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We recently proposed a semi-stochastic approach to converging high-level coupled-cluster (CC) energetics, such as those obtained in the CC calculations with singles, doubles, and triples (CCSDT), in which the deterministic CC(P;Q) framework is merged with the stochastic configuration interaction Quantum Monte Carlo propagations [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the ability of the semi-stochastic CC(P;Q) methodology to recover the CCSDT energies of the lowest singlet and triplet states and the corresponding singlet–triplet gaps of biradical systems using methylene, (HFH)−, cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as examples. [ABSTRACT FROM AUTHOR]

The catalytic activity of cyclobutadiene rhodium complexes was investigated in various transformations of alkynes. Complexes [Rh(C4Et4)Hal]n (at 1 mol% loading) were found to promote hydrosilylation of 1-octyne selectively giving 1-Z-, 1-E-, or 2-Et3Si-1-octene depending on the nature of the halide ligand and the presence of additional ligand PPh3. Complex [Rh(C4Et4)(p-xylene)]+ (1 mol%) also efficiently catalyzed the polymerization of phenylacetylene. At the same time, we found that various cyclobutadiene rhodium complexes do not catalyze the Diels–Alder reaction, alkyne hydration, and 1,4-addition of boronic acids to cyclohexanone. New complexes [Rh(C4Et4)(PPh3)2]BF4 and [Rh(C4Et4)(C10H8)]PF6 were also synthesized and their structures were established by X-ray diffraction. [ABSTRACT FROM AUTHOR]

[Display omitted] Reaction of [(coe) 2 IrCl] 2 (coe = cyclooctene) with AgPF 6 , arene (toluene, p -xylene, mesitylene), and diphenylacetylene, followed by counter anion exchange to BARF–, gives cyclobutadiene iridium complexes [(C 4 Ph 4)Ir(arene)]BARF in 10–22% yield. Replacement of toluene in [(C 4 Ph 4)Ir(toluene)]BARF by various ligands (trimethyl-phosphite, 1,3,5-triaza-7-phosphaadamantane, tert -butyl isonitrile) provides access to the half-sandwich cyclobuta-diene iridium complexes [(C 4 Ph 4)Ir(L) 3 ]BARF. Complex [(C 4 Ph 4)Ir(toluene)]BARF in combination with [NBnEt 3 ]Cl catalyses the insertion reaction between phenyl diazoacetate and various amines. [ABSTRACT FROM AUTHOR]

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Understanding the bonding of gold(I) species has been central to the development of gold(I) catalysis. Herein, we present the synthesis and characterization of the first gold(I)-cyclobutadiene complex, accompanied with bonding analysis by state-of-the-art energy decomposition analysis methods. Analysis of possible coordination modes for the new species not only confirms established characteristics of gold(I) bonding, but also suggests that Pauli repulsion is a key yet hitherto overlooked element. Additionally, we obtain a new perspective on gold(I)-bonding by comparison of the gold(I)-cyclobutadiene to congeners stabilized by p-, d-, and f-block metals. Consequently, we refine the gold(I) bonding model, with a delicate interplay of Pauli repulsion and charge transfer as the key driving force for various coordination motifs. Pauli repulsion is similarly determined as a significant interaction in AuI -alkyne species, corroborating this revised understanding of AuI bonding.

Inspired by our earlier semi-stochastic work aimed at converging high-level coupled-cluster (CC) energetics [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017) and J. E. Deustua, J. Shen, and P. Piecuch, J. Chem. Phys. 154, 124103 (2021)], we propose a novel form of the CC(P; Q) theory in which the stochastic Quantum Monte Carlo propagations, used to identify dominant higher-than-doubly excited determinants, are replaced by the selected configuration interaction (CI) approach using the perturbative selection made iteratively (CIPSI) algorithm. The advantages of the resulting CIPSI-driven CC(P; Q) methodology are illustrated by a few molecular examples, including the dissociation of F2 and the automerization of cyclobutadiene, where we recover the electronic energies corresponding to the CC calculations with a full treatment of singles, doubles, and triples based on the information extracted from compact CI wave functions originating from relatively inexpensive Hamiltonian diagonalizations. [ABSTRACT FROM AUTHOR]

We recently proposed a novel approach to converging electronic energies equivalent to high-level coupled-cluster (CC) computations by combining the deterministic CC(P;Q) formalism with the stochastic configuration interaction (CI) and CC Quantum Monte Carlo (QMC) propagations. This article extends our initial study [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)], which focused on recovering the energies obtained with the CC method with singles, doubles, and triples (CCSDT) using the information extracted from full CI QMC and CCSDT-MC, to the CIQMC approaches truncated at triples and quadruples. It also reports our first semi-stochastic CC(P;Q) calculations aimed at converging the energies that correspond to the CC method with singles, doubles, triples, and quadruples (CCSDTQ). The ability of the semi-stochastic CC(P;Q) formalism to recover the CCSDT and CCSDTQ energies, even when electronic quasi-degeneracies and triply and quadruply excited clusters become substantial, is illustrated by a few numerical examples, including the F–F bond breaking in F2, the automerization of cyclobutadiene, and the double dissociation of the water molecule. [ABSTRACT FROM AUTHOR]

