Wenxu Zheng

Some theoretical observations on the 1:1 glycine zwitterion–water complex

Abstract The structural transformation of three minima on the HF/6-31G∗ glycine zwitterion–water complex potential energy surface have been investigated at the Hartree-Fock (HF), second order Moller–Plesset perturbation theory (MP2)and the density functional theory using the B3LYP hybrid exchange-correlation functional employing several Poples standard basis sets. Contrary to the previous computational results, our results clearly confirm that the zwitterionic structure of glycine does not exist in the 1:1 complex with a water molecule in the gas phase. Several above mentioned computational methods were examined. HF calculations performed at various levels (up to 6-311++G(3df,3pd)) demonstrated the difficulty of obtaining reliable results. Generally, the agreement between B3LYP and MP2 calculations was good. With accurate basis sets, they got the proper results. At the same time, the results at different levels from CP-corrected gradient optimization and CP-uncorrected were also compared.

Theoretical Studies on the Nonlinear Optical Properties of Octupolar Tri-s-triazines.

The first-, second-, and third-order static and frequency-dependent polarizabilities of a series of octupolar tri-s-triazines have been investigated by using the ab initio coupled perturbed Hartree-Fock (CPHF) method. Effects of substitution have also been considered. The results show that α, β, and γ values for octupolar tri-s-triazines are much larger than those for s-triazine in both static and frequency-dependent cases. Attaching groups containing π systems such as azide and ethenyl to the tri-s-triazine molecule results in a significant increase of first-, second-, and third-order polarizabilities. Our calculations suggest that the octupolar tri-s-triazines may be prospective candidates for nonlinear optical materials.

The 1:1 glycine-water complex: some theoretical observations

Abstract The structures, interaction energies, electronic properties for different conformers of glycine(Ip)–water complex have been determined employing density functional theory using the B3LYP hybrid exchange-correlation functional with the Poples standard basis sets. Ab initio MP2 calculations were carried out to verify the appropriateness of the B3LYP methods for glycine(Ip)–water system. The basis set superposition error has been eliminated by using the full counterpoise correction method. The results from CP-corrected gradient optimization and the ones from CP-uncorrected were also compared. At last, the vibrational frequencies of the most stable conformer A(C1) were discussed.

Hyperconjugation versus intramolecular hydrogen bond: origin of the conformational preference of gaseous glycine

Abstract Three experimentally detected low-energy conformers of gaseous glycine are selected as models to investigate the origin of the conformational preference of nonionized glycine, employing the atoms in molecules (AIM) and natural bond orbital (NBO) analysis methods. At the B3LYP/6-311++G(3d,3p) theory level, it is found that the importance of intramolecular hydrogen bond was overemphasized in the previous studies and it is hyperconjugation not intramolecular hydrogen bond that determines the order and relative energy of the conformers considered in this Letter.

Theoretical study of tri-s-triazine and some of its derivatives

Density functional theory has been used to study the geometries, electronic structures, harmonic vibrational frequencies, and high energy density material properties of tri-s-triazine and ten derivatives (2-R-5,8-dihydrogen-tri-s-triazine with Ru2006=u2006NH2, OH, N3, NO2, F, Cl, Br, –CN, –CHCH2 and –CCH) at the B3LYP/aug-cc-pVDZ level of theory. The results show that the tri-s-triazine ring maintains a planar and rigid structure in all the compounds and that there exists considerable conjugation over the parent ring, which is advantageous for the stability of these compounds. Substituent effects on the geometry, electronic structure, conjugation and HOMO/LUMO of the parent ring are discussed in detail. Vibrational frequency studies indicate that the parent ring has a characteristic frequency and that all the studied substituents shift it to lower wave numbers. Moreover, our study shows that some of the discussed compounds may be potential candidates for high energy density materials (HEDMs).

An ab initio study of P-H...P interactions using the PH3... PH3 model complex

Abstract The P–H⋯P interactions were investigated using a model complex phophine dimmer with its three minimum energy conformers on the counterpoise corrected potential energy surface. At the MP2(full)/6-311++G(3df,3pd) level, the basis set superposion error corrected interaction energy of the three minimum energy conformers is−1.26, −0.85 and −0.85xa0kcal/mol, respectively, similar to the MP2 values previously found for the C–H⋯S, S–H⋯C, C–H⋯O and O–H⋯C interactions. The Natural Bond Orbital analysis confirms that they are not of weak hydrogen bonding and bound only by van der Waals forces.

