A new global reaction route map on the potential energy surface of H2CO with unrestricted level

One-to-One Correspondence between Reaction Pathways and Reactive Orbitals

The one-to-one correspondence between reaction pathways in the potential energy theory and reactive orbitals in the electronic theory for reactions is presented. In this study, the reactive orbital energy method is applied to the intrinsic reaction coordinates of the global reaction route map generated by an automated reaction path search method. The reactive orbital energy method specifies the pairs of occupied and unoccupied reactive orbitals driving chemical reactions and determines whether the reactions are electron transfer-driven or dynamics-driven. Surprisingly, it is found that the reactive orbital pairs are determined one by one for the electron transfer-driven reaction pathways from an identical molecule. The reactive orbital energy method is also found to provide the sophisticated interpretations of reactions for the electronic motions. This one-to-one correspondence is expected to trigger the unification of the potential energy theory and the electronic theory for reactions that have been independently developed.

High Temperature Accelerated Stone-Wales Transformation and the Threshold Temperature of IPR-C60 Formation

The Stone-Wales bond rotation isomerization of nonicosahedral C₆₀ (C_2v-C₆₀) into isolated-pentagon rule following icosahedral C₆₀ (I_h-C₆₀ or IPR-C₆₀) is a limiting step in the synthesis of I_h-C₆₀. However, extensive previous studies indicate that the potential energy barrier of the Stone-Wales bond rotation is between 6 and 8 eV, extremely high to allow for bond rotation at the temperatures used to produce fullerenes conventionally. This is also despite data indicating a possible fullerene road mechanism that necessitates low-temperature annealing. However, these previous investigations often have limiting factors, such as using the harmonic approximation to determine free energies at high temperatures or considering only the reverse I_h-C₆₀ to C_2v-C₆₀ transition as a basis. Indeed, when the difference in energy between I_h-C₆₀ and C_2v-C₆₀ is accounted for, this barrier is generally reduced by ∼1.5 eV. Thus, utilizing the recently developed density functional tight binding metadynamics (DFTB-MTD) interface, the effects of temperature on the bond rotation in the conversion of C_2v-C₆₀ to I_h-C₆₀ have been investigated. We found that Stone-Wales bond rotations are complex processes with both in-plane and out-of-plane transition states, and which transition path dominates depends on temperature. Our results clearly show that at temperatures of 2000 K, the free energy for a C_2v-C₆₀ to I_h-C₆₀ transition is only ∼4.21 eV and further reduces to ∼3.77 eV at 3000 K. This translates to transition times of ∼971 μs at 2000 K and ∼34 ns at 3000 K, indicating that defect healing is a fast process at temperatures typical of arc jet or laser ablation experiments. Conversely, below ∼2000 K, bond rotation becomes prohibitively slow, putting a lower threshold limit on the temperature of fullerene formation and subsequent annealing.

High performance global exploration of isomers and isomerization channels on quantum chemical potential energy surface of H5 C2 NO2

High performance global exploration of isomers and isomerization channels on the quantum chemical potential energy surface (PES) is performed for H₅ C₂ NO₂ by using the scaled hypersphere search-anharmonic downward distortion following (SHS-ADDF) method. A multi-node operation, NeoGRRM, has achieved high performance exploration calculations for the large system by submitting SHS-ADDF sub-jobs into many cores in parallel and unifying the results of sub-jobs into the total lists of the main-job. Global exploration of equilibrium (EQ) and transition-state structures at the level of B3LYP/6-31G(d) gave 3210 EQs and 23278 TSs. Nine compounds were found in the low energy regions of 0-100 kJ/mol; the lowest energy compound is N-methylcarbamic acid, the second is methyl carbamate, and the third is glycine (the most fundamental amino acid). Interconversion pathways between the conformers of each of the low energy compounds were surveyed. Isomerization channels around glycine were explored in detail. The lowest energy barriers around some of the EQs turned to be negative after zero-point energy corrections. This indicates that those structures cannot exist as independent structures because they spontaneously collapse into more stable structures. The global PES search showed various interesting dissociating channels which indicate synthon reaction pathways in the reverse directions.

