14178
J. Phys. Chem. 1996, 100, 14178-14183
Ionization Energy of Cl2O and ClO, Appearance Energy of ClO+ (Cl2O), and Heat of Formation of Cl2O R. Peyton Thorn, Jr.*,† and Louis J. Stief‡ Laboratory for Extraterrestrial Physics (Code 690), NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
Szu-Cherng Kuo§ and R. Bruce Klemm⊥ Department of Applied Science, Bldg. 815, BrookhaVen National Laboratory, P.O. Box 5000, Upton, New York 11973-5000 ReceiVed: May 3, 1996X
The photoionization efficiency (PIE) spectrum of Cl2O was measured over the wavelength range λ ) 98117 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. A value of 10.909 ( 0.016 eV was obtained for the adiabatic ionization energy (IE) of Cl2O from analysis of the photoionization thresholds. The ClO radical was produced Via the reaction F + Cl2O f FCl + ClO and the PIE spectrum of ClO was measured over the wavelength range λ ) 105-117 nm. Analysis of the photoionization threshold yields a value of IE of ClO(X2Π3/2) ) 10.885 ( 0.016 eV. The appearance energy (AE) of ClO+ (from the dissociative ionization of Cl2O) was determined from the PIE spectrum of ClO+ over the wavelength range λ ) 98-104 nm. Combining the AE298 value, 12.296 ( 0.032 eV, with known thermodynamic quantities yields a value for ∆fH°298(Cl2O) ) 77.2 ( 3.4 kJ mol-1. From the value for ∆fH°298(Cl2O) and the equilibrium constant for the reaction Cl2O + H2O T 2HOCl, a value for ∆fH°298(HOCl) ) -76.8 ( 3.5 kJ mol-1 is obtained. These results for IE(Cl2O), IE(ClO), AE298(ClO+, Cl2O), ∆fH°298(Cl2O), and ∆fH°298(HOCl) are compared with previous estimates or measurements.
Introduction Although chlorine oxides are generally well characterized, uncertainties in their thermochemistry and spectroscopy remain because of their low thermal stability and the difficulty of obtaining them in a pure state.1 One such compound is dichlorine monoxide, Cl2O, which is synthesized in the laboratory through the reaction of gaseous chlorine with mercury(II) oxide at room temperature and below.2,3 A simple triatomic molecule having C2V symmetry, Cl2O has been studied extensively by spectroscopists.4 Previous studies of Cl2O have included IR,5 Raman,6 and UV-visible spectroscopy.7 The Cl2O structure has been determined by microwave spectroscopy8 and electron diffraction studies.9 It is used in industry as an intermediate in the manufacture of (hypochlorite-based) disinfectant powders and as a bleaching agent in the wood pulp and textile industries.4 Cl2O is also used as a precursor for the synthesis of hypochlorous acid, HOCl, a significant chlorine reservoir species in the atmosphere.10-12 There have been three previous determinations of the adiabatic ionization energy (IE) of Cl2O13,14 and several determinations of the enthalpy of formation of Cl2O.15-20 A value for IE(Cl2O) ) 10.94 eV was obtained Via photoelectron spectroscopy13 (PES), while less precise electron impact mass spectrometry (EIMS) measurements yield 11.1614a and 10.52 eV.14b Early measurements, mostly from the 1930s, yield ∆fH°298(Cl2O) values of 89.5,15 103.3,16 87.9,17,18 and 79.217,19 kJ mol-1 based on the heat of explosion15,16 and heat of solution/ * Author to whom correspondence should be sent. † NAS/NRC Resident Research Associate. Email: ysrpt@ lepvax.gsfc.nasa.gov. ‡ Email:
[email protected]. § Email:
[email protected]. ⊥ Email:
[email protected]. X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)01262-2 CCC: $12.00
formation measurements.17 A more modern determination14a is based on an EIMS measurement of the appearance energy (AE) of ClO+ from the dissociative ionization of Cl2O. From AE298(ClO+, Cl2O) ) 12.5 eV,14a a value of ∆fH°298(Cl2O) ) 99 kJ mol-1 may be derived. The most recent measurement20 is based on the enthalpy of the reaction ClNO + Cl2O T ClNO2 + Cl2 and yields ∆fH°298(Cl2O) ) 81.4 kJ mol-1. To date, three measurements of the adiabatic ionization energy of the ClO radical have been reported. Fisher14a obtained a value of IE(ClO) ) 11.1 eV while Alekseev et al.14b report IE(ClO) ) 11.08 eV, both obtained Via EIMS. Bulgin et al.21 determined IE(ClO X2Π3/2) ) 10.87 eV Via PES. The value 