Philip R. Brooks

Reactions of oriented molecules.

Beams of oriented molecules have been used to directly study geometrical requirements in chemical reactions. These studies have shown that reactivity is much greater in some orientations than others and demonstrated the existence of steric effects. For some reactions portions of the orientation results are in good accord with traditional views of steric hindrance, but for others it is clear that our chemical intuition needs recalibrating. Indeed, the information gained from simultaneously orienting the reactants and observing the scattering angle of the products may lead to new insights about the detailed mechanism of certain reactions. Further work must be done to extend the scope and detail of the studies described here. More detailed information is needed on the CH3I reaction and the CF3I reaction. The effects of alkyl groups of various sizes and alkali metals of various sizes are of interest. In addition, reactions where a long-lived complex is formed should be studied to see if orientation is important. Finally, it would be of interest to apply the technique to the sort of reactions that led to our interest in the first place: the SN2 displacements in alkyl halides where the fascinating Walden inversion occurs.

Reactive Scattering of K Atoms from Oriented CH3I Molecules

Potassium atom reactive scattering from oriented methyl iodine molecules, determining variation of chemical reactivity over molecular surface

Translational excitation of the molecular beam reaction K+HCl→KCl+H

The translational energy dependence of the reaction K+HCl→KCl+H has been studied in crossed beams in the c.m. energy range 2.1–12.1 kcal/mole (0.09–0.53 eV). Beams of HCl were generated at different translational energies by hydrodynamic expansion of various H2/HCl mixtures and the laboratory energy of the HCl was measured by time‐of‐flight techniques. Angular distributions of the product were measured and integrated for each energy and the total reactive cross section was found to increase monotonically with energy. The functional form of the energy dependence is well approximated by the traditional line‐of‐centers relation. Although the cross section increases with energy, translational excitation is not as effective as vibrational excitation and modest extrapolation of the data suggests that still larger amounts of translational energy will be similarly ineffective. The cross section might be expected to depend upon the number of states which are accessible to the products. But this number grows rapidl...

Orienting Polar Molecules in Molecular Beams. Symmetric Tops

Symmetric‐top molecules exist in various orientations in even very weak electric fields, in contrast to diatomic molecules which are oriented only in fields so large as to be impractical. For symmetric tops these orientations can be separated in an inhomogeneous electric field, and calculations are presented for the separating properties of a hexapole field. The transmission and the final distributions of velocity and orientations have been calculated for several different molecules over a range of conditions. Experimental transmission measurements were made and are in good agreement with calculations. By fitting experimental points to calculated transmission curves dipole moments in good agreement with literature values have been determined. The molecules separated by the hexapole field are experimentally shown to make adiabatic transitions into a homogeneous electric field, providing evidence that the molecules can be oriented in the laboratory reference frame.

Steric hindrance in potassium atom-oriented molecule reactions. Methyl iodide and tert-butyl iodide

The reaction of K atoms with oriented CHJ or t-CqH9I molecules has been studied via the crossed molecular beam method. Oriented molecules are produced by passing a molecular beam through an inhomogeneous electric field which rejects unwanted orientations. The remaining molecules are oriented with respect to a weak electric field and can be reversed in the laboratory by changing the direction of the applied field, The reaction is studied for impact a t the two ends of the molecule and for both reactions the iodine end is most reactive. A simple model is used to interpret the results and suggests that the hindering size which can be ascribed to the R groups is only roughly compatible with van der Waals radii. Very few gas phase bimolecular chemical reactions proceed on every gas kinetic-collision. Most reactions have an activation energy which (presumably) restricts reaction to those collisions with energy greater than the activation energy, E,. But counting only those collisions with energy greater than E , still gives a rate faster than the rate of almost any chemical reaction. To account for this discrepancy between theory and fact the notion was advanced that only certain orientations of the reagents were effective in promoting reaction, and the “steric factor”, p , was introduced as the fraction of gas-kinetic collisions which had the right orientation to react. Journal of the American Chemical Society / 97:7 / April 2, 1975

Photoexcitation of reaction complexes in the reaction K+NaCl→KCl+Na

Emission at the Na D lines has been observed from the intersection of crossed molecular beams of K and NaCl irradiated by a cw dye laser at wavelengths from 590 and 735 nm. The three‐beam signal exhibits a threshold near 735 nm and is linearly dependent on the laser and molecular beam intensities. This three‐beam signal has been observed under experimental conditions in which all two‐beam signals are accounted for, and is attributed to the formation of Na* by photoexcitation of the KNaCl reaction complex. The effective two‐body cross section for the process is approximately 10−21 cm2 in a laser field of 1.5 kW/cm2.

Molecular beam reaction of K atoms with sideways oriented CF3I

Abstract Reactive scattering is observed for K + CF3I → KI+CF3 where the CF3I is oriented “sideways” with the I end pointing towards or away from the detector. Angular distributions are extremely broad and completely different from “heads” and “tails” orientations. Maximum signal corresponds to the “towards” orientation although reactivity of the two orientations is equivalent.

Focusing and Orienting Asymmetric‐Top Molecules in Molecular Beams

Asymmetric‐top molecules can be deflected in an inhomogeneous electric field, and the deflected molecules can be oriented by allowing them to pass adiabatically into a uniform field. It is possible to account quantitatively for the transmission in a hexapole field if only first order and intermediate (or fast) Stark effects are considered. Rigid rotors with the dipole moment parallel to the a axis and internal rotors with low barriers to rotation have many levels which so interact and these types of molecules are easily deflected. Rigid rotors with the dipole parallel to the b axis have far fewer levels which interact; for these molecules the deflection is weak and may be undetectable. For rigid rotors and free internal rotors it is possible to obtain dipole moments by matching theoretical curves with experimental points.

Electron transfer to oriented molecules : K + CF3I and K + CH3I

K+ ions have been detected from the intersection of a beam of K atoms (5–30 eV) with beams of CH3I and CF3I molecules which had been oriented prior to the collision. Collisional ionization is found to be favored for both molecules when the fast K is incident at the I end of the molecule, even though the electrical polarity of the I end is different for the two molecules. For both molecules, the effect of molecular orientation is most pronounced near threshold (≊5 eV) and almost disappears at higher (30 eV) energies. For CF3I, the threshold for impact at the I end is ≊0.7 eV less than the threshold for impact at the CF3 end. We interpret these results using a ‘‘harpoon’’ mechanism in which the electron jump during the initial approach is probably independent of orientation, but as the charged particles separate, the electron may jump back to the K+. For impact at the I or ‘‘head’’ end, the I− is ejected backwards towards the incoming K+. This increases the final relative velocity of the ions and lowers the...

Experimental evidence for the role of the πCO∗ 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.