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The cobalt carbonyl nitrosyls Co(NO)(CO)n(n= 4, 3, 2, 1) and Co2(NO)2(CO)n(n= 5, 4, 3, 2) have been studied by density functional theory. The lowest energy structures for the mononuclear derivatives Co(NO)(CO)n(n= 3, 2, 1) parallel those of the corresponding isoelectronic Ni(CO)n+1derivatives. In addition, a thermodynamically unstable Co(NO)(CO)4structure is predicted with a bent nitrosyl group. The potential energy surfaces of the binuclear cobalt carbonyl nitrosyl derivatives Co2(NO)2(CO)n(n= 5, 4, 3, 2) are complicated by the similar energies of analogous structures with bridging carbonyl or bridging nitrosyl groups. However, these structures are analogous to those previously predicted for the isoelectronic binuclear nickel carbonyls Ni2(CO)n+2. Thus the lowest energy structures of Co2(NO)2(CO)5, Co2(NO)2(CO)4, and Co2(NO)2(CO)3have one, two, and three bridging groups, respectively, just like the isoelectronic Ni2(CO)n+2. For Co2(NO)2(CO)5and Co2(NO)2(CO)4the otherwise analogous structures with bridging carbonyl and/or nitrosyl groups are of very similar energies. However, for Co2(NO)2(CO)3there appears to be a particularly stable triply carbonyl bridged structure with an unusually short CoCo distance consistent with the formal triple bond required to give the cobalt atoms the favored 18-electron configuration. [ABSTRACT FROM AUTHOR]

For the saturated carbonyl trimer Ru3(CO)12theoretical methods predict a doubly bridged Ru3(CO)10(μ-CO)2structure to lie only 0.3 kcal mol−1above the unbridged global minimum in accord with the fluxional properties observed experimentally by carbon-13 NMR on the very stable unbridged Ru3(CO)12. For M3(CO)11and M3(CO)10the global minima for the Fe, Ru, and Os derivatives each have totally different arrangements of the carbonyl groups. Thus the global minimum of Ru3(CO)11is singly edge-semibridged Ru3(CO)10(μ-CO) in contrast to the doubly face-bridged Fe3(CO)9(μ3-CO)2and triply edge-bridged Os3(CO)8(μ-CO)3found as global minima for the iron and osmium analogues, respectively. Furthermore, comparison of our predicted ν(CO) frequencies for low-lying unbridged, singly bridged, and doubly bridged isomers of Ru3(CO)11with the 1987 experiments of Bentsen and Wrighton indicates that three different Ru3(CO)11isomers are generated in these low-temperature inert matrix Ru3(CO)12photolysis experiments depending on the conditions. For Ru3(CO)10the global minimum is a triply bridged structure Ru3(CO)7(μ-CO)3, which is very different from the Fe3(CO)9(μ3-CO) and Os3(CO)8(μ3-CO)(μ-CO) global minima found for the iron and osmium analogues, respectively. For Ru3(CO)9the global minimum is a singly bridged structure Ru3(CO)8(μ-CO) analogous to the global minimum for the osmium analogue but totally different from the triply bridged global minimum of Fe3(CO)9. [ABSTRACT FROM AUTHOR]

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic binuclear rhenium carbonyls Re2(CO)n (n = 10, 9, 8, 7) were determined using the MPW1PW91 and BP86 methods from density functional theory (DFT) with the effective core potential basis sets LANL2DZ and SDD. In all cases triplet structures for Re2(CO)n were found to be unfavorable energetically relative to singlet structures, in contrast to corresponding Mn2(CO)n derivatives, apparently owing to the larger ligand field splitting of rhenium. For M2(CO)10 (M = Mn, Re) the unbridged structures (OC)5M–M(CO)5 are preferred energetically over structures with bridging CO groups. For M2(CO)9 (M = Mn, Re) the two low energy structures are (OC)4M(μ-CO)M(CO)4 with an M–M single bond and a four-electron donor bridging CO group and (OC)4MM(CO)5 with no bridging CO groups and an MM distance suggesting a double bond. The lowest energy structures for Re2(CO)8 have ReRe distances in the range 2.6–2.7 Å suggesting the triple bonds required to give the Re atoms the favored 18-electron configuration. Low energy structures for Re2(CO)7 are either of the type (OC)4MM(CO)3 with short metal–metal distances suggesting triple bonds or have a single four-electron donor bridging CO group and longer M–M distances consistent with single or double bonds. The 18-electron rule thus appears to be violated in these highly unsaturated Re2(CO)7 structures. [ABSTRACT FROM AUTHOR]

