78
J. Phys. Chem. 1988, 92, 78-81
Infrared Spectra of the 03--HF and S02--HF Complexes in Solid Argon Lester Andrews,* Robert Withnall, and Rodney D. Hunt Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: May 8, 1987)
Argon matrix spectra of the 03-HF and SOz--HF complexes are similar, and the structure deduced for 0,- -HF from the infrared spectrum is similar to the microwave structure for SO2--HF. All three ozone submoleculefundamentals were observed for the complex, and v,' was considerably enhanced in intensity relative to vJc. The asymmetric mixed isotopic ozone complexes exhibited split u I c bands which indicate inequivalent terminal oxygen atoms and attachment of HF at this position in the complex.
Introduction Weakly bound molecular complexes are of interest for their structure and the perturbing effect of one submolecule on the other. Rotational and vibrational spectroscopy of the complexes can answer these questions. Structure has been determined for the related COz, N 2 0 , and S O 2 - - H F complexes in pulsed nozzle beams,'-3 and vibrational spectra can in principle be obtained in the gas phase,4 in supersonic jet^,^,^ and in noble gas matrices.',' The present study focuses on O3and SO2complexes with H F from the vibrational perspective. Ozone is a particularly important molecule in chemistry and astrophysics, and matrix studies can provide information on the interaction of ozone with small molecules and the photochemistry of these c o m p l e ~ e s . Fur~ thermore, matrix experiments require small quantities of sample and mixed isotopic ozone species can be employed to determine the site of interaction in ozone complexes. Experimental Section The vacuum and cryogenic apparatus and techniques used for matrix studies of complexes have been reported.',' Infrared spectra were recorded on a Nicolet 7 199 FTIR at 1 -cm-I resolution from 4000 to 400 cm-I or at 2 cm-l from 425 to 125 cm-' or on a Perkin-Elmer 983 from 4000 to 180 cm-I. H F (Matheson), DF (prepared from D2 and F2), and SOz (Matheson) were frozen and evacuated at 77 K. Ozone was prepared from oxygen gas (normal isotopic or 55% " 0 ) by Tesla coil discharge in a Pyrex finger immersed in liquid nitrogen; unreacted oxygen was removed by pumping.1° The reagents were diluted with argon to give Ar/reagent mole ratios between loo/ 1 and 400/ 1. The gas mixtures were deposited from separate stainless steel lines onto a CsI window at 10-12 K. Approximately 20 mmol of each gas sample was codeposited in 3-4 h in each experiment. To foster further diffusion and association of H F within the matrix, the window was warmed to 20-25 K and recooled for more spectra. Results Matrix infrared experiments will be described for hydrogen fluoride with ozone and sulfur dioxide. Ozone. A series of experiments was performed with ozone and hydrogen fluoride samples at several concentrations to minimize ( I ) Baiocchi, F. A,; Dixon, T. A.; Joyner, C. H.; Klemperer, W. J . Chem. Phys. 1981, 74, 6544. (2) Joyner, C. H.; Dixon, T. A.; Baiocchi, F. A,; Klemperer, W. J . Chem. Phys. 1981, 74, 6550. ( 3 ) Fillery-Travis, A . J.; Legon, A. C. J . Chem. Phys. 1986, 85, 3180. (4) Pine, A. S.; Lafferty, W. J. J . Chem. Phys. 1983, 78, 2154. (5) Kolenbrander, K. D.; Lisy, J. M. J . Chem. Phys. 1986, 85, 2463. (6) Lovejoy, C. M.; Schuder, M. D.; Nesbitt, D. J. J . Chem. Phys. 1986, 85, 4890. (7) Andrews, L. J . Phys. Chem. 1984,88, 2940. Andrews, L.; Johnson, G. L. J. Chem. Phys. 1982, 76, 2875. (8) Davis, S. R.; Andrews, L. J . Chem. Phys. 1987, 86, 6027. (9) Withnall, R.; Hawkins, M.;Andrews, L. J . Phys. Chem. 1986, 50, 575. (10) Andrews, L.; Spiker, R. C. J . Phys. Chem. 1972, 76, 3208.
