Interchange Of Atoms Research Articles

Isomorphic heteromagnetism of an Fe2MnGa alloy in a face-centered cubic structure

The magnetic properties of isomorphic heterogeneous γ-Fe2MnGa alloys were investigated by means of experiments and calculations. Fe2MnGa alloys subject to rapid cooling and annealing treatment show the same face-centered cubic structure at room temperature, but significantly different magnetic properties: the molecular magnetic moments are 2.11 and 4.49 μB, while the Curie temperatures are 500 and 785 K. By combining the experimental and theoretical calculation results, it can be determined that disordered atomic occupation results in magnetic diversity in the same matrix structure. Rapid solidification introduces dislocated atoms, speculated to be due to partial interchange of Mn and Fe atoms with Ga atoms, which leads to an antiferromagnetic phase, while annealing can improve the degree of atomic order and transform the antiferromagnetic into a ferromagnetic phase. The change in the atomic occupation also leads to variation of the coercivity and a magnetic phase transition.

Inferring thermospheric composition from ionogram profiles: a calibration with the TIMED spacecraft

Abstract. We present a method for augmenting spacecraft measurements of thermospheric composition with quantitative estimates of daytime thermospheric composition below 200 km, inferred from ionospheric data, for which there is a global network of ground-based stations. Measurements of thermospheric composition via ground-based instrumentation are challenging to make, and so details about this important region of the upper atmosphere are currently sparse. The visibility of the F1 peak in ionospheric soundings from ground-based instrumentation is a sensitive function of thermospheric composition. The ionospheric profile in the transition region between F1 and F2 peaks can be expressed by the “G” factor, a function of ion production rate and loss rates via ion–atom interchange reactions and dissociative recombination of molecular ions. This in turn can be expressed as the square of the ratio of ions lost via these processes. We compare estimates of the G factor obtained from ionograms recorded at Kwajalein (9∘ N, 167.2∘ E) for 25 times during which the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) spacecraft recorded approximately co-located measurements of the neutral thermosphere. We find a linear relationship between G and the molecular-to-atomic composition ratio, with a gradient of 2.55±0.40. Alternatively, using hmF1 values obtained by ionogram inversion, this gradient was found to be 4.75±0.4. Further, accounting for equal ionisation in molecular and atomic species yielded a gradient of 4.20±0.8. This relationship has potential for using ground-based ionospheric measurements to infer quantitative variations in the composition of the neutral thermosphere via a relatively simple model. This has applications in understanding long-term change and the efficacy of the upper atmosphere on satellite drag.

Atomic-scale investigation on the interface structure of {2[formula omitted]01} α2-Ti3Al deformation twins in polysynthetically twinned TiAl single crystals

The interface structure of {22‾01} α2-Ti3Al deformation twins formed during tensile deformation of polysynthetically twinned (PST) TiAl single crystals at 900 °C has been studied systematically by aberration-corrected scanning transmission electron microscopy. Numerous superlattice intrinsic stacking faults (SISFs) are formed on the (0001) basal plane of α2 twins, thereby resulting in the formation of steps on the twin boundaries. The height of steps equals to the spacing of the (22‾01) twinning plane. The basal-plane SISFs formed in α2 twins can't be attributed to the glide of Shockley superpartial dislocations. Lattice correspondence analyses based on classical twinning theory reveal that complex atomic shuffle and interchange shuffling are responsible for the formation of twinning and basal-plane SISFs in α2 twins. In addition, basal-plane antiphase boundaries (APB) are formed at the vicinity of α2 twin tips to relieve the stress concentration imposed by misfit dislocations.

