Bond-stretch Isomerism Research Articles

From loose to tight: unveiling bond stretch isomerism in π-complexes of Li, Na and K.

The study investigates the phenomenon of bond stretch isomerism (BSI) in complexes formed between alkali metals (Li, Na, K) and various non-aromatic, aromatic hydrocarbons, as well as heteroaromatic systems. The research employs density functional theory (DFT) calculations to optimize complex geometries and analyze their electronic structures using molecular electrostatic potential (MESP), charge, and spin density analyses. The results reveal that these complexes can exist in two distinct configurations: 'loose' long-bond isomers (lbi) that retain the original hydrocarbon geometry and 'tight' short-bond isomers (sbi) that undergo geometrical distortion upon complexation, with sbi generally being more stable. The interconversion between lbi and sbi occurs through a transition state. The study highlights the crucial role of electron transfer in BSI, with sbi involving valence electron transfer from the metal to the hydrocarbon, leading to zwitterionic radical complexes. In contrast, lbi exhibit a slight electron density transfer from the hydrocarbon to the metal. The presence of low-energy transition states between lbi and sbi suggests a dynamic shuttling mechanism for alkali metals, particularly Li, on hydrocarbon surfaces. The study identifies Li complexes as potential candidates for anode materials in batteries due to their stability and electron transfer properties, offering valuable insights into the design of advanced materials for energy storage applications.

Atomic Clusters: Structure, Reactivity, Bonding, and Dynamics.

Atomic clusters lie somewhere in between isolated atoms and extended solids with distinctly different reactivity patterns. They are known to be useful as catalysts facilitating several reactions of industrial importance. Various machine learning based techniques have been adopted in generating their global minimum energy structures. Bond-stretch isomerism, aromatic stabilization, Rener-Teller effect, improved superhalogen/superalkali properties, and electride characteristics are some of the hallmarks of these clusters. Different all-metal and nonmetal clusters exhibit a variety of aromatic characteristics. Some of these clusters are dynamically stable as exemplified through their fluxional behavior. Several of these cluster cavitands are found to be agents for effective confinement. The confined media cause drastic changes in bonding, reactivity, and other properties, for example, bonding between two noble gas atoms, and remarkable acceleration in the rate of a chemical reaction under confinement. They have potential to be good hydrogen storage materials and also to activate small molecules for various purposes. Many atomic clusters show exceptional opto-electronic, magnetic, and nonlinear optical properties. In this Review article, we intend to highlight all these aspects.

Boron Tunneling in the "Weak" Bond-Stretch Isomerization of N-B Lewis Adducts.

Some nitrile-boron halide adducts exhibit a double-well potential energy surface with two distinct minima: a "long bond" geometry (LB, a van der Waals interaction mostly based on electrostatics, but including a residual charge transfer component) and a "short bond" structure (SB, a covalent dative bond). This behavior can be considered as a "weak" form of bond stretch isomerism. Our computations reveal that complexes RCN-BX3 (R=CH3 , FCH2 , BrCH2 , and X=Cl, Br) exhibit a fast interconversion from LB to SB geometries even close to the absolute zero thanks to a boron atom tunneling mechanism. The computed half-lives of the meta-stable LB compounds vary between minutes to nanoseconds at cryogenic conditions. Accordingly, we predict that the long bond structures are practically impossible to isolate or characterize, which agrees with previous matrix-isolation experiments.

Structural and energetic properties of OC-BX3 complexes: unrealized potential for bond-stretch isomerism.

