ABSTRACT
A stable aluminum tris(dithiolene) triradical (3) was experimentally realized through a low-temperature reaction of the sterically demanding lithium dithiolene radical (2) with aluminum iodide. Compound 3 was characterized by single-crystal X-ray diffraction, UV-vis and EPR spectroscopy, SQUID magnetometry, and theoretical computations. The quartet ground state of triradical 3 has been unambiguously confirmed by variable-temperature continuous wave EPR experiments and SQUID magnetometry. Both SQUID magnetometry and broken-symmetry DFT computations reveal a small doublet-quartet energy gap [ΔEDQ = 0.18 kcal mol-1 (SQUID); ΔEDQ = 0.14 kcal mol-1 (DFT)]. The pulsed EPR experiment (electron spin echo envelop modulation) provides further evidence for the interaction of these dithiolene-based radicals with the central aluminum nucleus of 3.
ABSTRACT
E2H2 (E = As, Sb, Bi) structures involving multiple bonds have attracted much attention recently. The E2H3+ cations (protonated E2H2) are predicted to be viable with substantial proton affinities (>180 kcal/mol). Herein, the bonding characters and energetics of a number of E2H3+ isomers are explored through CCSD(T) and DFT methods. For the As2H3+ system, the CCSD(T)/cc-pVQZ-PP method predicts that the vinylidene-like structure lies lowest in energy, with the trans and cis isomers higher by 6.7 and 9.3 kcal/mol, respectively. However, for Sb2H3+ and Bi2H3+ systems, the trans isomer is the global minimum, while the energies of the cis and vinylidene-like structures are higher, respectively, by 2.0 and 2.4 kcal/mol for Sb2H3+ and 1.6 and 15.0 kcal/mol for Bi2H3+. Thus, the vinyledene-like structure is the lowest energy for the arsenic system but only a transition state of the bismuth system. With permanent dipole moments, all minima may be observable in microwave experiments. Besides, we have also obtained transition states and planar-cis structures with higher energies. The current results should provide new insights into the various isomers and provide a number of predictions for future experiments.
ABSTRACT
The "gold standard" CCSD(T) method is adopted along with the correlation consistent basis sets up to aug-cc-pV5Z-PP to study the mechanism of the hydrogen abstraction reaction H2Te + OH. The predicted geometries and vibrational frequencies for reactants and products are in good agreement with the available experimental results. With the ZPVE corrections, the transition state in the favorable pathway of this reaction energetically lies 1.2 kcal mol-1 below the reactants, which is lower than the analogous relative energies for the H2Se + OH reaction (-0.7 kcal mol-1), the H2S + OH reaction (+0.8 kcal mol-1) and the H2O + OH reaction (+9.0 kcal mol-1). Accordingly, the exothermic reaction energies for these related reactions are predicted to be 47.8 (H2Te), 37.7 (H2Se), 27.1 (H2S), and 0.0 (H2O) kcal mol-1, respectively. Geometrically, the low-lying reactant complexes for H2Te + OH and H2Se + OH are two-center three-electron hemibonded structures, whereas those for H2S + OH and H2O + OH are hydrogen-bonded. With ZPVE and spin-orbit coupling corrections, the relative energies for the reactant complex, transition state, product complex, and the products for the H2Te + OH reaction are estimated to be -13.1, -1.0, -52.0, and -52.6 kcal mol-1, respectively. Finally, twenty-eight DFT functionals have been tested systematically to assess their ability in describing the potential energy surface of the H2Te + OH reaction. The best of these functionals for the corresponding energtics are -9.9, -1.4, -46.4, and -45.4 kcal mol-1 (MPWB1K), or -13.1, -2.4, -57.1, and -54.6 kcal mol-1 (M06-2X), respectively.
