832
C. M. Ellison and B. S. Ault
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
Infrared Spectra of the M'H12-, M'HCII-, and M'HBrI- Ion Pairs in Argon Matrices C. Michael Ellison and Bruce S. Ault * Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1 (Received September 8, 1978)
The salt-molecule reaction technique has been used to synthesize the HI;, HClI-, and HBrI- anions in ion pairs in argon matrices. Infrared spectra of the HI2- anion are characterized by an intense band near 673 cm-' due to the antisymmetric stretching mode u3 of the anion in a centrosymmetric Dmhgeometry. Weaker bands near 803 and 929 cm-' are assigned to the combination modes v1 + v3 and 2vl + u3. These spectra resemble closely the literature spectra of the HI2 radical, and suggest reassignment of the radical to the anion. Infrared spectra of the HClI- and HBrI- anions contained four bands, two of which were characteristic of hydrogen stretching vibrations, and have been assigned to a symmetric and an asymmetric form of the anion in these ion pairs.
Introduction The phenomenon of hydrogen bonding has interested chemists in all areas over the years as an interaction which is stronger than van der Waals and dipolar interactions, but weaker than normal chemical bonds, and also due to its importance in chemical and biological systems.lJ The simplest hydrogen bonded systems are the hydrogen dihalide anions, XHX-, which have been well studied since the discovery of HF2- in the 1950's. With all of this interest, however, few if any spectra have been reported3 for the iodine analogue, HI2-. One report has been made of the HI2neutral species? produced through the microwave discharge of Ar/H2/12 samples, but in view of the recent discoveries concerning the charged nature of the products of microwave discharge experiment^,^^^ there is reason to question the assignment to the neutral species. Recently, the inception of the salt-molecule matrix isolation technique has led to the synthesis of isolated ion pairs in low temperature mat rice^,^-^ allowing thorough spectroscopic investigation of the anion without crystal lattice interactions and distortions. The first of these matrix ~ t u d i e son , ~ HCL- in ion pairs with alkali metal cations, demonstrated a significant difference between the isolated ion pair and the crystalline anion. These matrix studies have been since extended to HF2-? HBr2-,9and the mixed anion HBrCl-. With the lack of good spectra of the HI2- anion, the effectiveness of the salt-molecule reaction technique, and the general interest in these strongly hydrogen bonded anions, a complete study was undertaken to thoroughly characterize the HI2- anion, as well as the mixed anions HBrI- and HClI- in low temperature argon matrices. Also, crystalline studies of HX2- have demonstrated two forms of the anions, type I and type I1 in the notation of Evans and Lo. Type I is characterized by a strongly asymmetric hydrogen bond, and hydrogen stretching frequency in the 1500-cm-' region while type I1 hydrogen dihalide anions have a centrosymmetric hydrogen bond, and a hydrogen stretching frequency between 700 and 800 cm-l. The factors which lead to a type I or type I1 anion are not well understood, and the application of matrix isolation may resolve this question. Moreover, the crystalline studies of the ClHI- and BrHI- anions are quite incomplete, lead to the type I anion exclusively, and point to a need for more complete study under matrix isolation conditions. Experimental Section All of the experiments performed here were conducted using a conventional matrix isolation apparatus which has been described previously.'" The salts employed in this 0022-3654/79/2083-0832$01 .OO/O
study were CsCl (Fisher), CsBr (Orion), CsI (Harshaw), RbCl (Fairmount), RbI (Fisher), KI (Fisher), and NaI (Fisher). HI (Matheson) was found to be very impure, but with repeated vacuum line distillations, a pure sample could be obtained. DI (Merck, 98%) was found to be quite pure, but was still distilled before sample preparation. HC1 (Matheson), DCl (Merck), and HBr (Matheson) were also employed in several experiments, and were usually used after two vacuum line distillations. Metallic sodium was employed in one experiment, and was handled in the usual manner." Argon served as the matrix gas in all of these experiments and was used without further purification. The salts (or sodium) were loaded in a stainless steel Knudsen cell, and outgassed for several hours behind a closed door, before the start of an experiment. Deposition temperatures for the different salts were as follows: CsC1, 500 "C; CsBr, 500 "C; CsI, 500 "C; RbC1,525 "C; RbI, 500 "C; KI, 530 "C; and NaI, 550 "C. Na was deposited at about 240 "C, and gave a characteristic dark red matrix. Matrices were deposited for 18-24 h, and complete scans were then recorded on a Beckman IR 12 infrared spectrophotometer over the range 4000-200 cm-l, a t normal scale, followed by high resolution scans a t expanded scale over the regions of interest.
