Diego Troya

Bottlebrush Polymer Synthesis by Ring-Opening Metathesis Polymerization: The Significance of the Anchor Group.

Control over bottlebrush polymer synthesis by ring-opening metathesis polymerization (ROMP) of macromonomers (MMs) is highly dependent on the competition between the kinetics of the polymerization and the lifetime of the catalyst. We evaluated the effect of anchor group chemistry-the configuration of atoms linking the polymer to a polymerizable norbornene-on the kinetics of ROMP of polystyrene and poly(lactic acid) MMs initiated by (H2IMes)(pyr)2(Cl)2Ru═CHPh (Grubbs third generation catalyst). We observed a variance in the rate of propagation of >4-fold between similar MMs with different anchor groups. This phenomenon was conserved across all MMs tested, regardless of solvent, molecular weight (MW), or repeat unit identity. The observed >4-fold difference in propagation rate had a dramatic effect on the maximum obtainable backbone degree of polymerization, with slower propagating MMs reducing the maximum bottlebrush MW by an order of magnitude (from ∼10(6) to ∼10(5) Da). A chelation mechanism was initially proposed to explain the observed anchor group effect, but experimental and computational studies indicated that the rate differences likely resulted from a combination of varying steric demands and electronic structure among the different anchor groups. The addition of trifluoroacetic acid to the ROMP reaction substantially increased the propagation rate for all anchor groups tested, likely due to scavenging of the pyridine ligands. Based on these data, rational selection of the anchor group is critical to achieve high MM conversion and to prepare pure, high MW bottlebrush polymers by ROMP grafting-through.

Classical trajectory study of collisions of Ar with alkanethiolate self-assembled monolayers: potential-energy surface effects on dynamics.

We have investigated collisions between Ar and alkanethiolate self-assembled monolayers (SAMs) using classical trajectory calculations with several potential-energy surfaces. The legitimacy of the potential-energy surfaces is established through comparison with molecular-beam data and ab initio calculations. Potential-energy surfaces used in previous work overestimate the binding of Ar to the SAM, leading to larger energy transfer than found in the experiments. New calculations, based on empirical force fields that better reproduce ab initio calculations, exhibit improved agreement with the experiments. In particular, polar-angle-dependent average energies calculated with explicit-atom potential-energy surfaces are in excellent agreement with the experiments. Polar- and azimuthal-angle-dependent product translational energies are examined to gain deeper insight into the dynamics of Ar+SAM collisions.

Ab initio and direct quasiclassical-trajectory study of the F+CH4→HF+CH3 reaction

We present an electronic structure and dynamics study of the F+CH4-->HF+CH3 reaction. CCSD(T)/aug-cc-pVDZ geometry optimizations, harmonic-frequency, and energy calculations indicate that the potential-energy surface is remarkably isotropic near the transition state. In addition, while the saddle-point F-H-C angle is 180 degrees using MP2 methods, CCSD(T) geometry optimizations predict a bent transition state, with a 153 degrees F-H-C angle. We use these high-quality ab initio data to reparametrize the parameter-model 3 (PM3) semiempirical Hamiltonian so that calculations with the improved Hamiltonian and employing restricted open-shell wave functions agree with the higher accuracy data. Using this specific-reaction-parameter PM3 semiempirical Hamiltonian (SRP-PM3), we investigate the reaction dynamics by propagating quasiclassical trajectories. The results of our calculations using the SRP-PM3 Hamiltonian are compared with experiments and with the estimates of two recently reported potential-energy surfaces. The trajectory calculations using the SRP-PM3 Hamiltonian reproduce quantitatively the measured HF vibrational distributions. The calculations also agree with the experimental HF rotational distributions and capture the essential features of the excitation function. The results of the SRP semiempirical Hamiltonian developed here clearly improve over those using the two prior potential-energy surfaces and suggest that reparametrization of semiempirical Hamiltonians is a promising strategy to develop accurate potential-energy surfaces for reaction dynamics studies of polyatomic systems.

In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.

Theoretical study of the dynamics of the H+CH4 and H+C2H6 reactions using a specific-reaction-parameter semiempirical Hamiltonian

We present a theoretical study of the reactions of hydrogen atoms with methane and ethane molecules and isotopomers. High-accuracy electronic-structure calculations have been carried out to characterize representative regions of the potential-energy surface (PES) of various reaction pathways, including H abstraction and H exchange. These ab initio calculations have been subsequently employed to derive an improved set of parameters for the modified symmetrically-orthogonalized intermediate neglect of differential overlap (MSINDO) semiempirical Hamiltonian, which are specific to the H+alkane family of reactions. The specific-reaction-parameter (SRP) Hamiltonian has then been used to perform a quasiclassical-trajectory study of both the H+CH4 and H+C2H6 reactions. The calculated values of dynamics properties of the H+CH4-->H2+CH3 reaction and isotopologues, including alkyl product speed distributions, diatomic product internal-state distributions, and cross sections, are generally in good agreement with experiment and with the results provided by the ZBB3 PES [Z. Xie et al., J. Chem. Phys. 125, 133120 (2006)]. The results of trajectories propagated with the SRP Hamiltonian for the H+C2H6-->H2+C2H5 reaction also agree with experiment. The level of agreement between the results calculated with the SRP Hamiltonian and experiment in both the H+methane and H+ethane reactions indicates that semiempirical Hamiltonians can be improved for not only a specific reaction but also a family of reactions.