0022-3654/88/2092-0078$01.50/0
TABLE I: New Product Absorptions (cm-I) Observed in Argon Matrix Samples Containing Ozone and Hydrogen Fluoride HF + 0, DF + 0 2 assinnt
3802.6
2788.1
3787 3595
u, (site) us* (1 :2)
1115.7
2715 267 1 2128 11 16.6
1026.9
1026.9
~ 3 '
713.2
7 13.2
uZ'
384
305 286
UP
2128
367
US
VI
+ ~3
(2108)'
ulc (1103.7)"
(1039.9)" (703.6)"
UP
Unperturbed precursor absorptions in parentheses. The superscript c denotes O3 absorptions in the complex. (HF), species in the sample and maximize product yield. Figure 1 shows a representative spectrum, and new absorptions are listed in Table I. The major new band, us at 3802.6 cm-', is stronger than HF, N2--HF, (HF),, and (HF), bands" (labeled HF, N, D, and T, respectively). In the ozone fundamental region, a new satellite, u l C ,was observed at 11 15.7 cm-' above v i of ozone and with greater intensity than the ozone band. Product satellites noted u~~ and v2c were resolved from their respective ozone precursor fundamentals labeled u3 and v 2 in the figure. In the far infrared new bands were observed at 384 and 367 cm-' with weak (HF), absorption at 401 cm-I. Sample annealing to 25 K produced a 10% growth in these bands and doubled a weak 3595-cm-I band. Similar experiments were done with DF. The major differences were us at 2788.1 cm-', the small shift of v,' to 11 16.6 cm-I, and the up modes in the far-infrared region, which are shown in Figure 2. This sample was prepared with low H F and DF concentrations to minimize spectral interference from higher clusters, and only weak isotopic trimer species are noted. The ue(HF) modes were observed as in the pure H F experiment, and two new bands noted U,(DF) appeared at 305 and 286 cm-I. The N,--HF and N2--DF complexes (N) were also observed at 262 and 213 cm-l, respectiveIy.I2 Mixed isotopic ozone samples were also condensed with HF, and the u1 ozone region is illustrated in Figure 3. Uncomplexed ozone absorptions are given their isotopic identification; notice the marked intensity enhancement for the asymmetric isotopes. Bars denote seven distinct new product absorptions at 11 15.7, 1110.3, 1101.5, 1085.6, 1079.8, 1071.4, and 1053.5 cm-'. The bar sextet blue shifted 12 cm-I from the ozone fundamentals is drawn for equivalent oxygen atoms in the asymmetric isotopes. In addition, vJc for IgO3--HFwas observed at 970.7 cm-', but unfortunately the mixed isotopic u3c products were masked by other isotopic precursor bands. A similar DF experiment gave a similar septet 1 cm-l to the blue of the one in Figure 3 with resolved weaker splittings identical with the H F septet owing to H F in the DF sample. ( 1 1 ) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984,88, 425. (12) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J . Chem. Phys. 1983, 75, 2488.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 1 , 1988 19
Infrared Spectra of 03- H F and SO2--HF Complexes '9
-1
7.
Ln
... N.
't
no
3900
shoo
3$oo i i o o
WRVENUMBERS
iioo
1600
son
WAVENUMBERS
'loo
Boo
Figure 1. FTIR spectrum of Ar/03 = 100/1 and Ar/HF = 200/1 samples deposited at 12 K. 5 3 : -
r B
32
t
oo-+
IiOO
lion
IO00
boo
600
io0
WRVENUMBERS
WAVENUMBERS
Figure 4. FTIR spectrum of Ar/S02 = 400/1 and Ar/HF = 400/1 samples codeposited at 12 K. TABLE 11: New Product Absorptions (cm-') Observed in Argon Matrix Samples Containing Sulfur Dioxide and Hydrogen Fluoride HF + SO2 DF + SO1 assignt 3807.0 3799.2 3793.4 3722.1 3706.6 3687.3 3659.4 3604.0 3588.9 3547.5 2498
2794.4 2788.6 2783.9 2726.5 2713.5 2699 2688 2654 2640.7 2613.1 2498
U.