Results Blank experiments were conducted with samples of Ar/HI, Ar/DI, Ar/HCl, Ar/DCl, and Ar/HBr, before salt reactions were carried out. One weak band was observed in the Ar/HI experiment, a t 2120 cm-', and may be due to monomeric HI. No bands were observed in the Ar/DI experiment, undoubtedly due to very low intensity. The blank experiments involving HC1, DC1, and HBr, all resulted in spectra which were virtually identical with previously reported spectra for these compounds and their dimers. CsI + HI. Five experiments were conducted in which CsI was codeposited with a sample of Ar/HI, with M / R values ranging from 250/1 to 2000/1. When CsI was deposited with a sample of Ar/HI = 400, three prominent new features were detected, at 673,803, and 929 cm-', with intensities of 0.39, 0.18, and 0.04 OD, respectively. In addition, two weak bands were observed at 644 and 1065 cm-l, which disappeared in all subsequent experiments, and can be attributed (as shown below) to the reaction product of CsI with impurity HC1. In the following experiments, with concentrations of M / R = 250,500,1000, and 2000, the three major bands remained present, and maintained the same intensity ratios throughout. At M/R = 2000, the three bands were still seen with intensities of 0 1979 American Chemical Society
Infrared Spectra of MH ’ ,;I
M’HCII-,
and M’HBrI-
Ion Pairs
TABLE I : Vibrational Absorptions and Band Assignments (cm-’) for the M+HI,- and M’D1,- Ion Pairs
a
ion pair
v3
Na+HI,K+HI,Rb+HI,Cs+HI,c s + DI, -
603 67 0 67 4 67 3 468
VI
t v3
725 799a 805 803 587
2v,
The Journal of Physical Chemlstty, Vol. 83, No. 7, 1979 833
7/
Na+HI
+ VI
-
927 928 929
V
Average of two site-split bands.
0,31,0,14, arid 0.03 OD, respectively. In addition, even at M/R = 250, no new bands were produced, despite the fact that the 673- and 803-cm-l bands were fully absorbing. CsI DI. CsI was codeposited with a sample of Ar/DI = 400, and two new product bands were observed in this experiment, as well as the three due to the “H” counterpart. These two new bands were located at 468 and 587 cm-l, with intensities of 0.91 and 0.13 OD, respectively. In addition, the ratio of intensities of the “H” and “D” bands suggested a deuteration ratio of about D / H = 3/1. No other product bands were observed in this experiment. N u l HI. NaI was codeposited with a sample of Ar/HI = 400 in one experiment and two product bands were observed in the final spectrum, at 603 and 725 cm-l, with intensities of 0.25 and 0.03 OD, respectively. Na HI. Metallic sodium was deposited with a similar sample of Ar/HI := 400 in one experiment, and the resultant spectrum was nearly identical with that obtained using NaI and HI. Both the bands at 603 and 725 cm-l were observed, and with the same intensity ratio. A weak band was observed a few cm-l above the 603-cm-l band in this experiment, and appears to be due to a site splitting of the 603-cm-l band. RbI HI. RbI was codeposited with a sample of Ar/HI in three different experiments, with M/R values of 500 and 1000. In the experiment using M/R = 1000, three bands were observed, at 674, 805, and 928 cm-l, with intensities of 0.38, 0.15, and 0.03 OD, respectively. The more concentrated experiment, at M/R = 500, yielded a similar spectrum, with the three bands maintaining a constant intensity ratio. K I HI. KI was deposited with sample of Ar/HI = 400 and 1500 and, in each case, product bands; were observed a t 670 cm-l, a doublet at 788 and 809 cm-‘, and 927 cm-l, with intensities of 0.24, 0.07, and 0.01 OD in the M/R = 400 experiment. In addition, a weak doublet was observed at 611,622 cm-l. This doublet did not maintain a constant intensity ratio to the other three bands in the two experiments, and can be assigned to the reaction product of KI with impurity HC1. Spectra of the reaction products of alkali iodide salts with HI and DI are shown in Figure 1, and tabulated in Table I. CsI + HCI. Cs[ was codeposited with a sample of Ar/HCl = 500, and the spectrum of this sample showed four new product bands, at 644,1065,1240, and 1560 cm-’, with intensities of 0.16, 0.13, 0.04, and 0.15 OD, respectively. The 1560-cm-’ band was quite broad, approximately 40-cm-l half-width, while the remaining bands were relatively sharp. CsCl + HI. These two reaction partners were codeposited in one experiment, at Ar/HI = 400, to provide a comparison to the above experiment. The final spectrum of this mixture showed the same four product bands, in approximately the same intensity ratio. The band positions were shifted by no more than 1 cm-’, and the band shapes were likewise comparable. In addition, one weak band was observed at 721 cm-l, which has been previously assigned to the Cs+HC12-ion pair.
