High-level Electronic Structure Calculations Research Articles

The role of the intermediate triplet state in iron-catalyzed multi-state C-H activation.

Efficient activation and functionalization of the C-H bond under mild conditions are of a great interest in chemical synthesis. We investigate the previously proposed spin-accelerated activation of the C(sp2)-H bond by a Fe(II)-based catalyst to clarify the role of the intermediate triplet state in the reaction mechanism. High-level electronic structure calculations on a small model of a catalytic system utilizing the coupled cluster with the single, double, and perturbative triple excitations [CCSD(T)] are used to select the density functional for the full-size model. Our analysis indicates that the previously proposed two-state quintet-singlet reaction pathway is unlikely to be efficient due to a very weak spin-orbit coupling between these two spin states. We propose a more favorable multi-state quintet-triplet-singlet reaction pathway and discuss the importance of the intermediate triplet state. This triplet state facilitates a spin-accelerated reaction mechanism by strongly coupling to both quintet and singlet states. Our calculations show that the C-H bond activation through the proposed quintet-triplet-singlet reaction pathway is more thermodynamically favorable than the single-state quintet and two-state singlet-quintet mechanisms.

Radical-Radical Reaction Dynamics Probed Using Millimeterwave Spectroscopy: Propargyl + NH2/ND2.

We apply chirped-pulse uniform flow millimeterwave (CPUF-mmW) spectroscopy to study the complex multichannel reaction dynamics in the reaction between the propargyl and amino radicals (C3H3 + NH2/ND2), a radical-radical reaction of importance in the gas-phase chemistry of astrochemical environments and combustion systems. The photolytically generated radicals are allowed to react in a well-characterized quasi-uniform supersonic flow, and mmW rotational spectroscopy (70-93 GHz) is used for simultaneous detection of the reaction products: HCN, HNC, HC3N, DCN, DNC, and DC3N, while spectral intensities of the measured pure-rotational lines allow product branching to be quantified. High-level electronic structure calculations were used for theoretical prediction of the reaction pathways and branching. Experimentally deduced product branching fractions were compared with the results from statistical simulations based on the RRKM theory. Product branching was found to be strongly dependent on the excess internal energy of the C3H3 and NH2/ND2 reactants.

Electronic absorptions of C5 + detected in the visible through action spectroscopy in a cryogenic trap

The electronic spectrum of C in the gas-phase with origin band at is reported following experiments in a cryogenic ion trapping instrument. Buffer gas-cooled C ions, generated by laser vaporisation of graphite, were investigated using two action spectroscopy approaches. Laser-induced dissociation of weakly bound complexes synthesised in the trap reveal a linear dependence of the absorption energies on n allowing prediction of those of the bare ion C. These results are confirmed in two colour experiments on C, by monitoring fragmentation into the product channel. The data are supplemented with high-level electronic structure calculations which support the assignment of symmetry to the ground electronic state. These laboratory results provide the requisite data needed for spectroscopic detection of this structure in terrestrial and extraterrestrial environments.

Electronic Structure of RhO2+, Its Ammoniated Complexes (NH3)1-5RhO2+, and Mechanistic Exploration of CH4 Activation by Them.

High-level electronic structure calculations are initially performed to investigate the electronic structure of RhO2+. The construction of potential energy curves for the ground and low-lying excited states allowed the calculation of spectroscopic constants, including harmonic and anharmonic vibrational frequencies, bond lengths, spin-orbit constants, and excitation energies. The equilibrium electronic configurations were used for the interpretation of the chemical bonding. We further monitored how the Rh-O bonding scheme changes with the gradual addition of ammonia ligands. The nature of this bond remains unaffected up to four ammonia ligands but adopts a different electronic configuration in the pseudo-octahedral geometry of (NH3)5RhO2+. This has consequences in the activation mechanism of the C-H bond of methane by these complexes, especially (NH3)4RhO2+. We show that the [2 + 2] mechanism in the (NH3)4RhO2+ case has a very low energy barrier comparable to that of a radical mechanism. We also demonstrate that methane can coordinate to the metal in a similar fashion to ammonia and that knowledge of the electronic structure of the pure ammonia complexes provides qualitative insights into the optimal reaction mechanism.

Crossed-Beam and Theoretical Studies of the O(3P, 1D) + Benzene Reactions: Primary Products, Branching Fractions, and Role of Intersystem Crossing.