ABSTRACT
Three different pathways for the atomic iodine plus water trimer reaction I + (H2O)3 â HI + (H2O)2OH were preliminarily examined by the DFT-MPW1K method. Related to previous predictions for the F/Cl/Br + (H2O)3 reactions, three pathways for the I + (H2O)3 reaction are linked in terms of geometry and energetics. To legitimize the results, the "gold standard" CCSD(T) method was employed to investigate the lowest-lying pathway with the correlation-consistent polarized valence basis set up to cc-pVQZ(-PP). According to the CCSD(T)/cc-pVQZ(-PP)//CCSD(T)/cc-pVTZ(-PP) results, the I + (H2O)3 â HI + (H2O)2OH reaction is predicted to be endothermic by 47.0 kcal mol-1. The submerged transition state is predicted to lie 43.7 kcal mol-1 above the separated reactants. The I···(H2O)3 entrance complex lies below the separated reactants by 4.1 kcal mol-1, and spin-orbit coupling has a significant impact on this dissociation energy. The HI···(H2O)2OH exit complex is bound by 4.3 kcal mol-1 in relation to the separated products. Compared with simpler I + (H2O)2 and I + H2O reactions, the I + (H2O)3 reaction is energetically between them in general. It is speculated that the reaction between the iodine atom and the larger water clusters may be energetically analogous to the I + (H2O)3 reaction. The iodine reaction I + (H2O)3 is connected with the analogous valence isoelectronic bromine/chlorine reactions Br/Cl + (H2O)3 but much different from the F + (H2O)3 reaction. Significant difference with other halogen systems, especially for barrier heights, are seen for the iodine systems.
Subject(s)
Iodine , Quantum Theory , Water , Chlorine , ChloridesABSTRACT
Three different reaction pathways are found for the reaction of bromine atom (Br) with the lowest-energy structure of the water trimer [uud-(H2O)3], initially using the MPW1K-DFT method. The three bromine pathways have closely related geometries and energetics, analogous to those found for the fluorine and chlorine reactions. The lowest-energy pathway of the Br + uud-(H2O)3 reaction was further investigated using the "gold standard" CCSD(T) method and the correlation-consistent basis sets up to cc-pVQZ(-PP). Based on the CCSD(T)/cc-pVQZ(-PP)//CCSD(T)/cc-pVTZ(-PP) results, the Br + (H2O)3 reaction is endothermic by 33.3 kcal mol-1. The classical barrier height is 29.0 kcal mol-1 between the reactants and the exit complex, and there is no barrier for the reverse reaction. The Brâ¯(H2O)3 entrance complex is found to lie 4.7 kcal mol-1 below the separated reactants, and the HBrâ¯(H2O)2OH exit complex is bound by 6.4 kcal mol-1 relative to the separated products. This potential energy profile is further corrected by the zero point energies and spin-orbit coupling effects. Structurally, the Br + (H2O)3 stationary points can be derived from those of the simpler Br + (H2O)2 reaction by judiciously appending a H2O molecule. The Br + (H2O)3 potential energy profile is compared with the Br + (H2O)2 and Br + H2O reactions, as well as to the valence isoelectronic Cl + (H2O)3 and F + (H2O)3 systems. It is reasonable to expect that the reactions between the bromine atom and larger water clusters would be similar to the Br + (H2O)3 reaction.
ABSTRACT
A carbene-stabilized dithiolene zwitterion (3) activates ammonia, affording 4⢠and 5, through both single-electron transfer (SET) and hydrogen atom transfer (HAT). Reaction products were characterized spectroscopically and by single-crystal X-ray diffraction. The mechanism of the formation of 4⢠and 5 was probed by experimental and computational methods. This discovery is the first example of metal-free ammonia activation via HAT.
Subject(s)
Ammonia , Hydrogen , Electron Transport , Hydrogen/chemistry , Methane/analogs & derivativesABSTRACT
Herein, we report a comprehensive study of CO2 hydroboration catalyzed by Mn pincer complexes. The traditional metal-ligand cooperation (MLC) mechanism based on the H-Mn-N-Bpin pincer complex is not viable due to the competing abstraction of the Bpin group from the H-Mn-N-Bpin complex by NaOtBu. Instead, we propose an ionic mechanism based on the H-Mn-N-Na species with a low energy span (22.5 kcal/mol) and unveil the acceleration effect of bases. The X groups in the H-Mn-N-X catalyst models are further modulated, and the steric hindrance and HâB donor-acceptor interactions of the X group increase the energy barrier of the hydride transfer. The hydrogen bond and electrostatic interactions of the X group can accelerate the hydride transfer to HCOOBpin and HCHO molecules except for the nonpolar CO2 molecule. Based on these discoveries, we designed a pyridine-based Mn pincer catalyst system, which could achieve CO2 hydroboration in low-temperature and base-free conditions through a metal-ligand cooperation mechanism.
