F. Fleming Crim

Controlling bimolecular reactions: Mode and bond selected reaction of water with hydrogen atoms

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖12〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction H+H2O→OH+H2. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Relative rate measurements for the three vibrational states ‖03〉−, ‖02〉−‖2〉, and ‖12〉−, which have similar total energies but correspond to very different distributions of vibrational energy, demonstrate the control that initially selected vibrations exert on reaction rates. The local mode stretching state ‖03〉− promotes the H+H2O reaction much more efficiently than either the state having part of its energy in bending excitation...

Bond-selected bimolecular chemistry : H + HOD(4νOH)→OD + H2

The reaction of HOD containing four quanta of O–H bond stretching vibration with H atoms produces OD fragments almost exclusively. Vibrational overtone excitation prepares HOD(4νOH) that reacts with H atoms formed in a microwave discharge. The endothermic reaction of water with hydrogen atoms does not occur for ground vibrational state water but proceeds at roughly the gas kinetic collision rate for the vibrationally excited molecule. The production of OD fragments from HOD(4νOH) in the reaction is at least two orders of magnitude more efficient than the production of OH, indicating very selective reaction of the vibrationally excited bond.

Selectively breaking the O--H bond in HOD

Vibrationally mediated photodissociation of HOD, in which one photon excites an O–H stretching vibration and another photon dissociates the vibrationally excited molecule, preferentially breaks the O–H bond for some photolysis wavelengths. Excitation of the third O–H stretching overtone (4νOH ) of HOD followed by photolysis with a 239.5 or 266 nm photon produces at least 15 times more OD than OH product, as determined by laser induced fluorescence detection of both species. Dissociation of HOD(4νOH ) with a 218.5 nm photon produces comparable amounts of OH and OD fragments. This large selectivity and strong dependence on the wavelength of the photolysis photon is consistent with qualitative models of vibrationally mediated photodissociation and with recent calculations.

Specific rate constants k(E,J) and product state distributions in simple bond fission reactions. II. Application to HOOH→OH+OH

Detailed and simplified statistical adiabatic channel calculations of specific rate constants k(E,J) and product quantum state distributions for the simple bond fission reaction HOOH→2 OH are compared with recent measurements of state‐resolved dissociation rates, product state distributions, and thermally averaged rate coefficients. A simple modification of phase space theory based on the statistical adiabatic channel model successfully predicts product state distributions and rate constants as well. Because of the amount of experimental data and theoretical analysis available, the dissociation of hydrogen peroxide is becoming a model case for simple unimolecular bond fission processes.

An experimental and theoretical study of the bond selected photodissociation of HOD

Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O–H bond in HOD molecules containing four quanta of O–H stretching excitation. Dissociation of HOD(4νOH) with 266 or 239.5‐nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5‐nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.Experimental and theoretical studies of the photodissociation of single vibrational states in HOD provide a qualitative and quantitative understanding of the dissociation dynamics and bond selectivity of this process. Vibrationally mediated photodissociation, in which one photon prepares a vibrational state that a second photon dissociates, can selectively cleave the O–H bond in HOD molecules containing four quanta of O–H stretching excitation. Dissociation of HOD(4νOH) with 266 or 239.5‐nm photons produces OD fragments in at least a 15 fold excess over OH, but photolysis of the same state with 218.5‐nm photons produces comparable amounts of OH and OD. Wave packet propagation calculations on an ab initio potential energy surface reproduce these observations quantitatively. They show that the origin of the selectivity and its energy dependence is the communication of the initial vibrational state with different portions of the outgoing continuum wave function for different photolysis energies.

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces

Experimental studies of the chemical reaction dynamics of vibrationally excited molecules reveal the ability of different vibrations to control the course of a reaction. This Perspective describes those studies for the prototypical reaction of vibrationally excited methane and its isotopologues in gases and on surfaces and looks to the prospects of similar studies in liquids. The influences of vibrational excitation on the CH bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities. Both reactions are bond-selective processes in which the initial preparation of a molecular eigenstate containing a large component of CH stretching results in preferential cleavage of that bond. It is possible to cleave either the CH bond or CD bond in the reaction of Cl with CH3D, CH2D2, or CHD3 and, similarly, to use initial excitation of the CH stretch to promote dissociation of CHD3 to CD3 and H on a Ni surface. Different vibrational modes, such as the symmetric and antisymmetric stretches in CH3D or CH4, lead to very different reactivities, and molecules with the symmetric stretching vibration excited can be as much as 10 times more reactive than ones with the antisymmetric stretch excited. The origin of this behavior lies in the change in the vibrational motion induced by the interaction with the atomic reaction partner or the surface.

Controlling bimolecular reactions: Mode and bond selected reaction of water with translationally excited chlorine atoms

Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction Cl+H2O→OH+HCl. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Comparison of relative OH product yield from Cl+H2O(‖04〉−) and the calibration reaction O(3P)+CH3OH shows that vibrational excitation of water to the ‖04〉− state leads to a near gas kinetic reaction rate (k(‖04〉−)=2×10−10 cm3 molecule−1 s−1). Relative rate measurements for the two vibrational states ‖03〉− and ‖02〉−‖2〉, which have similar total energies but correspond to very different distributions of vibrational excitation, demonstra...

State- and bond-selected unimolecular reactions

Unimolecular reactions are crucial chemical events that have been the focus of increasingly sophisticated investigation in the past decade. Unraveling their details is one fundamental goal of experimental and theoretical studies of chemical dynamics. New techniques are revealing the possibilities, and challenges, of eigenstate- and bondspecific unimolecular reactions. These experiments clearly demonstrate the intimate connection between intramolecular processes and unimolecular reaction dynamics and suggest means of exploiting molecular properties to study and control reactions at the level of individual quantum states.

Selectively breaking either bond in the bimolecular reaction of HOD with hydrogen atoms

We have determined the branching ratio for the reaction of hydrogen atoms and HOD with either the O–H bond excited or the O–D bond excited. In both cases, the initially excited bond reacts preferentially. Excitation of the third O–H stretching overtone, 4νOH, favors breaking the O–H bond by a factor of ∼200, and excitation of the fourth O–D stretching overtone, 5νOD, favors breaking the O–D bond by a factor of ∼220. Thus vibrational excitation can control the H+HOD reaction to produce either product almost exclusively. A simple model using the calculated wave function for each state and the measured reaction cross section for a particular vibrational excitation predicts the high selectivity observed for the two reactions.

Vibrational overtone spectroscopy of bound and predissociative states of hydrogen peroxide cooled in a supersonic expansion

The vibrational overtone excitation spectra of both bound and predissociative states of hydrogen peroxide molecules cooled in a supersonic expansion show features that are obscured otherwise. Spectra of predissociative states are measured by detecting the decomposition product following excitation of an overtone vibration. Spectra of bound states are obtained by a two‐photon excitation technique in which a second photon excites the molecule from its bound vibrational overtone state to a dissociative state. The features in the bound state (4νOH) spectrum are 0.08 to 0.13 cm−1 wide, reflecting small inhomogeneous broadening, but those to the predissociative state (6νOH) are 1.5±0.3 cm−1 wide. This width, which corresponds to a lifetime of about 3.5 ps, reflects coupling into the dissociative continuum.