Theoretical insights into the mechanism of carbon monoxide (CO) release from CO-releasing molecules

We used density functional theory to investigate the capacity for carbon monoxide (CO) release of five newly synthesized manganese-containing CO-releasing molecules (CO-RMs), namely CORM-368 (1), CORM-401 (2), CORM-371 (3), CORM-409 (4), and CORM-313 (5). The results correctly discriminated good CO releasers (1 and 2) from a compound unable to release CO (5). The predicted Mn-CO bond dissociation energies were well correlated (R(2) ≈0.9) with myoglobin (Mb) assay experiments, which quantified the formation of MbCO, and thus the amount of CO released by the CO-RMs. The nature of the Mn-CO bond was characterized by natural bond orbital (NBO) analysis. This allowed us to identify the key donor-acceptor interactions in the CO-RMs, and to evaluate the Mn-CO bond stabilization energies. According to the NBO calculations, the charge transfer is the major source of Mn-CO bond stabilization for this series. On the basis of the nature of the experimental buffers, we then analyzed the nucleophilic attack of putative ligands (L' = HPO(4)(2-), H(2)PO(4)(-), H(2)O, and Cl(-)) at the metal vacant site through the ligand-exchange reaction energies. The analysis revealed that different L'-exchange reactions were spontaneous in all the CO-RMs. Finally, the calculated second dissociation energies could explain the stoichiometry obtained with the Mb assay experiments.     

Evolved gas analysis–ion attachment mass spectrometric analysis of decacarbonyl dimanganese pyrolysis

Evolved gas analysis?Cion attachment mass spectrometric analysis of the principal species produced by the pyrolysis of Mn₂(CO)₁₀ in an infrared image furnace indicated the presence of Mn(CO)₅ in the gas phase. This observation indicates that Mn₂(CO)₁₀ was in equilibrium with Mn(CO)₅. We also studied the temperature dependence of the mass spectrum to obtain information about the kinetics of the Mn₂(CO)₅ dissociation reaction. From the temperature dependence of the peak for Mn(CO)₅Li⁺ (m/z 202), we calculated the apparent activation energy of Mn(CO)₅ dissociation from solid Mn₂(CO)₁₀. The calculated activation energy (274.57?kJ/mol) is compared with previously reported experimental and calculated values of Mn?CMn bond dissociation energies.     

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4‐triazol‐3‐ylidenes with a N‐2,4‐dinitrophenyl (N‐DNP) substituent are described. IR, NMR, single‐crystal X‐ray diffraction and computational analyses of the Rh complexes revealed that the N‐heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE_bb) to the bond dissociation energies (BDE) of the Rh? C_carbene bond for [RhCl(NHC)(cod)] (cod=1,5‐cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2‐methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom.     

Ligand Site Preference in Iron Tetracarbonyl Complexes Fe(CO)4L (L = CO,CS, N2, NO+, CN–, NC–, η2‐C2H4, η2‐C2H2, CCH2, CH2, CF2, NH3, NF3, PH3, PF3, η2‐H2)

Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(CO)₄L (L = CO, CS, N2, NO⁺, CN^–, NC^–, η²‐C₂H₄, η²‐C₂H₂, CCH₂, CH₂, CF₂, NH₃, NF₃, PH₃, PF₃, η²‐H₂) are calculated in order to investigate whether or not the ligand site preference of these ligands correlates with the ratio of their σ‐donor/π‐acceptor capabilities. Using density functional theory and effective‐core potentials with a valence basis set of DZP quality for iron and a 6‐31G(d) all‐electron basis set for the other elements gives theoretically predicted structural parameters that are in very good agreement with previous results and available experimental data. Improved estimates for the (CO)₄Fe–L bond dissociation energies (D₀) are obtained using the CCSD(T)/II//B3LYP/II combination of theoretical methods. The strongest Fe–L bonds are found for complexes involving NO⁺, CN^–, CH₂ and CCH₂ with bond dissociation energies of 105.1, 96.5, 87.4 and 83.8 kcal mol^–1, respectively. These values decrease to 78.6, 64.3 and 64.2 kcal mol^–1, respectively, for NC^–, CF₂ and CS. The Fe(CO)₄L complexes with L = CO, η²‐C₂H₄, η²‐C₂H₂, NH₃, PH₃ and PF₃ have even smaller bond dissociation energies ranging from 45.2 to 37.3 kcal mol^–1. Finally, the smallest bond dissociation energies of 23.5, 22.9 and 18.5 kcal mol^–1, respectively are found for the ligands NF₃, N₂ and η²‐H₂. A detailed examination of the (CO)₄Fe–L bond in terms of a semi‐quantitative Dewar‐Chatt‐Duncanson (DCD) model is presented on the basis of the CDA and NBO approach. The comparison of the relative energies between axial and equatorial isomers of the various Fe(CO)₄L complexes with the σ‐donor/π‐acceptor ratio of their respective ligands L thus does not generally support the classical picture of π‐accepting ligands preferring equatorial coordination sites and σ‐donors tending to coordinate in axial positions. In particular, this is shown by iron tetracarbonyl complexes with L = η²‐C₂H₂, η²‐C₂H₄, η²‐H₂. Although these ligands are predicted by the CDA to be stronger σ‐donors than π‐acceptors, the equatorial isomers of these complexes are more stable than their axial pendants.     

