Dibromine Monoxide Br2O and Bromine Dioxide OBrO Spectroscopic Properties Molecular Structures and Harmonic Force Fields.код для вставкиСкачать
f17, 9441 -9447; E. Kimura, Y Kodama, T. Koike, M. Shiro, ibid. 1995, f f 7 , 8304-8311; 1.0.Kady, B. Tan. 2. Ho, T. Scarborough, J. Chem. Suc. Chem. Conmun. 1995, 1137 1138; J. R. Morrow. K. Awes, D. Eppstein, ibid. 1995, 2431 -2432.  V. Jubian, R . P. Dixon, A. D. Hamilton, J. Am. Chem. Soc. 1992, 114. 11201121; J. Smith, K . Ariga, E. V. Anslyn, ibid. 1993, 1f5,362-364; V. Jubian, A. Veronese. R. P. Dition. A. D. Hamilton, A 7 p w C h m . 1995, 107. 1343-1345; Angen:. Chew!.Inl. Ed. Engl. 1995,34. 1237-1239. M. W. Hosseini, A. J. Blakker, J.-M. Lehn. J. Am. Cliem. Suc. 1990,112.3896-3904; B. Barbier, A. Brack, ibid. 1992, f14,3511-3535. [S] Complexation of phosphates by bis(guanidines) and related compounds: B. Dietrich, D. L. Fyles. T. M. Fyles, J.-M. Lehn, Helv. Chin?. Arta 1979, 62, 2763-2787; R. P. Dixon, S. J. Geib. A. D. Hamilton, J. Am. Chrm. SOC.1992, ff4. 365-366; D. M. Kneeland, K. Ariga, V. M. Lynch, C.-Y Huang. E. V. Anslyn, ;hid. 1993. f f . 5 , 10042-10055: F. Chu, L. S. Flatt, E. V. Anslyn, ?hid. 1994, 116. 4194-4204;V. Kral. H. Furuta, K . Shreder, V. Lynch. J. L. Sessler, %id. 1996, f f 8 , 1595 - 1607; B. L. Iverson, K. Shreder, V. Kril, P. Sansom, V. Lynch, J. L. Sessler, ibid. 1996, flR, 1608-1616; W. Peschke. F. P. Schmidtchen, Tetrahedron Lett. 1995. 5155-5158; G. Deslongcharnps, A. Galan, J. de Mendoza, J. Rebek, Jr.. Anpew. Chem. 1992, 104, 5X -60; Angrii. Chcni. Znl. Ed. Engl. 1992, 3 f , 61-63.  A superposition of first and higher ordcr reaction channels was also evident in the case of the amidinium alcohol 1. Here, k , was determined exactly by extrapolating k,,, to ~ ( ~ (=1 0) [2d]. This has not yet been successful with 5. [lo] F. M. Menger. M. Ladika, J. Am. Chern. SOC.1987, fU9, 3145-3146. [ l l ] C. A. Maryanoff, J. N. Plampin, R. C. Stanzione, US 4.656.291, 1988; Chem. Ahsrr. 1988, fU8, 21911.  M. S . Bernatowica, Y Wu, G . R. Matsueda, J. Org. Chem. 1992, 57, 24972502. Dibromine Monoxide, Br,O, and Bromine Dioxide, OBrO : Spectroscopic Properties, Molecular Structures, and Harmonic Force Fields"" Holger S. P. Miiller,* Charles E. Miller, and Edward A. Cohen The halogen oxides show great variations in structural parameters and other physical as well as chemical properties. Over a dozen chlorine oxides are known, and many of them are quite well characterized. Bromine oxides are unstable at room temperature; thus few of them are known.['] Even fewer have been studied structurally, generally in the solid state. Structure determinations of free molecules have been confined to the BrO radical.[21 Recently the chlorine and bromine oxides have attracted considerable attention because of their implication in reaction cycles leading to the destruction of atmospheric ozone. Of particular importance are the catalytic cycles involving C10 and BrO. Recently, bromine dioxide, OBrO, has been observed in the bromine-sensitized photodecomposition of 0, .[3,41 Dibromine monoxide, Br,O, was obtained in low yields from the reaction of Br, with Hg0.[51Bromine dioxide was first reported by Schwarz and Schmeisser as an egg-yellow deposit from a Br,/O, discharge.I6] The stoichiometry of the oxide was [*I [**I Dr. H. S. P. Miiller, Dr. C. E. Miller, Dr. E. A. Cohen Jet Propulsion Laboratory California Institute of Technology Mail Stop 183-301 4800 Oak Grove Drive, Pasadena. CA 91 109-8099 (USA) Fati: Int. code +(818)3548460 e-mail: firstname.lastname@example.org H. S. P. M. and C. E. M . thank the National Research Council for NRCNASA Resident Research Associateships. The research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Aizgew. Cheni. Znt. Ed. Engl. 1996, 35. No. 18 '(3 deduced by quantitative analysis. Its controlled decomposition below 0°C enabled Schmeisser and Wiele to obtain pure Br,O.