J. Phys. Chem. lQ81, 85, 608-613
increase in icelike water is responsible for the observed higher endothermic AH. At the same time, the large alkyl groups of these species and the hydrocarbon tails of the lipid form a lot of mutual icebergs due to hydrophobic interactions. This makes TAS a dominant term over AH in the equation, AG = AH - TAS.33 As a result of this, the free energy of the melting is lowered, which drives t downward. At the phase transition temperature the free energy of transition is zero. In order to have AH compensate for TAS so as to give AG = 0, the phase transition temperature of melting has to be lowered, i.e., t - to < 0. A complementary explanation can also account for the lower AH and higher t in the presence of a water structure (33)G.Nemethy, H.A. Scheraga, and W. Kauzmann, J. Phys. Chem., 42,1842 (1968).
breaker such as MelNBr or methanol. The partition coefficient of a solute between water and lipid phases can also be used to account for a lowering in t. The free energy of transfer of a solute from an aqueous medium to the lipid medium may be determined by the partition coefficient of a solute between water and lipid. The solubility of a solute in the lipid phase will lower its free energy of transfer. This will contribute to a lowering in the free energy of melting, which results in driving t downward. Nevertheless, the information on the partition coefficient or the free energy of transfer is not sufficient to provide a solid rationale for the change in AH of the melting. Acknowledgment. The author thanks Professor Henry S. Frank for very helpful suggestions and Dr. Donald S. Berns for comments on this manuscript.
Double-Salt Reactions In Matrix Isolation Studies. Applications to the Tin Trihalide Anions Camille J. Kallendorf and Bruce S. Ault Deparfment of Chemlstry, University of Clnclnnatl, Clnclnnatl, Ohio 4522 7 (Received September 22, 1980)
A technique has been developed involving the reaction of two salts, one an alkali halide, for the formation of ion pairs in matrices, as a route to anionic intermediates. Double-salt reactions can be carried out either by allowing the two salts to react in a single Knudsen cell followed by vaporization of the product ion pair or by vaporizing the salts from two separate Knudsen cells, followed by gas-phase reaction during condensation into an inert matrix. The single-oven reactions are particularly appealing in that, for the system studied, vaporization takes place at a temperature well below that needed to vaporize either parent salt, and the spectra obtained are free from any parent absorption. Application of this technique has allowed for the study of SnFC and its chloro-fluor0 analogues. Spectra and normal coordinate calculations for the SnF'f anion agreed well with literature values, while spectral characterization of the anions SnClFz and SnC12F-was accomplished for the first time. Tentative identification of the SnF42-anion was also made.
Introduction Considerable interest has been expressed in recent years concerning the spectroscopy of ions in other than crystalline or solution environments.l In the last 10 yr, the matrix isolation technique has been employed in a variety of studies aimed at the investigation of isolated ions. One approach, the salt/molecule reaction technique, has been used successfully for the formation of anions ion-paired with alkali metal cations in inert matrices. Species such as HF2-, F3-, PF4-,and COF3- have been studied in this f a s h i o P ' through the cocondensation of an alkali metal fluoride salt MF with a suitable gaseous Lewis acid diluted in argon. However, many compounds which exhibit at least some Lewis-acid character are solids themselves at room temperature, while the salt/molecule technique has been limited up to this point to gas-phase Lewis acids. The reaction of an alkali metal fluoride with such a solid might also lead to ion-pair formation, and this ion pair might then be vaporized and deposited into an argon matrix. Cer(1)L. Andrews, Annu. Rev.Phys. Chem., 30,79 (1979). (2)B. S.Ault, J.Phys. Chem., 83,837 (1979). (3) B.S. Ault and L. Andrews, Inorg. Chem., 16, 2024 (1977). (4)P.Wermer and B. S. Ault, Inorg. Chem., in press. (5)B. S.Ault, J. Phys. Chem., 84,3448 (1980). 0022-3654/81/2085-0608$01.25/0
tainly, the phase diagrams of CsF and a number of salts indicates distinct 1:l and 2:l compound formation,8,' suggestive of ion-pair formation. Alternatively, two salts could be vaporized from separate ovens and allowed to react on the surface of the cold window during the condensation process. Either approach, single or double oven, might lead to the formation of new and interesting ions through this double-salt technique. It should be noted, also, that this approach is not entirely new; there are a few literature reports of such double-salt reactionsF9 but the potential for this approach has not been fully exploited. An appropriate test system, where the potential product anion is known, is the MF/SnF2 system. The SnFf anion has been formed in crystals, although spectra were taken with two large bulky cations, due to cation interference.1° In addition, mass-spectral data indicate that the SnF3anion is quite stable.ll A vaporization-mass-spectral study (6)0.Sshmitz-Dumontand G. Bergerhoff, Z . Anorg. A&. Chem., 283, 314 (1956). (7)J-C. Vouillon, M-T. Saugier, J-J. Counioux, and R. Cohen-Adad, Bull. SOC.Chim. Fr., 10, 1669 (1976). (8) A. S.Kana'an, R. H. Hauge, and J. L. Margrave, J. Chem. Soe., Faraday Trans., 72,1991 (1976). (9)A. Snelson, B.N. Cyvin, and S. L. Cyvin, J. Mol. Struct., 24,165 (1975). (10)I. Wharf and D. F. Shriver, Inorg. Chem., 8,914 (1969).