Structural and electrical properties of Mg-doped vanadium dioxide thin films via room-temperature ion implantation

We report on the structural and electrical properties of Mg ion implanted semiconductor-metal transition of vanadium dioxide thin films with three different ion fluences. The effect of introducing Mg implants showed interchange, rearrangement, and modification of atoms in the lattice structure of VO2. The (011) monoclinic signature of VO2 is significantly affected; (i) peak broadening and shift of the peak position to lower angles at 1 × 1015 ions/cm2, (ii) revert and shift towards high angles at 1 × 1016 ions/cm2 and (iii) ceasing to exist at 1 × 1017 ions/cm2 suggesting a complete devolution of the VO2 thin films at higher fluence. Composition and profiling of the V-O matrix revealed a corresponding decrease of the vanadium edge peak at 1 × 1017 ions/cm2 and a decrease of the oxygen content in implanted films. Whilst the carrier concentration is increased, the magnitude of the semiconductor-metal transition of implanted VO2 thin films is significantly affected with a decrease of career mobility. Whilst increasing ion fluence above 1 × 1017 ions/cm2 at a depth of 40 nm, the Mg implants move relatively towards the surface of VO2 resulting in the loss of the characteristic thermochromism of VO2 films.

Large anomalous Hall angle in the Fe60Al40 alloy induced by substitutional atomic disorder

The electronic and transport properties of the ordered B2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase which undergoes a continuous transition into the disordered A2-(${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$) phase are studied from first principles. The disordering is characterized as a gradual interchange of atoms between Fe and Al sublattices under the condition that the total amount of Fe and Al atoms is kept unchanged. This is the simplest model of gradual disordering of the ordered phase due to the ion irradiation. The physical properties are strongly influenced by varying local environment of Fe atoms on both sublattices. This leads to the transition between a high moment at large disorder and a very low moment in the ordered phase. Similar behavior is found also for anomalous Hall conductivity and anomalous Hall angle. Unusual behavior of the longitudinal conductivity as a function of degree of disorder is due to the shift of the Fermi level of majority states from the $sp$-like part to the $d$ states with increasing ordering. The disordered phase has a large anomalous Hall angle as contrasted with a negligible anomalous Hall angle for well ordered samples, which is in agreement with recent experiment.

New insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys

The paper describes the microstructure evolution by high-temperature compression of a high Nb-containing TiAl alloy. The paper extends a previous publication [Song, et al. Intermetallics 109 (2019) 91–96], in which a unique twin-like morphology in the α2 (Ti3Al) phase was reported. However, the origin of these structures could not be clarified without doubt. The present study is focused on phase transformations that in this multiphase alloy can be associated with deformation. Particular attention is paid to local transformations of the α2 phase into O phase or ω-related phases, which, because of structural and chemical similarity of these phases with α2, can easily occur and could mistakenly be considered as a twin structure. The details of the atomic processes involved are elucidated by electron microscopy. Given the large shufflings and the atomic site interchanges required for the operation of this twinning system, it is concluded that twinning of the α2 phase is a diffusive-displacive process. Within the α2 phase, ωo is heterogeneously nucleated. The nucleation sites are defect-rich areas, which are subjected to high local stresses. The study strongly emphasizes the close relationship between high-temperature deformation and phase transformations in multiphase titanium aluminide alloys.

Effect of Molecular Structure in the Chain Mobility of Dichalcogenide-Based Polymers with Self-Healing Capacity.

Recently, it has been shown that the reaction mechanism in self-healing diphenyl dichalcogenide-based polymers involves the formation of sulfenyl and selenyl radicals. These radicals are able to attack a neighbouring dichalcogenide bond via a three-membered transition state, leading to the interchange of chalcogen atoms. Hence, the chain mobility is crucial for the exchange reaction to take place. In this work, molecular dynamics simulations have been performed in a set of disulfide- and diselenide-based materials to analyze the effect of the molecular structure in the chain mobility. First of all, a validation of the computational protocol has been carried out, and different simulation parameters like initial guess, length of the molecular chains, size of the simulation box and simulation time, have been evaluated. This protocol has been used to study the chain mobility and also the self-healing capacity, which depends on the probability to generate radicals (), the barrier of the exchange reaction () and the mobility of the chains (). The first two parameters have been obtained in previous quantum chemical calculations on the systems under study in this work. After analyzing the self-healing capacity, it is concluded that aromatic diselenides (PD-SeSe) are the best candidates among those studied to show self-healing, due to lower reaction barriers and larger values.

