Radicals masquerading as electrophiles: a computational study of the intramolecular addition reactions of acyl radicals to imines

Ab initio calculations using 6-311G**, cc-pVDZ, and aug-cc-pVDZ, with (MP2, QCISD, CCSD(T)) and without (UHF) electron correlation, and density functional methods (BHandHLYP and B3LYP) predict that cyclization of the 5-aza-5-hexenoyl and (E)-6-aza-5-hexenoyl radicals proceed to afford the 5-exo products. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVDZ level of theory, energy barriers (deltaE(double dagger)) of 36.1 and 47.0 kJ mol(-1) were calculated for the 5-exo and 6-endo pathways for the cyclization of the 5-aza-5-hexenoyl radical. On the other hand, at the same level of theory, deltaE(double dagger) of 38.9 and 45.4 kJ mol(-1) were obtained for the 5-exo and 6-endo cyclization modes of (E)-6-aza-5-hexenoyl radical, with exothermicities of about 27 and 110 kJ mol(-1) calculated for the exo and endo modes, respectively. Under suitable experimental conditions, the 6-endo cyclization product is likely to dominate. Analysis of the molecular orbitals involved in these ring-closure reactions indicate that both reactions at nitrogen are assisted by dual orbital interactions involving simultaneous SOMO-pi* and LP-pi* overlap in the transitions states. Interestingly, the (Z)-6-aza-5-hexenoyl radical, that cannot benefit from these dual orbital effects is predicted to ring-close exclusively in the 5-exo fashion.     

Multiorbital interactions during Acyl radical addition reactions involving imines and electron-rich olefins

Ab initio and DFT calculations reveal that acyl radicals add to imines and electron-rich olefins through simultaneous SOMO --> pi*, pi --> SOMO, and HOMO --> pi*C=O interactions between the radical and the radicalophile. At the CCSD(T)/aug-cc-pVDZ//QCISD/cc-pVDZ level, energy barriers of 15.6 and 17.9 kJ mol(-1) are calculated for the attack of the acetyl radical at the carbon and nitrogen ends of methanimine, respectively. These barriers are 17.1 and 20.4 kJ mol(-1) at BHandHLYP/cc-pVDZ. In comparison, barriers of 34.0 and 23.4 kJ mol(-1) are calculated at BHandHLYP/cc-pVDZ for reaction of the acetyl radical at the 1- and 2-positions in aminoethylene, repectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO --> pi*imine, pi imine--> SOMO, and LPN --> pi*C=O interactions are worth 90, 278, and 138 kJ mol-1, respectively, in the transition state (2) for reaction of acetyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving aminoethylene. These multiorbital interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions. NBO analyses for the remaining systems in this study support the hypothesis that the acetyl radical is ambiphilic in nature.     

Theoretical study on the isomerization behavior between alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals

[reaction: see text] Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis set, with (QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional methods (BHandHLYP, B3LYP) predict that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals exist as isomers. At the CCSD(T)/cc-pVDZ//BHandHLY/cc-pVDZ level of theory, energy barriers of 15.1 and 17.7-21.7 kJ mol(-)(1) are calculated for the isomerization of s-trans-propenoyl and s-trans-crotonoyl radical to ketenylmethyl and 1-ketenylethyl radical, respectively. Similar results are obtained for the reactions of s-trans isomers involving silyl, germyl, and stannyl groups with energy barriers (DeltaE++) of 12.2-12.4, 13.1-13.9, and 12.9-18.2 kJ mol(-)(1) at the CCSD(T)/DZP//BHandHLYP/DZP calculation, respectively. These results suggest that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals are not canonical forms but are isomeric species that can rapidly interconvert.     

An ab initio and DFT study of some halogen atom transfer reactions from alkyl groups to acyl radical

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that the transition states for the reaction of acetyl radical with several alkyl halides adopt an almost collinear arrangement of attacking and leaving radicals at the halogen atom. Energy barriers (DeltaE(double dagger)) for these halogen transfer reactions of between 89.2 (chlorine transfer from methyl group) and 25.3 kJ mol(-1) (iodine transfer from tert-butyl group) are calculated at the BHandHLYP/DZP level of theory. While the difference in forward and reverse energy barriers for iodine transfer to acetyl radical is predicted to be 15.1 kJ mol(-1) for primary alkyl iodide, these values are calculated to be 6.7 and -4.2 kJ mol(-1) for secondary and tertiary alkyl iodide respectively. These data are in good agreement with available experimental data in that atom transfer radical carbonylation reactions are sluggish with primary alkyl iodides, but proceed smoothly with secondary and tertiary alkyl iodides. These calculations also predict that bromine transfer reactions involving acyl radical are also feasible at moderately high temperature.     