0 1981 American Chemical Society
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Double-Salt Reactions in Matrix Isolation Studies
Schematic diagram of the vacuum vessel and oven arrangement used for double-ovenexperiments in double-salt reactions. Figure 1.
has also been carried out for KSnF3,and the predominant gas-phase species12above the solid was the monomeric ion pair KSnF3. In addition, with the MF/SnFz system, the possibility exists for the isolation of the SnFt- anion, which has been postulated in the literature but never conclusively identified or characterized.13J4 Finally, the mixed anions SnCIFz-and SnClzFhave not been characterized, as disproportionation occurs in condensed phases. Formation through the double-salt technique, using either a single oven or two ovens, might provide a direct synthetic route to these intermediate anions. With this background, a study was undertaken of the SnF:, anion through the double-salt technique and matrix isolation spectroscopy. Experimental Section The experiments carried out during this investigation were conducted by using a conventional matrix isolation apparatus, which has been described previously.16 The system was modified to house two ovens for some experiments, each aimed at a 45O angle to the 15 K window, as shown in Figure 1. The salts employed in this study were loaded in a stainless-steel Knudsen cell and outgassed before the start of an experiment. However, in the single-oven experiments, the temperature necessary for vaporization was not known ahead of time, and caution was taken not to outgas these samples more than necessary. Consequently, small amounts of H20 and COzwere present in some experiments. The salts employed were CsF (Alfa), CsCl (Fisher), SnF2 (Ozark-Mahoning), SnClz (Alfa), SnClF (Alfa), KF (Fisher), RbF (Alfa), NaF (Allied (11) S.L. Bennett, J. L-F. Wang, J. L. Margrave, and J. L. Franklin, High Temp. Sci., 7, 142 (1975). (12) J. W. Hastie, K. F. Zmbov, and J. L. Margrave, J. Inorg. Nucl. Chem., 30,729 (1968). (13) A. N. Kruglov and V. P. Kochergin, Nauchn. Dokl. Vyssh. Shk., Khim. Khim. Tekhnol., 70 (1969). (14) US. Patent 3 184326 (Republic Steel Corp.), 1960. (15) B. S.Ault, J. Am. Chern., 100,2426 (1978). (16) R. H. Hawe, J. W. Hastie, and J. L. Margrave,J. Mol. Spectrosc., 45, 420 (1973).