ABSTRACT
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
Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H2O)3 â HCl + (H2O)2OH using a standard DFT method. As found for the analogous fluorine reaction, the geometrical and energetic results for the four chlorine pathways are closely related. However, the energetics for the Cl reaction are very different from those for fluorine. In the present paper, we investigate the lowest-energy chlorine pathway using the "gold standard" CCSD(T) method in conjunction with correlation-consistent basis sets up to cc-pVQZ. Structurally, the stationary points for the water trimer reaction Cl + (H2O)3 may be compared to those for the water monomer reaction Cl + H2O and water dimer reaction Cl + (H2O)2. Based on the CCSD(T) energies, the title reaction is endothermic by 19.3 kcal mol-1, with a classical barrier height of 16.7 kcal mol-1 between the reactants and the exit complex. There is no barrier for the reverse reaction. The Clâ¯(H2O)3 entrance complex lies 5.3 kcal mol-1 below the separated reactants. The HClâ¯(H2O)2OH exit complex is bound by 8.6 kcal mol-1 relative to the separated products. The Cl + (H2O)3 reaction is somewhat similar to the analogous Cl + (H2O)2 reaction, but qualitatively different from the Cl + H2O reaction. It is reasonable to expect that the reactions between the chlorine atom and larger water clusters may be similar to the Cl + (H2O)3 reaction. The potential energy profile for the Cl + (H2O)3 reaction is radically different from that for the valence isoelectronic F + (H2O)3 system, which may be related to the different bond energies between HCl and HF.
ABSTRACT
In 2019, Diaz-Urrutia and Ott developed a high-yield method for direct conversion of methane to methanesulfonic acid and proposed a cationic chain reaction mechanism. However, Roytman and Singleton questioned this mechanism, and they favored a free-radical mechanism. In the present paper, we studied both the cationic chain and radical mechanisms and found the radical mechanism is more favorable, since it has a much lower energy barrier. However, the radical mechanism has not considered the effect of ions for the reaction taking place in oleum. Thus, we studied a simple model of a protonated radical mechanism, which further lowers the energy barrier. Although the true mechanism for the CH4 + SO3 reaction could be more complicated in electrolyte solutions, this model should be helpful for the further study of the mechanism of this reaction.
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A series of reactions between Lewis bases and an imidazole-based dithione dimer (1) has been investigated. Both cyclic(alkyl)(amino)carbene (CAAC) (2) and N-heterocyclic carbene (NHC) (4), in addition to N-heterocyclic silylene (NHSi) (6), demonstrate the capability to cleave the sulphur-sulphur bonds in 1, giving carbene-stabilized dithiolene (L0 ) zwitterions (3 and 5) and a spirocyclic silicon-dithiolene compound (7), respectively. The bonding nature of 3, 5, and 7 are probed by both experimental and theoretical methods.
ABSTRACT
The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C4H6)3M (M = Zr-Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C4H6)3Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C4H6)3M structures of the late second-row transition metals couple to form a C12H18 ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C4H6)3M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C4H6)3Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C4H6)3Cr analog relative to quintet (C12H18)Cr isomers with an open-chain C12H18 ligand.
ABSTRACT
Important recent experimental studies have allowed the isomer-selective identification of the 2-, 3-, and 4-picolyl radicals. The picolyl radicals and their valence isoelectronic P, As, Sb, and Bi congeners are investigated here. For the three observed parent radicals, the theoretical ionization potentials agree with experiment to within 0.02 eV. Two rules are proposed for predicting vertical ionization potentials (EVIE) and relative energies. The EVIE values for these radicals will be higher when large percentages of the SOMO orbitals are distributed on the atoms with greater electronegativities. The cations of these systems were also studied along with the closed-shell methylpyridines and their P, As, Sb, and Bi analogs. The energies for the cationic species will lie lower when high percentages of π natural localized molecular orbitals occur on the more electronegative atoms. The structures of the 2- and 4-isomers strongly depend upon the heteroatoms, with the C-C linkages adopting a single-double alternating bond manner when the heteroatoms become heavier. The 3-isomers adopt roughly equal C-C bond distances with small changes from N to Bi.
