J. Phys. Chem. 1996, 100, 12199-12203
12199
Photoionization Efficiency Spectrum, Ionization Energy, and Heat of Formation of Br2O R. Peyton Thorn, Jr.,† Paul S. Monks,†,‡ and Louis J. Stief*,# Laboratory for Extraterrestrial Physics (Code 690), NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
Szu-Cherng Kuo,§ Zhengyu Zhang,⊥ and R. Bruce Klemm*,@ Department of Applied Science, Building 815, BrookhaVen National Laboratory, P.O. Box 5000, Upton, New York 11973-5000 ReceiVed: February 8, 1996; In Final Form: April 5, 1996X
The photoionization efficiency (PIE) spectrum of Br2O was measured over the wavelength range λ ) 100125 nm by using a photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. A value of 10.264 ( 0.011 eV was obtained for the adiabatic ionization energy of Br2O from analysis of the photoionization thresholds. Also, the appearance energy (AE) of BrO+ (from the dissociative ionization of Br2O) was determined from the PIE spectrum of BrO+ over the wavelength range λ ) 104-110 nm. Combining the AE298 value, 11.714 ( 0.014 eV, with known thermodynamic quantities yields a value for ∆fH°298(Br2O) ) 107.1 ( 3.5 kJ mol-1. The value for ∆fH°298(Br2O) may be related to ∆fH°298(HOBr) through the equilibrium constant for the reaction Br2O + H2O h 2HOBr. This leads to ∆fH°298(HOBr) ) -60.0 ( 3.5 kJ mol-1.
Introduction The thermochemistry and spectroscopy of bromine oxides are not well-characterized mainly because of their low thermal stability and the difficulty of obtaining them in a pure state.1 One such compound is dark brown dibromine monoxide, Br2O, which was first synthesized through the reaction of bromine with mercury(II) oxide at low temperatures.2,3 Previous spectroscopic studies of Br2O have included solid phase IR spectra,4,5 UV-visible spectra both in CCl42 and in a N2 matrix5 and structural characterization by bromine K-edge EXAFS.5 Most frequently, Br2O is used as a precursor for the synthesis of hypobromous acid, HOBr, a significant bromine containing species in the atmosphere.6-8 There have been three values for the heat of formation of Br2O published in the literature. In a theoretical study of the molecular and spectroscopic properties of Br2O, Novak9 estimated the heat of formation to be 83 ( 8 kJ mol-1 at T ) 0 K via a crude “sum of bond enthalpies” procedure. In the same paper, ab initio calculations were employed to derive a value of 10.23 eV for the adiabatic ionization energy (IE) of Br2O. Orlando and Burkholder10 measured the equilibrium constant (Keq) for reaction 1. From Keq and estimates for ∆fH°298(HOBr),
Br2O + H2O h 2HOBr
(1)
they derived a value for ∆fH°298(Br2O) of 113-159 kJ mol-1. Very recently, Lee11 combined ab initio calculations of reaction energies with experimental and theoretical heats of formation to obtain a value of ∆fH°298 (Br2O) ) 122 kJ mol-1. There † NAS/NRC Resident Research Associate. Email: ysrpt@ lepvax.gsfc.nasa.gov. ‡ Present address: Chemistry Department, University of Leicester, University Road, Leicester, LE1 7RH, England. Email:
[email protected]. § Email:
[email protected]. ⊥ Present address: Philips Lighting, Nanjing Branch Office, 8th floor, Huaxin Building, Nanjiang, Peoples Republic of China. # Email:
[email protected]. @ Email:
[email protected]. * To whom correspondence should be sent. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00405-4 CCC: $12.00
appear to be no other published reports on measurements or calculations for either ∆fH°298(Br2O) or IE(Br2O).12,13 In the present study, we report the first direct determination of the photoionization efficiency (PIE) spectrum of Br2O and the photoionization threshold from which the IE was derived. From a direct determination of the appearance energy (AE) of BrO+ from Br2O a value for the heat of formation of Br2O is also derived. Direct photoionization was accomplished with dispersed synchrotron radiation and was followed by detection of the mass selected ions. Experimental Section Experiments were performed by employing a discharge flowphotoionization mass spectrometer (DF-PIMS) apparatus coupled to beamline U-11 at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. In the present study, the microwave discharge was not employed. The apparatus and experimental procedures have been described in detail in previous publications.14-20 Br2O was prepared via the heterogeneous reaction of bromine vapor on solid mercuric oxide:1-3,10
2Br2 + HgO(s) f HgBr2(s) + Br2O
(2)
Approximately 50 g of HgO powder was placed in a 5 L glass sample bulb, which was evacuated to a pressure of about 10-5 Torr. Bromine vapor (∼40 Torr) was then expanded into the bulb. After waiting a few (99.0%) was outgassed by repeated freezepump-thaw cycles. Results and Discussion As an example of the PIMS experiment, the PIE spectrum of Br2 (m/z ) 160) was measured over the wavelength range of λ ) 110-120 nm. As shown in Figure 1, the onset for ionization at λ ) 117.90 nm corresponds to an adiabatic ionization energy of 10.516 ( 0.011 eV. This result is in good agreement with the recommended value of 10.515 eV,13 and it
Thorn et al.