10.95 eV given in a later paper22 from the same laboratory was a typographical error.23 Unfortunately, the erroneous value from ref 22 has been used in data compilations (see for example ref 24). In the present study, we report the first determination of the photoionization efficiency (PIE) spectrum of Cl2O and the photoionization threshold from which the IE was derived. The PIE spectrum of the ClO radical, prepared by reaction of atomic fluorine with Cl2O, was also determined for the first time. From the photoionization threshold, a value for IE(ClO) is obtained. From the value for IE(ClO) and a determination of the appearance energy of ClO+ (from Cl2O), a value for the heat of formation of Cl2O is derived. From the latter value and the literature value for the equilibrium constant for the reaction Cl2O + H2O T 2 HOCl, a value for the heat of formation of HOCl is obtained. 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. The apparatus and © 1996 American Chemical Society
PIE Spectrum of Cl2O
J. Phys. Chem., Vol. 100, No. 33, 1996 14179
Figure 1. Mass spectrum of the sample mixture Cl2/Cl2O/HOCl at an excitation wavelength of λ ) 105.0 nm (11.81 eV) and with 0.1 m/z steps (m/z range ) 39-41, 50-55, 69-76, 85-92). Since a LiF filter is not used to block the second-order light, the peaks at m/z ) 40 (Ar+), 51 and 53 (ClO+), are due to photons at λ ) 52.5 nm. The data demonstrates the excellent mass resolution among the ClO+ and HOCl+ peaks. The ratio for the Cl2 isotopes (m/z ) 70, 72, and 74) is 0.61: 0.34:0.05, which is in good agreement with the calculated ratio, 0.57: 0.37:0.06, using 35Cl ) 75.77% and 37Cl ) 24.23% (ref 19). The same ratio holds for the Cl2O isotopes (m/z ) 86, 88, and 90).
experimental procedures have been described in detail in previous publications.25-29 Cl2O was prepared Via the heterogeneous reaction of chlorine on solid mercuric oxide:2,3
2Cl2 + HgO(s) f HgCl2(s) + Cl2O
(1)
Approximately 50 g of HgO powder was placed in a 2 L glass sample bulb, which was evacuated to a pressure of about 10-5 Torr. Chlorine (5% in He) was then expanded into the bulb (∼800 Torr) along with a small amount of Ar as a reference compound. The resulting mixtures of Cl2/Cl2O/HOCl/He were analyzed Via photoionization mass spectrometry. A typical mass spectrum is shown in Figure 1. The byproduct HOCl was presumably generated by reaction of Cl2O with water adsorbed on the HgO sample. Mass spectrometric analysis indicated that about 95% of the initial [Cl2] was consumed after addition to the flask containing HgO. Typical residual [Cl2] in the flow tube was about 6 × 1012 molecules cm-3, and the signals due to Cl2 and Cl2O were comparable (see Figure 1). Also, we estimate roughly that only a few percent of the consumed Cl2 ended up as Cl2O in the flow tube; loss of Cl2O in the transfer lines and in the flow controller was probably severe. Based on these considerations, a very rough estimate of [Cl2O] in the flow tube would be of the order of 1013 molecules cm-3. Chlorine monoxide radical, ClO, was produced in situ by the reaction
F + Cl2O f FCl + ClO
(2)
k2(298 K) ) 1.28 × 10-10 cm3 molecule-1 s-1 (ref 30) in a Teflon-lined flow reactor. The F radicals were produced by passing dilute mixtures of F2 through a microwave discharge cavity at the upstream end of the flow tube (about 100 cm from the sampling nozzle). The initial concentration of F2 was about 5 × 1013 molecules cm-3, and dissociation of F2 in the discharge was routinely greater than 90%. The Cl2O was introduced through the tip of the sliding injector at a distance of ≈5 cm from the sampling nozzle. Experiments were conducted at ambient temperature (T ) 298 ( 2 K) and at a flow tube
Figure 2. Photoionization efficiency spectrum of Cl2 (m/z ) 70) between λ ) 102 and 111 nm at a nominal resolution of 0.18 nm and with 0.05 nm steps. Photoionization efficiency is ion counts divided by light intensity in arbitrary units. The arrow indicates the onset of ionization at λ ) 108.00 nm (IE ) 11.480 ( 0.019 eV). The superposed lines (at 108.00, 107.25, 106.50, and 105.80 nm) indicate the vibrational steps (ν′ ) 0-3) in the Cl2 cation.