ABSTRACT
While the 1 : 1 reaction of 3 with an N-heterocyclic carbene ({(Me)CN(i-Pr)}2C:) in THF resulted in ligand-substituted product 4, the corresponding 1 : 2 reaction (in the presence of H2O) gives the first structurally characterized germanium tris(dithiolene)dianion 5 as the major product and the "naked" dithiolene radical 6Ë as a minor by-product. The structure and bonding of 4 and 5 were probed by experimental and theoretical methods. Our study suggests that carbene-mediated partial hydrolysis may represent a new method to access tris(dithiolene) complexes of main-group elements.
ABSTRACT
Reaction of the lithium dithiolene radical 2⢠with the imidazolium salt [{(Me)CN(i-Pr)}2CH]+[Cl]- (in a 1:1 molar ratio) gives the first stable naked anionic dithiolene radical 3â¢, which, when coupled with hexasulfide, [{(Me)CN(i-Pr)}2CH]+2[S6]2- (4), and N-heterocyclic silylene 5, unexpectedly results in synergic THF ring-opening via a radical mechanism.
Subject(s)
Heterocyclic Compounds/chemistry , Imidazoles/chemistry , Sulfhydryl Compounds/chemistry , Sulfides/chemistry , Anions/chemistry , Free Radicals/chemistry , Furans/chemistry , Models, Molecular , Molecular Structure , Solvents/chemistryABSTRACT
This research reports a search for peculiar monobridged structures of the E2H2 molecules (E = Be, Mg, Ca, Sr, Ba). For Be2H2 and Mg2H2, the monobridged geometry is not an equilibrium but rather a transition state between the vinylidene-like structure and the global minimum HE-EH linear geometry. However, for Ca2H2, Sr2H2, and Ba2H2, this situation changes significantly; the linear structure is no longer the global minimum but lies higher in energy than two other equilibria, the dibridged and monobridged structures. The planar dibridged structures of both Sr2H2 and Ba2H2 should be observable via IR spectroscopy. Although the remarkable monobridged structures lie 8.3 (Sr) and 7.6 kcal/mol (Ba) higher, the large IR intensities of the terminal E-H stretching frequencies may make the monobridged structures observable. The monobridged structures have sizable permanent dipole moments (3.07 and 3.06 D for Sr and Ba, respectively) and also should be observable via microwave spectroscopy.
ABSTRACT
Taxanes (paclitaxel, docetaxel, cabazitaxel) are anticancer drugs as microtubule inhibitors. Following our previous studies on paclitaxel and docetaxel, in this work, we examine cabazitaxel and compare these three taxenes. The binding interaction of three taxanes with various ß-tubulin isotypes is studied by homology modeling, molecular docking, and molecular dynamics simulations. The results show that the effects of docetaxel on ßI-tubulin (- 29.5 kcal/mol) and of paclitaxel on ßIIa-tubulin (- 25.5 kcal/mol) are much stronger than their effects on ßIII-tubulin (- 17.8 kcal/mol and - 8.6 kcal/mol, respectively). However, the effect of cabazitaxel on ßIII-tubulin (- 23.0 kcal/mol) is comparable with that on ßI-tubulin (- 24.0 kcal/mol) and ßIIa-tubulin (- 25.9 kcal/mol), consistent with the fact that overexpression of ßIII-tubulin increases the drug resistance to paclitaxel and docetaxel, but has little influence for cabazitaxel. This theoretical research supports the use of cabazitaxel for patients who are resistant to the action of paclitaxel and docetaxel.
