Synthesis, structure and redox properties of bis(cyclopentadienyl)dithiolene complexes of molybdenum and tungsten

The compounds [Cp(2)M(S(2)C(2)(H)R)] (M = Mo or W; R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl) and [Cp(2)Mo(S(2)C(2)(Me)(pyridin-2-yl)] have been prepared by a facile and general route for the synthesis of dithiolene complexes, viz. the reaction of [Cp(2)MCl(2)] (M = Mo or W) with the dithiolene pro-ligand generated by reacting the corresponding 4-(R)-1,3-dithiol-2-one with CsOH. These Mo compounds were reported previously (Hsu et al., Inorg. Chem. 1996, 35, 4743); however, the preparative method employed herein is more versatile and generates the compounds in good yield and all of the W compounds are new. Electrochemical investigations have shown that each compound undergoes a diffusion controlled one-electron oxidation (OX(I)) and a one-electron reduction (RED(I)) process; each redox change occurs at a more positive potential for a Mo compound than for its W counterpart. The mono-cations generated by chemical or electrochemical oxidation are stable and the structures of both components of the [Cp(2)Mo(S(2)C(2)(H)R)](+)/[Cp(2)Mo(S(2)C(2)(H)R)] (R = Ph or pyridin-3-yl) redox couples have been determined by X-ray crystallography. For each redox related pair, the changes in the Mo-S, S-C and C-C bond lengths of the {MoSCCS} moiety are generally consistent with OX(I) involving the loss of an electron from a π-orbital that is Mo-S and C-S antibonding and C-C bonding in character. These results have been interpreted successfully within the framework provided by DFT calculations accomplished for [Cp(2)M(S(2)C(2)(H)Ph)](n) (M = Mo or W; n = +1, 0 or -1). The HOMO of the neutral compounds is derived mainly from the dithiolene π(3) orbital (65%); therefore, OX(I) is essentially a dithiolene-based process. The similarity of the potentials for OX(I) (ca. 30 mV) for analogous Mo and W compounds is consistent with this interpretation and the EPR spectra of each of the Mo cations show that the unpaired electron is coupled to the dithiolene proton but relatively weakly to (95,97)Mo. The DFT calculations indicate that the unpaired electron is more localised on the metal in the mono-anions than in the mono-cations. In agreement with this, the EPR spectrum of each of the Mo-containing mono-anions manifests a larger (95,97)Mo coupling (A(iso)) than observed for the corresponding mono-cation and RED(I) for a W compound is significantly (ca. 300 mV) more negative than that of its Mo counterpart. [Cp(2)W(S(2)C(2)(H)(quinoxalin-2-yl))] is anomalous; RED(I) occurs at a potential ca. 230 mV more positive than expected from that of its Mo counterpart and the EPR spectrum of the mono-anion is typical of an organic radical. DFT calculations indicate that these properties arise because the electron is added to a quinoxalin-2-yl π-orbital.     

Energetics and kinetics of the reaction of HOCO with hydrogen atoms

The potential energy surface for the reaction of HOCO radicals with hydrogen atoms has been explored using the CCSD(T)/aug-cc-pVQZ ab initio method. Results show that the reaction occurs via a formic acid (HOC(O)H) intermediate, and produces two types of products: H(2)O+CO and H(2)+CO(2). Reaction enthalpies (0 K) are obtained as -102.0 kcalmol for the H(2)+CO(2) products, and -92.7 kcalmol for H(2)O+CO. Along the reaction pathways, there exists a nearly late transition state for each product channel. However, the transition states locate noticeably below the reactant asymptote. Direct ab initio dynamics calculations are also carried out for studying the kinetics of the H+HOCO reaction. At room temperature, the rate coefficient is predicted to be 1.07x10(-10)cm(3) molec(-1) s(-1) with a negligible activation energy E(a)=0.06 kcalmol, and the branching ratios are estimated to be 0.87 for H(2)+CO(2), and 0.13 for H(2)O+CO. In contrast, the product branching ratios have a strong T dependence. The branching ratio for H(2)O+CO could increase to 0.72 at T=1000 K.     

A computational investigation of HCN2+ isomeric structures: implications for the chemistry of Titan's atmosphere.