The complete-active-space self-consistent field (CASSCF) method is a canonical electronic structure theory that holds a central place in conceptualizing and practicing first principles simulations. For application to realistic molecules, however, the CASSCF must be approximated to circumvent its exponentially scaling computational costs. Applying the many-body expansion—also known as the method of increments—to CASSCF (iCASSCF) has been shown to produce a polynomially scaling method that retains much of the accuracy of the parent theory and is capable of treating full valence active spaces. Due to an approximation made in the orbital gradient, the orbital parameters of the original iCASSCF formulation could not be variationally optimized, which limited the accuracy of its nuclear gradient. Herein, a variational iCASSCF is introduced and implemented, where all parameters are fully optimized during energy minimization. This method is able to recover electronic correlations from the full valence space in large systems, produce accurate gradients, and optimize stable geometries as well as transition states. Demonstrations on challenging test cases, such as the oxoMn(salen)Cl complex with 84 electrons in 84 orbitals and the automerization of cyclobutadiene, show that the fully variational iCASSCF is a powerful tool for describing challenging molecular chemistries. [ABSTRACT FROM AUTHOR]

We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 1–4, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest reference compounds, CDTs, reveals that the local aromaticity of their thiophene rings is influenced by their fused position (b- or c-bond) to the four-membered ring (4MR). A thiophene ring fused at the b-position (b-TR) retains its aromatic character to some extent, whereas the aromatic character of one fused at the c-position is attenuated. The 4MR with two fused b-TRs retains a strong anti-aromatic character. Thiophene analogues of anti-kekulene with six to eight thiophene rings 1–3 favor bowl-shaped structures, in contrast to the planar structure of anti-kekulene, because of the shorter distances of the sulfur bridges. Compound 4, with nine thiophene rings, adopts a planar structure. The local aromaticity and anti-aromaticity of the thiophene ring and 4MR are significantly attenuated in 1–4 compared with the reference compounds, the CDTs and anti-kekulene. This can be attributed to the considerable contribution of the quinoidal electronic structure in 1–4. The present study provides new insight into the aromatic and electronic nature of systems containing cyclobutadienothiophene.

The reduction of the boryl‐substituted SnII bromide {(HCDippN)2B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2[Sn4{B(NDippCH)2}4], a homo‐metallic heavier analogue of the cyclobutadiene dianion. This system is non‐aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=−0.28, NICS(1)=−3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2‐localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2‐Sn4{B(NDippCH)2}4H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4{B(NDippCH)2}4]2− dianion. [ABSTRACT FROM AUTHOR]

Context: In 2000, a remarkably simple relationship was introduced, which connected the calculated geometries of isomolecular states of three different multiplicities. These encompass a ground single state, the first excited triplet state, as well as related radical anion and radical cation. The rule allows the prediction of the geometry of one of the species if the three remaining ones are known. Here, we verify the applicability of this bond length rule for two small planar cyclic organic molecules, i.e., benzene and cyclobutadiene, which stand as prototypical examples of, respectively, aromatic and antiaromatic systems. We see that the rule works fairly well to benzene, and it works independently for quinoid as well as for anti-quinoid minima, despite the fact that radical anion species poses challenges for correct theoretical description. Methods: To obtain chosen electronic state equilibrium geometries, three types of computational approaches were utilized: fast and efficient density functional theory DFT, the coupled cluster method CC2, the complete active space self-consistent field (CASSCF) approach, and the most accurate but also resource-consuming perturbation theory with multireference wavefunction (CASPT2) with a default value and without IPEA-shift. Dunning and co-workers correlation-consistent basis sets (aug-)cc-pVXZ (X = D, T, Q) were employed. Gaussian 16 revision A.03, Turbomole 7.1, and Molcas 8.0 computational software were used. [ABSTRACT FROM AUTHOR]

The readily available cyclobutadiene platinum complex [(C4Et4)PtCl2] catalyzes hydrosilylation of alkenes and alkynes. The reaction typically proceeds at room temperature and does not require any solvent. The target products are obtained from both terminal and internal alkynes, as well as from terminal alkenes in 43–99% yields. The scope of silanes includes various tertiary silanes (Et3SiH, (EtO)3SiH, Ph3SiH) and the disubstituted silane Ph2SiH2. The hydrosilylation proceeds with typical anti‐Markovnikov regioselectivity giving mainly β(E)‐vinylsilanes from alkynes and linear alkyl silanes from alkenes. [ABSTRACT FROM AUTHOR]