Chemical),and LiF (Fisher). Argon was used as the matrix gas in all experiments and was used without purification. Matrix samples were deposited for 16-24 h, with an argon deposition rate of 2 mmol/h, before final survey, and high-resolution scans were recorded on a Beckman IR 12 infrared spectrophotometer,with 1-cm-l resolution. In the single-oven experiments, approximately equal amounts of the two salts were mixed in the Knudsen cell, although a number of experiments demonstrated that the relative amounts of the two salts mixed did not affect the vaporization product, as long as roughly comparable amounts were employed. Results Before the reaction products were investigated through this double-salt technique, blank experiments were conducted on each of the new salts employed in this study, namely, SnF2, SnClF, and SnC1,. The spectra obtained after vaporization at 500, 375,and 250 "C, respectively, agreed well with the literature matrix spectra for SnFzand SnCl2.l6t1' The spectra obtained after vaporization of SnClF showed that some disproportionation had occurred in the Knudsen cell, as bands due to SnClz were present. In addition, a band was observed at 576 cm-l, which can be assigned to the Sn-F stretch of SnClF, while a band corresponding to the Sn-C1 stretch was not observed, because of either low intensity or overlap with the SnClz bands at 351 and 334 cm-l. CsF SnFP When CsF and SnFz powders were mixed together in a single Knudsen cell and the mixture was heated, the vaporization product in an argon matrix was characterized by two bands, at 454 and 509 cm-l. In all of the experiments described here, a window band was observed at -474 cm-', as can be seen in Figure 1. However, this band was present before the start of each experiment and cannot be assigned to a product species. Both bands were quite intense and moderately sharp, with bandwidths of 5-10 cm-l, although the 509-cm-' band was slightly sharper than the band at 454 cm-'. In a number of experiments, over a range of slightly different vaporization temperatures, the relative intensities of these two bands remained nearly constant. More importantly, the temperature necessary to vaporize the absorbing species was nearly 200 "C below that needed to vaporize either CsF or SnFz (both require near 500 OC). Thus, only two bands were observed in the spectrum, and no parent bands were detected. When the vaporization temperature was increased or the length of deposition was increased, a third, weak band was detected, near 256 cm-l. Although intensity measurement was difficult under these conditions, this weak band did appear to maintain a constant intensity ratio to the bands at 454 and 509 cm-l. In addition, bands at 385 and 1024 cm-l were occasionally observed in these experiments and are assigned to impurity SiF4 formed through the reaction of the vaporized product with the quartz sleeve of the oven. When the same reactants, SnF2 and CsF, were independently vaporized from two different Knudsen cells, quite similar results were obtained. Both ovens were heated to nearly 500 OC, and bands due to CsF and SnFz were both observed. In addition, the same bands at 509 and 454 cm-l were detected, although with considerably less intensity than in the single-oven experiments. RbF + SnF,. These two reactants were investigated in a single-oven experiment, and in this case also the va-
(17) D.L.Frederickson and L.Andrews, J . Am. Chem. SOC.,92,775 (1970). (18) B. S. Ault and L. Andrews, J. Chem. Phys., 63, 2466 (1975).
The Journal of Physical Chemistry, Vol. 85, No. 5, 1981
Kallendorf and Auk
TABLE I: Band Positionsa and Assignments for the Matrix-Isolated Tin Trihalide Anions
509 454 256
510 278 265
Band positions in cm-'.
assignmentC Sn-F stretch Sn-F stretch
Sn-C1 stretch symmetry. C, sym I
porization product appeared at a temperature well below that needed to vaporize either reactant. Here, too,product bands were observed at 454 and 509 cm-', with the same relative shapes and intensities as in the cesium reactions. The overall yield was slightly less, and the weak band in region was not detected. Because of the the 200-300-~rn-~ extreme difficulty in vaporizing RbF (650 "C is needed), double-oven experiments were not conducted on this system. K F + SnFP When KF and SnF2were mixed in a single Knudsen cell and the mixture was heated, the results were nearly identical with those obtained with CsF and RbF. Two product bands were observed, 455 and 509 cm-l, with intensities and bandwidths similar to those of the previous experiments. Again, no parent bands were detected in the spectrum, and the overall band yield continued to drop off with decreasing radius of the alkali metal cation. NaF + SnF2 Somewhat different results were obtained when these two reactants were investigated in a single-oven experiment. When this mixture was heated, bands near 508 and 454 cm-l were detected, as in the previous experiments. However, in some experiments, a very intense, broad band (half-width 30-40 cm-l) was observed near 590 cm-'. When present, this band dominated the spectrum and is clearly due to a chemical species different from that responsible for the 454- and 509-cm-l absorptions. LiF + SnF2 When these two reactants were loaded in a single Knudsen cell and heated, only a single product band was detected in each experiment, at 590 cm-l. Just as in the sodium experiments, this was an extremely intense, broad band, with a possible counterpart band near 210 cm-'. However, this is just at the instrumental limit, and the existence of this band cannot be determined with certainty. Bands were not detected at 509 and 454 cm-'. Spectra of the reaction products of some of the alkali fluoride-SnF2 single-oven reaction products are shown in Figure 2, while band positions are listed in Table I. CsCl SnC12. The study of the reaction products of these two reactants presented some additional difficulties; SnC1, absorbs at 334 and 351 cm-l, and the absorption of the reaction products is anticipated below that, in the region 200-300 cm-l. This spectral region is partially obscured by absorption of atmospheric H 2 0 and is also a region of low instrumental sensitivity. Nonetheless, when CsCl and SnC12were loaded in a single Knudsen cell and heated, bands were observed near 252 and 285 cm-l, which could be clearly delinated by comparison to a background spectrum of atmospheric H20 recorded immediately after the end of the experiment. In addition, parent bands of SnC1, were also observed quite strongly at 334 and 351 cm-l, indicating that the reaction product was about as volatile as the parent SnC12.
