J . Phys. Chem. 1988, 92, 3769-3774
3769
Infrared Spectra of Diatomic Halogen Complexes with Hydrogen Fluoride in Solid Argon and Neon Rodney D. Hunt and Lester Andrew$* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: October 5. 1987; In Final Form: December 18, 1987)
Chlorine was condensed at 12 K with H F in excess argon, which produced two 1:l complexes of the form HF-C12 and C12-HF on the basis of infrared spectra. Increasing the H F concentration favored secondary 1:2 complexes with both (HF),-C12 and Cl,-(HF), arrangements. The H F submolecule stretching frequency for the 1:1 anti-hydrogen-bonded complex was observed at 3902 cm-I; molecular beam electric resonance studies observed only the anti-hydrogen-bonded form, which ab initio studies predict is the more stable arrangement. The 1:l hydrogen-bonded complex, which is similar to N2-HF, displayed an H F stretching fundamental at 3860 cm-I. With DF the hydrogen-bonded complex population fraction was substantially larger than with HF on the basis of infrared band absorbances. Similar results were obtained for chlorine monofluoride complexes with hydrogen fluoride and deuterium fluoride. In contrast, hydrogen fluoride and fluorine produced only one product complex, with an H F stretching mode at 3915 cm-'. This complex is identified as Fz-HF on the basis of ab initio calculations. Photolysis of F2 and H F produced two new radical complexes, F-(HF) and F-(HF)S, with HF stretching modes at 3908 and 3901 cm-' for HF2 and 3795 cm-' for H2F3, which are markedly different from those of their anions. Similarly, the H F interaction with bromine and bromine monofluoride formed predominantly the hydrogen-bonded Br2-HF and FBr-HF complexes, respectively.
Introduction The hydrogen-bonding and Lewis acid-base interactions have been subjects of numerous investigations, since they play significant roles in determining the physical properties of a vast array of complexes. Since hydrogen fluoride can act as a Brransted acid or a Lewis base, relatively simple HF complexes serve as models for more complicated acid-base systems. Both types of H F complexes have been characterized in numerous theoretical and experimental studies. Investigations using molecular beam electric resonance spectroscopy'*2 have determined the most stable structures of C12 and ClF complexes with H F to be anti-hydrogen-bonded HF-C12 and HF-ClF. However, theoretical calculations suggested the possibility of a hydrogen-bonded species of almost identical ~tability,~"and the most recent ab initio calcul a t i o n ~agree ~ with the molecular beam studies on greater stability for the anti-hydrogen-bonded isomers. In contrast, a b initio calculations suggest that the hydrogen-bonded form of F2-HF is the more stable On the basis of these results, HF can serve as a Brransted acid or a Lewis base in complexes with diatomic halogens. The matrix isolation technique is especially suited for characterizing both types of weak interactions. HF complexes with the bases H2, 02,N2, and ClCN have been characterized by using infrared spectroscopy and the matrix The purpose of this study is to investigate different complexes between HF and halogen molecules and to examine the role that the diatomic halogens play in determining whether HF serves as an acid or a base. Infrared spectra of complexes of hydrogen fluoride with diatomic halogens will be described. (1) Baiocchi, F.A,; Dixon, T. A.; Klemperer, W. J. Chem. Phys. 1982,77, 1632. (2) Novick, S. E.; Janda, K. C.; Klemperer, W. J. Chem. Phys. 1976,65, 5115. (3) Umeyama, H.;Morakuma, K.; Yamabe, S. J . A m . Chem. SOC.1977, 99, 330. (4) Hobza, P.;Szczesniak, M. M.; Latajka, Z . Chem. Phys. Left. 1981, 82, 469. ( 5 ) Rendell, A. P. L.; Bacskay, G. G.; Hush, N. S.J. Chem. Phys. 1987, 87, 535. ( 6 ) Spase, A. M. J. Chem. Phys. 1983, 78, 5733. (7) Reed, A. E.;Weinhold, F.; Curtiss, L. A.; Pochatko, D. J. J . Chem. Phys. 1986.84, 5687 . ( 8 ) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J. Chem. Phys. 1983, 79, 2488. (9) Andrews, L.;Davis, S.R. J. Chem. Phys. 1985,83, 4983. (10) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1987, 86, 3781. (11) Hunt, R. D.; Andrews, L. J . Phys. Chem. 1987, 91, 5594.