The structures of the tetranuclear osmium carbonyl derivatives Os4(CO)n (n = 16, 15, 14, 13, 12) have been investigated using the density functional theory method MPW1PW91 with the SDD effective core potential basis set, found to be effective in previous work for the study of Os3(CO)12. The Os4 clusters in the lowest energy structures for Os4(CO)16, Os4(CO)15, and Os4(CO)14 are found to be rhombi, butterflies, and tetrahedra with four, five, and six Os–Os bonds, respectively, in accord with structures determined by X-ray diffraction as well as the 18-electron rule. The fluxionality of tetrahedral Os4(CO)14, suggested by experimental work of Johnston, Einstein, and Pomeroy, is confirmed by our DFT studies, which find four Os4(CO)14 structures within 1.5 kcal mol−1 of each other with similar tetrahedral Os4 frameworks but with different arrangements of bridging and semibridging carbonyl groups. The lowest energy structures for the more unsaturated Os4(CO)13 and Os4(CO)12 are also based on Os4 tetrahedra but with shorter Os–Os edge lengths than in Os4(CO)14 suggesting delocalized multiple bonding in the more highly unsaturated systems. Thus the global minimum for Os4(CO)12 is predicted to have tetrahedral symmetry, with all terminal carbonyl groups analogous to the experimentally known structure of (µ3-H)4Re3(CO)12, but without the face-bridging hydrogen atoms. [ABSTRACT FROM AUTHOR]

The synthesis and structure of a Zn–Zn-bonded compound supported by a doubly reduced α-diimine ligand, [Na(THF)2]2·[LZn–ZnL] (L = [(2,6-iPr2C6H3)N(Me)C]22−) are reported, with a Zn–Zn bond length of 2.399(1) Å. [ABSTRACT FROM AUTHOR]

Neutral bis(pentalene)metal sandwich compounds have been synthesized for the first row transition metals titanium and iron. In this connection, the complete series of first row transition metal bis(pentalene)metal complexes (C8H6)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) has now been investigated by density functional theory in order to evaluate the effect of the metal electronic requirements on the hapticity of the pentalene ligands. The lowest energy structure for the titanium complex is the 18-electron complex (η8-C8H6)(η6-C8H6)Ti with one octahapto and one hexahapto pentalene ligand rather than the previously suggested 20-electron complex (η8-C8H6)2Ti with two octahapto pentalene ligands. The experimental NMR observations on (C8H6)2Ti can then be reinterpreted as interchange of octahapto and hexahapto bonding of the pentalene units through a low energy bis(octahapto) (η8-C8H6)2Ti transition state. The lowest energy structure for (C8H6)2V has two fulvene-like hexahapto pentalene ligands and a local vanadium environment similar to the well-known dibenzenevanadium. The lowest energy structures of the later first row transition metals have two pentahapto pentalene ligands with local metal environments similar to those in the corresponding metallocenes. In the manganese and iron structures of this type, the remaining unpaired electrons on one of the uncomplexed carbon atoms in each η5-C8H6system form a C–C single bond to couple the two pentalene ligands. This coupling of the two pentalene systems through a C–C single bond is similar to that found experimentally by X-ray crystallography in bis(pentalene)iron, (η5-C8H6)2Fe. [ABSTRACT FROM AUTHOR]