The structure and stability of various HCN2+ isomeric structures have been investigated at the complete active space SCF (CASSCF) and multireference-configuration interaction [MR-Cl-SD(Q)] levels of theory with the 6-31G(d) and 6-311G(d,p) basis sets. The investigated species include the singlet (S) and triplet (T) open-chain H-N-C-N+ ions 1S, 1S', and 1T, the open-chain H-C-N-N+ ions 2S, 2S', and 2T, the HC-N2+ cyclic structures 3S and 3T, and the HN-CN+ cyclic structures 4S and 4T. All these species have been identified as true energy minima on the CASSCF(8,7)/6-31G(d) potential energy surface, and their optimised geometries, refined at the CASSCF(8,8)/6-31G(d) level of theory, have been used to perform single point calculations at the [MR-Cl-SD(Q]/6-311G(d,p) computational level. The most stable structure was the H-N-C-N+ ion 1T, whose absolute enthalpy of formation at 298.15 K has been estimated as 333.9 +/- 2 kcalmol(-1) using the Gaussian-3 (G3) procedure. The two species closest in energy to 1T are the triplet H-C-N-N+ ion 2T and the singlet diazirinyl cation 3S, whose G3 enthalpies of formation at 298.15 K are 343.5 +/- 2 and 340.6 +/- 2 kcalmol(-1), respectively. Finally, we have discussed the implications of our calculations for the detailed structure of the HCN2+ ions formed in the reaction between N3+ and HCN, experimentally observed by flowing after-glow-selected ion flow/drift tube mass spectrometry and possibly occurring in Titan's atmosphere.     

Electrochemical oxidations: Part IV. Electrochemical investigations into the behaviour of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)-phenol part 1. Phenol and the species derived from it: Phenoxy radical,phenolate anion and phenoxenium cation

The anodic oxidation of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)phenol (IIa) was investigated by cyclic voltammetry. The stable products resulting from the oxidation are phenoxenium cation (IId) (which itself is reduced in two one-electron steps to the phenolate anion (IIc) via the phenoxy radical, IIb) and the “protonated phenol (III) (which is oxidized to (IId) in a second peak at higher potential). A mechanism for the electrochemical oxidation of phenol (IIa) is suggested.     

Conformational analysis and stereoelectronic effects in trans-1,2-dihalocyclohexanes: 1H NMR and theoretical investigation

The conformational equilibrium of trans-1,2-difluoro- (1), trans-1,2-dichloro- (2) and trans-1,2-dibromo-cyclohexane (3) was studied through a combined method of NMR, theoretical calculations and solvation theory. The solvent dependence of the 3JH1,H2 NMR coupling constants together with theoretical calculations allow the direct determination of the conformational equilibria without recourse to model compounds. The coupling constants were obtained with the aid of spectrum simulation, since these symmetric molecules present complex coupling systems. The observed couplings, when analysed by solvation theory and utilising DFT geometries (B3LYP/6-311+G**), gave energy values of E(ee) -E(aa) of 0.10, 0.95 and 1.40 kcalmol(-1) in the vapour phase for 1, 2 and 3, respectively, decreasing to -0.63, 0.36 and 0.93 kcalmol(-1) in CCl4 and to -1.91, -0.80 and -0.05 kcalmol(-1) in DMSO solution. The diaxial preference for all compounds is explained by natural bond orbital (NBO) analysis, which shows important hyperconjugative effects in this conformation. The "gauche effect" for compounds with more electronegative substituents, which are in gauche arrangement in the ee conformation, also plays a relevant role in more polar solvents.     

Acyclic and cyclic forms of the radicals HO4? CH3O4? and C2H5O4·: ab initio quantum chemical calculations

Acyclic and cyclic structures and total energies of radicals HO₄⋅ CH₃O₄⋅ and C₂H₅O₄· were calculated by ab initio quantum chemical methods. Depending on the computational method and basis sets used, the cyclic conformer of the HO₄· radical is 4.8 to 7.3 kJ mol^-1 more stable than the acyclic one. For the first two representatives of the homologous series of alkyl tetraoxyl radicals, CH₃O₄· and C₂H₅O₄, MP2/6-311++G** calculations predict insignificant energy differences (1.2 kJ mol^-1) between six-membered cyclic and acyclic conformers. Apparently, these radicals can exist in both forms.     