E NE RGY km.' I
Figure 2. Infrared spectra of the single-oven reaction products of some of the alkali fluoride salts with SnF,. The top traces show blank experiments of SnF, abne and of CsF alone. The middle trace shows the spectrum of the vaporidon products above a solid mixture of CsF and SnF,. The fourth trace shows the analogous results with KF, while the last trace shows the resuits with NaF. The band near 474 cm-' in most traces is a window band (see text).
When the same pair of reactants were investigated in a two-oven experiment, bands due to SnC12were again detected (CsC1 does not absorb in the region above 200 cm-', the instrumental cutoff). In addition, apparent weak bands were observed near 250 and 280 cm-l, although these were quite weak, and only separated from the atmospheric H20 background with difficulty. Evidence for the presence of CsCl was detected in the observation of a band at 462 cm-l which has been assigned to a complex between CsCl and impurity H20.16 Mixed Halide Anions. Numerous synthetic routes are available for the synthesis of the two mixed haloanions, SnClF2- and SnC12F-. These included a variety of precursors, as well as single- and two-oven experiments. When CsCl was mixed with SnF2in a single-oven experiment and the vaporization products were condensed into an argon matrix, bands were observed at 454,490, and 509 cm-' (see Figure 3). The bands at 454 and 509 cm-l resembled closely the bands obtained in the CsF/SnF2 single-oven experiment. The overall yield in this experiment was not extremely high, and no bands were observed below 300 cm-l where an Sn-C1 stretch might be located. A similar experiment was conducted in which CsF was mixed with SnClF in a single Knudsen cell and the vaporization products were condensed. The same set of three bands were observed, with the bands at 454 and 509 cm-' being roughly twice as intense as the band at 490 cm-'. CsF and SnC12were reacted in both single- and double-oven experiments, and similar results were obtained in all experiments. When CsF and SnC12were vaporized from separate ovens and cocondensed into an argon matrix, three intense product bands were observed, at 265, 278, and 510 cm-l. In addition, bands due to the parent species CsF and SnC12were observed at 313,334, and 351 cm-l. In the analogous single-oven experiment, product bands were detected at 265, 278, 454, 490, and 510 cm-l, but bands due to the parent salts CsF and SnF2 were not detected. Discussion When CsF was mixed with SnF2powder and the mixture heated, the bands observed in the matrix spectrum clearly indicate that a new product has been formed, absorbing at 509 and 454 cm-l. The fact that these two bands showed
The Journal of Physicai Chemistry, Vol. 85, No. 5, 198 1 611
Double-Salt Reactions in Matrix Isolation Studies :sF/SnCIF
420 380 340 WAVENUMBER (crn")
Figure 3. Infrared spectra of the mixed chloro-fluoro tin trlhallde anions, formed through both single and double oven experiments. The top trace shows the singleoven reaction products of a mixture of CsF and SnCIF, while the second trace shows the single-oven reactlon products of CsCl wlth SnF,. The third trace shows the single-oven reactlon products of CsF with SnCI,, while the bottom trace shows the double-oven reaction products of the same two reactants. Atmospheric water background between 200 and 350 cm-I has been subtracted for clarity in the last two traces. The band near 474 cm-' in most experiments is a window band.