We predict the geometries, electronic properties, and aromaticity of thiophene analogues of anti-kekulene with six to nine thiophene rings 1–4, together with those of cyclobutadithiophenes (CDTs) and anti-kekulene as reference compounds, using density functional theory calculations. Investigation of the simplest reference compounds, CDTs, reveals that the local aromaticity of their thiophene rings is influenced by their fused position (b- or c-bond) to the four-membered ring (4MR). A thiophene ring fused at the b-position (b-TR) retains its aromatic character to some extent, whereas the aromatic character of one fused at the c-position is attenuated. The 4MR with two fused b-TRs retains a strong anti-aromatic character. Thiophene analogues of anti-kekulene with six to eight thiophene rings 1–3 favor bowl-shaped structures, in contrast to the planar structure of anti-kekulene, because of the shorter distances of the sulfur bridges. Compound 4, with nine thiophene rings, adopts a planar structure. The local aromaticity and anti-aromaticity of the thiophene ring and 4MR are significantly attenuated in 1–4 compared with the reference compounds, the CDTs and anti-kekulene. This can be attributed to the considerable contribution of the quinoidal electronic structure in 1–4. The present study provides new insight into the aromatic and electronic nature of systems containing cyclobutadienothiophene. [ABSTRACT FROM AUTHOR]

A one-pot reaction starting with C5Ph5Br, n-BuLi and [CoCl(PPh3)3] followed by the addition of diphenylethyne produces the title compound with 12% yield. Spectroscopic characterization involved 1H, 13C-NMR, UV-Vis and mass spectrometry. A crystal structure determination showed that the central aromatic rings are exactly parallel with the cyclobutadiene ring further apart from the metal as usual. The pentaphenyl–cyclopentadienyl ligand shows an usual paddlewheel orientation, whereas in the tetraphenyl–cyclobutadiene ligand, two of the phenyl groups are nearly coplanar with the four-membered ring. There are also numerous C–H...C(π) interactions between the phenyl groups on the same ring, as well as between rings. [ABSTRACT FROM AUTHOR]

The synthesis and characterisation of [Co2(η4-P2C2tBu2)2(μ:η4:η4-P2C2tBu2)] (1) is described, featuring two cobalt atoms and three diphosphacyclobutadiene ligands. This compound is the first triple-decker sandwich complex which exclusively contains cyclobutadiene ligands. [ABSTRACT FROM AUTHOR]

The interaction of the seven‐membered zirconacyclocumulene complex of zirconocene with acenaphthenequinone leads to the cyclobutadiene formation with its subsequent dimerization in cyclooctatetraene in addition to tetraoxadizirconacycle consists of two zirconocene and two acenaphthenequinone fragments. If this reaction is carried out in the presence of the excess of cyclopentadiene or acenaphthylene, the trapping of cyclobutadiene by the Diels‐Alder reaction takes place. Cyclobutadiene can act both as dienophile and diene source in these reactions. [ABSTRACT FROM AUTHOR]

Singlet-triplet gaps in diradical organic π-systems are of interest in many applications. In this study, we calculate them in a series of molecules, including cyclobutadiene and its derivatives and cyclopentadienyl cation, by using correlated participating orbitals within the complete active space (CAS) and restricted active space (RAS) self-consistent field frameworks, followed by second-order perturbation theory (CASPT2 and RASPT2). These calculations are evaluated by comparison with the results of doubly electron-attached (DEA) equation-of-motion (EOM) coupled-cluster (CC) calculations with up to 4-particle-2-hole (4p-2h) excitations. We find active spaces that can accurately reproduce the DEA-EOMCC(4p-2h) data while being small enough to be applicable to larger organic diradicals. [ABSTRACT FROM AUTHOR]

In electronic structure theory, the availability of an analytical derivative is one of the desired features for a method to be useful in practical applications, as it allows for geometry optimization as well as computation of molecular properties. With the recent advances in the development of symmetryprojected Hartree-Fock (PHF) methods, we here aim at further extensions by devising the analytic gradients of post-PHF approaches with a special focus on spin-extended (spin-projected) configuration interaction with single and double substitutions (ECISD). Just like standard single-reference methods, the mean-field PHF part does not require the corresponding coupled-perturbed equation to be solved, while the correlation energy term needs the orbital relaxation effect to be accounted for, unless the underlying molecular orbitals are variationally optimized in the presence of the correlation energy. We present a general strategy for post-PHF analytical gradients, which closely parallels that for single-reference methods, yet addressing the major difference between them. The similarity between ECISD and multi-reference CI not only in the energy but also in the optimized geometry is clearly demonstrated by the numerical examples of ozone and cyclobutadiene. [ABSTRACT FROM AUTHOR]

