16566
J. Phys. Chem. 1995,99, 16566-16570
Discharge Flow-Photoionization Mass Spectrometric Study of HOI: Photoionization Efficiency Spectrum and Ionization Energy Paul S. Monks' and Louis J. StieP Laboratory f o r Extraterrestrial Physics (Code 690), NASMGoddard Space Flight Center, Greenbelt, Maryland 20771
Dwight C. Tardy Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Joel F. Liebman Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, Maryland 21228-5398
Zhengyu Zhang, Szu-Cherng Kuo, and R. Bruce Klemm* Brookhaven National Laboratory, Bldg. 815, P.O. Box 5000, Upton, New York 11973-5000 Received: July 12, 1995; In Final Form: August 25, 1995@
Photoionization efficiency (PIE) spectra of HOI were measured over the wavelength range A = 115-130 nm and in the ionization threshold region, A = 123-129 nm, using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. HOI was generated, in situ but in varying amounts, by three separate reactions: OH I,; OH CF3I; O(3P) C2HJ The PIE spectra displayed steplike behavior near threshold, and the HO-I stretching frequency in the cation was determined to be 702 f 60 cm-I. A value of (9.811 & 0.020) eV was obtained for the adiabatic ionization energy (IE) of HOI from photoion thresholds, corresponding to the HOI+(X2A") HOI(X' A') transition. Even though the present result appears to be the first reported determination of IE(HOI), the experimental value is compared to an estimated value previously derived via a trend analysis and it is considered in terms of trends in the series IE(HOX), where X = F, C1, Br, and I. The branching ratio for [HOI]/[IO] in the O(3P) CzHJ reaction was estimated to be about 8/1 at T = 298 K if we assume that the photoionization efficiencies for HOI and IO are the same at 10.3 eV (A = 120 nm). Also, based on the value for IE(HO1) derived in the present study, a value for IE(I0) % 9.66 & 0.10 eV has been predicted.
+
+
+
-
+
Introduction Since HOI is the least stable hypohalous acid,' it is not surprising that little is known about the gas-phase chemistry of the molecule. In the troposphere, HOI is potentially important in marine boundary regions as a reservoir species for Since CF31 is a candidate replacement for CF3Br in halon fire ~uppressants,~ there is interest in HOI that may be formed as a combustion and atmospheric degradation intermediate. Further interest in the tropospheric fate of HOI centers around its role as a possible radioiodine (I3'I) carrier.6 Previously, HOI was thought to have little importance in stratospheric However, the possible role of iodine in ozone depletion has been discussed recently by Solomon et a1.l who showed that, under certain conditions, reaction 1, IO
+ HO, -.
HOI
+ 0,
(1)
k,(298 K) = (6.4-10.3) x lo-'' cm3 molecule-' s-' (ref 8) and the subsequent formatioddestruction of HOI via a catalytic cycle (for the HOBr analogy, see ref 9) could have an ozone
* Authors to whom correspondence should be addressed. ' NAS/NRC Resident Research Associate. Present address: School of Environmental Sciences, University of East Anglia. Norwich, NR4 7TJ, England. @Abstractpublished in Aduance ACS Abstracts, October 15, 1995. 0022-365419512099-16566$09.00/0
destruction efficiency nearly 100 times greater than that of chlorine at an altitude of about z = 20 km. HOI has been positively identified in the gas phase by Maguin et al.8b.'0via mass spectrometry and by Barnes et al.'I in an infrared study. Earlier kinetic studies have inferred HOI as a product from reactions such as OH with CF3I,I2 and alkyl halides.I4,l5 More recently, Leone and co-workers (KLL)I6have identified HOI, via high-resolution emission spectra (OH stretch), as a product of the reaction between O(3P) and alkyl iodides. Still, little data appear to exist on both the kinetics and thermochemistry of HOI in the gas phase. To date, the only value for the ionization energy of HOI is an estimate by Ruscic and Berkowitz (RB)" that was derived via a complex set of trend analyses. In this study, which continues our work on both X018.'9and HOXzO species, we present the first determination of the HOI photoionization efficiency (PIE) spectrum and the photoionization threshold from which the ionization energy (IE) may be derived.
