Ab initio mechanistic studies of radical reactions. Addition of methyl radical to acetylene and ethylene

Addition of methyl radicals to ethylene and acetylene has been studied by ab initio molecular-orbital calculations. In agreement with experimental data, we find that, although addition to ethylene is characterized by a lower activation energy, addition to acetylene is faster due to the opposite and larger trend of pre-exponential factors. The reaction barriers have been analyzed by the energy decomposition scheme proposed by Morokuma.     

Ab initio molecular orbital studies of nonidentity allyl transfer reactions

Ab initio molecular orbital (MO) calculations are carried out on the nonidentity allyl transfer processes, X^? + CH₂CHCH₂Y ? CH₂CHCH₂ X + Y^?, with X^? = H, F, and Cl and Y = H, NH₂, OH, F, PH₂, SH, and Cl. The Marcus equation applies well to the allyl transfer reactions. The transition state (TS) position along the reaction coordinate and the TS structure are strongly influenced by the thermodynamic driving force, whereas the TS looseness is originated from the intrinsic barrier. The intrinsic barrier, ΔE, looseness, %L?, and absolute asymmetry, %AS?, are well correlated with the percentage bond elongation, %CY? = [(d ? d)/d] × 100 and/or %CX?. The %CY? and the bond orders indicate that a stronger nucleophile and/or a stronger nucleofuge (or a better leaving group) leads to an earlier TS on the reaction coordinate with a lesser degree of bond making as well as bond breaking. These are consistent with the Bell-Evans-Polanyi principle and the Leffler-Hammond postulate. © 1995 by John Wiley & Sons, Inc.     

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.     

Proton affinity of methyl peroxynitrate

The equilibrium structures, harmonic vibrational frequencies of methyl peroxynitrate, and structures of protonated methyl peroxynitrate have been investigated using ab initio methods. The methods include the single- and double-excitation quadratic configuration (QCISD) methods and the QCISD(T) method, which incorporates a perturbational estimate of the effects of corrected triple excitation. The lowest-energy gas-phase form of protonated methyl peroxynitrate is a complex between CH3OOH and NO2+. The CH3OOH.NO2+ complex is bound by 22 +/- 2 kcal/mol. The estimated proton affinity of methyl peroxynitrate is 178.8 +/- 3 kcal/mol. A general trend for the proton affinity of ROO-NO2 (peroxynitrates) compounds is discussed.     

Ab initio studies of hydrogen bond formation in methyl cyanide or methyl isocyanide and methanol systems

CH₃CN ? HOCH₃ and CH₃NC ? HOCH₃ hydrogen-bonded systems have been studied theoretically by ab initio MO methods using a 4–31 G basis set at their equilibrium geometries. The stabilization energies of these hydrogen bonds are 5.4 and 5.9 kcal/mol, respectively. The nature of these hydrogen bonds is discussed in the light of frontier orbital theory and the topological properties of the charge density of the chemical bond.     

Ab initio quantum dynamical study of the vinylidene-acetylene isomerization

The dynamics of vinylidene-acetylene isomerization has been studied quantum mechanically, based on a new ab initio calculated potential energy surface (PES). The grid underlying the PES and the dynamic treatment rest on planar 4-atom Jacobi-like coordinates. Three degrees of freedom, the C-C bond length and the two Jacobi-like angles are treated explicitly, while the two other coordinates relating to the C-H bond lengths have been optimized in the ab initio treatment. The energies of the resulting 3D grid have been computed with the CCSD(T) method, using a cc-pVTZ basis set, while selected stationary points have been characterized at a higher level of theory. The subsequent dynamic treatment reproduces very satisfactorily the experimental photodetachment spectrum of Lineberger and coworkers. Preliminary results are also reported for the lifetime of vinylidene. Received: 5 June 1998/Accepted: 23 July 1998 / Published online: 9 October 1998     

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH₃CC₂H₅^·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.     

Ab initio studies on calicene

Optimum geometries of planar and 90°-twisted C_2v calicene are calculated with single-configuration STO-3G and 3-21-G wavefunctions. The barrier to ring–ring rotation is computed. Bond alternation is pronounced in the planar form and decreases in the twisted form, while dipolar character increases on twisting.     

Ab initio study on the photochemical isomerization of furan derivatives

The photochemical isomerization reactions of furan, 2-methylfuran, 2-trimethylsilylfuran, and furan-2-carbonitrile were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. In particular, the sensitized irradiation of furan derivatives populates the first excited triplet state of the furan, and this triplet state can evolve only through O-Ca cleavage. The selection of the bond to be broken can depend on energetic factors (furan-2-carbonitrile) or on kinetic factors (2-methylfuran, 2-trimethylsilylfuran). The direct irradiation of furan derivatives populates the singlet excited state and leads to a conversion into the Dewar isomer or into the corresponding triplet state through the usual intersystem crossing procedure. The efficiency of these processes determines the presence or the absence of isomerized furan derivatives in the reaction mixtures.     

Ab initio studies of arginine and deaminoarginine

L-Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO -3G basis set for arginine and the STO -3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.     