Transition‐Metal Complexes [(PMe3)2Cl2M(E)] and [(PMe3)2(CO)2M(E)] with Naked Group 14 Atoms (E=C–Sn) as Ligands; Part 1: Parent Compounds

The equilibrium geometries and bond dissociation energies of 16‐valence‐electron(VE) complexes [(PMe₃)₂Cl₂M(E)] and 18‐VE complexes [(PMe₃)₂(CO)₂M(E)] with M=Fe, Ru, Os and E=C, Si, Ge, Sn were calculated by using density functional theory at the BP86/TZ2P level. The nature of the M? E bond was analyzed with the NBO charge decomposition analysis and the EDA energy‐decomposition analysis. The theoretical results predict that the heavier Group 14 complexes [(PMe₃)₂Cl₂M(E)] and [(PMe₃)₂(CO)₂M(E)] with E=Si, Ge, Sn have C_2v equilibrium geometries in which the PMe₃ ligands are in the axial positions. The complexes have strong M? E bonds which are slightly stronger in the 16‐VE species 1ME than in the 18‐VE complexes 2ME . The calculated bond dissociation energies show that the M? E bonds become weaker in both series in the order C>Si>Ge>Sn; the bond strength increases in the order Fe<Ru<Os for 1ME , whereas a U‐shaped trend Ru<Os<Fe is found for 2ME . The M? E bonding analysis suggests that the 16‐VE complexes 1ME have two electron‐sharing bonds with σ and π symmetry and one donor–acceptor π bond like the carbon complex. Thus, the bonding situation is intermediate between a typical Fischer complex and a Schrock complex. In contrast, the 18‐VE complexes 2ME have donor–acceptor bonds, as suggested by the Dewar–Chatt–Duncanson model, with one M←E σ donor bond and two M→E π‐acceptor bonds, which are not degenerate. The shape of the frontier orbitals reveals that the HOMO?2 σ MO and the LUMO and LUMO+1 π* MOs of 1ME are very similar to the frontier orbitals of CO.     

The reactions of Cr(CO)6, Fe(CO)5, and Ni(CO)4 with O2 yield viable oxo‐metal carbonyls

Transition metal complexes with terminal oxo and dioxygen ligands exist in metal oxidation reactions, and many are key intermediates in various catalytic and biological processes. The prototypical oxo‐metal [(OC)₅Cr? O, (OC)₄Fe? O, and (OC)₃Ni? O] and dioxygen‐metal carbonyls [(OC)₅Cr? OO, (OC)₄Fe? OO, and (OC)₃Ni? OO] are studied theoretically. All three oxo‐metal carbonyls were found to have triplet ground states, with metal‐oxo bond dissociation energies of 77 (Cr? O), 74 (Fe? O), and 51 (Ni? O) kcal/mol. Natural bond orbital and quantum theory of atoms in molecules analyses predict metal‐oxo bond orders around 1.3. Their featured ν(MO, M = metal) vibrational frequencies all reflect very low IR intensities, suggesting Raman spectroscopy for experimental identification. The metal interactions with O₂ are much weaker [dissociation energies 13 (Cr? OO), 21 (Fe? OO), and 4 (Ni? OO) kcal/mol] for the dioxygen‐metal carbonyls. The classic parent compounds Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ all exhibit thermodynamic instability in the presence of O₂, driven to displacement of CO to form CO₂. The latter reactions are exothermic by 47 [Cr(CO)₆], 46 [Fe(CO)₅], and 35 [Ni(CO)₄] kcal/mol. However, the barrier heights for the three reactions are very large, 51 (Cr), 39 (Fe), and 40 (Ni) kcal/mol. Thus, the parent metal carbonyls should be kinetically stable in the presence of oxygen. © 2014 Wiley Periodicals, Inc.     

