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THE JOURNAL OF CHEMICAL PHYSICS 132, 044307 共2010兲

Crossed-beam studies of electron transfer to oriented trichloronitromethane, CCl3NO2, molecules Peter W. Harland and Philip R. Brooksa兲 Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251, USA

共Received 26 October 2009; accepted 6 January 2010; published online 27 January 2010兲 Fast potassium atoms donate an electron to CCl3NO2 molecules to form K+ ions and the negative ions O−, Cl−, NO2−, CCl3−, CCl2NO2−, CCl3NO−, and CCl3NO2−. Threshold energies are measured for these ions and electron affinities for CCl2NO2−, CCl3NO−, and CCl3NO2− are estimated to be 2.35, 2.35, and 1.89 eV 共⫾0.6 eV兲, respectively. The threshold energies show that the C–N and N–O bonds are substantially weaker than in nitromethane. The CCl3NO2 molecules are oriented before the collision and at energies near 2.5 eV the electron appears to transfer to the ␲ⴱNO orbital forming the parent negative ion, CCl3NO2−, which is stabilized by interacting with the K+ donor. As the collision energy increases the parent negative ion fragments and the orientation dependence of the fragment ions helps understand the fragmentation pathway. © 2010 American Institute of Physics. 关doi:10.1063/1.3299280兴 I. INTRODUCTION

Reagent orientation is assumed to be important in chemical reaction. As part of a general study of the effects of orientation on reaction, we recently studied electron transfer collisions between alkali metal atoms Na and K and nitromethane, CH3NO2, molecules oriented before the collision.1,2 Transfer of the electron produces a stable negative ion, as well as a transient negative ion that can autodetach the electron. In addition, chemical bonds can be broken to produce NO2− or O−. Despite the fact that the electron can be bound by the electrostatic potential of the very high dipole moment,3 3.46 D,4 the steric data showed no evidence for participation of the dipole bound state, instead showing evidence for the role of the ␲ⴱNO orbital. We have extended these studies to trichloronitromethane, CCl3NO2, also known as chloropicrin. The dipole moment is too low, 1.88 D,5 to support a dipole-bound negative ion, but as expected, the chlorine atoms open new ionic channels and close others. For example, the parent negative ion, CCl3NO2− is formed with an electron affinity of 1.89⫾ 0.6 eV, but the transient negative ion lives long enough for bonds to break and the electron is not autodetached as in nitromethane. Previous studies of CCl3NO2 reactivity seem to be limited to the crossed-beam experiments of Herm et al.,6 who studied reactions of alkaline earth M atoms with 共CH3兲2CHNO2 and CCl3NO2. The metal oxide, MO, was formed from 共CH3兲2CHNO2, whereas MCl 共or possibly MCl2兲 was formed from CCl3NO2 peaked sharply forward in the center-of-mass system and proceeding via a direct, electron transfer mechanism. CCl3NO2 is mainly used as an agricultural pesticide. Its vapor pressure is 24 torr at room temperature and it easily evaporates. It is a powerful irritant used in WWI as a chemical weapon, so the destruction of unexploded ordinance and a兲

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of the vapor emanating from treated soil is of current interest. Phosgene is the principle decomposition product as identified in early photolysis7 and pyrolysis8 investigations. More recent photolysis experiments, however, show that the primary decomposition step is the cleavage of the C–N bond to produce CCl3 and NO2 and that production of phosgene is likely a result of secondary reactions. This cleavage is also seen in the present experiments. II. EXPERIMENTAL

A beam of fast potassium atoms crosses a beam of oriented CCl3NO2 molecules at right angles inside a coincidence time-of-flight mass spectrometer 共TOFMS兲. Electron transfer collisions produce a positive and negative ion pair which are separated and detected in coincidence when the collision energy E can overcome the Coulomb attraction between the ions, i.e., if E ⱖ IE+ BDE-EA, where IE is the ionization energy of K, BDE is the bond dissociation energy of any bonds broken, and EA is the ionization energy of the negative ion. As in earlier experiments,9 atoms are accelerated by charge exchange inside a small oven filled with potassium vapor: atoms are surface-ionized on a hot rhenium filament, accelerated toward a grounded slit by a voltage V, and then allowed to drift field-free inside the oven. The beam emerging from the oven consists of ions 共deflected by charged plates兲, thermal atoms, and fast atoms, but only the fast atoms have enough energy to produce ion pairs resulting in signal. Helium passes through a cylinder containing CCl3NO2 at 22 ° C, where the vapor pressure is 22 torr. The gas mixture expands through a nozzle at total pressure of 200 torr, the molecular beam is collimated by a skimmer and directed along the axis of a 1.4 m hexapole electric field where lowfield seeking states are deflected toward the axis and focused when the rods are charged, typically to ⫾10 kV. For symmetric top molecules such as CH3Br, the focused beam con-