Recently synthesized by the group of Sadler, the platinum(iv) diazido complexes [Pt(N3)2(OH)2(L′)(L′′)] (L′ and L′′ are N-donor ligands) have potential to be used as photoactivatable metallodrugs in cancer chemotherapy. In the present study optimized structures and UV-Vis electronic spectra of trans,trans,trans- and cis,trans,cis-[Pt(N3)2(OH)2(NH3)2] (1tand 1c, respectively) as well as cis,trans,cis-[Pt(N3)2(OH)2(L)2] (L = NH3, NH2CH3, NF3, PH3, PF3, H2O, CO, OH−, CN−, py, imid; 2c–11c) and cis,trans-[Pt(N3)2(OH)2(bpy)] (12c) complexes were predicted using density functional theory (DFT). The ground state electronic structures of all complexes were analyzed with the help of the natural bond orbital analysis (NBO). The electronic spectra of 1cand 1twere computed using time-dependent density functional theory (TDDFT) with five different density functionals and the ab initioCASSCF/CASPT2 method (for the five lowest energy transitions). The best agreement with available experiments was found in the case of the long-range corrected ωB97X functional. The electronic transitions were characterized by the analysis of the natural transition orbitals (NTO). The low-lying excited singlet states of 1tand 1chave significant azide-to-platinum(iv) charge-transfer character (LMCT). Geometry optimization of the three lowest singlet excited states performed using TDDFT results in the simultaneous dissociation of two azide ligands with the formation of the azidyl radicals N3 and photoreduction of PtIVto PtII. Variation of the ligand L does not strongly affect the nature and the relative energies of the low-lying states. It is shown that the replacement of the OH−groups in 1cby OPh−ligands results in the red shift of the intense N3−→Pt LMCT band and the appearance of transitions with significant intensity in the visible region of the spectrum. The dissociative nature of the low-lying unoccupied orbitals remains unaffected. These theoretical results may suggest new experimental routes for the improvement of the photochemical activity of PtIVdiazido complexes. [ABSTRACT FROM AUTHOR]

Azulene is reported to react with Mn2(CO)10to give trans-C10H8Mn2(CO)6, which has been shown by X-ray crystallography to have a bis(pentahapto) structure with no metal-metal bond. This structure is found by density functional theory to be the lowest energy C10H8Mn2(CO)6structure. However, a corresponding bis(pentahapto) cis-C10H8Mn2(CO)6structure, also without an MnMn bond, lies within ∼1 kcal mol−1of this global minimum. The lowest energy C10H8Mn2(CO)5structure is singlet cis-η5,η5-C10H8Mn2(CO)5with an Mn→Mn dative bond from the Mn(CO)3group to the Mn(CO)2group. However, a singlet cis-η6,η4-C10H8Mn2(CO)5structure with a normal Mn–Mn single bond lies within ∼6 kcal mol−1of this global minimum. The lowest energy structures of the more highly unsaturated C10H8Mn2(CO)n(n= 4, 3, 2) systems all have cisgeometries and manganese-manganese bonds of various orders. The corresponding global minima are triplet cis-η5,η3-C10H8Mn2(CO)3(η2-μ-CO) for the tetracarbonyl with a four-electron donor bridging carbonyl group, singlet cis-η5,η5-C10H8Mn2(CO)3for the tricarbonyl, and triplet cis-η6,η4-C10H8Mn2(CO)(η2-μ-CO) for the dicarbonyl. [ABSTRACT FROM AUTHOR]

Recent work has resulted in the discovery of the fluorocarbyne complex (η5-C5H5)Mo(CO)2(CF), in which the CF ligand behaves as a formal three-electron donor like the isoelectronic well-known NO ligand. In this connection the related fluorocarbyne cobalt carbonyls Co(CF)(CO)n(n= 3, 2, 1) and Co2(CF)2(CO)n(n= 6, 5, 4, 3, 2) have been studied by density functional theory. The lowest energy structures for the mononuclear Co(CF)(CO)nderivatives parallel those of the isoelectronic Co(NO)(CO)nand Ni(CO)n+1derivatives. The mononuclear fluorocarbyne complex Co(CF)(CO)3is predicted to be thermodynamically unstable with respect to dimerization to the difluoroacetylene complex (FCCF)Co2(CO)6by ∼46 kcal mol−1. The binuclear Co2(CF)2(CO)nstructures can broadly be divided into two classes: (1) Structures in which the two CF ligands are coupled to form a Co2C2tetrahedrane derivative containing an FCCF ligand derived from difluoroacetylene; (2) Structures maintaining separate CF ligands. With important exceptions, the structures of the difluoroacetylene complexes (FCCF)Co2(CO)n(n= 6, 5, 4) parallel for the most part the structures predicted for the corresponding unsubstituted acetylene complexes. The relative energies of the Co2(CF)2(CO)nstructures with separate CF ligands indicate that the CF ligand is a more favorable bridging ligand than the ubiquitous CO ligand. [ABSTRACT FROM AUTHOR]