pressure of about 4 Torr using He carrier gas. With the tip of the injector at 5 cm from the nozzle and a flow velocity of about 800 cm s-1, reaction 2 was >99% complete within the available reaction time (about 6 ms). The gaseous mixture in the flow reactor was sampled as a molecular beam into the source chamber and subsequently into the photoionization region of the mass spectrometer. Ions were mass selected with an axially aligned quadrupole mass filter, detected with a channeltron/pulse preamplifier, and thence counted for preset integration times. Measurements of PIE spectra, the ratio of ion counts/light intensity vs wavelength, were made using tunable vacuum-ultraviolet (VUV) radiation at the NSLS. A monochromator with a normal incidence grating (1200 lines/mm) was used to disperse the VUV light28 and, where applicable, a LiF filter (λ g 105 nm) was used to eliminate second- and higher-order radiation. For photoionization experiments performed at λ < 105 nm, it was necessary to correct the PIE data for signal due to ionization by secondorder light.27,29 The intensity of the dispersed VUV light was monitored Via a sodium salicylate coated window with an attached photomultiplier tube. The helium (MG Industries, scientific grade, 99.9999%), mercuric oxide (Aldrich Chemical Co., HgO, yellow, 99+%, ACS reagent), chlorine and fluorine (MG Industries, 99.99% purity, 5.0% in He), and argon (MG Industries, scientific grade, 99.9999%) were used without further purification. Results and Discussion As an example of the PIMS experiment, the PIE spectrum of Cl2 (m/z ) 70) was measured over the wavelength range of λ ) 102-111 nm at a nominal resolution of 0.18 nm (fwhm). As shown in Figure 2, the onset for ionization at λ ) 108.00 nm, taken as the half-rise point of the first step, corresponds to an adiabatic ionization energy of 11.480 ( 0.019 eV. This result is in good agreement with the recommended value of 11.480 eV24 and it demonstrates that the wavelength calibration, established by location of zero order, is excellent.27 In addition to the onset, the PIE spectrum of Cl2 displays the vibrational steps (ν′ ) 0-3) 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 PES study.31
14180 J. Phys. Chem., Vol. 100, No. 33, 1996
Figure 3. (a, top) Photoionization efficiency spectrum of Cl2O (m/z ) 86) between λ ) 98.0 and 117.0 nm at a nominal resolution of 0.19 nm and 0.2 nm steps. (b, bottom) Photoionization threshold region of Cl2O (m/z ) 86) between λ ) 112.0 and 116.0 nm at a nominal resolution of 0.17 nm and with 0.1 nm steps. The arrow indicates the onset of ionization at λ ) 113.65 nm (IE ) 10.909 ( 0.016 eV). The superposed lines at 113.65 and 112.75 nm indicate the vibrational steps (ν′ ) 0, 1) in the Cl2O cation.
A. Determination of IE (Cl2O). The PIE spectrum for Cl2O, at m/z ) 86, is shown in Figure 3a over the wavelength range of λ ) 98-117 nm at 0.19 nm resolution and at 0.2 nm intervals. The Cl2O 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 was analyzed by taking the half-rise point of the step indicated by the arrow in Figure 3b. The threshold wavelength is λ ) 113.65 nm and therefore IE(Cl2O) ) 10.909 ( 0.016 eV, where the uncertainty is determined by the wavelength resolution. This PIMS value agrees well with the only previous precise measurement of 10.94 ( 0.01 eV, which was obtained by PES.13 The less precise value obtained by conventional EIMS,14a IE(Cl2O) ) 11.16 ( 0.1 eV, appears to be too high. The value reported by Alekseev et al.,14b 10.52 ( 0.06 eV, was presumably perturbed by hot band effects since they generated Cl2O Via pyrolysis of Cl2O7. B. Determination of IE(ClO). The PIE spectrum for ClO, at m/z ) 51, is shown in Figure 4a over the wavelength range of λ ) 105 to 117 nm at 0.26 nm resolution and 0.2 nm steps. The ClO spectrum displays some vibrational structure above the threshold (ν′ ) 0-2) along with features that presumably arise from transitions to excited states of the cation as well as from autoionizing Rydberg states. The photoionization threshold region of ClO is shown in Figure 4b over the wavelength
Thorn et al.