+
+
+
+
+
,
920
880
840
800
WAVENUMBERS
760
720
,I, ,
680
,
840
,
,
600
(cm-1)
Figure 1. Representative infrared spectra of the reaction products of alkali iodide salts with HI in argon matrices over the spectral region 580-960 cm-‘.
TABLE 11: Antisymmetric Stretch Band Positions (cm-’) for the M’HClI- and M’HBrI- Ion Pairs and Their Deuterium Analogues HCIIRb
+
HBrIcs
+
CS’
v 3 (type I,
(type I, H ) D) v,(H)iv,(D)
1530 1210 1.26
1560 1219 1.28
920 728 1.26
(type 11, H ) (type 11, D) v,(H)lv,(D)
647 458 1.41
644 456 1.41
666 470 1.42
u3
u,
v3
+
CsI DCI. CsI was codeposited with Ar/DCl= 400 and the resulting spectrum showed new bands at 456,762,888, and 1219 cm-l, in addition to the “H” bands described above at 644, 1065, 1240, and 1560 cm-l. The intensities indicated a deuteration ratio of about D / H = 1. The 1219-cm-l band was also much broader than the remaining deuterium product bands, analogous to the 1560-cm-’ band in the “H” experiments. CsCl + DI. These reaction partners were investigated in two experiments, at M/R = 250 and 400. In the more concentrated experiment, new bands were observed at 456, 505, 762, 888, and 1219 cm-’, just as in the experiment described above, with the addition of the 505-cm-’ band which can be ascribed to Cs+DC12-. The “H” bands were also observed, and the deuteration ratio was about D / H = 1. RbI + HCI. RbI was codeposited with samples of Ar/HI = 400 and 500, and similar spectra were observed in both experiments, showing new bands at 647, 1065,1250, and 1530 cm-’, with the 1530-cm-l band much broader than the remaining bands. The intensities were 0.14,0.10,0.03, and 0.08 OD, respectively, in the M/R = 500 experiment, with somewhat greater intensities in the M/R = 400 experiment. RbI + DC1. Four new product bands were observed when these two species were reacted that were not present in the “H” experiment, at 458, 763, 895, and 1210 cm-l, with the 1210-cm-1band being considerably broader than the other three bands. The deuteration ratio in this experiment was about 211. The spectra of the reaction
034
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979 Rbl + H C I /
C. M. Ellison and
B. S. Auk
I
1200
1000 80C 600 W AV E N U V BE R S ( c m-1
>
Flgure 3. Infrared spectra of the reaction products of CsI with HBr, CsBr with HI, and CsBr with DI, over the spectral region 400-1200 cm-’. I 1600
1400 1200 1000 W A V E N U M B E R S (cm-1)
800
600
Figure 2. Infrared spectra of the reaction products of alkali iodide salts with HCI and DCI, and alkali chloride salts with HI, over the spectral region 400-1800 cm-I.