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FIG. 1. Quantum probability distribution P共cos ␪兲 vs cos ␪ for the HVON − HVOFF change in beam intensity for beams of CH3NO2 and CCl3NO2 transmitted through the hexapole field.

sists of molecules in M, K states such that MⴱK ⬍ 0 and if these molecules are adiabatically transported into a uniform electric field they are oriented in that field. A weak, randomly oriented beam is transmitted when no voltage is applied. Even though CCl3NO2 is an asymmetric top, the barrier to internal rotation of the NO2 group with respect to the CCl3 group is very low5 共25.6 cm−1兲 and the molecule displays first-order Stark effects with a dipole moment of 1.88 D. We expect that the –NO2 group spins freely about the CCl3 making the CCl3NO2 molecule effectively a symmetric top as in nitromethane,1,2,10 CH3NO2, where the barrier to internal rotation is even smaller11 共2.11 cm−1兲. Molecules with significantly larger barriers to internal rotation, such as acetic acid CH3CO2H, where the barrier12 is 170 cm−1, still appear13 to be roughly symmetric due to the internal rotation, so we believe that it is reasonable to conclude that CCl3NO2 should behave as a symmetric top. Assuming the rotation to be truly free and averaging the moments of inertia about the B and C axis allows us to calculate the orientation probability distribution for CCl3NO2 and CH3NO2 using methods described previously.14 The calculated distributions in Fig. 1 show that CCl3NO2 with 具cos ␪典 ⬇ .73 is somewhat better oriented than CH3NO2 for which 具cos ␪典 ⬇ .60, apparently because the heavy Cl’s stabilize the rotation about the direction of the dipole moment. These molecules travel adiabatically from the hexapole field and intersect the atomic beam in a uniform field ⬇300 V/cm defined by two oppositely charged 共but otherwise identical兲 Wiley–McLaren TOFMSs.9 The accelerating field of the TOFMSs lies in the plane of the crossed beams roughly along the relative velocity. The state-selected molecules are in low-field seeking states, and in this uniform field the negative end of the molecule points toward the negatively charged TOFMS. Reversing the polarity of the TOFMSs reverses E and the direction of orientation. All voltages are dc and the beams are continuous so there is no time zero, but each electron transfer event simultaneously produces an ion pair. The positive ion signal starts a time to digital converter 共TDC兲 and the negative ion signal 共delayed 4 ␮s to allow detection of electrons兲 stops the TDC, giving the difference in flight times between the posi-

FIG. 2. Coincidence TOF mass spectra for two energies for negative attack orientation, showing relative importance of the parent ion at the lower energy.

tive and negative ions. The mass spectrometers are identical, and the positive ion is assumed to be K+, so the time difference is a signature of the mass of the negative ion. We acquire coincidence TOF spectra for each laboratory energy for positive or negative end attack with the hexapole field on and off for each orientation. If the hexapole field is off a randomly oriented beam is transmitted, and its signal is used to eliminate any differences in collection or detection efficiency arising from different TOFMS polarities.14 The experimental conditions are computer-controlled in random sequence. III. RESULTS A. Ions

Electron transfer from the accelerated K atoms to CCl3NO2 molecules produces K+ and negative ions at m/e corresponding to e−, O−, O2−, Cl−, NO2−, CCl3−, CCl2NO2−, CCl3NO−, and the parent negative ion, CCl3NO2−. With the exception of the electron, the intensity increases for all of these ions when high voltage is applied to the hexapole field, showing that they arise from the CCl3NO2 monomer. We believe that O2− is probably formed from trace amounts of oxygen in the beam or background rather than by breaking two N–O bonds. It probably appears to focus because the mass peaks are not completely separated from those of the nearby very strong Cl− peak. Each ion signal depends strongly on energy and representative mass spectra for negative attack orientation are shown in Fig. 2. IV. THRESHOLDS AND CALIBRATION

Fast K atoms are produced by accelerating K+ ions through a voltage VA and then letting the fast ions exchange charge with neutral K atoms as the ions drift through a fieldfree region containing potassium vapor. The nominal laboratory energy of the fast neutral atoms is thus VA but the actual laboratory energy may differ from the accelerating voltage, VA, because of voltage drops, the charge-exchange process, and contact potentials. It is thus necessary to calibrate the energy using thresholds of ions with known thermochemistry as described below.