Reliable modeling of hydrocarbon oxidation relies critically on knowledge of the branching fractions (BFs) as a function of temperature (T) and pressure (p) for the products of the reaction of the hydrocarbon with atomic oxygen in its ground state, O(3P). During the past decade, we have performed in-depth investigations of the reactions of O(3P) with a variety of small unsaturated hydrocarbons using the crossed molecular beam (CMB) technique with universal mass spectrometric (MS) detection and time-of-flight (TOF) analysis, combined with synergistic theoretical calculations of the relevant potential energy surfaces (PESs) and statistical computations of product BFs, including intersystem crossing (ISC). This has allowed us to determine the primary products, their BFs, and extent of ISC to ultimately provide theoretical channel-specific rate constants as a function of T and p. In this work, we have extended this approach to the oxidation of one of the most important species involved in the combustion of aromatics: the benzene (C6H6) molecule. Despite extensive experimental and theoretical studies on the kinetics and dynamics of the O(3P) + C6H6 reaction, the relative importance of the C6H5O (phenoxy) + H open-shell products and of the spin-forbidden C5H6 (cyclopentadiene) + CO and phenol adduct closed-shell products are still open issues, which have hampered the development of reliable benzene combustion models. With the CMB technique, we have investigated the reaction dynamics of O(3P) + benzene at a collision energy (Ec) of 8.2 kcal/mol, focusing on the occurrence of the phenoxy + H and spin-forbidden C5H6 + CO and phenol channels in order to shed further light on the dynamics of this complex and important reaction, including the role of ISC. Concurrently, we have also investigated the reaction dynamics of O(1D) + benzene at the same Ec. Synergistic high-level electronic structure calculations of the underlying triplet/singlet PESs, including nonadiabatic couplings, have been performed to complement and assist the interpretation of the experimental results. Statistical (RRKM)/master equation (ME) computations of the product distribution and BFs on these PESs, with inclusion of ISC, have been performed and compared to experiment. In light of the reasonable agreement between the CMB experiment, literature kinetic experimental results, and theoretical predictions for the O(3P) + benzene reaction, the so-validated computational methodology has been used to predict (i) the BF between the C6H5O + H and C5H6 + CO channels as a function of collision energy and temperature (at 0.1 and 1 bar), showing that their increase progressively favors radical (phenoxy + H)-forming over molecule (C5H6 + CO and phenol stabilization)-forming channels, and (ii) channel-specific rate constants as a function of T and p, which are expected to be useful for improved combustion models.

C-H Bond Activation by the Excited Zinc Atom: Gas-Phase Formation of Methylzinc Hydride (HZnCH3) Based on Multireference Second-Order Perturbation Theory and Coupled Cluster Calculations.

The pioneering spectroscopic observations of the methylzinc hydride [HZnCH3(X1A1)] molecule were reported previously by the Ziurys group [J. Am. Chem. Soc.2010, 132, 17186–17192], and the possible formation mechanisms were suggested therein, including those with the participation of excited zinc atoms in reaction with methane. Herein, the ground singlet state and the lowest excited triplet state potential energy surfaces of the Zn + CH4 reaction have been explored using high-level electronic structure calculations with multireference second-order perturbation theory and coupled cluster singles and doubles with perturbative triples (CCSD(T)) methods in conjunction with all-electron basis sets (up to aug-cc-pV5Z) and scalar relativistic effects incorporated via the second-order Douglas–Kroll–Hess (DK) method. Based on the ab initio results, a plausible scenario for the formation of HZnCH3(X1A1) is proposed involving the activation of the C–H bond of methane by the lowest excited 3P state atomic zinc. Calculations also highlight the importance of an agostic-like Zn···H–C interactions in the pre-activation complex and good agreement between the structure of the HZnCH3(X1A1) molecule predicted at the DK-CCSD(T)/aug-cc-pVQZ-DK level of theory and that derived from rotational spectroscopy, as well as the discrepancies between the ab initio and density functional theory predictions.

Probing the gas-phase oxidation of ammonia: Addressing uncertainties with theoretical calculations

The kinetics of the reactions H2NO + O2(3Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(3Σg−), which are, respectively, very sensitive chain-propagation and chain-termination reactions in ammonia kinetic models, have been revisited by means of high-level electronic structure and variational transition state theory calculations with the goal of improving former predictions and the performance of ammonia kinetic models. In addition, the rate constants of the reactions H2NO + O2(3Σg−) → HNO(a˜3A″) + HO2, NH2 + HO2 → H2NO + OH, and NH2 + HO2 → NH3 + O2(1Δg), which take place on excited-state potential energy surfaces and/or yield the electronically excited species HNO(a˜3A″) and O2(1Δg), have been also calculated for the first time in order to assess their importance in ammonia oxidation. We observed that spin contamination and multi-reference character are pronounced in many of the investigated reactions, and these features were handled by performing post-CCSD(T) electronic structure calculations with the W3X-L composite method as well as restricted open shell coupled cluster calculations. Branching ratios were also analyzed, and indicate that the contribution of the electronically excited species HNO(a˜3A″) and O2(1Δg) are of little importance even at very high temperatures; however, we do not preclude an effect of those species at certain conditions that contribute to their yield. The calculated rate constants were implemented in two recent kinetic models to perform jet stirred reactor, rapid compression machine, and flow reactor simulations, concluding that the model predictions are very sensitive to the reactions H2NO + O2(3Σg−) → HNO(X˜1A′) + HO2 and NH2 + HO2 → NH3 + O2(3Σg−) .