Dedicated to Professor Manfred Scheer on the occasion of his 65th birthday The reaction of (1)Ni(η2‐cod), 2, incorporating a chelating bis(N‐heterocyclic carbene) 1, with P4 in pentane yielded the dinuclear complex [(2)Ni]2(μ2,η2 : η2‐P4), 3, formally featuring a cyclobutadiene‐like, neutral, rectangular, π‐bridging P4‐ring. In toluene, the butterfly‐shaped complex [(1)Ni]2(μ2,η2 : η2‐P2), 4, with a formally neutral P2‐unit was obtained from 2 and either P4 or 3. Computational studies showed that a haptotropic rearrangement involving two isomers of the μ2,η2 : η2‐P4 coordination mode and a low‐energy μ2,η4 : η4‐P4 coordination mode, as previously predicted for related nickel cyclobutadiene complexes, could explain the coalescence observed in the low‐temperature NMR spectra of 3. The insertion of the (1)Ni fragment into a P−P bond of P7(SiMe3)3, forming complex 5 with a norbornane‐like P7 ligand, was also observed. [ABSTRACT FROM AUTHOR]

Chosen from István Mayer's very impressive canon of important work on bond orders and related quantities, we explore a paper from 2012 which introduced a "pseudo spin density matrix" for correlated singlet‐state wave functions, leading to "improved" definitions of bond orders and free valence. Examining such "improved" bond orders for the singlet ground states of H2, N2 and CH+ we find that all of them exhibit sensible geometry dependences. Mayer's free valence index works well for H2 and N2; the asymptotic behavior for CH+ turns out to be slightly more complicated but can easily be explained. Using B2H6 to examine three‐center bonding, a simple generalization of Mayer's approach produces numerical results close to those based on the "pseudo spin density matrix." As expected, the various "correction" terms remain small, albeit some of them are larger for the multicenter indices of ground and excited singlet states of benzene, S2N2 and square (D4h) cyclobutadiene. [ABSTRACT FROM AUTHOR]

This Review surveys the experimental and synthetic history of three classic antiaromatic cores: cyclobutadiene, borole, and pentalene. The coverage extends from the bare molecular cores to fused or substituted variants that impart thermodynamic or kinetic stability to the antiaromatic systems. Recent studies to utilize the unusual properties of these antiaromatic cores in the context of advanced electronic materials are discussed. [ABSTRACT FROM AUTHOR]

Cyclobuta[1,2-b:3,4-b′]ditetracene – an analogue of nonacene with a cyclobutadiene unit embedded in the central part has been synthesized by the combination of solution and on-surface chemistry. The atomic structure and electronic properties of the product on Au(111) have been determined by high resolution scanning tunnelling microscopy/spectroscopy corroborated by density functional theory calculations. Structural and magnetic parameters derived from theoretical calculations reveal that π conjugation is dominated by radialene-type contribution, with an admixture of cyclobutadiene-like antiaromaticity. [ABSTRACT FROM AUTHOR]

The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory (DFT). The lowest energy (C4H4)2V2(CO)8 structure consists of two C4H4V(CO)4 units linked by a V-V single bond of length 3.4 Å. The two lowest energy (C4H4)2V2(CO)7 structures also have formal V-V single bonds. The "extra" two electrons to give each vanadium atom in these heptacarbonyls the favored 18-electron configuration can come from either an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings or from a four-electron donor bridging η2-µ-CO group with a short V–O distance. The lowest energy (C4H4)2V2(CO)6 structure has a formal V≡V triple bond of length 2.52 Å similar to the V≡V triple bond of length 2.46 Å found in the experimentally known cyclopentadienyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structures for the more highly unsaturated (C4H4)2V2(CO)n (n = 5, 4, 3, 2) have at least two four-electron donor bridging η2-µ-CO groups and a vanadium-vanadium bond order sufficient to give each vanadium atom at least a 16-electron configuration. The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory. The two lowest energy (C4H4)2V2(CO)7 structures include one with an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings and another with a four-electron donor bridging η2-µ-CO group with a short V–O bonding distance. [ABSTRACT FROM AUTHOR]

The title compound was made by the reaction of the CpCo-complex of 2,3,7,8-tetrakis(trimethylsilyl) linear [3]phenylene, in which the metal coordinates one of the cyclobutadiene rings, with CpCo(C2 H4)2. Because of its relative stability, a syn -dicobalt-bound configuration is indicated rather than its anti alternative. [ABSTRACT FROM AUTHOR]

The interplay between aromaticity and antiaromaticity is maximized in [n]phenylenes. In their smaller isomer sets, some isomers can be aromatic while others can be antiaromatic. The sum of the superimposed constituent circuits contributes to the individual ring aromaticity. Both cyclic conjugated energy (CCEi = efi ) of Bosanac and Gutman and Aihara's topological bond resonance energy (t-BRE) are good measures of relative degree of ring aromatic/antiaromatic character. Topological resonance energy (TRE) is a reliable measure of global molecular aromaticity/antiaromaticity because it is independent of selection of frame-of-reference molecules for calibration. [ABSTRACT FROM AUTHOR]