N-glycosidesindolo[2,3,-a]pyrrolo[3,4,-c]carbazole derivatives chemical structure influence on antitumor activity

Introduction . The report considers the prospect of rational approach to the new anticancer agents creation based on indolocarbazole derivatives.Objective . To conduct a comparative study of 12 domestic N-glycosides, indolo[2,3-a]pirrolo[2,3-a]carbazole derivatives in the course of “structure – activity” bond analysis.Materials and methods . The investigation of influence of 12 carbohydrate – containing indolocarbazoles, synthesized in N. N. Blokhin Russian Cancer Research Center of the Ministry of Health of Russia, performed on models of solid transplantable mouse tumors: lung Lewis epidermoid carcinoma and B16 melanoma. The antitumor effect was assessed by Lewis epidermoid carcinoma and B16 melanoma tumor growth inhibition (TGI %) criterion.Results . A variety of indolocarbazoles modifications allowed revealing the dependence of their antitumor properties on the structure of both, the aglycone and the glycoside residue. Imino-nitrogen interchange of atoms in upper heterocycle influences on indocarbazole derivatives antitumor activity change. During a comparative study of 12 N-glycosides indolocarbazole derivatives on lung Lewis epidermoid carcinoma and B16 melanoma models, 8 derivatives showed antitumor activity.Conclusion . The formulated concepts on the modification features in indolocarbazole derivatives structure can be used for more active compounds creation with greater action selectivity.

An oxygen-17 dynamic NMR study of the Pr–DOTA complex

The complex between (17)O-enriched DOTA (tetraazacyclododecanetetraacetic acid) and praseodymium(III) (Pr(3+)) was studied in aqueous solution by variable-temperature (17)O NMR at 14.1 T. pH effects as well as the influence of metal ions free in solution were investigated. At low temperature, the so-called TSAP and SAP conformations give rise to distinct signals for the oxygen atoms coordinated to the metal ion (O2); coalescence occurs between 20 and 30 °C. In contrast, a single signal was detected for the noncoordinated oxygen atoms (O1) in the entire investigated temperature range, i.e. between -3 and 135 °C. At high temperature, the spectra exhibit signal broadening that reveals the interchange of the O1 and O2 oxygen atoms of the carboxylate groups. The linewidths measured for O1 were deconvolved into contributions from quadrupole relaxation and chemical exchange, allowing the corresponding activation barriers to be determined. The present (17)O dynamic NMR study provides the first quantitative experimental data characterizing the interchange of the oxygen atoms in a DOTA chelate of a lanthanide metal ion. The activation entropy of this process is negligible and the activation enthalpy is found to range between 66 and 77 kJ mol(-1), depending on the pH and the presence of free Pr(3+) ions in solution. These data support the results of a previous computational study according to which the exchange mechanism involves the internal rotation of the carboxylate groups.

Mechanical behavior under cyclic loading of the 18R-6R high-hysteresis martensitic transformation in Cu-Zn-Al alloys with nanoprecipitates