ui
(site)
us (site) usb
(site) (site) (SO,),(HF)y
usb usb
uSa (site) vsa
(SOZ),WF)y
+
~ j ) ' (2493, 2485)' (1355, 1351)" u I C (1 152, 1147)" 2Ve v2 (519, 517)'
(VI ~j
3-P
430
24e
2-3
1158.5 712
1158.5 549
41 1 391
306 290
2 3L
WAVENUMBERS
Figure 2. Far-infrared spectrum of argon matrix containing 03, DF, and HF with Ar/Oj/acid = 300/1/1 in final sample. m
Ye Ye
Unperturbed precursor absorptions.
atoms like ozone precursor.
and 2782 cm-', and broader new bands were observed at 3767 and 2767 cm-I. Sulfur Dioxide. An analogous group of experiments was done with SO2, and Figure 4 shows a representative spectrum. The dominant product band, vp, and minor bands, v, and Vsb, appeared at 3807.0, 3722.1, and 3588.9 cm-', respectively. No satellite was observed near the v3 band presumably due to site (and aggregate) splittings on the low-energy side; however, a sharp new band, vlc, was resolved at 1158.5 cm-I above the vl doublet at 1 152 and 1147 cm-I. In addition, new bands were observed at 712 (2vC),41 1, and 391 cm-I (ve). Sample warming caused slight increases in the vs, vlC, 2vc, and vc bands, caused a 2-fold increase in the v,, and vSb bands, and produced a myriad of site splittings for the above bands, much more than in the ozone experiments. Experiments run at higher SO2 concentration favored these site splittings, and samples with higher relative H F concentration gave increased vsBand vsb bands relative to v,. Similar experiments were done with SO2 and DF including the far infrared. In addition to the HF bands mentioned above, new D F product bands were observed and are given in Table 11.
The D F sample was subjected to high-pressure mercury arc photolysis for 2 h with Pyrex and water filters and 1 h with only a water filter. The strong v,(HF) and v,(DF) bands were reduced 20% by the first photolysis and 50% by the second irradiation. Isotopic ozone submolecule modes followed this photolysis behavior. Sharp, strong new bands were produced in the HF region at 3917 cm-I and in the D F region at 2876 cm-I; sharp, weak new bands appeared in each region at 3856 and 3796 cm-' and at 2821
Discussion Complexes of hydrogen fluoride with ozone and sulfur dioxide will be identified from infrared spectra, and implications about structure will be considered. Ozone. The new product bands described above were not present when either reagent was deposited separately. Accordingly, these bands are due to specific complexes involving the reagent molecules. The very strong us band and the weak v, band exhibit
I
1130
iiio I
I iiio
iioo
io90
I
I
/I io60
WAVENUMBERS
io'io
io60
ioso
1030
Figure 3. Expanded scale spectrum of mixed oxygen isotopic ozone sample with HF in solid argon, 55% lSO,Ar/Oj/HF = 400/1/1 in final sample. Stick spectrum contrasts complex with equivalent terminal
80
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988
relative intensity behavior appropriate for 1:1 and 1:2 complexes, 1 and 2, respectively, on sample annealing and concentration changes. ,O--H-F
90
,o--Ha-F\,
O,
Hb
0
F '
1
2
