摘要:
Chemical Corn mu n i cat ions NUMBER 24/1968 18 DECEMBER The Br,+ and Braf Cations: an Example of a Resonance Raman Spectrum By R. J. GILLESPIE* and M. J. MORTON (Departme rtt of Chemistry, McMaster Univevsity, Hamilton, Ontmio) IODISE can be oxidised in solution in a strong acid, such as H,SO,, HSO,F, or HzS20,, either by the acid itself or by an added oxidising agent, e.g. S,06F,, to the cations I,+ and I,+. Evidence for I,- was obtained thirty years ago1 but the evidence for I,+ is more recent., There has been no previous evidence for analogous stable cations of bromine but we now show that in media of suf- ficiently high acidity both Br,+ and Br,+ exist as stable species. When bromine nionofl~orosulphate~ is dissolved in the superacid4 HSO3F-SbF,-3SO, (I) a new species is formed which is not obtained in fluoro- siilphuric acid alone.This species has a strong visible absorption a t 510 m,x (Figure 1) and the Ranian spectrum of its solution in the superacid solvent shows, in addition to weak peaks due to bromine monofluorosulphate and the solvent, a new intense fundamental band a t 360 cm.-l. That both spectra arise from the same species was confirmed by changing the Ranian exciting wave- length from the 6325 laser line whichogives a pre-resonance Raman spectrum to 5145 A which gives a resonance Raman spectruni.5 The inten- sity of the 360 cm.-l peak is considerably enhanced relativc to the other peaks a t the resonance frequency (Figure 2) and strong overtones are observed siinilx- to those observed previously6 in the resonance Raman spectrum of 12+.Dtcreasing the exciting wavelength further to 4880 A caused a decrease in the intensity of the 360cm.-l peak corresponding to the shift from the resonance condition (Figure 2). As iodine monofluoro- sulphate is completely disproportionated to I,+ I 1 I I I 600 500 400 3 00 (mJJ) FIGURE 1. Path lengtlz 0.0144 cm. Ultraviolet and visible spectrum of 6.91 x mold BrS0,F and 0.176 m SbF,-SSO, in HS0,F. and I(S0,F) undergoes a similar disproportionation when dissolved in fluorosulphuric it is reasonable to assume that BrOS0,F 2BrOS02F 2 C.SBr,+ + 0.4Br(S03F), + 0.8S03F- (1) The 510 nip peak in the visible spectrum and the 360 cm.-l peak in the Raman spectrum may then be attributed to the Br,+ cation. In comparison 15651566 CHEMICAL COMMUNICATIONS, 1968 I I 400 360 div (cm.-ij FIGURE 2.Raman spectrum of 360cm.-l Br,+ funda- mental and 400 cm.-l solvent band in a 1.30 x m BrS0,F and 0.176m SbF5-3S0, solution in HS0,F. Excited by A 6328 A, B 5145 A, C 4880 fi laser. with the following data for I,,I,+, and Br, these values are very reasonable for Br2+ hmax(mp.c) v(cm.-l) 1, .. I,+ . . Br, . . Br,+ . . . . . , 520 215 . . .. 640 238 . . .. 410 320 .. .. 510 360 A solution of Br,-S,O,F, in the mole ratio 3 : 1 in solution in the superacid (I) gave a spectrum which had an intense absorption at 300 mp with a shoulder a t 375 mp and no absorption at 510 mp. Conductivity measurements on such solutions are in agreement with the quantitative formation of Br,+. Since the superacid solvent has a high conductivity due to the highly conducting H,SO,F+ the ionisation of a substance that behaves as a base of the fluorosulphuric acid solvent system may be investigated by following the decrease in conductivity, when the basic solute neutralises the H,SO,F+ ion in the superacid, and comparing this decrease in conductivity with that caused by the strong base KS0,F.In this way, 7, the number of moles of fluorosulphate ions formed by one mole of solute, was found to be 0.7 for the 3: 1 Br,- S,O,F, solution which is consistent with the formation of the Br,+ cation according to the equation. 3Br, + S,O,F, --f 2Br,+ + 2S0,F- (2) The visible spectrum attributed to Bra+ can also be obtained in fluorosulphuric acid. Figure 3 shows the spectrum of bromine in fluorosulphuric acid and also the changes that occur in the spectruni on addition of S,O,F, to give the mole ratios S,0,F2:Br2= 1:3, 1:1, 3:1,and5:1.Theseratios I I I 600 500 400 300 FIGURE 3. Ultraviolet and visible spectra in HS0,F. Path length 0.100 cm. Br, = 1.422 x 10-2m; ratio Br,-S,O,F,: A, 1 : O ; B, 1 :0.33; C , 1 : 1; D, 1 :3and 1 : 5. correspond to oxidation to Br3+, BrOSO,F, and Br(S0,F) , respectively. Apparently Br(S0,F) is not further oxidised by excess S20,F, and we note also that there is no trace of the 510 mp peak due to the red Br," ion. -\ solution of Br,-S,0,F2 in the mole ratio 1: 1 has exactly the same visible and Raman spectra as the solution of BrOS0,F in the superacid (1). The 300 mp and 376 mp peaks attributed to Bra- are also present in the absorption spectrum of this solution implying that, in addition to the dispro- portionation (l), it is also disproportionated to some extent according t o the following equation : 4BrOS0,F -;f Br3- f Br(SO3F) 3 f SO3F - (3) Still another equilibrium is involved in these systems as conductivity measurements show that Br(SO,F), behaves as a base in the superacid system and is approximately ' i 5 O o ionised accord- ing to the equation: Br(SO,F), -+ H,S03F+- ", Br(SO,F),+ + 2HS0,F (4) I t is noteworthy that although the I,+ cation is stabilised by the acidity of sulphuric acid, the 1,-CHEMICAL COMMUNICATIONS, 1968 1567 cation is almost completely disproportionated in this solvent and it needs the greater acidity of fluorosulphuric acid to enable it to exist in any appreciable concentration.The bromine cations are still less stable and Bra+ can be obtained in fiuorosulphuric acid although not in sulphuric acid, while the Br,+ cation exists as a stable species only in the superacid (I). We thank Dr. A. Anderson for kindly allowing us the use of the argon-ion laser at the Department of Physics, University of Waterloo. (Received, September 24th, 1968; Corn. 1305.) I I . Masson, J . Chem. Soc., 1938, 1708. * R. J. Gillespie and J. B. Milne, Inorg. Chem., 1966, 5, 1577. * F. Aubke and R. J. Gillespie, Inorg. Chern., 1968,7, 599. R. C. Thompson, J. Barr, R. J. Gillespie, J. B. Milne, and I<. A. Kothenbury, Inorg. Chem., 1965, 4, 1641; l<. J. J. Behringer, in “Raman Spectroscopy,” ed. H. A. Szymanski, Plenum Press, New York, 1967, p. 168. l i . J. Gillespie and M. J. Morton, J . Mol. Spectroscopy, in the press. F. Aubke and G. H. Cady, Inorg. Chem., 1965, 4, 269. Gillespie, Accounts Chem. Res., 1968, 1, 202.
ISSN:0009-241X
DOI:10.1039/C19680001565
出版商:RSC
年代:1968
数据来源: RSC