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 experimental procedures have been described in detail in previous publication^.'^-^^
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 45, 1995 16567
Mass Spectrometric Study of HOI HOI was produced in a Teflon-lined flow reactor by three different reactions, two of which have been inferred to yield HOI: OH
+ I,
-
HOI
+I
(2)
k,(298 K) = 1.8 x lo-’’ cm3 molecule-’ s-’ (ref 13) OH
+ CF31- HOI + CF3 k3(298 K) = 3.1 x
cm3 molecule-’ s-I (ref 12)
OH radicals were generated by passing a H20/02/He mixture through a microwave discharge ( 1 7 0 W, 2450 MHz) at the upstream end of the flow tube (about 100 cm from the nozzle). The I2/He or CF3I/He mixture was introduced through the tip of the movable injector at a distance of 4-5 cm from the sampling nozzle. In a recent study of reactions of O(3P) atoms with alkyl radicals using alkyl iodides as precursors, KLLI6identified HOI to be a product of the reaction of O(3P) C2H5I and suggested that reaction 4a
+
+
O(3P) CH3-CH21
- HOI +
3
0.54
(3)
CH,=CH2
(44
125
130
135
WAVELENGTH (nm)
Figure 1. Photoionization efficiency spectrum of C2H5I between ,I=
124.0 and 136.0 nm at a nominal resolution of 0.1 1 nm and 0.05 nm steps. The photoion efficiency is ion counts at mlz = 156 divided by the light intensity in arbitrary units. The onset of ionization, at ,I = 132.75 nm, yields IE = 9.340 f 0.008 eV where the uncertainty is = 5.17 x derived from the wavelength resolution, 0.11 nm. [CZH~I] 1013 molecule cme3. The superposed lines indicate autoionization features due to two Rydberg series that converge to the upper spinorbit state, X2A”~,2,at 9.935 eV: solid lines at 132.50, 130.10, 128.60, 127.75, 127.00, and 126.65 nm; dashed lines at 131.80, 129.60, and 128.30 nm. iodide (Aldrich, 99%), and ethyl iodide (Fluka, Puris. Grade, >99.5%) were all outgassed by repeated freeze-pump-thaw cycles.
Results and Discussion k4(298 K) x (2-5) x
As an example of the PIMS experiment, the PIE spectrum of C2H51was measured over the wavelength range J. = 124.0136.0 nm, as shown in Figure 1. The onset at A = 132.75 nm corresponds to IE(C2H5I) = 9.340 f 0.008 eV, in good must involve a 1,4-H atom shift mechanism. In the present agreement with the recommended value of 9.35 f 0.01 eV.25 study, reaction 4 was employed as the third source of HOI with This level of agreement indicates that (1) the wavelength O(3P) atoms generated in an O2/He discharge and C2H5I calibration, established by the location of zero order, is reliable admitted via the movable injector at 4 cm from the sampling and (2) the threshold is not significantly perturbed by thermal nozzle. effects. The PIE spectrum displays autoionizing structure due All experiments were conducted at ambient temperature (T to two Rydberg series, as indicated, that converge to the upper = 298 f 2 K) and at a flow reactor pressure of about 4.3 f 0.2 spin-orbit state (X2A”1/2) at 9.935 eV (A = 124.80 nm), in Torr with helium carrier gas. Flow velocities were in the range agreement with earlier photoelectron and optical spectroscopy 1300-1500 cm s-I. The concentrations of 0 2 , H20,12, CF31, and CzH5I in the flow reactor were, in units of molecule ~ m - ~ , studies.26 There appear to be no previous photoionization studies of ethyl iodide.25 1014,(1.5-6) x 10’3, 1 x 1012, (2.6-3.7) x 10’4, (1-10) A mass scan (at J. = 120 nm) of the reactant, C2H5I, and and 1.5 x lOI4, respectively. products, HOI and IO, is shown in Figure 2. The fraction of The gaseous mixture in the flow reactor was sampled as a C2H51 lost due to reaction 4 (Le., discharge off signal minus molecular beam into the sample chamber and subsequently into discharge on signal) was about 18%. Although the relative the photoionization source of the mass spectrometer. Ions were photoionization efficiencies for HOI and IO are not known, it mass selected with an axially aligned quadrupole mass filter, might be reasonable to assume they are about the same at 10.3 detected with a channeltron/pulsepreamp, and then counted for eV (A = 120 nm). (Maguin et aLsb presented evidence that the preset integration times. Measurements of PIE spectra, the ratio ionization efficiencies of HOI and