The strong v, band exhibits a red shift from the 3919-cm-' value for isolated HF and a v,(HF)/v,(DF) = 3802.6/2788.1 = 1.364 ratio which are appropriate for the H F stretching fundamental in a hydrogen fluoride complex such as 1. The low-frequency doublet at 384 and 367 cm-I is assigned to the librational motions of the HF submolecule, vi, one in and one out of the molecular plane. The vt(HF)/vi(DF) isotopic ratios 1.283 and 1.259 are slightly higher than found for this motion in the NO2--HF complex* but lower than found for the C 0 2 --HF and N 2 0 - - H F complexes.7 The ozone submolecule in the 03-HF complex exhibits all three of its vibrational fundamentals perturbed enough to be resolved from the precursor bands.I0 The valence angle bending mode v; (the v2 mode of the ozone submolecule in the complex) was blue shifted 10 cm-' in the complex. The v I c mode was blue shifted by 12.0 cm-' while the v3c mode was red shifted 13.0 cm-I, giving 25 cm-I more separation between vlCand v~~than found for ozone itself. Increased separation between the symmetric and antisymmetric stretching modes suggests a small increase in the valence angle of ozone on complex formation. The vlc mode was substantially more intense relative to v3c in the complex than in O3 itself. The attachment of H F to one terminal oxygen may effectively reduce the symmetry and impart some antisymmetric character to this former totally symmetric stretching mode. A similar enhancement in v 1 intensity is observed for the asymmetric mixed isotopes 16-16-18 and 16-18-18. The HF ligand is bound to a terminal oxygen atom in ozone on the basis of the scrambled isotopic spectrum in Figure 3. The asymmetric mixed ozone isotopes exhibit stronger v I type modes than the symmetric ozone isotopes; ozone isotopic absorptions are noted in Figure 3. Based on the 12-cm-' blue shift for vIc, the stick spectrum is drawn for similar shifts in all ozone isotopes giving double weight to the asymmetric isotopes (Le., assuming equivalent terminal oxygen atoms as in ozone). The experimental complex spectrum clearly does not match the stick spectrum. In fact, two new bands, one strong and one weak, were observed for each asymmetric isotopic species. We assume that the stronger asymmetric mixed isotopic band is due to the lighter terminal isotope (16-16-18- -HF) and that the weaker band is due to the heavier terminal isotopic species (1 8-16-16- -HF). Apparently the middle component for the symmetric mixed isotopes 16-18-16 and 18-16-18 cannot be resolved into two bands, the same as found for isolated ozone,1° so only seven distinct bands were recorded for the complex. Finally, D F blue shifts v l Cfor ozone in the complex 1 cm-I more than HF whereas the v2cand v3cmodes are identical for the HF and D F complexes. It is anticipated that HF binds to a terminal oxygen, and if the two-lone-pair model3 discussed for SO2 is appropriate, a trans or cis configuration with an approximately 120° 0-0- -H angle - H F is drawn in 1 as the would be formed. The structure for 03cis form because GAUSSIAN 82 calculations have shown the optimized cis form to be more stable than the optimized trans form,I3 and the microwave spectrum of SO2--HF is interpreted in terms of a planar geometry with HF forming a cis hydrogen bond to a terminal oxygen atom.3 The 1:2 complex is expected to have the same structure as the 1:l complex plus the additional Hb-F submolecule bonded to Ha-F with an unknown relative orientation. The v, band at 3595 cm-I is clearly due to an H-F stretching fundamental in the 1:2 complex as substantiated by the HF/DF = 3595/2671 = 1.344 ratio. This absorption falls below both the v, mode of the 1:l complex (3803 (13) Davis, S . R., unpublished results.
Andrews et al. TABLE 111: HF Submolecule Modes (cm-') in Solid Argon for Simple Inorganic Oxide- -HF Comulexese
3907 3871 3851 3845 3807 3803 3789 3555
315 328, 325 315, 305 41 1, 391 384, 361 389 721, 614
"References 7-9 and Andrews, L.; Hunt, R. D., unpublished experiments.
