides. Since the differences in the frequencies of these product absorptions are insignificant, the intermolecular interaction between the cyclic H F polymers and the hexafluoride should be (23) Hunt, R. D.; Andrews, L.; Toth. L. M. Manuscript in preparation. (24) LIU.S. Y . ;Dykstra, C. E. Private communication.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1187 the same for each hexafluoride. However, the type of interaction cannot be determined from these experimental results. Bonding Trends and Physical Properties. Several interesting bonding trends have been observed in this investigation. The hydrogen-bonded complexes provide useful information about the relative proton affinities of the hexafluorides. The frequency shift of the Y, mode gives a measure of the strength of the base-HF interaction for similar Since the us fundamentals for the hexafluorides are 391 1 cm-’ for WF6 and MoF,, 3903 cm-’ for SF,, and 3848 cm-’ for UF,, the order of base strength (proton affinity) is WF, = MoF, < SF6 C UF,, with H F serving as a proton donor. The proton affinity of SF6 is an important Value for kineticists, since SF, is frequently used as a thermal electron scavenger in ionized systems in order to modify the charge neutralization mechanism. Radiolysis studies have reported conflicting results on the proton affinity of SF,. According to Van der Linde and Freeman,2sthe proton affinity of SF6 was on the order of 184 kcal/mol while Gorden and Seck2, determined that the proton affinity of SF6 was less than 146 kcal/mol. H F complexes with H F and CH3F which have known proton affinities?’ 116 and 151 kcal/mol, respectively, serve as useful standards in approximating the proton affinity of SF,, since the three H F complexes involve the same basic site, the fluorine lone pair. The frequencies of the us modes for the HF-HPJO and CH3F-HF2* complexes are 3826 and 3774 cm-I, respectively. Even though the correlations of the frequency shifts of the us modes to the base’s proton affinities is not ideal, the results clearly support the work of Gorden and Seck2, and indicate that the proton affinity of SF6 may be considerably less than 146 kcal/mol. The au, modes for UF6 (3903 cm-I), WF, (3E84 cm-I), and MoF, (3878 cm-I) provide information about the relative Lewis acidity of the metal hexafluorides. The order of acidity is UF6 < WF, C MoF,, with H F serving as the Lewis base. This trend determines the relative yield of the anti-hydrogen-bonded 1 :I complexes in this study on the basis of infrared absorption intensities for similar complexes. In addition, the frequencies of the av, bands can be used to predict the fluoride affinity of MoF,, since the fluoride affinities of UF6 (46 k ~ a l / m o l ) and ’ ~ WF6 (69 kcal/mol)Is have been experimentally determined. Assuming a linear relationship between the frequency shift of the aus mode and the hexafluoride’s fluoride affinity, the fluoride affinity of MoF, is predicted to be greater than 70 kcal/mol. The assumption of linearity is justified for this series of complexes since each metal hexafluoride complex involves the same sites for the Lewis acid-base interaction. This assumption would be much harder to justify if the acid and base sites were different. A comparison of the us and au, modes can be used to determine the suitability of the hexafluorides as fluorine atom sources for H F chemical lasers. Both SF629and MoF,’O have been used in H F lasers. Ideally, this fluorine atom source must be very inefficient in deactivating HF. Even though the MoF,-HF complex is much weaker than its SF, counterpart, the strong MoF,-FH complex makes MoF6 less desirable as a fluorine atom source than SF6. Conclusions
In summary, UF6 and H F react in solid argon to form a well-defined 1: 1 hydrogen-bonded complex, UF,-HF, as well as a much weaker anti-hydrogen-bonded complex, UF,-FH. The us and au, modes are very comparable to the chlorine-HF observations for both complexes. In experiments with more concentrated samples, a single 1 :2 complex, UF,-(HF),, was Observed
v.
(25) Van der Linde, J.; Freeman, G. R. J . Am. Chem. Soc. 1970,92, 4417. (26) Gorden, R., Jr.; Sieck, L. W. J . Res. Nad. Bur. Stand., Sect A 1972, 76, 655.
