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Chloroacetone photodissociation at 193 nm and the subsequent dynamics of the CH3C(O)CH2 radical—an intermediate formed in the OH + allene reaction en route to CH3 + ketene
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10.1063/1.3525465
/content/aip/journal/jcp/134/5/10.1063/1.3525465
http://aip.metastore.ingenta.com/content/aip/journal/jcp/134/5/10.1063/1.3525465
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Figures

Image of FIG. 1.
FIG. 1.

Time-of-flight spectrum taken at m/e = 43 (CH3CO+) of the products resulting from the photodissociation of chloroacetone. The data are shown in open circles, and the total forward convolution fit to the data (shown in solid black line) is the sum of two contributions, shown here in long-dashed black and dash-dot-dashed black line. The long-dashed black line shows the contribution from the primary C–C fission of chloroacetone resulting in CH3CO, detected at this mass-to-charge ratio, and its momentum-matched partner, CH2Cl. Forward convolution fitting of this signal was used to derive the P() shown in Fig. 2. The dash-dot-dashed line depicts the signal rising from molecular clusters in the beam.

Image of FIG. 2.
FIG. 2.

The total recoil kinetic energy distribution, P(), of the CH3CO and CH2Cl fragments resulting from C–C bond fission in the photodissociation of chloroacetone. This fit was derived by forward convolution fitting of the fastest portion of the m/e = 43 (CH3CO+) signal in Fig. 1. This distribution was then used to predict the arrival time of the momentum-matched CH2Cl partner photofragments, as seen in Fig. 3.

Image of FIG. 3.
FIG. 3.

Time-of-flight spectrum taken at m/e = 49 (CH2Cl+) of the signal resulting from the photodissociation of chloroacetone. The data are shown in open circles, and the total forward convolution fit to the data (shown in solid black line) is the sum of two contributions, shown here in long-dashed black and dash-dot-dashed black lines. The long-dashed black line shows the contribution from the primary C–C fission of chloroacetone resulting in CH2Cl. This fit is derived from the P() shown in Fig. 2. The signal shown in dash-dot-dashed lines is from molecular clusters in the beam.

Image of FIG. 4.
FIG. 4.

Time-of-flight spectra corresponding to m/e = 35 (Cl+) products resulting from the photodissociation of chloroacetone. The three frames displayed here are from data acquisition at 15°, 30°, and 45° (from top to bottom frame, respectively). The data are shown in open circles, and the total forward convolution fits to the data are shown in solid black line. The signal from Cl atoms produced in the C–Cl bond fission of chloroacetone at 193 nm to yield Cl atoms and CH3C(O)CH2 radicals is shown in solid gray line. This fit is used to derive the total recoil kinetic energy distribution shown in Fig. 5. The top frame, taken with a source angle of 15°, also shows a contribution from the photodissociation of molecular clusters in the beam; this contribution is shown in dash-dot-dashed black line.

Image of FIG. 5.
FIG. 5.

The total recoil kinetic energy distribution, P(), derived from forward convolution fitting of the data taken at m/e = 35 (Cl+), as shown in Fig. 4. The portion shown in dotted gray line has vibrational energies [based on Eq. (4)] below the barrier to dissociation to CH3 + ketene—and thus the radicals formed with these kinetic energies are expected to be stable to dissociation. Likewise, those radicals formed with the range of recoil kinetic energies shown in long-dashed gray line have enough vibrational energy to dissociate. This distribution does not exhibit an abrupt cutoff = 21.9 kcal/mol, as is calculated from Eq. (4), because here we include a thermal distribution of vibrational energies for E chloroacetone, rather than just the average of the distribution. The arrow marks the expected energetic limit for radicals in the excited 12A state based on the calculated (Ref. 15) adiabatic excitation energy of 22 kcal/mol assuming the rotational energy partitioning is impulsive.

Image of FIG. 6.
FIG. 6.