products of alkali chloride salts with HI and alkali iodide salts with HCI and DC1 are shown in Figure 2, and tabulated in Table 11. CsI t HBr. When CsI was codeposited with a sample of Ar/HBr = 500, four new product bands were observed also, a t 667, 799, 922, and 1171 cm-l, with intensities of 0.18,0.02,0.10, and 0.06 OD. In this experiment, the band a t 922 cm-l was considerably broader than the remaining bands. CsBr t HI. In this experiment, with Ar/HI = 400, the same four product bands were observed as in the CsI t HBr experiment, at 666, 799, 920, and 1171 cm-’, with intensities of 0.20, 0.04, 0.19, and 0.16 OD. CsBr t DI. When these two species were reacted with Ar/DI = 500, three new bands not present in the “H” experiment were observed at 470,728, and 862 cm-l, while the bands described above from the “H” experiment were also present with roughly the same intensity. Spectra of the reaction products of the mixed bromide/iodide experiments are shown in Figure 3, and tabulated in Table
11. Discussion HIz-. A clear trend emerges in all of the experiments in which an alkali iodide salt was codeposited with Ar/HI; namely, three product bands were observed in almost all cases. The most intense band was near 673 cm-l, with less intense bands near 800 and 930 cm-l. Since the experiments employing the cesium salt were studied most thoroughly the product bands in those experiments, at 673, 803, and 929 cm-’, will be used as a prototype for analysis. In the five experiments reacting CsI and HI, the three product bands maintained a constant intensity ratio to one another, even with a variation in the M/R from 250 to 2000. Thus, the bands can be assigned to the same absorbing species. Moreover, considering the high dilution used, assignment to a 1:l reaction product is reasonable. Two product bands were observed in the deuterium experiment, at 468 and 587 cm-l. These are shifted to positions which are correct for deuterium counterparts of the two more intense bands in the “H” experiment, the
673- and 803-cm-l bands. The deuterium shift ratios are u H / v D = 1.44 and 1.36 for the 673- and 803-cm-I bands, respectively. Also important in the deuterium experiment is the fact that no intermediate bands were observed half-way between the H and D counterparts, even though both were quite intense. This suggests that the absorbing species contains only a single hydrogen atom, again consistent with a 1:l reaction pair. Finally, the deuterium shift of the 673-cm-’ band, 1.44, is quite unusual, in that only a species with quartic anharmonicity can lead to a shift greater than the harmonic value of 1.41. However, cubic anharmonicity dominates the potential function for most molecules, unless this term must be zero by reason of symmetry, in which case the quartic term can then appear. The symmetry condition for a vanishing cubic term is an antisymmetric vibration of a species with a center of symmetry. Putting these facts together, the deuterium data plus the 1:l reaction pair, the assignment of the product band a t 673 cm-l to the antisymmetric stretch of a Dmhcentrosymmetric HI2- anion is the Cs+H12-ion pair is made. This band is characteristically very intense in hydrogen bonded system,12and was the most intense band observed here. This is also in agreement with the general results of salt-molecule reactions, in which an ion pair is formed, and in agreement with the earlier studies on HCl;, HBrz-, and HF2-. In the HCL- and HBrz ~ t u d i e s ,an ~ ,intense ~ band near 725 cm-I was observed in each case, and assigned to u3 of the appropriate HXL anion, in a centrosymmetric configuration, corresponding to the type I1 species of Evans and The large shift of the hydrogen stretching frequency here from free HI near 2120 cm-l is indicative of a very strong, completely shared hydrogen bond, as is often found in these systems. The sodium atom study confirms this as well, in that earlier workers have shown that the reaction of a sodium atom with HC1 can lead to the formation of the HC12anion,14 either through abstraction of a C1 from HC1 to form NaC1, which then can react with a second HC1 to form the Na+HCl; ion pair, or through direct reaction with (HCl), to form the ion pair plus an H atom. In the current study, reaction of Na atoms with HI lead to the same spectrum as did the reaction of NaI with HI; either reaction leading to the formation of Na+H12-. The band positions for the K+, Rb+, and Cs” ion pairs were very close, while the Na+ complex was shifted somewhat to
Infrared Spectra of M+H12-, M'HCII-, and M'HBrI- Ion Pairs
lower energy. This results has been noticed in previous salt-molecule reactions and attributed to the relatively small size of the Na+ cation perturbing the neighboring anion, more than the larger alkali metal cations. With this assignment, the two weaker bands in the spectrum need to be assigned. However, only one more vibration in a centrosymmetric triatomic anion is infrared active, the bending mode, which is expected to lie well below 800 cm-'. Rather, the 803-cm-l band showed a strong, but not complete deuterium shift, and no counterpart was observed for the 929-cm-l band. A similar situation was observed for the HBr; anion in salt-molecule ~ t u d i e s and , ~ the assignment was made to combination bands vl v3 and 2 v 1 v3. A similar assignment is reasonable here as well. This leads to a value of 130 cm-l for the fundamental transition of v1 of HI2-, and 126 cm-l for the 1 2 transition of vl, well below the values of 260 cm-l for HC12-14and 177 cm-' for HBI+;.