Trigonal‐Bipyramidal and Square‐Pyramidal Chromium?Manganese Chalcogenide Clusters, [E2CrMn2(CO)n]2− (E=S,Se, Te; n=9, 10): Synthesis,Electrochemistry, UV/Vis Absorption,and Computational Studies

The reactions of E powder (E=S, Se) with a mixture of Cr(CO)₆ and Mn₂(CO)₁₀ in concentrated solutions of KOH/MeOH produced two new mixed Cr? Mn? carbonyl clusters, [E₂CrMn₂(CO)₉]^2? (E=S, 1 ; Se, 2 ). Clusters 1 and 2 were isostructural with one another and each displayed a trigonal‐bipyramidal structure, with the CrMn₂ triangle axially capped by two μ₃‐E atoms. The analogous telluride cluster, [Te₂CrMn₂(CO)₉]^2? ( 3 ), was obtained from the ring‐closure of Te₂Mn₂ ring complex [Te₂Mn₂Cr₂(CO)₁₈]^2? ( 4 ). Upon bubbling with CO, clusters 2 and 3 were readily converted into square‐pyramidal clusters, [E₂CrMn₂(CO)₁₀]^2? (E=Se, 5 ; Te, 6 ), accompanied with the cleavage of one Cr? Mn bond. According to SQUID analysis, cluster 6 was paramagnetic, with S=1 at room temperature; however, the Se analogue ( 5 ) was spectroscopically proposed to be diamagnetic, as verified by TD‐DFT calculations. Cluster 6 could be further carbonylated, with cleavage of the Mn? Mn bond to produce a new arachno‐cluster, [Te₂CrMn₂(CO)₁₁]^2? ( 7 ). The formation and structural isomers, as well as electrochemistry and UV/Vis absorption, of these clusters were also elucidated by DFT calculations.     

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N,P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)₂(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)₂(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)₂(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X‐ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner‐type complex, that is, with an electrostatic interaction between the Re^I and the NCO group. The phosphaethynolate complex [Re(P?C?O)(CO)₂(triphos)] is best described as a metallaphosphaketene with a Re^I–phosphorus bond of highly covalent character.     

An N‐Acetyl Cysteine Ruthenium Tricarbonyl Conjugate Enables Simultaneous Release of CO and Ablation of Reactive Oxygen Species

We have designed and synthesised a [Ru(CO)₃Cl₂(NAC)] pro‐drug that features an N‐acetyl cysteine (NAC) ligand. This NAC carbon monoxide releasing molecule (CORM) conjugate is able to simultaneously release biologically active CO and to ablate the concurrent formation of reactive oxygen species (ROS). Complexes of the general formulae [Ru(CO)₃(L)₃]²⁺, including [Ru(CO)₃Cl(glycinate)] (CORM‐3), have been shown to produce ROS through a water–gas shift reaction, which contributes significantly, for example, to their antibacterial activity. In contrast, NAC‐CORM conjugates do not produce ROS or possess antibacterial activity. In addition, we demonstrate the synergistic effect of CO and NAC both for the inhibition of nitric oxide (formation) and in the expression of tumour‐necrosis factor (TNF)‐α. This work highlights the advantages of combining a CO‐releasing scaffold with the anti‐oxidant and anti‐inflammatory drug NAC in a unique pro‐drug.     

The π‐Bonding Trend in VIB M(CO)6 Observed by NMR Spectroscopy — A Natural Bond Orbital View

Analysis of carbonyl's 2π orbital populations, [2π], obtained by NMR relaxation time experiments of VIB M(CO)(?⁶‐C₆H₆) reveals the 3d < 4d < 5d trend for M r? CO back‐donation, as reported values of [2π] for VIB M(CO)₅(quinuclidine). The same analysis performed on Mn(CO)₃(?⁵C₅H₅) and Re(CO)₃(?⁵‐C₅H₅) also gives 3d < 5d order of back‐donation. The distinctive 3d ～ 5d > 4d trend reported for VIB M(CO)₆ has been investigated by second‐order perturbation theory analysis within the natural bond orbital (NBO) scheme to search for orbital‐based explanations. Besides the conventional d_π r? 2π donor‐acceptor (DA) interaction in the trend 3d < 4d < 5d, the other DA interaction arising from three‐center‐hyperbond (3CHB) hyperconjugation has been found in the trend 3d >> 5d ～ 4d. Within the VIB M(CO)₆ family, this 3CHB hyperconjugation is so much higher in Cr(CO)₆ than in W(CO)₆ as to render the overall 2π populations exhibiting the 3d ～ 5d > 4d trend.     

