Synthesis and structure of an asymmetric copper(I) dimer with two-coordinate and four-coordinate copper(I) sites

Journal ArticleCopper(I) coordination compounds have been studied intensively due to their biological relevance, catalytic properties, and theoretical interests. Numerous mononuclear and polynuclear copper(I) complexes with different coordination numbers have been prepared and characterized. Recently the possible d10-d10 interactions in copper(I) dimers have been the subjects of many experimental and theoretical studies. Among the dimers and even polynuclear complexes, asymmetric ones are rare, especially when the compounds are synthesized from a single copper(I) source. Because of their potential of being selective catalysts and biomimetic models, asymmetric polynuclear complexes are of great interest, and several examples have been reported.3 An example is CuI 2Cl2(PPh3)3 (1), which has both three- and four-coordinate copper(I).4 Herein, we report an unusual asymmetric copper(I) dimer with both 2- and 4-coordinate copper(I) sites (2a and its most important resonance form 2b). 2 is an unusual homobimetallic complex with two isolated donor sets of different coordinate numbers,3d and more importantly it was prepared in a way to not enforce asymmetry, i.e., it self assembles. Hence, asymmetric 2 appears to be more stable than symmetric 3, while our calculations of those of others5 show that 3 is a possible stable structure

On the existence of long C-C bonds between pairs of anions which repel: when and why? A test case on the [TCNE]22- dimers found in ionic crystals*

Journal ArticleMany of the Cm[TCNE]n (C = cation) salts have intradimer C-C interactions in the range of 2.9 to 3.5 A(°) and show the electronic fingerprints associated with C-C bond formation (IR and UV spectra, magnetic properties, structural changes), despite the fact that two [TCNE].- anions should repel each other due to purely Coulombic considerations. Herein, we analyze these pairwise interactions in detail for a particular crystal in which such a C-C bond is found, to understand when and why these bonds are formed in a general case

Origin of bistability in the butyl-substituted spirobiphenalenyl-based neutral radical material

One of the most remarkable bistable materials reported so far is made of pi dimers of a butyl-substituted spirobiphenalenyl boron radical (butyl-SBP). The phase transition of this material, which is accompanied by changes in its optical, conductive, and magnetic properties, occurs with a hysteretic loop 25 K wide centered at 335 K. Herein, a computational study is presented aimed at deciphering the origin of this hysteresis. The phase transition of butyl-SBP consists of a spin transition of the constituent pi dimers coupled with an order-disorder transition involving the butyl chains linked to the nitrogen atoms of the superimposed phenalenyl rings of the pi dimer. Below 335 K, the terminal methyl group of the butyl chains adopts a gauche conformation with respect to the methylene unit bonded to the nitrogen atom. Above 335 K, the methyl group is in an anti conformation and exhibits dynamic disorder. The gauche -> anti conformational rearrangement triggers the spin transition of the pi dimers and is responsible for the hysteretic behavior of butyl-SBP. Specifically, the onset of the phase transition in the heating mode, and thus, the width of the hysteresis loop, are governed by the high energy cost and strong structural cooperative effects associated with this conformational change. Our results show that coupling a spin switch with a conformational switch in a molecular crystal provides a promising strategy in the design of new bistable materials

An ab initio analytical potential energy surface for the O(3P) + CS(X1Σ+) → CO(X1Σ+) + S(3P) reaction useful for kinetic and dynamical studies

The N(4Su) + NO(X 2Π) → N 2(X 1Σg+) + O( 3Pg) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie-Murrell expression for the lowest 3A″ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. Moreover, thermal rate constants derived from trajectories are in excellent accordance with experimental value

An analytical representation of the ground potential energy surface (2A') of the H + Cl2 → HCl + Cl and Cl + HCl → HCl + Cl reactions, based on ab initio calculations

In this work we have studied at an ab initio level the lowest 2A′ potential energy surface (PES) of the HCl2 system. This PES is involved in the H(2S)+Cl2(X 1Σ+g)→HCl(X 1Σ+)+Cl(2P) and Cl(2P)+HCl(X 1Σ+)→HCl(X 1Σ+)+Cl(2P) gas phase elementary chemical reactions. The former reaction is an important chemical laser while the second one is the most frequently used prototype of heavy-light-heavy reaction. A large number of points on the 2A′ PES have been calculated at the PUMP2/6-311G(3d2 f,3p2d) ab initio level. The ab initio calculations show the existence of two angular transition states with negligible or very small barriers to collinearity. This and other properties of the PES are in agreement with previous studies. An analytical expression based on a many-body expansion has been used to obtain a satisfactory fit of the 740 ab initio points calculated, with a root-mean-square deviation within the range of the estimated ab initio method error margin. This analytical representation of the 2A′ PES has been used to evaluate the variational transition state theory thermal rate constants of the above-mentioned reactions, including also the Cl+DCl reaction, and quite good agreement has been obtained when comparing with experimental results. The analytical PES obtained in this work is suitable for use in studies on the kinetics and dynamics of the HCl2 system

