Potential energy surfaces reaction pathways

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

Cee M L and M Page 1993. Computing Reaction Pathways on Molecular Potential Energy Surfaces. In Lipkowitz K B and D B Boyd (Editors). Reviews in Computational Chemistry Volume 4. New York, VCH Publishers, pp. 35-65. 

A disadvantage of this technique is that isotopic labeling can cause unwanted perturbations to the competition between pathways through kinetic isotope effects. Whereas the Born-Oppenheimer potential energy surfaces are not affected by isotopic substitution, rotational and vibrational levels become more closely spaced with substitution of heavier isotopes. Consequently, the rate of reaction in competing pathways will be modified somewhat compared to the unlabeled reaction. This effect scales approximately as the square root of the ratio of the isotopic masses, and will be most pronounced for deuterium or... 

The potential energy surface [47] for this reaction (Fig. 5) shows many potentially competitive pathways, labeled A-F, leading to the two most exothermic product channels. Many of these pathways can be isotopically separated by reaction of 02 with HCCO in normal abundance, as diagramed in Fig. 5. Zou and Osbom used time-resolved Fourier transform emission spectroscopy to detect the CO and CO2 products of this reaction [47]. Rotationally resolved infrared (IR) spectroscopy can easily identify all the possible isotopologs. For example. Fig. 6 shows a single... 

For both statistical and dynamical pathway branching, trajectory calculations are an indispensable tool, providing qualitative insight into the mechanisms and quantitative predictions of the branching ratios. For systems beyond four or five atoms, direct dynamics calculations will continue to play the leading theoretical role. In any case, predictions of reaction mechanisms based on examinations of the potential energy surface and/or statistical calculations based on stationary point properties should be viewed with caution. 

The study of multiple pathways leading to a single product channel provides a stringent test of our understanding of the potential energy surface and the calculations that use it to predict reaction outcomes. Although there are not many examples to date of pathway competitions, the increasing prominence of such systems, coupled with advances in experiment and theory that facilitate their study, promises a rich future in this normally hidden facet of reaction mechanisms. 

Figure 4. Schematic potential energy surface for the reaction of FeO" " with methane. The sohd line indicates the sextet surface the quartet surface is shown with a dotted line, in each case leading to the production of Fe + CH3OH. The dashed line leads to formation of FeOET + CH3. The pathway leading to the minor FeCH2" + H2O channel is not shown. Schematic structures are shown for the three minima the [OFe CHJ entrance channel complex, [HO—Fe—CH3] insertion intermediate, and Fe" (CH30H) exit channel complex. See text for details on the calculations on which the potential energy surface is based.

Studies of kinetic energy release distributions have implications for the reverse reactions. Notice that on a Type II surface, the association reaction of ground state MB+ and C to form MA+ cannot occur. In contrast, on a Type I potential energy surface the reverse reaction can occur to give the adduct MA+. Unless another exothermic pathway is available to this species, the reaction will be nonproductive. However, it is possible in certain cases to determine that adduct formation did occur by observation of isotopic exchange processes or collisional stabilization at high pressures. 

The 3Fe(CO)3(H2) species is the one that can be formed most readily from the separated reactants, but there are several other minima on the [Fe,C3,03,H2] singlet and triplet potential energy surfaces that could conceivably also be formed in the reaction. Most notably, we located isomers 1Fe(CO)3(H2), 3Fe(CO)3(H)2, and 1Fe(CO)3(H)2, located, respectively, at 4.8, 14.9, and —1.4 kcalmol-1 with respect to 3Fe(CO)3(H2) (see Table III and Scheme 5, and Ref. (24)). The triplet dihydride species lies higher in energy than separated H2 and Fe(CO)3, and hence is unlikely to be formed in this system. However, the two singlet species are lower in energy and might be formed hence, we have also explored the pathways for interconversion. 

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