1,3,2,4‐Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground‐ and excited‐state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4‐diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence. [ABSTRACT FROM AUTHOR]

For the investigation of molecular systems with electronic ground states exhibiting multi-reference character, a spin-flip (SF) version of the algebraic diagrammatic construction (ADC) scheme for the polarization propagator up to third order perturbation theory (SF-ADC(3)) is derived via the intermediate state representation and implemented into our existing ADC computer program adcman. The accuracy of these new SF-ADC(n) approaches is tested on typical situations, in which the ground state acquires multi-reference character, like bond breaking of H2 and HF, the torsional motion of ethylene, and the excited states of rectangular and square-planar cyclobutadiene. Overall, the results of SF-ADC(n) reveal an accurate description of these systems in comparison with standard multireference methods. Thus, the spin-flip versions of ADC are easy-to-use methods for the calculation of "few-reference" systems, which possess a stable single-reference triplet ground state. [ABSTRACT FROM AUTHOR]

Recently developed routes to the synthesis of mono‐ and polyethynylated azulenes and their transformations into linear oligoazulenes with ethynyl and butadiynyl bridges as well as azulenyl‐substituted benzenes, cyclobutadiene complexes, and azulene‐substituted tetracyanobutadienes are reviewed. The utility of ethynylazulene derivatives for the synthesis of azulene‐substituted heterocycles has also been reviewed. [ABSTRACT FROM AUTHOR]

Cyclobutadiene rhodium complexes [(C4R4)RhX]2 (R=Et, Ph; X=I, Cl) at 0.5 mol % loading catalyze the formation of aromatic amides from various indoles, tosyl azide, and carbon monoxide (1 atm). The reaction gives good yields of the target products (typically 60–85 %) and tolerates both donor and moderately acceptor substituents in the indoles (alkyl, OMe, Hal, COOMe, but not CN). The DFT calculations at the PBE/3ζ level revealed a new plausible mechanism for the reaction of TsN3 with CO via cycloaddition and formation of RhCON3 metallacycle, which is more favorable than the commonly assumed formation of nitrene intermediate. [ABSTRACT FROM AUTHOR]

The 18-electron (η4-C4Me4)Co(CO)2TeI2Ph compound noticeably decomposes upon prolonged storage and also unusually transforms into the 17-electron (η4-C4Me4)CoI2(PhTeI) complex (I). The X-ray diffraction study of the molecular and crystal structures of complex I (CIF file CCDC no. 1991837) reveals interesting analogies with the molecular structures of the known metal complexes containing organotellurium halide ligands and with the crystal packing of the related one-component crystals of (η4-C4Me4)Co(СО)2TeI2Ph and (η4-C4Me4)Co(СО)2I and two-component crystals of (η4-C4Me4)Co(СО)2I–1,4-C6H4I2. An analysis of the crystal packing of molecules of complex I elucidates a new supramolecular synthone I···(η4-C4Me4) determining the structural motifs of self-assembly in complex I and related compounds (η4-C4Me4)Co(CO)2I, (η4-C4Me4)Co(CO)2I···1,4-C6H4I2, and (η4-C4Me4)Co(CO)2TeI2Ph. The EPR study of compound I shows that cobalt exists in the low-spin state with the total spin S = 1/2. [ABSTRACT FROM AUTHOR]

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C4H4•+ ion in the cluster beam. The measured average collision cross section of the C4H4•+ isomers in helium (38.9 ± 1 Å2) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C4H4•+ ion [methylenecyclopropene (39.9 Å2), 1,2,3-butatriene (41.1 Å2), cyclobutadiene (38.6 Å2), and vinyl acetylene (41.1 Å2)]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C4H4•+ ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C4H4•+ ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C2H2)2•+ [R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys. 131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C4H4•+(H2O)n clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C4H4•+H2O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene•+·H2O cluster (41 kJ/mol). The binding energies of the C4H4•+(H2O)n clusters change little from n = 1 to 5 (39-48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water-C4H4•+ and water-water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C4H4•+(H2O)5 cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C4H4•+(H2O)5 cluster presumably consisting of a cyclobutadiene•+ cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C2H2)2•+ ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation. [ABSTRACT FROM AUTHOR]

The multi-ionization equation-of-motion coupled-cluster (CC) method is developed for multireference (MR) problems. It is operationally single reference, depending upon a formal matrix diagonalization step to define the coefficients in the wavefunction in an unbiased way that allows for important MR character. The method is illustrated for the autoisomerization of cyclobutadiene, which has a very large multireference effect and compared to other MR-CC results. The newly implemented methods are also used to obtain the vertical double ionization (DI) potentials of several small molecules (H2O, CO, C2H2, C2H4). Also, the performance of the new methods is analyzed by plotting the potential energy curve for twisted ethylene as a function of a dihedral angle between two methylenes. Evaluation of the total molecular energy via MR-DI-CC calculations makes it possible to avoid an unphysical cusp. [ABSTRACT FROM AUTHOR]