Figure 2. Photoionization efficiency spectrum of Br2O (m/z ) 176) between λ ) 100.0 and 125.0 nm at a nominal resolution of 0.13 nm and with 0.2 nm steps. The arrow indicates the onset of ionization at λ ) 120.80 nm (IE ) 10.264 ( 0.011 eV).
demonstrates that the wavelength calibration, established by location of zero order, is excellent.18,19 In addition to the onset, the PIE spectrum of Br2 displays the vibrational steps (V′ ) 0-8) in the cation. Even though the superimposed structure (presumably due to vibrational autoionization) perturbs these transitions, there is generally good agreement with results from a recent photoelectron spectroscopy study.21 Determination of IE (Br2O). The PIE spectrum for Br2O, at m/z ) 176, is shown in Figure 2 over the wavelength range λ ) 100-125 nm at 0.13 nm resolution and at 0.2 nm intervals. The Br2O spectrum displays some structure above threshold even at moderate resolution. This structure arises not only from the direct transition to the ground state of the cation but also from transitions to excited states of the cation and a number of autoionizing Rydberg states. The threshold, which appears to be a step function, was analyzed by determining the half-rise point, as indicated by the arrow in Figure 2. The threshold wavelength is λ ) 120.80 nm and therefore IE(Br2O) ) 10.264 ( 0.011 eV. This value for IE(Br2O) is in good agreement with the only published value of 10.23 eV which is based on an ab initio calculation.9 There is also an unpublished experimental result quoted by Ruscic and Berkowitz22 in their study of the threshold photoelectron spectrum of HOBr. They refer to an “approximate adiabatic” value, IE(Br2O) ) 10.18 ( 0.03 eV, which is in moderate agreement with this determination. Appearance Energy of BrO+. The PIE spectrum of BrO+ at m/z ) 95, formed via dissociative ionization of Br2O, is shown in Figure 3 over the wavelength range λ ) 90-110 nm at 0.14 nm resolution and at 0.2 nm intervals. The threshold for the appearance of BrO+ was examined in more detail, as shown in Figure 4, over the wavelength range λ ) 104-110 nm at a resolution of 0.13 nm (fwhm) and 0.1 nm intervals. At the onset, BrO+ formation displays a linear buildup that is expected for a process such as this.23 Thus, the threshold was determined by making a simple extrapolation of the spectrum to the background at 105.845 nm; this corresponds to an appearance energy of 11.714 ( 0.014 eV. Although HOBr is also present in this system (as mentioned in the Experimental Section), the BrO+ signal observed here is not due to dissociative ionization of HOBr since the process HOBr f BrO+ + H is calculated to have a threshold above 14 eV. Also, the presence of HOBr requires sufficient mass resolution to avoid contribution to the BrO+ signal from the neighboring HOBr+ signal. In Figure 5,
Properties of Br2O
J. Phys. Chem., Vol. 100, No. 30, 1996 12201
Figure 3. Photoionization efficiency spectrum of BrO+(Br2O) (m/z ) 95) between λ ) 90.0 and 110.0 nm at a nominal resolution of 0.14 nm and with 0.2 nm steps.