Subject(s)
Taxoids/pharmacology , Tubulin/metabolism , Antineoplastic Agents/pharmacology , Docetaxel/pharmacology , Drug Resistance, Neoplasm/drug effects , Humans , Microtubules/drug effects , Molecular Docking Simulation , Molecular Dynamics Simulation , Paclitaxel/pharmacology , Thermodynamics , Tubulin Modulators/pharmacologyABSTRACT
There is considerable interest, from both experimental and theoretical perspectives, in molecules incorporating multiple bonds between main group elements. Herein, we not only consider the parent molecules HE=EH (E=As, Sb, Bi), but also a number of their isomers. For each E2 H2 molecule, a number of different structures were optimized with four different DFT methods. Final structures were determined with the coupled cluster method CCSD(T) using large basis sets, namely cc-pVQZ-PP, incorporating relativistic psuedopotentials. All feasible dissociation pathways are examined. For all three E2 H2 molecules the trans isomer lies lowest in energy, with the cis isomer higher by 2.7 (As), 2.1 (Sb), and 1.8 (Bi)â kcal mol-1 , respectively. However, both cis and trans structures should be observable, as large barriers (27.7, 20.5, and 17.7â kcal mol-1 ) separate them. For both the cis and trans structures, in the infrared the strong E-H stretching frequencies should also be observable. Only the cis structures have dipole moments (0.62, 0.01, and 0.83 debye, respectively), and their observation by microwave spectroscopy would be stunning. Also considered were the higher energy vinylidene-like, pyramidal, monobridged, and linear structures. We conclude that molecules such as HSb=SbH-Fe(CO)4 , HBi=BiH-Fe(CO)4 , and related systems, should be feasible synthetic targets.
ABSTRACT
All important stationary points on the potential energy surface (PES) for the reaction OH + (H2O)3â (H2O)2OH + H2O have been fully optimized using the "gold standard" CCSD(T) method with the large Dunning correlation-consistent cc-pVQZ basis sets. Three types of pathways were found. For the pathway without hydrogen abstraction, the barrier height of the transition state (TS1) is predicted to lie 5.9 kcal mol-1 below the reactants. The two major complexes (H2O)3OH (CP1 and CP2a) are found to lie 6.3 and 11.0 kcal mol-1, respectively, below the reactants [OH + (H2O)3]. For one of the H-abstraction pathways the lowest classical barrier height is predicted to be much higher, 6.1 kcal mol-1 (TS2a) above the reactants. For the other H-abstraction pathway the barrier height is even higher, 15.0 (TS3) kcal mol-1. Vibrational frequencies and the zero-point vibrational energies connected to the PES are also reported. The energy barriers for the H-abstraction pathways are compared with those for the OH + (H2O)2 and OH + H2O reactions, and the effects of the third water on the energetics are usually minor (0.2 kcal mol-1).
ABSTRACT
Fluorocarbons have been shown experimentally by Baker and coworkers to combine with the cyclopentadienylcobalt (CpCo) moiety to form fluoroolefin and fluorocarbene complexes as well as fluorinated cobaltacyclic rings. In this connection density functional theory (DFT) studies on the cyclopentadienylcobalt fluorocarbon complexes CpCo(L)(CnF2n) (L = CO, PMe3; n = 3 and 4) indicate structures with perfluoroolefin ligands to be the lowest energy structures followed by perfluorometallacycle structures and finally by structures with perfluorocarbene ligands. Thus, for the CpCo(L)(C3F6) (L = CO, PMe3) complexes, the perfluoropropene structure has the lowest energy, followed by the perfluorocobaltacyclobutane structure and the perfluoroisopropylidene structure less stable by 8 to 11 kcal mol-1, and the highest energy perfluoropropylidene structure less stable by more than 12 kcal mol-1. For the two metal carbene structures Cp(L)Co[double bond, length as m-dash]C(CF3)2 and Cp(L)Co[double bond, length as m-dash]CF(C2F5), the former is more stable than the latter, even though the latter has Fischer carbene character. For the CpCo(L)(C4F8) (L = CO, PMe3) complexes, the perfluoroolefin complex structures have the lowest energies, followed by the perfluorometallacycle structures at 10 to 20 kcal mol-1, and the structures with perfluorocarbene ligands at yet higher energies more than 20 kcal mol-1 above the lowest energy structure. This is consistent with the experimentally observed isomerization of the perfluorinated cobaltacyclobutane complexes CpCo(PPh2Me)(-CFR-CF2-CF2-) (R = F, CF3) to the perfluoroolefin complexes CpCo(PPh2Me)(RCF[double bond, length as m-dash]CF2) in the presence of catalytic quantities of HN(SO2CF3)2. Further refinement of the relative energies by the state-of-the-art DLPNO-CCSD(T) method gives results essentially consistent with the DFT results summarized above.