Kezun Xu

How many uracil tautomers there are? Density functional studies of stability ordering of tautomers

Abstract Eight stable tautomers of the molecular uracil were presented as the results of density functional theory (DFT) studies with various exchange-correlation functionals such as the local density functional SL, the nonlocal density functional BLYP, the hybrid functional B3LYP, and the half-and-half functional BHandH, using the 6-31G ∗ , 6-31+G ∗ , and 6-31G ∗∗ basis sets. The geometries of the eight tautomers were fully optimized at the B3LYP/6-31G ∗ , B3LYP/6-31+G ∗ , and BLYP/6-31G ∗∗ level, and it was noted that the polarized p-orbital function added to hydrogen atom and the diffuse functions added to the heavy atoms (nitrogen and oxygen atoms) did not contribute to the geometrical optimization of the ring structures except for the variance within 2° for HOC bond angles. All of the present DFT results suggested that the stability order should be: U1 > U2 > U4 > U5′ > U5 > U3 > U6′ > U6, where 2,4-dioxo-dilactam tautomer (U1) was the most stable. The relative energies including corrections of zero-point vibrational energies (ZPVEs) and the dipole moments and polarizabilities of the tautomers were tabulated in detail, with references to the experimental data and ab initio data in literature.

Conformational Stability of 1-Butene: An Electron Momentum Spectroscopy Investigation

The valence-shell electron momentum distributions for 1-butene are measured by electron momentum spectroscopy (EMS) employing non-coplanar symmetric geometry. The experimental electron momentum distributions are compared with the density functional theory (DFT) calculations using different-sized basis sets. Although the two conformers of 1-butene in the gas phase, namely the skew and syn, have very close ionization potentials, the electron momentum distributions, especially in the low momentum region, can show prominent differences for some of the valence orbitals. By comparing the experimental electron momentum profiles with the theoretical ones, the skew conformer is found to be more stable than the syn and their relative abundances at room temperature are estimated to be (69 +/- 6)% and (31 +/- 6)%, respectively. It demonstrates that EMS has the latent potential to study the relative stability of conformers.

Electron momentum spectroscopy of CF2Cl2: experimental and theoretical momentum profiles for outer valence orbitals.

Electron momentum distributions for outer valence orbitals of CF2Cl2 have been obtained by (e,2e) electron momentum spectroscopy at an incident energy of 1200 eV + binding energy. The experimental electron momentum profiles are compared with Hartree-Fock and density functional theory (DFT) calculations using B3LYP hybrid functional with the 6-31G and 6-311+G* basis sets. Generally, the shapes of the experimental momentum profiles are well reproduced by DFT calculations using larger basis sets 6-311 + G*. An attempt has been made to clarify the ordering of the outer valence orbitals, which have been in controversy, by comparing experimental results with B3LYP/6-311 + G* calculations.

Application of a multichannel detection system to the high-resolution fast electron energy loss spectrometer

A microchannel-plate based resistive-anode position sensitive detection system is applied to a high-resolution fast electron energy loss spectrometer. The biasing voltage divider network for the detector is investigated. The measurement efficiency is increased by about 20 times, and to some extent, the energy resolution is also improved. The static mode and the scanning mode of the spectrometer are discussed in detail. In the scanning mode, a spectrum with higher energy resolution can be obtained through a coarse scanning. As a testing experiment, the optical oscillator strength spectrum for the Rydberg series of helium is measured and the results are compared.

THEORETICAL STUDY ON INFRARED VIBRATIONAL SPECTRA OF BORIC-ACID IN GAS-PHASE USING DENSITY FUNCTIONAL METHODS

Abstract Density functional theory (DFT) methods with various exchange-correlation functionals such as SVWN, BVWN, BVWN5, BLYP, B1LYP, B3LYP, B3PW91, and BH and H are employed in a theoretical study of molecular boric-acid in gas-phase. In the calculations, the split valence 6-311++G** and 6-31G* basis sets were used. The geometry, zero-point vibrational energies (ZPVEs), and harmonic infrared vibrational (IR) frequencies are predicted. The calculated C 3h -symmetry geometrical parameters are compared with Hartree–Fock (HF) calculation results and experimental data. IR frequencies predicted by the BLYP, B3LYP, and B3PW91 calculations are in good agreement with experimental data. The frequency calculations presented here also suggest that the C 3h -symmetrical structure corresponds to a minimum in the potential energy surface, but neither is D 3h - or C 3 -symmetrical structure.

