J. Phys. Chem. 1993,97, 6220-6225
6220
Photodissociation of tert-Butyl Hypochlorite and Decomposition of the tert-Butoxy Radical Fragment M.-A. Thelen, P. Felder, J. G. Frey,? and J. Robert Huber' Physikalisch- Chemisches Institut der Uniuersitiit Ziirich, Winterthurerstrasse 190, CH-8057 Ziirich. Switzerland Received: January 20, 1993; In Final Form: March 29, I993
The photochemistry of tert-butyl hypochlorite (TBOCl) has been studied in a molecular beam by photofragment translational spectroscopy. After laser excitation a t 248 or 308 nm, the TBOCl molecule decays exclusively into a tert-butoxy radical (TBO) and a C1 atom. The photolysis a t 248 nm releases ~ 6 0 % of the available energy into the photofragment recoil which is highly atisotropic (/3 = 1.9f O.l), implying a direct dissociation mechanism and an electronic transition dipole moment p in the parent molecule that is oriented along the O-cl bond. Approximately 90% of the nascent TBO radicals formed a t 248 nm have sufficient internal energy to undergo a secondary decay into methyl radicals and acetone, while at 308 nm TBO is stable. From a comparison of the time-of-flight distributions of the primary fragments C1 and TBO and on the basis of the dissociation energy Di')(TBO - C1) = 50 kcal/mol of the primary reaction, the threshold energy for the unimolecular decay of TBO is found to be EO= 20 f 3 kcallmol, a value which is higher than the previously reported gas kinetic activation energy E, = 15 kcal/mol.
The gas-phase photochemistry of tert-butyl hypochlorite (CH3),COC1 (TBOCl) is of interest because of its relation to HOCl. TBOCl is rather stable and can readily be obtained as a gaseous sample, and, in contrast to the smaller organic hypochlorites which show a strong tendency to decompose, its photofragmentation can be studied in a molecular beam. The simplest hypochlorite HOCl has been studied extensively owing to its potential role in the chemistry of the stratospheric ozone layer.112 The hypochlorites ROCl (R = CH3, CH'CH2, etc.) are not expected to be present in the upper atmosphere, but with the increased importance of hydrochlorofluorocarbons (HCFC) as a replacement' of the chlorofluorocarbons(CFC), more chlorine containing compounds will be degraded in the troposphere since the C-H bond in HCFC's provides a pathway for attack by OH radicals. This degradation could lead to an increased concentration of C10, which in analogy to the formation reaction of HOCl in the atmosphere,HO2 + C10 HOCl + 0214 may react with RO2 to produce ROCl species. The latter reaction becomes efficient when the concentration of C10 is high enough for C10 to compete with the reaction of NO, with R02. The possible formation and decomposition of ROCl compounds thereforemerit some attention. The results of experimental studiesw indicate that the formation of OH and C1 is the exclusive primary photochemical step of HOCl excited in the near-UV region. The analogous reaction is expected for
-
hr
(CH3),COC1
-
lecular decay proceeds via the loss of a methyl radical
I. Introduction
(CH,),CO
+ C1
(1) with the possibility of other primary processes competing with reaction 1. Of special importance here is the formation of the alkoxy radical (CH3)3CO (TBO). Alkoxy radicals are formed in the lower atmosphere and are known to participate in various reactions, including, most notably, the addition of NO, and unimolecular decompo~ition.~In the case of TBO the unimoDepartmentof Chemistry,University of Southampton,SouthamptonSO9 SNH, England.