Watching a hydroperoxyalkyl radical (•QOOH) dissociate.

A prototypical hydroperoxyalkyl radical (•QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of •QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate-again increased by heavy-atom tunneling-which are required for global models of atmospheric and combustion chemistry.

Spectral fingerprint of stabilized •QOOH

Chemical Kinetics Carbon-centered radicals containing the hydroperoxy group, commonly denoted as •QOOH, are elusive but are among the most critical intermediate species for kinetic modeling of hydrocarbon oxidation in various atmospheric and combustion processes. Their direct experimental observation is a long-standing challenge, with only one successful previous attempt. Using a combination of infrared activation spectroscopy and ultraviolet laser–induced fluorescence detection, Hansen et al. directly characterized the vibrational structure of a •QOOH intermediate in isobutane oxidation, collisionally stabilized and isolated, and followed its dissociative evolution under infrared activation with time and energy resolution. High-level electronic structure calculations revealed an important role of heavy-atom tunneling in this process. Science , abj0412, this issue p. [679][1] [1]: /lookup/doi/10.1126/science.abj0412

First principles reactive simulation for equation of state prediction.

The high cost of density functional theory (DFT) has hitherto limited the ab initio prediction of the equation of state (EOS). In this article, we employ a combination of large scale computing, advanced simulation techniques, and smart data science strategies to provide an unprecedented ab initio performance analysis of the high explosive pentaerythritol tetranitrate (PETN). Comparison to both experiment and thermochemical predictions reveals important quantitative limitations of DFT for EOS prediction and thus the assessment of high explosives. In particular, we find that DFT predicts the energy of PETN detonation products to be systematically too high relative to the unreacted neat crystalline material, resulting in an underprediction of the detonation velocity, pressure, and temperature at the Chapman-Jouguet state. The energetic bias can be partially accounted for by high-level electronic structure calculations of the product molecules. We also demonstrate a modeling strategy for mapping chemical composition across a wide parameter space with limited numerical data, the results of which suggest additional molecular species to consider in thermochemical modeling.

Extending Non-Hysteretic Electrochemistry Beyond Conventional Transition Metal Capacity in Li and Na-Ion Cathodes Using Pi-Redox

The anomalous electrochemical capacity observed in some Li- and Na-excess materials, extending beyond the conventional limit of transition metal redox, represents one of the most promising sources of untapped energy density in intercalation cathodes. Unfortunately, the majority of materials known to exhibit this behavior suffer from voltage hysteresis and structural degradation. A prominent exception is Na2Mn3O7, which shows no voltage hysteresis despite operating beyond the conventional limit of Mnoxidation. The redox mechanism of this material and the source of the non-hysteretic behavior remain controversial with significant inconsistencies between experimental observations and the commonly invoked "oxygen-redox" theories. We use high-level electronic structure calculations to demonstrate that the origin of the anomalous capacity in Na2Mn3O7 is the oxidation of a delocalized redox center consisting of pi-bonded Mn-d and O-p orbitals, which we term pi-redox. This hybridized redox is unique in that capacity, voltage, and stability are determined by the long-range structure of the metal-oxygen network, rather than any local bonding environment. The pi-redox mechanism competes with oxidation-driven transition metal migration which underlies the hysteresis seen in most excess-capacity compounds, such as those based on Li2MnO3. The new pi-redox mechanism is the first to reconcile all experimental observations of non-hysteretic anomalous capacity, and sets forth rigorous materials design principles for extending this behavior from model systems to high-performance compounds.

Methane to Methanol Conversion Facilitated by Anionic Transition Metal Centers: The Case of Fe, Ni, Pd, and Pt.

