Quantum chemical calculation studies on 4-phenyl-1-(1-phenylethylidene)thiosemicarbazide

Ab initio calculations of the structure, atomic charges, natural bond orbital and thermodynamic functions have been performed at HF/6-311G^** and B3LYP/6-311G^** levels of theory for the title compound of 4-phenyl-1-(1-phenylethylidene) thiosemicarbazide. The calculated results show that the sulfur atom and all of the nitrogen atoms have bigger negative charges and that they are the potential sites to react with the metallic ions, which make the title compound a multidentate ligand. The coordination ability of the sulfur atom and the nitrogen atom of C=N double bond increases with the increase of polarity of the solvent. Electronic absorption spectra have been calculated by time-dependent density functional theory (TD-DFT) method. The calculation of the second-order optical nonlinearity has also been carried out with the PM3 semi-empirical method, resulting in the molecular hyperpolarisability is 5.477×10⁻³⁰ esu.     

Evaluation of an ab initio quantum mechanical/molecular mechanical hybrid-potential link-atom method

 Hybrid potentials have become a common tool in the study of many condensed-phase processes and are the subject of much active research. An important aspect of the formulation of a hybrid potential concerns how to handle covalent bonds between atoms that are described with different potentials and, most notably, those at the interface of the quantum mechanical (QM) and molecular mechanical (MM) regions. Several methods have been proposed to deal with this problem, ranging from the simple link-atom method to more sophisticated hybrid-orbital techniques. Although it has been heavily criticized, the link-atom method has probably been the most widely used in applications, especially with hybrid potentials that use semiempirical QM methods. Our aim in this paper has been to evaluate the link-atom method for ab initio QM/MM hybrid potentials and to compare the results it gives with those of previously published studies. Given its simplicity and robustness, we find that the link-atom method can produce results of comparable accuracy to other methods as long as the charge distribution on the MM atoms at the interface is treated appropriately. Received: 27 September 2002 / Accepted: 21 October 2002 / Published online: 8 January 2003 Correspondence to: M. J. Field e-mail: mjfield@ibs.fr Acknowledgements. The authors thank the Institut de Biologie Structurale – Jean-Pierre Ebel, the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique for support of this work.     

Bonding Energy of a Molecular Orbital(Ⅱ)——Ab initio Calculation

Calculation of the bonding energy of a molecular orbital for a series of small molecules has been carried out by using ab initio STO-3G method. The results obtained demonstrate that the concept of the molecular orbital bonding energy is applicable for judging whether a molecular orbital is bonding, nonbonding or antibonding besides Mulliken overlap criterion.     

7-氮杂吲哚衍生物分子基态和激发态性质的理论研究

用从头算HF和密度泛函B3LYP方法对7-氮杂吲哚衍生物1,3-二(N-7-氮杂吲哚基)苯、1,3,5 三(N-7-氮杂吲哚基)苯和4,4′-二(N-7-氮杂吲哚基)联苯进行全优化, 计算分子的电离势IP和电子亲和势EA等相关能量, 并用ZINDO和TDDFT方法计算吸收光谱, 用CIS优化三种化合物分子的S1激发态结构, 并分析其能量与发射光谱的关系, 计算溶剂中分子的吸收和发射光谱, 并与实验结果对照. 计算结果表明, 从7-氮杂吲哚到上述三种衍生物依次愈来愈容易接受空穴, 吸收和发射光谱红移.     

Theoretical study of photoinduced ring-isomerization in the 1,2,4-oxadiazole series

A theoretical study of photoinduced ring-isomerization of 3-amino-5-methyl- and 3-amino-5-phenyl-1,2,4-oxadiazoles is reported. The results well agree with the reported experimental data: in particular, they explain the ring-photoisomerization into the corresponding 2-amino-1,3,4-oxadiazoles through a ring contraction-ring expansion route; moreover, the occurrence of competing pathways involving both the ring contraction and the internal cyclization-isomerization mechanism during irradiation of the 5-alkyl substituted substrates in the presence of a base has been also substantiated.     