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Electron transfer to oriented CCl3NO2

TABLE I. Threshold accelerating voltages and center of mass thresholds 共eV兲.

FIG. 3. Threshold energies 共in LAB system兲 calculated from thermodynamic data plotted vs accelerating voltage thresholds.

Nominal 共uncorrected兲 experimental thresholds, VT, for all ions are determined by fitting plots of signals, S 共HV on and HV off for both positive and negative attack orientations兲 versus the accelerating voltage, VA with the empirical second-order relation S=a

共VA ⱕ VT兲,

S = a + b共VA − VT兲 + c共VA − VT兲2

共VA ⬎ VT兲.

Examples of plots of signal versus VA are shown in Figs. 5 and 6. The K atom LAB threshold energy for the standards is EK = Ethresh共M/mG兲 − EG共mK/mG兲,

共1兲

where Ethresh is the center of mass 共CM兲 threshold energy given by Ethresh ⱖ IE + BDE − EA,

共2兲

where IE is the ionization energy of K, BDE is the bond dissociation of any bonds broken, and EA is the electron affinity of the fragment receiving the electron. 共The inequality applies if the fragments are internally excited.兲 M is the total mass of the colliding pair, mK and mG the mass of K and gas, respectively, and EG the energy of the gas beam calculated assuming a complete isenthalpic expansion in helium. This theoretical K laboratory energy is plotted versus the accelerating voltage thresholds, VT, as shown in Fig. 3 giving a corrected K laboratory energy. The present signals arise from species for which IEs or BDEs are not well known, so the calibration uses thresholds for various ions from acetic acid13 and trifluoroacetic acid,15 which were determined with the identical electrode geometry in the K oven used here. We use points from the present experiments for e− and Cl−, where BDE for C–Cl is approximated as 3.16 eV, the mean of that for CCl3CCl3, 3.14 eV, and CCl3CF3, 3.19. The values for O2− and C4H9 + CN− are from our preliminary data on t-C4H9CN. Using the laboratory reference frame allows us to use more than one reagent for calibration.

Ion

VT

␴V

Ethresh

␴E

共e−兲 O− 共O2−兲 Cl− NO2− CCl3− CCl2NO2− CCl3NO− CCl3NO2−

7.12 5.44 6.68 5.77 5.55 6.63 7.60 4.16 4.13

0.3 0.06 0.18 0.11 0.05 0.08 0.07 0.02 0.03

4.51 3.35 4.21 3.58 3.43 4.17 4.84 2.46 2.45

0.66 0.61 0.66 0.62 0.61 0.64 0.67 0.58 0.58

The calibration shown in Fig. 3 enables us to determine EK the calibrated LAB threshold corresponding to each accelerating voltage threshold, VT. The CM energy threshold Ethresh is then calculated from the corrected K laboratory energy by rearranging Eq. 共1兲 to yield Ethresh = EK共mG / M兲 + EG共mK / M兲. Accelerating voltage thresholds and center-of-mass threshold energies are shown in Table I. The ions listed in Table I arise from the CCl3NO2 molecule, except for the electron and O2−. The electron signal does not change with focusing voltage indicating that it does not depend on CCl3NO2. It probably arises from a collision of the fast atoms with a surface. Although the O2− signal does depend on the focusing voltage we believe it does not arise from CCl3NO2 for the following reasons: 共a兲 O2− is seen in many experiments where it clearly arises from impurity O2, 共b兲 it was not observed in CH3NO2, and 共c兲 the O2− mass peak is not completely resolved from the nearby tenfold bigger Cl− signal which does focus. 共The O2− calibration point in Fig. 3 is from experiments with C4H9CN, where the O2 is clearly an impurity.兲 A. Bond dissociation energies and electron affinities

Except for the parent ion, CCl3NO2−, the rest of the ions listed in Table I arise by breaking a C–Cl bond, a C–N bond,

FIG. 4. Ion fractions for various ions vs ECM. At energies much above threshold, the parent negative ion, CCl3NO2−, becomes unstable and fragments, producing mainly Cl− and a small proportion of 关P – O兴−. Other ions are produced in much smaller proportions.