We have explored the structural and energetic properties of OC-BX3 (X = F, Cl, or Br) complexes using computations and low-temperature infrared spectroscopy. Quantum-chemical calculations have provided equilibrium structures, binding energies, vibrational frequencies, and B-C potential energy curves. The OC-BF3 system is a weak, long-bonded complex with a single minimum on the B-C potential (R(B-C) = 2.865 Å). For the remaining two complexes, OC-BCl3 and OC-BBr3, computations predict two stable minima on their B-C potential curves. The BCl3 system is a weak complex with a long bond (R(B-C) = 3.358 Å), but it exhibits a secondary, meta-stable minimum with a short bond length of 1.659 Å. For OC-BBr3, the system is a weak complex with a relatively short bond of 1.604 Å (according to wB97X-D/aug-cc-pVTZ), but also has a secondary minimum at R(B-C) = 3.483 Å. This long-bond structure is the global minimum according to CCSD/aug-cc-pVTZ. In addition, the long-bond forms of both OC-BCl3 and OC-BBr3 were observed in matrix-isolation IR experiments. The measured CO stretching frequencies were 2145 cm-1 and 2143 cm-1, respectively. No signals due to the short-bond forms of OC-BCl3 and OC-BBr3 were observed.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes.

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX—phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Intriguing structural, bonding and reactivity features in some beryllium containing complexes.

Although the toxicity of beryllium compounds causes impediments in experiments involving them, beryllium chemistry has seen a recent upsurge of interest and considerable progress. Computations play a very important complementary role in analyzing the structure, stability and bonding of these compounds. In this perspective article, we highlighted our contribution to beryllium chemistry which is either completely through theoretical results or sometimes supported by experimental findings. It starts with the smallest 2π aromatic system, Be32-, which also exhibits rare bond-stretch isomerism. Furthermore, its reactivity towards different transformations is mentioned. Because of the ability of beryllium to attain a high ionic potential, the beryllium center in an appropriate situation can act as an excellent Lewis acid which is utilized to bind noble gas (Ng) atoms, carbon monoxide and dinitrogen through donor-acceptor types of interactions. We made several efforts to have strong Ng-Be bonds which led us to NgBeNCN that is recorded to have the strongest Ng-Be bond among the neutral Ng-Be complexes reported so far. Significant dinitrogen activation was also achieved in (NN)2Be(η2-N2) and OCBeNN complexes. In the latter case, a complete cleavage of the N-N bond producing the most stable NBeNCO molecule has occurred. We also found viable M2(NHBMe)2 (M = Be, Mg) complexes having unusual bonding where the interacting fragments are best described as the neutral M2 and (NHBMe)2 but M2 still possesses a single bond. We finally discussed the complex comprising an unusual Be(i) oxidation state, [BeI(cAACAr)2]+˙ and di-ortho-beryllated carbodiphosphorane exhibiting Be⇇C double dative bonds.

Switchable dual bonding nature in silabicyclo[1.1.0]butanes that exhibit bond stretch isomerism

AbstractSilicon analogues of bicyclo[1.1.0]butanes are known to exhibit a bond stretch isomerism where the bridgehead SiSi bond changes in both length and geometry. The bridgehead silicon atom of long bond (lb) isomers exhibits an unusual geometry where all the bonds are located on the same hemisphere, making the geometry of the bridgehead SiSi bond the so‐called inverted σ bond in contrast with the typical σ bond geometry found in the short bond (sb) isomers. Herein, the bonding nature of bridgehead SiSi bonds was theoretically investigated in detail using valence bond theory calculation at SL‐BOVB/6‐31G(d) level of theory. Our calculation demonstrates that the bond type of the bridgehead bond depends on the geometry around the bridgehead atoms with those in sb and lb isomers classified as covalent (COV) and charge‐shift (CS) bonds, respectively. The different bond types between two bond stretch isomer indicate that the bonding nature is switchable in one molecule. The detailed examination of the bond stretch isomerization in parent 1,3‐disilabicyclo[1.1.0]butane demonstrated that the enhanced CS bond nature in the lb isomer is mainly attributed to the geometry inversion of the σ bond rather than the bond elongation. This switchable dual bonding nature provides an important platform to compare covalent and CS bond on equal footing for both theoretical and experimental investigations.

Bond stretch isomerism in Be32- driven by the Renner-Teller effect.