Density functional theory is employed in understanding the reactivity in the TiCl4 catalyzed Friedel–Crafts benzylation of benzene with substituted benzyl chlorides in nitromethane solvent. A series of ten substituted (in the aromatic ring) benzyl chlorides are characterized by theoretical reactivity indices. The theoretical parameters are juxtaposed to experimental relative rates of benzylation. It is established that the carbon–chlorine ionic bond dissociation energy and the Hirshfeld charge at the chlorine atom for the benzyl chlorides reactants – describe quite satisfactorily the reactivity trends. These results provide further insights into the factors governing reactivity in EAS reactions, which so far have been mostly focused on rate variations induced by changes in the structure of the aromatic substrate. The EAS benzylation investigated is quite unusual since, in contrast to most EAS reactions, the latest experimental kinetic results suggest that the aromatic substrate does not participate in the kinetic equation of the process. To shed more light on this unexpected result, we also conducted a theoretical study on the mechanistic pathway by applying M06-2X density functional computations combined with several basis sets: 6-311+G(d,p), 6-311+G(2df,2p), and def2-TZVPP. Because of the well-known difficulties in evaluating realistic free energy barriers for organic reactions, we tested two solvent models in determining the barrier for the TiCl4-catalyzed Friedel–Crafts benzylation of benzene by benzyl chloride. Since all methods employed did not provide satisfactory results for the free energy barriers, we used a combination of theoretically estimated enthalpy barriers and the available (from kinetic experiments) entropy contribution. This approach enabled us to verify that indeed the rate of this EAS reaction does not depend on the nature of the aromatic substrate. The computations revealed the structure and relative energies of the critical structures along the mechanistic pathway. Four intermediates were established along the reaction route. [ABSTRACT FROM AUTHOR]

Fulminic acid (HCNO) played a critical role in the early development of organic chemistry, and chemists have sought to discern the structure and characteristics of this molecule and its isomers for over 200 years. The mercurial nature of the extremely flat H–C–N bending potential of fulminic acid, with a nearly vanishing harmonic vibrational frequency at linearity, remains enigmatic and refractory to electronic structure theory, as dramatic variation with both orbital basis set and electron correlation method is witnessed. To solve this problem using rigorous electronic wavefunction methods, we have employed focal point analyses (FPA) to ascertain the ab initio limit of optimized linear and bent geometries, corresponding vibrational frequencies, and the HCN + O(³P) → HCNO reaction energy. Electron correlation treatments as extensive as CCSDT(Q), CCSDTQ(P), and even CCSDTQP(H) were employed, and complete basis set (CBS) extrapolations were performed using the cc-pCVXZ (X = 4–6) basis sets. Core electron correlation, scalar relativistic effects (MVD1), and diagonal Born–Oppenheimer corrections (DBOC) were all included and found to contribute significantly in determining whether vibrationless HCNO is linear or bent. At the all-electron (AE) CCSD(T)/CBS level, HCNO is a linear molecule with ω5(H–C–N bend) = 120 cm−1. However, composite AE-CCSDT(Q)/CBS computations give an imaginary frequency (51i cm−1) at the linear optimized geometry, leading to an equilibrium structure with an H–C–N angle of 173.9°. Finally, at the AE-CCSDTQ(P)/CBS level, HCNO is once again linear with ω5 = 45 cm−1, and inclusion of both MVD1 and DBOC effects yields ω5 = 32 cm−1. A host of other topics has also been investigated for fulminic acid, including the dependence of re and ωi predictions on a variety of CBS extrapolation formulas, the question of multireference character, the N–O bond energy and enthalpy of formation, and issues that give rise to the quasibent appellation. [ABSTRACT FROM AUTHOR]

Five structures of Ge2H2 and Ge2H2+ are investigated in this study. Optimized geometries at the CCSD(T)/cc-pwCVQZ-PP level of theory were obtained. Focal point analyses were performed on these optimized geometries to determine relative energies using the CCSD(T) method with polarized basis sets up to quintuple-zeta. Energy corrections include full T and pertubative (Q) coupled-cluster effects plus anharmonic corrections to the zero-point vibrational energy. Relative ordering in energy from lowest to highest of the five Ge2H2+ structures is butterfly, germylidene, monobridged, trans, then linear. In neutral Ge2H2, the monobridged structure lies lower in energy than the germylidene structure. Fundamental vibrational frequencies and IR intensities were computed for the minima at the CCSD(T)/cc-pwCVTZ-PP level of theory to compare with experimental research. Partial atomic charges and natural bonding orbital analyses indicated that the positive charge of Ge2H2+ is contained in the region of the Ge–Ge bond. [ABSTRACT FROM AUTHOR]

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9 via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