Figure 4. (a, top) Photoionization efficiency spectrum of ClO (m/z ) 51) between λ ) 105.0 and 117.0 nm at a nominal resolution of 0.26 nm and with 0.2 nm steps. The superposed lines at 114.1, 112.8 and 111.5 nm indicate the vibrational steps (ν′ ) 0-2) in the ClO cation. (b, bottom) Photoionization threshold region of ClO (m/z ) 51) between λ ) 113.0 and 116.0 nm at a nominal resolution of 0.17 nm and with 0.1 nm steps. The first arrow indicates the onset of ionization of the lower state ClO (X2Π3/2) at λ ) 113.9 nm (IE ) 10.885 ( 0.016 eV). The second arrow indicates the onset of ionization of the higher state ClO(X2Π1/2) at λ ) 114.3 nm (IE ) 10.847 ( 0.016 eV).
range of λ ) 113-116 nm at a higher resolution of 0.17 nm and with 0.1 nm steps. The spin-orbit splitting of the ClO molecule is resolved and appears as a double threshold in Figure 4b: ClO(X3Σ- r X2Π3/2) at λ ) 113.9 nm (IE ) 10.885 ( 0.016 eV), and ClO(X3Σ- r X2Π1/2) at λ ) 114.3 nm (IE ) 10.847 ( 0.016 eV). The ratio of the spin-orbit state signals, ClO(X2Π3/2)/ClO(X2Π1/2), is about 4:1. These PIMS results agree, within the combined experimental uncertainties, with the PES results21 of 10.87 ( 0.01 eV for ionization from the lower X2Π3/2 state and 10.83 ( 0.01 eV for ionization from the higher X2Π1/2 state. The present result also agrees with that of a very recent PIMS study that was performed at the synchrotron in Berlin. Schwell et al.32 generated ClO in the same manner as Bulgin et al.,21,27 i.e., by the reaction of atomic chlorine with ozone; they report a value for IE(ClO) of 10.85 ( 0.05 eV. As was the case for Cl2O, the less precise values obtained by conventional EIMS, IE(ClO) ) 11.1 ( 0.1 eV14a and 11.08 ( 0.05 eV,14b are too high. In a very recent theoretical study, McGrath and Rowland33 calculated thermodynamic properties for the halogen oxides; they obtain a value for IE(ClO) of 10.80 eV at the highest level of theory that was employed [G2(QCI/QCI)]. Although they do not quote a specific uncertainty for this IE value, they do indicate an “average deviation” of about (0.04 eV for calcula-
PIE Spectrum of Cl2O
J. Phys. Chem., Vol. 100, No. 33, 1996 14181 by about 0.2 eV. The EIMS results from ref 14a for IE(Cl2O), IE(ClO), and AE(ClO+, Cl2O) are all uniformly higher by about 0.2 eV than the corresponding PIMS or PES results. Even allowing for the poorer energy resolution inherent in the electron impact work and hence the larger error, the 0.2 eV difference is outside the stated uncertainty. Errors in EIMS results that are systematically high by a few hundred meV in magnitude have been noted previously by Rosenstock.35 The appearance energy determined here may be used to derive the heat of formation of Cl2O. First, the AE must be corrected for the internal energy present at room temperature in Cl2O as discussed by Traeger and McLoughlin:36,37
AE0(ClO+, Cl2O) ) AE298 + Ei Figure 5. Photoionization threshold region for the appearance of ClO+ (Cl2O) (m/z ) 51) between λ ) 98.0 and 104.0 nm at a nominal resolution of 0.26 nm and with 0.02 nm steps. The linear extrapolation of the spectrum to the baseline yields an onset at 100.83 nm (AE298 ) 12.296 ( 0.032 eV). The data used in the linear extrapolation (100.28100.70 nm) and to establish the baseline (101.60-103.20 nm) are represented by filled circles. Data not employed in the extrapolations are represented by solid triangles. The peak near 101 nm (open circles) is residual signal due to the second-order light (i.e., λ ≈ 50.5 nm) which was incompletely corrected. It is presumably caused by fluorescence from atomic helium (because we observed signal at this wavelength at all m/z).