The gauge-including magnetically induced current method for calculating the components of the current-density tensor using gauge-including atomic orbitals has been extended to treating open-shell molecules. The applicability of the method is demonstrated by calculations of first-order induced current densities on cyclobutadiene, Al3, and B3 at correlated ab initio levels of theory. For comparison, current-density calculations were also performed on the lowest closed-shell singlet state of cyclobutadiene as well on the closed-shell Al3- and B3- anions. The ring-current susceptibilities of the open-shell species are computed at the Hartree-Fock self-consistent-field, second-order Mo\ller- Plesset perturbation theory, and coupled-cluster singles and doubles levels, whereas for the closed-shell systems also density functional theory calculations were employed. Explicit values for the current strengths caused by α and β electrons as well as the difference, representing the spin current, were obtained by numerical integration of the current-density contributions passing a plane perpendicular to the molecular ring. Comparisons of the present results to those recently obtained for the lowest triplet state of biphenyl emphasize that electron correlation effects must be considered for obtaining an accurate description of spin-current densities. [ABSTRACT FROM AUTHOR]

The automerization of cyclobutadiene (CBD) is employed to test the performance of the reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest-size MR CISD wave function as an external source for the most important (primary) triples and quadruples in order to account for the nondynamic correlation effects in the presence of quasidegeneracy, as well as of its perturbatively corrected version accounting for the remaining (secondary) triples [RMR CCSD(T)]. The experimental results are compared with those obtained by the standard CCSD and CCSD(T) methods, by the state universal (SU) MR CCSD and its state selective or state specific (SS) version as formulated by Mukherjee et al. (SS MRCC or MkMRCC) and, wherever available, by the Brillouin–Wigner MRCC [MR BWCCSD(T)] method. Both restricted Hartree-Fock (RHF) and multiconfigurational self-consistent field (MCSCF) molecular orbitals are employed. For a smaller STO-3G basis set we also make a comparison with the exact full configuration interaction (FCI) results. Both fundamental vibrational energies—as obtained via the integral averaging method (IAM) that can handle anomalous potentials and automatically accounts for anharmonicity– and the CBD automerization barrier for the interconversion of the two rectangular structures are considered. It is shown that the RMR CCSD(T) potential has the smallest nonparallelism error relative to the FCI potential and the corresponding fundamental vibrational frequencies compare reasonably well with the experimental ones and are very close to those recently obtained by other authors. The effect of anharmonicity is assessed using the second-order perturbation theory (MP2). Finally, the invariance of the RMR CC methods with respect to orbital rotations is also examined. [ABSTRACT FROM AUTHOR]

We have formulated and implemented the multireference Mukherjee’s coupled cluster method with connected singles, doubles, and perturbative triples [MR MkCCSD(T)] in the ACES II program package. Assessment of the new method has been performed on the first three electronic states of the oxygen molecule and on the automerization barrier of cyclobutadiene, where a comparison with other multireference CC treatments and with experimental data where available. The MR MkCCSD(T) method seems to be a promising candidate for an accurate, yet computationally tractable, treatment of systems where the static correlation plays an important role. [ABSTRACT FROM AUTHOR]

A self-consistent field (SCF) London-orbital computational scheme to perform gauge-origin independent nonperturbative calculations for molecules in strong magnetic fields is presented. The crucial difference in the proposed approach with respect to common-origin finite-field SCF implementations consists in the evaluation of molecular integrals over the field-dependent molecular basis functions, which is tantamount to computing molecular integrals in a hybrid Gaussian and plane-wave basis set. The implementation of a McMurchie–Davidson scheme for the calculation of the molecular integrals over London orbitals is discussed, and preliminary applications of the newly developed code to the calculation of fourth-rank hypermagnetizabilities for a set of small molecules, benzene, and cyclobutadiene are presented. The nonperturbative approach is particularly useful for studying the highly nonlinear response of paramagnetic closed-shell systems such as boron monohydride, or the π-electron response of cyclobutadiene. [ABSTRACT FROM AUTHOR]

An approach to treat static correlation within a density-functional framework is presented. To that end, a multiconfiguration optimized effective potential (MCOEP) method is derived. In contrast to standard multiconfiguration self-consistent field (MCSCF) methods and previous combinations of MCSCF procedures with density-functional theory, the MCOEP method yields well-defined physically meaningful orbital and eigenvalue spectra. In addition to the electronic ground state also excited electronic states can be described. The MCOEP method is implemented invoking the localized Hartree–Fock approximation, leading to a multiconfiguration localized Hartree–Fock approach. Applications of the new method to the dissociation of the hydrogen molecule and the isomerization of ethene and cyclobutadiene show that it is capable of describing situations that are characterized by strong static correlation [ABSTRACT FROM AUTHOR]

