Science
Atomic reactivity hinges on approach geometry Researchers show that the
pendence holds also for atomic reagents. The Stanford researchers, with support from the National Science Foundation and the Air Force Office of Scientific Research, have studied three reactions all leading to the unstable species, CaCl. The reactions are those of calcium atoms with hydrogen chloride, calcium atoms with molecular chlorine, and calcium atoms with carbon tetrachloride. Each of these reactions proceeds differently with different alignment of the calcium. In the experiment, a beam of calcium atoms in the ground state passes through a polarized laser tuned to excite one outer-shell s electron to the p orbital. Because the laser's energy is polarized, all the dumbbell-shaped p orbitals produced by this energy lie parallel in the direction of the laser's polarization. The beam of excited atoms collides with the target gas— hydrogen chloride, for example—to produce CaCl in one or more excited molecular orbital states. By measuring the chemiluminescence of the reactions, the researchers can determine which excited states of CaCl have formed. In practice, the polarized laser
reactivity of calcium atoms depends on the geometry of their approach to various molecular targets Using a plane-polarized laser to excite a beam of calcium atoms, chemists at Stanford University, Palo Alto, Calif., have demonstrated that the alignment of a reactant's electron orbitals affects the outcome of a reaction. The result isn't startling, says chemist Charles T. Rettner, who conducted the experiments with chemistry professor Richard N. Zare. Chemists have accepted for some time that reactivity depends on the geometry with which reactants approach each other; indeed, researchers often use a "steric factor" to rationalize observed reaction rates with the theoretical rates. However, direct experimental evidence for the phenomenon is difficult to obtain and, until now, has been limited to molecular species. Rettner and Zare's work is the first to show that this de-
Geometry of reactant collision affects reaction outcome
Ca CaCl
+ X
molecular orbital Perpendicular approach
Ca
4s /
Polarized laser light, X - 423 nm
Polarized light excites a calcium 4s electron to the 4p orbital, which is aligned with the polarization of light
18
C&ENJuly5, 1982
0gQ
4p i
+@
/ /
> Z T - CaCI I
Parallel approach
+ X
molecular orbital
When X =r= hydrogen (black arrows), perpendicular approach favors formation of CaCl with FT molecular orbital, but parallel approach favors formation of CaCl with a Imolecular orbital. When X = chlorine (green arrows), however, perpendicular approach is more reactive, yielding greater amounts of both forms of product than the parallel approach
beam is rotated at 10 Hz. As the experiment proceeds, the target molecules, which are oriented randomly, are struck by calcium atoms whose p orbitals cycle from parallel to perpendicular, relative to the axis between the two reactants. Thus, if the excited state of the product depends on the alignment of the p orbital, cyclic modulation in the chemiluminescence should be observed. That is exactly what happens, Rettner says. In the case of calcium reacting with hydrogen chloride, the total chemiluminescence is independent of calcium alignment. However, parallel approach favors formation of electronically excited CaCl in a 2 molecular orbital (no angular momentum projected along the axis connecting the two atoms); perpendicular approach favors formation of excited CaCl in a II molecular orbital (one unit of angular momentum projected along the internuclear axis). In the case of calcium reacting with molecular chlorine, total chemiluminescence depends on the calcium alignment—more total product is formed when the calcium p orbital approaches perpendicular to chlorine. The chemiluminescence of the excited 2 and II states shows that both form in greater amounts with perpendicular approach. In the reaction with carbon tetrachloride, there is no dependence on approach geometry. Rettner and Zare explain the results in terms of intersecting reaction potential energy surfaces and the electron jump "harpoon" mechanism. Reaction potential energy surfaces describe the energy relationships of the reactants in space. Where different surfaces cross, according to quantum mechanics, a reaction is likely to occur. In the reactions studied, Rettner says, a harpoon mechanism applies. At the crossing point of the potential energy surfaces for the neutral species (reactants) and the ionic species (products), an electron jumps from calcium to the other reactant, effectively harpooning it. Such a description is very dependent on the symmetry of the system of reactants, and Rettner and Zare explain their results in terms of differing symmetries of the three reaction systems studied. Rudy Baum, San Francisco