~ This assignment also suggests why the dueterium counterpart of the 803-cm-' band was so weak relative to the v3 band, and why the deuterium counterpart of the 929-cm-l band was not observed. The combination bands are observed only through breakdown in selection rules due to anharmonicity. A major contribution to the anharmonicity in these species is the large amplitude of vibration of the hydrogen atom in the antisymmetric stretching motion. However, the amplitude is decreased by a factor of 2 in the deuterium species, leading to a more harmonic vibration, and less intense combination bands. These data also suggest a slight deuterium shift for v l of the anion, from 130 to 119 cm-l. A harmonic, centrosymmetric structure cannot account for such a shift, but it may possibly be accounted for by the reasonably large uncertainty in the measurement of the band positions of the combination bands as a result of low intensity and considerably band width. Comparison to prior studies is of interest; the one previous report3 on HI2- in crystals with the N(C4Hp)4+ cation located v3 at about 1650 cm-', and its deuterium counterpart at 1300 cm-l. This by analogy with HC12-must be the asymmetric type I form proposed for these anions, rather than the centrosymmetric form observed here. The deuterium shift in the crystalline species, 1.27, supports this conclusion, as it indicates large cubic anharmonicity, and an asymmetric anion. Comparison to the work of Noble4 on the HI2 radical should be made as well. Noble observed three bands in his reported spectrum of HI2,at 682,803, and 923 cm-l, just a few wavenumbers from the values obtained here for the HI2- anion. Moreover, the intensity ratios are almost identical, as are his assignments to v3, v1 + ul, and 2vl v3. As a result, using the analogy to the other HX2- species and the microwave discharge studies of these systems, the HI2 radical species of Noble must be reassigned to the corresponding anion. Finally, since the counterion in these microwave discharge studies is not known, it is noteworthy here that there is a very slight dependence at best of u3 upon the alkali metal cation and that the unknown counterion in the discharge experiment produces about the same magnitude of shift. HClI-. A number of experiments were conducted which might lead to the HClI- anion, with the Rb+ and Cs+ cations. In each case, four bands were observed in the spectrum; the most intense was a moderately sharp band at 644 cm-l, followed by a moderately intense band at 1065 cm-', a weak band a t 1240 cm-l, and a relatively intense, broad band a t 1560 cm-l, when the Cs+ cation was employed. These bands all persisted at high dilution, and maintained a relatively constant ratio with respect to one
+
+
-+
+
The Journal of Physical Chemistry, Vol. 83,No. 7, 1979 835
another. This, as with the HI2- anion, is indicative of a 1:l reaction complex. More important, the product species formed was independent of the synthetic route used; i.e. the reaction of CsI with HC1 yielded an identical spectrum, within 1 cm-l, to the reaction of CsCl with HI. This immediately demonstrates that a distinct species is being formed, not a perturbed salt molecule, and that the two halogen positions are chemically equivalent in the transition state. The deuterium studies showed a distinct deuterium counterpart for each of the four bands, at 456, 762,888, and 1219 cm-', with the same respective band shapes, and deuterium shift ratios of 1.41, 1.40, 1.40, and 1.28, respectively. Following the analogy to the study of HI;, and the earlier studies of HC1; and HClBr-, these four bands are assigned to the HClI- anion in the Cs+HClI- ion pair. However, a linear triatomic anion cannot account for four fundamental vibrations, and certainly not four in this spectral region. Moreover, the bands do not appear to fit any combination that might be anticipated for such an anion, based on known frequencies for similar species. The band at 644 cm-I, the most intense in the spectrum, seem appropriate for the v3 antisymmetric stretching mode of such an anion, based on the values of 673 cm-' for HI; and 723 cm-' for HCL-. However, the band at 1560 cm-' also has a shape and intensity appropriate for a hydrogen stretching motion in a hydrogen bonded species. The shift ratio of the 644-cm-l band is 1.41, which is nearly harmonic (the molecule cannot have a center of symmetry, so that values greater than 1.41 are not anticipated), while the shift ratio for the 1560-cm-l band is 1.28, showing very strong cubic anharmonicity. Since a single product cannot account for all four bands, and since two bands each show the characteristics of us, the results suggest that two different forms of the anion are formed in these salt reactions. This is identical with the study of HBrC1-, where bands at 742 and 1115 cm-I were assigned to the v3 mode of two different forms of the anion.g The two forms of the anion fit well into the categories of Evans and Lo, type I and type 11. Type I was characterized by an asymmetric anion, with the hydrogen favoring one side of the anion, and u3 was found in the region 1500-1600 cm-l, with a deuterium shift ratio between 1.26 and 1.30. The type I1 species was characterized by a single broad, nearly harmonic potential well, with v3 below 800 cm-l, and the observed value, 644 cm-', fits well here. All of these observations, and the analogy to