0022-3654/88/2092-3769$01.50/0
Experimental Section The vacuum and FTIR spectroscopic techniques as well as the cryogenic apparatus used for the argon matrices have been described in earlier ~ o r k s . ' ~ *A' ~Helipex (Air Products) three-stage closed-cycle helium refrigerator provided 5 K refrigeration for condensing Ne matrices. All spectra were recorded with a Nicolet 7 199 Fourier transform infrared spectrometer between 4000 and 400 cm-' at 1-cm-' resolution. A single-beam spectrum of the CsI window at 5 or 12 K was recorded and used as a background for a single-beam spectrum of the matrix to produce a simulated double-beam spectrum for each experiment. The observed absorptions are accurate to k0.3 cm-'. Hydrogen fluoride (Matheson), deuterium fluoride, chlorine monofluoride (Ozark Mahoning), chlorine (Matheson), and bromine (Mallinckrodt) were each evacuated at 77 K to remove volatile impurities. Deuterium fluoride was prepared by reacting F2 (Matheson) with D2 (Air Products) at low pressures in a passivated stainless steel vacuum system. Mixtures of Br2 and F2 gave BrF in equilibrium, which was used without further purification. The acid and base samples were diluted between 100/ 1 and 400/1 mole ratios with argon (Air Products) or neon (Cryogenic Rare Gas). In the experiments with Ar matrices, reagent samples were codeposited on the CsI window (12 K) at approximately 14-15 mmol/h for 4 h. The matrices containing F2 and ClF were then photolyzed at 12-16 IC with a mercury arc. Finally each Ar matrix was annealed to 19-22 K for 10 min and then recooled to 12 K. In the N e matrix work, F2, ClF, and C12 samples were condensed with H F on the CsI window ( 5 K) at approximately 13 mmol/h for 2-3 h. The matrices containing F2 were then photolyzed at 5 K. Every N e matrix was warmed to 11-12 K and then recooled to 5 K. For both Ar and Ne matrices, spectra were taken before, during, and after sample preparation, photolysis, and annealing. Blank samples in Ar and N e were prepared and examined for each reagent separately. Results Fourier transform matrix infrared spectra of HF and HF/DF mixtures with diatomic halogens will be described in turn. Chlorine. Four experiments were performed with chlorine and hydrogen fluoride at different dilutions in argon at 12 K. With the most dilute samples, the spectrum, displayed in Figure 1b, (12) Andrews, L.;Johnson, G. L. J . Chem. Phys. 1982, 76, 2875. (13) Johnson, G.L.;Andrews, L. J . A m . SOC.1983, 105, 163.
0 1988 American Chemical Society
3770 The Journal of Physical Chemistry, Vol. 92, No. I S , 1988
Hunt and Andrews
TABLE I: Absorptions (cm-I) Produced on Codeposition and Photolysis of Diatomic Halogens and Hydrogen Fluoride or Deuterium Fluoride with Excess Argon at 12 K F2 HF F, DF CIF+ HF CIF+ DF CI2 HF C1, HF BrF+ HF BrF DF Br, HF Br, DF assgmnt 3896 2859 3902 2864 a 3892’ 2864‘ av, 3890 2852 av, (site) 3915 2874 3870 2840 3860 2831 2857 2826 3851 2824 v, 3913 287 1 3863 3853‘ 2826‘ 3845 2821 3841 2816 us (site) 3908 2867 3858 2830 3830 281 1 R = X-HF 3901 2863 R (site) 3804 279 1 3786 2775 a”sb 3799 2785 aulb (site) 3795 2783 R‘ = F(FH)2 3817 2799 374s 2748 3740 2743 (vsb) 3676 2693 3642 2677 3662 2689 T‘
+
+
+
+
+
+
+
“ I f any of the HF-BrF arrangement is trapped, it probably absorbs under the N2-HF band at 3881 cm-l. bSharp weak bands are the most likely candidate for the HF-Br, arrangement if trapped in these experiments. ‘A third site was also observed at 3848 and 2822 cm-’ in the 5 K experiment.
0
..