tions of this type. In this case, the agreement between theory and experiment appears to be less precise than their average deviation. The heat of formation of ClO+ may be computed readily from their IE value and ∆fH°0(ClO):34
∆fH°0(ClO+) ) ∆fH°0(ClO) + IE(ClO)
(3) -1
) 101.04 + 1050.26 kJ mol
) AE298 + (H298 - H0)Cl2O - 5/2RT
(4) (5)
) 1191.91 ( 3.09 kJ mol-1 (12.353 ( 0.032 eV) From this value for AE0, ∆fH°0(Cl2O) can be computed using the heat of formation of ClO+ that was derived from the ionization energy for ClO (determined separately as discussed in the preceding section) and the heat of formation of Cl. For the dissociative ionization of Cl2O to form ClO+
Cl2O f ClO+ + Cl, ∆rH°0 ) AE0
(6)
the heats of formation are related to AE0 by eq 7
AE0 ) ∆fH°0(ClO+) + ∆fH°0(Cl) - ∆fH°0(Cl2O) (7) Rearranging and using the ∆fH°0(Cl) value from Chase et al.38
∆fH°0(Cl2O) ) ∆fH°0(ClO+) + ∆fH°0(Cl) - AE0 (8)
) 1151.30 ( 1.54 kJ mol-1
) 1151.30 + 119.62 - 1191.91 mol-1
where the uncertainties in ∆fH and IE are 0.11 and 1.54 kJ and the combined uncertainty was derived from the square root of the sum of the squares. C. Appearance Energy of ClO+ and Determination of ∆fH°298(Cl2O). The PIE spectrum of ClO+ at m/z ) 51, formed Via dissociative ionization of Cl2O, is shown in Figure 5 over the wavelength range λ ) 98-104 nm at 0.26 nm resolution and at 0.02 nm intervals. The onset for ClO+ formation, determined by linear extrapolation of the spectrum to the background, occurs at 100.83 nm which corresponds to an appearance energy of 12.296 ( 0.032 eV. The nonzero baseline in Figure 5 is due to ClO+ signal from ion-pair formation (ClO+ + Cl- r Cl2O), which we determined to have a threshold near λ ) 116.1 nm. This same nonzero baseline is also evident in Figure 4b. Although HOCl is also present in this system (as mentioned in the Experimental Section), the ClO+ signal observed here is not due to dissociative ionization of HOCl since the process HOCl f ClO+ + H is calculated to have a threshold above 15 eV. However, the presence of HOCl does require sufficient mass resolution to avoid contribution to the ClO+ signal from the neighboring HOCl+ signal. In Figure 1, the mass scan of ClO+ (from dissociative ionization of Cl2O) and HOCl+ over the range m/z ) 50-55 at an excitation energy of 11.81 eV (λ ) 105 nm) demonstrates that excellent mass resolution was attained in these DF-PIMS experiments. A previous measurement of AE(ClO+, Cl2O) ) 12.5 ( 0.1 eV Via conventional EIMS14a is higher than the present result
∆fH°0(Cl2O) ) 79.01 kJ mol-1 leads to
∆fH°298(Cl2O) ) 77.18 ( 3.45 kJ mol-1 by applying the correction of -1.83 kJ mol-1 given in Gurvich et al.19 for the integrated heat capacities of Cl2O and the elements. The uncertainty associated with this derivation of ∆fH°298(Cl2O) was derived from the combined uncertainties.39 The value derived for ∆fH°298(Cl2O) from eq 8 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 Cl2O to ClO+ + Cl. However, this dissociative ionization process is the lowest energy one (except for ion-pair formation, i.e., ClO+ + Cl-) and involves only a simple O-Cl bond rupture. Since little or no energy barrier is expected, AE0 should be a good measure of ∆rH°0 as indicated in eq 6. The present value for ∆rH°298(Cl2O), 77.2 ( 3.4 kJ mol-1, is compared with nine previous values in Table 1. Although our result is lower than all previous measurements, calculations or recommendations, the differences are rather small compared with those of refs 19 and 20. We are thus in essential agreement with the experimental result of Alqasmi et al.20 (81.4 ( 1.7 kJ mol-1), the calculation of Gurvich et al.19 (79.2 ( 4.2 kJ mol-1) based on the data of ref 17, and the value (79 ( 10 kJ mol-1)
14182 J. Phys. Chem., Vol. 100, No. 33, 1996
Thorn et al.