Figure 5. Mass scan of the products BrO (formed Via dissociation ionization of Br2O) and HOBr from the sample mixture at an excitation wavelength of λ ) 104 nm (11.92eV), m/z range ) 93-99 at 0.1 m/z steps.
eV)15 and the heat of formation of BrO.28 For the dissociative ionization of Br2O to form BrO+,
Br2O f BrO+ + Br,
∆rH°0 ) AE0
(5)
the heats of formation are related to AE0 by
AE0 ) ∆fH°0(BrO+) + ∆fH°0(Br) - ∆fH°0(Br2O) (6) Rearranging and using suitable ∆fH values from the latest NISTJANAF evaluation:29
∆fH°0(Br2O) ) ∆fH°0(BrO+) + ∆fH°0(Br) - AE0 (7) ) 1142.6 + 117.9 - 1136.4 Figure 4. Photoionization threshold region for the appearance of BrO+ (Br2O) (m/z ) 95) between λ ) 104.0 and 110.0 nm at a nominal resolution of 0.13 nm and with 0.1 nm steps. The linear extrapolation of the spectrum to the baseline yields an onset at 105.845nm (AE298 ) 11.714 ( 0.014 eV).
we show a mass scan of the BrO+ (from dissociative ionization of Br2O) and HOBr+ over the range m/z ) 93-99 at an excitation energy of 11.92 eV (λ ) 104 nm). These data demonstrate the excellent mass resolution attained in the DFPIMS experiments. Determination of ∆fH°298(Br2O) and ∆fH°298(HOBr). The appearance energy determined here may be used to derive the heat of formation of Br2O. First, the AE must be corrected for the internal energy present at room temperature in Br2O as discussed by Traeger and McLoughlin:24-26
AE0(BrO+,Br2O) ) AE298 + Ei ) AE298 + (H298 - H0)Br2O - 5/2RT
(3) (4)
) 1136.4 kJ mol-1 (11.778 eV) The uncertainty in this AE0 value is estimated to be (1.5 kJ mol-1.27 From this value for AE0, ∆fH°0(Br2O) can be computed using the heat of formation of ∆fH°0(BrO+) that has been derived from the ionization energy for BrO (10.46 ( 0.02
∆fH°0(Br2O) ) 124.1 kJ mol-1 leads to
∆fH°298(Br2O) ) 107.1 kJ mol-1 by applying the correction of 17 kJ mol-1 given in ref 29 for the integrated heat capacities of Br2O and the elements. The uncertainty associated with this derivation of ∆fH°298(Br2O) is estimated to be (3.5 kJ mol-1.30 The value derived here for ∆fH°298(Br2O) from eq 7 is a lower limit since AE0 is an upper limit to ∆rH°0. The upper limit allows for the possibility of an energy barrier to the dissociative ionization of Br2O to BrO+ + Br. However, this dissociative ionization process is the lowest energy one (except for ion pair formation, i.e. BrO+ + Br-) and involves only a simple O-Br bond rupture. Since little or no energy barrier is expected, AE0 should be a good measure of ∆rH°0 as indicated in eq 5. The present value, ∆fH°298(Br2O) ) 107.1 ( 3.5 kJ mol-1, is compared with the previous calculations or estimates listed in Table 1. The recent values of Orlando and Burkholder10 and Lee,11 and the evaluation of Chase,29 depend on the value for ∆fH°298(HOBr) which has recently undergone revision. Our result depends on the heats of formation for BrO+ and Br which are more precisely determined. It may also be instructive to compare our experimental value with one estimated via a trend analysis for the species XO, HOX, XNO, and XONO where X ) Cl and Br. The heats of formation for these species are collected in Table 2 (which includes refs 13, 26, 31, 32, and
12202 J. Phys. Chem., Vol. 100, No. 30, 1996
Thorn et al.
TABLE 1: Comparison of Values for ∆fH°298(Br2O) ∆fH°298(Br2O) (kJ mol-1) 66 ( 8
a
reference Novak9
113-159
Orlando and Burkholder10
108 ( 15 122
Chase29 Lee11
107.1 ( 3.5
this study
a
method/comment calculated via “sum of bond enthalpies” from Br2O + H2O a 2HOBr equilibrium constant/depends on ∆fH°298(HOBr) data evaluation ab initio calculation of reaction energies/depends on ∆fH°298(HOBr) AE(BrO+,Br2O)/depends on ∆fH°298 for BrO+ and Br
Corrected from the value at T ) 0 K.