(CH,),CO
(CH,),C=O
+ CH,
(2) a reaction which has been the subject of many investigations in the gas phase."JJl Very recently it has also been investigated under collision-free conditions as a secondary process in the photolysis of tert-butyl nitrite.12 The gas-phase absorption spectrum of TBOC1, displayed in Figure 1, consists of a weak band centered around 3 10 nm (e 9 L mol-' cm-1; u = 3.5 X 10-20 cm2) and a stronger absorption with a maximum near 240 nm (c = 64 L mol-' cm-1; u 2.5 X 10-19 cm2). The absence of any vibrational structure suggests that the excited electronic states are strongly repulsive. Except for a wavelength shift, the similarity of the near-UV spectra of HOCl and TBOCl in Figure 1 is striking. For HOCl the assignment of the weak absorption near 300 nm has been the subject of some debate, but recent ab initio calculations clearly indicate that this band is due to an lA" IA' (a*m/ T * X / ) transition located close to 320 nm and a stronger 'A' 1A' ( u * w l + ncr) transition at 250 nm.13 In order to elucidate the primary and secondary photodissociation processes of TBOCl, we have carried out an investigation by photofragment translational spectroscopy.14*15 Furthermore, the findings for the secondary dissociation of the tert-butoxy radicals complement the TOF results of the correspondingprocess from the photolysis of tert-butyl nitrite, as the production of the TBO radical with an atom as the counter fragment provides in a most direct way the internal energy distribution of this important radical from translational energy data.
-
-
--
II. Experimental Section A detailed description of the molecular beam apparatus and experimental procedures is found in previous articles.16 The apparatus consists of a rotatable pulsed molecular beam source and a spatially fixed quadrupole mass spectrometer. The pulsed laser beam is directed along the rotation axis of the molecular beam source and the variable scattering angle 0 is given by the direction of the molecular beam and the detector axis. Most of the experimentswere performed at 248 nm using a KrF excimer laser (Lambda Physik EMG 101 MSC) with a pulse energy of 220 mJ. The laser beam was collimated with an iris of 15-mm
0022-365419312097-6220$04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6221
Photodissociation of tert-Butyl Hypochlorite
248 nm m/e = 35
- i li
8 = 21"
P
200
2Eo
300
350
400
Wavelength !nm)
Figure 1. Gas-phase absorption spectra of HOCP and TBOCl in the
near-UV region at room temperature.
0
diameter and then softly focused by spherical and cylindrical lenses to a spot size of about 3 X 4 mm at the intersection with the molecular beam, yielding a laser fluence of ~ 5 4 mJ/cm2. 0 Linearly polarized light with a polarization degree of 92% was produced by directing the beam through a pile of 10 quartz plates at Brewster's angle. The polarization angle ep, defined as the angle between the electricvectorof the laser light and the detection axis, was varied by rotating the plane of polarization with a X/2 plate. An additional set of experiments was carried out with unpolarized laser light at a photolysis wavelength of 308 nm using an XeCl excimer laser (Lambda Physik EMG 101 MSC) which provided a fluence at the focus of 2 J/cm2. TBOCl was prepared from the reaction of terr-butyl alcohol with sodium hydroxide and chlorine according to a published procedure.*7The molecular beam was formed by flowing a stream of He at 370 mbar through a sample of degassed TBOCl kept at -20 O C . In order to avoid decomposition the sample was protected against light exposure and the inlet system consisted of glass, Teflon, and stainless steel. Stable beam conditions were achieved only after passivating the inlet system for at least 1 h. The velocity distribution of the molecular beam was determined from the laser induced depletion of the signal measured with the molecular beam source pointing toward the detector. Least squares fits to the density distributionflu) a u2 exp(-[(u - UO)/ yielded parameter values of DO H 1300 m/s and a H 70 m/s. TOF distributions of the photofragments were obtained at severalmass-to-charge ratios m/e and laboratory scattering angles 8. Over the range of 26Cb540 mJ/cm2 no power dependence of the TOF spectra was observed at 248 nm; at 308 nm the low signal-to-noise ratio prevented power dependence measurements. The recoil anisotropy ,8was determined by recording the integrated photofragment signal as a function of the polarization angle eP. All the TOF distributions shown in this paper have been corrected for the ion flight time through the mass filter.