IO are about the same at an of ion countsllight intensity vs wavelength, were made using electron impact energy of 30 eV.) Applying this assumption, tunable vacuum-ultraviolet (VUV) radiation at the NSLS. A the ratio of integrated peak heights, ((mlz = 144)/(m/z = 143)) monochromator with a normal incidence grating (1200 lines/ = 7.6, may be equated to the concentration ratio, [HOI]/[IO], mm) was used to disperse the VUV light, and a LiF filter (A 2 which in turn is the branching fraction for these products of 103 nm) was used to eliminate second- and higher-order reaction 4. It is also assumed that loss of both HOI and IO, radiation. The intensity of the VUV light was monitored via a within the 4 cm reaction zone, has an insignificant effect on sodium salicylate-coated window with an attached photomulthe derived branching fraction. tiplier tube. PIE spectra for HOI, generated via reactions 3 and 4a, are The oxygen and helium were of the highest purity obtainable shown in Figure 3. The peak signal to background is more (MG Industries, Scientific Grade, 99.999% and 99.9999%, than 100:1 . The “tail” that extends toward longer wavelengths respectively) and were used directly from cylinders. The water may have contributions from (1) a nonideal slit function (e.g., sample was of triply distilled quality and it was thoroughly a Gaussian function with “wings”), (2) a small contribution from outgassed. The iodine (Mallinckrodt, AR grade), trifluoromethyl
IO-’’ cm3 molecule-’
s-l
(ref 24)
Monks et al.
16568 J. Phys. Chem., Vol. 99, No. 45, 1995 0.10
,
,
5
0.08
,
I
i
I
I
gG g 7
0.02
E
0.00
140
I45
150
155
160
123
124
m/z
-
127
128
129
WAVELENGTH (nm)
Figure 2. Mass spectrum of reactant and products of reaction 4 at an excitation wavelength of 120 nm (10.33 eV) after a reaction time of 2.6 ms with [C~H~IIO = 1.49 x lOI4 molecule cm-3 and [02]0 = 1.22 x lOI4 molecule ~ m - ~ (a): mass scan over the range mlz = 141-145 at mlz = 0.1 steps (scale on left); (b) mass scan over the range mlz = 155-157 at mlz = 0.1 steps (scale on right). 0.10
126
125
I
I
.“Y
5.
Figure 4. Photoionization threshold region of HOI at a nominal resolution of 0.13 nm and 0.1 nm steps between J. = 123.0 and 129.0 nm. The onset of ionization is at A = 126.35 nm (9.813 f 0.010 eV). = 1.47 x lOI4 molecule cm-3 and [02]0 = 1.17 x 1014 [C~HSIIO molecule ~ m - ~ . TABLE 1: Threshold Wavelengths and Ionization Energies for HOI threshold IE nominal 1 step fwhm run no.a (nm) (eV) (nm) (nm) HOI+ (HOI 12 + OH) 1 126.30 9.817 f 0.018 0.20 0.23 0.23 0.10 2 126.15 9.828 f 0.018 0.05 0.23 3 126.35 9.813 f 0.018 4 126.35 9.813 i0.018 0.10 0.23 mean 9.818 f 0.012 ( 2 4 HOI’ (HOI CFJ + OH) 0.20 0.23 5 126.30 9.817 f 0.018 0.23 0.10 6 126.35 9.813 f 0.018 7 126.35 9.813 f 0.018 0.10 0.23 8 126.55 9.797 i 0.01 1 0.10 0.14 mean 9.810 f 0.015 ( 2 4 HOI’ (HOI CzHsI + 0) 0.20 0.14 9 126.30 9.817 f 0.011 0.10 0.13 10 126.35 9.813 f 0.010 11 126.40 9.809 f 0.009 0.05 0.11 12 126.42 9.807 0.009 0.05 0.11 mean 9.812 f 0.009 (2a)
-
G 5
0.05 i
U 0
z
3
5
+
L
0
E 0.00
120
115
I25
130
WAVELENGTH (nm)
-
Figure 3. Photoionization efficiency spectra of HOI between 1 = 115.0 and 130.0nm at 0.2 nm steps. The photoion efficiency is the ion counts at mlz = 144 divided by the light intensity in arbitrary units. The HOI molecule was produced in (a) by the reaction 4a with [C~H~IIO = 1.47 x lOI4molecule cm-3 and [ 0 2 ] 0 = 1.17 x lOI4molecule cm-3 and in (b) by the reaction 3 with [CF& = 2.59 x 1014molecule cm-.’, [H20]o a Average of runs 5- 12: 9.81 1 f 0.020 (3a). See text. = 6.09 x lOI3 molecule cm-3, and [O& = 5.95 x lOI4molecule ~ m - ~ . example is plotted in Figure 4 where HOI was generated via The positions of the “steps” in the PIE curves (126.3, 125.2, and 124.1 reaction 4a. The spectrum for HOI was obtained in the nm) are indicated by the superposed lines. The average vibrational spacing (0 0 to 1 0 and 1 0 to 2 0) is 702 & 60 cm-’ wavelength region 1 = 123.0-129.0 nm at 0.10 nm intervals (where the estimated uncertainty reflects the resolution of the measureand a nominal resolution of 0.13 nm (fwhm). The threshold ment). (see Figure 4) was analyzed by taking the half-rise point of the step to derive the ionization