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Catalytic Reactions on Solid Surfaces
Published in Qingmin Ji, Harald Fuchs, Soft Matters for Catalysts, 2019
Huihui Kong, Xinbang Liu, Harald Fuchs
On-surface Bergman cyclization could not only be used for construction of one-dimensional conjugated nanostructures, but also for in situ synthesis of novel organic molecules. In 2013, de Oteryza et al. investigated Bergman reaction of individual oligo-(phenylene-1,2-ethynylenes) on Ag(100) by heating. By using non-contact AFM, the molecular morphologies of several different complex products were characterized in atiomic resolution as shown in Fig. 7.14a. Moreover, based on DFT calculations, they further demonstrated the possible surface reaction mechanisms and the reaction pathways [34]. Besides heating, Schuler et al. found that Bergman reaction could be realized by atomic manipulation [35], which is verified by high-resolution, non-contact AFM. They revealed that an individual aromatic diradical could convert into a highly strained 10-membered diyne; moreover, the 10-membered diyne could transform back to the diradical form as shown in Fig. 7.14b–c. Because of the different physicochemical properties between the diradical and the diyne, the reversibility of the two reactive intermediates may open up the field of radical chemistry for on-surface reactions by atomic manipulation.
Sulfur Carriers
Published in Leslie R. Rudnick, Lubricant Additives, 2017
Thomas Rossrucker, Sandra Horstmann, Achim Fessenbecker
For the black sulfurization, a radical mechanism is postulated. The first reaction step is the ring opening of the sulfur flower (S8-ring structure) by forming a diradical (chain initiation, Equation 9.1). In the second step, the sulfur diradical abstracts an allylic hydrogen atom from the unsaturated organic compound (Equation 9.2). Thereby, a new radical is formed and the growth reaction is started. The addition of a sulfur ring on the organic radical is another growth reaction step (Equation 9.3). The organic radical polysulfide can react with another organic radical polysulfide by forming a sulfur-bridged compound (Equation 9.4). This is one possible termination reaction. Besides the termination reaction mentioned earlier, also other known radical termination reactions like allylic hydrogen abstraction or disproportionation can happen. Due to the fact that these termination reactions can occur on both allylic carbon atoms, higher sulfur-bridged compounds could be formed.
Double electron-attachment equation-of-motion coupled-cluster methods with up to 4-particle–2-hole excitations: improved implementation and application to singlet–triplet gaps in ortho-, meta-, and para-benzyne isomers
Published in Molecular Physics, 2021
The equation-of-motion (EOM) [1–4] and linear response [5–10] extensions of the single-reference coupled-cluster (CC) theory [11–15] are nowadays widely used to study excited electronic states of molecular systems [16]. One of the interesting features of the EOMCC formalism is the possibility to extend it to electronic spectra of open-shell species that can formally be obtained by adding electrons to or removing electrons from the parent closed-shell cores [17–44] (an operation generating the appropriate multi-configurational reference space within a single-reference framework, while relaxing the remaining electrons). In particular, the double electron-attachment (DEA) and double ionisation potential (DIP) EOMCC approaches have been developed [32–43] as elegant and physically appealing ways of determining singlet and triplet manifolds of diradical systems. Being formally single reference, the DEA- and DIP-EOMCC methods are conceptually simpler and easier to use than genuine multi-reference approaches [45–47]. At the same time, they offer several advantages over the conventional particle-conserving CC/EOMCC treatments that rely on the spin-integrated, spin-orbital formulation employing unrestricted or restricted open-shell reference determinants, such as rigorous spin and symmetry adaptation of the calculated states and the ability of handling high-spin and low-spin states in an accurate and balanced manner.
Carbon-13 studies of sulphur-terminated carbon chains: chemical bonding, molecular structures, and formation pathways
Published in Molecular Physics, 2021
Michael C. McCarthy, Kin Long Kelvin Lee, Jessica P. Porterfield, P. Bryan Changala, André K. Eckhardt
Small carbon–sulphur molecules are also of fundamental interest. Relative to oxygen, sulphur differs in three important respects: it is substantially less electronegative (2.58 and 3.44, respectively), its atomic radius is about 60% larger (1.00 Å vs. 0.60 Å; Ref. [22]), and it possesses a five times larger atomic spin-orbit coupling constant (288 vs. 57 cm; Ref. [23]). For these reasons, substitution of O with S can affect the electronic structure and nuclear potential, and demonstratively alter the structure and chemical bonding in the ground state equilibrium configuration. Such differences may be manifest in the heavy atom bonding, and, for radicals, the distribution of the unpaired electron along the chain. A vivid example is provided by the shorter cumulenones and cumulenethiones. Although and are known to have kinked heavy atom backbones [24–26], substitution of the oxygen atom with sulphur instead yields structures possessing linear heavy atom backbones and symmetries [27,28]. This difference is thought to arise from electron correlation effects, whereby the energy difference between singlet and triplet states of the diradical fragment decreases significantly in the sulphur versus oxygen variant so as to stabilise the linear geometry instead of a kinked one [29].
Theory of chemical bonds in metalloenzymes XXI. Possible mechanisms of water oxidation in oxygen evolving complex of photosystem II
Published in Molecular Physics, 2018
Kizashi Yamaguchi, Mitsuo Shoji, Hiroshi Isobe, Shusuke Yamanaka, Takashi Kawakami, Satoru Yamada, Michio Katouda, Takahito Nakajima
Transition-metal oxo species play important roles for various oxygenation reactions [45–47,55–57]. In this section, we briefly revisit basic notions for metal-oxo bonds of SCES in Figure S1. As shown in our series of papers [45–47,58–62], the metal-diradical character (•M–O•) of the high-valence transition-metal oxo (M=O) bonds is one of the key concepts for theoretical understanding of chemical reactivity of several metalloenzymes such as p450 [46,55–57]. In OEC of PSII (see Scheme 5 later) [32], the nature of the high-valence manganese oxo bonds (Mn=O) is closely related to possible roles of the O(5) and O(4) sites, namely Mn=O(5) and Mn=O(4), [33] for the catalytic function of the CaMn4O5 cluster. Historically, the transition-metal oxo (MO) bonds have been regarded as M2+O2−, indicating that the oxygen site is oxygen dianion (O2−) responsible for the nucleophilic reactivity [63]. In early 1980s, the high-valent transition-metal oxo bonds were found to exhibit the radical or electrophilic reactivity [64,65]. Discovery of the unexpected radical reactivity was a topic at the Hawaii Pacific Chemical Conference in 1984. Therefore, we have extended BS MO theoretical methods of diradical species [66] to high-valent transition-metal oxo species M=O (M = Ti,V, Cr, Mn, Fe, Ni, Cu) to elucidate the nature of their dσ-pσ and dπ-pπ bonds [45,46,67] with metal-diradical character (•M–O•).