a constant intensity ratio over a number of experiments indicates that they can be ascribed to the same species. The vaporization of the new product took place, in most cases, nearly 200 "C below that needed to vaporize either parent salt, so that these were the only bands observed in the spectrum. The vaporized species can be viewed as either a distinct reaction product or a mixed-salt dimeric species. However, it would be anticipated that the vibrations of a mixed-salt dimer would be dependent on which of the alkali metal cations was employed, while a distinct reaction to form an ion pair might have no metal cation dependence. Within the accuracy of band position measurement, no metal cation dependence was observed, suggesting that a distinct reaction had occurred, yielding a 1:1, OF possibly 2:1, species. Experiments in which the two reactant salts were vaporized from separate ovens and allowed to react during condensation into the matrix confirm the single-oven results. The same set of bands at 509 and 454 cm-, were observed when CsF and SnFzwere reacted, along with the expected parent bands. This, in part, rules against 1:2 reaction product, or higher aggregate, as at high dilution usually only the 1:l product is observed. The anticipated anion formed through fluoride ion transfer from CsF to SnF, is the SnF, anion, in an ion pair with Cs'.. The SnF,- anion has been observed in crystals'l and hence provides a test of this reaction technique. The crystalline spectrum of SnF,- has been observed by using two large cations, as each cation obscures a portion of the region of interest. The composite spectrum of SnFs under these conditions shows two bands in the Sn-F stretching region, at 520 and 478 cm-l, which have been assigned to the symmetric and antisymmetric stretches (A, and E under CBusymmetry), respectively. The bands observed here at 509 and 454 cm-I show only slight shifts to lower energies, and the evidence clearly suggests that the Cs+-
SnFT ion pair is formed and vaporized through the double-salt technique. The slight shifts in band positions can easily be rationalized, either by the fact that a much smaller monoatomic cation was employed here or by the general effect of matrix shifts to lower energy for ionic compounds, which is well k n 0 w n . ~ ~ 3In ~ ~addition, vaporization studies above solid KSnF, have shown that the dominant vapor-phase species is KSnF,, lending support to the assignment here.', Finally, it is noteworthy that vaporization occurred nearly 200 "C below that needed to vaporize either reactant salt to 1 pmHg premure, indicating that the reaction to form M+SnFr is a condensed-phase reaction, not a vapor-phase reaction within the Knudsen cell. Certiinly, however, formation of the Cs+SnF3-ion pair in double-oven experiments is a gas-phase reaction in front of the surface of the cold window. The single-oven reactions of NaF and LiF with SnF2 gave rise to an additional band near 590 cm-'; with NaF this band was a second product, while with LiF this band represented the only product produced, and represents a new chemical species. One possibility is SnF,, which can be formed through the partial disproportionation of SnF2 to metallic Sn plus SnF,. However, the matrix spectrum of SnF4has been recorded16and shows the intense triply degenerate Sn-F stretch near 671 cm-'. Moreover, the disproportionation of SnF, to Sn and SnF, should be relatively independent of the alkali metal fluoride salt employed. This is clearly not the case; only LiF and NaF gave rise to the 590-cm-' product band. Another possible species produced in a solid mixture would be a 1:2 product; the SIIF,~-anion has been proposed as a stable anion for years but has never been isolated and characterized.13J4 The gas-phase vaporization study of NaSnF3indicated that the primary vaporization species was NaSnF,, but the next most abundant species was Na2SnF4,so that, in the vapor above a solid mixture of NaF and SnFz, the Na2SnF4 species might be anticipated.12 It is also noteworthy that in the vaporization study of KSnF, the analogous species K2SnF4was not observed, suggesting that only the small alkali metal cations can stabilize this species. When LiF was employed, the 590-cm-' product band was the only band observed, lending further credence to the assignment of this intense band to the SnF42-anion in the M2SnF4 triple ion. The ionic nature of the species is supported f i s t by the consideration of the requirement for very small alkali metals and secondly by the fact that the band did not shift position as the alkali metal cation was changed. Little can be deduced about the structure of the SIIF,~-anion; however, the observation of a single, broad, intense Sn-F stretching mode suggests a geometry which is tetrahedral, or nearly so, and the degeneracy of the F2 mode is not destroyed, within the bandwidth of the 590-cm-' band. Since SIIF,~- is isoelectronic with PF4- or SF,, such a pseudotetrahedral structure is not anticipated. However, the very large central atom may stabilize this structure, although any discussion of the structure of the SnF42anion on the basis of a single absorption must be taken as very tentative. Simple normal coordinate calculations were undertaken for the SnF3- anion, to provide some comparison to those obtained previously. However, with only two stretching frequencies and a possible bending mode, a very simple force field was employed: a stretching force constant FR and a stretch-stretch interaction term FR-k A fit was (19) S. Craddock and A. J. Hinchcliffe, "Matrix Isolation", Cambridge University Press, New York, 1976. (20) M. J. Linevaky, J. Chern. Phys., 34, 587 (1961).
