Crossed-beam studies of electron transfer to oriented trichloronitromethane, CCl(3)NO(2), molecules.

Fast potassium atoms donate an electron to CCl(3)NO(2) molecules to form K(+) ions and the negative ions O(-), Cl(-), NO(2) (-), CCl(3) (-), CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-). Threshold energies are measured for these ions and electron affinities for CCl(2)NO(2) (-), CCl(3)NO(-), and CCl(3)NO(2) (-) are estimated to be 2.35, 2.35, and 1.89 eV (+/-0.6 eV), respectively. The threshold energies show that the C-N and N-O bonds are substantially weaker than in nitromethane. The CCl(3)NO(2) molecules are oriented before the collision and at energies near 2.5 eV the electron appears to transfer to the pi( *) (NO) orbital forming the parent negative ion, CCl(3)NO(2) (-), 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.

Electron transfer collisions with oriented trifluoroacetic acid (CF3CO2H).

Electron transfer collisions between neutral K atoms and neutral, oriented trifluoroacetic acid molecules, CF(3)CO(2)H, are studied in crossed molecular beams at center of mass energies from 6 to 18 eV. An electron transfer produces a pair of ions with enough energy to escape the Coulomb attraction, and the ions are detected in separate time-of-flight mass spectrometers. The principle ions formed are K(+) and the trifluoroacetate ion, CF(3)CO(2)(-) ion, and this channel is favored for attack at the positive (-CO(2)H) end of the molecule. The steric asymmetry suggests that the electron is transferred into the pi*(CO) orbital. The nascent K(+) perturbs the molecular symmetry, allowing electron migration to the sigma*(OH) orbital to break the O-H bond and form CF(3)CO(2)(-).

Experimental evidence for the role of the pi(CO)* orbital in electron transfer to gas phase acetic acid CH3CO2H: effects of molecular orientation.

Electron transfer from K atoms to oriented acetic acid molecules produces acetate ions (and K(+)) when the CO(2)H end of the molecule is attacked. The electron enters the pi(CO)(*) orbital and the donor atom distorts the molecule to allow migration to the sigma(OH)(*) orbital, thereby breaking the bond.

Steric effects in electron transfer from potassium to pi-bonded oriented molecules CH3CN, CH3NC, and CCl3CN.

Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.

Steric asymmetry in electron transfer from potassium atoms to oriented nitromethane (CH3NO2) molecules.

Electrons are transferred in collisions between potassium atoms and CH(3)NO(2) molecules that have been oriented in space prior to collision. The electron transfer produces K(+) ions, parent negative ions CH(3)NO(2)(-), and the fragment ions e(-), NO(2)(-), and O(-) in amounts that depend on the energy. The positive and negative ions are detected in coincidence by separate time-of-flight mass spectrometers at various collision energies for both CH(3)-end attack and NO(2)-end attack. The steric asymmetry for electrons and CH(3)NO(2)(-) is essentially zero, but the steric asymmetry for NO(2)(-) shows that NO(2)(-) is formed mainly in CH(3)-end collisions. There is evidence that the electrons and NO(2)(-) have the same transient precursor, despite having different steric asymmetries. It appears likely that the precursor is formed by electron transfer mainly in collisions normal to the molecular axis leading to near zero steric asymmetry for the electron. This transient precursor can also eject an NO(2)(-) ion, which is more likely to be removed as KNO(2) salt when K(+) ions are near the NO(2) end of the molecule, with the result that CH(3)-end collisions seem to produce more NO(2)(-).

Electron transfer from sodium to oriented nitromethane, CH3NO2: probing the spatial extent of unoccupied orbitals.

Beams of sodium atoms with energies of a few eV are crossed with a beam of oriented CH3NO2 molecules to study the effect of collision energy and orientation on electron transfer. The electron transfer produces Na+ ions and free electrons, parent negative ions (CH)NO2-), and fragmentation ions NO2- and O- in proportions that depend on the collision energy. The steric asymmetry is very small or zero and suggests that production of all of the ions is favored by sideways attack with respect to the permanent dipole along the C-N axis. In these experiments, the electron appears to be transferred into the 2B1 state of the anion comprising mainly the pi*NO LUMO, producing a valence-bound state rather than a dipole-bound state.

Threshold behavior in electron-transfer collisions between rubidium atoms and C2F5Cl or C2F5I molecules.

Rubidium atoms are accelerated in a high-temperature expansion of hydrogen to produce beams with energies high enough to observe collisional ionization with a cross beam. The speed of the atoms is directly measured by time-of-flight techniques, and the positive and negative ions produced are detected in separate mass spectrometers and detected in coincidence. Chloroperfluoroethane produces C(2)F(5)(-) and Cl(-) ions, whereas iodoperfluoroethane produces I(-), C(2)F(5)(-), and C(2)F(5)I(-) ions. When the measured speed distributions are used, the signal versus energy may be deconvolved to yield thresholds and electron affinities (EAs). The EA for C(2)F(5)I is measured to be 0.96 +/- 0.1 eV. Anomalously high EA values result for C(2)F(5) apparently because C(2)F(5)(-) is produced by parts per million concentrations of Rb(2).

Dramatic steric behavior in electron transfer from various donors to CF3Br.

Different alkali metal atoms are observed to donate electrons to CF(3)Br molecules that are oriented in space. For collision energies high enough to overcome the Coulomb attraction, a positive ion/negative ion pair is observed and mass-analyzed using coincident time-of-flight mass spectroscopy. The alkali metal cation and various negative ions are observed. The most abundant negative ion is the bromide ion, Br(-), formed preferentially by attack at the Br end of the molecule. The steric asymmetry to produce Br(-) is almost identical for all of the alkali metal donors. Fluoride ions are formed in smaller abundance and reflect completely different behavior among the donors. Sodium and potassium have high thresholds and prefer the CF(3) end of the molecule, whereas cesium prefers the Br end of the molecule. Sodium and potassium apparently interact with the transient CF(3)Br(-) molecular negative ion while it is in the process of decomposing.

Evidence for orbital-specific electron transfer to oriented haloform molecules.

Beams of hyperthermal K atoms cross beams of the oriented haloforms CF(3)H, CCl(3)H, and CBr(3)H, and transfer of an electron mainly produces K(+) and the X(-) halide ion which are detected in coincidence. As expected, the steric asymmetry of CCl(3)H and CBr(3)H is very small and the halogen end is more reactive. However, even though there are three potentially reactive centers on each molecule, the F(-) ion yield in CF(3)H is strongly dependent on orientation. At energies close to the threshold for ion-pair formation ( approximately 5.5 eV), H-end attack is more reactive to form F(-). As the energy is increased, the more productive end switches, and F-end attack dominates the reactivity. In CF(3)H near threshold the electron is apparently transferred to the sigma(CH) antibonding orbital, and small signals are observed from electrons and CF(3)(-) ions, indicating "activation" of this orbital. In CCl(3)H and CBr(3)H the steric asymmetry is very small, and signals from free electrons and CX(3)(-) ions are barely detectable, indicating that the sigma(CH) antibonding orbital is not activated. The electron is apparently transferred to the sigma(CX) orbital which is believed to be the LUMO. At very low energies the proximity of the incipient ions probably determines whether salt molecules or ions are formed.