(27) Beauchamp, J. L. In Interactions Between Ions and Molecules; Ausloos, P., Ed.; Plenum: New York, 1975. (28) Johnson, G. L.; Andrews, L. J . Am. Chem. Soc. 1980, 102, 5736. (29) Amimoto, S. T.; Whittier, J. S.; Lundquist, M. L.; Ronkowski, F. G.; Hofland, R., Jr.; Ortwerth, P. J. Appl. Phys. Lett. 1982. 40, 20. (30) Inagaki, H.; Suda, A.; Obara, M. Appl. Phys. Lett. 1986, 49, 122.
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J . Phys. Chem. 1991, 95, 1188-1 194
and characterized by considering the perturbation of the second H F submolecule on the 1:l complex. Although similar 1:l complexes were observed in samples of WF6 and MoF6 condensed with H F and DF, the relative population of the 1:l complexes was dominated by the anti-hydrogen-bonded complexes, WF,-FH and MoF6-FH. In addition, several HF-perturbed WF6 and MoF, modes that are normally infrared inactive were identified. Furthermore, a different structural arrangement of the 1:2 complexes, WF,-FH-FH and MoF6-FH-FH, was observed. In contrast to the complexity of the metal hexafluorides, SF6 and H F produced only one 1 : l complex that probably has a hydrogen-bonded structure. Even though SF6-HF and SF6-HF-HF complexes exhibited significantly weaker principal interactions than the other
hexafluorides, the band positions of the 1:3 and 1:4 complexes for SF6 were very similar to the metal hexafluoride counterparts, which indicates the structures of these complexes are primarily determined by the cyclic nature of H F trimer and tetramer.
Acknowledgment. This research was sponsored by the Division of Chemical Sciences, US. Department of Energy, under Contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. Registry No. UF,, 10049-14-6; HF, 7664-39-3; UF6-FH, 131 15314-5; UFb-(HF)z, 131 153-15-6; WFb-FH, 131 153-16-7; WF,-(HF),, 131 153-18-9; MoFb-FH, 131 153-17-8; MoF,-(HF)Z, 131 153-19-0; SF,, 2551-62-4; SF,-FH, 131 153-20-3; SF,-(HF),, 131153-21-4.
Spectroscopy of the Ionic Ground State of Monohalogenated Benzenes K. Walter, K. Scherm, and U. Boesl* Institut fur physikalische und theoretische Chemie, der Technischen Universitat Munchen. Lichtenbergstrasse 4, 0-8046 Garching, Federal Republic of Germany (Received: July 23, 1990; In Final Form: October 4 , 1990)
We present multiphoton ionization photoelectron (PE) spectra of monofluoro-, -chloro-, and -bromobenzene. The population of the vibrational levels in the cation after one-color, two-photon ionization via various vibronic intermediate states of the neutral molecule has been investigated. The observed structure has been assigned, and the frequencies of some vibrations in the ionic X-state were determined, providing new data for these molecular ions. Our results show that the PE spectra reflect Fermi resonances and Duschinsky rotations in the neutral intermediate states, allowing an interpretation of these states. The consequences of our results for neutral as well as ion spectroscopy will be discussed.