The Journal of Physlcal Chemistry, Vol. 85, No. 5, 1981
obtained by using C3,symmetry for F R = 2.16 and FR-R = 0.17 mdyn/A, which compares to the values of F R = 2.38 and FR-R = 0.16 mdyn/A obtained previously.’l The slightly lower value of F R is a consequence of the lower values for both the Al and E frequencies relative to the crystal, which is due either to a difference in cation or to a red matrix shift. The calculations, which used literature Sn-F bond lengthdl and an F-Sn-F angle of 90°, also confirmed that the 454-cm-l band is doubly degenerate and that the 509-cm-l band is the singly degenerate Al mode. The bending mode a t 256 cm-l was not included in the calculations, as this band was very weak and cannot be assigned with the same degree of confidence. Numerous studies have been conducted in recent years on the mode of interaction between cation and anion in matrix isolated ion pair^.^'-^' A bidentate arrangement is common, although a monodentate configuration has been observed.24However, in the previously studied examples the possibility of coordination to a lone pair on the central atom has not been present. The spectroscopic effects of ion pairing are not apparent in the present study; the degeneracy of the E mode is not removed, within the bandwidth of this absorption. Monodentate and bidentate arrangements, which both yield C, symmetry, are expected to split the E mode%into A’ A”, and this splitting is not observed. Normal coordinate calculations were conducted to determine the magnitude of splitting for a specific change in force constant of the coordinated fluorine or fluorines. A change AFR between coordinated and noncoordinated fluorines of 0.10 mdyn/A gave a splitting between A’ and A” of -8 cm-l, while a value of AFR = 0.20 gave a splitting Au of -15 cm-l for both mono- and bidentate geometries. Since the bandwidth of the 454-cm-’ band was 4-5 cm-l in the more dilute experiments, a splitting of more than 5, and certainly more than 10 cm-’, should be detectable. This suggests either that the interaction is not monodentate or bidentate or that AFR is less than 0.10 mdyn/A. In other ion-pairing studies, A F R / F R values of 0.25-0.50 have been2’-%observed, so that a value less than 0.10/2.16 = 0.05 here is very unlikely. The interaction is more likely C3,, with the metal cation lying on the C3axis either tridentate with the three fluorines or interacting directly with the lone pair on the tin center. It would be very difficult to distinguish between these two configurations with the data presently available. It is somewhat suprising that there is no detectable shift when the alkali metal cation is changed certainly in other ion-pair studies this is not the case. However, of the many matrix isolation investigations of ion pairing, there have been no reports of the effect of coordination to a lone pair rather than to a ligand. Hence, it is difficult to anticipate the magnitude of cation shift with such an interaction. This argument perhaps slightly favors a lone-pair cationanion interaction but is certainly not definitive. There is no other ready explanation as to the lack of cation shift, but this should not detract from the assignment to the SnF3- anion ion paired with alkali metal cations. Overall, the effects of ion pairing are minimal, with no removal of degeneracy and virtually no shift in band positions with a change in metal cation. However, the product yield did seem to be cation dependent, with the yield decreasing as the alkali metal decreased in size. This
(21) D. Smith, D. W. James, and J. P. Devlii, J.Chem.Phys., 64,4437 (1971). (22) G.Ritzhaupt and 3. P. Devlin, J. Chem. Phys., 62,1982 (1976). (23)N. Smyrl and J. P. Devlin, J. Chem. Phys., 60, 2640 (1974). (24) R.L. Hunt and B. S. Ault, Spectrochim. Acta, Part A, in press. (26) K. Nakamoto, “Infrared and Raman Spectra of Inorganic and Coordination Compounds”, 3rd ed., Wiley-Interscience,New York, 1978.