['] Later, Schmeisser and Jorger reported that the ozonolysis of Br, in cooled CC1,F solution yielded OBrO.['] In contrast, Seppelt et al. obtained single crystals of orange Br,0,[91 and colorless Br,O,['ol from that reaction but found no evidence for molecular OBrO. OBrO was detected during mass spectrometric investigations of the 0 + Br, reaction system,[' 'I ESR studies of X-ray irradiated perbromates,l'2] and UV/Vis spectroscopic investigations of the Br + 0, reaction system.[3.41 The v1 ['31 and v3[13, l 4 1 stretching fundamentals of the oxide were observed in Ar matrices. Studies on Br,O included determination of the melting point,L71 the solution ~ ' 1and gas-phase UV/Vis spectra,["] the solid-state Raman spectrum,[' 'I the solid-state structure from EXAFS (extended X-ray absorption,finestructure) data,['71and the heat of formation." The symmetric stretching Vibration v11141 or both stretching modes['7%' I were observed in Ar[14,18] or N, matrices.['71 We have observed the rotational spectra of the triatomic oxides Br,O and OBrO in the millimeter- and submillimeterwave regions using the spectrometer described in reference . We report here some of the fundamental spectroscopic constants as well as derived properties such as structure and harmonic force field. Initially, Br,O was obtained when Br, was passed through a column of HgO at room temperature, and the products were flowed through the microwave absorption cell. Although the yields were low, in agreement with earlier work,[51many transitions were observed with no or little signal averaging. OBrO was formed when the reaction products of an 0, discharge plus Br, were condensed onto the wall of the microwave absorption cell at -20 "C. After the flow of 0, and Br, was stopped, spectra of the vapor were recorded while pumping on the condensate. The identity of the product on the wall is not clear. It might be OBrO, a more complex bromine oxide, or a mixture of oxides that decompose slowly to yield OBrO. At low temperatures ( Z - 20 "C) and pressures ( ~ 0 . Pa) 1 essentially all absorption features were due to OBrO, although weak features of BrO were also seen. The amount of OBrO increased at higher temperatures because of a larger vapor pressure or an enhanced decomposition of its precursor. The BrO/OBrO ratio increased as well, possibly because of decomposition of OBrO. Strong absorptions of Br,O were also seen at higher pressures (reduced flow rate) and higher temperatures. This method was subsequently used for further study of Br,O. The hyperfine patterns of 79Br,0 and *'Br,O are consistent with their C,, symmetry, whereas 79'81Br20has C9symmetry. At low J or K, quantum numbers the splittings between single hyperfine components can be several tens of MHz, and the patterns often are complex. At high J and K, quantum numbers usually several components overlap and symmetric triplets or quartets, or even single lines are seen, as shown in Figure 1. Additional complications arise from perturbations of the quadrupole patterns due to effects of the off-diagonal quadrupole coupling constant xab. The OBrO rotational spectrum is that of a C,,-symmetric molecule in the 2Bl electronic ground state, as is that of OCIO. In general, each rotational transition is observable as a doublet of quartets. This is illustrated. in Figure 2 for a transition with rather small splittings. The doublets indicate the presence of an unpaired electron, the quartets are due to one nucleus with spin 3/2 (79Br or *'Br). A large