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 Br⋯(H2O)3 entrance complex is found to lie 4.7 kcal mol−1 below the separated reactants, and the HBr⋯(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 FROM AUTHOR]

Hierarchical, convergent ab initio benchmark computations were performed followed by a systematic analysis of DFT performance for five pericyclic reactions comprising Diels-Alder, 1,3-dipolar cycloaddition, electrocyclic rearrangement, sigmatropic rearrangement, and double group transfer prototypes. Focal point analyses (FPA) extrapolating to the ab initio limit were executed via explicit quantum chemical computations with electron correlation treatments through CCSDT(Q) and correlation-consistent Gaussian basis sets up to aug′-cc-pV5Z. Optimized geometric structures and vibrational frequencies of all stationary points were obtained at the CCSD(T)/cc-pVTZ level of theory. The FPA reaction barriers and energies exhibit convergence to within a few tenths of a kcal mol−1. The FPA benchmarks were used to evaluate the performance of 60 density functionals (eight dispersion-corrected), covering the local-density approximation (LDA), generalized gradient approximations (GGAs), meta-GGAs, hybrids, meta-hybrids, double-hybrids, and range-separated hybrids. The meta-hybrid M06-2X functional provided the best overall performance [mean absolute error (MAE) of 1.1 kcal mol−1] followed closely by the double-hybrids B2K-PLYP, mPW2K-PLYP, and revDSD-PBEP86 [MAE of 1.4–1.5 kcal mol−1]. The regularly used GGA functional BP86 gave a higher MAE of 5.8 kcal mol−1, but it qualitatively described the trends in reaction barriers and energies. Importantly, we established that accurate yet efficient meta-hybrid or double-hybrid DFT potential energy surfaces can be acquired based on geometries from the computationally efficient and robust BP86/DZP level. [ABSTRACT FROM AUTHOR]

Four different reaction pathways are initially located for the reaction of Cl atom plus water trimer Cl + (H2O)3 → 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 Cl⋯(H2O)3 entrance complex lies 5.3 kcal mol−1 below the separated reactants. The HCl⋯(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 FROM AUTHOR]

In the Comment on our paper on the description of the Gibbs Free energy profile of Claisen–Schmidt condensation, it is claimed that our calculations are flawed due to inconsistencies with experimental isotope effects in aqueous acetonitrile. In this Reply, we presented rigorous arguments, ambiguities in the Comment and new calculations confirming the consistency of our results: (i) small differences in the relative energetic barriers in both experimental and theoretical curves make the assignment of the rate-limiting step debatable, making the concept of RCS questionable; (ii) it is shown how the misinterpretation of the elementary steps and of the overall processes rate constants led the Comment to incorrect conclusions about the behavior of the inverse isotopic effect; (iii) neglect in the Comment of the inverse kinetic isotope effect in step R2 due to the hybridization conversion, and of the inverse equilibrium isotopic effect for step R1 to describe an overall iKIE > 1, (iv) an erroneous suggestion in the Comment that the disagreement between experimental kinetic parameters is due to the fact that acetonitrile is not used in previous experimental works, when contradictorily the literature recommends it as being indispensable to allow kinetic accuracy; and (v) new calculations improved by explicit-implicit hybrid treatment again ensure that step R4, and not step R5, can assume the role of RCS in protic solvents. Recognizing that questioning is an excellent path for promoting understanding, we hope that the answers provided here will help to clarify and expand the pertinent topics under discussion. [ABSTRACT FROM AUTHOR]