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TABLE II. Bond dissociation energies and electron affinities 共eV兲 determined from K + CCl3NO2. Products

ECM

BDE

CCl3 + NO2− CCl3− + NO2 Cl− + CCl2NO2 Cl+ CCl2NO2− CCl3NO2− O + CCl3NO− O− + CCl3NO

3.43 4.17 3.58 4.84 2.45 2.46 3.35

1.36 2.00 2.85

EA

2.35 1.89 2.35 0.47

or an N–O bond with the electron residing on one or the other of the complementary pairs. The fraction of various ions is shown in Fig. 4 where it is clear that at CM energies ⬇2.5 eV the important ions are the parent negative ion, CCl3NO2− and 关P – O兴−, where 关P – O兴− denotes the parent ion minus a neutral O. These ion fractions decrease following the opening of the channel to Cl− at 3.6 eV whereupon Cl− becomes the major ion. The various reactions arising from electron transfer to trichloronitromethane allow us to determine bond dissociation energies for the molecule, as well as electron affinities for several fragments. These data are collected in Table II, where Ethresh, the threshold energy from Table I is Ethresh ⱖ IEK + BDE-EA. The ionization energy of K and known electron affinities from the NIST webbook are shown in Table III. Our estimate of the uncertainties for the BDEs and EAs in Table II is ⫾0.6 eV due almost entirely to the uncertainty in calibration of Fig. 3. The electron affinities of the various chlorinated radicals are high, but not surprising in view of the many electronwithdrawing groups. Our estimates for the dissociation energies of the C–N and N–O bonds, 1.7⫾ 0.4 and 0.5⫾ 0.6 eV are remarkably low, but these bonds appear to be weaker than corresponding bonds in nitromethane. We shall first discuss the C–N bond. Considerable uncertainty in threshold energies arises from the calibration procedure, but the raw experimental data suggest that the NO2− threshold is lower in CCl3NO2. In Fig. 5 we plot the NO2− and e− signal rate versus the accelerating voltage, VA, for both molecules. The CH3NO2 data were taken with different accelerator geometries and the energy scales for the two molecules are not to be compared, but the e− and NO2− signals for a given molecule are simultaneously acquired and can be compared to each other. It is clear that the NO2− threshold for CCl3NO2 is lower than that for the electron, whereas in nitromethane 共open symbols兲 the NO2− threshold is higher than that for the electron. The raw data

FIG. 5. Electron and NO2− signal rates plotted vs accelerating voltage. Data for CCl3NO2 共closed symbols兲 and nitromethane 共open symbols兲 are taken using different electrode geometries and are not comparable. The electron and NO2− signals for a single molecule are acquired simultaneously and can be compared. The NO2− threshold is lower than that for the electron for CCl3NO2 and vice versa for CH3NO2. The NO2− signals from CCl3NO2 are about six times smaller than from nitromethane.

thus demonstrate that the C–N bond dissociation energy is lower in CCl3NO2 than in nitromethane and this conclusion is independent of any calibration errors. The C–N bond dissociation energy is 2.70 eV in CH3NO2 as calculated from heats of formation in the NIST webbook.4 Comparable data appears to be unavailable for CCl3NO2, although B3LYP calculations16 with a 6-31G共d兲 basis give a CCl3 – NO2 bond dissociation energy of 1.35 eV 共and 2.37 eV for nitromethane兲. Thus the raw data and calculation show that the C–N bond in CCl3NO2 is weaker than that in nitromethane, and our experimental assessment 1.7⫾ 0.4 eV is in fair agreement with the theoretical prediction of 1.35 eV. 共Recent electron diffraction experiments report an unusually long C–N bond distance, 1.59 Å in CCl3NO2.17兲 Our raw data also show that the N–O bond is weaker in CCl3NO2 than in nitromethane. Figure 4 shows that ion fractions for the parent ion and 关P – O兴− maximize at nearly the same low energy and consequently EA-BDE must be large, 1.88 eV from values in Tables I and III. The analogous ion, CH3NO− is not observed in the mass spectrum of ni-

TABLE III. Ionization energies and electron affinities.