cm-') and the usb mode of (HF), (3826 cm-l)" and is better described as the Ha-F stretching mode in 2. The effect of Hb-F attachment to 1 is to substantially reduce the Ha-F stretching mode whereas the effect of O3 on the Hb-F mode in (HF), is expected to be small, as is found for the SO,-(HF), complex produced in larger yield. Ultraviolet photolysis produced a marked growth in the 02-HF ~ o m p l e x absorbing '~ at 3917 cm-I at the expense of 0,-- H F as expected from the photochemical behavior of ozone. In addition, three sets of new bands were observed in the HF and DF regions, which exhibited H F / D F ratios of 1.367, 1.364, and 1.361, respectively, for H-F stretching modes in weak complexes. The first band at 3856 cm-I is tentatively assigned to the 0 atom complex 0--HF; since the proton affinity of 0 (1 16 f 1 kcal/mol) is greater than the proton affinity of 0, (101 f 1 kcal/mol),15 the 0 atom complex is expected to be stronger and exhibit a larger displacement in v,(HF). Similar behavior has been found for halogen atom and molecule complexes; the Cl- -HF species absorbs at 3858 cm-' with Cl--(HF), at 3739 cm-'.16 The second and third bands at 3796 and 3767 cm-' are tentatively assigned to the 02-(HF), and 0--(HF), complexes, respectively. Sulfur Dioxide. The spectrum of the SO2-- H F complex is - H F complex spectrum. The strong us band at similar to the 033807.0 cm-I exhibits a similar v,(HF)/v,(DF) = 3807.0/2794.4 = 1.362 ratio, but the v p doublet at 411 and 391 cm-' exhibits higher vi(HF)/vp(DF) = 1.343 and 1.348 ratios comparable to those of the C0,- -HFand N,O- - H F complexes.* One overtone was observed for vp(HF) and one for vp(DF), and it is not possible to relate it uniquely to one of the up fundamentals, although the 2vp(HF)/vp(HF) = 712/391 = 1.821 ratio is preferred. The corresponding 2vc(DF)/ve(DF) = 549/290 = 1.893 ratio is larger owing to less anharmonicity for the DF motion lower in the potential well. Only one perturbed SO, fundamental was observed in the complex; vIc was blue shifted from the precursor vl doublet. A much larger yield of the 1:2 complex was produced for SOz, and both H-F stretching modes were observed. The structure of the SO2--HF complex deduced from microwave studies3 is the cis form as depicted in 1 for 0,--HF. This is also in accord with GAUSSIAN 82 calculations, which show the optimized cis structure to be more stable than the best trans c~nfiguration.'~ Comparisons. Since the proton affinity of SO2 (162 f 3 k ~ a l / m o l ) 'is~ larger than that for O3 (124 kcal/mol theoretical)17 it might be expected that SOz forms a stronger hydrogen bond with H F than 0,. On the basis of the almost identical us modes for these complexes, which are compared in Table 111, this is apparently not the case. The SO2--HF complex is, however, slightly more rigid, based on the higher ve doublet at 41 1 and 391 cm-' as compared to the ut doublet at 384 and 367 cm-' for 0,- -HF. The v,(HF) absorption for SOz-- H F is substantially stronger than its ozone counterpart even at half the reagent concentration; this suggests that the SO2 complex is formed more readily than the O3 complex, since these H F modes should have (14) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1987, 86, 3781. (15) Lias, S . G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref: Data 1984, 13, 695.
J. Phys. Chem. 1988, 92, 81-85 approximately the same absolute intensity. Finally, the 03-HF complex is considerably stronger than the 02-HF complex; the latter exhibited a slightly perturbed v, mode at 3917 cm-' in soIid argon and was much better defined in solid neon.14 Conclusions The codeposition of HF and O3 with excess argon forms a well-defined 0 3 - - H F complex with v,(HF) at 3803 cm-' and ve(HF) = 384 and 367 cm-'. All three perturbed ozone fundamentals were observed for the complex. Two vlc modes were observed for the 16-16-18 and two for the 16-18-18 mixed (16) Hunt, R. D.; Andrews, L., to be published. (17) Kausch, M.; Schleyer, P. yon R. J . Comput. Chem. 1980, 1, 94.
81
isotopes, which show that the terminal oxygen atoms are not equivalent and indicate attachment of HF to a terminal oxygen atom in the complex. The structure justifies the considerable enhancement of vlC in the complex relative to vJC as compared to the infrared intensity of v1 to O3 relative to v3. This spectra of SO2-- H F and 0,- -HF are similar, and the microwave structure for the former is in agreement with the structure deduced from the latter FTIR spectrum.
Acknowledgment. We gratefully acknowledge financial support from N S F Grant C H E 85-1661 1, preliminary experiments performed by R. T. Arlinghaus, and communication of unpublished results by S.R. Davis and A. J. Downs. Registry No. Ar, 7440-37-1; 03, 10028-15-6; SOz, 7446-09-5; HF, 7664-39-3.