3F
-
I ill
f
iSC2
395C 99OC 285C NPVENUMBERZ
360C
Figure 1. FTIR spectra of hydrogen fluoride mixtures with chlorine monofluoride, chlorine, or bromine in solid argon: (a) after codeposition of 32 mmol of Ar/HF = 200/1 and 26 mmol of Ar/Br2 = 200/1 for 4 h (absorbance scale must be doubled for this spectrum): (b) after codeposition of 27 mmol of Ar/HF = 400/1 and 27 mmol of Ar/CI, = 200/1 at 12 K for 4 h; (c) after codeposition of 29 mmol of Ar/HF = 200/1, 36 mmol of Ar/HF = 200/1, and 36 mmol of Ar/CIF = 200/1 at 12
K for 4 h.
showed HF monomer and dimer bands (labeled H F and D),I4 absorptions due to water,15 and a band due to the N2-HF complex (labeled N).9 New product absorptions included a sharp band at 3902 cm-’ ( A = absorbance = 0.23, labeled av,) and a weaker absorption at 3860 cm-l ( A = 0.06, labeled vs). The matrix was annealed to 23 K and then recooled to 12 K; hydrogen fluoride dimer and impurity complexes increased, while absorptions for (HF)314and N,(HF)? appeared. The av, and v, product bands increased in intensity by 50%, a second site for the v, complex appeared at 3853 cm-’, and new bands appeared at 3786 (avSb), 3740 (vsa), and 3662 (T’) cm-’. With more concentrated H F samples the latter secondary product absorptions were weak on deposition and grew on annealing. The relative absorbance of a (14) Andrews, L.; Johnson, G.L. J . Phys. Chem. 1984, 88, 425. (15) Andrews, L.; Johnson, G.L. J . Chem. Phys. 1983,79, 3670
Figure 2. FTIR spectra of HF/DF and C12mixtures in solid argon: (a) after codeposition of Ar/(HF + DF) = 200/1 (60% DF) and Ar/CI2 = 200/1 at 5 K for 3 h; (b) after codeposition of Ar/(HF + DF) = 200/1 (90% DF) and Ar/CI2 = 200jl at 12 K for 4 h: (c) after warming to 23 K and recooling to 12 K. vs to v, was not affected by the relative concentrations of H F and CI, in the sample. Finally, the 550-cm-I region revealed no absorption. Two additional experiments were conducted with D F / H F (>90% DF) mixtures and chlorine samples, and the principal absorptions observed in the H F experiments were again present as shown in Figure 2b. New absorptions specific to the DF system appeared at 2864 ( A = 0.12, labeled av,) and 2831 cm-’ ( A = 0.10, labeled v,) with a component at 2826 cm-’. Annealing these samples first to 18 K and then to 23 K (Figure 2c) produced 30% increases in both major bands, doubled the 2826-cm-’ component, and produced the secondary product absorptions listed in Table I. Another argon/HF/DF (60% DF) mixture was codeposited with argon/Cl, at 5 K, and the same absorptions were observed but with different relative intensities. Figure 2a shows substantially more intensity in the u,(HF) and v,(DF) regions relative to the av, regions; notice also that the v, fraction of the total product absorbance has increased in both regions for 5 K deposition (Figure 2a) relative to 12 K (Figure 2b). Parallel chlorine experiments were conducted with HF and HF/DF mixtures at high dilution in neon codeposited at 5 K. The
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3771
Diatomic Halogen Complexes with HF in Ar and N e
TABLE 11: Absorptions (cm-') Produced on Codeposition and Photolysis of Diatomic Halogens and Hydrogen Fluoride Isotopes with Excess Neon at 5 K F2 + HF F2 + DF CIF HF C1F + DF C12 + DF Cl2 + DF assgmnt
+
3950 3942 3927 3832
289 1 2885 2880 2812
3697 892
2715
3886 3878 3868" 3822 3778 3762 3679
2885
3940 3930 3878
2887 2882 2847 2843 2837b 2798 2764 2759 2708
3930
avs au, (site)
2843
vs
(site) R = F-HF vi
;.
3838 3790 3743 3680 554 550
OR = CI-HF. bShoulder, R = C1-DF. W
2816 2779 2746 2708
vsb avsb
Ysa
T'
Y
v
(neon) (neon)
D
,I'
r
S
c I/ I
- I
11 4
,A00
3950
3900
3650 3800 WAVENUMBERS
3750
3jOO
3650
- Figure 3.
