Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed ''roaming,'' has been described in the photodissociation of H 2CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH 3CHO to CH4 ؉ CO. The CH 4 product is found to have extreme vibrational excitation, with the vibrational distribution peaked at Ϸ95% of the total available energy. Quasiclassical trajectory calculations on fulldimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH 4 ؉ CO products is via a roaming pathway where a CH 3 fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH 4 ؉ CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH 3 roaming mechanism is attributed to the fact that the CH3 ؉ HCO radical asymptote and the TS saddle-point barrier to CH 4 ؉ CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H 2CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.reaction dynamics ͉ roaming mechanisms ͉ photochemistry ͉ quasiclassical trajectories ͉ transition state S ince its introduction by Eyring in 1935 (1), the concept of the ''transition state'' (TS) has been central to chemistry because the products, rates, and dynamics of a reaction are often determined by this special molecular configuration (2). For reactions with potential barriers, the TS is a transient molecular structure at the highest point along the minimum energy path connecting reactants to products and thus is a central construct in reaction rate theory and the classification of reaction types. In transition state theory (TST), the reaction rate coefficient is obtained from the ''one-way'' flux through a dividing surface containing the TS. In the more general variational version of TST, denoted VTST, the TS dividing surface is chosen to minimize the reactive flux. This theory is widely used in mathematical modeling of reaction rates in combustion, atmospheric, and biological chemistry, impacting fields as diverse as energy production (3), climate change (4), and enzyme function (5).Although the conventional transition state paradigm will remain essential to chemists, several reaction pathways have been reported in the last 8 years (6-13) in which it is not obvious how to use present implementations of TST or VTST. If such mechanisms are common, they may represent a significant challenge for reaction rate theories. In this report, we show that the ''roaming atom mechanism'' in formaldehyde (H 2 CO) dissociation (9) is not unique to H 2 CO but also occurs in acetaldehyde (CH 3 CHO) dissociation. Moreover, in CH 3 CHO, we find that the roa...

We present a method for expressing a potential energy surface (PES) for polyatomic molecules as an interpolation of local Taylor expansions in internal coordinates. This approach extends and replaces an earlier method which was only directly applicable to molecules of no more than four atoms. In general, the local Taylor expansions are derived from ab initio quantum calculations. Here, the methodology is evaluated by comparison with an analytic surface for the reactions H+CH4⇌H2+CH3. Approximately 1000–1300 data points are required for an accurate 12-dimensional surface which describes both forward and backward reactions, at the energy studied.

Anharmonicity and matrix effects play important roles in determining the proton-stretching frequencies in hydrogen-bonded complexes of HCl and HBr with NH 3 and N(CH 3 ) 3 . These effects have been investigated through ab initio calculations carried out at MP2/aug′-cc-pVDZ for complexes with HCl and at MP2/6-31+G-(d,p) for complexes with HBr. The potential surfaces of these complexes are very anharmonic, since the region surrounding the global minimum may be very broad and relatively flat, or a second region of the surface, displaced from the global minimum, can be accessed in either the ground (V ) 0) or the first excited (V ) 1) state of the proton-stretching mode. As a result, two-dimensional anharmonic frequencies, particularly for the proton-stretching vibration, can be dramatically different from the corresponding harmonic frequencies. Moreover, the zero-point energy contribution to binding enthalpies based on harmonic vibrational frequencies can be significantly overestimated in some complexes. To model the effects of matrices on the structures and spectra of these complexes, potential surfaces have been generated in the presence of external electric fields applied along the hydrogen-bonding X-H-N direction. These fields preferentially stabilize more polar hydrogen-bonded structures. The changes in anharmonic frequencies computed from these surfaces depend on the strength of the field and the nature of the equilibrium structure at zero field. Comparisons between computed frequencies for these complexes and experimental frequencies obtained in Ar and N 2 matrices provide insight into the dependence of proton-stretching frequencies on the environment. It is now possible to understand the apparently disparate effects of Ar and N 2 matrices on the spectra of closely related complexes.

Current atmospheric models underestimate the production of organic acids in the troposphere. We report a detailed kinetic model of the photochemistry of acetaldehyde (ethanal) under tropospheric conditions. The rate constants are benchmarked to collision-free experiments, where extensive photo-isomerization is observed upon irradiation with actinic ultraviolet radiation (310 to 330 nanometers). The model quantitatively reproduces the experiments and shows unequivocally that keto-enol photo-tautomerization, forming vinyl alcohol (ethenol), is the crucial first step. When collisions at atmospheric pressure are included, the model quantitatively reproduces previously reported quantum yields for photodissociation at all pressures and wavelengths. The model also predicts that 21 ± 4% of the initially excited acetaldehyde forms stable vinyl alcohol, a known precursor to organic acid formation, which may help to account for the production of organic acids in the troposphere.