We investigate the underlying principle behind the occurrence of bond stretch isomerism in Be32-, which has not been revealed yet. Various computational studies of the different isomers are carried out at the complete active space self-consistent field (CASSCF) level of theory in addition to the B3LYP level in conjunction with the 6-311++G(d) basis set. The potential energy surfaces linking the different isomers through transition states and conical intersections are investigated at the CASSCF level, connecting various geometrical isomers of Be32-. The linear intermediate of the Be32- cluster is considered to be of profound importance since its excited state is found to be degenerate and undergoing the Renner-Teller effect, producing two triangular bond stretch isomers. Ab initio molecular dynamics simulations based on the Atom Centered Density Matrix Propagation (ADMP) method also further elucidate the phenomenon of isomerization via the linear intermediate. The variation of the global reactivity descriptors and free energy profile along the bond stretch isomerization path is also investigated. The estimated aromatic stabilization energy also corroborates the stability ordering of the bond stretch isomers.

Theoretical evidence for bond stretch isomerism in Grubbs olefin metathesis.

A comprehensive density functional theory study on the dissociative and associative mechanisms of Grubbs first and second generation olefin metathesis catalysis reveals that ruthenacyclobutane intermediate (RuCB) observed in the Chauvin mechanism is not unique as it can change to a non-metathetic ruthenacyclobutane (RuCB') via the phenomenon of bond stretch isomerism (BSI). RuCB and RuCB' differ mainly in RuCα , RuCβ , and Cα Cβ bond lengths of the metallacycle. RuCB is metathesis active due to the agostic type bonding-assisted simultaneous activation of both Cα Cβ bonds, giving hypercoordinate character to Cβ whereas an absence of such bonding interactions in RuCB' leads to typical CC single bond distances and metathesis inactivity. RuCB and RuCB' are connected by a transition state showing moderate activation barrier. The new mechanistic insights invoking BSI explains the non-preference of associative mechanism and the requirement of bulky ligands in the Grubbs catalyst design. The present study lifts the status of BSI from a concept of largely theoretical interest to a phenomenon of intense importance to describe an eminent catalytic reaction. © 2017 Wiley Periodicals, Inc.

Metal-Metal Bonding in Trinuclear, Mixed-Valence [Ti3X12](4-) (X = F, Cl, Br, I) Face-Shared Complexes.

Metal-metal bonding in structurally characterized In4Ti3Br12, comprising linear, mixed-valence d(1)d(2)d(1) face-shared [Ti3Br12](4-) units with a Ti-Ti separation of 3.087 Å and strong antiferromagnetic coupling (Θ = -1216 K), has been investigated using density functional theory. The antiferromagnetic configuration, in which the single d electron on each terminal Ti(III) (d(1)) metal center is aligned antiparallel to the two electrons occupying the central Ti(II) (d(2)) metal site, is shown to best agree with the reported structural and magnetic data and is consistent with an S = 0 ground state in which two of the four metal-based electrons are involved in a two-electron, three-center σ bond between the Ti atoms (formal Ti-Ti bond order of ∼0.5). However, the unpaired spin densities on the Ti sites indicate that while the metal-metal σ interaction is strong, the electrons are not fully paired off and consequently dominate the ground state antiferromagnetic coupling. The same overall partially delocalized bonding regime is predicted for the other three halide [Ti3X12](4-) (X = F, Cl, I) systems with the metal-metal bonding becoming weaker as the halide group is descended. The possibility of bond-stretch isomerism was also examined where one isomer has a symmetric structure with identical Ti-Ti bonds while the other is unsymmetric with one short and one long Ti-Ti bond. Although calculations indicate that the latter form is more stable, the barrier to interconversion between equivalent unsymmetric forms, where the short Ti-Ti bond is on one side of the trinuclear unit or the other, is relatively small such that at room temperature only the averaged (symmetric) structure is likely to be observed.