Matrix Trapping of Two Structural Arrangements of Weak Complexes Lester Andrews* and Rodney D. Hunt Chemistry Department, University of Virginia, Charlottesuille, Virginia 22901 (Received: June 17, 1987)
F H R spectra of Ar/CO/HF mixtures rapidly frozen at 12 and 5 K contained a weak new absorption at 3907 cm-' in addition to the strong OC- -HF complex band at 3789 cm-'. The DF counterpart absorbance at 2866 cm-' was a larger fraction of the absorbance for OC- -DF at 2781 cm-'. The former new bands were generally favored by slower spray-on rates and were destroyed on annealing that increased the latter bands. These weaker new bands are assigned to the reverse complexes CO- -HF and CO- -DF that calculations predict to be less stable than the carbon-bonded structures observed in the gas phase and in larger yield in matrices. An analogous situation was found for Ar/HCN/HF mixtures which revealed both HF--HCN and HCN--HF. Apparently matrix formation relaxes these molecules fast enough to trap the less stable complex structures that are not observed in supersonic nozzle expansion experiments.
Introduction One of the most interesting questions concerning weak complexes is intermolecular structure. Both theory and experiment have sought answers to this important question for a large number of simple systems involving HF plus a small molecule. Where theoretical calculations have shown two possible stable intermolecular arrangements, gas-phase studies involving seeded supersonic molecular beams have characterized only' the lower energy However, the matrix isolation technique can trap a complex in both stable structural forms where the matrix cage quenches internal energy and prevents rearrangement of the less stable to the more stable structural form. In the first such example, HF and H C N were trapped in the HF- - H C N and HCN- - H F arrangements, where each can act as both acid and base,6 and S C F calculations predicted the latter form to be more stable by 3.8 kcal/mol.' Of particular interest is the relative population of the two HCN complexes with DF, which was not determined in the argon matrix study owing to the especially small yield of DF- -HCN compared to H C N - -DF. There is evidence from both bond length4,' and (1) Curtiss, L. A.; Pople, J. A. J . Mol. Spectrosc. 1973, 48, 413. (2) Benzel, M. A.; Dykstra, C. E. J. Chem. Phys. 1982,77, 1602; J . Chem. Phys. 1983, 78, 4052; Chem. Phys. 1983, 80, 213. (3) Curtiss, L. A.; Pochatko, D. J.; Reed, A. E.; Weinhold, F. J . Chem. Phys. 1985,82, 2619. (4) Legon, A. C.; Soper, P. D.; Flygare, W. H. J . Chem. Phys. 1981, 74, 4944. ( 5 ) Legon, A. C.; Millen, D. J.; Rogers, S . C. Proc. R. SOC.London, A 1980, 370, 213. (6) Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1983, 105, 163. (7) Reed, W. G.; Campbell, E. J. J . Chem. Phys. 1983, 78, 6515.
0022-3654/88/2092-0081$01.50/0
TABLE I: Infrared Absorptions (cm-I) Containing CO, HF, and DF in Solid Neon at 5 K
HF
DF
assignt
3992.1 3953.8 3936.5 3928.8 3914.4 3898.4 3848.4 3807.4 3705.0 3671.2
2921.7 2900.1 2886.0 2880.6 2872.9 2857.9 2819.9 2793.9 2720.3 272 1.8 2172.0 2166.4 2141.1
HF RfO) HF(Q) induced 0 2 - -HF CO- -HF (v,) (HF), (Dl1 N2- -HF (HF), D2 OC- -HF (u,) (HQ3 T (HF), T (mixed) OC- -HF OC- -HF
co
vibrational frequency measurements8q9that DF approaches a base submolecule slightly closer than H F in the hydrogen bond; this suggests a preference for D F as the acid in the hydrogen bond. A noteworthy example is HF- -DF, which is more stable than the DF--HF form.1° Five analogous cases of role reversal have resulted in two complex arrangements being trapped for H F with H 2 0 , FC,H, C4H,, ClF, and Cl2.I1-l4 (8) Andrews, L.; Arlinghaus, R. T.; Johnson, G.L. J . Chem. Phys. 1983, 78, 6341.
(9) Andrews, L.; Davis, S. R. J . Chem. Phys. 1985, 83, 4983. (10) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1985, 82, 4442. (11) Andrews, L.; Johnson, G. L.J . Chem. Phys. 1983, 79, 3670. (12) Andrews, L.; Johnson, G. L. J. Phys. Chem. 1982,86, 3380. (13) Patten, K. O., Jr.; Andrews, L. J . Phys. Chem. 1986, 90, 3910.
0 1988 American Chemical Society