Ib. A 1 1
III. Results and Analysis The main body of experimental data was obtained at 248 nm. TheTOFdistributionsmeasuredatm/e = 35 with theunpolarized photolysis laser are displayed in Figure 2 for 8 = 21° and 33O. Each spectrum consists of a single narrow peak due to the C1 photofragments of reaction 1. The total translational energy distribution P(&) of the fragment pair in the center of mass (c.m.) system of the parent molecule was determined with an iterative forward convolution method.19 The best fit of the TOF data, shown by the solid lines in Figure 2, was obtained with the P(ET) distribution given as a solid line in Figure 3. This P(ET) is of roughly Gaussian shape and has an average translational energy (ET)= 37 kcal/mol and a full width at half-maximum (fwhm) of 8 kcal/mol. Neglecting the small residual internal energy (50.1 kcal/mol) of the parent moleculesin the supersonicbeam, the energy balance
100
200
300
400
m/e = 35
0 = 33"
100
0
200
300
400
flight time (ps) Figure 2. TOF distribution of the C1 fragments measured -following photolysis at 248 nm under two different scattering angles (a) 8 = 21° and (b) 8 = 3 3 O . The fit (solid line) was calculated from the total translational energy distribution P(E,) given as a solid line in Figure 3.
0
10
20
30
40
50
60
70
I
0
c.m. translational energy (kcal/mol) Figure 3. Total translational energy distribution P(&) after 248-nm photolysis. The hatched region pertains to the unstable TBO radicals; the small blank portion, to the stable radicals (see text). E,,,,! marks the maximum available fragment energy and E p is the threshold energy at which secondary dissociation occurs.
of the photodissociation process (1) is given by
E,/ = hv - 06''= ET
+ E , + E,,
(3)
In this equation hu = 115 kcal/mol is the photon energy at 248 nm and @,') is the dissociation energy of the 0 4 1 bond in TBOC1. The available energy (Em,) is partitioned between the fragment translational energy ET, the electronic energy E a of the C1atom and the rovibrational energy ETBoof the TBO radical.
Thelen et al.
6222 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 Taking the best estimate of the dissociation energy Dhl) = 50 kcal/mol (see Discussion), we obtain E,l= 65 kcal/mol. E a is zero for ground-state Cl(2P312) and 2.5 kcal/mol for the spinorbit excited-state C1(2P1/2).20Although the experiment does not allow us to selectively detect the two spin-orbit states of the C1 atom because the internal energy distribution of the counter fragments yields too broad a translational energy distribution, previous photodissociation results lead us to expect that a fraction of the C1 atoms is formed in the spin-orbit excited-state 2P1pas discussed below. The measured average translational energy of 37 kcal/mol corresponds to an average internal energy of the nascent TBO radicals of 28 or 25.5 kcal/mol, depending on the spin+rbit state of the C1 atom formed as partner. Moreover, since the highest translational energy observed is 49 kcal/mol, we conclude from the energy balance (3) that all the nascent TBO radicals have an internal energy ETBO1 13.5 kcal/mol. The recoil anisotropy of the primary photofragments was determined from the polarization dependence of the TOF signal measured at m / e = 35. The angular distribution in the c.m. system has the form21*22 w(9)
1
100
200
300
400
300
400
+ j3P2(cos 8)
(4) with P2 being the second Legendre polynomial. After transformation from the c.m. to the laboratory system and correction for the incomplete laser polarization, the anisotropy parameter was found to be fl = 1.9 f 0.1, which is very close to the maximum possible value +2. Attempts to measure the TOF distribution of the ( C H W O fragments at the parent ion mass m / e = 73 were not successful. Since the average internal energy of the TBO radicals (25.5 or 28 kcal/mol, see above) clearly exceeds the threshold of the secondary decay reaction (2) which is 20 kcal/mol (see Discussion), most of these radicals are fragmentedto acetoneand methyl radicals before reaching the detector. Moreover, the small fraction of surviving TBO radicals are then fragmented in the electron bombardment ionizer. The signal from these radicals should, therefore, be observable