Geometric analysis of anharmonic downward distortion following paths

A mathematical aspect of the anharmonic downward distortion following (ADDF) path is discussed. The ADDF method is utilized as an automated reaction path search method, which can explore transition state geometries on a potential energy surface from a potential minimum. We show that the maximum number of the ADD stationary paths intersecting the potential minimum is 2^f + 1  - 2, where f denotes the degree of freedom of the system. We also show that the bifurcation of the ADD stationary path is essential to detect all the transition states connected from a given minimum. The ADDF computation is demonstrated for a H₂ O molecule in which pitchfork bifurcation is observed.

Exploration of Carbon Allotropes with Four-membered Ring Structures on Quantum Chemical Potential Energy Surfaces

The existence of a new carbon allotrope family with four-membered rings as a key unit has been recently predicted with quantum chemical calculations. This family includes carbon allotropes in prism-, polymerized prism-, sheet-, tube-, and wavy-forms. An atypical bond property has been observed in this series of carbon structures, which differs from the typical sp³ , sp² , and sp hybridizations. The lowest energy barrier from some of the equilibrium states of the carbon structures has been determined with the SHS-ADDF (scaled-hypersphere-search combined with the anharmonic downward distortion following) method within the GRRM software program package. The height of the barriers indicates that the well is deep enough for the carbon structures to exist. This class of carbon allotropes is expected to be energy-reservoirs with extra energy of 100-350 kJ mol^-1 per one carbon atom. This article presents the structures, energies and reactivity of the carbon allotropes with four-membered ring structures as well as the background of the findings in the context of the global exploration of potential energy surfaces. © 2018 Wiley Periodicals, Inc.

TGMin: An efficient global minimum searching program for free and surface-supported clusters

In this article, we introduce an efficient global-minimum structural search program named Tsinghua Global Minimum 2 (TGMin-2), which is the successor of the original TGMin algorithm that was developed in our group in 2011. We have introduced a number of new features and improvements into TGMin-2, including a symmetric structure generation algorithm that can produce good initial seeds for small- and medium-size clusters, the duplicated structure identification algorithm, and the improved structure adaption algorithm that was implemented in the original TGMin code. To predict the simulated photoelectron spectrum (PE spectrum) automatically, we also implemented a standalone program named AutoPES (Auto Photoelectron Spectroscopy), which can be used to simulate PE spectra and compare them with experimental results automatically. We have demonstrated that TGMin-2 and AutoPES are powerful tools for studying free and surface-supported molecules, clusters, and nanoclusters. © 2018 Wiley Periodicals, Inc.

Quantum chemical exploration of formaldehyde clusters (H2 CO)n (n = 2-4)

Global exploration of equilibrium structures and interconversion pathways on the quantum chemical potential energy surface (PES) is performed for (H₂ CO)n (n = 2-4) by using the Scaled Hypersphere Search-Anharmonic Downward Distortion Following (SHS-ADDF) method. Density functional theoretical (DFT) calculations with empirical dispersion corrections (D3) yielded comparable results for formaldehyde dimer in comparison with recent detailed studies at CCSD(T) levels. Based on DFT-D3 calculations, trimer and tetramer structures and their stabilities were studied. For tetramer, a highly symmetrical S₄ structure was found as the most stable form in good accordance with experimentally determined tetramer unit in the formaldehyde crystal. © 2018 Wiley Periodicals, Inc.