TABLE 1: Comparison of Values for ∆fH°298(Cl2O) ∆fH°298(Cl2O) (kJ mol-1)
reference
method/comment
89.5 ( 2.5 103.3 ( 0.5 87.9 ( 2.5
Wallace and Goodeve (1931) Gunther and Wekua16 (1931) Stull and Prophet18 (1971)
79.2 ( 4.2 99 ( 10 81.4 ( 1.7 79 ( 10 87.9 ( 6.7 81.0 ( 2.0 77.2 ( 3.4
Gurvich et al.19 (1989) Fisher14a (1968) Alqasmi et al.20 (1978) Gurvich et al.19 (1989) Chase et al.38 (1985) Abramowitz and Chase48 (1991) This Study
15
heat of explosion as above calculation based on enthalpy of solution17a and enthalpy of formation in solution17b at 283 K calculation similar to above EIMSa/AE(ClO+,Cl2O)b enthalpy of reaction CINO + Cl2O T CINO2 + Cl2 literature review literature review thermodynamic calculation PIMSc/AE(ClO+, Cl2O)b
c a EIMS ) electron impact mass spectrometry. b ∆ H° f 298 calculated from the measured AE value, as described in text. PIMS ) photoionization mass spectrometry.
recommended by Gurvich et al.19 (It should be noted that the value reported by Alqasmi et al.,20 81.4 ( 1.7 kJ mol-1, is misquoted by Gurvich et al.19 as 87.4 ( 1.7 kJ mol-1.) D. Determination of ∆fH°298(HOCl). The value for ∆fH°298(Cl2O) may be related to ∆fH°298(HOCl) through the equilibrium constant for reaction 9:
Cl2O + H2O T 2HOCl
(9)
From the value of Keq at 298 K40-43 ((8.2 ( 1.0) × 10-2)44 we obtain ∆rG ) 6.20 kJ mol-1 with an uncertainty of 0.36 kJ mol-1 (ref 45). The entropy of reaction 9 is 16.38 J mol-1 K-1 (ref 46) and thus ∆rH ) ∆rG + T∆rS ) 6.20 ((0.36) + 4.88 ((0.15) ) 11.08 ( 0.39 kJ mol-1. The enthalpy of formation of HOCl is thus computed from eq 10:
2∆fH°298(HOCl) - ∆fH°298(Cl2O) - ∆fH°298(H2O) ) ∆rH (10) ∆fH°298(HOCl) ) [∆fH°298(Cl2O) - 230.75]/2 ) -76.78 ( 3.47 kJ mol-1 where the uncertainty was derived from the root-sum-square of the individual uncertainties. Correcting this value by +2.94 kJ mol-1 (ref 19) for the integrated heat capacity of HOCl and the elements, we obtain ∆fH°0(HOCl) ) -73.84 ( 3.47 kJ mol-1. The present result for ∆fH°298(HOCl), -76.8 ( 3.5 kJ mol-1, is in excellent agreement with the recent calculations of Glukhovtsev et al.47 (-76.0 kJ mol-1), and the recommended value of Gurvich et al.19 (-75.7 ( 5.0 kJ mol-1); there is poorer agreement with the value recommended in the JANAF tables38 (-71.5 ( 2.1 kJ mol-1). Note Added in Proof: Referring to Figure 2, the ratio of signal-minus-background for the peaks that correspond to the hot band (0 r 1 transition, at λ ≈ 108.4 nm) and the ionization threshold (0 r 0 transition, at λ ≈ 107.8 nm) is consistent with a vibrational temperature of about 298 K (see: Deibler; et al. Int. J. Mass Spectrom. Ion Phys. 1971, 7, 209). Referring to Figure 4b, the spin-orbit splitting (10.885 - 10.847 ) 0.038 eV) of 306 cm-1 is in very good agreement with values reported previously by Basco and Morse (318 cm-1, J. Mol. Spectrosc. 1973, 45, 35) and by Duignan and Hudgens (≈323 cm-1, J. Chem. Phys. 1985, 82, 4426). Acknowledgment. R.P.T., Jr., thanks the NAS/NRC for an award of Resident Research Associateships. The work at GSFC was supported by the NASA Upper Atmosphere Research