FTIR spectra of hydrogen fluoride-chlorine samples in solid neon: (a) after codeposition of 8 mmol of Ne/HF = 200/1 and 8 mmol of Ne/CI, = 200/1 and at 5 K for 1 h; (b) after codeposition of 18 mmol of Ne/HF = 100/1 and 18 mmol of Ne/CI2 = 200/1 at 5 K for 2 h.
product band positions are listed in Table 11. Figure 3 is representative of the spectra following deposition of C12 with HF. Several major differences were observed between the Ar and N e experiments. In Ne, the us absorption was much stronger than the av, absorption while the reverse was true for Ar matrices, and the v, absorption increased more on annealing than did the au, bands. An absorption at 3838 cm-' (labeled uSb) was observed in the Ne work, while its counterpart in Ar was probably obscured. With N e matrices, the secondary product absorptions were strong even after deposition. Finally, in all Ne/Cl, experiments including the C12 blank experiment, the ClZfundamental was observed as an isotopic doublet at 554 and 550 cm-', but this doublet was intensified two-fold with HF. Chlorine Monofuoride. A similar series of experiments was performed with chlorine monofluoride and hydrogen or deuterium fluoride in both Ar and Ne matrices, and the product band positions are listed in Tables I and 11, respectively. In the most concentrated A r / H F samples, Figure IC displays the spectrum following deposition. A strong 3896-cm-I band ( A = 0.24, labeled au,) with a weaker 3890-cm-' component was the major product absorption, and a sharp weak 3870-cm-' band ( A = 0.01, labeled us) was a minor product absorption; no new absortions were observed in the CIF fundamental region. Photolysis of this matrix produced a sharp absorption at 3858 cm-', which is due to the C1-HF radica1.l6J7 In D F experiments with ClF, the strong av, bands at 2859 and 2852 cm-' also dominated the weak us band (16) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1988, 88, 3599 (17) Ault, B. S. J . Chem. Phys. 1987, 68, 4012.
'1800
335C
3600
3650 3bCC WAVENUMBERS
3j5C
370G
3650
Figure 4. FTIR spectra of hydrogen fluoride-fluorine samples in solid argon: (a) after codeposition of 26 mmol of Ar/HF = 100/1 and 32 mmol of Ar/F2 = 100/1 at 12 K for 4 h; (b) after photolyzing at 12-16 K for 1 h; (c) after warming to 22 K and recooling to 13 K.
at 2840 cm-', much the same as the relative band intensities with HF. Neon matrix studies with H F and C12 gave the strong au, band at 3930 cm-I ( A = 0.44) and the weaker us bands at 3886 and 3878 cm-' ( A = 0.05),but the corresponding DF experiments gave a slightly enhanced v, band at 2843 cm-' ( A = 0.04) relative to au, at 2887 cm-' ( A = 0.15). Complementary experiments with FZ,HC1, and DCl in solid neon gave new photolysis product bands at 3868 and 2837 cm-' for the C1-HF and Cl-DF radicals, respectively. For both Ar and Ne matrices, the major difference between C12 and ClF was the relative intensity of the v, bands, which were much weaker in the C1F experiments. Fluorine. A parallel study was conducted with fluorine and hydrogen fluoride. The frequencies of the product absorptions are listed in Tables I and I1 for the Ar and Ne work, respectively. In the most concentrated samples with use of Ar, the spectrum, displayed in Figure 4a, showed the H F and impurity bands described earlier. New absorptions included an extremely strong and sharp band at 3915 cm-' ( A = 1.2, labeled v,) with a satellite at 3913 cm-' ( A = 0.08), a sharp doublet at 3817 and 3814 cm-' (labeled uSb),and a weaker band (labeled T') at 3676 cm-I. Figure 4, parts b and c, displays the spectra following photolysis and
3712
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
sample annealing (12-23-12 K). A sharp doublet at 3908 and 3901 cm-' (labeled R) and a strong absorption at 3795 cm-' (labeled R') appeared following photolysis and grew sharply during sample annealing. A similar growth pattern was observed for the 1489-cm-' band, which has been assigned to F 0 2 formed from reaction of F atoms with trace O2impurity in the fluorine sample.'* In the experiments using Ne, the results were slightly different from those in Ar. New, sharp absorptions, which have no Ar counterparts, appeared at 3832 (labeled vsb) and at 893 cm-' (labeled u ) , while no Ne counterpart was seen for the R' band. Several experiments were performed with DF/HF mixtures, and the DF counterpart absorptions are given in the tables. Bromine and Bromine Monofluoride. Hydrogen fluoride experiments with bromine and bromine monofluoride were performed in Ar matrices. The major product absorption in the H F experiments with Br, was a strong band at 3841 cm-' ( A = 1.0, labeled v,) with a weaker component at 385 1 cm-' ( A = 0.2), and a sharp new 3892-cm-I band ( A = 0.05 labeled (av,)) may be due to a minor product, which are compared in Figure 1 with the product distribution from C1F and C1, experiments with HF. In the BrF experiments, which also contained F2 and Br,, a sharp new v, doublet was observed at 3857 and 3845 cm-'. Band positions for the DF counterparts observed in separate experiments are presented in Table I.