Dynamics of collisions of hydroxyl radicals with fluorinated self-assembled monolayers

We present a classical trajectory study of the dynamics of collisions between OH radicals and fluorinated self-assembled monolayers (F-SAMs). The gas/surface interaction potential required in the simulations has been derived from high-level ab initio calculations (focal-point-CCSD(T)/aug-cc-pVQZ) of various approaches of OH to a model fluorinated alkane. The two lowest-energy doublet potential energy surfaces considered in the electronic structure calculations have been averaged to produce a pairwise analytic potential. This analytic potential has been subsequently employed to propagate classical trajectories of collisions between OH and F-SAMs at initial conditions relevant to recent experiments on related systems. The calculated rotational distributions of the inelastically scattered OH agree well with the experiment, which serves to validate the accuracy of the simulations. Investigation of the dynamics of energy transfer for different initial rotational states of OH indicates that an increase in the initial rotation of OH results in increases in both the final average OH rotational and translational energy and in a slight decrease in the amount of energy transferred to the surface. Analysis of the dynamics as a function of the desorption angle of OH from the surface shows that while there is a correlation between the final scattering angle and OH’s amount of final translational energy, the amount of rotational energy in OH is largely independent of the desorption angle. The mechanism of the collisions is found to be mostly direct; in about 90% of most trajectories, OH only collides with the surface once before desorbing, which exemplifies the rigidity of fluorinated monolayer surfaces and their inability to efficiently accommodate gas species.

Quasi-classical trajectory study of the dynamics of the Cl + CH4 → HCl + CH3 reaction

We present an on-the-fly classical trajectory study of the Cl + CH(4)→ HCl + CH(3) reaction using a specific reaction parameter (SRP) AM1 Hamiltonian that was previously optimized for the Cl + ethane reaction [S. J. Greaves et al., J. Phys Chem A, 2008, 112, 9387]. The SRP-AM1 Hamiltonian is shown to be a good model for the potential energy surface of the title reaction. Calculated differential cross sections, obtained from trajectories propagated with the SRP-AM1 Hamiltonian compare favourably with experimental results for this system. Analysis of the vibrational modes of the methyl radical shows different scattering distributions for ground and vibrationally excited products.

Direct-dynamics study of the F + CH4, C2H6, C3H8, and i-C4H10 reactions.

We present a theoretical study of the dynamics of the first few members of the F + alkane --> HF + alkyl family of reactions (alkane = CH(4), C(2)H(6), C(3)H(8), and i-C(4)H(10)). Quasiclassical trajectories have been propagated employing a reparameterized semiempirical Hamiltonian that was derived in this work based on ab initio information of the global potential-energy surfaces of all reactions studied. The accuracy of the Hamiltonian is probed via comparison of the calculated dynamics properties with experimental results in the F + CH(4) --> HF + CH(3), F + CD(4) --> DF + CD(3), and F + C(2)H(6) --> HF + C(2)H(5) reactions. Additional calculations on the F + C(3)H(8) --> HF + C(3)H(7) and F + i-C(4)H(10) --> HF + C(4)H(9) reactions have been analyzed with emphasis on the difference in the dynamics of reactions occurring at primary, secondary, and tertiary sites. We learn that at low collision energies, the amount of energy going into HF vibration increases very slightly along the primary --> secondary --> tertiary sequence. In addition, reactions involving larger alkane molecules tend to channel more energy toward alkyl products at the expense of the rest of the degrees of freedom. Angular distributions are also dependent on the abstraction site, with tertiary abstractions resulting in slightly more backward scattering than reactions at primary sites.

Chemical Warfare Agent Surface Adsorption: Hydrogen Bonding of Sarin and Soman to Amorphous Silica

Sarin and soman are warfare nerve agents that represent some of the most toxic compounds ever synthesized. The extreme risk in handling such molecules has, until now, precluded detailed research into the surface chemistry of agents. We have developed a surface science approach to explore the fundamental nature of hydrogen bonding forces between these agents and a hydroxylated surface. Infrared spectroscopy revealed that both agents adsorb to amorphous silica through the formation of surprisingly strong hydrogen-bonding interactions with primarily isolated silanol groups (SiOH). Comparisons with previous theoretical results reveal that this bonding occurs almost exclusively through the phosphoryl oxygen (P═O) of the agent. Temperature-programmed desorption experiments determined that the activation energy for hydrogen bond rupture and desorption of sarin and soman was 50 ± 2 and 52 ± 2 kJ/mol, respectively. Together with results from previous studies involving other phosphoryl-containing molecules, we have constructed a detailed understanding of the structure-function relationship for nerve agent hydrogen bonding at the gas-surface interface.

Experimental and theoretical study of CO collisions with CH3- and CF3-terminated self-assembled monolayers

We present an experimental and theoretical study of the dynamics of collisions of the CO molecule with organic surfaces. Experimentally, we scatter CO at 60 kJ mol(-1) and 30 degrees incident angle from regular (CH(3)-terminated) and omega-fluorinated (CF(3)-terminated) alkanethiol self-assembled monolayers (SAMs) and measure the time-of-flight distributions at the specular angle after collision. At a theoretical level, we carry out classical-trajectory simulations of the same scattering process using CO/SAM potential-energy surfaces derived from ab initio calculations. Agreement between measured and calculated final translational energy distributions justifies use of the calculations to examine dynamical behavior of the gas/surface system not available directly from the experiment. Calculated state-to-state energy-transfer properties indicate that the collisions are notably vibrationally adiabatic. Similarly, translational energy transfer from and to CO rotation is relatively weak. These trends are examined as a function of collision energy and incident angle to provide a deeper understanding of the factors governing state-to-state energy transfer in gas/organic-surface collisions.