Program. 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) Cady, G. H. Inorg. Synth. 1957, 5, 156 and references therein. (3) Schack, C. J.; Lindahl, C. B. Inorg. Nucl. Chem. Lett. 1967, 3, 387. (4) Renard, J. J.; Bolker, H. I. Chem. ReV. 1976, 76, 487 and references therein. (5) Bailey, C. R.; Cassie, A. B. D. Proc. R. Soc. (London) 1933, A142, 192. Hedberg, K. J. Chem. Phys. 1951, 19, 509. Rochkind, M. M.; Pimetel, G. C. J. Chem. Phys. 1965, 42, 1361. (6) Gardiner, D. J. J. Mol. Spectrosc. 1971, 38, 476. Chi, F. K.; Andrews, L. J. Phys. Chem. 1973, 77, 3062. (7) Finkelnburg, W.; Schumacher, H. J.; Stieger, G. Z. Phys. Chem. Abt. B 1931, 15, 127. Goodeve, C. F.; Wallace, J. I. Trans. Faraday Soc. 1930, 26, 254. (8) Jackson, R. H.; Millen, D. J. Proc. Chem. Soc., London, 1959, 10. Jackson, R. H.; Millen, D. J. AdV. Spectrosc. 1962, 3, 1157. Herberich, G. E.; Jackson, R. H.; Millen, D. J. J. Chem. Soc. A 1966, 336. (9) Dunitz, J. D.; Hedberg, K. J. Am. Chem. Soc. 1950, 72, 3108. Beagley, B.; Clark, A. H.; Hewitt, T. G. J. Chem. Soc. A 1968, 658. (10) Cicerone, R. J. ReV. Geophys. Space Phys. 1981, 19, 123. (11) Prather, M. J.; McElroy, M. B.; Wofsy, S. C. Nature 1984, 312, 227. (12) Ko, M. K. W.; Sze, N. D. J. Geophys. Res. 1984, 89, 11619. (13) Cornford, A. B.; Frost, D. C.; Herring, F. G.; McDowell, C. A. J. Chem. Phys. 1971, 55, 2820. (14) (a) Fisher, I. P. Faraday Trans. 1968, 64, 1852. (b) Alekseev, V. I.; Zyubina, T. S.; Zyubin, A. S.; Baluev, A. V. IzV. Akad. Nauk. SSR, Ser. Chim. 1989, 2278. (15) Wallace, J. I.; Goodeve, C. F. Trans. Faraday Soc. 1931, 27, 648. (16) Gu¨nther, P.; Wekua, K. Z. Phys. Chem. 1931, A154, 193. (17) (a) Yost, D. M.; Felt, R. C. J. Am. Chem. Soc. 1934, 56, 68. (b) Kustodina, V. A.; Mishchenko, K. P.; Flis, I. E. Zh. Prikl. Khim. 1962, 35, 1374. Flis, I. E.; Mishchenko, K. P.; Kustodina, V. A. Zh. Prikl. Khim. 1961, 34, 306. (18) Stull, D. R.; Prophet, H. JANF Thermochemical Tables, 2nd ed., NSRDS-NBS, N 37, Washington, 1971. (19) Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of IndiVidual Substances, 4th ed.; Hemisphere Publishing Corp.: New York, 1989; Vol. 1 (Parts 1 and 2). (20) Alqasmi, R.; Knauth, H.-D.; Rohlack, D. Ber. Bunsenges. Phys. Chem. 1978, 82, 217. (21) Bulgin, D. K.; Dyke, J. M.; Jonathan, N.; Morris, A. Mol. Phys. 1976, 32, 1487. (22) Bulgin, D. K.; Dyke, J. M.; Jonathan, N.; Morris, A. J. Chem. Soc., Faraday Trans. 2 1979, 75, 456. (23) Dyke, J. M. Personal communication, 1996. (24) Lias, S. G.; Liebman, J. F.; Levin, R. D.; Kafafi, S. A. PositiVe Ion Energetics Version 2.0; NIST Standard Reference Database 19A; NIST: Gaithersburg, MD, 1993. (25) Tao, W.; Klemm, R. B.; Nesbitt, F. L.; Stief, L. J. J. Phys. Chem. 1992, 96, 104. (26) Monks, P. S.; Stief, L. J.; Krauss, M.; Kuo, S.-C.; Klemm, R. B. Chem. Phys. Lett. 1993, 211, 416. (27) 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. (28) Thorn, R. P., Jr.; Monks, P. S.; Stief, L. J.; Kuo, S.-C.; Zhang, Z.; Klemm, R. B. J. Phys. Chem., in press.