at the m / e ratios of the daughter ions. Parts a and b of Figure 4 show the TOF spectra measured at m / e = 58 (CH3COCH3+)and m / e = 15 (CH3+)with B = 39'. The spectrum at these ion masses is complicated by the fact that the stable TBO primary radicals as well as the secondary dissociation products CH3 and acetone formed in the decay of unstable TBO radicals contribute to the signal. The different fragment flight times allow us, however, to disentangle these contributions. On the basis of the momentum correlation between C1 and TBO, the sharp feature denoted as 1 in Figure 4a,b can be assigned to the stable TBO radicals. The large signal at m / e = 58 (denoted by 2 in Figure 4a) is due to acetone formed in the secondary decay of TBO. The fits shown as the dashed and thick solid lines drawn through the data in Figure 4a were obtained as explained below. Starting with the P(&) distribution derived from the C1 atom data, the TOF spectrum of the TBO radicals was calculated and compared with the data at m / e = 58. To account for the loss of TBO radicals through secondarydissociation, the lower energy part of the P(ET)was then set equal to zero. The fit of feature 1 in Figure 4a was obtained by "zeroing" the P(ET) portion at translational energies below E p h = 42.5 kcal/mol. The portion of theP(ET)distribution pertaining to the unstable radicals is shown as the hatched region in Figure 3, whereas the blank region pertains to the stable radicals. From a comparison of the integrated probabilities of the hatched and blank regions in Figure 3, we estimate that about 90% of the TBO radicals are unstable. In a second analysis step the TOF contribution of the secondary product, acetone, was calculated as the three-dimensional superposition of the primary recoil distribution of unstable TBO radicals and the secondary recoil distribution of acetone. The three-dimensional velocity distribution of the unstable TBO radicals was determined from the hatched part of the P(ET) a
0
I
0
1
100
"
"
l
"
"
I
200
"
"
I
"
Flight time (ps) Figure 4. TOF distribution after photolysis at 248 nm measured at (a) m / e = 58 (CH$XCH3+) and (b) m/e = 15 (CH3+) with B = 39O. The fits (thick solid line) and the components (broken lines) were calculated with P(&) shown in Figure 3.
distribution by taking the anisotropic angular distribution into account. For the translational energy distributionofthe secondary fragments we used the di~tribution~~
P ( E g ) )= C(Eg)- @'(A -
(5) which provides the appropriate flexibility for fitting purposes. Here, E?) is the translational energy of the secondary fragment pair with respect to thec.m. frameoftheTBOradica1, Brepresents a low energy threshold, and Cis a normalization constant. The exponents r and ware fitting parameters employed to model the shape of the secondary distribution, and A is the maximum translational energy of the secondary step given by
where D${ = 5.6 kcal/moP is the dissociation energy of reaction 2. The parametric dependence of A on ET accounts for the fact that the internal energy content of the TBO radical depends on the translational energy released in the primary dissociation step. For simplicity, we carried out the analysis by absorbing the constant terms on the right-hand side of (6) into a single constant
which was then treated as a fitting parameter. The best fit shown in Figure 4a was obtained with an isotropic secondary fragment distribution and the parameter values r = 1.5, w = 1.0, B = 13 kcal/mol, and E!!! = 57 kcal/mol. This result will be discussed in the next section. The contribution from acetone is also evident as feature 2 in the TOF spectrum recorded at m / e = 15 (Figure 4b), which is
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6223
Photodissociation of tert-Butyl Hypochlorite
TABLE I: Fragment Translational Energy Distribution in the Photodissociation of TBOCl at 248 and 308 MI.
308 nm m/e = 35
248nm 308nm
65 43
37 26
28 17
25.5
22.5
20
14.5
"The energy unit is kilocalories per mol. bBased on a primary dissociation energy D t ) = 50 kcal/mol; see text. Average translational energy. Average internal energy. * Correspondsto the activationenergy EO for the secondary dissociation of the TBO radical (see text).