Species K Cl O

IE or EA 共eV兲

Species

EA 共eV兲

4.34 3.61 1.46

CCl3 NO2 O2

2.17 2.27 0.45

FIG. 6. Electron and O− signal rates plotted vs accelerating voltage similar to Fig. 5. The O− threshold is lower than that for the electron for CCl3NO2 and vice versa for CH3NO2.


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FIG. 7. Steric asymmetry factor for Cl− ion and the parent ion CCl3NO2−. Lines are linear least-square fits to the data.

tromethane because it is not resolved from NO2−, but the complementary ion, O−, is observed for both species. The signal rates for O− compared with that for the electron are shown plotted versus accelerating voltage in Fig. 6. The threshold for O− in nitromethane is above the electron threshold, whereas the O− threshold in CCl3NO2 is below the electron threshold, providing evidence that the N–O bond in CCl3NO2 is weaker than that in nitromethane. The N–O bond strength in nitromethane is apparently not well known. Simple N–O bond rupture would yield kinetically stable nitrosomethane, CH3NO, calculated to be about 50 kJ/mole unstable18 with respect to formaldoxime, H2C = N – OH. Reliable thermodynamic data for these compounds are not available, but the nitrosomethaneformaldoxime isomerization suggests that a similar isomerization might occur in CCl3NO and thus our value of 0.47⫾ 0.6 eV in Table II might represent the energy required for N–O bond dissociation followed by a bond rearrangement. V. STERIC ASYMMETRY

The steric asymmetry factor G = 共␴− − ␴+兲 / 共␴− + ␴+兲, where ␴⫾ is the cross section for attack at the positive 共nega-

FIG. 8. Steric asymmetry factors for the complementary ions NO2− and CCl3−. Lines are linear least-square fits to the data.

J. Chem. Phys. 132, 044307 共2010兲

FIG. 9. Steric asymmetry factor for the complementary ions Cl− and CCl2NO2−. Line for Cl− is a linear least-square fit to the data and that for CCl2NO2− is a smooth curve to guide the eyes.

tive兲 end of the molecule. This is calculated from the signal rates as previously described.14 Steric asymmetry factors for the Cl− ion and parent negative ion, CCl3NO2−, are shown in Fig. 7. The steric asymmetry for Cl− seems to be zero and formation of Cl− is likely for attack at either end of the molecule. The steric asymmetry for formation of the parent negative ion 共PNI兲 is small but not zero, and reaction is slightly favored for approach at the positive 共CCl3兲 end. Figure 8 shows the steric asymmetry factors for the complementary ions CCl3− and NO2− formed upon rupture of the C–N bond. The asymmetry is small, but identical, for the two ions showing that they arise from the same precursor. The steric asymmetry for the complementary pair, Cl−, CCl2NO2−, formed on rupturing the C–Cl bond is shown in Fig. 9. These asymmetries are not identical. Rupture of the N–O bond gives the complementary pair CCl3NO− and O− but the O− signal is very weak 共1% of the total ion signal兲 and the steric asymmetry of O− is wildly scattered. So in Fig. 10 we compare the steric asymmetry of the CCl3NO− ion and the parent CCl3NO2− ion. Positive end attack favors the fragment resulting from N–O bond rupture, CCl3NO−.

FIG. 10. Steric asymmetry factor for 关P – O兴− ion, CCl3NO−, and the parent ion CCl3NO2−. Lines are linear least-square fits to the data.


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FIG. 11. Steric asymmetry factors for formation of the parent negative ions, CCl3NO2− and CH3NO2−. The steric asymmetry for Br− from CF3Br is included for comparison.