17O NMR spectroscopic characterization and the mechanism of formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the low-temperature ozonation of isopropyl alcohol and isopropyl methyl ether: water-assisted decomposition

Low-temperature ozonation of isopropyl alcohol (1a) and isopropyl methyl ether (1b) in [D6]acetone, methyl acetate, and tert-butyl methyl ether at -78 degrees C produced the corresponding hydrotrioxides, Me2C(OH)(OOOH) (2a) and Me2C(OMe)(OOOH) (2b), along with hydrogen trioxide (HOOOH). All the polyoxides investigated were characterized for the first time by 17O NMR spectroscopy of highly 17O-enriched species. The assignment was confirmed by GIAO/MP2/6-31++G* calculations of 17O NMR chemical shifts, which were in excellent agreement with the experimental values. Ab initio density functional (DFT) calculations at the B3LYP/ 6-31G*+ZPE level have clarified the transition structure (TS1, deltaE = 7.4 and 10.6 kcalmol(-1), relative to isolated reactants and the complex 1a-ozone, respectively) for the ozonation of 1a: this, together with the formation of HOOOH and some other products, indicates the involvement of radical intermediates (R*, *OOOH) in the reaction. The activation parameters for the decomposition of the hydrotrioxides 2a and 2b (Ea, = 23.5+/-1.5 kcalmol(-1), logA = 16+/-1.8) were typical for a homolytic process in which cleavage of the ROOOH molecule occurs to yield a radical pair [RO* *OOH] and represents the lowest available energy pathway. Significantly the lower activation parameters for the decomposition of HOOOH (Ea = 16.5+/-2.2 kcalmol(-1), logA = 9.5+/-2.0) relative to those expected for the homolytic scission of the HO-OOH bond [bond dissociation energy (BDE) = 29.8 kcalmol(-1), CCSD(T)/6-311++G**] are in accord with the proposal that water behaves as a bifunctional catalyst and therefore participates in a "polar" (non-radical) decomposition process of this polyoxide. A relatively large acceleration of the decomposition of the hydrotrioxide 2a in [D6]acetone, accompanied by a significant lowering of the activation energies, was observed in the presence of a large excess of water. Thus intramolecular 1,3-proton transfer probably also involves the participation of water and is similar to the mechanism proposed for the decomposition of HOOOH. This hypothesis was further substantiated by the B3LYP/6-31++ G*+ZPE calculations for the participation of water in the decomposition of CH3OOOH, which revealed two stationary points on the potential energy surface corresponding to a CH3OOOH-HOH complex and a six-membered cyclic transition state TS2. The energy barriers were comparable with those calculated for HOOOH, that is, deltaE = 15.0 and 21.5 kcalmol(-1) relative to isolated reactants and the CH3OOOH-HOH complex, respectively.     

Mechanisms of [1,3]-Sigmatropic Migrations of the Nitroso Group in the ON-X-CH=X Systems (X = O,S, Se,NH, CH<Subscript>2</Subscript>)

[1,3]-Sigmatropic migrations of the nitroso group in the systems ON-X-CH=X (X = O, S, Se, NH, CH₂) were studied by MP2(fc)/6-311+G** and B3LYP/6-311+G** quantum-chemical calculations. The energy barrier in the process was estimated at 2.4 (2.5), 20.0 (25.0), and 22.3 (23.4) kcal/mol for X = O, NH, and CH₂, respectively. The energy minima for X = S and X = Se correspond to cyclic structures with two-coordinate NO group, which are more stable than acyclic structures by 9.3 (4.3) (X = S) or 13.1 (5.7) kcal/mol (X = Se).     

Density functional investigation of reactive intermediates derived from alkyne-Co2(CO)6 complexes

By applying the hybrid density functional method B3LYP and a flexible all-electron basis set, structures and energies of reactive intermediates derived from the 1-butyne complex of Co2(CO), (1) were calculated. In particular, the geometry, electronic distribution, and configurational stability of the cationic, radical, and anionic Co2(CO)6-complexed propargylic species were studied. The calculations revealed that the configurational barrier, that is, the racemization barrier for the antarafacial migration of the CHCH3 group, is low (7.6 kcalmol(-1)) for the radical and is similar to the experimental value for the corresponding cation (ca. 10 kcalmol(-1)). However, a high racemization barrier (23.7 kcalmol(-1)) for the anionic intermediate suggests the possibility of stereospecific reactions involving Co2(CO)6-complexed propargylic anions.     