HClBr-, suggest that two forms of the anion are formed. Two bands remain, near 1065 and 1240 cm-', each showing a strong deuterium shift. Since these do not shift in a manner appropriate for a combination with v l (as observed for HI;), the bending fundamental of each anion is a likely choice. These bending modes are possibly in this region of the spectrum, with medium to weak intensity. Which bending mode is assigned to which form of the anion is not clear, but a close monitoring of the intensity ratios of the two bands relative to the 644-cm-l band suggests that the 1240-cm-l band is associated with the 644-cm-' band, and the 1065-cm-l band is not, and thus is associated with the 1560-cm-' band. While this matching of v 2 with u3 is not definitive, it does not alter the major conclusion that two different forms of the anion are produced in these reactions. Finally, it must be considered that the two bands assigned to v 2 for each anion may in fact be due to 2v2. In many hydrogen bonded species, the overtone of the bending mode has been shown to have greater intensity2J5than the fundamental due to a large dipole moment derivative for the overtone. Nibler and
830
C. M. Ellison and B. S.Ault
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
Pimente12 assign v2 for the type I HClI anion at 485 cm-l, such that the overtone 2v2 would fall in the region where the two present bands are observed. Again, this point, which cannot be resolved clearly, does not alter the major conclusions. Nibler and Pimentel observed v3 for their HClI- anion at 2200 cm-l, which is clearly a type I anion, and also well above the value of 1560 cm-l observed here. This suggests that the matrix environment is helping to stabilize the anion as the shift downward of the hydrogen vibration in a hydrogen bond from the free acid is proportional to strength of the hydrogen bond.12 Why, then, are two forms of the anion produced in these salt-molecule reactions? This was not the case for any of the symmetric XHX- species, but has been observed for the unsymmetrical XHX'- species. The answer may lie with the placement of the cation in the ion pair. In the XHX'- species, two nonequivalent positions for the cation exist, whereas in the symmetric XHX- anion, only one position occurs. So, whether the cation favors the X side or the X' side of the XHX'- anion may be the factor that determines whether the type I or type I1 species is formed. If this is the case, then the relative amounts of type I and type I1 formed might be altered with different alkali metal cations. It was observed in the HClBr- study that smaller cations favored the type I1 species, while the larger cations, and in particular Cs' favored the type I anion. The same effect is observed here: the 1560-cm-l band is almost as intense as the 644-cm-l band with Cs', but in the Rb' experiment, the 647-cm-l band is almost twice as intense as the 1530-cm-l band. Thus, the relative amounts, and the formation itself, of two forms of the anion stems from the counterion in the ion pair. A second interpretation must be considered for the spectra of the ClHI- anion, and indeed for the BrHC1anion published previo~sly.~ The bending mode in a linear triatomic species is doubly degenerate, but this degeneracy may be lifted in the presence of an alkali metal cation, leading to an additional hydrogenic vibration from a single absorbing species. Using this interpretation, the band assigned above to the v3 mode (644 cm-l) of the type I1 anion would be assigned instead to one component of the split v2 vibration. This interpretation only requires the presence of a single species, rather than both the type I and type I1 anions postulated above. This interpretation cannot be ruled out completely, but several factors weigh the evidence against this view. First, the bending mode in hydrogen bonded species is known to be very weak, often a factor of 100 weaker than u3 antisymmetric stretching vibration.12 However, in the case of the ClHI- anion presented here (and the ClHBr- anion published earlier) the 644-cm-l band is as intense as the vg band (1560 cm-l) of the type I anion. It is hard to envision a mechanism by which one component of v2 could have an intensity equal to that of v3 in a strongly hydrogen bonded anion. Second, four hydrogenic vibrations were observed for both the ClHI- anion and the ClHBr- anions, and while the model of a split v2 band can account for three hydrogenic vibrations, it cannot account for the four which are observed for these anions. Certainly, for some of the XHY- anions studied here (see below, and the following paper), only three bands were observed, and could be accounted for with this model. However, whichever model is correctly should apply to all of the XHY- anions. Finally, the splitting of the vq mode would be attributed to the presence of the alkali metal cation. However, such a splitting was not observed for the HX,- anions, where
TABLE 111: Correlation of Type I u g Band Positions with the Proton Affinities of the Two Halide Anionsa v3
anion
(type!), cm-
HBrC1HBrIHIC1HClFHBrFHIF-
1110 920 1560 2500e 2803e 2960e
APA,~ A V , ~ kcal cm-' 10 10 20 37 47 57
1700 1530 1250 1460 1157 1000
Av/vd
0.60 0.62 0.44 0.37 0.29 0.26
a Cesium cation. Difference in proton affinities, from ref 17. Shift in hydrogen stretching frequency from the free HX acid (of the halide with the higher proton affinFractional shift in the hydrogen stretching freity). quency. e From ref 16.