Critical points on the S0 and S1 electronic surfaces of chloroacetone as calculated by Liu and Fang (Ref. 17) at the CAS (10,8)/cc-pVDZ level of theory. None of the energies include zero-point corrections. Their work reveals an excited electronic state of chloroacetone, the S1 state, that lies 97.1 kcal/mol above the ground electronic state, the S0 state. There is then a 4.4 kcal/mol barrier, relative to the minimum of the S1 state, as the C–Cl bond stretches out to CH3C(O)CH2 + Cl. This S1 state is easily accessible with the 147.8 kcal/mol photon imparted to the system, most likely via internal conversion from the S2 state. We use the geometry of the transition state on the S1 excited state, labeled here as TSC–Cl (S1), in our impulsive model to predict the partitioning of rotational energy to the CH3C(O)CH2 radicals. This allows us to estimate the velocity spectra of the CH3C(O)CH2 radicals that are stable to subsequent dissociation to CH3 + ketene.

Image of FIG. 7.
FIG. 7.

The internal energy distribution of the nascent CH3C(O)CH2 radicals formed from C–Cl bond fission in the photodissociation of chloroacetone. The upper frame shows the internal energy distribution of the CH3C(O)CH2 radicals, as derived from the measured total recoil kinetic energy distribution in Fig. 5 and using the conservation of energy as given by Eq. (3). The lower frame explicitly shows the vibrational energy distribution of the nascent CH3C(O)CH2 radicals. Both of the distributions are superimposed on the zero-point corrected energies for the relevant minima and transition states of the OH + allene potential energy surface, where the OH adds to the center C atom of allene. All points are calculated at the G3//B3LYP level of theory. The structures were optimized at the B3LYP level of theory, using an aug-cc-pVTZ basis set, and they were converged to a root-mean-square (rms) force below 1 × 10−5 and a rms displacement below 4 × 10−5, both in atomic units. The zero-point corrected energies are presented relative to the CH3C(O)CH2 radical. The radicals formed with lower internal energies, shown here in shaded gray, are predicted to be stable to dissociation to CH3 + ketene. The fact that, in the upper frame, some have internal energies above the barrier to dissociation, but are stable, is again due to the fact that some energy has been partitioned to rotations. For the bottom frame, the rotational energy has been subtracted, and we present the pure P(E vib) distribution as calculated from Eq. (4). The higher recoil kinetic energy C–Cl bond fission events, producing radicals with a total internal energy below 30 kcal/mol, cannot dissociate from the assumed geometry, so we simply truncate the prediction in the lower frame of Fig. 7. We of course retain these radicals among the ones predicted to be stable to subsequent dissociation.

Image of FIG. 8.
FIG. 8.

Time-of-flight spectra of the signal at m/e = 42 (COCH2 +) resulting from the photodissociation of chloroacetone and the subsequent dissociation of CH3C(O)CH2 radicals. The data are shown in open circles, and both the top and bottom frames have the overall fit shown in solid black line. The signal from ketene is shown by the broad component extending from 160 to 500 μs. These spectra also contain the contribution from neutral CH3CO photoproducts dissociatively ionizing to m/e = 42, as well as the stable mass 57 CH3C(O)CH2 radicals dissociatively ionizing to m/e = 42, both upon 200 eV electron bombardment ionization; these fits are shown in long-dashed black line and dotted gray line, respectively. The top frame shows, in black dotted line, the signal assigned to ketene based on the P(v ketene)and β(v ketene) from the imaging lab. The fits account for the fact that we are more sensitive to products with a parallel angular distribution, as they preferentially scatter in the detection plane. That same component is fit in the lower frame by considering the primary and secondary recoil velocities explicitly (see text for detail). A comparison between the fits to this broad component for ketene is given in the upper frame of Fig. 11.

Image of FIG. 9.
FIG. 9.

Raw image of the m/e = 42 (COCH2 +) products. It was obtained with 118 nm photoionization following photodissociation of chloroacetone at 193 nm. The photodissociation laser was polarized in the vertical plane of the image, as shown with the arrow. The image is 901 × 901 pixels and is the result of subtracting the background images obtained with 193 nm only and 118 nm only from the raw data.

Image of FIG. 10.
FIG. 10.

Velocity distribution of the ketene products from the imaging data. The top frame shows the background-subtracted velocity distribution of the products detected at m/e = 42 (COCH2 +). This velocity distribution is derived from the image shown in Fig. 9, which contains velocity-dependent anisotropies. In the scattering lab, the data are more sensitive to products with a parallel anisotropy, so we scale the individual points by (1 + β/4) to generate the corresponding fits shown in the top frame of Fig. 8 and the top frame of Fig. 11. To derive the velocity-dependent anisotropy for this correction, we fit the β(v ketene) measured in the imaging apparatus to a line (shown in the bottom frame).