Conflict in the Mechanism and Kinetics of the Barrierless Reaction between SH and NO2 Radicals

There are some unsettled issues regarding the mechanism and kinetics of an important atmospheric reaction of NO₂ radical with the SH radical. The existing mechanism is based on the formation of HSO and NO radicals, both of which can result only along one barrierless channel. However, the detection of NO radical has never been reported though the formation of HSO radical has been followed in some studies to determine the rate constants. The latter are mainly obtained by monitoring the SH decay, but rate constants are reported to be highly conflicting among the existing studies reporting its value ranging from 10^-10 to 10^-12 cm³ molecule^-1 sec^-1. The present work attempts to resolve these issues by exploring various reaction pathways through the global reaction route mapping of the potential energy surface at the level of spin-unrestricted and spin-restricted coupled-cluster and density functional theories. The initial association of two radicals was found to proceed via two barrierless modes: (1) S-N association leading to HSNO₂ and, (2) S-O association resulting in HSONO, in particular the cis-isomer. The kinetics of the barrierless pathways was investigated through rate constants computed using canonical variational transition state theory (CVTST) along with their temperature and pressure dependence investigated using the master equation. The rate constants calculated using spin-unrestricted methods are found to be in agreement with experimentally observed range of rate constant, and the formation of cis-HSONO (via mode 2) is observed to be the main contributing channel. Contrary to the results of spin-restricted calculations, the barrierless channel (mode 1) leading to the formation of HSNO₂ is predicted to involve two bottlenecks when results using spin-unrestricted calculations were analyzed. Notably, the spin-unrestricted calculations predict a prereaction complex for the formation of S-N bond (via mode 1) which has been treated using Miller's unified transition state theory with a two transition state model. The fate of all the species involved in the reaction is critically evaluated in the present work, and the predictions made can be a subject of further experimental and theoretical studies involving radical-radical reactions.

Study of Potential Energy Surfaces towards Global Reaction Route Mapping

The potential energy surface (PES) is just a theoretical construct based on the Born-Oppenheimer approximation, but it underlies various phenomena, including molecular vibrations, collisional ionizations, and chemical reactions. This account describes how a new idea for global reaction route mapping (GRRM), which had seemed to be impossible for chemical systems with more than three atoms, was born and has been developed during the course of the study of the PES. GRRM has pioneered new fields of chemistry. Furthermore, techniques for GRRM are still developing, and GRRM is further extending its application to various areas of chemistry and chemical physics.

Density Functional Exploration of C4H3N Isomers

Molecules having C4H3N stoichiometry are of astrophysical interest. Two of these, methylcyanoacetylene (CH3C3N) and its structural isomer allenyl cyanide (H2CCCHN), have been observed in interstellar space, while several more have been examined in laboratories. Here we describe, for a broad range of C4H3N isomers, density functional calculations (B3LYP/aug-cc-pVTZ) of molecular parameters including the energetics, geometries, rotational constants, electric dipole moments, polarizabilities, vibrational IR frequencies, IR absorption intensities, and Raman activities. Singlet-triplet splittings as well as singlet vertical electronic excitation energies are given for selected species. The identification of less stable C4H3N molecules, generated in ongoing spectroscopic experiments, relies heavily on these quantum chemical predictions.

An automated transition state search using classical trajectories initialized at multiple minima

TS finding using iterative TSSCDS with trajectories initialized at different minima.

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

In theoretical studies of chemical reactions involving multiple potential energy surfaces (PESs) such as photochemical reactions, seams of intersection among the PESs often complicate the analysis. In this paper, we review our recipe for exploring multiple PESs by using an automated reaction path search method which has previously been applied to single PESs. Although any such methods for single PESs can be employed in the recipe, the global reaction route mapping (GRRM) method was employed in this study. By combining GRRM with the proposed recipe, all critical regions, that is, transition states, conical intersections, intersection seams, and local minima, associated with multiple PESs, can be explored automatically. As illustrative examples, applications to photochemistry of formaldehyde and acetone are described. In these examples as well as in recent applications to other systems, the present approach led to discovery of many unexpected nonadiabatic pathways, by which some complicated experimental data have been explained very clearly.