Anmin Tian

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.

An isomeric study of N5+, N5, and N5−: a Gaussian-3 investigation

Abstract An isomeric study for N 5 , N 5 + , and N5− has been carried out using the Gaussian-3 (G3) method. At the MP2(FU)/6-31G(d) level, six N5, five N5+, and four N5− isomers are identified. Of these 15 species, 12 of them have not been reported previously. The most stable N5 isomer is a weak N2⋯N3 complex with C2v symmetry, while the most stable N5− isomer is also a weak N2⋯N3− complex with the same symmetry. The most stable species for N5+ has an open-chain structure with C2v symmetry, which has been synthesized and characterized spectroscopically. Natural bond orbital (NBO) analysis suggests that the stability of this cation is enhanced by hyperconjugation.

N18 : A COMPUTATIONAL INVESTIGATION

Abstract Ab initio quantum mechanics methods have been applied to study the cage-like polynitrogen cluster N 18 . The previously reported cage-like structure of the N 18 molecule with symmetry C 2v has been optimized at the RHF/4-31G ∗ , RHF/6-31G ∗ , DFT(B3LYP, B3P86, BHLYP)/6-31G ∗ and MP2(full)/6-31G ∗ levels of theory. The harmonic vibration frequencies and their infrared (IR) and Raman intensities have also been reported at RHF/4-31G ∗ , RHF/6-31G ∗ , as well as the B3P86/6-31G ∗ level. The results show that the investigated cage-like structure is stable on the potential energy hypersurface, lying above separated nitrogen molecules by about 50 kcal mol −1 of nitrogen atoms, with a stability comparable with the dodecahedral N 20 molecule. The resultant structure also suggests the aromaticity of conjugated pentagons which stabilizes the cage-like polynitrogen clusters.

Theoretical investigation of four conformations of HNIW by B3LYP method

Abstract The density function theory method has been used to study the geometry and electronic structure of hexanitrohexaazaisowurtzitane (HNIW). Four conformations of HNIW were obtained. Full geometry optimization, calculation of vibrational frequency, and NBO analyses for the four conformations of HNIW were performed at the B3LYP/6-31G ∗∗ level. The geometry and electronic structures of four conformations were analyzed. The C–C bond linking two five-membered rings and the boat-shaped six-membered ring are important factors in stabilizing the molecular skeleton. The N–N bonds in N-nitro group are possibly more easily broken than the other bonds. The spatial orientation of NO 2 has some influence on the molecular structure and energy.

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.

On the correlation between bond-length change and vibrational frequency shift in halogen-bonded complexes

The C-Hal (Hal = Cl, Br, or I) bond-length change and the corresponding vibrational frequency shift of the C-Hal stretch upon the C-Hal···Y (Y is the electron donor) halogen bond formation have been determined by using density functional theory computations. Plots of the C-Hal bond-length change versus the corresponding vibrational frequency shift of the C-Hal stretch all give straight lines. The coefficients of determination range from 0.94366 to 0.99219, showing that the correlation between the C-Hal bond-length change and the corresponding frequency shift is very good in the halogen-bonded complexes. The possible effects of vibrational coupling, computational method, and anharmonicity on the bond-length change-frequency shift correlation are discussed in detail.

Structural Competition between Halogen Bonds and Lone-Pair⋅⋅⋅π Interactions in Solution

Structural competition between halogen bonds and lone-pair···π interactions in solution is studied using (13)C NMR combined with density functional theory calculations. Among the halogen bonds considered, only the iodine bonds and a few bromine bonds are strong enough to compete successfully with the lone-pair···π interactions.

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.

Hydrogen bond and halogen bond inside the carbon nanotube.

The hydrogen bond and halogen bond inside the open-ended single-walled carbon nanotubes have been investigated theoretically employing the newly developed density functional M06 with the suitable basis set and the natural bond orbital analysis. Comparing with the hydrogen or halogen bond in the gas phase, we find that the strength of the hydrogen or halogen bond inside the carbon nanotube will become weaker if there is a larger intramolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom donor to the antibonding orbital of the X-H or X-Hal bond involved in the formation of the hydrogen or halogen bond and will become stronger if there is a larger intermolecular electron-density transfer from the electron-rich region of the hydrogen or halogen atom acceptor to the antibonding orbital of the X-H or X-Hal bond. According to the analysis of the molecular electrostatic potential of the carbon nanotube, the driving force for the electron-density transfer is found to be the negative electric field formed in the carbon nanotube inner phase. Our results also show that the X-H bond involved in the formation of the hydrogen bond and the X-Hal bond involved in the formation of the halogen bond are all elongated when encapsulating the hydrogen bond and halogen bond within the carbon nanotube, so the carbon nanotube confinement may change the blue-shifting hydrogen bond and the blue-shifting halogen bond into the red-shifting hydrogen bond and the red-shifting halogen bond. The possibility to replace the all electron nanotube-confined calculation by the simple polarizable continuum model is also evaluated.