Copolymers of carbon dioxide and nitrogen: an AM1 and ab initio computational study

Extending work by various groups on possible dimers, trimers, etc. of dinitrogen and of carbon dioxide, the authors have studied analogous copolymers of N₂ and CO₂ computationally. Twelve cyclic structures were examined with the AM1, HF/3-21G, HF/6-31G* and MP2(FC)/6-31G* methods, and the acyclic “monomer” to “tetramer” HO(C(O)O–N= N–)_nH, n=1–4, were studied at the AM1 and HF/3-21G levels; the cyclic species included 2-oxa-3,4-diazacyclobut-3-ene-1-one, 2-oxa-3,4,5,6-tetraazacyclohexa-3,5-diene-1-one, and various aza/oxa bicyclo[2.2.0] and bicyclo[2.2.2] systems. For the cyclic species, it was concluded that only the MP2(FC)/6-31G* results, which differ considerably from those at the other three levels, are likely to be reliable. These MP2 calculations indicate that only seven of the 12 cyclic structures studied are stationary points (one is a transition structure), and none of them is kinetically stable at room temperature. Although some have high energy densities (ca. 7–10 kJ g⁻¹), their expected low kinetic stabilities seems to make this of little practical value. The acyclic “copolymers” were all relative minima at the AM1 and HF/3-21G levels; unlike the cyclic species, their kinetic stabilities were not investigated directly by comparing the energies of reactants and decomposition transition states. The energy density of the infinite acyclic polymer was found by extrapolation to be 5.1 (AM1) or 5.6 (3-21G) kJ g⁻¹. The calculated vibrational spectra of the MP2 stationary points and of the acyclic molecules gave some indication of instability by the presence of low-frequency modes leading in the limit to decomposition.     

Thermal rearrangements of 2-vinylcyclopropylidene to cyclopentadiene and vinylallene: a theoretical investigation

In an attempt to clarify the favored rearrangement reaction of vinylcyclopropylidenes, the prototype thermal rearrangements of singlet 2-vinylcyclopropylidene (1) leading to 1,3cyclopentadiene (2) and 1,2,4-pentatriene (vinylallene) (3) were investigated by means of ab initio quantum-mechanical electronic-structure calculations. The B3LYP functional with the 6-31G(d) basis set was employed for geometry optimization of the equilibrium and transition-state structures relevant to the two reaction pathways and for computing their harmonic vibrational frequencies. Final energies were evaluated by single-point calculations at the CCSD(T) level of theory with the 6-311 + G(3df,2p) basis set. The rearrangement of s-cis 1 to 2 is found to occur by a three-step pathway. The first step involves the formation of a nonclassical carbene (5), which is an internal pi complex between the pi molecular orbital of the double bond and the empty p atomic orbital of the carbene carbon. In the second step, the nonplanar five-membered ring geometry of 5 flattens to reach the planar structure of 3-cyclopentenylidene (4). The last step is the 1,2-migration of a alpha-hydrogen atom to the carbene center in 4. The rate-determining step for the rearrangement of s-cis 1 to 2 is the formation of 5, with a predicted global deltaG++(220 K) of only 0.6 kcalmol(-1). The rearrangement of s-trans 1 to 2 requires an initial conversion of s-trans 1 to the s-cis conformer, with a predicted deltaG++(220 K) of 1.8 kcalmol(-1). The transition structure for the ring-opening of s-trans 1 into s-trans 3 (deltaG++(220 K)=4.7 kcalmol(-1)) is more energetic than that for the ring-opening of s-cis 1 into s-cis 3 (deltaG++(220 K)=2.5 kcalmol(-2)) due to larger repulsive nonbonded H...H interactions in the former transition structure. On the basis of these results, it is suggested that if the reaction of 1,1-dibromo-2-vinylcyclopropane with methyllithium at -78 degrees C leads to the initial formation of carbene 1, then the reaction should yield 2 as the main product together with small amounts of 3. This theoretical prediction nicely agrees with experimental findings.     