Ab initio study of 1,4-pentadienyl electrocyclic reactions

The thermochemistry and transition states of the electrocyclic ring closures of the resonance-stabilized 1,4-pentadienyl radical to cyclopenten-3-yl, cyclobut-2-enylmethyl, and 2-vinylcyclopropyl are investigated at Hartree-Fock and coupled-cluster levels of theory. The CCSD(T)//QCISD/cc-pVDZ calculations predict activation barriers of 130, 169, and 236 kJ/mol, respectively, and DeltaH values of -60, 115, and 155 kJ/mol. Experimental evidence for the appearance of vinylcyclopropyl following photolytic generation of pentadienyl is more likely the result of a distinct electrocyclic reaction than quenching of a two-step mechanism for formation of cyclopentenyl. Higher energy pathways for formation of polycyclic structures are also briefly examined.     

Ozonolysis of geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal: kinetics and mechanisms

A combined density functional theory and transition-state theory study of the mechanisms and reaction coefficients of gas-phase ozonolysis of geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal is presented. The geometries, energies, and harmonic vibrational frequencies of each stationary point were determined by B3LYP/6-31(d,p), MPW1K/cc-pVDZ, and BH&HLYP/cc-pVDZ methods. According to the calculations, the ozone 6-methyl-5-hepten-2-one reaction is faster than the ozone 6-hydroxy-4-methyl-4-hexenal reaction, but both are slower than the ozone geraniol-trans reaction. By using the BH&HLYP/cc-pVDZ data, a global rate coefficient of 5.9 x 10(-16) cm(3) molecule(-1) s(-1) was calculated, corresponding to the sum of geraniol-trans, 6-methyl-5-hepten-2-one, and 6-hydroxy-4-methyl-4-hexenal reactions with the ozone. These results are in good agreement with the experimental studies.     

Methyl radical also reacts by the frontside mechanism: an ab initio study of some homolytic substitution reactions of methyl radical at silicon,germanium and tin

Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (B3LYO) calculations predict that homolytic substitution reactions of the methyl radical at the silicon atom in disilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (delta E) of 47.4 and 48.6 kJ mol-1 are calculated for the backside and frontside reactions respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (delta E) of between 46.5 and 67.3, and 41.0 and 73.3 kJ mol-1 for the backside and frontside mechanisms, respectively. These data suggest that homolytic substitution reactions of methyl radical at silicon, germanium, and tin can proceed via either homolytic substitution mechanism.     

A computational study of organic polyradicals stabilized by chromium atoms

Density functional theory has been used to investigate the properties of organic high spin molecules. The M05/cc-pVDZ calculations predict a septet ground state for the 2,3,6,7,10,11-hexahydro-1,4,5,8,9,12-hexaoxocoronene-2,3,6,7,10,11-hexayl radical (coronene-6O). The computations show further that the formation of intermolecular carbon-carbon bonds yields a singlet ground state for the dimer rather than a possible tridectet state as expected from the monomer's multiplicity. A benzene molecule placed between coronene-6O molecules leads to the desired high-spin cluster, but the overall stability of the cluster is low. A chromium atom inserted between two peripheral C(6) rings of coronene-6O yields a sandwich structure with the expected tridectet ground state and a binding energy which is 15 times larger than the corresponding tridectet dimer stabilized by a benzene molecule. The presented DFT calculations suggest that a chromium atom can effectively link organic polyradicals to larger magnetic units.     