number of lines was observed for Br,O (>700) and OBrO (> 1400), resulting in the precise determination of VCH Verlfi~sgesell~c/iaft mhH. D-69451 Weinhrim, 1996 + U570-o833~96!35lX-2129 3 15.00 ,2510 2129 COMMUNICATIONS The principal planar moments have been used to calculate the ro structural parameters. They are given with data for the related chlorine compounds in Table 2. The structural parameters of Table 2. Structural parameters [pm, '1 [a] and harmonic force coiistaiits [ N n - ' 1 of Br,O and OBrO in comparison with related chlorine compounds. Parainctcr Br,O [aj C1,O [b,c] OBrO[a] I 184.29 (20) 112.24 (20) 288.2 103.3 38.7 29.6 169.59 110.88 294.9 123.7 50.4 31.2 164.91 (15) 11 4.44 (25) 530.5 99.5 -3.1 -4.1 U f, fa .A. .fr - I 412 550 412 555 vIGHz I I 412560 412565 0 . . I I 411.62 411 66 411.64 v/GHz 411 hK 411 70 Fig. 2. Section from the submillimeter spectrum of the bromine dioxide radical. The 311,30-302,29transition of 0"BrO in the ground vibrational state (filled circles) and ofO'"Br0 in the first excited bending mode (open circles) are indicated. In general each transition appears as a doublet of quartets; see text for details. rotational, centrifugal distortion, fine (OBrO only), and Br hyperfine structure constants as well as distortion terms on fine and some hyperfine constants. Rotational and quartic centrifugal distortion constants for the vibrational ground state are given in Table 1 for "Br,O and 07'Br0. Table I. Rotational [MHz] and quartic centrifugal distortion constants [a] [kHz] of 79Br,0 and 0 7 9 B r 0in the ground vibrational states. Parameter 7 9 ~ ~ 33220.6172 (64) 1368.30433 (29) 1313.37607 (39) 0.285030 (82) -19.1365 (26) 1051.31Y (126) - 19.6723 (124) -0.3533 (29) A B C D, DJK DK 1000 d , 1000 d , ~ 0 0 7 9 ~ ~ 0 28024.51786 (111) 8233 17265 (32) 6345.43314 (32) 7.13486 (48) -70.6925 (33) 714.380 (27) -2637.543 (124) - 156 555 (53) [a] Numbers in parentheses arc two standard deviations in units of the least significant figures. Watson's S reductioii is used in the representation I' . 21 30 C) 146.984 117.403 71 1.0 137.2 -25.0 -2.7 [a] This work; r,, structurc with estimated upper limits for uncertainties, see text. [b] Equilibrium structure. [c] Ref. . [d] Ref$. [22,23] Fig. 1. Section from the submillimeter spectrum of Br,O, which demonstrates significant intensity and quadrupole splitting for high J transitions. The asymmetric pattern of the 475,,, -46,,., transition is caused by the off-diagonal quadrupole coupling constant z.~,,. I = intensity in arbitrary units. 411 60 OClO[h,d] VCH l4icrlu~fi,gereNwhnfl mbH D-69451 Wernlierm, 1996 different isotopomers agree well within the quoted digits. However, structural ambiguities due to vibrational effects are of up to the order of i 0 . 2 pm and 0.2" for Br,O and i 0 . 1 5 pm and 0.25" for OBr0.[201The equilibrium ( r e )structures of Br,O and OBrO have not been determined. For both molecules the re bond lengths are about 0.5 pm smaller than the ro parameters, as estimated from the changes of related molecules; the re bond angles are expected to agree with the yo data to within the uncertainties ascribed to vibrational effects. The present structural parameters of Br,O support the EXAFS results (185 (1)pm and 112 (2)") for the molecular solid.However, the present values are more precise and refer to the free molecule. The changes in structural parameters are smaller from C1,O to Br,O than from C1,O to F,O, which is in agreement with the corresponding results in the HOBr, HOCI, HOF series.