energy. From Figure 4 a threshold residual intemal energy (Le., “rotational tailing”), and (3) a small wavelength of 1 = 126.35 nm is obtained and therefore an IE “hot band” around 1 = 126.8 nm. By analogy with HOBr,20 of 9.813 eV. Table 1 lists 12 independent determinations of the threshold transition is presumed to be HOI+(X*A”) HOIIE(HO1) where HOI was produced by three different reactions. (XIA’), where the highest energy occupied orbital in the neutral is antibonding between the C-type atomic orbitals of iodine and Of the HOI-producing methods, the reaction between OH and I2 proved to be the least reliable. For example, the signal level oxygen. The A2A’ and B2A” cation states20 may contribute would decrease over a period of time. Since the maximum [I21 to the observed structure (above threshold) that can arise from concentration was only about lo’* molecule ~ m - the ~ , HOI both direct transition and autoionization of Rydberg states of signal obtained via reaction 2 was very low. In order to obtain the neutral that converge to the ion excited states. In the case a sufficient signal to background ( S B ) ratio, an 0 2 discharge of HOBr, RBI7 tentatively suggested that the main autoionizing was used to “clean” the flow tube between experimental runs. series arises from the A2A’ state, which was supported by the derived excitation frequency and ‘by analogy to the He I Since the OH 1 2 method gave the lowest SIB ratios, the results are not used in the overall derivation of the ionization energy. photoelectron spectroscopy (PES) spectrum of HOCL2’ Since the autoionizing region of the HOI PIE spectrum is quite Taking only the data resulting from reactions 3 and 4a, we obtain congested and not fully resolved, further speculation here on a simple average value of (9.811 f 0.0%) eV for IE(HOI), where the contributing states is not warranted. the error limit is at the 3 0 level. In order to better determine the ionization energy, detailed In their pulsed photolysis experiment, KLLI6 observed high vibrational excitation of the OH ( V I )bands in nascent HOI (their examinations near the threshold were carried out, and an
*
-
-
-
-
-
+
J. Phys. Chem., Vol. 99, No. 45, 1995 16569
Mass Spectrometric Study of HOI vibrational temperature was estimated to be between 6000 and 8000 K); they estimate that 50-60% of the HOI is not vibrationally excited. However, we do not observe highly populated hot bands in the PIE spectrum. Emission from the mode was observed by KLLI6 up to u = 3 (-10 000 cm-I); the thermodynamic limit for reaction 4a is -20000 cm-’ (estimated). Without direct observation from the HOI bending (v2)and 01 stretch (v3) modes, the individual nascent vibrational temperatures of v:,and vg cannot be estimated. Under flow conditions employed in the present study (residence time of 2.5 ms and 4.5 Torr He), there are about 2 x lo5 collisions between the formation and sampling of HOI. These collisions, predominately with He, can enhance vibrational to vibrational (VV) intramolecular energy transfer and/or deactivate HOI by vibrational to translational (VT) intermolecularenergy transfer;28VV relaxation is faster than VT. Rapid VV transfer would produce equilibration in the V I , v2, and v3 vibrational modes. In the limiting case that the nascent energy distribution is only in V I at T = 8000 K, VV equilibration would produce an overall vibrational temperature of 2880 K. At low pressures, where VT relaxation is slow, the hot HOI could decompose to OH f I; the fraction of HOI with energy in excess of the HO-I bond strength (estimated to be 45 kcal mol-’) changes from -0.3 to 0.006 for 8OOO L T 2 2880 K. (Our experiments do not provide information on the OH yield from decomposition, and in any event, the abstraction of H atom from C2H5I by 0 atom, to produce OH, is expected to be of negligible importance.) At higher pressures, VV relaxation provides a path to populating v3, which, having the lowest vibrational frequency (-580 cm-I), would be the doorway mode for VT r e l a ~ a t i o n . ~For ~ . small ~~ molecules with similar vibrational frequencies, the relaxation time is on the order of 102-104 collision^;^' this is an upper limit since fewer collisions