Quantum chemical study on enantioselective reduction of keto oxime ether with borane catalyzed by oxazaborolidine. Part 4. The reduction process in which oxime is reduced first and carbonyl is reduced through an intermediate containing a B(2)–N(3)–BBH2–OCO–CCO–CCN–NCN 7-membered ring

Abstract As the fourth part of quantum chemical study on enantioselective reduction of keto oxime ether with borane catalyzed by chiral oxazaborolidine, the reduction path in which oxime is reduced first and the carbonyl is reduced through an intermediate containing a B(2)–N(3)–B BH2 –O C ue605 O –C C ue605 O –C C ue605 N –N C ue605 N 7-membered ring is discussed by means of density functional theory (DFT) method in this paper. The structures of the main intermediary and transition states are optimized completely at the B3LYP/6-31g(d) level. The harmonic frequencies of all the structures are calculated, and corresponding vibrational modes are decided. The reduction path discussed in this paper mainly goes through the formation of the catalyst-borane adduct 2 , catalyst-borane-keto oxime ether adduct 3b , adduct 4b* with a B(2)–N(3)–B BH2 –O C ue605 O –C C ue605 O –C C ue605 N –N C ue605 N 7-membered ring and adduct 5b* . As shown from the studies on transition states, the controlling steps for the chirality of products are the transfers of hydride from the borane moiety to the oxime carbon and carbonyl carbon. From the energy point of view, the reaction should mainly go through 1→2→TS 1b -1→3b-1→TS 2b -1→4b*-1→TS 5b -1→5b*-1 and get cyclic(1S,2R)- cis amino-alcohols, which is in accordance with the experiment.

Effect of CP-corrected gradient optimization on the water-radical (anion) dimmer hypersurface

Abstract Geometry optimizations are carried out for the CN–H 2 O, CN − –H 2 O, NO–H 2 O, NO − –H 2 O, HO–H 2 O, and OH − –H 2 O intermolecular complexes on both the uncorrected and CP-corrected potential energy hypersurfaces. Because of the correction of the basis set superposition error (BSSE) during the gradient optimization, CP-corrected gradient optimization is more prior than normal optimization in structure research. But there is no significant difference between CP-corrected gradient optimization and normal optimization at interaction energies and BSSE. The diffuse basis functions are necessary for all the present systems. 6-311++g** basis set is efficient to these systems for its good results with low time consumption.

The gas-phase conformations of (N–Cl)-glycine: some theoretical observations

Abstract B3LYP and MP2 methods were employed to optimize thirteen conformers of (N–Cl)-glycine using 6-311++G** basis set. Single-point MPn (n=2–4) calculations using the same basis set were carried out in order to confirm the reliabilities of B3LYP and MP2 on energies and relative stabilities for the glycine derivative. On the other hand, the effects of zero-point vibrational energy and electron correlation on the relative stabilities of different (N–Cl)-glycine conformers were discussed by comparing results from these theoretical calculations. It was found from the calculations above that B3LYP method could provide geometry structure very similar to MP2 theory for (N–Cl)-glycine. But the relative stabilities of the conformers of (N–Cl)-glycine predicted by MP2 method are in more agreement with high-level MP4(SDTQ) than B3LYP. The zero-point vibrational energy plays an important role on relative stabilities of some conformers for MP2 methods, but not for B3LYP. Significant effects of electron correlation on the relative energies of (N–Cl)-glycine conformation were observed by means of different treatments for electron correlation. Natural Bond Orbital and Atoms in Molecule analyses were performed to determine the existence of intramolecular hydrogen bond, which indicate that only three conformers have intramolecular hydrogen bonds. Furthermore, NBO is also employed to investigate quantitatively hyperconjugation in the conformers in order to account for conformational preference. Our results point out that hyperconjugations are responsible for the relative stability of most conformers. While for conformers containing hydrogen bond, the origin of relative stability comes from the balance of H-bond and hyperconjugative effect.