Unique bonding pattern and resulting bond stretch isomerism in Be32−

The bond-stretch isomers are characterized by a principal change in the bond-length with the rest of the molecule being unaltered. The electronic structure regulates the bond stretch isomerism phenomenon in which has been investigated with density functional theory, ab initio CASSCF, highly efficient n-electron valence state perturbation theory and multireference configuration interaction calculations. Two isomers are distinguished on different potential energy surfaces and the corresponding avoided crossing is also studied in details. The bonding pattern in two isomers are analysed through adaptive natural density partitioning analysis and quantum theory of atoms in molecules analysis. The bonds in both the isomers primarily involve the 2p orbitals, which overlap face-to-face in long-bond isomer. Whereas, in-plane π-bonding occurs at the short-bond isomer leading to unusual bent bond. © 2015 Wiley Periodicals, Inc.

Is bond stretch isomerism in mononuclear transition metal complexes a real issue? The misleading case of the MoCl5/tetrahydropyran reaction system.

Distinct batches of orange (1a-e) and green crystals (2a-e) were isolated from the reactions of MoCl5 with tetrahydropyran (thp), respectively at room temperature (in CH2Cl2) and at ca. 80 °C (in ClCH2CH2Cl). Crystals 2a-e are isomorphous to 1a-e and the IR spectra are almost superimposable. 1a-e were identified by X-ray studies as cis-MoCl4(thp)2, in agreement with previous findings. Careful refinement of the X-ray data of 2a-e by modeling one position as disordered between chlorine (minor component) and oxygen (major component) led to the conclusion that 2a-e consisted of a mixture of cis-MoCl4(thp)2 and mer-MoOCl3(thp)2, the latter being largely prevalent.

Invariant Kekulé structures in fullerene graphs

Abstract Fullerene graphs are trivalent plane graphs with only hexagonal and pentagonal faces. They are often used to model large carbon molecules: each vertex represents a carbon atom and the edges represent chemical bonds. A totally symmetric Kekule structure in a fullerene graph is a set of independent edges which is fixed by all symmetries of the fullerene and molecules with totally symmetric Kekule structures could have special physical and chemical properties, as suggested in [Austin, S.J, and J. Baker, P. W. Fowler, D. E. Manolopoulos, Bond-stretch Isomerism and the Fullerenes, J. Chem. Soc. Perkin Trans. 2 (1994), 2319–2323] and [Rogers, K.M., and P. W. Fowler, Leapfrog fullerenes, Huckel bond order and Kekule structures, J. Chem. Soc. Perkin Trans. 2 (2001), 18–22]. All fullerenes with at least ten symmetries were studied in [Graver, J.E. The Structure of Fullerene Signature, DIMACS Series of Discrete Mathematics and Theoretical Computer Science 64, AMS (2005), 137–166.] and a complete catalog was given in [Graver, J. E. Catalog of All Fullerene with Ten or More Symmetries DIMACS Series of Discrete Mathematics and Theoretical Computer Science 64 AMS (2005), 167–188]. Starting from this catalog in [Bogaerts, M., and G. Mazzuoccolo, G.Rinaldi, Totally symmetric Kekule structures in fullerene graphs with ten or more symmetries, MATCH Communications in Mathematical and in Computer Chemistry 69 (2013), 677–705] we established exactly which of them have at least one totally symmetric Kekule structure.

Bond-stretch isomerism in 2-chalcogen-trimetallabicyclo[1.1.0]butane derivatives

Electronic and steric factors that control the bond-stretch isomerism phenomenon in 2-chalcogen-trimetallabicyclo[1.1.0]butane systems have been investigated using quantum chemical calculations. Beside the short-bond and long-bond extremes we found and characterized a third, stable conformer with significantly elongated central bond and anti arrangement of the bridgehead substituents. Electronic structures of the isomers have been described and interpreted in details within the framework of MO theory. Germanium analogues of trisilirene have been predicted to also react with sulphur to form the corresponding 2-thia-trimetallabicyclo[1.1.0]butane analogues. The systematic analysis of the relative stability dependence on the ring composition as well as the bridgehead substituents revealed that the hitherto unknown anti isomer is a promising synthetic target in germanium bridged bicyclo compounds with bridgehead substituents containing carbon contact atom.

