Density functional theory calculations are used to study the energetics of the electrochemical oxygen evolution reaction (OER) of water and the reverse oxygen reduction reaction (ORR) on metal-porphyrin-like centers incorporated into graphene layers or single-walled carbon nanotubes (SWCNTs). The objective is to explore the reductions in computational thermodynamic overpotential that can be achieved relative to catalysis on metal oxide surfaces (OER) or platinum (ORR) by varying the metal center and axial ligand. This permits a degree of simultaneous control over the free energy gap between the lowest energy OH and highest energy OOH intermediates, and the position of the oxo (O) intermediate in this gap. Optimal choice of metal toward the right of the first transition series largely controls the gap. Given a suitable metal such as Fe, the overpotential for OER can be tuned over a range greater than 0.35 V by choice of the axial ligand. For OER occurring within the SWCNTs, a minimum predicted overpotential of 0.35 V is found, very close to the gap-imposed limit of 0.30 V for this system. Similarly, the overpotential of ORR can be tuned over a range more than 0.30 V by selection of the axial ligand. While the calculations necessarily have limited accuracy, the principles should provide a transferable path toward overpotential optimization for the OER and ORR.

Detailed mechanisms for the synthesis of p-xylene as well as the primary byproducts observed experimentally, 2,5-hexadione and 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan, from ethylene and 2,5-dimethylfuran (DMF) mediated by H-BEA are obtained using an extended QM/MM model containing 208 tetrahedral atoms. The formation of p-xylene proceeds via Diels-Alder cycloaddition of ethylene and DMF, which is rate-limiting, followed by Brønsted acid-catalyzed dehydration. Secondary addition of DMF to the substrate following the Diels-Alder reaction leads to 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan. The analysis of the free energies associated with the mechanisms suggests that the secondary addition can be eliminated by introducing n-heptane as an inert solvent to decrease the loading of DMF in the zeolite or by using a weak Brønsted acid site to facilitate the dehydration of the Diels-Alder product, for which the rate is determined by the deprotonation via the conjugate base of the active site. Water formed in the dehydration process can react directly with DMF to form 2,5-hexadione, thereby decreasing the yield of p-xylene. However, the free-energy barriers for the formation of 2,5-heaxdione compared to the Diels-Alder reaction indicate that DMF and 2,5-hexadione will be equilibrated. Therefore, the 2,5-hexadione yield can be minimized by operating at a high conversion of DMF.

A hybrid quantum mechanics/molecular mechanics (QM/MM) model and the quasiclassical trajectory (QCT) method have been combined to study the reaction of alkene methylation by methanol catalyzed by the zeolite H-MFI. The rate-limiting step of this reaction is the methylation of the alkene, and the apparent activation energy calculated at the ωB97X-D/6-31G(d,p)//ωB97X-D/6-311++G(3df,3pd) level of theory for this step agrees well with experiment and previous full QM studies. Following the ethene methylation transition state toward the products along the intrinsic reaction coordinate reveals the existence of a protonated cyclopropane (PCP+) carbocation intermediate. A similar protonated methylcyclopropane (mPCP+) carbocation intermediate is found for propene methylation. The intermediates produced during the alkene methylation reaction are metastable with a lifetime of O(1 ps) obtained from QCTs. Because of the short lifetime of these intermediates, the available energy in the carbocation is not in thermal equilibrium distribution with the zeolite lattice before subsequent reaction occurs. The qualitative difference between product distributions obtained by static and dynamic reaction pathways suggests the pathways of zeolite-catalyzed reactions proceed through high-temperature pathways that differ from the 0 K potential energy surface. The transformation of the m-PCP+ intermediate to the longer-lived secondary 2-butyl carbocation observed during QCTs suggests that more stable carbocations can properly thermalize and exist as reaction intermediates for longer than 1 ps.

Accurate simulation of electronic excitations and deexcitations are critical for complementing complex spectroscopic experiments and can provide validation to theoretical approaches. Using a generalized framework, we contrast the accuracy and validity of orbital-constrained and linear-response approaches that build upon Kohn-Sham density functional theory (DFT) to simulate emission spectra of electronic origin and propose an efficient approximation, named many-body x-ray emission spectroscopy or MBXES, for simulating such processes. We show analytically as well as with computed examples that for electronic (de)excitation leading to an appreciable change in polarization (i.e., density rearrangement), the adiabatic approximation in a response-based formalism will be inadequate for the calculation of oscillator strength. Thus, such a change (e.g., in the net electrostatic dipole moment of a finite system) can be used as a metric for evaluating the applicability of the adiabatic response-based approach and can be particularly valuable in x-ray emission spectroscopy. On the other hand, MBXES, the flexible method introduced in this paper, can compute oscillator strengths accurately at a much lower computational expense on the basis of two DFT-based self-consistent field calculations. Using illustrative examples of emission spectra, the efficacy of the MBXES method is demonstrated by comparison with its parent theory, orbital-optimized DFT, and with experiments.