are required as the excitation energy increases (at T = 2880 K the average excitation energy corresponds to u = 3). Thus, it is expected, under the present experimental conditions, that the nascent energy distribution for HOI should be thermalized and that hot bands resulting from nascent HOI will not be observed. Since the present study represents the first experimental determination of the ionization energy of HOI, our discussion will be limited to comparison to a previously estimated value for IE(HO1) and a brief summary of trends in IE(HOX), where X = F, C1, Br, and I. A value of IE(HO1) = 9.71-9.80 eV was inferred by RBI7 based on a consideration of OH as a pseudohalogen similar to C1 and on a trend analysis for the ratio IE(XCl)/IE(HOX) for X = F, C1, and Br. Even though the , ~ ~agreederivation of the RB estimate may be q ~ e s t i o n e dthe ment with our experimental value, IE(HO1) = (9.81 f 0.02) eV, suggests that the basis for the estimate may have been a reasonable one. The experimental value for IE(HO1) may also be considered in terms of trends in the series IE(HOX), where X = F, C1, Br and I. A set of values for IE(H0X) was listed by RBI7 in their Table V, but they did not comment on them. Our set of values for the series is given in Table 2 (which includes refs 33-36) and differs slightly from that of RBI7 in two of the entries: IE(HOBr) and IE(HO1). For IE(H0Br) we use an average of all the published experimental values, and for IE(HO1) we use the present experimental result of (9.81 f 0.02) eV rather than the one estimated by RB.I7 Qualitatively, the observed trend in IE(H0X) is for IE to decrease with increasing atomic size, A(F C1) being the largest (1.59 eV), A(C1- Br) being the smallest (0.49 eV), and A(Br I) being intermediate between the other two (0.82 eV). A comparable trend in IE is exhibited for the series X = F, C1, Br, and I in a large number of molecular
-
-
TABLE 2: Ionization Energies for HOX and XO and Vibrational Frequencies for HOX and HOX+ IE(H0X) IE(X0) v,(HOX) VI(HOX+) X (eV) (ev) (cm-I) (cm-I) F 12.71 iO.O1”,b 12.78 f 0.03‘ 889d 1O5Ob 10.95 f 0.015 724d 830‘ C1 11.12 i 0.01‘ Br 10.63 f 0.019 10.46 f 0.04h 620d 720’ I 9.81 f 0.021 9.66 f 0.10’ 577/575d 7021 Berkowitz et ~ 1 . ’ ~ van Lonkhuyzen and de Lar~ge.)~Zhang et aE.I9 dBarnes et a/.11(and references therein). ‘Colbourne et dZ7 f Bulgin et a/.35g Average of three values, from Ruscic and Berkowitz,” Monks et a1.,20and Ruscic and Berko~itz.’~Monks et a1.l8 ’ Ruscic and Berk0~itz.I~ 1 Present study. The estimated uncertainty in this frequency is f 6 0 cm-’. See text, Table 1, and caption to Figure 3. TABLE 3: Ionization Energy Trends for Some Halogen Compound9 IE(HX) IE(CH3X) IE(FX) IE(C1X) IE(BrX) IE(1X) X (ev) (eV) (eV) (eV) (eV) (eV) F 16.05 12.47 15.70 12.65 11.77 10.62 C1 12.75 11.22 12.65 11.48 11.01 10.09 Br 11.66 10.54 11.77 11.01 10.52 9.79 9.79 9.40 I 10.39 9.54 10.62 10.09 a
All ionization energy values were taken from Lias et a/.25a
groups such as HX, CH3X, FX,ClX, BrX, and IX (see Table 3). For example, in the case of CH3X, A(F C1) = 1.25 eV, A(C1- Br) = 0.68 eV, and A(Br I) = 1.00 eV.25 Lack of experimental IE data, especially for the iodine compounds, precluded any comparison with groups such as XNO, XONO, X20, OXO, and XO. In the latter case, there is only an estimate of IE(IO), but it would be of dubious value for us to use the RBI7 trend analysis that depends on their estimate for IE(I0) in order to evaluate the reasonableness of the present experimental value for IE(HO1). Instead, it would be more reasonable to employ our experimental value for IE(HO1) to estimate one for IE(I0). Following the reverse of the scheme employed by RB,l7 we obtain IE(I0) = 9.66 f 0.10 eV.37 Finally, in Table 2, we compare the HO-X vibrational frequencies for neutrals and cations. In all cases, the cation frequency is 15-18% larger than the neutral frequency owing to ejection of an electron from an antibonding orbital. Similar behavior is now observed for HOI, although it may not be quantitatively rigorous to compare the present result for HOI+ (obtained in the gas phase) to the literature value for HOI (obtained in the condensed phase, frozen matrix).