Discussion The new product bands will be identified, and vibrational assignments will be made. Bonding trends for these diatomic halogen complexes will be discussed, and competing effects between various halogen subunits will be examined. Identification. The new product absorptions listed in Tables I and I1 were not observed in argon or neon matrix samples of diatomic halogens or hydrogen fluoride alone; however, these bands were produced in high yields when the reagents were mixed during the condensation process. Five groups of HF product absorptions can be identified on the basis of concentration, annealing behavior, band position, and theoretical calculations. Absorptions of the first group were generally strong after sample deposition regardless of concentration, and their positions were between the N2-HF9 and (HF),I4 bands except for the case of F,. Absorptions in the second group were strong after reagent condensation at all concentration levels, and these absorptions were slightly red shifted from the HF-induced Q branch at 3919 cm-I but not as far as the N,-HF complex. bands of the third and fourth groups were weak in Ar and strong in N e following sample deposition; these absorptions increased in intensity by approximately 50% on sample annealing. Weak absorptions in the fifth group were red shifted from the T bands. The first group of absorptions (labeled v,) were usually strong after sample deposition, which indicates these bands belong to the major product species. This group of bands maintained constant relative intensities over the wide range of sample concentrations and annealing conditions. The first group of absorptions can be assigned to the 1:l complex with a hydrogenbonded structure 1 taken from ab initio calculations5 primarily x-x
3
2
1
7-1
F,
x2
H-F' 5
H
F-H-F 6
4
F
-H-F \ H\
F
Hunt and Andrews in the u,(DF) band intensity with C1, relative to av,(DF) (0.10 to 0.12) as compared to the weaker v,(HF) band intensity relative to av,(HF) (0.06 to 0.23) further supports assignment of the v, bands to a hydrogen-bonded complex owing to the known enhancement of stability for deuterium in the hydrogen-bonding p ~ s i t i o n . 'The ~ v, absorptions in the HF and F2experiments have been placed in the first group solely on the basis of ab initio calculation^^^^ even though their band positions are closer to those in the second group. The absorptions (labeled av,) of the second group exhibited the same concentration dependence as observed for the first group, which indicates that these absorptions can be attributed to a second 1:l complex most likely with the anti-hydrogen-bonded structure 2. This identification is in good agreement with the spectra of the complexes between cyanogen halides and HF. The third group of H F product absorptions with labels v, and v,b displayed a higher order concentration dependence on HF, which is characteristic of a 1:2 complex, 3. Absorptions of the N2(HF), complex9 served as a useful model for the identification of the bands of the third group. The bands (labeled avSb) exhibited the same concentration dependence as the third group; this observation indicates that these absorptions can be attributed to a second 1.2 complex most likely with structure 4. The fifth group of H F product bands (labeled T') displayed an even higher order concentration dependence on HF, and these absorptions are probably due to the 1.3 complex with structure 5. Two additional groups of H F product absorptions (labeled R and R') can be identified by band positions and straightforward F atom addition in F, photolysis experiments. Absorptions of both groups did not appear until photolysis, and they had the same growth on annealing as did FO,, which is a useful monitor of F atom addition. On the basis of their band positions, these groups of absorptions can be assigned to the 1:l and 1:2 complexes of atomic F and HF even though these absorptions were not observed in earlier H F and F2 photolysis e x p e r i m e n t ~using ' ~ a Beckman IR-12, which has less sensitivity in the H F stretching region. The analogous Cl-HF radical was also observed on photolysis of the FCl-HF complex. In H F experiments with Fz and C1, in solid neon, the molecular halogen fundamental was detected in the