An ab initio and DFT study of radical addition reactions of imidoyl and thioyl radicals to methanimine

Ab initio and DFT calculations reveal that both imidoyl and thioyl radicals add to the nitrogen end of methanimine through simultaneous SOMO-π*(imine), SOMO-π(imine), SOMO-LP(N) and π*(radical)-LP(N) interactions between the radical and the imine. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVTZ level of theory, barriers of 13.8 and 26.1 kJ mol(-1) are calculated for the attack of the methylimidoyl radical at the carbon- and nitrogen- end of methanimine, respectively, indicating that the imidoyl radial has a preference for addition to the nitrogen end of imine. On the other hand, barriers of 25.1 and 13.4 kJ mol(-1) are calculated at the same level of theory for the addition reaction of the methanethioyl radical at the carbon- and nitrogen- end of methanimine, respectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO-π*(imine), SOMO-π(imine), SOMO-LP(N) and π*(radical)-LP(N) interactions are worth 111, 89, 115 and 17 kJ mol(-1), respectively, in the transition state (4) for the reaction of methylimidoyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving all the radicals studied here. These multi-component interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions.     

A theoretical study of the kinetics of the benzylperoxy radical isomerization

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.     

Computational study on the 1,2-rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals

The 1,2-nitroxyl and 1,2-acetoxyl rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals 13a and 13b, respectively, has been studied for the gas phase with various ab initio and density functional methods. The energetically most favorable pathway for 13a is calculated to proceed via reversible fragmentation/radical addition through transition state I-19a. In the case of 13b, rearrangement through a five-membered ring transition state III-16b and the fragmentation/radical addition pathway via transition state I-19b are competing processes. Mulliken and natural population analysis reveal a certain degree of charge separation in III-16a/b that may indicate a potential solvent effect on the rearrangement rate. A stepwise group migration through a cyclic radical intermediate V-18a/b or rearrangement through a three-membered ring transition state II-15a/b can be ruled out for both vinyl radicals. A comparison of the results of the calculations with experimental findings provides important insights into the kinetics of "self-terminating radical oxygenations". A significant method dependence on the outcome of the calculations was observed, which revealed the unsuitability of the UHF, MP2, B3LYP, and mPW1PW91 methods for computing these radical rearrangement processes. The results from BHandHLYP/cc-pVDZ calculations showed the best agreement with single-point energy calculations performed at the QCISD and CCSD(T) levels of theory.     

Vibrational spectra of germanium-carbon clusters in solid Ar: identification of the nu4(sigma(u)) mode of linear GeC5Ge

The linear GeC(5)Ge cluster has been detected in Fourier transform infrared spectra observed when the products from the dual laser evaporation of carbon and germanium rods were trapped in solid Ar at approximately 10 K. Comparison of (13)C isotopic shift measurements with the predictions of density functional theory calculations at the B3LYP/cc-pVDZ level confirms the identification of the nu(4)(sigma(u)) mode of GeC(5)Ge at 2158.0 cm(-1).     

An ab-initio study of some homolytic substitution reactions of acyl radicals at silicon, germanium and tint

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that homolytic substitution reactions of acetyl radicals at the silicon atoms in disilane can proceed via both backside and frontside attack mechanisms. At the highest level of theory (CCSD(T)/cc-pVDZ//MP2/cc-pVDZ), energy barriers (deltaE double dagger) of 77.2 and 81.9 kJ mol(-1) are calculated for the backside and frontside reactions respectively. Similar results are obtained for reactions involving germanium and tin with energy barriers (deltaE double dagger) of 53.7-84.2, and 55.0-89.7 kJ mol(-1) for the backside and frontside mechanisms, respectively. These data suggest that both homolytic substitution mechanisms are feasible for homolytic substitution reactions of acetyl radicals at silicon, germanium, and tin. BHandHLYP calculations provide geometries and energy barriers for backside and frontside transition states in good agreement with those obtained by traditional ab initio techniques.     

Ab initio study of the cyclooctatetraenyl radical

Ab initio calculations have been carried out on the 1,3,5,7- and 1,2,4,7-tetraene configurations of the cyclooctatetraenyl radical at UHF, ROHF, MCSCF, ROCISD, QCISD, and CCSD(T) levels of theory with 6-311G(d,p) and cc-pVDZ basis sets. Although spin contamination is present, the ROCISD calculations support the energies obtained from less intensive, UHF-based coupled cluster calculations over the energies obtained from MCSCF analysis of the pi-electron orbitals. The 1,3,5,7-form is a local minimum at the coupled cluster levels, higher in energy than the resonance-stabilized 1,2,4,7-form by 10-13 kJ/mol, but bounded by a barrier of less than 0.5 kJ/mol. The isomerization surface connecting these two structures is described and results reported from integration of the vibrational Schr?dinger equation on that surface. Excited vibrational states at energies just above the isomerization barrier are dominated by the character of the 1,3,5,7-tetraenyl radical, which suggests that chemistry involving this intermediate at typical combustion temperatures may branch at this juncture.     

