Page 148 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN003H-565 June 13, 2001 20:37

210 Coherent Control of Chemical Reactions

HOD. A 722.5 nm laser pulse (λ 1 ) excites the third by the umbrella vibration of the molecule. The d z , d xz ,
2
overtone stretch of OH. After a short delay, a pulse of and d yz orbitals have lobes perpendicular to the plane of
ultraviolet radiation of frequency ν 2 (wavelength λ 2 ) dis- the C Rydberg state of NH 3 . Excitation of the out-of-
˜
sociates the molecule, and a third pulse with a wavelength plane umbrella mode of the molecule promotes vibrational
near 308 nm (λ 3 ) probes the OH or OD fragments by autoionization of electrons in these orbitals but has little
laser-induced fluorescence. It is observed that with a dis- effect on the poorly overlapping d x −y and d xy electrons.
2
2
sociation wavelength of 266 or 239.5 nm, the products are It is also possible to use localized electronic excitation
almost exclusively H + OD, to promote reactions selectively. An example studied by
Laurie Butler and coworkers is the ultraviolet photodisso-
HOD(4ν OH ) + hν 2 → H + OD, (2)
ciation of CH 2 IBr. This molecule has absorption maxima
as seen in the Q 1 (4,4 ) and R 2 (4) fluorescence lines of at 270, 215, and 190 nm, corresponding to localized ex-

OD, whereas with 218.5 nm equal amounts of OH and citation of a nonbonding iodine electron to an antibond-
∗
OD are formed. Because a stationary state of the molecule ing orbital localized on the C–I bond, (n I → σ C−I ), pro-
∗
is excited by the first laser, the bond remains energized motion of a nonbonding bromine electron (n Br → σ C−Br ),
indefinitely until it collides with another particle. The and a Rydberg transition, respectively. Photodissociation
∗
excited molecule can then react, breaking preferentially at 248.5 nm, at the edge of the n I → σ C−I transition, yields
the activated bond. For example, collision of HOD(4ν OH ) 60% I atoms and 40% Br. At 210 nm only Br atoms are
with a chlorine atom produces primarily HCl rather than formed, even though the C–I bond is the weakest bond in
DCl: the molecule. In addition, some concerted IBr elimination
occurs. At 193 nm all three products are formed.
HOD(4ν OH ) + Cl → HCl + OD. (3)
Another example of electronic control studied by
The same principles have been applied to molecules Butler is the photodissociation of methyl mercaptan,
CH 3 SH. Although the CH 3 –SH bond is the weakest bond
with four atoms. For example, Fleming Crim and cowork-
ers showed that excitation of the NH stretch of isocyanic in the molecule, CH 3 S + H are the primary photodisso-
acid enhances its reaction with Cl atoms, ciation products at 193 nm. Bond selectivity in this case
occurs even though the initially excited state is not re-
HNCO(3ν 1 ) + Cl → HCl + NCO, (4) pulsive along the reaction coordinate. Here selectivity
results from non-adiabatic coupling of the initially ex-
whereas excitation of the bending mode inhibits the re-
1
cited metastable 2 A Rydberg state to the dissociative

action. For the reactions of ammonia ions with neutral
n I → σ ∗ state. Another case where nonadiabatic cou-
ammonia molecules, S−H
pling results in bond-selective chemistry is the photodis-
+
NH + ND 3 sociation of bromoacetyl chloride, BrCH 2 COCl. It was
3
found for this molecule that at 248 nm the C–Cl bond is
+
→ NH 3 D + ND 2 (hydrogen extraction) (5a)
preferentially broken, even though the barrier for C–Br
→ NH 3 + ND + (charge transfer) (5b) scission is lower than that for C–Cl scission. The reason
3
for bond selectivity in this case is that the splitting between
+
→ NH 2 + ND 3 H , (proton transfer) (5c)
the adiabatic potential energy surfaces is much smaller for
Richard Zare and coworkers found that excitation of the C–Br scission, so that nonadiabtic crossing and recrossing
+
umbrella mode of NH selectively enhances the proton withoutreactionismuchfasterinthischannelascompared
3
transfer reaction; however, in this case the projection of with adiabatic motion along the C–Cl reaction coordinate.
the nuclear motion onto the reaction coordinate is not as These examples illustrate that, although bond-selective
obvious. photoexcitation is a general phenomenon, its mechanism
Vibrational mode selectivity can also be used to pro- depends strongly on the details of the potential energy sur-
mote electronic processes. Vibrational autoionization is a faces. Studies of mode-selective reactions are, therefore,
process whereby a bound electron acquires sufficient en- a valuable source of information about the structure of
ergy to escape by extracting one quantum of vibrational potential energy surfaces and their interactions.
energy from the ionic core of the molecule. For such an en-
ergy transfer to occur, the electron must first collide with
the core. Scattering of the electron with the core can be III. COHERENT PHASE CONTROL
promoted if the amplitude of the nuclear motion overlaps
the electronic charge density. An example of this process The underlying principle of coherent phase control is
studied by Steven Pratt is vibrational autoionization of that the probability of an event occurring is given by the
the 3d Rydberg electrons of ammonia, which is enhanced square of the sum of the quantum mechanical amplitudes