Bond selective dissociation of methane (CHD) on the steps and terraces of Pt(211).

The dissociative chemisorption of singly deuterated methane (CHD) has been studied on the steps and terraces of a Pt(211) surface by quantum state resolved molecular beam methods. At incident translational energy (E) below 50 kJ/mol, CHD dissociates only on the more reactive steps of Pt(211), where both C-H and C-D cleavage products CHD(ads) and CH(ads) can be detected by reflection absorption infrared spectroscopy. Vibrational excitation of a slow beam of CHD (E = 10 kJ/mol), prepared with one quantum of antisymmetric C-H stretch excitation by infrared laser pumping, allows for fully bond- and site-selective dissociation forming exclusively CHD(ads) on the step sites.

First Evidence of Vibrationally Driven Bimolecular Reactions in Solution: Reactions of Br Atoms with Dimethylsulfoxide and Methanol.

We present evidence for vibrational enhancement of the rate of bimolecular reactions of Br atoms with dimethylsulfoxide (DMSO) and methanol (CHOH) in the condensed phase. The abstraction of a hydrogen atom from either of these solvents by a Br atom is highly endoergic: 3269 cm for DMSO and 1416 or 4414 cm for CHOH, depending on the hydrogen atom abstracted. Thus, there is no thermal abstraction reaction at room temperature.

Solvent Dependent Dynamics of Salicylidene Aniline in Binary Mixtures of Supercritical CO with 1-Propanol or Cyclohexane.

The role of different solvent environments in determining the behavior of molecules in solution is a fundamental aspect of chemical reactivity. We present an approach for exploring the influence of solvent properties on condensed-phase dynamics using ultrafast transient absorption spectroscopy in supercritical CO. Using supercritical CO permits adjustment of the density, by varying the temperature and pressure, whereas varying the concentration or identity of a second solvent, the cosolvent, in a binary mixture allows for adjustments of the degree of interaction between the solute and the solvent.

Dynamics and yields for CHBrCl photodissociation from 215-265 nm.

We investigate the Ã-band photodissociation of CHBrCl at 215, 225, 235, 245, 255, and 265 nm. Following C-Br bond cleavage, resonance enhanced multiphoton ionization and time of flight mass spectrometry provide selective detection of the two product channels, from which we quantify the relative quantum yield of Br/Br* production. Velocity-map imaging of the photofragments allows us to determine the energy partitioning as a function of the photolysis energy for different exit channels.

Comparative Study of Cl-Atom Reactions in Solution Using Time-Resolved Vibrational Spectroscopy.

A Cl atom can react with 2,3-dimethylbutane (DMB), 2,3-dimethyl-2-butene (DMBE), and 2,5-dimethyl-2,4-hexadiene (DMHD) in solution via a hydrogen-abstraction reaction. The large exoergicity of the reaction between a Cl atom and alkenes (DMBE and DMHD) makes vibrational excitation of the HCl product possible, and we observe the formation of vibrationally excited HCl (v = 1) for both reactions. In CCl4, the branching fractions of HCl (v = 1), Γ (v = 1), for the Cl-atom reactions with DMBE and DMHD are 0.

Picosecond Dynamics of Avobenzone in Solution.

Avobenzone, a dibenzoylmethane compound commonly found in sunscreens, can photoisomerize after exposure to near-ultraviolet light. At equilibrium, avobenzone exists as a chelated enol characterized by a strong intramolecular hydrogen bond. Many nanosecond- to microsecond-scale experiments have shown that the photoisomerization involves several nonchelated enol (NCE) isomers and reaction paths, including some that reduce avobenzone's efficacy as a sunscreen.

Vibrational predissociation and vibrationally induced isomerization of 3-aminophenol-ammonia.

We investigate the vibrational predissociation dynamics of the hydrogen-bonded 3-aminophenol-ammonia cluster (3-AP-NH3) in the OH and NH stretching regions. Vibrational excitation provides enough energy to dissociate the cluster into its constituent 3-AP and NH3 monomers, and we detect the 3-AP fragments via (1 + 1) resonance-enhanced multiphoton ionization (REMPI). The distribution of vibrational states of the 3-AP fragment suggests the presence of two distinct dissociation pathways.

The influence of translational and vibrational energy on the reaction of Cl with CH3D.

The reaction of Cl atoms with CH3D proceeds either by abstraction of hydrogen to produce HCl + CH2D or by abstraction of deuterium to produce DCl + CH3. Using Cl atoms with different amounts of translational energy, produced by photolysis of Cl2 with 309, 355, or 416 nm light, reveals the influence of translational energy on the relative reaction probability for the two channels. These measurements give an estimate of the energy barrier for the reaction for comparison to theory and indicate that tunneling is the dominant reaction mechanism at low collision energies.

Probing the photoisomerization of CHBr3 and CHI3 in solution with transient vibrational and electronic spectroscopy.

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C-H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C-H stretching transitions of these isomers, in which the emergent halogen atom returns to form a C-X-X bonding motif, appear about 9 ps after photolysis for iso-CHBr2-Br and in about 46 ps for iso-CHI2-I.

Molecular reaction dynamics across the phases: similarities and differences.

This introduction to the 157th Faraday Discussion describes features of bimolecular reaction and photodissociation in gases and liquids and at interfaces. Two unifying ideas are the concepts of a transition state on a single potential energy surface and of a conical intersection between two surfaces, the former being important in bimolecular reactions and the latter often being important in photodissociation. State-resolved studies of the reactions of methane and its isotopologues with F, Cl, and Br illustrate many aspects of bimolecular reactions including the ability of excitation in vibrational modes to enhance or inhibit a reaction and to control the cleavage of selected bonds.

