Xiao-Yuan Fu

Theoretical study of structures and energies of [HCOO]+ and [COOH]+ and their rearrangement

Abstract The structures of [HCOO] + , [COOH] + and the transition state on the rearrangement path are obtained by the MC SCF gradient method with a 6-31G ★ basis set. The energy barrier of the singlet rearrangement reaction, 8.47 kcal mol , calculated by the MC SCF MRSDQ CI method with a 6-311G ★★ basis set suggests that [HCOO] + is a stable biradical and experimentally observable.

Electron distribution in cationic fragments generated mass spectrometrically from peptides

Ab initio Hartree‐Fock (HF/6-31G, HF/6-31G* and HF/6-31G**) methods have been used to study the mechanism of the formation of cationic fragments from peptides. The results show that the simplest B 3 ion of nominal structure HCONHCH2CONHCH2CO 1 , can be fragmented into the A3 ion in the first step by the loss of CO, but not directly into the B2 ion. In addition, the A3 ion can be further fragmented into the B2 ion. The calculated activation barrier of the first reaction (from B3 to A3 )i s 26.2 kcal/mol at the HF/6-31G** level of theory. The second reaction (from B3 to B2) has much higher energy barrier (77.15 kcal/mol at HF/6-31G** level of theory). The third reaction (from A3 to B2) and the fourth reaction (from B2 to A2) have barriers of 28.82 and 25.59 kcal/mol, respectively. In addition to the energetics of the fragmentation, the electronic structure and bonding of the main stationary points have been analyzed by Bader’s theory of atoms in molecules (AIM). q 1999 Elsevier Science B.V. All rights reserved.

Ab initio studies on the mechanism of the cycloaddition reaction of fluoroketene with imines: substituent effects

Abstract The cycloaddition reactions of fluoro-substituted ketenes and imines (H2Cue5fbNH, CH3CHue5fbNH, CH3OCHue5fbNH and NH2CHue5fbNH) leading to 2-azetidinones were studied theoretically using RHF/3-21G. For H2Cue5fbNH, CH3CHue5fbNH and CH3OCHue5fbNH the reactions are believed to be non-synchronous and concerted. Four orbitals are involved in these reactions, which are of “2 × [1 + 1]” type. However, in the case of NH2CHue5fbNH the reaction proceeds in a two-step manner. The order of activity of the substituted imines studied is: NH2CHue5fbNH ⪢ CH3OCHue5fbNH ⪢ CH3CHue5fbNH ⪢ H2Cue5fbNH. The effect of substituents on the mechanism and activity of these reactions is explained in terms of frontier orbital interactions.

Ab initio study on the reaction 2NH(X3Σ−) → NH2(X2B1) + N(4S)

Abstract The geometries of the stationary points of the reaction 2 NH ( X 3 Σ − ) → NH 2 ( X 2 B 1 ) + N ( 4 S ) have been optimized at the UHF, UMP2, and UMP4(SDQ) levels with polarized double and triple zeta basis sets. The best calculation of the potential barrier for this reaction is 6.4 kcal/mol, obtained with the UMP-SAC4 theory. The intrinsic reaction coordinate (IRC) and vibrational frequencies along the IRC were calculated at the UMP2/6-311G ∗∗ level, and the UMP2 energy profile was refined with the UQCISD(T) and UMP-SAC4 levels of theory. Theoretical rate constants calculated by conventional and variational transition state theories are in good agreement with the experimental values in the available temperature range.

Ab initio studies on the mechanism of cycloaddition reactions of isocyanates and ethylene

Abstract The cylcoaddition reactions of ethylene and isocyanates (HNue5fbCue5fbO, FNue5fbCue5fbO or CH3Nue5fbCue5fbO) leading to 2-azetidiones were studied theoretically. For the HNue5fbCue5fbO + H2C ue5fbCH2 system, the stationary points are located by RHF/3-21G, RHF/6-31G, RHF/6-31G∗, MP2/3-21G and MP2/6-31G respectively. However, for the FNue5fbCue5fbO + H2C ue5fb CH2 or CH3Nue5fbCue5fbO + H2C ue5fb CH2 systems, HF/6-31G and HF/6-31G∗ have been used. All reactions studied are believed to be concerted. Four orbitals are involved in these reactions, which are of the “2 × [1 + 1]” type. The order of reactivity of the different isocyanates is FNue5fbCue5fbO > HNue5fbCue5fbO or CH3Nue5fbCue5fbO, which is explained in terms of frontier orbital interactions.