Ab initio modified Gaussian-2 G2M(RCC,MP2) calculations have been performed for various isomers and transition states on the singlet C4H4 potential energy surface. The computed relative energies and molecular parameters have then been used to calculate energy-dependent rate constants for different isomerization and dissociation processes in the C4H4 system employing Rice-Ramsperger-Kassel-Marcus theory and to predict branching ratios of possible products of the C2(1Σg+)+C2H4, C(1D)+H2CCCH2, and C(1D)+H3CCCH reactions under single-collision conditions. The results show that C2 adds to the double C==C bond of ethylene without a barrier to form carbenecyclopropane, which then isomerizes to butatriene by a formal C2 “insertion” into the C–C bond of the C2H4 fragment. Butatriene can rearrange to the other isomers of C4H4, including allenylcarbene, methylenecyclopropene, vinylacetylene, methylpropargylene, cyclobutadiene, tetrahedrane, methylcyclopropenylidene, and bicyclobutene. The major decomposition products of the chemically activated C4H4 molecule formed in the C2(1Σg+)+C2H4 reaction are calculated to be acetylene+vinylidene (48.6% at Ecol=0) and 1-buten-3-yne-2-yl radical [i-C4H3(X 2A′),H2C==C==C==CH·]+H (41.3%). As the collision energy increases from 0 to 10 kcal/mol, the relative yield of i-C4H3+H grows to 52.6% and that of C2H2+CCH2 decreases to 35.5%. For the C(1D)+allene reaction, the most important products are also i-C4H3+H (55.2%) and C2H2+CCH2 (30.1%), but for C(1D)+methylacetylene, which accesses a different region of the C4H4 singlet potential energy surface, the calculated product branching ratios differ significantly: 65%–69% for i-C4H3+H, 18%–14% for C2H2+CCH2, and ∼8% for diacetylene+H2. [ABSTRACT FROM AUTHOR]

The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol-1, which is higher than the best earlier theoretical estimate of 4.0 kcal mol-1. This result is confirmed by including into the reference space the orbitals of the CC σ bonds beyond the standard π orbital space. It places the present value into the middle of the range of the measured data (1.6–10 kcal mol-1). An adiabatic singlet-triplet energy gap of 7.4 kcal mol-1 between the transition state 1Btg and the first triplet 3A2g state is obtained. A low barrier height for the CBD automerization and a small ΔE(3A2g,1B1g) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed. [ABSTRACT FROM AUTHOR]

We developed and implemented the state-specific Brillouin-Wigner coupled cluster method with singles, doubles, and noniterative perturbative triples, called MR BWCCSD(T), for a general number of closed- and open-shell reference configurations. To assess the accuracy of the method, we performed calculations of the three lowest electronic states of the oxygen molecule and of the automerization barrier of cyclobutadiene. For the oxygen molecule, the results were in a good agreement in comparison with those of the iterative MR BWCCSDTα method. For cyclobutadiene, the effect of connected triples was found to be minor, which is in agreement with the previous study by and Balková and Bartlett [J. Chem. Phys. 101, 8972 (1994)]. [ABSTRACT FROM AUTHOR]

Analytic gradient expressions for the spin-conserving and spin-flipping equation-of-motion coupled-cluster models with single and double substitutions are derived using a Lagrangian approach for the restricted and unrestricted Hartree–Fock references, both for the case of all orbitals being active in correlated calculations and for the frozen core and/or virtual orbitals. Details of the implementation within the Q-CHEM electronic structure package are discussed. The capabilities of the new code are demonstrated by application to cyclobutadiene. [ABSTRACT FROM AUTHOR]

In this work, the behavior of the Colle-Salvetti correlation functional is examined for strongly correlated systems with non-negligible nondynamic effects. Used with an appropriate multideterminantal wave function, it is able to reproduce accurately previous multireference coupled-cluster results for the problem of the automerization of cyclobutadiene, as well as to provide the correct energetical profiles for diatomic molecules under dissociation. The results confirm the current quality of the functional for complicated chemical problems, in spite of the fact that the functional does not satisfy some known exact properties. [ABSTRACT FROM AUTHOR]

Investigates various atomic and molecular multiplet states using a proposed spin-restricted open-shell Kohn-Sham (ROKS) method. Range of multiplets considered; ROKS equations; States of atomic species; Angular momentum states of linear molecules; Diradicaloid states of perpendicular ethylene and of square cyclobutadiene.

Investigates the vibrational spectra of cyclobutadiene (CB) and CB:carbon dioxide van der Waals molecule isotopomers. Analysis on the automerization energy levels of [sup 12]C[sub 4]H[sub 4]; Application of the argon/CB matrix-isolation spectra; Details on the multiconfiguration-computed energy of CB saddle-point.