[" The differences in bond lengths are smaller between Br,O and C1,O (14.2 pin) than between OBrO and OClO (17.4 pm). This can be explained by the decreased tendency of Br compared to CI to form double bonds. For both chlorine oxides, known re values were employed, for the two bromine oxides the estimates of r e . The quartic distortion constants have been used to calculate the harmonic force fields for both bromine oxides. Vibrational data for Br,O were taken from reference . The force constants are also given in Table 2 together with those of related molecules. The stretching force constantsf; of Br,O and C1,O are remarkably similar, possibly a reflection of the pure single-bond character. The larger stretching force constants and shorter bond lengths in OBrO and OClO compared to those of Br20 and C1,O are indicative of substantial double-bond character in the dioxides. For Br,O both,f, andf,, and the values of the quartic centrifugal distortion constants are strongly interdependent. The present force field indicates a value of 180 cm-I for the v2 band center, which may be compared to 197 cm-' observed for the solid.[' We have observed the v3 band of OBrO in the gas phase (Fig. 3). The Q-branch feature of 0 7 ' B r 0 at 848.6 cm-' and the isotopic shift of 2.4cm-' agree well with values from the force field (851.2 and 2.35 cm-') as does the position of the v I band in an argon matrix (794.6 versus 794.1 c n - I from reference ). The calculated isotopic shift for v1 is 1.15 cm-'; an observed splitting of 2.9 cm-' is more likely due to a matrix effect; see for example reference , where these effects have 0570-0833/96/3518-2130$ 15.00+ ,2510 Angen,. Clwni. Int. Ed. Engl. 1996. 35,N o . 18 ondary chemistry that may be of atmospheric importance. We are currently investigating the spectroscopy and kinetics of these interesting molecules. <;eriii;nii K S C C I \ C d ,~pl-ll 10. 1996 Re\i\cd bersioti Jiiiie 3. 1996 [ZPO5XIE] bcrsiiin . . l i i ~ ( , i x ( % w i 1996. 10s. 77x5 22XX Keywords: bromine oxides . halogen compounds spectroscopy . structure elucidation becn studied in detail for OCIO. Although the bending mode has n o t yet becn observed directly. our calculated value of 31 1 c m for 0'"BrO is consistent with relative intensities observed for rotational transitions i n the ground state compared to those in the = I state (Fig. 2 ) . Thc principal Br quadrupole constants of Br,O have been obtained by diagonalization of the quadrupole tensor. The valLK x,< = 938 MHi. is close to that of Br2 (810 M H Z [ ' ~ ~indicat). ing ;I prcdominantly covalent Br --0 bond.[201Contributions from ionic tornis Br 'OBr and BrO.-Br' are about 12% and x bonding is almost negligible (ca. 2 % ) . The ionic character is slightly lurgcr- than i n BrCl ( 9 ( j / 0 ) ( and ~ ' ~ essentially the same as in HOBr.['"lwhich isconsistent with the view that theOX group ( X = H. halogen) IS slightly more electronegative than the C1 atom. The qu~idrupolar:axis differs substantially from the Br 0 bond (by cii. 2.4 ; Fig. 4). I f one would assume the z axis t o be identical with the Br --0bond. one would derive an ionic character of about 45% for the Br- 0 bond in BrzO, exceeding that of B1-F. (35'h1).''-l a n d a substantial degree of n bonding. ' Although a detailed discussion of the fine and hyperfine structure constants is beyond the scope of a brief communication, it should be noted that earlier ESR results" 2 1 are consistent with those of this study. Thus. the conclusion given in reference [I21 that. iis in OCIO. halfof the spin-density is at the halogen atom. and essentially ;ill of' this in the /i-orbital perpendicular to the inoiecular plane. remuins unchanged. I11 conclusion. we h a w used the 0 + Br2 reaction system to study BrzO and the O B r O radical. The presence of a mixture of bromine oxides under cci-t;iin conditions indicates complex sec- . radicals .