Theoretical studies on nucleophilic vinylic fluoride substitution

Abstract The reaction of vinyl fluoride with CN − has been studied by using the DFTB3YLP/6-31 + G(d) method. Topological features of the electron density distribution ϱ ( r ) and the analysis of the Laplacian concentration − ▽ 2 ϱ ( r ) have also been utilized for describing this reaction. The results show that this nucleophilic substitution is mainly of π attack with a barrier height of 14.8 kcal mol −1 which is 17.91 kcal mol −1 more favourable than the σ attack.

Ab initio studies on the mechanism of cycloaddition reactions of isocyanic acid and methylenimine

Abstract The cycloaddition reactions of isocyanic acid and methylenimine leading to four- or six-membered ring products were studied theoretically. If the reaction is equimolar, a two-step mechanism via a cis intermediate is confirmed. The second step is a rate-controlling one with an energy barrier of 28.3 kcal mol−1. In addition, the cis intermediate (INT) can further react with another methylenimine or isocyanate molecule to form a six-membered ring adduct. For the system INT + H2Cue5fbNH, the energy barrier is only 2.76 kcal mol−1, while for the system INT + HNue5fbCue5fbO, it is 5.02 kcal mol−1. This is consistent with the experimental fact that when one of the reactants is in excess, a six-membered ring adduct is formed exclusively.

Theoretical studies on the substituent effect of the thermal cycloaddition of ethylene and formaldehyde

Abstract Ab initio HF molecular orbital calculations using 3-21G and 6-31G∗ basis sets and including electron correlation (MP2 to MP4) have been applied to investigate the concerted supra-antara [2s + 2a] thermal cycloaddition reaction of some substituted ethylenes and formaldehyde. The stepwise reaction paths have also been studied for one of the reactions by using UHF/3-21G and MP2/3-21G. The calculated results indicate that although the presence of some particular substituents can decrease the activation barriers obviously, the title reaction is difficult to carry out via the supra-antara [2s + 2a] thermal cycloaddition pathway under moderate conditions. However, the reactions are probably carried out via stepwise pathways when there are strong electron-withdrawing groups in formaldehyde and/or electron-releasing groups in ethylene.

Hydrogen-bonded dimers with methanol as a proton donor: Ab initio MO studies

Abstract The hydrogen-bonded open chain dimers CH3OHue5f8CH3CN(I), CH3OHue5f8CH3NC(II), CH3OHue5f8H2O(III), CH3OHue5f8CH3OH(IV) and CH3OHue5f8CH3F(V) are studied by the ab initio SCF-MO method. The geometries of both isolated molecules and dimers are optimized (STO-2G level). It is found that all the above hydrogen bonds have a nearly colinear moiety. The hydrogen bond stabilization energies ΔE are calculated by means of a double-zeta 4–31G basis set. The hydrogen bond energy components at equilibrium geometries of these complexes are calculated according to Morokumas energy decomposing scheme. By inspection of the calculated results, one can see that all five complexes have electrostatic interaction (ES) greater than charge-transfer interaction (CT). Besides, ES(%) of complexes (I), (III) and (IV) are greater than that of (II) and (V), while CT(%) of (II) and (V) is greater than that of (I), (III) and (IV). The trend of CT(%) is (II) &>; (V) &>; (IV) &>; (I) &>; (III), which is the same as the increasing trend of the bond order of H⋯X and the decreasing trend of the bond order of proton donating Oue5f8H bonds. This tendency indicates that charge transfer occurs mainly within the region Oue5f8H⋯X.

MCSCF study on the IRC and the dynamic properties of the dehydrogenation reaction of vinyl radicals

Abstract The intrinsic reaction coordination (IRC) for the dehydrogenation reaction of vinyl radicals was traced by means of MCSCF/6–31G (210 configurations). The activation barrier of this reaction is 40.0 kcalmor −1 . Using the zero-point energy correction, the activation energy is 33.5 kcal mol −1 , which is in much better agreement with the experimental value (31.5 kcal mol −1 ). The rate constants at five temperatures from 800 to 1200 K were calculated by CVT, ICVT and μ VT. The reverse process is also discussed.