initial Ne/F, and Ne/Cl, samples, on the basis of close proximity to the gas-phase fundam e n t a l ~ but , ~ ~these ~ ~ ~weak bands were intensified (at least twofold) in the codeposition experiments with N e / H F samples. However, no absorption was observed in these regions in argon matrix experiments. It appears that repulsive interaction with the small neon matrix cage, which may necessarily have vacancies, gives a small amount of asymmetry and infrared intensity to the halogen fundamental, and this effect is enhanced by the presence of the HF ligand. Similar activation of normally infrared-inactive modes has been observed for H 2 and 0, in solid Ne.Io Assignments. The strong, sharp product absorptions (labeled v,) have similar D F counterparts with H F / D F ratios of 1.360-1.366, which is characteristic of the fundamental vibration of the H-F ligand in the primary complex 1. These bands are displaced up to 75 cm-' from the 3953- and 3919-cm-I H F fundamentals in solid neon2' and argon,I4 respectively, due to the hydrogen-bonding interaction. Similarly, the strong, sharp bands (labeled av,), which were less red shifted than their hydrogenbonded counterparts, exhibited like D F absorptions with HF/DF ratios of 1.361-1.366. These absorptions assigned to the fundamental vibration of the H F Lewis base ligand in complex 2 are only slightly red shifted (1 3-29 cm-') from the HF matrix fundamental due to the Lewis acid-base interaction with the fluorine in HF, which perturbs the largely hydrogen motion of the H-F stretching fundamental substantially less than does the hydrogen-bonding interaction. The v,, vSbbands are assigned to the fundamental vibrations of the H F submolecules in the 1:2 complex 3. The HF/DF ratios,
?
on the basis of band position. In addition the marked enhancement (18) Arkell, A. J . A m . Chem. SOC.1965, 87, 4057
(19) Barrow, R. F.; Ketteringham, J. M. Can. J . Phys. 1963, 4 1 , 419. (20) Andrychuk, D. J . Chem. Phys. 1950, 18, 233. (21) Andrews, L.; Bondybey, V. E.; English, J. E. J . Chem. Phys. 1984, 81, 3453.
Diatomic Halogen Complexes with H F in Ar and N e
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3773
c o m p l e ~ e s . 'In ~ ~the ~ ~HF-HCN case, a definite isotopic prefwhich ranged from 1.362 to 1.366, again verify the HF nature erence was found for the HCN-DF arrangement,24as was obof the vibrations. These assignments appear quite reasonable when served here for C12-DF. compared to the u, and v,b bands in the Nz(HF)2 complex.' The A final interesting comparison of the us fundamental for the v,b absorption for F,-(HF), at 3817 cm-' exhibits a very small HF2 radical complex (3908 and 3901 cm-') should be made with displacement from Y,b of (HF), at 3826 cm-' owing to the weak the centrosymmetric HFz- anion (1377 cm-'), which was produced interaction with the F, base, which shifts us in Fz-HF to 3915 cm-', by photoionization and electron impact of A r / H F sample^.^^^^^ only 4 cm-' below the HF-induced Q branch in solid argon.14 The extremely large difference in v, modes is expected since Likewise, the auSbassignment to complex 4 follows. These ausb theoretical calculations have predicted quite different hydrogenabsorptions are red shifted from the corresponding u,b bands at bond strengths for the (F-H-F) radical (3 kcal/mol) and the 3848 and 3826 cm-' for the (HF), dimer in solid neon,' and (F-H-F)- anion (40 k c a l / m ~ l ) . ~The ~ removal of an electron argon,I4 respectively, due to the Lewis acid-base interaction befrom HF, eliminates the symmetrical electron-electron repulsion tween Cl, (or CIF) and the (HF), dimer. Furthermore, it is readily that forces HF, to be centrosymmetric and reduces charge seen that the ausbmode in complex 4 is more perturbed by the separation within the electrostatic approximation. Both of these diatomic halogen than the Usb mode in complex 3. factors weaken bonding in the HF, radical,27which is an asymBonding Trends and Comparisons. Several interesting bonding metric, weak, hydrogen-bonded complex.28 A similar effect was trends were observed in this study. First, the hydrogen-bonded found for the F-(HF), radical and (F-H-F-H-F)anion.26 complexes provide information about the relative basicity of the A final comment is in