Mechanical damping applications could benefit from the large hysteresis, large pseudoelastic strain and the fact that the transformation stresses of the 18R↔6R martensite–martensite transformation depend very little on temperature in Cu-based alloys. This work presents the 18R↔6R mechanical cycling behavior of CuZnAl shape-memory alloy single crystals with electronic concentration e/a=1.48. A fine distribution of gamma phase nanoprecipitates is introduced to prevent plastic deformation of the 6R phase. Results show that, although significant 6R stabilization is observed at very low frequencies (below 10−2Hz), it is possible to obtain more than 1000 stable pseudoelastic cycles with only minor changes in transformation stresses and hysteresis width at frequencies above 10−1Hz. A more pronounced decrease in transformation stresses is observed after 1000 cycles. Nevertheless, the decrease in hysteresis is small up to 2000 cycles. Reported and present results indicate that pair interchange of atoms can explain the stabilization of 6R under quasistatic experimental conditions. However, at higher frequencies of cycling, stabilization of this martensite shows additional features, leading to a dynamic stabilization with slight effects on the mechanical behavior at the required frequency and number of cycles. On the whole, the behavior of this transformation is unique and very promising.

Atomic Interchange Potentials Calculation of Ni-Al-Cr Alloy Based on Microscopic Phase Field Method

The effects of increasing atomic interchange potentials to the precipitation process and microstructure of Ni-Al-Cr alloy have been simulated based on the microscopic phase field theory. The first nearest neighbour atomic interchange potentials of Ni-Al-Cr alloys for L12 and D022 phase was calculated out according to the formula which were referenced on the relation equation between atomic interchange potentials and long range order parameters by Khachaturyan. The results indicated that Ni-Al (WNi-Al) and Ni-Cr (WNi-Cr) s first nearest neighbor atomic interaction potentials will increase linearly while the temperatures rose. Moreover WNi-Al increased but WNi-Cr decreased roughly linearly if Al atoms concentration rose, and conversely inversed. In addition, these atomic interchange potentials changing with temperature and concentration were in good agreement with earlier study.

Modified Feed-Forward Neural Network Structures and Combined-Function-Derivative Approximations Incorporating Exchange Symmetry for Potential Energy Surface Fitting

The classical interchange (permutation) of atoms of similar identity does not have an effect on the overall potential energy. In this study, we present feed-forward neural network structures that provide permutation symmetry to the potential energy surfaces of molecules. The new feed-forward neural network structures are employed to fit the potential energy surfaces for two illustrative molecules, which are H(2)O and ClOOCl. Modifications are made to describe the symmetric interchange (permutation) of atoms of similar identity (or mathematically, the permutation of symmetric input parameters). The combined-function-derivative approximation algorithm (J. Chem. Phys. 2009, 130, 134101) is also implemented to fit the neural-network potential energy surfaces accurately. The combination of our symmetric neural networks and the function-derivative fitting effectively produces PES fits using fewer numbers of training data points. For H(2)O, only 282 configurations are employed as the training set; the testing root-mean-squared and mean-absolute energy errors are respectively reported as 0.0103 eV (0.236 kcal/mol) and 0.0078 eV (0.179 kcal/mol). In the ClOOCl case, 1693 configurations are required to construct the training set; the root-mean-squared and mean-absolute energy errors for the ClOOCl testing set are 0.0409 eV (0.943 kcal/mol) and 0.0269 eV (0.620 kcal/mol), respectively. Overall, we find good agreements between ab initio and NN prediction in term of energy and gradient errors, and conclude that the new feed-forward neural-network models advantageously describe the molecules with excellent accuracy.

A molecular dynamics study of the structural change differences between Au225 and Au369 clusters on MgO surfaces at low temperature

The differences in structural change between Au225 and Au369 clusters with their (111) facets supported on MgO(100) surfaces at 5 K are studied by using molecular-dynamics simulations with the atomic interchange potentials of the Au/MgO interface. The parameters are obtained from the ab initio energies using the Chen—Möbius inversion method. Analyses of the pair distribution functions show that the two Au clusters use different deformation processes to adjust the distances between the interface atoms, owing to the misfit between the atom distances among the clusters and the substrates. The local structural changes are identified by atomic density profiles.