Stable structures of 12-crown-O3N complexes with Li or Na in aqueous and acetonitrile solutions

Molecular geometries of crown ether derivatives play an important role in capturing and transporting alkali metal ions such as Li⁺ and Na⁺. As the selectivity of ions is observed in solutions, it is necessary to know their molecular structures in solutions. Recently, we investigated stable conformations of 12-crown-O₃N and its Li⁺ complex in aqueous solution by the combination of three programs, the CONFLEX, Gaussian 98, and BOSS programs. In the present study, we applied the same procedure to investigate stable structures of 12-crown-O₃N complexes with an alkali ion in aqueous and acetonitrile solutions. It was confirmed that the stable structures of Li⁺ and Na⁺ complexes in solutions are highly dependent on the polarity of the solvents.     

Cl_2 2HI=2HCl I_2反应机理的理论研究

分别在MP2/3-21G!!、CCSD(T)/3-21G!!//MP2/3-21G!!和B3LYP/3-21G!!3种水平上,计算研究了气相反应Cl2 2HI=2HCl I2的机理,求得一系列四中心和三中心的过渡态.通过比较六种反应通道的活化能大小,得到了相同的结论:双分子基元反应Cl2 HI"HCl ICl和ICl HI"I2 HCl的最小活化能小于Cl2、HI和ICl的解离能,从理论上证明了反应Cl2 2HI=2HCl I2将优先以分子与分子作用形式分两步完成.用内禀反应坐标(IRC)验证了MP2/3-21G!!方法计算得到的过渡态.     

Quantum chemical and spectroscopic analysis of calcium hydroxyapatite and related materials

Amorphous calcium hydroxyapatite was examined by vibrational spectroscopy (Raman and infra-red (IR)) and quantum chemical simulation techniques. The structures and vibrational (IR, Raman and inelastic neutron scattering) spectra of PO₄³⁻ ion, Ca₃(PO₄)₂, [Ca₃(PO₄)₂]₃, Ca₅(PO₄)₃OH, CaHPO₄, [CaHPO₄]₂, Ca₃(PO₄)₂·H₂O, Ca₃(PO₄)₂·2H₂O and Ca₃(PO₄)₂·3H₂O clusters were quantum chemically simulated at ab initio and semiempirical levels of approximation. A complete coordinate analysis of the vibrational spectra was performed. The comparison of the theoretically simulated spectra with the experimental ones allows to identify correctly the phase composition of the amorphous calcium hydroxyapatite and related materials. The shape of the bands in the IR spectra of the hydroxoapatite can be used in order to characterize the structural properties of the material, e.g., the PO₄³⁻ ion status, the degree of hydrolysis of the material and the presence of hydrolysis products.     

Ab initio and DFT computer studies of complexes of trimethylphosphine and products of substituted phosphonium cations with hydroxide, chloride and fluoride anions: Phosphorus analogues of choline and acetylcholine

Theoretical calculations (DFT, MP2) are reported for up to four sets of reaction products of trimethylphosphine, (CH₃)₃P, each with H₂O, HCl and HF together with DFT calculations on up to three sets of reaction products of substituted phosphonium cations, (CH₃)₃P–R⁺. These products comprise (a) P(III) normal complexes (CH₃)₃PHY, (b) P(IV) ‘reverse’ complexes Y(H–CH₂)₃P–R, (c) P(IV) ylidic complexes YHCH₂(CH₃)₂P–R and (d) P(V) covalent compounds Y–P(CH₃)₃–R for Y=HO, Cl and F and R=H, CH₃, C₂H₅, C₂H₄OH and C₂H₄OC:OCH₃. Calculations are carried out at the B3LYP/6-31+G(d,p) level in all cases and also at the MP2/6-31+G(d,p) level for systems in which R=H. Minimum energy structures are determined for predicted complexes or structures and geometrical properties, harmonic vibrations and BSSE corrected binding energies are reported and compared with the limited experimental information available. Potential energy scans predict equilibria between covalent trigonal bipyramidal P(V) forms and reverse complexes comprising hydrogen bonded or ion pair, tetrahedral P(IV) forms separated by low potential energy barriers. Similar scans are also reported for equilibria between reverse complexes and ylidic complexes for Y=OH and R=CH₃, C₂H₅, C₂H₄OH and C₂H₄OC:OCH₃. Corrected binding energies, structures and values of harmonic modes are discussed in relation to bonding The names ‘pholine’ and ‘acetylpholine’ are suggested for phosphorus analogues to choline and acetylcholine.