Bond strengths in cyclopentadienyl metal carbonyl anions

Energy-resolved collision-induced dissociation of metal cyclopentadienyl carbonyl anions CpM(CO)_x⁻(Cpc-C₅H₅, MV, Cr, Mn, Fe, Co) is used to determine metal–carbonyl bond energies in these systems. These bond energies are, in general, slightly stronger than those for the corresponding homoleptic metal carbonyl anions. The bond strength in CpCo(CO)₂⁻, a 19-electron complex, is notably weaker than most of the others. D[CpMn⁻-CO] is also weak; this is attributed to a mismatch in the electronic ground states of CpMn⁻ and CpMnCO⁻. D[CpCo⁻-CO], on the other hand, is substantially larger than the others, and is comparable to the bond energy measured in solution for CpMn(CO)₃.     

A Computational Approach to the Effects of Solvent on the Structural,Electronic, Spectroscopic (195Pt NMR and IR), and Thermochemical Properties of a Third‐Generation Anticancer Drug: Trans‐Platinum(II) Complex of 3‐Aminoflavone

This study evaluated the structural, electronic and thermochemical properties of an anticancer active molecule, i.e. trans‐bis‐(3‐aminoflavone)dichloridoplatinum(II) (trans‐Pt(3‐af)₂Cl₂; TCAP) in the gas and solution phases. The polarizable continuum model (PCM) model was used to perform the required calculations in five solvents with different polarities. Moreover, the dependencies of energetic aspects, structural, thermodynamic parameters and frontier orbital energies of the complex were also examined. Dependencies of the frequency shifts of u(CO), u(NH) and ¹⁹⁵Pt Chemical shifts on the solvent dielectric were investigated by Kirkwood–Bauer–Magat equation (KBM). The energies of platinum d‐orbitals and formal electron configurations of Pt atom were calculated by natural bond analysis (NBO).     

Stibinidene and Bismuthinidene as Two‐Electron Donors for Transition Metals (Co and Mn)

The reaction of stibinidene and bismuthinidene ArM [where Ar=C₆H₃‐2,6‐(CH=NtBu)₂; M=Sb ( 1 ), Bi ( 2 )] with transition metal (TM) carbonyls Co₂(CO)₈ and Mn₂(CO)₁₀ produced unprecedented ionic complexes [(ArM)₂Co(CO)₃]⁺[Co(CO)₄]^? and [(ArM)₂Mn(CO)₄]⁺[Mn(CO)₅]^? [where M=Sb ( 3 , 5 ), Bi ( 4 , 6 )]. The pnictinidenes 1 and 2 behaved as two‐electron donors in this set of compounds. Besides the M→TM bonds, the topological analysis also revealed a number of secondary interactions contributing to the stabilization of cationic parts of titled complexes.     

Structures and thermochemistry of ions formed by methane chemical ionization of Fe(CO)5. Site of protonation and determination of Fe?H and Fe?CO bond dissociation energies of HFe(CO)n+ ions

High-energy collisional activation mass spectrometry of HFe(CO)₅⁺ ions shows that Fe(CO)₅ is protonated on the iron atom rather than on one of the ligands. This finding is supported by ab initio quantum chemical calculations. The value of the proton affinity of Fe(CO)₅ was measured by high-pressure mass spectrometry to be 857 kJ mol^?1. The Fe? CO bond dissociation energies for HFe(CO)_n⁺ (n = 1–5) were measured by energy-variable low-energy collisional activation mass spectrometry. The Fe? H bond dissociation energies in HFe(CO)_n⁺ ions were also determined. A synergistic effect on the strengths of the Fe? H and Fe? CO bonds in HFe(CO)⁺ is noticed. It is demonstrated that the electronically unsaturated species HFe(CO)_n⁺ (n = 3, 4) formed in exothermic proton-transfer reactions with Fe(CO)₅ form adducts with CH₄. Adducts between C₂H₅⁺ or C₃H₅⁺ and Fe(CO)_n are observed. These adducts are probably formed in direct reactions between the respective carbocations and Fe(CO)₅.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

Why are the Homoleptic Diyl Complexes M(InR)4 with M = Ni and Pt Stable Compounds while only Ni(CO)4 but not Pt(CO)4 can become Isolated? A Theoretical Study of M(EMe)44 and M(CO)4 (M = Ni,Pd, Pt; E = B,Al, Ga,In, Tl)