0
100
200
300
400
0
100
200
300
400
of the TOF data given by the dashed line in Figure 5a was achieved with the translational energy distribution P(ET) displayed in Figure 5c. This P(ET) has a width (fwhm) of 6 kcal/mol and an average translational energy ( E T ) = 26 kcal/mol. Taking DC) = 50 kcal/mol and hu(308 nm) = 93 kcal/mol yields E,I = 43 kcal/mol from which we derive an average internal energy of the TBO radicals (Etnt)= 17 or 14.5 kcal/mol, depending on the excitation of the C1 atom. The TBO radicals were monitored at m/e = 58 by the (CH&CO+ daughter ions which provided the TOF distribution shown in Figure 5b. In contrast to the complex signal found at 248 nm, we detected at 308 nm a single TOF peak at 170 1 s . The broad feature at longer flight time is attributed to the photodissociationof TBOCl clusters. The more distinct cluster signal at 308 nm as compared to 248 nm suggests that the relative absorption cross sections of the monomer and cluster species are different at these two photolysis wavelengths. The fit indicated by a dashed line in Figure 5b was calculated with the P(ET)obtained from the chlorine radicals (m/e = 35) and hence confirms the TBO fragments to be momentum matched to the chlorine radicals produced in reaction 1. On the basis of these findings, we conclude that most of the TBO radicals formed at 308 nm do not undergo secondary dissociation.
Flight time (,us) I
I
IV. Discussion
A
L
i
I " " / " " " " ' " ' ' ' I ' ' " / " ' ' l ' ' " l " "
0
1 0
20
30
40
50
60
10
80
c.m. translational energy (kcal/mol) Figure 5. TOF distribution after excitation of 308 nm of (a) chlorine fragments measured at m/e = 35 and of (b) their counterfragmentsTBO measuredatm/e=58withB=21°. Thefits(dashed1ine)werecalculated with the total translational energy distribution P(&) displayed in c. due to CH3+ ions formed upon electron impact ionization of acetone. An additional feature, denoted as 3, appears at very short arrival times and can be attributed to CH3, the counter fragment of acetone. Because of linear momentum conservation, the signal of the methyl radicals must be modeled with the same recoil parameters as those used for the acetone signal. The result is shown as a solid line drawn through feature 3 in Figure 4b and accommodates most of the fast TOF signal. In order to corroborate the threshold energy for the secondary dissociationobserved at 248 nm, we have also measured the TOF distributions of m/e = 35 and 58 using unpolarized light at 308 nm. The spectrum of the chlorine photofragments measured with 0 = 21° is displayed in Figure Sa. The distribution appears as a single peak, similar to that observed at 248 nm. The best fit
The only primary dissociationchannel observed after excitation of TBOCl at 248 nm is 0-C1 bond fission (1). Other primary dissociation pathways such as the loss of a methyl group or the C-0 bond rupture can be excluded since these would produce CI containing fragments which would inevitably contribute to the TOF signal at m/e = 35. The latter, however, consists of a single narrow peakwhich is exclusively due to the C1atoms from reaction 1. The energy disposal in the photodissociation process is summarized in Table I, where the available energy was calculated with eq 3. The dissociation energy D t ) was taken from the work of Walling and PapaioannouZS who determined the enthalpy change of reaction 1 to be A H 2 9 8 = 48 f 2 kcal/mol. Two of the reaction enthalpies they used in the thermochemical cycle have in the meantime been revised*C2*so that after consideration of the new values and neglecting the minor temperature dependence of the reaction enthalpy we obtained D t ) = 50 2 kcal/mol. This dissociation energy is lower than that for HO-Cl, which is 57 & 3 kcal/mol,26 and, hence, parallels the behavior of TBONO and HO-NO, where D?) was found to be 41 and 50 kcal/ mol, respectively.26~29 The average translational energy ( E T ) = 37 kcal/mol corresponds to approximately 60% of E,/. This rather large translational energy release points to a direct dissociation on a repulsive potential energy surface, a mechanism that is supported by the very high anisotropy of the fragment recoil. The value of p = 1.9 f 0.1 being very close to the upper limit of +2 implies that the dissociation process is considerably faster than the rotational period of the par$nt molecule21~2293O and that the electronic transition moment p of the parent is oriented parallel to the asymptotic recoil directi0n2~9~~ which is approximatelyalong the 0 4 1 bond. This latter finding contributes to the similarity between the electronic spectra of TBOCl and HOC1 (see Figure