Theoretical study of the thermal decomposition of the 5-methyl-2-furanylmethyl radical

The thermal decomposition of the 5-methyl-2-furanylmethyl radical (R(1)), the most important primary radical formed during the combustion and thermal decomposition of 2,5-dimethylfuran (a promising next-generation biofuel), was studied using CBS-QB3 calculations and master equation (ME)/RRKM modeling. Because very little information is available in the literature, the detailed potential energy surface (PES) was investigated thoroughly. Only the main pathways, having a kinetic influence on the decomposition of R(1), were retained in the final ME/RRKM model. Among all the channels studied, the ring-opening of the 5-methyl-2-furanylmethyl radical, followed by ring enlargement to form cyclohexadienone molecules is predicted to be the easiest decomposition channel of R(1). The C(6) cyclic species formed can undergo unimolecular reactions to yield phenol and to a lesser extent cyclopentadiene and CO. Our calculations predict that these species are important products formed during the pyrolysis of 2,5-dimethylfuran (DMF). Other channels involved in the decomposition of R(1) lead directly to the formation of linear and cyclic unsaturated C(5) species and constitute an additional source of cyclopentadiene and CO. High-pressure limit rate constants were computed as well as thermochemical properties for important species. ME/RRKM analysis was performed to probe the influence of pressure on the rate coefficients and pressure dependent rate coefficients were proposed for pressures and temperatures ranging, respectively, from 10(-2) bar to 10 bar and 1000 to 2000 K.     

AM1, PM3, and PM5 calculations of the absorption maxima of basic organic dyes

The absorption maxima, λ_max, of various organic dyes such as indigo, azobenzene, phenylamine, hydrazone, anthraquinone, naphthoquinone, and malachite green were calculated using the AM1, PM3, and PM5 semiempirical molecular orbital theories with the configuration interaction singles (CIS) and random phase approximation (RPA) approaches. The calculated λ_max were then compared with the values obtained by CNDO/S, INDO/S, ab initio CIS, and time-dependent density functional theory (TD-DFT). We found that the λ_max values calculated by AM1, PM3, and PM5 were in good correlation with the observed λ_max values. When B3LYP/cc-pVDZ optimized geometries were used, the square of the correlation coefficients between the calculated and observed λ_max, , at the AM1-RPA, PM3-RPA, and PM5-RPA levels were 0.891, 0.897, and 0.927, respectively. In particular, at PM5-RPA//B3LYP/cc-pVDZ was the largest among those obtained from all the other calculations including TD/B3LYP/cc-pVDZ//B3LYP/cc-pVDZ . Accordingly, the standard deviation of the difference between observed and calculated λ_max by the linear regression function at PM5-RPA//B3LYP/cc-pVDZ was the smallest. It was therefore concluded that this method was the most promising for the prediction of λ_max of various dyes among the computational methods studied here. When AM1 optimized geometries were used, at the AM1-RPA, PM3-RPA, and PM5-RPA levels were 0.822, 0.841, and 0.901, respectively, and they were also comparable to that at TD/B3LYP/cc-pVDZ//B3LYP/cc-pVDZ. Therefore, although some calibration efforts may be needed for AM1 geometries, PM5-RPA(CIS)//AM1 may be a second candidate available for the prediction of the absorption maxima of dyes, especially in the case of emphasizing computational cost.     

Mechanistic insights into NO3* induced self-terminating radical oxygenations, part 1: a computational study on NO3* and its addition to alkynes

The geometry of the nitrate radical, NO3*, for which unrestricted Hartree-Fock (HF) breaks spatial symmetry of the wave function, was optimized using hybrid density functionals that include varying fractions of Hartree-Fock exchange. Although symmetry breaking was not observed even when the functional with the highest HF exchange (BHandHLYP) was used, only B3LYP correctly describes the D(3h) symmetry of NO3* as ground-state structure with the lowest energy. Further, geometries and energies of the stationary points in the addition of NO3* to ethyne, propyne, and 2-butyne were calculated using ab initio and density functional methods. The reactions proceed through Z-configurated transition states leading to Z-configurated vinyl radicals with the activation barrier decreasing with increasing methyl substitution at the C[triple bond]C by ca. 11 kJ mol(-1) per methyl group. It was found that the results obtained at the BHandHLYP/cc-pVDZ level of theory are in good agreement with the data from single-point QCISD and CCSD(T) calculations.     