Image of FIG. 11.
FIG. 11.

Contributions of ketene to the m/e = 42 time-of-flight spectrum (upper frame) and the momentum-matched fit to the m/e = 15 (CH3 +) observed signal (lower frame). In the upper frame, the dotted black line shows the portion of the time-of-flight at m/e = 42 attributed to ketene that is predicted by the imaging P(v ketene) and β(v ketene); this is the same fit that is shown in dotted black line in the upper frame of Fig. 8. The long-dashed gray line is the fit obtained by explicitly considering the velocities imparted in the primary photodissociation, along with those imparted as the higher internal energy radicals dissociate to CH3 + ketene. The P() that gives this fit, depicted in Fig. 12, is used to predict the time-of-flight in the scattering lab; this same fit is shown in the bottom frame of Fig. 8. Similarly, the lower frame shows the portion of the m/e = 15 time-of-flight that is predicted from the imaging P(v methyl) and β(v methyl); the long-dashed gray line is the momentum-matched fit based on the P() used to fit the ketene signal. This is the same fit as shown in the bottom frame of Fig. 15.

Image of FIG. 12.
FIG. 12.

The recoil translational energy distribution, P(), for the dissociation of vibrationally excited CH3C(O)CH2 radicals to CH3 + ketene. Forward convolution fitting of the data shown in dotted black line in the upper frame of Fig. 11 was used to derive this P(). For the primary C–Cl bond fission, the low kinetic energy portion of the overall P() shown in Fig. 5 was considered; this portion contains C–Cl fission recoil kinetic energies between 0 and 24 kcal/mol and is shown in long-dashed gray line in that figure. The P() shown here is used to calculate the additional velocity imparted as the nascent CH3C(O)CH2 radicals dissociate to CH3 + ketene. The resulting fits are shown in long-dashed gray line in Fig. 11.

Image of FIG. 13.
FIG. 13.

Raw image of the m/e = 15 (CH3 +) products. It was obtained with 118 nm photoionization following photodissociation of chloroacetone at 193 nm. The photodissociation laser was polarized in the vertical plane of the image, as shown with the arrow. The image is 901 × 901 pixels and is the result of subtracting the background images obtained with 193 nm only and 118 nm only from the raw data.

Image of FIG. 14.
FIG. 14.

Velocity distribution of the CH3 products from the imaging data. The top frame shows the background-subtracted velocity distribution of the products detected at m/e = 15 (CH3 +). This velocity distribution is derived from the image shown in Fig. 13, which contains velocity-dependent anisotropies. In the scattering lab, the data are more sensitive to products with a parallel anisotropy, so we scale the individual points by (1 + β/4) to generate the corresponding contributions shown in the lower frame of Fig. 11 and in black dotted line in the top frame of Fig. 15. To derive the velocity-dependent anisotropy for this correction, we fit the β(v methyl) measured in the imaging apparatus to a line (shown in the bottom frame).

Image of FIG. 15.
FIG. 15.

Time-of-flight spectra taken at m/e = 15 (CH3 +). The data are shown in open circles, and both the top and bottom frames have the overall fit shown in solid black line. These spectra also contain the contribution from neutral CH3CO photofragments dissociatively ionizing to m/e = 15, as well as stable CH3C(O)CH2 radicals (mass 57) dissociatively ionizing to m/e = 15, both upon 200 eV electron bombardment ionization; these fits are shown in long-dashed black line and dotted gray line, respectively. The top frame has the contribution predicted based on the from the imaging lab. The long-dashed gray line in the bottom frame is the predicted time-of-flight based on the fit from Fig. 11, where the primary (initial C–Cl bond photofission) and secondary (subsequent C–C bond fission in the nascent CH3C(O)CH2 radical) processes have been explicitly considered.

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/content/aip/journal/jcp/134/5/10.1063/1.3525465
2011-02-01
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Chloroacetone photodissociation at 193 nm and the subsequent dynamics of the CH3C(O)CH2 radical—an intermediate formed in the OH + allene reaction en route to CH3 + ketene
http://aip.metastore.ingenta.com/content/aip/journal/jcp/134/5/10.1063/1.3525465
10.1063/1.3525465
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