Quantum mechanics/molecular mechanics (QM/MM) simulations provide an efficient avenue for studying reactions catalyzed in zeolite systems; however, the accuracy of such calculations is highly dependent on the zeolite MM parameters used. Previously reported parameters (P1), which were chosen to minimize the root mean square (RMS) deviations of adsorption energies compared with full QM ωB97X-D/6-31+G∗∗ adsorption energies, are shown to overestimate binding energies compared with experimental values, particularly for larger substrates. To address this issue, a new parameter set (P2) is derived by rescaling the previously reported characteristic energies of the Lennard-Jones potential in P1. The accuracy of the thermal correction for adsorption enthalpies determined by the rigid rotor-harmonic oscillator approximation (RRHO) is examined and shown to be improved by treating low-lying vibrational modes as free translational and rotational modes via a quasi-RRHO model. With P2 and quasi-RRHO, adsorption energies calculated with QM/MM agree with experimental values with an RMS error of 1.8 kcal/mol for both nonpolar and polar molecules adsorbed in MFI, H-MFI, and H-BEA. By contrast, the RMS error for the same test sets obtained using parameter set P1 is 8.3 kcal/mol. Glucose-fructose isomerization catalyzed by Sn-BEA is taken as an example to demonstrate that improved values for apparent activation energies can be obtained using the methodology reported here. With parameter set P2, the apparent activation energy calculated with QM/MM reproduces the experimental value to within 1 kcal/mol. By contrast, using parameter set P1, the error is -12.9 kcal/mol. (Graph Presented).

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. A highly efficient water-tolerant, solid-base catalyst for the self-condensation of biomass-derived methyl ketones to jet-diesel fuel precursors was developed by grafting site-isolated secondary amines on silica-alumina supports. It is shown that apart from the nature and density of amine groups and the spatial separation of the acidic and basic sites, the acidity of the support material plays a critical role in defining the catalytic activity. It is also found that a combination of weakly acidic silanol/aluminol with secondary amine groups can mimic proline catalysts and are more effective in catalyzing the selective dimerization reaction than the combination of amines with organic acids. In situ FTIR measurements demonstrate that acidic groups activate methyl ketones through their carbonyl groups leading to a favorable C-C bond formation step involving an enamine intermediate. DFT analysis of the reaction pathway confirms that C-C bond formation is the rate-limiting step.

The solvent dependent electronic excited state deactivation dynamics of Indigo Carmine is mapped by the two-dimensional electronic-vibrational spectroscopy (2DEV). The H-bonding ability of the solvent has been found to assist the excited state proton transfer.

Quantum mechanics/molecular mechanics (QM/MM) models are applied to investigate the adsorption and cracking of n-hexane on ZSM-5 and Faujasite zeolite structures. These simulations account for the long-range electrostatic and midrange van-der-Waals interactions in the zeolite and provide energy barriers that are close to experimental data. The active acidic site was modeled by dispersion corrected density functional theory (DFT, ω B97X-D6-311/G∗). The long-range interactions were calculated by molecular mechanics (MM). The adsorbed molecules under investigation are characterized by their thermodynamic properties (adsorption energy and enthalpy). The influence of the zeolite type on the thermodynamic properties is also pointed out. The results reveal that the kinetics of cracking is insensitive to differences in acid strengths. The thermodynamic data obtained are mainly influenced by the adsorption energy of n-hexane on ZSM-5 and/or Faujasite (Y) structures. The pore sizes of the zeolite types can lead to a stronger or weaker adsorption energy. Except for the thermodynamic property investigations in this article, the quasi-classical trajectory method (QCT) is used for investigating the pathways along metastable intermediates toward various cracking products. Not only the reaction barriers but also the reaction dynamics determine the reaction selectivity.

We have investigated the solvent response after photoexcitation using Optical Pump THz Probe (OPTP) spectroscopy with sub-ps time resolution in combination with quantum-chemical modeling. Excited State Proton Transfer (ESPT) is found to be facilitated by excitation of an intermolecular stretch (Hpyranine-Owater) triggered by an instantaneous (< 100 fs) redistribution of charge density. Initiation of proton transfer is observed on a time scale of 0.4 ps, i.e. on a much shorter time scale than anticipated before. Our results demonstrate the exceptional sensitivity of OPTP, which enables us to follow the proton motion into the hydrogen bond network of water exploring charge transport in transient solvent configurations in real time.

In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO2 catalyzed by the macrocyclic complex [CoIIN4H]2+ (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [CoIIN4H]2+ resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)3]2+ sensitizer yields an infrared band at 1670 cm-1 attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN4H(MeCN)]+ in CO2 saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to CoI in the first reduction, which is consistent with experimental d-d spectra of square Co(I) macrocycle complexes. The energy of the CO2 adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN4H(MeCN)]+, which implies that only a fraction of the latter forms an adduct with CO2. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [CoI(N4H) -•], and show that [CoI(N4H) -•] binds partially reduced CO2 fairly strongly. Experimentally accessing [CoI(N4H) -•] with an Ir(bpy)3 sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C18O2 also shows clear evidence for 18O-substituted byproducts from CO2 reduction to CO.