Discovery of conical intersection mediated photochemistry with growing string methods

Cody Aldaz, Joshua A. Kammeraad and Paul M. Zimmerman (2018)

Highlighted by Jan Jensen

Photochemistry is becoming an increasing important synthetic tool but is significantly harder to study computationally than thermal chemistry. Zimmerman and co-workers have developed a new tool that promises to help change that.

The method uses a growing string method (usually used to find TSs) to locate minimum energy conical intersections (MECI), the lowest energy point where the excited state PES intersects the ground state PES. Ground state geometry optimisation starting from the MECI structures are then used to identify the products of the photochemical reaction. Crucially, the method doesn't just find the MECI closest to the reactant structure, but considers several search directions.

One has to define a driving coordinate but this can be automatically determined by generating several possible products, e.g. using Zimmerman's ZStruct method. As far as I know the molecule is not in thermal equilibrium on the excited state PES, so I am not sure one can use the relative energies of the MECIs to predict a product distribution.  Still, an important step forward.



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Exceptionally Long C−C Single Bonds in Diamino-o-carborane as Induced by Negative Hyperconjugation

Li, J.; Pang, R.; Li, Z.; Lai, G.; Xiao, X.-Q.; Müller, T., Angew. Chem. Int. Ed. 2019, 58, 1397-1401

Contributed by Steven Bacharach
Reposted from Computational Organic Chemistry with permission

Chemists are constantly checking the limits of theories, and the limits of bonding is one that has been subject to many tests of late. I have posted on two recent papers (here, here) that probe just how long a C-C bond can be, and now Li, Müller, and co-workers report a structure that pushes that limit even further out.¹

They prepared and obtained the x-ray structure of five derivatives of o-carborane, namely compounds 1, 2a, 3a, 3b and 4. In all of these, the C-C bond in the carborane is stretched well beyond that of a typical C-C bond (see Table 1). The longest case is in 3b where the C-C bond length is a whopping 1.931 Å (see Figure 1), which obliterates the previous record holder at 1.798 Å.² B3PW91-D3/cc-pVTZ computations corroborate these structures and the long C-C bond.

Scheme 1: Carboranes with long C-C bonds (highlighted in blue)

Table 1. C-C bond distance (Å)

cmpd	r(C-C) expt	r(C-C) DFT
1	1.829	1.839
2a	1.720	1.710
3a	1.893	1.917
3b	1.931	1.936
4	1.627	1.607

Figure 1. B3PW91-D3/cc-pVTZ optimized structure of 3b.

Topological electron density analysis locates a bond path between the two carbons in all five structures. The Wiberg bond index is small, with a value of only 0.34 in 3b. Natural bond orbital (NBO) analysis identifies a negative hyperconjugation interaction between the nitrogen lone pair and the σ*_C-C orbital. This rationalizes both the very long C-C bond and the very short C-N bonds, and the trends associated with the variation between 1° amine, 2° amine and imine.

References

1. Li, J.; Pang, R.; Li, Z.; Lai, G.; Xiao, X.-Q.; Müller, T., “Exceptionally Long C−C Single Bonds in Diamino-o-carborane as Induced by Negative Hyperconjugation.” Angew. Chem. Int. Ed. 2019, 58, 1397-1401, DOI: 10.1002/anie.201812555.

2. Ishigaki, Y.; Shimajiri, T.; Takeda, T.; Katoono, R.; Suzuki, T., “Longest C–C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å.” Chem 2018, 4, 795-806, DOI: 10.1016/j.chempr.2018.01.011.

InChIs

3b: InChI=1S/C22H28B10N2/c1-13-7-15(3)19(16(4)8-13)11-33-21-22(34-12-20-17(5)9-14(2)10-18(20)6)25(21)23-27(21)24-30(23,25)28(22,25)29(22)26(21,22,27)31(24,27,29)32(24,28,29)30/h7-10,33-34H,11-12H2,1-6H3
InChIKey=UEZUONSMPNIZRQ-UHFFFAOYSA-N

'
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Understanding Combustion of H/O Gases inside Nanobubbles Generated by Water Electrolysis Using Reactive Molecular Dynamic Simulations.

S. Jain and L. Qiao, The Journal of Physical Chemistry A,122, 5261 2018
Highlighted by Tina Mihm, Colleen Lasar, Matthew Emerson

It was found (by accident) that nanobubbles containing both H₂ and O₂ gas would form during the electrolysis of water and spontaneously combust. This is surprising because at smaller scales, the surface-to-volume ratio is large enough that heat loss becomes a real factor when trying to create and sustain a combustion reaction. Jain et al. believe this nanobubble combustion reaction is due to the low temperature and high pressure zone that takes place in the bubble. The initial thought with this discovery was that the combustion reaction inside said nanobubbles could be used to produce energy. However, most of the temperature from the reaction was found to be lost to the walls of the bubbles indicating low energy yields.