A conceptual density functional study of structure, bonding, reactivity and the possibility of bond-stretch isomerism in some neutral sulfur clusters, S n (n=3–8)

Geometries of different isomers of various neutral sulfur clusters, S n (n=3–8) are optimized at the B3LYP/6-311+G* level of theory. Their stability and aromaticity behavior are analyzed in terms of the conceptual density functional theory-based reactivity descriptors and the associated electronic structure principles. The nucleus-independent chemical shift lends additional support. Possibility of bond-stretch isomerism in these clusters is explored.

Bonding, aromaticity, and structure of trigonal dianion metal clusters

Various isomers of the trigonal dianion metal clusters, X(3)(2-), X = Be, Mg, Ca, and their mono- and disodium complexes are optimized at the B3LYP/6-311+G(d) level. Different conceptual density functional theory based reactivity descriptors as well as the induced magnetic field values are calculated to understand the stability and aromaticity of these systems. Possibility of bond stretch isomerism is explored. Genetic algorithm results lend additional insights into the structures of these isomers.

Bonding, aromaticity and reactivity patterns in some all-metal and non-metal clusters

Several sandwich-like metal clusters have been studied at the B3LYP/6-311 + G* level of theory. Bonding and reactivity have been analysed through various geometrical parameters and conceptual density functional theory based global reactivity descriptors. Aromaticity patterns have been understood in terms of the associated nucleus independent chemical shift values. Possibility of bond-stretch isomerism in some doped clusters is explored. Preferable sites for electrophilic and nucleophilic attacks have been identified using different local reactivity descriptors.

The Two Structures of the Hexafluorobenzene Radical Cation C6F6.+

Ring, ring! C6F6.+ Os2F11− and C6F6.+Sb2F11− have isomorphous crystal structures and each contain two different C6F6.+ cations. One is a quinoid structure, the other bisallyl structure. Calculations show that indeed two such Jahn–Teller distorted structures coexist with essentially the same energy. This seems to be a case of bond-stretch isomerism.

The Isomerization Barrier in Cyanocyclobutadienes: An ab Initio Multireference Average Quadratic Coupled Cluster Study

The energy profiles of the isomerization of mono, di-, and tetracyano-substituted cyclobutadienes (CBDs) are computed at the multireference average quadratic coupled cluster/complete active space self-consistent field level of theory. It was found that the energy barrier heights for the automerization reaction are 2.6 (tetracyano-CBD), 5.1 (1,3-dicyano-CBD), and 6.4 (cyano-CBD) kcal mol(-1), implying that they are lowered relative to that in the parent CBD (6.4 kcal mol(-1)), the monosubstituted derivative being an exception. Since the free CBD shuttles between two equivalent structures even at low temperature of 10 K, it follows that bond-stretch isomerism does not take place in cyanocyclobutadienes. Instead, these compounds exhibit rapid fluxional interconversion at room temperature between two bond-stretch isomers by the double bond flipping mechanism. The reason behind the decrease in the barrier heights is identified as a slightly enhanced resonance effect at the saddle points separating two (equivalent) bond-stretch isomers, compared to that in the equilibrium structures, predominantly due to the diradical character of the former. It is also shown that the energy gap between the singlet ground state saddle point structure and the first triplet equilibrium geometry decreases upon multiple substitution by the cyano groups. The splitting of the S and T energy is small being within the range of 6.5-8.2 kcal mol(-1).

Relative energies of C 2O 2H 2 isomers and their ionized counterparts: possibility of bond stretch isomerism

Hybrid density functional theory and post-SCF ab initio calculations were employed to explore the isomers of glyoxal, C 2O 2H 2, along with the corresponding cation and anion radicals. The dependence of relative energies with respect to the methods and quality of basis set was critically analyzed, all the way up to cc-pV5Z. Bond stretch isomerism between 5a and 6 as well as 5C and 6C was clearly established. While the glyoxal isomer 1 and 1A is a global minima on neutral and anionic surfaces, 8C a ketene analog was found to be the global minima among the cation isomers.