VI. DISCUSSION

CCl3NO2 was chosen for study to complement and contrast the nitromethane studies. The dipole moments for nitromethane and CCl3H are 3.46 and 1.04 D, respectively. Vector addition of the dipoles gives 2.42 D, close to the experimental value of 1.88 D, showing that the NO2-end is the negative end for both molecules. Figure 11 shows that the steric asymmetry for the PNI is the same for CCl3NO2 and for nitromethane using either a Na or a K donor atom. The steric asymmetry is small and essentially constant indicating that the electron has been transferred into a ␲ⴱ orbital. 关The steric asymmetry for Br− from CF3Br 共Ref. 19兲 shows behavior characteristic of transfer into a ␴ⴱ orbital.兴 In nitromethane the ␲ⴱNO orbital is the lowest unoccupied molecular orbital 共LUMO兲 and the steric asymmetry suggests that it also is in CCl3NO2. The dipole moment of CCl3NO2 is too small to support a dipole bound state, and the formation of its PNI does not proceed through a dipole bound doorway state. Since the steric asymmetry for CCl3NO2 and nitromethane are the same it is apparent that formation of the PNI in nitromethane in our earlier charge-transfer experiments also does not proceed through a dipole bound doorway state, reinforcing the conclusions reached in the earlier papers.1,2 Adding the electron to the ␲ⴱNO LUMO produces a transient negative ion in both nitromethane and CCl3NO2, and the PNI is produced in both molecules because the donor atom is able to deactivate the transient negative ion.1,13,15,20 As the collision energy is increased the donor is less able to stabilize the PNI and other dissociation channels open but the further behavior of the two molecules is completely different. In nitromethane the transient negative ion detaches an electron as the energy is increased, but in CCl3NO2 the three strongly electron withdrawing Cl atoms favor the production of various Cl-containing ions and autodetachment is minimal or nonexistent. Figure 4 shows that the PNI, CCl3NO2−, is the dominant ion near threshold 共⬇3 eV兲 and that it starts to disappear as the channel for formation of Cl− is opened. In an amazingly low energy process, the PNI can lose an O atom to give

FIG. 12. Steric asymmetry factors for formation of NO2− from nitromethane and trichloronitromethane.

CCl3NO− with the same energy decay as the PNI. The steric asymmetry of the PNI and CCl3NO− are similar 共Fig. 10兲, with CCl3NO− favoring Cl-end attack, perhaps because decomposition of the PNI with the K+ near the NO2-end might favor KO salt formation, which would diminish the ion signal. The complementary O− ion was observed and was ⬇1% of the total signal, possibly accounting for the very small CaO product signals in Herm’s thermal energy reactions.6 Above 3.4 eV, Cl− is the dominant ion, whereas in nitromethane the dominant ion is NO2−. This is in accord with the thermal energy reactive scattering experiments which found Cs共Li兲NO2 from Cs 共Ref. 21兲 or 共Li兲共Ref. 22兲, +CH3NO2 and CaCl 共Ref. 6兲 from Ca+ CCl3NO2. As shown in Fig. 7, the steric asymmetry for production of Cl− from CCl3NO2 is even smaller than that for the PNI. We expect the electron to enter the ␲ⴱNO LUMO, and expect the incipient donor to perturb the symmetry of the transient negative ion allowing density to flow into the ␴ⴱCN orbital and break the C–N bond.13,20 Thus the steric asymmetry factors for production of CCl3− and NO2− should be 共and are兲 identical and about the same as for the PNI. The NO2− signal is about three times larger than the CCl3− signal, apparently a consequence of NO2 having a slightly larger electron affinity than CCl3. In nitromethane the rupture of the C–N bond produces only NO2−, but as shown in Fig. 12, the steric asymmetries for the two molecules are basically the same. Electron density might travel further along the molecule to populate the ␴ⴱCCl orbital, but that orbital probably lies close in energy to the LUMO and might be populated by direct electron transfer. There are three chlorines and the ␴ⴱCCl orbitals will appear almost spherical 关as in CCl3H 共Ref. 23兲兴, and since transfer to the ␲ⴱNO orbitals favors near sideways orientation, we expect that the steric asymmetry for this combination process to be nearly zero. VII. SUMMARY

Electrons are transferred from K atoms to oriented CCl3NO2 molecules and the steric asymmetry for CCl3NO2 is indistinguishable from that for CH3NO2 showing that the electron is transferred to the ␲ⴱNO orbital in both molecules. At CM energies near 2.5 eV, the incipient positive ion deac-


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tivates the transient negative ion producing the parent ion, CCl3NO2−, and the species lacking an O atom, CCl3NO−. As the collision energy is raised new fragment channels become available to O−, NO2−, Cl−, CCl3−, and CCl2NO2− in that order. Threshold measurements give electron affinities for CCl3NO2, CCl3NO, and CCl2NO2 and show that the C–N bond in CCl3NO2 is weaker than the corresponding bond in nitromethane. The O− threshold is unusually low suggesting either that the NO bond is unusually weak or that the perchloro analog of formaldoxime, Cl2 = NOCl, is strongly bound. ACKNOWLEDGMENTS

We gratefully acknowledge support of this research by the National Science Foundation 共Grant No. CHE 0411498兲. 1

J. Chem. Phys. 132, 044307 共2010兲


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