Changing the mechanism of the widely used 1,3-dipolar cycloaddition reaction from its usual asynchronous one-step pattern to the rarely observed stepwise form leads to the emergence of intermediates, side products, and other impurities. Thus, it is crucial to determine the nature of the mechanism of the 1,3-dipolar cycloaddition reaction between a special 1,3-dipole and a specified dipolarophile (by theoretical methods) before using them for synthesizing a desired product. In this study, therefore, we have investigated the possibility of some probable intermediates emergence in the 1,3-dipolar cycloaddition reaction between cyclobutadiene and thioformaldehyde S-oxide. The results showed that emergence of Int (B) (−52.1 kcal mol−1) via transition state (B-1) is favorable both thermodynamically and kinetically (in comparison with all other stepwise routes). That is, developing probable impurities should not be neglected at least in the cases of the reactions between some thioformaldehyde S-oxide and some dipolarophiles. [ABSTRACT FROM AUTHOR]

The serendipitous isolation of very small amounts of two naphthocyclobutadiene (NCB) derivatives has led to the computational re‐examination of the electrocyclization of Z,Z‐3,5‐octadiene‐1,7‐diyne as well as the experimental and computational study of diethynylindeno[2,1‐a]fluorene derivatives that contain the 3,5‐octadiene‐1,7‐diyne motif as part of a larger π‐framework. In both cases the calculated potential energy surface strongly implicates two successive electrocyclic reactions to afford the antiaromatic products. With the octadienediyne fragment locked in the reactive conformation, the postulated diethynylindeno[2,1‐a]fluorene intermediates afford the NCBs in modest to good yields. X‐ray crystallography of four NCBs as well as NICS‐XY scan calculations show that the paratropic motif is located primarily in the benzocyclobutadiene fragment within the larger π‐scaffold. [ABSTRACT FROM AUTHOR]

Polycyclic conjugated hydrocarbons containing antiaromatic four‐membered cyclobutadienoids (CDB) are of great fundamental and technical interest. However, their challenging synthesis has hampered the exploration and understanding of such systems. Reported herein is a modular and efficient synthesis of novel CBD‐containing acene analogues, dinaphthobenzo[1,2:4,5]dicyclobutadiene (DNBDCs), with orthogonally tunable electronic properties and molecular packing. The design also features strong antiaromaticity of the CBD units, as revealed by nucleus‐independent chemical shift and anisotropy of the induced current density calculations, as well as X‐ray crystallography. Tuning the size of silyl substituents resulted in the most favorable "brick‐layer" packing for triisobutylsilyl‐DNBDC and a charge mobility of up to 0.52 cm2 V−1 s−1 in field‐effect transistors. [ABSTRACT FROM AUTHOR]

The objective of this investigation was to develop two cyclobutadiene-steroid derivatives (compounds 6 or 7) to evaluate its theoretical interaction on μ, δ and k opioid-receptors. The synthesis of 6 or 7 was carried out using a series of reactions which involves. 1) addition/cyclization: 2) imination, 3) etherification and 4) oxy-functionalization. Chemical structure of all compounds was confirmed using elemental analysis and NMR spectra. In addition, a theoretical analysis on the interaction of compounds 6 or 7 with μ, δ and k opioid-receptors was evaluated using a docking model. The results showed that 6 or 7 may interact with different type of amino acids residues on surface of the μ, δ and k opioid-receptors. Other data, indicated that inhibition constant (Ki) involved in the interaction of compounds 6 or 7 with k-receptor was less compared with the Ki present in the interaction with μ, δ receptors. These data indicated that 1) compounds 6 or 7 show a high affinity by k-receptor; 2) the cyclobutadiene analogs are particularly interesting, because these drugs may constitute a novel therapy for pain. [ABSTRACT FROM AUTHOR]

The electronic structure of the ground state and several low-lying excited states of cyclobutadiene are studied using the new state-universal multireference coupled-cluster method with single and double excitations (MR-CCSD) augmented by a noniterative inclusion of the triple excitations [MR-CCSD(T)]. Two possible ground state configurations are examined, namely the square and the distorted rectangular geometries, and the multireference coupled-cluster energy barrier for the interconversion between the two rectangular ground state structures is estimated to be 6.6 kcal mol-1 compared with the best theoretical value, 6.4 kcal mol-1 obtained using the highly accurate coupled-cluster method with full inclusion of the triple excitations (CCSDT). The ordering of electronic states for the square geometry is determined, with the ground state singlet being located 6.9 kcal mol-1 below the lowest triplet electronic state. We also examine the potential energy surface for the interconversion between the two equivalent second-order Jahn–Teller rhombic structures for the first excited singlet state. When comparing the MRCC energies with the results provided by various single- and multireference correlation methods, the critical importance of including both the dynamic and nondynamic correlation for a qualitatively correct description of the electronic structure of cyclobutadiene is emphasized. We also address the invariance properties of the present MRCC methods with respect to the alternative selections of reference orbital spaces. [ABSTRACT FROM AUTHOR]