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

The recombination of CF(2)Cl with CH(2)Cl and CFCl(2) with CH(2)F were employed to generate CF(2)ClCH(2)Cl* and CFCl(2)CH(2)F* molecules with 381 and 368 kJ mol(-1), respectively, of vibrational energy in a room-temperature bath gas. The unimolecular reactions of these molecules, which include HCl elimination, HF elimination, and isomerisation by interchange of chlorine and fluorine atoms, were characterized. The three rate constants for CFCl(2)CH(2)F were 2.9×10(7), 0.87×10(7) and 0.04×10(7) s(-1) for HCl elimination, isomerisation and HF elimination, respectively. The isomerisation reaction must be included to have a complete characterization of the unimolecular kinetics of CFCl(2)CH(2)F. The rate constants for HCl elimination and HF elimination from CF(2)ClCH(2)Cl were 14×10(7) and 0.37×10(7) s(-1), respectively. Isomerisation that has a rate constant less than 0.08×10(7) s(-1) is not important. These experimental rate constants were matched to calculated statistical rate constants to assign threshold energies, which are 264, 268, and 297 kJ mol(-1), respectively, for isomerisation, HCl elimination, and HF elimination for CFCl(2)CH(2)F and 314, 251, and 289 kJ mol(-1) in the same order for CF(2)ClCH(2)Cl. Density functional theory was used to evaluate the models that were needed for the statistical rate constants; the computational method was B3PW91/6-31G(d',p'). Threshold energies for the unimolecular reactions of CF(2)ClCH(2)Cl and CFCl(2)CH(2)F are compared to those for CF(2)ClCH(3) and CFCl(2)CH(3) to illustrate the elevation of threshold energies by F- or Cl-atom substitution at the beta carbon atom (identified by C(H)). The DFT calculations systematically underestimate the threshold energy for HCl elimination.

Modeling of the bis(glycinato)copper(ii) cis-trans isomerization process: Theoretical analysis

The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 52, No. 5, pp. 906-6-916, September-October, 2011. Original article submitted August 5, 2010. In a number of theoretical works, the cis-trans isomerization of bis(glycinato)copper(II) and its interaction with water molecules is investigated. The interactions with the water medium modeled either by adding water molecules explicitly or through the Polarized Continuum Model (PCM) are investigated by the density functional (B3LYP) method in order to question the reliability of theoretical results. The crucial dependence of theoretical energies on the accuracy of PCM corrections is established. It is shown that for bis(glycinato)copper(II) the differences of isomer energies are of the order of the upper limit of the reliability of PCM corrections. Based on the calculation results, two possible mechanisms for the cis-trans isomerization are proposed. It is shown that only the inclusion of two explicit water molecules enables the modeling of the isomerization mechanism involving the interchange of glycine and water oxygen atoms in copper coordination.

Structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane

In this work, we present a complete structural and vibrational analysis of the OH torsional motion in difluorohydroxyborane (BF2OH) at the HF/aug-cc-pVTZ, MP2(full)/aug-cc-pVTZ, and CCSD/aug-cc-pVTZ theory levels. After full relaxation of the geometry, the equilibrium structure is found in a planar conformation of Cs symmetry. The difference in the two BF distances suggests the existence of a nonbonded interaction between the fluorine and oxygen atoms. The structural and energetic variation of BF2OH as a function of the OH torsional angle is considered. The torsional barrier, at the CCSD/aug-cc-pVTZ level, and including the effect of the zero-point energy of the remaining vibrations, is found 2,728 cm−1. In addition, an anharmonic Hamiltonian for the OH torsional mode is presented and variationally solved. To simplify the treatment and to classify the energy levels, BF2OH is classified under a G4 nonrigid group accounting for the inversion symmetry of the molecule and the interchange of the fluorine atoms. The computed torsional energy levels exhibit a very small inversion splitting. The torsional spectrum is simulated considering the dipole moment components along the principal axes of inertia as an explicit function of the torsional coordinate. We observe three dominant bands in the spectrum formed by doublets corresponding to ν9 = 0 → 1, 2 transitions. The fundamental is an a-type, Franck–Condon, transition. This is the strongest and appears at 466.80 cm−1 with relative intensity 0.4312. The ν9 = 0 → 2 bands correspond to doublets of b- and c-type, i.e., Herzberg–Teller transitions. These are two overlapping bands found at 890.92 and 890.94 cm−1 with intensity 0.2207 for the b-type band and 0.2193 for the c-type band. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Two unusual mixed-valent trinuclear CuII2CuI complexes containing copper(i) tribromide dianion as bridging ligand: Identification of an unprecedented doubly hydrogen-bonded water dimer