Kallendorf and Auk
is likely because of the higher lattice energy associated with a smaller cation and the need for more heat to vaporize the product ion pair. Certainly, as noted above, the formation of the SnFd2-anion was quite cation dependent. The analogous chloride system proved considerably more difficult, partly because of experimental difficulties in the 200-300-cm-’ spectral region and the lower absorption coefficients for Sn-C1 stretches. Nonetheless, in both single-ovenand two-oven experiments, product bands were observed near 252 and 285 cm-l, which can be ascribed to the SnClf anion. These band positions compare well to the crystalline spectrum of this anion:’ which absorbs at 258 and 292 cm-l. A number of experiments were conducted which might lead to the mixed halogenated anions SnClF; and SnCI2F, in a fashion analogous to the perfluoro and perchloro experiments discussed above. When either CsCl was mixed with SnF2or CsF was mixed with SnClF in a single oven, the same set of bands were observed, at 454,490, and 509 cm-l. The bands at 454 and 509 cm-l match well those of SnFc and indicate that, in single-oven experiments involving two salts with different halogen atoms, reaction and disproportionation do occur. The remaining band, at 490 cm-l, can be assigned to one of the two Sn-F stretching modes of the SnC1F2-anion. The location of the second mode is not clear; this mode derives from the 454-cm-l band of SnF, and may be hidden under this band, which was also present in these experiments. The 509-cm-l band of SnFf transforms into the Sn-C1 stretch of SnC1F2- and was not observed, probably because of lack of intensity and difficulty in detection between 200 and 300 cm-’. When CsF was reacted with SnC12in either a single-oven or two-oven experiment, product bands were detected at 265,278, and 510 cm-’. These bands are appropriate for assignment to the SnC12F-anion; the lower two are assigned to the two Sn-C1 stretching modes (near those of SnC13- at 252 and 285 cm-’) while the 510-cm-’ band is assigned to the Sn-F stretch of this anion. It should be noted that, upon reduction of symmetry from C3, (Le., SnF3-) to C, (SnC12F),the symmetric stretching mode at 509 cm-’ transforms into the stretch of the unique halogen,2SLe., the Sn-F stretch of SnC12F-, and this mode shows virtually no shift,helping to confirm the assignment. That this band is not due to SnFf due to disproportionation is apparent from the absence of the counterpart SnF3- band at 454 cm-l, in the two-oven experiment.
Conclusions The reaction of two salts, one an alkali halide, has provided a route to the synthesis of new anionic intermediates in inert matrices. This reaction may be carried out either by mixing the two salts in a single Knudsen cell and heating the resulting solid-state mixture or by vaporization from two separate ovens and cocondensation into an inert matrix. The former method, employing a single Knudsen cell, leads to vaporization at a temperature well below that needed to vaporize either parent salt. This indicates that the reaction between the two salts is occurring in the condensed phase and leads to very clean spectra, free from absorption bands due to the parent species. This method has been applied here to the study of the tin trihalide anions and demonstrates that the spectra obtained are very close to those obtained for the SnF3- and SnC13- anions in crystals. In addition, the use of mixed halide salts has allowed for the synthesis of the mixed halogenated anions and provided the first spectroscopic information on these anions. When these anions were studied via single-oven reactions, some disproportionation was observed, but reactions using two separate
J. Phys. Chem. 1981, 85, 613-618
ovens confirmed the identity of these mixed anions. Application of this double-salt method to other chemical systems is apparent, and studies aimed at new anions accessible through this method are currently underway.
Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation under Grant No. CHE78-27643, and by the Research Corporation under Grant No. 8305.
Circular Dichroism Spectra and the Interaction between Acridine Dyes and Deoxyribonucleic Acid Danlel Fornaslero and Tomas Kurucsev’ Depaement of Physical and Inorganic Chemlstty, The Unlversty OF Adekkle, AdelaMe 500 1, South Australla (Recelved: April 2, 1080; In Final Form: July 3, 1980)
The circular dichroism of proflavin and acridine orange complexed to DNA has been studied at several ionic strengths. The spectra were interpreted entirely and successfully in terms of the long-axis-polarized electronic transitions of the dyes. The experimental spectra were attributed to the circular dichroism of intercalated and nonintercalated bound “monomeric” dye species superimposedby degenerate vibronic exciton interactions between them.