Ab initio kinetics of gas phase decomposition reactions

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process. To overcome this ambiguity, a theoretical study of a complete picture of gas phase thermo-decomposition, including reaction energies, activation barriers, and reaction rates, is illustrated with the example of the β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) molecule by means of quantum-chemical calculations. We study three types of major decomposition reactions characteristic of nitramines: the HONO elimination, the NONO rearrangement, and the N-NO(2) homolysis. The reaction rates were determined using the conventional transition state theory for the HONO and NONO decompositions and the variational transition state theory for the N-NO(2) homolysis. Our calculations show that the HMX decomposition process is more complex than it was previously believed to be and is defined by a combination of reactions at any given temperature. At all temperatures, the direct N-NO(2) homolysis prevails with the activation barrier at 38.1 kcal/mol. The nitro-nitrite isomerization and the HONO elimination, with the activation barriers at 46.3 and 39.4 kcal/mol, respectively, are slow reactions at all temperatures. The obtained conclusions provide a consistent interpretation for the reported experimental data.     

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.     

Ab initio studies of methylenecarbene and isoelectronic species

Optimum equilibrium geometries, energetics, harmonic vibrational frequencies, and infared intensities within the double harmonic approximation are computed for methylenecarbene, CCH₂, and isoelectronic species involving silicon and germanium at both the SCF level of theory and the level of second-order perturbation theory using a 6-311G(2df, 2p) basis set or its equivalent. Optimum equilibrium geometries and energetics are also computed at both levels of theory using a smaller 6-311G(d, p) basis set or its equivalent. This investigation of these species is the first to include all of the systems with germanium. In addition, this present work is the first study to includef-type polarization functions in a systematic investigation of the molecular structure and properties of all the molecules in the series. Acetylenic structures are also computed for energy comparisons. Of all the linear isomers, only acetylene is found to be a minimum on the potential energy surface. However, all of the C_2v structures are found to be local minima. Both the C_2v and linear structures will serve as a basis for future work involving mapping the entire hyperenergy surfaces of all of the molecular systems in the series.     

Ab initio studies of small AlmFen clusters

The equilibrium geometrical structures of small Al_mFe_n clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies, the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and experiments is reasonable. The ground stable geometrical structures of Fe₄, FeAl₃, Fe₃Al and Fe₂Al₂ clusters favor three-dimension configurations, but Al₄ tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is different from that of bulk alloys.     

Ab initio studies of peroxynitrite anion-water complexes

Quantum mechanical methods have been applied to thecis-ONOO^–-H₂O,cis-ONOO^–-(H₂O)₂ andtrans- ONOO^–-H₂O complexes. Equilibrium geometries, binding energies, net atomic charges and vibrational frequencies are presented for several different arrangements. The MØller-Plessett second-order perturbation (MP2) method predicted shorter hydrogen bonds than the SCF method, but the computed Hartree-Fock (HF) binding energies are similar to counterpoise corrected MP2 values. The geometry changes of ONOO^– and water after solvation are examined. The ONOO^– and H₂O bond length changes follow typical hydrogen bond structural trends, whereas bond angles in ONOO^– are unaffected when the hydrogen bond is formed, similar to the conclusions from NO ₂ ^– -(H₂O) _n HF/6-31G studies and Monte Carlo simulations. Thecis-ONOO^–-(H₂O) _n frequencies are compared with the solution Raman spectrum and with calculations on isolated ONOO^–.     

Ab initio studies of benzocyclopropenone, benzocyclopropenone-containing

Ab initio calculations were carried out on cyclopropenone, 1, benzocyclopropenone, 2, the benzocyclopropenone-containing [2.2]paracyclophane derivative tetracyclo[8.3.2.(4,7)O(11,13)]heptadeca-1(13),4,6,10,14,16-hexaen-12-one, 3, its decarbonylation product tricyclo[8.2.2.2(4,7)]hexadeca-1(12), 4,6,10,13-pentaen-15-yne, 5, a benzyne intermediate, and the bridged benzobarrelene derivative, pentacyclo[5.5.2.2.(1,4)O(4,14)O(10,13)]hexadeca-2,7,9,13,15-pentaene, 6. These calculations suggest that benzocyclopropenone-containing [2.2]paracyclophane, 3, and highly strained bridged benzobarrelene, 6, could exist as stable species. Both aryl rings of the benzocyclopropenone derivative 3 are predicted to be distorted from planarity. This distortion relieves some angle strain present in planar benzocyclopropenone due to the presence of the annulated three-membered ring. Calculations on benzobarrelene, 8, and [2.2]paracyclophane, 4, were performed for comparison to gain a better understanding of the strain borne in bridged benzobarrelene 6. The activation barrier for the intramolecular [4 + 2] cycloaddition of 5 to give 6 was estimated at 18.8 kcal/mol while that for the corresponding [2 + 2] cycloaddition, giving the less stable 9, was 54.5 kcal/mol. The [2 + 2] cycloaddition's transition state was twisted in a manner reminiscent of the conservation of orbital symmetry prediction for an unstrained system.     

Ab initio calculations of isomerization reaction of diphosphene 1-sulfide to thiadiphosphirane

Ab initio Calculations of the isomerization reaction of diphosphene 1-sulfide (2′) to thiadiphosphirane (3′) suggest that the energy barrier of the reaction in the ground state is 25 kcal/mol and that 2′ lies 21 kcal/mol above 3′, while the calculations show that there exists one local minimum on the lowest triplet energy surface.     

Ab initio calculations on the isomerization of the 1,5-hexadiyne radical cation

Ab initio calculations at the MRCI//ROHF/6-31G** level suggest that the 1,5-hexadiyne radical cation is not stable and isomerizes to the 1,2,4,5-hexatetraene radical cation without a barrier. After this rearrangement, the internal energy of the ions will be sufficient for a subsequent isomerization to the benzene structure. At energies of about 1 eV above the vertical ionization energy, the 1,5-hexadiyne radical cation may dissociate to linear neutral and ionic C₃H₃ fragments. These results are in excellent agreement with previous experiments.