The reactions between substituted isocyanates (RNCO) and other small molecules (e.g. water, alcohols, and amines) are of significant industrial importance, particularly for the development of novel polyurethanes and other useful polymers. We present very high-level ab initio computations on the HNCO + H2O reaction, with results targeting the CCSDT(Q)/CBS//CCSD(T)/cc-pVQZ level of theory. Our results affirm that hydrolysis can occur across both the N＝C and C＝O bonds of HNCO via concerted mechanisms to form carbamate or imidic acid with ΔH0K barrier heights of 38.5 and 47.5 kcal mol−1. A total of 24 substituted RNCO + H2O reactions were studied. Geometries obtained with a composite method and refined with CCSD(T)/CBS single point energies determine that substituted RNCO species have a significant influence on these barrier heights, with an extreme case like fluorine lowering both barriers by close to 15 kcal mol−1 and most common alkyl substituents lowering both by approximately 3 kcal mol−1. Natural Bond Orbital (NBO) analysis provides evidence that the predicted barrier heights are strongly associated with the occupation of the in-plane C–O* orbital of the RNCO reactant. Key autocatalytic mechanisms are considered in the presence of excess water and RNCO species. Additional waters (one or two) are predicted to lower both barriers significantly at the CCSD(T)/aug-cc-pV(T+d)Z level of theory with strongly electron withdrawing RNCO substituents also increasing these effects, similar to the uncatalyzed case. The 298 K Gibbs energies are only marginally lowered by a second catalyst water molecule, indicating that the decreasing ΔH0K barriers are offset by loss of translational entropy with more than one catalyst water. Two-step 2RNCO + H2O mechanisms are characterized for the formation of carbamate and imidic acid. The second step of these two pathways exhibits the largest barrier and presents no clear pattern with respect to substituent choice. Our results indicate that an additional RNCO molecule might catalyze imidic acid formation but have less influence on the efficiency of carbamate formation. We expect that these results lay a firm foundation for the experimental study of substituted isocyanates and their relationship to the energetic pathways of related systems. [ABSTRACT FROM AUTHOR]

Email chemcomm-rsc@rsc.org Chemical Communications (print: ISSN 1359-7345; electronic: ISSN 1364-548X) is published 100 times a year by the Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, UK CB4 0WF. HIGHLIGHTS 2441 Chorismate- and isochorismate converting enzymes: versatile catalysts acting on an important metabolic node Florian Hubrich,* Michael Mu ller and Jennifer N. Andexer* 2464 Electrochemical rearrangement protocols towards the construction of diverse molecular frameworks Debarshi Saha, Irshad Maajid Taily, Rakesh Kumar and Prabal Banerjee* IN THIS ISSUE ISSN 1359-7345 CODEN CHCOFS 57(20) 2431-2574 (2021) Cover See Kei Goto et al., pp. 2479-2482. ChemComm Chemical Communications rsc.li/chemcomm The Royal Society of Chemistry is the world's leading chemistry community. [Extracted from the article]

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 6˙ 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 FROM AUTHOR]

The cyano radical (CN) is an abundant, open-shell molecule found in a variety of environments, including the atmosphere, the interstellar medium and combustion processes. In these environments, it often reacts with small, closed-shell molecules via hydrogen abstraction. Both carbon and nitrogen atoms of the cyano radical are reactive sites, however the carbon is more reactive with reaction barrier heights generally between 2–15 kcal mol−1 lower than those of the analogous nitrogen. The CN + HX → HCN/HNC + X, with X = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, C2H, CN reactions have been studied at a high-level of theory, including CCSD(T)-F12a. Furthermore, kinetics were obtained over the 100–1000 K temperature range, showing excellent agreement with those rate constants that have been determined experimentally. [ABSTRACT FROM AUTHOR]

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H2O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H2 and CO2 (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FA⋯water complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol−1 plus 6.07, 3.79, 2.60, and 2.55 kcal mol−1 for the trans-FA⋯water and cis-FA⋯water complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol−1 and the barrier to decarboxylation by up to 15.90 kcal mol−1. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FA⋯H2O system as well as provide insight into future studies of similar atmospheric systems. [ABSTRACT FROM AUTHOR]

Criegee intermediates are important targets for study in atmospheric chemistry because of their capacity to oxidize airborne species. Among these species, ammonia has received critical attention for its presence in polluted agricultural or industrial areas and its role in forming particulate matter and condensation nuclei. Although methylamine has been given less attention than ammonia, both theoretical and experimental studies have demonstrated that the additional methyl substitution on the ammonia derivatives increases the rate constants for some systems. This suggests that the methylamine addition to Criegee intermediates could be more significant to atmospheric processes. In this work, geometries are optimized at the DF-CCSD(T)/ANO1 level for the methylamine addition reactions to the simplest Criegee intermediate and the anti- and syn-methylated Criegee intermediates. Energies for each stationary point were computed at the CCSD(T)/CBS level with corrections from the CCSDT(Q) method. Rate constants are obtained for each reaction using canonical transition state theory. Although methylamine addition proved to be a more favorable reaction relative to ammonia addition, the significantly lower concentration of atmospheric methylamine limits the prevalence of these reactions, even in the most optimal conditions. It is unlikely that the methylamine addition to Criegee intermediates will contribute significantly to the consumption of Criegee intermediates in the atmosphere. [ABSTRACT FROM AUTHOR]