*
6224 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
l), since for HOC1 the theoretical13 and experimental*evidence strongly indicates the 248-nm photolysis proceeds mainly through the 2 ‘A‘ 1 ’A‘ transition which involves an U*WI ncl type excitation and an electronic transition moment roughly parallel to the 0-C1 bond. It is well established from gas kinetic studies that hot alkoxy radicals undergo unimolecular decomposition into an alkyl radical and a ketone or aldehydemolecule.31 The observation of reaction 2 under collisionless conditions is, therefore, not unexpected, also because the analogous process has been observed after the photolysis of alkyl nitrites in a molecular beam.12.32 The unimolecular decay rate of TBO was the subject of numerous experimental studies, the results of which were collected by Batt.” The Arrhenius parameters of reaction 2 have been revised several times but have now converged to the values loglo A = 14.04 f 0.37 and E, = 14.9 f 0.2 kcal/mol,33 which can accommodate the decay rates measured in the temperature range between 303 and 443 K. In contrast to these experimental conditions, our measurementsprovide the activation energy at T = 0 K, henceforth denoted as Eo. The P(ET)distributionsof the primary dissociation at 248 and 308 nm, shown as the overall curves in Figures 3 and 5c, were determined from the TOF signals of the C1 atoms. The analysis of the signals at m/e = 58 and 15 led us to conclude that about 90%of the TBO radicals formed at 248 nm undergo the secondary dissociation (2). The remaining 10%are stable and give rise to a narrow TOF signal which is momentum matched to that of the C1 atoms. According to the analysis of the TOF distributions of the stable TBO radicals (features 1 in Figure 4a,b), the onset of secondary dissociation occurs at translational energies below EPah = 42.5 kcal/mol which, depending on the spin-orbit state of the C1 atom, corresponds to an internal energy E $.! = 22.5 or 20 kcal/mol (see eq 3). While the translational energy threshold is subject to an error of f l kcal/mol, the primary dissociation energy Dt)has an uncertainty of f 2 kcal/ mo1.25 Therefore,the uncertainty of the internal energy threshold EthnJh TBo is estimated to be f 3 kcal/mol with an additional ambiguity of 2.5 kcal/mol caused by the unknown spin-orbit state of the C1 atom. Under the collisionless conditions in our experiments and a negligible tunnelingprobability, a TBO radical formed with an internal energy below the activation barrier Eo will not decay. At energies above EO the radical decay rate increases strongly with the excess energy and it is unlikely that a radical with ETBO> EOsurvives the =lOO-ps flight time from the photolysis region to the detector. The experimentally determined threshold energy E$Zh should, therefore, closely agree with the activation barrier at T = 0 K, which is either EO = 22.5 f 3 kcal/mol or, if the C1 atoms are formed in the spinorbit excited state, Eo = 20 f 3 kcal/mol. On the basis of the results obtained with a number of chlorine containing compounds3636 and the relatively large available energy E,, = 65 kcal/mol, we can safely assume that a significant fraction of the C1 atoms is indeed formed in the excited state. Thus, we adopted the value of Eo = 20 kcal/mol. To support our findingsat 248 nm, we also camed out photolysis at 308 nm. The photofragment energy distributions found at the two wavelengths are summarizedin Table I. Parallel to the result at 248 nm the 0 4 1 bond fission is the only primary dissociation channel at 308 nm, but most of the TBO radicals do not undergo secondary dissociation. With use of EO= 20 kcal/mol the TBO radicals formed at 308 nm are stable (Le. ETBO< 20 kcal/mol) if the translational energy is larger than 23 kcal/mol (for C1(2P3/2) atoms) or 20.5 kcal/mol (for C1(2P1/2) atoms). A comparison of these threshold values with the P(ET) distribution of Figure 5c clearly shows that most of the TBO fragments are stable, a result which is confirmed by the good fit of the TOF signal at m/e 58 shown in Figure 5b. It is informative to consider these experimental findings by assuming a threshold value of 15 kcal/ mol as extracted from gas kinetic data.33 On the basis of this EO,