Density functional theory and high-level ab initio electronic structure calculations are performed to study the mechanism of the partial oxidation of methane to methanol facilitated by the titled anionic transition metal atoms. The energy landscape for the overall reaction M- + N2O + CH4 → M- + N2 + CH3OH (M = Fe, Ni, Pd, Pt) is constructed for different reaction pathways for all four metals. The comparison with earlier experimental and theoretical results for cationic centers demonstrates the better performance of the metal anions. The main advantage is that anionic centers interact weakly with the produced methanol. This fact facilitates the fast removal of methanol from the catalytic center and prevents the overoxidation of methane. Moreover, a moderate or high energy barrier for the M- + CH4 → HMCH3- reaction step is observed, which protects the metal center from deactivation. Future work should focus on the identification of proper ligands, which stabilize the negative charge on the metal (electronic factors) and prevent the formation of the global CH3MOH- minimum (steric factors). Finally, a composite electronic structure method (combining size extensive coupled clusters approaches and accurate multireference configuration interaction) is proposed for computationally demanding systems and is applied to Fe-.

Exploring the chemical kinetics on oxygen addition reactions of o-xylyl radical at the low temperature

o-Xylene oxidation displays an autoignition behavior similar to alkanes at low temperatures. This paper presents a detailed investigation of the chemical kinetics of oxygen additions with the o-xylyl radical that control the ignition reactivity of o-xylene at low temperatures. High-level electronic structure calculations, transition state theory, and master equation simulations are combined to predict the rate coefficients of main elementary reactions. For the initially formed o-methylbenzylperoxy (ROO) in the first oxygen addition o-xylyl + O2, its isomerization to o-hydroperoxymethyl-benzyl (QOOH) proceeds with a much smaller branching ratio compared to the counterpart ROO → QOOH in alkanes. Despite the slow formation of QOOH, the absence of fast dissociation pathways enable QOOH concentration to build up. QOOH that has the unpaired electron located on a side-chain carbon can readily react with a second molecular oxygen. The QOOH + O2 reaction then efficiently leads to the growth of the radical pool through a highly chain-branching reaction sequence QOOH + O2 → 2-hydroperoxymethyl-benzaldehyde + OH → 1,2-diformylbenzene + 2OH + H. The predicted oxygen addition kinetics offers a good explanation for the alkane-like autoignition behavior of o-xylene. Meanwhile, as the key intermediate to chain branching, the lower yield of QOOH results in its lower ignition reactivity. The present study shows that classical low temperature scheme is also valid for benzylic-type hydrogens and radicals of o-xylene where the transferred hydrogen from the ortho-methyl chain facilitates the isomerization ROO ↔ QOOH. It is also easy to deduce that for m- and p-xylenes, where no such isomerization step is available, little reactivity should be expected at the low temperature.

Local energy decomposition analysis and molecular properties of encapsulated methane in fullerene (CH4@C60)

High-level electronic structure calculations provide quantitative insights into noncovalent interactions in endohedral complexes of fullerene C60 (X@C60; X = He, He2, Ne, Ar, Kr, CH4).

Position of the Benzene Ring Substituent Regulates the Excited-State Deactivation Process of the Benzyluracil Systems.

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S2 → S1 → S0 two-step decay pattern, and the decay coordinates of the S2 → S1 and S1 → S0 processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S1 → S0 decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Be-Be Bond in Action: Lessons from the Beryllium-Ammonia Complexes [Be(NH3)0-4]20,2.

High-level electronic structure calculations are performed to elucidate the Be-Be chemical bond in the (NH3)nBe-Be(NH3)n species for n = 0-4. We show that the Be2 bond is explained as a resonance between two Lewis structures, where one beryllium atom donates an electron pair to the second one, and vice versa. The presence of ammonia ligands enhances the stability of this bond considerably. The ∼2.5 kcal/mol binding energy of Be2 becomes ∼30 kcal/mol for [Be(NH3)1-3]2 because of their more polarizable electron pairs. The larger Be(NH3)4 complex has been classified as a solvated electron precursor in the past and has an electron pair in the periphery of a Be(NH3)42+ core occupying a diffuse s-type orbital. The analogy of Be(NH3)4 to Be reflects into the electronic structure of their dimers. The two systems have identical bonding patterns and low-lying electronic states. The ground state binding energy of [Be(NH3)4]2 is 3 times larger than Be2, and its excitation energies are considerably lower by a factor of 3. We also studied the dimers of the cationic Be(NH3)n+ species, and we found that the Coulombic repulsion is counterbalanced by the formation of a single covalent bond in the cases of n = 1, 2 forming stable dicationic [(NH3)nBe-Be(NH3)n]2+ systems, unlike Be22+. We believe that our numerical results will allow the identification and characterization of these exotic species and their solid state (beryllium liquid metals) analogues in future experiments.

Directed Gas Phase Formation of Silene (H2 SiCH2 ).

