The breaking and making of bonds in a chemical reaction is expected to take the minimum-energy pathway. But not all reactions follow such textbook paths. According to a new report, reactions between the radical H∙ and D2 at high collision energies circumvent the minimum-energy pathway and show unusual, unexpected reaction dynamics on the way to forming HD and D∙.
The work is part of an emerging body of research showing that a variety of reactions have unusual mechanisms at elevated collision energies. Along with “roaming” reaction mechanisms (C&EN, March 19, page 42), such nontextbook reactions “open our scientific eyes to the possibility and even likelihood of unexpected pathways” to reaction products, says Emory University chemistry professor Joel M. Bowman, who was not involved in the H∙ + D2 work.
In the case of H∙ + D2, the minimum-energy pathway involves H∙ approaching D2 in line with the D–D axis on a collinear trajectory. After the two species react, HD rebounds backward along the original H∙ path with little rotational excitation. Bump energies a little higher, and the reaction can occur when H∙ approaches D2 at an angle. In this situation, product HD scatters sideways with more rotational excitation, as the initial H∙ angular momentum transfers into angular rotation of the product.
That was what scientists expected and observed, until now. A group led by Stanford University chemistry professor Richard N. Zare and graduate student Justin Jankunas looked at H∙ + D2 with high collision energies and found rotationally excited HD scattered backward (Science, DOI: 10.1126/science.1221329). Theoretical analysis showed that the hyperthermal collisions create a high-energy barrier for the angled approaches, reducing the probability of sideways-scattered reactions. Experimentally, the phenomenon shows up as more backward-scattered products.
Studying basic reactions such as H∙ + D2 or the analogous H∙ + H2 is important for fundamental understanding of chemistry. “Everyone thinks that H∙ + H2 is completely solved,” Zare says. “In some sense it is, but we’re still learning from it, and it’s because it’s so well solved that we have confidence about the new things we learn.” He and colleagues believe that the barrier effect likely applies to other reactions, given the right conditions.
In fact, a separate group led by Montana State University chemistry professor Timothy K. Minton, in collaboration with Xueming Yang, a professor at China’s Dalian Institute of Chemical Physics, observed something similar last year in the reaction of O with CH4 to form ∙OH and ∙CH3. Minton is interested in high-energy atmospheric reactions that degrade spacecraft materials and contribute to rocket plume signatures in Earth’s outer atmosphere. Looking at O + CH4, Minton and coworkers also found rotationally excited ∙OH scattered preferentially backward, although they didn’t have an explanation for it at the time (J. Phys. Chem. A, DOI: 10.1021/jp207137t).
Additionally, Minton and colleagues found an unusual mechanism when studying hyperthermal collisions of O with HCl to form ClO∙ and H∙ (J. Am. Chem. Soc., DOI: 10.1021/ja803080q). In this case, the minimum-energy pathway involves O reacting with the chlorine of HCl when the hydrogen is oriented away from the incoming O. Little energy is transferred to H∙, so increasing the collision energy increases the internal excitation of the product ClO∙. In hyperthermal collisions that follow the minimum-energy pathway, the internal excitation of ClO∙ gets high enough that the molecule dissociates into Cl∙ and O.
But ClO∙ is observed in hyperthermal collisions. Minton’s team concluded that in this case the reaction follows a higher-energy mechanism in which the hydrogen of HCl is oriented toward the incoming O. Thus, the hydrogen atom experiences a strong repulsive force from both the O and Cl, and H∙ carries away energy in the form of translation. This mechanism lowers the internal energy of product ClO∙, allowing it to remain intact.
Another example of how mechanisms may change at hyperthermal collision energies comes from the reaction of H∙ with CD4 to form HD and ∙CD3; the analogous H∙ + CH4 reaction is important in combustion. In the minimum-energy pathway for this reaction, hydrogen approaches in line with a C–D bond. As in the H∙ + D2 system, collisions at somewhat higher energies enable H∙ to approach at an angle to a C–D bond. Take the collision energies too high, however, and the collinear reaction doesn’t happen.
The reason has to do with the relative masses of the species involved, according to a study by Yang, his colleague Dong H. Zhang, Bowman, and colleagues (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1006910107). Because the mass of CD4 is so large relative to H∙, H∙ gets reflected in collisions, and reactions only happen when the target deuterium atom departs quickly enough to join it. In hyperthermal collinear collisions, H∙ moves too quickly for deuterium to keep up.
All the hyperthermal collision effects observed so far are likely to apply to other systems, the researchers note. And new ones may yet be discovered, providing still deeper understanding of just how atoms and molecules react.