1. Introduction Since its first application to polyatomic molecules,' multiphoton ionization photoelectron spectroscopy (MPI-PES) has proven to be a very useful technique for the investigation of the electronic ground state of polyatomic cations. Molecules, excited to a neutral intermediate vibronic level by the absorption of a first photon, are ionized from this level by the absorption of a second photon. The kinetic energy of the outgoing photoelectrons has discrete values, correlated with the internal energy levels of the ion. Especially for nonfluorescing molecular ions such as benzene and many substituted benzenes the only data concerning the vibronic structure of the X-state result from photoelectron spectroscopy; in particular, most highly resolved data are due to MPI-PES. Moreover, with MPI-PES not only can vibrational frequencies be determined but also the population of the vibronic levels in the ion after multiphoton ionization. This information is of great interest for the use of multiphoton ionization as an ion source, especially when used for the spectroscopy of excited ionic state^.^-^ On the one hand, the vibronic structure of these PE spectra is in general not very congested because of symmetryselection rules for the ionization process. On the other hand, these selection rules give access to a variety of vibrational modes in the ion if one uses different vibrational levels as intermediate states for multiphoton ionization. In future, ZEKE-PES (zero kinetic energy PES)5 may deliver much better resolved spectra. Nevertheless, MPI-PES will be useful for fast survey spectra and will be necessary for the investigation of ion ground-state populations after MPI. ( I ) For a review see: Kimura, K. Int. Reo. Phys. Chem. 1987, 6, 195. (2) Ripoche, X.; Dimicoli, 1.; LeCalve, J.; Piuzzi, F.; Botter, R. Chem. Phys. 1988, 124, 305. (3) Walter, K.; Weinkauf, R.; Boesl, U.;Schlag, E. W. Chem. Phys. Lea. 1989, 155. 8. (4) Walter, K.; Boesl, U.;Schlag, E. W . Chem. Phys. Lett. 1989, 162, 261. ( 5 ) Chewter, L. A.; Sander, M.; Muller-Dethlefs, K.; Schlag, E. W. J. Chem. Phys. 1987, 86. 4737.
In the past decade benzene6 and many substituted benzenes7-" have been studied by MPI-PES. The ionic ground state of monohalogenated benzenes, however, has not been investigated very thoroughly. No data are available for bromobenzene and iodobenzene. MPI-PE spectra of monochlorobenzene have been published by Anderson et but some of their results are in conflcit with the assignment of the B X transition of this cation investigated by Ripoche et aL2 In a recent work" we have published the MPI-PE spectrum of monofluorobenzene for the neutral @o(S, So) transition to confirm the assignment of our REMPD (resonance-enhanced multiphoton dissociation) spectrum of the B X transition of this molecular cation. These examples have shown that for an unambiguous assignment of ionic REMPD spectra it is essential to know the vibrational population in the ionic ground state. In this current work we present a detailed study of the ionic X-state of monofluoro-, monochloro-, and monobromobenzene by MPI-PES via a great variety of intermediate states in the neutral molecule. Spectra of the SI So transitions have been recorded by resonance-enhanced multiphoton ionization, and the corresponding assignments have been taken from the literature. lodobenzene has not been investigated because its neutral SIstate is not stable, and a resonant ionization is not possible with laser intensities as used by us.12 We observed richly structured MPI-PE
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(6) (a) Long, S.R.; Meek, J. T.; Reilly. J. P. J. Chem. Phys. 1983, 79, 3206. (b) Kuhlewind, H.; Kiermeier, A.; Neusser, H. J. In Resonance Ionizafion Spectroscopy 1986. Inst. Phys. Con/. Ser. 1986, No. 84, p 121. (7) Anderson, S.L.; Rider, D. M.; Zare, R. N . Chem. Phys. Lett. 1982, 93, 11. (8) Meek, J. T.; Long, S. K.;Reilly, J. P. J. Phys. Chem. 1982.86. 2809. (9) Meek, J. R.; Sektreta, E.;Wilson, W.; Viswanathan, K. S.; Reilly, J. P. J. Chem. Phys. 1985.82. 1741. (IO) Sektreta, E.; Viswanathan, K. S.; Reilly, J. P. J. Chem. Phys. 1989, 90, 5349. ( I I ) Walter, K.; Scherm, K.; Boesl, U. Chem. Phys. Leff. 1989,161,473. (12) Dietz, T. G.;Duncan, M. A.; Liverman, M. G.;Smalley, R. E. J. Chem. Phys. 1980, 73, 4816.
0022-3654/9l/2095-1188%02.50/00 1991 American Chemical Society
