Igor S. Ignatyev

Mechanism of the C2H5+O2 reaction

The geometries, energies, and vibrational frequencies of the reactants, transition states, intermediates, and products of the reaction of ethyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with experimental deductions and high-level coupled cluster results suggest that the B3LYP method is superior for the C2H5+O2 problem. Using the B3LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3 kcal mol−1 below reactants. This transition-state energy is consistent with the observed high yields of ethylene in the high-temperature reaction and is in good agreement with the height of the barrier estimated via modeling of the experimental kinetic data. However, this transition state (TS1) corresponds not to the internal proton transfer lead...The geometries, energies, and vibrational frequencies of the reactants, transition states, intermediates, and products of the reaction of ethyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with experimental deductions and high-level coupled cluster results suggest that the B3LYP method is superior for the C{sub 2}H{sub 5}+O{sub 2} problem. Using the B3LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3kcalmol{sup {minus}1} {ital below} reactants. This transition-state energy is consistent with the observed high yields of ethylene in the high-temperature reaction and is in good agreement with the height of the barrier estimated via modeling of the experimental kinetic data. However, this transition state (TS1) corresponds not to the internal proton transfer leading to the hydroperoxyethyl radical C{sub 2}H{sub 4}OOH but to the concerted elimination of ethylene. For the reverse reaction C{sub 2}H{sub 4}+HO{sub 2}{r_arrow}C{sub 2}H{sub 4}OOH, the TZ2Pf UB3LYP classical barrier is 11.2kcalmol{sup {minus}1}. {copyright} {ital 1997 American Institute of Physics.}

Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine

Abstract The complete harmonic force field of pyrimidine has been computed at the ab initio Hartree—Fock level using a 4–21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aforementioned level of quantum mechanical approximation, the theoretical force constants were empirically scaled by using nine scale factors. (The values of all these scale factors were previously determined by fitting the theoretical force field of benzene to the observed vibrational spectra of benzene.) The resulting a priori scaled quantum mechanical (SQM) force field is regarded as the most accurate and physically the most correct harmonic force field for pyrimidine. This force field was then used to predict the vibrational spectra of pyrimidine- h 4 and pyrimidine- d 4 . On the basis of these a priori vibrational spectra uncertain assignments have been confidently resolved. After a few reassignments, the mean deviations between the experimental and calculated frequencies are below 9 and 18 cm −1 for the non-CH stretching in-plane and the out-of-plane vibrations, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.

Competitive ring hydride shifts and tolyl-benzyl rearrangements in tolyl and silatolyl cations

Abstract Transition states for two plausible isomerization pathways of tolyl and silatolyl cations are predicted at the B3LYP/6-31(d,p) level of theory. The barrier heights for the interconversion of the ortho- and meta- isomers are high for both tolyl (50 kcal/mol) and silatolyl (57 kcal/mol) cations. Competitive hydride shifts from methyl and silyl groups to the positive charge center at the phenyl ring leading to benzyl and silabenzyl cations have sufficiently different barrier heights: the one for tolyl cation (26 kcal/mol) is close to the upper excess energy limit of the nucleogenic tolyl cation, while this barrier for the silatolyl cation is substantially lower (9 kcal/mol). Stationary points for the path leading from the benzyl cation to the global minimum of the C 7 H 7 + system, i.e. the tropylium cation, were also found.

Vibrational spectra of silatranes and germatranes XM(OCH2CH2)3N (X = F,Cl,H; M = Si,Ge). The problem of the theoretical prediction of condensed phase spectra