Electron Momentum Spectroscopy of Ethanethiol Complete Valence Shell

The binding energy spectra and electron momentum distributions for the complete valence orbitals of ethanethiol were measured for the first time by binary (e, 2e) electron momentum spectroscopy employing non-coplanar symmetric kinematics at an impact energy of 1200 eV plus binding energy. The experimental results are generally consistent with the theoretical calculations using density functional theory and HartreeFock methods with various basis sets. A possible satellite line at 17.8 eV in binding energy spectrum was observed and studied by electron momentum spectroscopy.

Photosynthetic performances of five Cypripedium species after transplanting

Photosynthesis and leaf traits of five species in genus Cypripedium were compared in natural habitats and transplant nursery to develop effective strategy for cultivation and conservation. Among five species, C. guttatum had the highest photosynthetic capacity (PNmax) in the natural habitat and nursery, while C. lichiangense the lowest. The differences in PNmax among species were correlated with leaf N content (LNC) and leaf dry mass per unit area (LMA). After transplanting from natural habitats to nursery, the PNmax of C. lichiangense and C. yunnanense decreased, that of C. guttatum increased, while those of C. flavum and C. tibeticum remained relatively constant. The variations in LNC and biochemical efficiency would be responsible for the differences in PNmax between plants in natural habitats and in the nursery, but not the relative stomatal limitation. After transplanting, the Fv/Fm of C. lichiangense and C. yunnanense were declined. Meanwhile, the temperature ranges maintaining 90 % PNmax of C. lichiangense and C. yunnanense were narrower than those of the other three species. Thus the biochemical process in five species played a major role in the differences of PNmax after transplanting, and the widespread species had higher photosynthetic adaptability than the narrow-spread species.

Which is the most stable one in WSin (n = 1-4)? A density functional investigation with pseudo-potential model

Abstract Geometries and stabilities of the clusters WSin (n=1–4) were investigated by the density functional method. Geometry optimizations and energy calculations were carried out at the B3LYP level with the LanL2DZ basis set for the tungsten atom and LanL2DZ, LanL2DZ+STO-3G and LanL2DZ+6-31G basis sets sequentially for the silicon atoms. Higher basis sets are used for the silicon atom, slightly lower total energies are obtained while modest changes of their geometries. From the calculations with different basis sets, WSi3 is expected as the most stable one according to the relatively higher dissociation energy among these clusters. Compared with the experimental result [Phys. Rev. Lett. 86 (2001) 1733], WSin (n=1–4) have the same stability order as that of WSin+ (n=1–4). In addition, charge transfer from tungsten 6s orbital and silicon 3s orbital to tungsten 5d orbital was found. Good agreements are achieved between the calculated ionization potentials and experimental results.

An investigation of electron momentum spectroscopy for ethane

The binding energies and electron momentum distributions of the five valence orbitals of have been measured at high momentum resolution. The measured binding energy spectrum has been compared favourably with the results of previously published experimental data, theoretical calculations and outer-valence Green function method calculation of this work. The measured momentum profiles have been quantitatively compared with momentum distributions predicted by Snyder and Basch molecular wavefunctions, self consistent field (SCF) wavefunctions up to the near-Hartree-Fock limit, density functional theory calculations and experimental data (Dey et al 1976 J. Electron Spectrosc. Relat. Phenom. 9 397). They are found to be in excellent agreement with Snyder and Basch molecular wavefunctions and density functional theory calculations, especially for the orbital. The density maps for oriented molecule in position and momentum space are presented.

Electron-impact study for the and ( n = 3 - 6) excitations in helium

Differential cross sections and generalized oscillator strengths for the and = 3 - 6) excitations overlapping neighbour non-dipole excitations in helium have been measured by an angle-resolved electron energy loss spectrometer at an impact energy of 1500 eV in a scattering angular range of . These data are first reported at such a high impact energy. Integral cross sections for the above (n = 3 - 6) excitations were also determined. Meanwhile, the absolute optical oscillator strength density spectrum in the valence region of helium was established at the same impact energy in a mean scattering angle of , and the absolute optical oscillator strengths for these discrete transitions corresponding to the valence region of helium are reported. All these experimental results are compared with previous appropriate published data.