Two very rare mixed-valent CuII2CuI trinuclear complexes, [Cu2(Br–L1)2CuBr3]·2H2O (1) and [Cu2(L2)2CuBr3]·2CH3OH (2) have been synthesized using CuBr2 and the tridentate reduced Schiff-base ligands HL1 (2-[(2-dimethylamino-ethylamino)-methyl]-phenol) and HL2 (2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol), respectively. The complexes have been characterized by X-ray structural analyses. In both complexes, the deprotonated tridentate reduced Schiff base forms a di-μ-phenoxo bridge between the two CuII centers and the CuBr32− anion acts as an additional bridge between them to result in the trinuclear structures. During complex formation, ligand HL1 has been brominated at the five position of the aromatic ring but HL2 did not undergo any such reaction. In complex 1 an unprecedented doubly hydrogen-bonded water dimer, which is considered as an important transition state for the interchange of hydrogen atoms within the water dimer, has been identified. The dimensions of this water dimer agree quite well to those obtained from theoretical calculations.

Au<SUB>603</SUB> and Au<SUB>675</SUB> Cluster Structures with (100) Facets Supported on MgO(010) Surfaces

Structural changes of Au clusters respectively containing 603 and 675 atoms with their (100) facts supported on MgO(010) surfaces at 5 K have been investigated by molecular-dynamics simulations on the atomic interchange potentials of the metal/MgO interface, which parameters are from the Ab initio energies using Chen-Möbius inversion method. As the interaction time evolves, the structural changes accompanying with atom movements are determined by decomposing peaks of pair distribution functions (PDFs) according to the pair analysis (PA) technique, and local structure patterns are identified for the supported clusters. It is shown that there exist deformation processes of the two Au clusters to adjust their atom distances owing to atom misfits among the clusters and the MgO support. Concerning these supported clusters, the obtained results reveal how the cluster size and the interaction affect the structural changes in the interaction between the supported cluster and the MgO support.

Ab initiostudy of stabilization of the misfit layer compound(PbS)1.14TaS2

In the present work we perform ab initio electronic-structure calculations of the ${(\text{PbS})}_{1.14}{\text{TaS}}_{2}$ misfit layer compound in order to understand the basic mechanism of its stabilization. Density-functional based calculations were carried out in commensurate unit cells containing 74 or 296 atoms using the WIEN2K code. The two experimentally predicted mechanisms of stabilization, namely, metal cross substitution (interchange of Pb and Ta atoms in the PbS and ${\text{TaS}}_{2}$ layers) and nonstoichiometry (substitution of Pb by Ta), were investigated. The results show clearly that the nonstoichiometry mechanism plays the significant role. When Pb inside the PbS layer is substituted by Ta with concentrations around 0.13--0.19, a stabilization is found with respect to the parent PbS and ${\text{TaS}}_{2}$ compounds. These results are explained by an analysis of partial densities of states. The calculated x-ray photoemission spectroscopy core-level shifts are in reasonable agreement with experiments.

Isomerization of Neopentyl Chloride and Neopentyl Bromide by a 1,2-Interchange of a Halogen Atom and a Methyl Group

The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol(-1) of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD(2)Cl and C(CH(3))(3) radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d',p') level to verify the Cl/CH(3) interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ± 2 kcal mol(-1) as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro-2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes.