Chaoyong Mang

Minimum Polarizability Principle Applied to Lowest Energy Isomers of Some Gaseous All-Metal Clusters

In comparison with the minimum energy criterion as an indicator of the most stable state, the minimum polarizability and maximum hardness principles have been examined to describe the relative stability of various isomers of nine gaseous all-metal clusters M4X- (Cu4Na-, Cu4Li-, Al4Cu-, Ag4Li-, Au4Li-, Ag4Na-, Au4Na-, Al4Ag-, Al4Au-) on the basis of MP2 calculations. In these species, there are two lowest energy isomers with near isoenergy that sometimes make it very difficult to determine which of them is more stable when we depend only on the minimum energy criterion. According to the minimum polarizability principle, however, the square-pyramidal structure is always more stable than the planar isomer at various computational levels, which was also confirmed by the results from the minimum energy principle that sometimes requires higher computational precision. Thus, there is an indication that, at least for our present cluster system, the minimum polarizability principle is less dependent on the computational levels compared to the minimum energy principle.

Electronic circular dichroism of some double-helical alkynyl cyclophanes with 1,1′-binaphthyl auxiliaries investigated using time-dependent density functional calculations

For several double-helical molecules, the dependence of electronic circular dichroism (ECD) spectra on the molecular and electronic structures is investigated on the basis of TDDFT computations with the B3LYP hybrid functional. The calculations of a model molecule reveal that an acute-angled rotational dihedral angle other than an obtuse-angled rotational dihedral angle between two naphthyls implies Davydov splitting of exciton coupling. For the R enantiomers, the acute-angled rotational dihedral angle implies a negative lowest-energy Cotton effect, which is induced by electron density transfer from binaphthyls to phenyethynyls in the S0 → S4 excitations. Due to the large positive rotational strength of the S0 → S1 transition, the lowest-energy Cotton effect of the simplest cyclophane R-1 is positive, where the electron density transfer from phenyethynyls to binaphthyls plays an important role.

Theoretical searches and spectral computations of preferred conformations of various absolute configurations for a cyclodipeptide, cordycedipeptide A from the culture liquid of Cordyceps sinensis.

A cyclic dipeptide often has the multiple configurations and the abundant conformations. The density functional theory (DFT) method is used to search the preferred conformation of the most probable configuration for cordycedipeptide A isolated from the culture liquid of Cordyceps sinensis. The time-dependent DFT approach is exploited to describe the profile of electronic circular dichroism (CD). The calculated results show that the most probable configuration is 3S6R7S, whose preferred conformation has a negative optical rotation and a positive lowest energy electronic CD band.

Structural characteristics and spectral behaviours of the preferred conformation of a cyclic pentapeptide, cycloaspeptide G from Cordyceps-colonising fungus Isoria farinose

The density functional theory (DFT) is exploited to search the stable conformations of a cyclic pentapeptide called cycloaspeptide G from Cordyceps-colonising fungus Isoria farinose. Its time-dependent version is employed to describe the profiles of electronic circular dichroism (ECD) of the preferred conformation, where the solvent effect in methanol is taken into account on the basis of the polarisable continuum model computation. Four stable conformers are optimised, and the results of the harmonically vibrational frequency calculations illustrate that they are the true minima. In the vibrational CD spectrum at the B3LYP/6-31G* level, the negative peak at 3334 cm−1 has its origin of the NH stretches of the peptide ring. In the ECD spectrum at the B3LYP/6-31G* level, three strong bands are negative, positive, and negative. Since the ECD spectrum at the B3LYP/6-31G* level is remarkably different from that at the B3LYP/6-31G level, it is necessary for the expended functions to be added to the 6-31G basis set.

Density functional study of stereo-chemistry and chiroptics of helical alkynyl cyclophanes with binaphthyls

Stereo-chemistry and chiroptics of several alkynyl cyclophanes with multiple binaphthyls are investigated using density functional calculations. It is revealed that the lower-energy Cotton effects of tetragon-shape cyclophanes originate from electronic transitions on two long sides while the higher-energy Cotton effects have important origin in electronic transitions on two short sides. According to spectral characteristic and relative energy, we deduce that two low-energy conformers of S5 with two meta-position-substituted benzene rings are likely to coexist. For S6 as a macrocyclic molecule including eight binaphthyls, we infer that only one conformer as S6c is populated. This conformer is of a groove-shape structure with the αβαβ orientation of four meta-position-substituted benzene rings.

Theoretical studies on hyperpolarizabilities and UV-vis-IR spectra of a diamminecobalt(III) tetripeptide transition-metal complex

The second-order polarizabilities and the UV-vis-IR spectra of a transition-metal complex Co(NH3)2(L-ala–gly–gly) have been studied by using the MP2 and TDHF methods. The complex has a maximum β component in the direction from the N(alanyl) group to the N(glycyl) groups. A transparent optical spectrum region from 0.55 to 5.5 µm was found, which offers potential applications as an optical material. The alkyl substitution of the glycyl group only slightly affected the β value and retained the IR transparent region but may cause the molecules to have a favorite packing fashion in the bulk crystal that leads to larger second-order nonlinear optical coefficients.

Density functional study of conformational states for burkholdac C, a biologically active bicyclic depsipeptide from the bacterium Burkholderia Thailandensis

Highlights The lowest energy conformer of the peptide backbone ring is used to design initial conformations of burkholdac C. Four low energy conformers of burkholdac C are characterised by conformational geometry optimisation and electronic circular dichrosim calculations. Taking into account the functional, basis and solvent factors, the calculated electronic circular dichrosim agrees with the experiment. ABSTRACT This is a theoretical investigation of conformational analysis leading to determination of the most abundant conformers of burkholdac C. For this purpose, we applied methods of molecular modelling at molecular mechanics, density functional theory and time-dependent functional theory levels. As a consequence, we determined in vacuum the pool of four lowest energy conformations. To provide correlation between theoretical and experimental signals, we compared the calculated electronic circular dichrosim spectrum with the B3LYP and CAM-B3LYP functionals to the experimental results. Taking into account the functional, basis and solvent factors, the CAM-B3LYP results are preferable

Relative stability and optical nonlinearity of multiple conformers of 1,1′-substituted ferrocenes

The relative stability and the first and second hyperpolarizabilities of various conformers of four 1,1′-diarylethenylferrocenes have been evaluated at the MP2 computational level. Relative stability on the basis of the minimum polarizability principle is in complete agreement with the conclusion from the minimum energy criterion at the higher computational levels. Since the pi–pi interaction is an attractive force, the conformer with the intramolecular pi–pi interactions belongs to the lowest energy structure and is most likely to be populated. The calculated second hyperpolarizability is in excellent agreement with the experimental data available. There appear two low-energy absorption bands in electronic absorption spectra, where the lower-energy band shifts to the red and is enhanced by molecular distortion. This band is responsible for the largest linear polarizability and second hyperpolarizability of the conformer with inversion symmetry and the magnitude of the latter is dominated by one-photon transition.

A theoretical study on the second-order nonlinear optical susceptibilities of lithium formate monohydrate crystal, HCOOLi . H2O

Abstract Ab initio calculations of the first hyperpolarizabilities of (HCOOLi·H 2 O) 2 n supermolecules, as the building-blocks of lithium formate monohydrate (LFM) crystal with extended system, were performed for the first time. The dependence of the static β ijk 0 values on chain length was explored, and the frequency dependence of β ijk (−2 ω ; ω , ω ) was measured, and the influences of electron correlation and basis set on β ijk 0 were evaluated. Finally, we predicted the second-order nonlinear optical coefficients of LFM crystal. The β ijk 0 value of (HCOOLi·H 2 O) 2 n is linearly dependent on the chain length of supermolecule, which is quite unusual for an extended system connected by the O–Li bonds with ionic characters. Although the static component of β zzz 0 tensor is the static largest in these three components under study, the absolute value of frequency-dependent β zyy (−2 ω ; ω , ω ) element, transforming the smallest into the largest, is the most sensitive to frequency. After the fundamental wavelength is smaller than 500 nm, it is found that the β ijk (−2 ω ; ω , ω ) value is resonantly enhanced to a great extent due to the double frequency lies in the region of resonance. In addition, the β zxx 0 value goes from negative to positive with changes of electron correlation and basis set. Obviously, it is very necessary to take into account the effect of electron correlation, if the hyperpolarizability tensor components must be accurately calculated. Moreover, it is also very important whether it is adopted a complete basis set with diffuse and polarization functions. The calculated nonlinear coefficients at high level suggest that the scaled set reported by Robert seem more reasonable.