Design and implementation of an ultrasonic sensor for rapid monitoring of industrial malolactic fermentation of wines

Ultrasound is an emerging technology that can be applied to monitor food processes. However, ultrasonic techniques are usually limited to research activities within a laboratory environment and they are not extensively used in industrial processes. The aim of this paper is to describe a novel ultrasonic sensor designed to monitor physical–chemical changes that occur in wines stored in industrial tanks. Essentially, the sensor consists of an ultrasonic transducer in contact with a buffer rod, mounted inside a stainless steel tube section. This structure allows the ultrasonic sensor to be directly installed in stainless steel tanks of an industrial plant. The operating principle of this design is based on the measurement of ultrasonic velocity of propagation. To test its proper operation, the sensor has been used to measure changes of concentration in aqueous samples and to monitor the progress of a malolactic fermentation of red wines in various commercial wineries. Results show the feasibility of using this sensor for monitoring malolactic fermentations in red wines placed in industrial tanks.Postprint (author's final draft

Towards the tailored design of benzotriazinyl-based organic radicals displaying a spin transition

The mechanism of the phase transition of 1-phenyl-3-trifluoromethyl- 1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (1), the first reported triazinyl radical to present such a feature, is unveiled. In so doing, we identify the key ingredients that are crucial to enable the phase transition in this family of radicals, and how those can be exploited by a rational design of the spin-carrying units

Tracing the sources of the different magnetic behavior in the two phases of the bistable (BDTA)2[Co(mnt)2] compound

A complete computational study of the magnetic properties of the two known phases of the bistable (BDTA)2[Co(mnt)2] compound is presented. The origin of their different magnetic properties can be traced to a variation in the values of the g tensor, together with a hitherto unknown change in the JAB values and their magnetic topology

Pitfalls on evaluating pair exchange interactions for modelling molecule-based magnetism

Molecule-based magnetism is a solid-state property that results from the microscopic interaction between magnetic centres or radicals. The observed magnetic response is due to unpaired electrons whose coupling leads to a particular magnetic topology. Therefore, to understand the magnetic response of a given molecule-based magnet and reproduce the available experimental magnetic properties by means of statistical mechanics, one has to be able to determine the value of the J(AB) magnetic exchange coupling between radicals. The calculation of J(AB) is thus a key point for modelling molecule-based magnetism. In this Perspectives article, we will build upon our experience in modelling molecular magnetism to point out some pitfalls on evaluating J(AB) couplings. Special attention must be paid to the cluster models used to evaluate J(AB), which should account for cooperative effects among J(AB) interactions and also consider the environment (counterions, hydrogen bonding) of the two radicals whose interaction has to be evaluated. It will be also necessary to assess whether a DFT-based or a wavefunction-based method is best to study a given radical. Finally, in addition to model and method, the J(AB) couplings have to be able to adapt to changes in the magnetic topology due to thermal fluctuations. Therefore, it is most important to appraise in which systems molecular dynamics simulations would be required. Given the large number of issues one must tackle when choosing the correct model and method to evaluate J(AB) interactions for modelling magnetic properties in molecule-based materials, the "human factor" is a must to cross-examine and challenge computations before trusting any result.MD, JRA, and JJN acknowledge financial support from MINECO (CTQ2017-87773-P/AEI/FEDER, UE), Spanish Structures Excellence Maria de Maeztu program (MDM-2017-0767), and Catalan DURSI (2017SGR348)

Formation of long, multicenter pi-[TCNE](2)(2-) dimers in solution: solvation and stability assessed through molecular dynamics simulations

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these pi-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of pi-[TCNE](2)(2-) (TCNE = tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an inter-planar separation of approximately 3.0 angstrom. The pi-[TCNE](2)(2-) dimers form dissociated ion pairs with the NBu4+ counterions, and their first solvation shell comprises approximately 20 CH2Cl2 molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C-H center dot center dot center dot N contacts. The calculated free energy of dimerization of TCNE center dot- in solution at 175 K is -5.5 kcal mol(-1). These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process