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters.

Infrared-action spectroscopy of small ammonia clusters obtained by detecting ammonia fragments from vibrational predissociation provides an estimate of the dissociation energy of the trimer. The product detection uses resonance enhanced multiphoton ionization (REMPI) of individual rovibrational states of ammonia identified by simulations using a consistent set of ground-electronic-state spectroscopic constants in the PGOPHER program. Comparison of the infrared-action spectra to a less congested spectrum measured in He droplets [M.

Photoisomerization and relaxation dynamics of a structurally modified biomimetic photoswitch.

Recent experimental and theoretical studies on N-alkylated indanylidene pyrroline Schiff bases (NAIP) show that these compounds exhibit biomimetic photoisomerization analogous to that in the chromophore of rhodopsin. The NAIP compounds studied previously isomerize rapidly and often evolve coherently on the ground-electronic surface after reaction. We present the results of transient electronic absorption spectroscopy on dMe-OMe-NAIP, a newly synthesized NAIP analogue that differs from other NAIP compounds in the substituents on its pyrrolinium ring.

Formation and relaxation dynamics of iso-CH2Cl-I in cryogenic matrices.

Photolysis of chloroiodomethane (CH(2)ClI) in cryogenic matrices followed by recombination of the nascent radical pair produces an isomer (CH(2)Cl-I) that features a halogen-halogen (Cl-I) bond. Using ultrafast laser pulses, it is possible to follow the formation of this isomer by transient electronic absorption in low-temperature matrices of N(2), CH(4), and Ar. Frequency-domain measurements provide vibrational and electronic spectra, and electronic structure calculations give the structures of the isomers and the minimum energy path that connects them.

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer.

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer.

The influence of vibrational excitation on the photoisomerization of trans-stilbene in solution.

Preparing electronically excited trans-stilbene molecules in deuterated chloroform using both one-photon excitation and excitation through an intermediate vibrational state explores the influence of vibrational energy on excited-state isomerization in solution. After infrared excitation of either two quanta of C-H stretch vibration |2ν(CH)> at 5990 cm(-1) or the C-H stretch-bend combination |ν(CH) + ν(bend)> at 4650 cm(-1) in the ground electronic state, an ultraviolet photon intercepts the vibrationally excited molecules during the course of vibrational energy flow and promotes them to the electronically excited state. The energy of the infrared and ultraviolet photons together is the same as that added in the one-photon excitation.

Ultrafast observation of isomerization and complexation in the photolysis of bromoform in solution.

Ultrafast photolysis of bromoform (CHBr(3)) with a 267 nm pulse of light followed by broadband transient electronic absorption identifies the photoproducts and follows their evolution in both neat bromoform and cyclohexane solutions. In neat bromoform, a species absorbing at 390 nm appears promptly and decays with a time constant of 13 ps as another species absorbing at 495 nm appears. The wavelength and time evolution of the first absorption is consistent with the formation of iso-bromoform (CHBr(2)-Br) by recombination of the fragment radicals within the solvent cage.

Time-resolved studies of the reactions of CN radical complexes with alkanes, alcohols, and chloroalkanes.

Ultrafast transient absorption experiments monitor the reaction of CN radicals with 16 different alkane, alcohol, and chloroalkane solutes in CH(2)Cl(2) and with a smaller number of representative solutes in CHCl(3) and CH(3)CCl(3). In these experiments, 267-nm photolysis generates CN radicals, and transient electronic absorption at 400 nm probes their time evolution. A crucial feature of the reactions of CN radicals is their rapid formation of two different types of complexes with the solvent that have different stabilities and reactivities.

Time-resolved studies of CN radical reactions and the role of complexes in solution.

Time-resolved studies using 100 fs laser pulses generate CN radicals photolytically in solution and probe their subsequent reaction with solvent molecules by monitoring both radical loss and product formation. The experiments follow the CN reactants by transient electronic spectroscopy at 400 nm and monitor the HCN products by transient vibrational spectroscopy near 3.07 microm.

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 C-H bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities.

Chemical reaction dynamics.

Understanding the motions of the constituent atoms in reacting molecules lies at the heart of chemistry and is the central focus of chemical reaction dynamics. The most detailed questions one can ask are about the evolution of molecules prepared in a single quantum state to products in individual states, and both calculations and experiments are providing such detailed understanding of increasingly complex systems. A central goal of these studies is uncovering the essential details of chemical change by removing the averaging over the initial conditions that occurs in many cases.

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited.

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.

Dynamics at conical intersections: the influence of O-H stretching vibrations on the photodissociation of phenol.

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d(5) with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d(5) molecules with an initially excited O-H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy.

Vibrational action spectroscopy of the C-H and C-D stretches in partially deuterated formic acid dimer.

Vibrational action spectroscopy of jet-cooled formic acid dimer measures the frequency of the C-H(D) stretching vibration and its coupling to nearby states. The action spectrum of (DCOOH)2 reveals a specific Fermi resonance between the C-D stretch and two antisymmetric combination states formed from the C-O stretch and DCO bend. A three-state deperturbation analysis shows that there is a relatively strong coupling between the fundamental vibration and each of the combination vibrations (mid R:13 cm(-1)mid R:) as well as between the combination states themselves (mid R:7 cm(-1)mid R:).