Decomposition of triacetone triperoxide is an entropic explosion

Both X-ray crystallography and electronic structure calculations using the cc-pVDZ basis set at the DFT B3LYP level were employed to study the explosive properties of triacetone triperoxide (TATP) and diacetone diperoxide (DADP). The thermal decomposition pathway of TATP was investigated by a series of calculations that identified transition states, intermediates, and the final products. Counterintuitively, these calculations predict that the explosion of TATP is not a thermochemically highly favored event. It rather involves entropy burst, which is the result of formation of one ozone and three acetone molecules from every molecule of TATP in the solid state.     

An evolved explanation for the molecular geometry and electronic structure of diphenyl-substituted cyclic trimethylenemethane in the ground state: a nearly planar conformation with a considerably localized electronic state

[structures: see text] We reinvestigated the molecular geometry and electronic structure of the diphenyl-substituted, five-membered cyclic trimethylenemethane (TMM) diradical (Berson's TMM, 3**) using UV/VIS absorption and emission spectroscopy combined with density functional theory (DFT) and time-dependent (TD)-DFT calculations. Two intense absorption bands, A and B, with lambda(ab) at 298 and 328 nm, respectively, a weak absorption band C, with lambda(ab) at 472 nm, and an intense emission band D, with lambda(em) at 491 nm, were observed for 3**. By comparing the spectrum of 3** with those of the 1,1-diphenylethyl (7*) and cyclopent-2-en-1-yl (9*) radicals, it was found that bands B, C, and D originated from the diphenylmethyl radical moiety (subunit I), while band A should most likely be assigned to an electronic transition related to an interaction between subunit I and residual subunit II, the cyclopentenyl radical moiety. An UB3LYP/cc-pVDZ calculation indicated that, in the ground state, the two unpaired electrons of 3** are mainly localized in subunits I and II, respectively, and the interaction between them is inefficient, despite the nearly planar conformation (theta = +23.5 degrees). Furthermore, a TD-UB3LYP/cc-pVDZ calculation suggested that absorption band A is assigned to an electronic transition involved with enhancement of the electron density of the C-2-C-3 bond. Substituent effects on the absorption and emission spectra of 3** using 11** and 13** support the conclusion based on the experiments and calculations. Therefore, we propose an evolved explanation for the molecular geometry and electronic structure of the ground state of 3** in a low-temperature matrix, a nearly planar conformation with a considerably localized electronic state, which alone accounts for the spectroscopic characteristics.     

Theoretical studies and rate constant calculations of the reactions C2F5CHO with OH radicals and Cl atoms

The hydrogen abstraction reactions of C₂F₅CHO with OH radicals and Cl atoms have been investigated theoretically by a dual-level direct dynamics method. In this study, the optimized geometries and frequencies of the stationary points are calculated at the MP2/cc-pVDZ level of theory. The energies of the stationery points and the selected points along the minimum energy paths are further refined at the MC-QCISD level using the MP2 geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of the two reactions. This result indicates that both of reactions proceed via indirect reaction mechanisms. The enthalpies of formation for the reactant C₂F₅CHO and the product radical C₂F₅CO are estimated by isodesmic reactions at the MC-QCISD//MP2/cc-pVDZ level. At the same level, the rate constants are calculated by canonical variational transition state theory (CVT) incorporating with the small-curvature tunneling correction (SCT) in the temperature range 200–1000 K. Good agreement between the calculated and experimental rate constants is obtained at the room temperature. Due to the lack of the kinetic data of these reactions, the fitted three-parameter expressions based on the CVT/SCT rate constants within 200–1000 K are k₁ = 1.64 × 10⁻²⁴ T^4.33 exp (−566.1/T) and k₂ = 6.33 × 10⁻¹⁵ T^1.35 exp (550.3/T) cm³ molecule⁻¹ s⁻¹, respectively.