The silene molecule (H2 SiCH2 ; X1 A1 ) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH4 ). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH2 SiH3 ; X2 A') complex followed by hydrogen migration to the methylsilyl radical (SiH2 CH3 ; X2 A'). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H2 SiCH2 ; X1 A1 ). The exploration of the elementary reaction of methylidyne with silane delivers a unique view at the widely uncharted reaction dynamics and isomerization processes of the carbon-silicon system in the gas phase, which are noticeably different from those of the isovalent carbon system thus contributing to our knowledge on carbon silicon bond couplings at the molecular level.

Benchmark studies on protonated benzene (BZH+) and water (Wn, n = 1–6) clusters: a comparison of hybrid DFT with MP2/CBS and CCSD(T)/CBS methods

The selection of a suitable density functional theory (DFT) method is critical to study the interfacial interactions between the protonated species and water clusters at the carbon surface. The interfacial interactions are crucial for the stability of complexes with the support of various kinds of noncovalent interactions. To model this environment, we consider excess proton with benzene [i.e., protonated benzene (BZH+)] and water clusters using high-level electronic structure calculations. These clusters were stabilized by different kinds of noncovalent interactions at the interface. Modeling these clusters is challenging as high-level electronic structure calculations are expensive, whereas less expensive DFT-based methods are inconsistent. Thus, in the present study, we have chosen various hybrid DFT functionals (such as B3LYP, PBE0, M05-2X, M06-2X, and M11) to study the geometries, energetics, and the importance of interfacial interactions. Furthermore, these methods performance is validated by using MP2/CBS and CCSD(T)/CBS approaches. It is found that the selected functionals are reliable to predict the structure, but the energetics of these clusters are varied. Also, each one of these methods has its own advantage in certain aspects. Scrutiny of result reveals that hybrid GGA (PBE0) and hybrid meta-GGA (M11) functionals are more consistent for the structure and stability with the benchmark obtained from MP2/CBS and CCSD(T)/CBS approaches. Besides, our benchmark report states that the selected DFT method can be suitable for the following individual interactions; (1) PBE0 suited for O–H+···O, O–H+···π and C–H+···O interactions, (2) M05-2X more suited for O–H···π, C–H···O, and (3) B3LYP method highly favor for the O–H+···O interactions (i.e., proton transfer). Overall, PBE0/aVTZ method has an excellent correlation with MP2/CBS and CCSD(T)/CBS methods based on our analysis using the mean signed error and correlation coefficient (r) analyses.

Permutationally Invariant, Reproducing Kernel-Based Potential Energy Surfaces for Polyatomic Molecules: From Formaldehyde to Acetone

Constructing accurate, high-dimensional molecular potential energy surfaces (PESs) for polyatomic molecules is challenging. Reproducing kernel Hilbert space (RKHS) interpolation is an efficient way to construct such PESs. However, RKHS interpolation is computationally most effective when the input energies are available on a regular grid. Thus, the number of reference energies required can become very large even for pentaatomic systems making such an approach computationally prohibitive when using high-level electronic structure calculations. Here, an efficient and robust scheme is presented to overcome these limitations and is applied to constructing high-dimensional PESs for systems with up to 10 atoms. Using energies as well as gradients reduces the number of input data required and thus keeps the number of coefficients at a manageable size. The correct implementation of permutational symmetry in the kernel products is tested and explicitly demonstrated for the highly symmetric CH4 molecule.

New insight into the photoinduced wavelength dependent decay mechanisms of the ferulic acid system on the excited states.

The ferulic acid (FA) is a kind of phenolic acid widely exists in nature plants. Apart from its medicinal values, the FA is also widely applied in cosmetic industry. Recently, it was found to have potential applications in commercial sunscreens for its strong photostability and photoprotection property from harmful UV rays. Such excellent property lies in the ultrafast decay process of the FA system when exposure to the UV light, but the underlying detailed relaxation pathway is still less clear-cut. In the current work, high-level ab initio electronic structure calculations and on-the-fly surface hopping dynamics simulations were employed to explore the photoinduced decay mechanism of the FA system both on the S1 and S3 states in the gas phase. The results provide a reasonable explanation for the wavelength dependent decay patterns of FA system. The S1 state decay pathway is driven by a re-emission process to dissipate excess energy. While for the S3 state deactivation process, the pathway is dominated by a non-adiabatic process driven by the internal conversion process through the conical intersection regions. A S3-S1-S0 two step decay pattern is proposed, and the pathways are mainly driven by a puckering distortion motion of the aromatic ring and a twisting motion around the bridging double bond. The calculation results contribute to a better understanding of detailed dynamics behavior of the FA deactivation process, and provide theoretical guidance for further design of efficient and environmentally friendly sunscreens.