The 1-norbornyl (nor) ligand is known experimentally to form stable transition metal alkyl derivatives through direct metal–carbon bond formation. This appears to be related to its steric bulk and inaccessibility towards β-hydrogen elimination, as exemplified by the tetraalkyls (nor)4M, some of which are very stable. In this connection we have used density functional theory and the DLPNO-CCSD(T) method to investigate the 1-norbornylcobalt carbonyl derivatives (nor)Co(CO)n (n = 4, 3, 2, 1) and (nor)2Co2(CO)n (n = 7, 6, 5). Low-energy structures of the unsaturated systems (nor)Co(CO)n (n = 3, 2) and (nor)2Co2(CO)n (n = 6, 5) are found to have agostic hydrogen atoms from a CH2 group adjacent to the Co–C bond. Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than ∼2 Å. In such structures unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH2 group. In addition, structures in which carbonyl migration from cobalt to carbon has occurred to form acyl norCO ligands are among the lowest energy structures. The resulting acyl carbonyl groups of the norCO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety. Furthermore, such neutral norCO acyl ligands can either be one-electron donors to a cobalt atom, bonding solely through the carbonyl carbon, or three-electron donor η2-μ-norCO groups bridging a central Co2 unit through both the acyl carbon and oxygen atoms. The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient (RDG) values. [ABSTRACT FROM AUTHOR]

All important stationary points on the potential energy surface (PES) for the reaction OH + (H2O)3 → (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)3⋯OH (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 FROM AUTHOR]

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＝C(CF3)2 and Cp(L)Co＝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＝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. [ABSTRACT FROM AUTHOR]

The lowest lying singlet and triplet states of HBCN−, HCCN, HNCN+, HAlCN−, HSiCN, HPCN+, HGaCN−, HGeCN, and HAsCN+ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet–triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet–triplet gap (ΔEST = Esinglet − Etriplet) of 3.5 kcal mol−1, 11.9 kcal mol−1, and 22.6 kcal mol−1, respectively, for HBCN−, HCCN, and HNCN+. The second row increasing favors the singlet state, with singlet–triplet gaps of −20.4 kcal mol−1 (HAlCN−), −26.6 kcal mol−1 (HSiCN), and −26.8 kcal mol−1 (HPCN+). The third row also favors the singlet state, with singlet–triplet gaps of −26.8 kcal mol−1 (HGaCN−), −33.5 kcal mol−1 (HGeCN), and −33.1 kcal mol−1 (HAsCN+). The HXCN species have larger absolute singlet–triplet energy gaps compared to their parent species XH2 except for the case of X = N+. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO. [ABSTRACT FROM AUTHOR]

Quantitative conformational analysis (Eliel, Stereochemistry of Organic Compounds, Wiley-Interscience, 1994) has been with us at least as long as Pitzer's landmark 1937 paper (J. Am. Chem. Soc., 1937, 59, 276) on ethane. Cyclohexanes have played a critical role in the quest for understanding. Notably, 1,2,3,4,5,6 hexachlorocyclohexane (C6H6Cl6) was apparently synthesized for the first time by Michael Faraday in 1825 (Philos. Trans. R. Soc., B, 1825, 115, 440). The γ-1,2,3,4,5,6 hexachlorocyclohexane molecule subsequently acquired the common name lindane. Although banned or limited by many countries in 2006, nearly one billion tons of lindane has been manufactured and employed, mostly in agriculture, but also for treatment of human diseases such as lice. Although not as well characterized as lindane, other chlorocyclohexanes have been made and to some degree characterized. The pioneering experimental conformational studies by LeFevre and coworkers (J. Chem. Soc. B, 1970, 1608) of 1,2 dichlorocyclohexane, 1,1,2 trichlorocyclohexane, and 1,2,3,4,5,6 hexachlorocyclohexane are particularly noteworthy. The chlorocyclohexanes have also played a role in the development of molecular mechanics methods by Allinger and coworkers (J. Am. Chem. Soc., 1983, 105, 1716 and 1723). In the present research, we report the first systematic studies of all the chlorocyclohexanes, excluding those with two chlorines attached to a single carbon atom. We make careful comparisons with previous experimental and computational studies. A simple system is established to estimate the relative energies of the different isomers of a particular molecular species. Predicted dipole moments range from identically zero to 5.7 Debye. [ABSTRACT FROM AUTHOR]