The structures of silatranes and germatranes XM(OCH(2)CH(2))(3)N (X=F,Cl,H; M=Si,Ge) were optimized and their vibrational spectra were calculated at the B3LYP/aug-cc-pVDZ level of theory. Theoretical frequencies of vibrations perpendicular to the C(3) axis (E type) are in good agreement with experimental values, while the axial vibrations (MX and M...N stretchings) demonstrate a significant discrepancy with experimental spectra recorded for the crystalline state. This discrepancy stems from the well-known difference in the MX and M...N bond lengths in gas and solid state. The force constant scaling procedure was used to compensate for this difference. As a result a set of scaling factors was refined for 1-Cl-germatrane (the unique atrane for which the distinction between A and E modes was experimentally established). This set was transferred to the theoretical force fields of other atranes, which provided a fair reproduction of their experimental frequencies. The analysis of the normal modes allowed us to assign the ν M...N mode to bands in the 180-270 cm(-1) frequency range, although large contributions of these coordinates are in two other modes in the 450-500 cm(-1) and 600-800 cm(-1) frequency ranges. The frequencies of degenerate vibrations (with vectors perpendicular to the C(3) axis) do not depend substantially on the axial atom (X and M) substitution, while those of A-type in the 200-700 cm(-1) frequency range vary significantly.

Radiochemical study of the gas phase reaction of nucleogenic diethylsilylium ions with methanol and butanol

Abstract The gas phase ion–molecule reactions between alcohols (methanol and n-butanol) and nucleogenic diethylsilylium ions generated by the β-decay of the tritiated diethylsilane were studied by the radiochromatographic method. Among the labelled neutral products of the silylation of alcohols diethylsilylalkoxysilanes exhibit the major yield. Diethyl-butoxysilane contains not only n-butyl groups as in the substrate (21%), but also small amounts of s-butyl (5%) and t-butyl (7%) groups. In other alkoxysilanes the diethylsilyl group of the nascent nucleogenic cation is rearranged and isomerized. Products with the similar distribution of the silylium group isomers were observed earlier in the reaction of benzene, however the yield of the products with rearranged silylium groups was substantially greater for benzene. This difference in the yield of the products with rearranged silylium groups between benzene and alcohols is rationalized taking into account the more acidic character of the proton in the adduct oxonium ions.

Condensation reactions in silanol–water clusters

Abstract Two types of complexes were found in the 2H 3 SiOH+ n H 2 O ( n =1,2) systems by B3LYP and MP2 methods. Global energy minima correspond to cyclic structures and less stable open isomers, in which one hydrogen bond is eliminated, are higher in energy by 3–7 kcal/mol. The decrease in the stability of open structures due to the rupture of one hydrogen bond is partly compensated by a weak silicon–oxygen intermolecular interaction. This interaction favors condensation and results in a substantially lower barrier height for the reaction path emerging from the open complex. The catalytic effect of water reveals in the substantial lowering of the barriers heights for both types of transition states in going from n =0 to n =2.

Transition states for inversion and retention of configuration channels in the reactions of alkyl and silyl fluorides with a water molecule

Abstract The potential energy surface of CH 3 F+H 2 O and SiH 3 F+H 2 O systems were scanned by B3LYP and MP2 methods. For each system two channels of reactions, i.e. with retention and inversion of configuration, were explored by modelling the front-side and the back-side attack of the water molecule. In each case the barrier heights for two channels were predicted. The inversion channels have similar and high energy barriers for both systems while the substantial difference was found for the retention channel barriers between methyl and silyl fluorides. The lower barrier for silyl fluoride is in keeping with experimental observations.

Quantum chemical study of silanediols as metal binding groups for metalloprotease inhibitors

DFT (B3LYP and M06L) as well as ab initio (MP2) methods with Dunning cc-pVnZ (n = 2,3) basis sets are employed for the study of the binding ability of the new class of protease inhibitors, i.e., silanediols, in comparison to the well-known and well-studied class of inhibitors with hydroxamic functionality (HAM). Active sites of metalloproteases are modeled by [R3M-OH2]2+ complexes, where R stands for ammonia or imidazole molecules and M is a divalent cation, namely zinc, iron or nickel (in their different spin states). The inhibiting activity is estimated by calculating Gibbs free energies of the water displacement by metal binding groups (MBGs) according to: [R3M-OH2]2+ + MBG → [R3M-MBG]2+ + H2O. The binding energy of silanediol is only a few kcal mol−1 inferior to that of HAM for zinc and iron complexes and is even slightly higher for the triplet state of the (NH3)3Ni2+ complex. For both MBGs studied in the ammonia model the binding ability is nearly the same, i.e., Fe2+(t) > Ni2+(t) > Fe2+(q) > Ni2+(s) > Zn2+. However, for the imidazole model the order is slightly different, i.e., Ni2+(t) > Fe2+(t) > Fe2+(q) > Ni2+(s) ≥ Zn2+. Equilibrium structures of the R3Zn 2+ complexes with both HAM and silanediol are characterized by the monodentate binding, but the bidentate character of binding increases on going to iron and nickel complexes. Two types of intermediates of the water displacement reactions for [(NH3)3M-OH2]2+ complexes were found which differ by the direction of the attack of the MBG. Hexacoordinated complexes exhibit bidentate bonding of MBGs and are lower in energy for M=Ni and Fe. For Zn penta- and hexacoordinated complexes have nearly the same energy. Intermediate complexes with imidazole ligands have only octahedral structures with bidentate bonding of both HAM and dimethylsilanediol molecules.