TABLE 2: Trends in Heats of Formation (T ) 298 K) for a Series of Related Compounds Containing Cl and Br ∆fH°298 (kJ mol-1)a compound
X ) Cl
X ) Br
∆(Cl f Br)
XO HOX XNO XONO XOX
100 -79 50 84 79
125 -58b 84 105 104 ( 6c
25 21 34 21 〈25 ( 6〉d
a Values for ∆ H° f 298 are from refs 13 and 26 except where noted. Average of refs 31-33. c Estimated from ∆fH°298(Cl2O) + 〈∆(ClfBr)〉. d Average of four values; error is (1σ. b
33) as well as the change for each of the four pairs as Cl is replaced by Br. The average change, ∆(ClfBr), is 25 ( 6 kJ mol-1. Combining this estimate with ∆fH°298(Cl2O)12 ) 79 ( 2 kJ mol-1 yields the estimated value ∆fH°298(Br2O) ) 104 ( 6 kJ mol-1 which is in good agreement with our experimental result of 107.1 ( 3.5 kJ mol-1. Thus, our value for Br2O is consistent with trends observed in ∆fH°298 for related species such as BrO, HOBr, BrNO, and BrONO. Finally, the value for ∆fH°298(Br2O) may be related to ∆fH°298(HOBr) through the equilibrium constant for reaction 1. Although Orlando and Burkholder10 assumed that ∆rS was zero, we may compute this quantity and therefore obtain a more accurate value for ∆fH°298(HOBr). From the measured value of the equilibrium constant for reaction 1, Keq ) 0.020 ( 0.002,10 we obtain ∆rG ) 9.69 kJ mol-1 with an uncertainty of (0.25 kJ mol-1. Taking entropy values for HOBr (247.79 J mol-1 K-1) and Br2O (290.22 J mol-1 K-1) from Chase29 and for H2O (188.83 J mol-1 K-1) from ref 26, we obtain ∆rS ) 16.53 J mol-1 K-1, with an uncertainty of (0.09 J mol-1 K-1. Thus, ∆rH ) ∆rG + T∆rS ) 9.69 + 4.93 kJ mol-1 ) 14.62 ( 0.25 kJ mol-1. The enthalpy of formation of HOBr is thus computed from26
2∆fH°298(HOBr) - ∆fH°298(Br2O) ∆fH°298(H2O) ) ∆rH (8) ∆fH°298(HOBr) ) [∆fH°298(Br2O) - 227.2]/2 ) -60.0 ((3.5) kJ mol-1 This result for ∆fH°298(HOBr) is in good to excellent agreement with values reported by McGrath and Rowland (59.4 ( 6.7 kJ mol-1), via calculation,31 later by Ruscic and Berkowitz (56.19 ( 1.76 kJ mol-1), via experiment,32 and more recently by Glukhovtsev et al. (58.3 ( 10.0 kJ mol-1), via calculation.33 This extent of agreement among the four independently derived values demonstrates internal consistency in the system under study; it also supports our earlier determination of IE(BrO).15
An average of the previous determinations and the present result gives ∆fH°298(HOBr) ) 58.5 kJ mol-1 with a 2σ uncertainty of (3.4 kJ mol-1. There is poorer agreement with the earlier value derived by Monks et al.,17 probably due to a small error (∼3%) in the BrO proton affinity calculation. An error of this size, in a calculation for an open-shell system containing a heavy atom, would not seem unreasonable. Note Added in Proof: Referring to Figure 1, the ratio of signal-minus-background for the peaks that correspond to the hot band (0-1, at λ ) 118.15 nm) and the ionization threshold (0-0, at λ ) 117.60 nm) is consistent with a vibrational temperature of about 298 K with an estimated uncertainty of (10 K (Kuo and Klemm, unpublished results; also, see Deibler et al., Int. J. Mass Spectrom. Ion Phys. 1971, 7, 209). Since this paper was submitted, a determination of ∆fH0298(HOBr) ) -60.0 kJ mol-1 was reported by Lock et al., J. Phys. Chem. 1996, 100, 7972. Acknowledgment. We thank Dr. M. W. Chase, Jr. for providing the NIST-JANAF review of bromine oxides prior to publication. We also thank both reviewers for a careful reading of the manuscript and for making helpful suggestions. R.P.T. and P.S.M. would like to thank the NAS/NRC for awards of Resident Research Associateships. The work at GSFC was supported by the NASA Upper Atmosphere Research Program. Z.Z. was supported under the Laboratory Directed Research and Development Program at Brookhaven National Laboratory. The work at BNL was supported by the Chemical Sciences Division, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC02-76CH00016. References and Notes (1) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry: A ComprehensiVe Text, 3rd ed., Interscience: New York, 1972. (2) Brenschede, W.; Schumacher, H.