Multireference configuration interaction calculations for the F(2P)+HCl-->HF+Cl(2P) reaction: a correlation scaled ground state (1 2A') potential energy surface

This paper presents a new ground state (1 (2)A(')) electronic potential energy surface for the F((2)P)+HCl-->HF+Cl((2)P) reaction. The ab initio calculations are done at the multireference configuration interaction+Davidson correction (MRCI+Q) level of theory by complete basis set extrapolation of the aug-cc-pVnZ (n=2,3,4) energies. Due to low-lying charge transfer states in the transition state region, the molecular orbitals are obtained by six-state dynamically weighted multichannel self-consistent field methods. Additional perturbative refinement of the energies is achieved by implementing simple one-parameter correlation energy scaling to reproduce the experimental exothermicity (DeltaE=-33.06 kcalmol) for the reaction. Ab initio points are fitted to an analytical function based on sum of two- and three-body contributions, yielding a rms deviation of <0.3 kcalmol for all geometries below 10 kcalmol above the barrier. Of particular relevance to nonadiabatic dynamics, the calculations show significant multireference character in the transition state region, which is located 3.8 kcalmol with respect to F+HCl reactants and features a strongly bent F-H-Cl transition state geometry (theta approximately 123.5 degrees ). Finally, the surface also exhibits two conical intersection seams that are energetically accessible at low collision energies. These seams arise naturally from allowed crossings in the C(infinityv) linear configuration that become avoided in C(s) bent configurations of both the reactant and product, and should be a hallmark of all X-H-Y atom transfer reaction dynamics between ((2)P) halogen atoms.     

Isomers of cyclo-heptasulfur and their coordination to Li(+): an ab initio molecular orbital study

The potential energy hypersurfaces (PESs) of heptasulfur (S7) and of [LiS7]+ have been investigated by ab initio molecular orbital calculations at the G3X(MP2) level of theory. Besides the chair-like seven-membered ring (1a) as the global minimum structure, eight local minimum structures and one transition state have been located on the PES of S7. The barrier for pseudorotation of 1a is only 5.6 kJ mol(-1). The boat-like S7 ring (1b) is 12.1 kJ mol(-1) less stable than 1a, followed by three isomers of connectivity S6=S and four open-chain isomers. On the basis of multireference calculations at the MRCI(4,4)+Q/6-311G(d) level, the most stable open-chain form of S7 is a triplet of relative energy 133.1 kJ mol(-1). Thus, the reaction energy (deltaE0) for the ring opening of 1a is 133.1 kJ mol(-1), halfway between those of the highly symmetrical rings S6 and S8. Because of their strong multireference characters, the stabilities of the biradicalic singlet chains are significantly overestimated by the single-reference-based G3X(MP2) method. The calculated vibrational spectrum of 1a is in good agreement with experimental data. The various isomers of S7 form stable complexes with Li+ with coordination numbers of 1-4 for the metal atom and binding energies in the range of -93.8 to -165.7 kJ mol(-1). A total of 15 isomeric complexes have been located, with 13 of them containing cyclic ligands. The global minimum structure (2a) is composed of 1a, with the Li+ cation linked to the four negatively charged sulfur atoms (symmetry C(s)). The corresponding complex 2c containing the ligand 1b is by 23.4 kJ mol(-1) less stable than 2a, and a bicyclic crown-shaped LiS7 cation (2e) is by 34.9 kJ mol(-1) less stable than 2a. Even less stable are four complexes with the branched S6=S ligand. SS bond activation by polarization of the valence electrons takes place on coordination of Li+ to cyclo-S7 (1a).     

Torquoselectivity in the cationic cyclopentannelation of (2Z)-hexa-2,4,5-trienal acetals

Inter- and intramolecular trapping experiments and density functional theory ab initio calculations for model systems are consistent with the acid-catalyzed rearrangement of 2-[(1Z)-hexa-1,3,4-trienyl]dioxolanes 1 to tetrahydroalkylidenecyclopenta-1,4-dioxins 4; this involves the electrocyclic ring closure of substituted hydroxypentadienyl carbocations. The reaction, which may be considered a variant of the Nazarov cyclization, occurs much more readily than the standard Nazarov cyclization, proceeding rapidly even at -30 degrees C. B3LYP/6-31G**//HF/6-31G** calculations for models 36, 38 and 40 predict that the two alternative conrotations at the cyclization termini are associated with activation energies differing by 0.55, 0.56 and 1.60 kcalmol(-1), respectively, in favour of the R-outwards rotation. This last value corresponds to an E-41/Z-41 product ratio of >99:1 at -60 degrees C, in consonance with the experimental observation that divinylallene 1a rearranges exclusively to E-4a at temperatures below -30 degrees C. At higher temperatures the torquoselectivity of the reaction 1a-->4a is masked by subsequent isomerization to the Z isomer, the greater stability of which is attributable to steric interaction between the substituent at the exocyclic double bond and the bulky neighbouring tBu group in the E isomer.     