Introduction The first report that, upon binding to DNA, acridine dyes may become optically active’ appeared in 1961. Since that time it has become generally accepted that the phenomenon, in particular the observed circular dichroism (CD) spectra, should be interpreted in terms of three major types of interactiom2 First, dichroism is induced by the asymmetric binding sites; secondly, degenerate exciton interactions between bound dyes produce a “conservative” spectrum with equal total contributions from positive and negative rotatory strengths; and, thirdly, exciton interactions between nondegenerate transitions may turn such spectra into “nonconservative” types. However, in spite of the above recognition and of the variety of empirical correlations established between the CD of dye/DNA systems and the dye to DNA concentration the temperature5?’and the chemical structure of the acridine dyesaJOthere has not emerged as yet a satisfactory unified model for the interpretation of the observations. In this work we shall show that, at least at the lower regions of dye loading, most of the difficulties encountered in the interpretation of the CD spectra of the acridine dyes proflavin (PF) and acridine orange (AO) bound to DNA are resolved provided that two major assumptions are made. First, the manifestation of exciton interactions in (1) Neville, D. M.; Bradley, D. F. Biochim. Biophys. Acta 1961,50,397. (2) For example, Jackson, K.; Mason, S. F. Trans. Faraday Soc. 1971, 67, 966. (3) Yamaoka, K.; Resnik, R. A. J. Phys. Chem. 1966, 70,4051. (4) Gardner, B. J.; Mason, S. F. Biopolymers 1967,5, 79. (5) Li, H. J.; Crothers, D. M. Biopolymers 1969,8, 217. (6) Dalgleish, D. G.; Fujita, H.; Peacocke, A. R. Biopolymers 1969,8, 633. (7) Zama, M.; Ichimura, S., Biopolymers 1970, 9, 53. (8) Fredericq, F.; Houssier, C. Biopolymers 1972,11, 2281. (9) Lee, C. H.; Chang, C. T.; Wetmur, J. G. Biopolymers 1973, 12, 1 M
(10) Dalgleish, D. G.; Peacocke, A. R.; Fey, G.; Harvey, C. Biopolymers 1971,10,1853. 0022-365418112085-0613$01.2510
the CD spectra should be attributed to the “dimeric” bound species described by Armstrong, Kurucsev, and Strauss’l and consisting of an intercalated and a nonintercalated dye. Secondly, it must be recognized that exciton coupling occurs between vibronic rather than pure electronic states of the dyes.12-14 Experimental Section The methods of purification of the acridine dyes used have been described before.’lJ5 The calf-thymus DNA preparation was that used previ~usly.’~Absorption and circular dichroism spectra were measured, respectively, with a Zeiss DMRlO recording spectrophotometer and a JASCO J-4W recording spectropolarimeter with 1-,2-, and 5-cm pathlength silica cells. All measurements refer to a temperature of 20 f 1 “C. Data Treatment Binding Model. The existence of the following binding equilibria was assumed AF
+ DNA & Al + AF & AlA2
where AF, Al, and AIAz refer to dye molecules which are, respectively, free and nonassociated in solution, bound to DNA in monomeric form (intercalated), and bound in dimeric form. The last symbol emphasizes that a dimer is taken to consist of an intercalated (A,) and a nonintercalated (A,) dye.” Due to aggregation of the dye the concentration of AF may be substantially less than the total free dye concentration. (11) Armstrong, R. W.; Kurucsev, T.;Straws, U. P. J.Am. Chem. SOC. 1970,92,3174. (12) Fulton, R. L.; Gouterman, M. J. Chem. Phys. 1964, 41, 2280. (13) Weigang, 0. E. J. Chem. Phys. 1966, 43, 71. (14) Gianneschi. L. P.: Kurucsev. T. J . Chem. Soc., Faraday Trans. 2 1976, 72, 2095. (15) Kelly, G. R.; Kurucsev, T. Biopolymers 1976,15, 1481.
0 1981 American Chemical Soclety