order concerning a possible role for the diatomic halogens. The us fundamental is 3915 an-'for (F,)(HF), matrix in determining the relative populations of the fluorine- and 3869 cm-I for (CIF)(HF), 3860 cm-' for (Cl,)(HF), 3857 cm-' hydrogen-bonded complex arrangements, particularly in the C1, for (BrF)(HF), and 3851 cm-' for (Br,)(HF), and insofar as the and HF case, where stabilization energies are close for the two shift in the u, mode provides a measure of the strength of the arrangement^,^ and the Cl, and DF case, where the stabilization base-HF interaction, the order of base strength is F, < C1F < energies may in fact favor the deuterium-bonded form. The major C1, < BrF < Br, with HF serving as a proton donor. As indicated interaction between these complexes and the matrix host is by these results, the basicity of Xz (X = F, C1, or Br) increases probably of dipole-induced dipole origin, and the structure with as size (or polarizability) increases. In addition, the basicity of Y F (Y = F, C1, or Br) increases as the size of Y and the difference the larger dipole moment is likely to be favored by the matrix. Since ab initio calculationsSpredict a slightly larger dipole moment in electronegatives between Y and F increases. Second, the au, (2.2 D) for the hydrogen-bonded form than observed (1.4 D) for fundamentals for C1, (3902 cm-') and CIF (3896 cm-') give the fluorine-bonded form,' the matrix may contribute slightly more information about the relative Lewis acidity of Clz and CIF. C1F is a stronger Lewis acid than C1, when HF serves as the Lewis to the stabilization energy of the hydrogen-bonded form. Any matrix enhancement of the hydrogen-bonded form appears to be base, and this trend dominates the relative yield of 1:l complexes in these studies on the basis of infrared band intensities for similar quite small, however, on the basis of the observation of even more relative population for the hydrogen bonded C1,-HF form in less complexes. polarizable solid neon at 5 K. It is interesting to speculate on the factors that govern the population fractions of the hydrogen-bonded (1) and anti-hyConclusions drogen-bonded (2) complexes in these diatomic halogen matrix experiments. We can only assume that the H-F stretching funIn summary, chlorine and HF interact in solid argon to form damental extinction coefficients are comparable in complexes 1 a well-defined 1:1 anti-hydrogen-bonded complex of the form and 2 for a given diatomic halogen; the hydrogen-bonded arHF-Cl, as well as a hydrogen-bonded complex of the form C12-HF rangement, however, probably has the higher infrared intensity, based on infrared spectra; only the former has been observed in and observation of this form is thereby favored in the matrix the gas phase,' which ab initio calculationss predict to be the more infrared experiments. Several comparisons can nevertheless be stable arrangement. The matrix au, mode is comparable to other made: the relative yield of the hydrogen bonded form (1) increases cyanogen halide-HF observations for the anti-hydrogen-bonded from C1F to Cl, to Br, and is clearly dominant for Br, (Figure product. The hydrogen-bonded complexes C1,-HF and N2-HF 1). Gas-phase beam studies should be able to observe the Br2-HF gave similar us modes, which sustain larger displacements due to structural arrangement. The population fraction of the hydrointeraction with the hydrogen that contributes much more to the gen-bonded complex with C1, is increased with D F over HF vibrational amplitude than fluorine. The yield of C12-DF is (Figure 2). This arises because of the greater stability of D F increased relative to DF-C1, compared to the yield of C1,-HF relative to H F in the "hydrogen-bonding" p o ~ i t i o n . ~ ~On' ~ * ~ ~relative ~ ~ ~ to HF-C1,. This suggests that the Cl,-DF and DF-C1, the basis of infrared band intensities the matrix experiments are arrangements have similar binding energies and that both should in agreement with gas-phase beam studies' and theoretical calbe observed in a gas-phase study. In experiments with more c u l a t i o n ~that ~ HF-Cl, is more stable than C1,-HF; however, the concentrated HF samples, two secondary reaction products were energy separation between these two arrangements is small (