the species were observed in ion pairs. In fact, in these systems the bending mode was observed weakly, and in several cases was not observed at all. As a result, while the possibility of a splitting of the bending mode contributing to the observed spectrum cannot be ruled out, it appears to be an unlikely possibility. A second alternative explanation for the observed spectra lies with aggregation in the matrix, and that one of the two observed species is either an aggregate species, or a species perturbed by a neighboring parent aggregate. However, there seems to be no concentration dependence of the two sets of bands; Le., one set of bands in not favored relative to the other at high concentrations. The concentration range studies was M/R = 100 to 500, which provides a sufficient range for testing. Moreover, the studies of the HX2- species were conducted from M / R values of 100 up to 3000, and no aggregation effects were observed in the M/R = 500 concentration range. Also, aggregation, or interaction with a parent aggregate, would likely lead to broad spectral features, which is not observed. This, plus past evidence from salt-molecule reactions, suggest that aggregation is not a problem at these concentrations. HBrI-. Several experiments were conducted in which the formation of the HBrI- anion was feasible. In these experiments, such as CsI + HBr, the spectra revealed four product bands, at 666, 799, 920, and 1171 cm-'. The deuterium experiment, however, yielded three bands, at 470,728, and 862 cm-l, which are apparently the deuterium analogues of the 666-, 920-, and 1171-cm-l bands, with shift ratios of 1.42, 1.26, and 1.36. In addition, the 920-cm-l band was much broader than the remaining bands. Using the same arguments employed above for the HClI- anion, the 666- and 920-cm-l bands appear to be hydrogen stretching modes in type I1 and type I anions, respectively. The shift ratio of 1.42, or nearly harmonic, for the 666-cm-' band is indicative of a type I1 species, while the band shape and shift ratio (1.26) of the 920-cm-l band indicates a type I anion. The remaining two bands at 799 and 1171 cm-' are best assigned t o either the bending mode of each of the anions, or to the overtone of the bending mode, analogous to the HClI- case above. Finally, it is noteworthy that here, too, the anion was formed from either reaction route, CsI HBr or CsBr + HI. Unfortunately, there are no known literature spectra of the HBrI- anion, so no direct comparison can be made. T y p e I Anions. The position of the antisymmetric strecth u3 in a number of type I anions is now available, including HClBr-, HClI-, HC1F-,16 HBrF-, and HIF-, and some correlations can be drawn. As shown in Table 111, the larger the difference in proton affinity of the two terminal halide anions, the smaller the shift of v3 from the
+
Infrared Studies of Ni+HF,- and M'XHF-
position of the free HX acid. When the terminal halide anions have almost identical proton affinities, such as HBrC1- or HIBr-, then the shift is very large, up to 1700 cm-l, and the closer the approach to the spectrum of the type I1 complex which, of course, represents the limit of zero proton affinity difference between the two halide anions. Type 11 Anions. The type I1 anions, just as originally characterized by Evans and Lo, have a u3 value between 650 and 750 cm-', are very intense, and show either quartic anharmonicity or a very harmonic potential function, in contrast to the type I. In the unsymmetrical anions, such as HClI-, a center (of symmetry is not possible, and quartic anharmonicity is not expected, but these anions show a potential function which is very close to harmonic, and certainly quite different from the large cubic anharmonicity term in the type I anions.
Conclusions The reaction of alkali iodide salt molecules with HI in argon matrices leads to the formation of the MfH12- ion pair, in which the HI2- anion contains a strong hydrogen bond, and has a D,, geometry. Similar reactions led to the formation of the HClI- and HBrI- anions, and the product species were found to be independent of direction of formation. The spectra indicate that two different forms of the anion are created, one with a nearly symmetrical
The Journal of Physical Chemistry, Vol. 83, No.