DFT study of the hydrolysis reaction in atranes and ocanes: the influence of transannular bonding

AbstractThermochemical kinetics of hydrolysis reactions of compounds with transannular intramolecular M…N bonds, i.e., atranes RM(OCH2CH2)3N and ocanes R2M(OCH2CH2)2NH (M = Si, Ge; R = F, Cl, Me), is studied at the B3LYP/aug-cc-pVDZ theoretical level. Several DFT methods are assessed for the reproduction of the experimental activation barrier for the Si-O bond cleavage of 1-methylsilatrane. Activation barriers for atranes and ocanes show the tendency for their growth with the decrease of the electronegativity of a substituent R on going from F to Me and their decrease from Si to Ge. Hydrolysis activation barriers of atranes and ocanes are compared with those of their acyclic analogs RM(OCH3)3 and R2M(OCH2)2NH in order to study the role of transannular M…N bonds in the stability of these molecules to hydrolysis. Substantially larger barriers for atranes support the opinion that stability of atranes may be explained by the formation of intramolecular bonds; however, the strengthening of transannular M…N bonds results in lower M-O cleavage barriers. It was proposed that the M-O cleavage barrier height is determined not by a weak M…N bonding itself, but rather by the contribution of a nitrogen lone pair to the antibonding orbitals of M-O bonds. The NBO analysis show that this interaction increases with the decrease of the electronegativity of a substituent R and decreases on going from atranes to ocanes. In ocanes, the presence of M…N bonds does not kinetically hinder the hydrolytic process; M-O cleavage activation barriers for acyclic analogs are higher. M-Hal cleavage barriers are substantially higher than those for M-O cleavage for R = F, but lower for R = Cl. Graphical AbstractThe experimental barrier height of the Si-O bond cleavage in 1-methylsilatrane is well reproduced when three explicit water molecules are included in the B3LYP/aug-cc-pVDZ theoretical model.

Rearrangements of [C6H7Si]+ Cations. A Radiochemical and Quantum Chemical Study

Nucleogenic cations were formed by beta-decay of phenylsilane tritiated at ortho- and para-positions of the benzene ring as well as at the silyl group and the products of their reactions with methyl tert-butyl ether were analyzed by radiochromatography. We found that the o-silatolyl cation was isomerized into the silabenzyl cation while the p-silatolyl cation was not. Furthermore, the silabenzyl cation was not converted into other isomers. The potential energy surface of the C(6)H(7)Si(+) system was constructed by B3LYP and MP2 methods using an aug-cc-pVDZ basis set. Theory predicts the low barrier for o-silatolyl-silabenzyl isomerization but high barriers for hydride shifts in the ring from para to meta- to ortho-isomers. It seems that nascent nucleogenic ions have enough internal energy to overcome the ortho-to-benzyl barrier but not enough to cross over the hydride-shift barriers. Theory also confirms that the isomerization of the silabenzyl cation to the [C(6)H(6).SiH](+) complex takes place in one step with a 62 kcal mol(-1) barrier, whereas that to either the global minimum [C(6)H(7).Si](+) or the silatropylium ion involves multisteps with 69-80 kcal/mol barriers. In addition, we find that the barrier of interconversion between the [C(6)H(6).SiH](+) complex and one of the low-lying [C(6)H(7).Si](+) complexes is only 29 kcal/mol.