- -
Acknowledgment. We thank Drs. S. R. Leone, J. J. Klaassen, A. R. Ravishankara, M. K. Gilles, and I. Slagle for providing their data prior to publication. 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. 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. P.S.M. thanks the NAS/NRC for the award of a research associateship. References and Notes (1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text; Interscience: New York, 1962. ( 2 ) Jenkin, M. E.; Cox, R. A,; Candelhand, D. E. J. Atmos. Chem. 1985, 2, 359. (3) Chameides, W. L.; Davis, D. D. J. Geophys. Res. 1980, 85, 7383. (4) Jenkin, M. E. The Tropospheric Chemistv of Ozone in the Polar Regions; Niki, H., Becker, K. H., Eds.; NATO AS1 Series Vol. 17;SpringerVerlag: New York, 1993.
16570 J. Phys. Chem., Vol. 99, No. 45, 1995 (5) Nyden, M. R.; Linteris, G. T.; Burgess, D. F. R.; Westmorland, P. R.; Tsang, W.; Zachariah, M. R. In Evaluation ofAltematiue In-Flight Fire Suppressantsfor Full-Scale Testing in Simulated Aircraft Engine Nacelles and Dry Bays; Grosshandler, W. L., Gann, R. G., Pitts, W. M., Eds.; NIST Soecial Publication 861: National Institute of Standards and Technoloev: ", Washington, DC, 1994; p 467. (6) Voilleaue, P. G.; Keller. J. H. Health Phvs. 1981, 40, 91. (7) (a) Sofomon, S . ; Garcia, R. R.; Ravishankara, A. R. J. Geophys. Res. 1994, 99, 20491. (b) Solomon, S . ; Burkholder, J. B.; Ravishankara, A. R.: Garcia, R. R. J. Geophys. Res. 1994, 99, 20929. (8) (a) Jenkin, M. E.; Cox, R. A,; Hayman, G. D. Chem. Phys. Lett. 1991, 177. 272. (b) Maguin, F.; Laverdet, G.; La Bras, G.; Poulet, G. J. Phys. Chem. 1992, 96, 1775. (9) Monks, P. S . ; Nesbitt, F. L.; Scanlon, M.; Stief, L. J. J. Phys. Chem. 1993, 97. 11699. (10) Maguin, F.; Mellouki, A,: Laverdet. G.; Poulet, G.: Le Bras, G. Int. J. Chem. Kinet. 1991, 23, 237. (1 1) Barnes, I.; Becker, K. H.; Starcke, J. Chem. Phys. Lett. 1992, 196, 578. (12) Brown, A. C.; Canosa-Mas, C. E.; Wayne. R. P. A m o s . Environ. 1990, 24, 361. (13) (a) Jenkin, M. E.; Clemitshaw, K. C.; Cox, R. A. J. Chem. Soc., Faraday Trans. 2 1984, 80, 1633. (b) Loewenstein, L. M.: Anderson, J. G. J. Phys. Chem. 1985, 89, 5371. (14) Slagle, I.; Kalinovski, 1. J.; Gutman, D.; Harding, L. B. J. Phys. Chem., submitted. (15) Gilles, M. K.; Turnipseed, A. A,; Rudich, Y.; Talukdar, R. K.; Villalta, P.; Huey, L. G.; Burkholder, J. B.; Ravishankara, A. R. Private communication (to be published). (16) (a) Leone, S. R. Time-Resolved FTIR Emission Studies of Laser Photofragmentation and Radical Reactions. 17th Combustion Research Contractors' Meeting, Lake Geneva, WI, May 31-June 2, 1995. (b) Klaassen, J. J.; Lindner, J. L.; Leone. S. R. Private communication (to be published). (17) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1994, 101, 7795. (18) Monks, P. S.; Stief, L. J.: Krauss, M.; Kuo, S.-C.; Klemm, R. B. Chem. Phys. Lett. 1993. 211, 416. (19) Zhang, Z.; Kuo, S.-C.; Klemm, R. B.; Monks, P. S.; Stief, L. J. Chem. Phys. Lett. 1994, 229, 377. (20) Monks, P. S . ; Stief, L. J.; Krauss, M.; Kuo, S.-C.; Klemm, R. B. J. Chem. Phys. 1994, 100, 1902. (21) Tao, W.; Klemm, R. B.; Nesbitt, F. L.; Stief, L. J. J. Phys. Chem. 1992, 96, 104. (22) Kuo, S.-C.; Zhang, Z.; Klemm, R. B.; Liebman. J. F.; Stief, L. J.; Nesbitt, F. L. J. Phys. Chem. 1994, 98, 4026. (23) Buckley, T. J.; Johnson, R. D., 111; Huie, R. E.; Zhang, 2.: Kuo, S.-C.; Klemm, R. B. J. Phys. Chem. 1995, 99, 4879.