-J. Z. Anorg. Allg. Chem. 1936, 226, 370. (3) Schwarz, R.; Weile, H. J. Prakt. Chem. 1939, 152, 157. (4) Campbell, C.; Jones, J. P. M.; Turner, J. J. J. Chem. Soc., Chem. Commun. 1968, 888. (5) Levason, W. L.; Ogden, J. S.; Spicer, M. D.; Young, N. A. J. Am. Chem. Soc. 1990, 112, 1019. (6) Yung, Y. L.; Pinto, J. P.; Watson, R. T.; Sander, S. P. J. Atmos. Sci. 1980, 37, 339. (7) Poulet, G.; Pirre, M.; Maguin, F.; Ramaroson, R.; LeBras, G. Geophys. Res. Lett. 1992, 19, 2305. (8) Monks, P. S.; Nesbitt, F. L.; Scanlon, M.; Stief, L. J. J. Phys. Chem. 1993, 97, 11699. (9) Novak, I. Struct. Chem. 1992, 3, 377. (10) Orlando, J. J.; Burkholder, J. B. J. Phys. Chem. 1995, 99, 1143 and references therein. (11) Lee, T. J. J. Phys. Chem. 1995, 99, 15074. (12) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. No. 1, 1. (13) Lias, S. G.; Liebman, J. F.; Levin, R. D.; Kafafi, S. A. PositiVe Ion Energetics Version 2.0, NIST Standard Reference Database 19A, Gaithersburg, MD, 1993. (14) Tao, W.; Klemm, R. B.; Nesbitt, F. L.; Stief, L. J. J. Phys. Chem. 1992, 96, 104. (15) Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo, S.-C.; Klemm, R. B. Chem. Phys. Lett. 1993, 211, 416. (16) Zhang, Z.; Kuo, S.-C.; Klemm, R. B.; Monks, P. S.; Stief, L. J. Chem. Phys. Lett. 1994, 229, 377. (17) Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo, S.-C.; Klemm, R. B. J. Chem. Phys. 1994, 100, 1902. (18) Kuo, S.-C.; Zhang, Z.; Klemm, R. B.; Liebman, J. F.; Stief, L. J.; Nesbitt, F. L. J. Phys. Chem. 1994, 98, 4026 and reference therein. (19) Buckley, T. J.; Johnson, R. D., III; Huie, R. E.; Zhang, Z.; Kuo, S.-C.; Klemm, R. B. J. Phys. Chem. 1995, 99, 4879 and reference therein. (20) Grover, J. R.; Walters, E. A.; Newman, J. K.; White, M. C. J. Am. Chem. Soc. 1985, 107, 7329 and references therein. (21) Yencha, A. J.; Hopkirk, A.; Hiraya, A.; Donovan, R. J.; Goode, J. G.; Maier, R. R. J.; King, G. C.; Kvaran, A. J. Phys. Chem. 1995, 99, 7231. (22) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1994, 101, 9215.
Properties of Br2O (23) Chupka, W. A. J. Chem. Phys. 1971, 54, 1936. (24) Traeger, J. C.; McLoughlin, R. G. J. Am. Chem. Soc. 1981, 103, 3647. (25) A value of 12.33 kJ mol-1 for the integrated heat capacity for Br2O was taken from ref 29. (26) Unless otherwise stated, the thermodynamic values employed in these derivations are taken from: (a) Chase, M. W.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data Suppl. 1 1985, 14. (b) Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of IndiVidual Substances, 4th ed.; Hemisphere Publishing Corp.: New York, 1991; Vol. 1. (27) The uncertainty in the AE298 measurement is (1.35 kJ mol-1 and that in the integrated heat capacity is estimated to be (0.50 kJ mol-1; thus the combined root-sum-square (RSS) uncertainty in AE0 is (1.5 kJ mol-1. In eV units, AE0 ) 11.778 ( 0.016.
J. Phys. Chem., Vol. 100, No. 30, 1996 12203 (28) A value of 133.3 kJ mol-1 for the heat of formation of BrO (at T ) 0 K) was taken from ref 29. (29) Chase, M. W., Jr. NIST-JANAF Thermochemical Tables for the Bromine Oxides,in press. (30) The uncertainties in ∆fH°0(BrO+), ∆fH°0(Br), and AE0 are (3.1, (0.5, and (1.5 kJ mol-1, respectively. Thus, the combined RSS uncertainty is (3.5 kJ mol-1. (31) McGrath, M. P.; Rowland, F. S. J. Phys. Chem. 1994, 98, 4773. (32) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1994, 101, 7795. (33) Glukhovtsev, M. N.; Pross, A.; Radom, L. J. Phys. Chem. 1996, 100, 3498.
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