-
-
Thelen et al. most of the TBO radicals formed at 308 nm would be predicted to be unstable with the consequence that the TOF peak at m/e = 58 would have to be much narrower than that found in our experiment. The recoil distribution of the secondary fragmentscreated upon 248-nm photolysis provides further information on the decomposition of the TBO radical. The best fit of the translational energy distribution was obtained with eq 5 and the shape parameters r = 1.5 and w = 1.0 and a minimum translational energy B = 13 kcal/mol. In principle, the high energy threshold A can be derived by inserting the known thermochemicalvalue E:; = 59 kcal/mol into eqs 5-7, but a better fit was found after the high energy threshold A was lowered by 2 kcal/mol. This small reduction can be ascribed to the fact that the distribution P(@)) does not reach the maximum possible translational energy because of residual internal energy. However, according to the nonzero value of B, a substantial fraction of the available energy must be channeled into translational motion of the fragments CH3 and acetone which, therefore, emerge from the dissociation process with little internal energy. Moreover, the isotropic angular distribution of the secondary fragments reflects a relatively long lifetime of the activated radical as well as the decay of the highly symmetric, i.e., near-spherical, TBO radical with three equivalent methyl groups. The activationenergy EO= 20 i 3 kcal/mol determined in this study is larger than the value E. = I5 kcal/mol obtained from gas kinetic data.33 Since this latter value was measured for the temperature range 303-443 K whereas our value pertains to T = 0 K, part of the discrepancy might be due to a relatively strong temperature dependence of E,. Furthermore, we cannot exclude the possibility that the decay path of a TBO radical created by UV photon absorption via an electronically excited surface could be different from that of a thermally prepared one with the corresponding activation energies being different. Finally, it is interesting to compare the above findings with the results from the 248-nm photodissociation of tert-butyl nitrite (TBONO). Effenhauseret al.I2have recently measured the total translational energy distribution of the primary fragments TBO and NO from which they inferred an average internal energy ETBO= 30 kcal/mol. This result was obtained by assuming an average internal energy of the NO counterfragments EN* = 14 kcal/mol taken from a photodissociation work using LIF probing.37 However, due to the partitioning of the total internal energy into the internal energy of the TBO and NO fragments, it is not feasible to extract the distribution of ETBOfrom the observed P(ET)distribution. Therefore, although the formation of both stable and unstable TBO radicals was observed with TBONO, determination of the threshold energy EO for the decomposition of TBO was not possible. In contrast to TBONO, the TBOCl molecule yields an atomic fragment species with only two accessible quantum states. This favorablecondition allowed us in the present experiment to determine EOfrom the kinetic energy of the slowest stable TBO fragments, albeit with an ambiguity of 2.5 kcal/mol, arising from the unknown spin-orbit state of the C1 atom.
V. Conclusion
The photodissociation of TBOCl at 248 and 308 nm has been investigated by photofragment translational spectroscopy. At 248 nm the photolysis of TBOCl produces TBO radicals and C1 atoms in a fast and direct dissociation process as revealed by a very high anisotropy with B = 1.9 f 0.1. From this recoil anisotropyof the fragments we infer that the electronic transition moment p(Sn SO)is parallel to the 0 4 1 bond in the parent molecule and that the dissociation time is