Monks et al.
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(24) We assume that the rate constant for O(3P) C Z H ~lies I between those for O(3P) CH3II4.l5( z 2 x cm3 molecule-' S-I)and O(3P) i-C3H7II5 ( a 5 x lo-" cm3 molecule-' s-l). A value of 2 x lo-" cm3 molecule-' s-' or greater for k4 would be consistent with our observation that the HOI signal remained constant while the tip of the movable injector was varied from 3 to 5 cm from the nozzle. (25) (a) 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. (b) Lias, S . G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref Data, Suppl. 1988, 17. (26) (a) Brogli, F; Heilbronner, E. Helv. Chim.Acta 1971, 54, 1423. (b) Boschi, R. A. A.; Salahub, D. R. Can. J. Chem. 1974, 52, 1217. (c) Carlson, T. A,; Gerard, P.; Pullen, B. P.; Grimm, F. A. J. Chem. Phys. 1988, 89, 1464. (d) Herzberg, G. Molecular Spectra and Molecular Structure; Krieger Publishing Company: Malabar, FL, 1991; Vol. 111. (27) Colboume, D.; Frost, D. C.; McDowell, C. A.; Westwood, N. P. C. J . Chem. Phys. 1978, 68, 3574. (28) Yardley, J. T. Introduction to Molecular Energy Transfer; Academic Press: New York, 1980. (29) (a) Tardy, D. C. J . Phys. Chem. 1993, 97, 5624. (b) Tardy, D. C.; Song, B. H. J. Phys. Chem. 1993, 97, 5628. (30) Clarke, D. L.; Gilbert, R. G. J. Phys. Chem. 1992, 96, 8450. (31) Lambert, J. D. Vibrational and Rotational Relaxation in GaAes; Clarendon Press: Oxford, 1977. (32) The value for IE(HO1) reported in ref 17 was 9.71-9.80 eV. However, only the smaller value, 9.71 eV, was derived directly for the adiabatic ionization energy that involves the transition from ground state HOI, XIA, to the lowest spin-orbit state of the cation, X2A"3,2. The larger value, 9.80 eV, was estimated (via an obfuscated scheme) from an average of spin-orbit state ionization energies for IC1, and thus it cannot be related to an experimental IE measurement that pertains only to the lowest spinorbit state of HOI+. (33) Berkowitz, J.; Appleman, E. H.; Chupka, W. A. J. Chem. Phys. 1973, 58, 1950. (34) van Lonkhuyzen, H.; de Lange, C. A. Mol. Phys. 1984, 51, 551. (35) Bulgin, D. K.; Dyke, J. M.; Moms, A. J. Chem. Soc., Faraday Trans. 2 1979, 75, 456. (36) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1994, 101, 9215. (37) A value of 9.66 0.10 eV may be derived for IE(I0) by assuming that IE(IO)iIE(HOI) is about equal to the ratios for the C1 and Br compounds (values from Table 2): IE(ClO)/IE(HOCl) = 0.985; IE(BrO)/IE(HOBr) = 0.984. Thus, IE(IO)/IE(HOI) = 0.9845 k 0.01 and IE(I0) = 9.66 5 0.10 eV.
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JF95 19474