The reaction mechanism is as follows: 2 H₂(g) + O₂(g)  →  2 H₂O(g). Both experimental and older computational methods have looked into the temperature changes and kinetics of this reaction, however, the mechanism has not been looked into in detail. Jain et al. uses molecular dynamic simulations to explore the mechanism of this phenomenon. They explored the characteristics of H₂/O₂ reactions at high pressure and low temperature as a function of Hydrogen radical concentration and found that H₂O₂ was the dominant species produced instead of the expected H₂O.

In the simulations, they used a force field designed specifically to investigate the reaction kinetics of H₂/O₂ system at high pressures and low temperatures. Specifically, the first-principles derived reactive force field ReaxFF was employed, as implemented in the open-source molecular dynamics simulation code LAMMPS.  After thermalizing the system to 300 K with a Nose-Hoover thermostat, production runs  of 100 fs were carried out, using a 0.1 fs time step. The model was then validated using existing more generalized force fields that were not designed for the H₂/O₂ system. They also found that increasing the concentration of H radical or the system pressure increased reactivity. While this result was initially thought to be able to increase energy output, it was found that most of the energy from the reaction was found to be lost to the walls of the combustion chamber. If this happened in an automobile, the engine would become so hot that the hood would melt off.

In conclusion, it was found that hydrogen and oxygen gas in nano bubbles formed during electrolysis of water and would spontaneously combust. The mechanism for this reaction was investigated using reactions at high pressure and low temperature as a function of Hydrogen radical concentration and found that H₂O₂ was the dominant species produced instead of the expected H₂O. The increase in reactivity due to increased pressure and H radical concentration during simulation was thought to increase energy output, and, therefore, create a source of clean energy. However, further computational simulations found that most of the heat was lost to the walls of the bubbles, greatly decreasing energy output, making a lousy nano-engine.

SERS, XPS and DFT investigation on palladium surfaces coated with 2,2′-bipyridine monolayers

M. Muniz-Miranda, F. Miranda-Muniz, S. Caporali, N. Calisi, P. Alfonso, Applied Surface Science, 457, 98-103 2018
Highlighted by Michaella Raglione, Sajeewani Kumarage, and Glorianne Dorce

Palladium (II) chloride complexes containing diimine ligands like what is shown in Figure 1 have been widely used as a reaction catalyst. One application of a palladium catalyst is the Heck reaction, which utilizes a Palladium (II) complex intermediate to activate the reaction of an unsaturated halide with an alkene in the presence of a base. Often, palladium is used in a suspension, but it lacks the colloidal stability to maintain a homogeneous mixture, which can lower its efficiency. Furthermore, heterogenous catalysis provides high yield, and facilitates the reusability of catalysts compared to homogeneous catalysis. Thus, Muniz-Miranda et al. recently investigated the use of Palladium coated with 2,2’-bipyridine monolayers for heterogenous catalysis.

Figure 1. Crystal structure of byp-PdCl2 from Table S1 supplementary material.

Their experiments centered around Palladium plate which was wetted with 2,2’-bipyridine (bpy). In order to test their work, Muniz-Miranda et al. utilized the surface plasmon effect of Ag nanoparticles (nps) to enhance their surface enhance Raman signal (SERS). The Ag nps were obtained through laser ablation in the bpy solution without free chloride anions. To determine the resulting complex, DFT calculations were performed using GAUSSIAN 09 software with a B3LYP/6-311++G(d,p) for non-metal atoms, and Lanl2DZ for palladium basis sets. To ensure their experimental methods would work, byp-PdCl₂ was collected and the Raman spectra was compared to the calculated spectra.

Muniz-Miranda et al.  have made bpy-PdX, and by comparing the resulting calculated Raman active modes to the experimental bpy-PdR (R=O,O₂,(OH)₂)  revealed that they created bpy-Pd(OH)₂.

Due to similar structural and spectroscopic characteristics of the bpy-Pd(OH)₂ and bpy-PdCl₂, the catalysts are expected to function similarly. The similarities in these catalysts opens the possibility of utilization of the much simpler heterogenous nucleation of the bpy-Pd(OH)₂ complex for reaction mechanisms. This suggests that combined benefits of both heterogenous and homogenous nucleation can be achieved: improved yield and reusability as well as selectivity control.