Ab initio calculations on the mechanism of the oxidation of the hydroxymethyl radical by molecular oxygen in the gas phase: a significant reaction for environmental science

The mechanism of the gas-phase reaction of *CH2OH+O2 to form CH2O+HO2* was studied theoretically by means of high-level quantum-chemical electronic structure methods (CASSCF and CCSD(T)). The calculations indicate that the oxidation of *CH2OH by O2 is a two-step process that goes through the peroxy radical intermediate *OOCH2OH (1), formed by the barrier-free radical addition of *CH2OH to O2. The concerted elimination of HO2* from 1 is predicted to occur via a five-membered ringlike transition structure of Cs symmetry, TS1, which lies 19.6 kcalmol(-1) below the sum of the energies of the reactants at 0 K. A four-membered ringlike transition structure TS2 of Cs symmetry, which lies 13.9 kcalmol(-1) above the energy of the separated reactants at 0 K, was also found for the concerted HO2* elimination from 1. An analysis of the electronic structures of TS1 and TS2 indicates that both modes of concerted HO2* elimination from 1 are better described as internal proton transfers than as intramolecular free-radical H-atom abstractions. The intramolecular 1,4-H-atom transfer in 1, which yields the alkoxy radical intermediate HOOCH2O*, takes place via a puckered ringlike transition structure TS3 that lies 13.7 kcalmol(-1) above the energy of the reactants at 0 K. In contrast with earlier studies suggesting that a direct H-atom abstraction mechanism might occur at high temperatures, we could not find any transition structure for direct H-atom transfer from the OH group of *CH2OH to the O2. The observed non-Arrhenius behavior of the temperature dependence of the rate constant for the gas-phase oxidation of *CH2OH is ascribed to the combined effect of the initial barrier-free formation of the *OO-CH2OH adduct with a substantial energy release and the existence of a low-barrier and two high-barrier pathways for its decomposition into CH2O and HO2*.     

The rearrangement of the cubane radical cation in solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.     

Structures of C4

Linear (1), cyclic (2) and bicyclic (3) alternatives are considered as possible ground-state structures for C₄. At the highest levels of theory, MP4SDQ/6-31¹//HF/6-31¹, 3, with two π electrons is found to be most stable.     

Ab initio and DFT studies on van der Waals trimers: the OCS.(CO2)2 complexes

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS.(CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm-1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer.     

From Eight‐Membered 10π Electron Sulfur–Nitrogen Cycles to Bicycles and Cages: A Theoretical Approach

A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur–nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight‐membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular S?S bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit. The final structures depend on the number of π electrons in the bridges. Intermediate biradicals are stabilized by Jahn–Teller distortion, giving transannular S?S bonds between the NSN units. This procedure may be summarized by two rules, which provide a rationale for the structures of a large number of sulfur–nitrogen‐based molecules. The long bonds between the NSN units show a p character of >95 %. The qualitative results have been compared with known molecular structures and the results of B3LYP/cc‐pVTZ calculations as well as CASSCF and CASVB calculations. B3LYP/cc‐pVTZ calculations have also provided the UV/Vis spectra and the NICS values of the planar 10π systems.     

The monoanionic pi-radical redox state of alpha-iminoketones in bis(ligand)metal complexes of nickel and cobalt

The electronic structures of nickel and cobalt centers coordinated by two alpha-iminoketone ligands have been elucidated using density functional theory calculations and a host of physical methods such as X-ray crystallography, cyclic voltammetry, UV-vis spectroscopy, electron paramagnetic resonance spectroscopy, and magnetic susceptibility measurements. In principle, alpha-iminoketone ligands can exist in three oxidation levels: the closed-shell neutral form (L)0, the closed-shell dianion (L(red))(2-), and the open-shell monoanion (L*)(-). Herein, the monoanionic pi-radical form (L*)(-) of alpha-iminoketones is characterized in the compounds [(L*)2Ni] (1) and [(L*)2Co] (3), where (L*)(-) is the one-electron-reduced form of the neutral ligand (t-Bu)N=CH-C(Ph)=O. The metal centers in 1 and 3 are divalent, high-spin, and coupled antiferromagnetically to two ligand pi radicals. These bis(ligand)metal complexes can be chemically oxidized by two electrons to give the dications [trans-(L)2Ni(CH3CN)2](PF6)2 (2) and [trans-(L)2Co(CH3CN)2](PF6)2 (4), wherein the ligands are in the neutral form.