The equilibrium geometries and first bond dissociation energies of the homoleptic complexes M(EMe)₄ and M(CO)₄ with M = Ni, Pd, Pt and E = B, Al, Ga, In, Tl have been calculated at the gradient corrected DFT level using the BP86 functionals. The electronic structure of the metal‐ligand bonds has been examined with the topologial analysis of the electron density distribution. The nature of the bonding is revealed by partitioning the metal‐ligand interaction energies into contributions by electrostatic attraction, covalent bonding and Pauli repulsion. The calculated data show that the M‐CO and M‐EMe bonding is very similar. However, the M‐EMe bonds of the lighter elements E are much stronger than the M‐CO bonds. The bond energies of the latter are as low or even lower than the M‐TlMe bonds. The main reason why Pd(CO)₄ and Pt(CO)₄ are unstable at room temperature in a condensed phase can be traced back to the already rather weak bond energy of the Ni‐CO bond. The Pd‐L bond energies of the complexes with L = CO and L = EMe are always 10 — 20 kcal/mol lower than the Ni‐L bond energies. The calculated bond energy of Ni(CO)₄ is only D_o = 27 kcal/mol. Thus, the bond energy of Pd(CO)₄ is only D_o = 12 kcal/mol. The first bond dissociation energy of Pt(CO)₄ is low because the relaxation energy of the Pt(CO)₃ fragment is rather high. The low bond energies of the M‐CO bonds are mainly caused by the relatively weak electrostatic attraction and by the comparatively large Pauli repulsion. The σ and π contributions to the covalent M‐CO interactions have about the same strength. The π bonding in the M‐EMe bonds is less than in the M‐CO bonds but it remains an important part of the bond energy. The trends of the electrostatic and covalent contributions to the bond energies and the σ and π bonding in the metal‐ligand bonds are discussed.     

Theoretical study of the red‐ and blue‐shifted hydrogen bonds of nitroxyl and acetylene dimers

Ab initio molecular orbital and density functional theory (DFT) in conjunction with different basis sets calculations were performed to study the C? H…O red‐shifted and N? H…π blue‐shifted hydrogen bonds in HNO? C₂H₂ dimers. The geometric structures, vibrational frequencies and interaction energies were calculated by both standard and counterpoise (CP)‐corrected methods. In addition, the G3B3 method was employed to calculate the interaction energies. The topological and natural bond orbital (NBO) analysis were investigated the origin of N? H…π blue‐shifted hydrogen bond. From the NBO analysis, the electron density decrease in the σ* (N? H) is due to the significant electron density redistribution effect. The blue shifts of the N? H stretching frequency are attributed to a cooperative effect between the rehybridization and electron density redistribution. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Heteronukleare Komplexverbindungen mit Metall—Metall-Bindungen. VIII. Neue heterodinukleare Komplexe mit Bindungen zwischen Kupfer(I) und Mangan(–I), Eisen(–I) oder Cobalt(–I)

Heteronuclear Coordination Compounds with Metal—Metal Bonds. VIII. New Heterodinuclear Complexes with Bonds between Copper(I) and Manganese(?I), Iron(?I), or Cobalt(?I) [(en)Cu? Mn(CO)₅] ( 1a ), [(dien)Cu? Mn(CO)₅] ( 1b ), [(en)Cu? Fe(CO)₃(NO)] ( 2a ), [(dien)Cu? Fe(CO)₃(NO)] ( 2b ), [(en)Cu? Co(CO)₄] ( 3a ), and [(dien)Cu? Co(CO)₄] ( 3b ) are new heterobinuclear metal—metal bonded complexes. The geometry of the [Mn(CO)₅]^?, [Fe(CO)₃(NO)]^?, and [Co(CO)₄]^? ions is distorted only to a less extend in accord with a heteropolar bond to copper.     

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

The 17e⁻ monoradical [Mn(CO)₅] is widely recognized as an unstable organometallic transient and is known to dimerize rapidly with the formation of a Mn Mn single bond. As a result of this instability, isolable analogues of [Mn(CO)₅] have remained elusive. Herein, we show that two sterically encumbering isocyanide ligands can destabilize the Mn Mn bond leading to the formation of the isolable, manganese(0) monoradical [Mn(CO)₃(CNAr^Dipp2)₂] (Ar^Dipp2=2,6‐(2,6‐(iPr)₂C₆H₃)₂C₆H₃). The persistence of [Mn(CO)₃(CNAr^Dipp2)₂] has allowed for new insights into nitrosoarene spin‐trapping studies of [Mn(CO)₅].     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007