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hydrogen bond, and one which is distinctly asymmetric, or type I1 and type I in the notation of Evans and Lo. It was also found that the smaller alkali metal cation gave the type I1 anion preferentially (but not totally) while the larger cations have relatively more type I anion. Acknowledgment. The author gratefully acknowledges support of this research by the Research Corporation under Grant 8305, and the University of Cincinnati Research Council. References a n d Notes (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14) (15) (16) (17)
D. G. Tuck, Prog. Inorg. Chem., 9, 161 (1968). J. W. Nibler and G. C. Pimentel, J. Chem. Phys., 47, 710 (1967). D. H. McDaniel and R. E. Vallee, Inorg. Chem., 2, 996 (1963). P. N. Noble, J. Chem. Phys., 56, 2088 (1972). C. A. Wight, 8. S.Auk, and L. Andrews, J. Chem. Phys., 65, 1244 (1976). B. S.Ault, J. Chem. Phys., 68, 4012 (1978). B. S. Auk and L. Andrews, J. Chem. Phys., 63, 2466 (1975). B. S.Auk, J . Phys. Chem., 82, 844 (1978). B. S. Ault and L. Andrews, J. Chem. Phys., 64, 1986 (1976). B. S. Auk, J. Am. Chem. Soc., 100, 2426 (1978). L. Andrews, J . Phys. Chem., 73, 3922 (1969). G. C. Pimentel and A. L. McClellan, "The Hydrogen Bond", W. H. Freeman, San Francisco, 1960. J. C. Evans and G. Y-S. Lo, J. Phys. Chem., 70, 543 (1966). D. E. Milligan and M. E. Jacox, J . Chem. Phys., 53, 2034 (1970). W. E. Thompsonand G. C. Pimentel, 2.€!&rochem., 64, 148 (1960). B. S. Auk, J. Phys. Chem., following paper in this issue. J. L. Beauchamp, Annu. Rev. Phys. Chem., 22, 527 (1971).
Infrared Matrix Isolation Studies of the M+HF2- and M'XHF- Ion Pairs for X = CI, Br, and I Bruce S. Ault Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received November 75, 1978)
The triatomic anions HF2-, ClHF-, BrHF-, and IHF- have been synthesized in alkali metal ion pairs in argon matrices. Infrared spectra of the M'HF; ion pairs indicated a strongly bound species with u3 near 1360 cm-l and u2 near 1200 cm-l. Infrared spectra of the mixed XHF- anions were characterized by an intense band between 2500 and 3000 cm-l, a second intense band between 750 and 950 cm-', and, in the case of ClHF- and BrHF-, a third band between 725 and 825 cm-l. These data are employed to suggest that two forms of the anion are present, a type I asymmetric form, and a type I1 symmmetric form, with the position of the alkali metal cation in the ion pair determining which type of anion is formed. Introduction In recent years, the matrix isolation technique has been applied to the study of ion pairs, including the hydrogen dihalide anions paired with alkali metal ~ations.l-~ These ion pairs, formed through the salt/molecule reaction technique, have provided considerable information about the anions in a more isolated environment than previous studies of the anions in crystals. To now, however, little has been reported on the fluorine-containing hydrogen dihalide anions, including HF2-. With the continuing interest in these anions, and in the general phenomenon of hydrogen a complete study was undertaken to characterize all of the fluorine-containing hydrogen dihalide anions, following up an earlier preliminary comm~nication.~ Experimental Section All of the experiments performed here were conducted using a standard matrix isolation apparatus, which has 0022-3654/79/2083-0837$01 .OO/O
been described previously.8 The salts employed in this study were CsF (Alfa), KF (Fisher), CsCl (Fisher), CsBr (Orion), KI (Fisher), and CsI (Harshaw). HF (Matheson) was found to be very impure, but could be purified with repeated vacuum line distillations. The same was true, to a lesser degree, for HCl, HBr, and HI (all Matheson). DF, DC1, and DI (all Merck, 99% D) were relatively pure, and were employed after two freeze-thaw cycles under vacuum. Metallic sodium was employed in several experiments and handled in the usual manner.g Argon served as the matrix gas in all of these experiments and was used without further purification. The salts (or sodium) were loaded in a stainless steel Knudsen cell, evacuated in the vacuum vessel, and outgassed for several hours behind a closed door, at temperatures higher than those used during the experiment. Deposition temperatures for the different salts were as follows: CsF, 500 "C; KF, 650 "C; KI, 500 "C; CsC1, 500 "C, CsBr, 500 "C; and CsI, 500 "C. Na was deposited at about 240 "C and gave a characteristic deep red color to 0 1979 American Chemical Society