PIE Spectrum of Cl2O (29) Grover, J. R.; Walters, E. A.; Newman, J. K.; White, M. C. J. Am. Chem. Soc. 1985, 107, 7329, and references therein. (30) Stevens, P. S.; Anderson, J. G. J. Phys. Chem. 1992, 96, 1708. (31) van Lonkhuyzen, H.; de Lange, C. A. Chem. Phys. 1984, 89, 313. (32) Schwell, M.; Jochims, H.-W.; Wassermann, B.; Rockland, U.; Flesh, R.; Ru¨hl, E. J. Phys. Chem. 1996, 100, 10070. (33) McGrath, M. P.; Rowland, F. S. J. Phys. Chem. 1996, 100, 4815. (34) The value for ∆fH°0(ClO) was taken from ref 19 and the value for IE(ClO) was taken from this work. (35) Rosenstock, H. M. Int. J. Mass. Spectrum. Ion Phys. 1976, 20, 139. (36) Traeger, J. C. and McLoughlin, R. G. J. Am. Chem. Soc. 1981, 103, 3647. (37) A value of 11.695 kJ mol-1 for the integrated heat capacity for Cl2O was taken from ref 19. (38) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985, 14 (Suppl. No. 1). (39) The uncertainties in ∆fH°0(ClO+), ∆fH°0(Cl), and AE0 are (1.54 kJ mol-1, (0.006 kJ mol-1, and (3.09 kJ mol-1. The combined uncertainty was computed from the square root of the sum of the squares. (40) Direct measurements for Keq for reaction 9 were taken from Niki et al.41 and Ennis and Birks.42 The value for Niki et al. was corrected from 6.8 × 10-2 at 295 K to 7.3 × 10-2 at 298 K following the correction procedure employed by Knauth et al.43
J. Phys. Chem., Vol. 100, No. 33, 1996 14183 (41) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1979, 66, 325. (42) Ennis, C. A.; Birks, J. W. J. Phys. Chem. 1985, 89, 186. (43) Knauth, H. D.; Alberti, H.; Clausen, H. J. Phys. Chem. 1979, 83, 1604. (44) The average of Keq values, at 298 K, from Niki et al.41 (7.3 × 10-2)40 and Ennis and Birks42 (9.2 × 10-2) is 8.2 × 10-2 with an uncertainty of 1.0 × 10-2. This result is in exact agreement with the value reported by Knauth et al.43 that was measured at 333 K and corrected to 298 K. (45) The uncertainties in Keq and T were taken into account. The combined uncertainty was obtained by computing the square root of the sum of the squares, i.e., the root-sum-square (RSS). (46) The entropy values were all taken from ref 19. The combined uncertainty in ∆rS is estimated to be (0.50 J mol-1 K-1. (47) Glukhovtsev, M. N.; Pross, A.; Radom, L. J. Phys. Chem. 1996, 100, 3498. (48) Abramowitz, S.; Chase, M. W., Jr. Appl. Chem. 1991, 63, 1449. Abramowitz, S.; Chase, M. W., Jr. Pure Appl. Chem. 1991, 63, 1829. The value quoted in our Table 1 is taken from Table 2 of Abramowitz and Chase. The value given in their Table 3 is for T ) 0 K, not T ) 298.15 K despite the table heading.
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