Theoretical study of C24N4 molecule

Both ab initio 6-31G, 3-21G and STO-3G basis sets and semiempirical PM3 and AM1 molecular orbital calculations are carried out on the C₂₄N₄ molecule of the T_d symmetry group. Results on the fully optimized structure which constrained T_d symmetry, molecular orbitals and vibrational frequency were obtained by both ab initio and semiempirical methods. The binding energy and various thermodynamic properties were also calculated via the PM3 and AM1 semiempirical methods. All the evidence of this work proves that the C₂₄N₄ molecule is stable and that its four six-membered rings with a remarkable delocalized C…C bond are similar to the related rings in the C₆₀ buckminsterfullerene structure.     

A semiempirical HAM/3 study of core-electron binding energies for uracil its related molecules

The semiempirical HAM/3 molecular orbital method is used to calculate core-electron binding energies of common pyrimidine bases and some of their aza analogs. Average percent difference (error) is found to be 0.91 ± 0.12% which is substantially smaller than the error obtained from ab initio calculations with the application of Koopmans' theorem. HAM/3 chemical shifts are as good as those of ab initio direct hole state calculations reported in the literature. The molecular charge distributions calculated by HAM/3 agree fairly well with those of experiment and other theoretical methods. A large error in the HAM/3 core-electron binding energy of carbon in a CH₃ group has been detected.     

Evidence that there is a future for semiempirical molecular orbital calculations

The frequency of use and discussion of semiempirical and ab initio software is traced with bibliometric data from the Current Journals of the American Chemistry Society (JCACS) database, which has complete papers published in 19 journals in various fields of chemistry. Not only is the use of all types of computational chemistry software increasing apace, but also the number of papers mentioning semiempirical programs as a percentage of all papers mentioning quantum chemistry programs is growing. This percentage referring to semiempirical molecular orbital software grew from 29% in 1989 to 34% in 1993.     

Performance of semiempirical methods in fullerene chemistry: relative energies and nucleus-independent chemical shifts

Semiempirical MNDO, AM1, and PM3 calculations are reported for 153 fullerene isomers in an attempt to assess the reliability of these methods through comparisons with ab initio and density functional results. B3LYP/6-31G^* relative energies are generally reproduced quite well by these calculations. Qualitative trends in ab initio nucleus-independent chemical shifts at the cage centers are captured by the semiempirical GIAO-MNDO approach while underestimating their absolute values. The agreement between the semiempirical results and the ab initio or density functional reference data is generally better for the larger fullerenes (C₆₀–C₁₀₂) than for the smaller ones (C₂₀–C₅₀). These systematic comparisons clarify the accuracy that may be expected from semiempirical computations in fullerene chemistry.     

Ab initio force field for simulations of proteins and nucleic acids

We present a new self-consistent set of ab initio analytical pair potential to predict specific nonbonded interactions of protein with nucleic acid, of protein with protein, and of nucleic acid with nucleic acid. The purpose of this study is to represent the interaction between biological molecules with an accuracy equivalent to the ab initio molecular orbital calculations, which are used as reference data to obtain the pair potentials. Atoms in nucleic acids and proteins are classified according to their chemical environments. An “effective charge,” a modification of a charge obtained from the Mulliken population analysis, is introduced and used to represent the electrostatic energy. More than 30,000 SCF interaction energies have been calculated to provide the reference data for the fitting procedure that we have adopted in the parameterization of the potentials. The standard deviation is 1.61 kcal/mol for interaction energies spanning the range from about ?220 kcal/mol to +20 kcal/mol. Molecular dynamics simulations, using the above new set of force field, have been performed successfully for the systems where adequate treatments of specific interactions are required: The stability of α-helix of C-peptide and the interaction of spermine with oligonucleotide are examined as preliminary examples.     

Theoretical study of the N---H tautomerism in free base porphyrin

The N---H tautomerism of free base porphyrin is investigated at the semiempirical spin-unrestricted AM1 (UAM1) and ab initio RHF/3-21G levels. The UAM1 method provides delocalized geometries for all stationary structures without imposing any symmetry constraint. RHF/3-21G geometry optimizations have to be performed under symmetry restrictions to ensure that realistic delocalized structures are obtained. Both the semiempirical and the ab initio calculations predict that the interconversion between trans tautomers proceeds in an asynchronous two-step process via intermediate cis tautomers. The cis tautomers are characterized as minima in the potential energy surface and are 8–10 kcal mol⁻¹ higher in energy. The activation energy for the trans → cis interconversion is calculated to be approximately 23 kcal mol⁻¹ at the 3-21G level. The activation energy for the synchronous trans → trans interconversion is higher and has a value of 30.5 kcal mol⁻¹. The activation energies obtained at the semiempirical UAM1 level are twice as large as the ab initio values.     

Hydrogen bonds: a comparison of semiempirical and ab initio treatments

H-bonding interactions calculated using the AM1, PM3 and SAM1 semiempirical molecular orbital methods are compared with the best available ab initio calculations for several intermolecular interactions of interest: acetic acid dimers, water/ acetylene, water/HCN, formaldehyde/acetylene, formaldehyde/HCN, ozone/acetylene, ozone/HCN, acetylacetone, melamine/ cyanuric acid, and nitromethane/ammonia. Experimental values are also presented where available. The energetic comparisons are based upon enthalpies of interaction from the ab initio calculations after counterpoise and vibrational corrections have been applied. Overall, AM1 seems to do best, except for O---H…O interactions, where none of the three methods excel.     

Laser spectroscopy in analytical chemistry:light on the next millennium

To date, lasers are widely accepted tools in analytical spectroscopy, involved in various stand-alone and hyphenated techniques. Furthermore, significant progress can be noted in this field. In this paper, first of all some laser characteristics are discussed. Subsequently, five selected topics are outlined to illustrate recent achievements and future developments:

1. Laser-induced fluorescence for detection in capillary electrophoresis, including the use of ultraviolet, continuous-wave lasers in combination with wavelength-resolved emission detection; the use of diode laser-induced fluorescence in the red region of the electromagnetic spectrum and the use of Ti:sapphire lasers for multiphoton-excited fluorescence detection.

2. Degenerate four-wave mixing for detection in liquid microseparation systems (based on the coherence of laser light).

3. Fluorescence line-narrowing spectroscopy for identification purposes, a cryogenic high-resolution molecular fluorescence technique with a high potential in environmental analysis.

4. Recent developments in Raman spectroscopy (including ultraviolet-Resonance Raman and hyphenation of liquid chromatography and Raman spectroscopy).

5. Use of lasers for sample introduction in inorganic analysis based on controlled material ablation.

From ephemers to monomers, oligomers and polymers. New methods for the generation and transformation of silanones

New efficient routes to the generation of silanones at relatively low temperatures are based on the following reactions:

1. (1) reaction of linear and branched perhydrocarbyldisiloxanes and oligosiloxanes, and of some of their C-functional derivatives, with gallium or indium iodides or bromides;

2. (2) reaction of hydrocarbylchlorosilanes R_4−nSiCl_n (n = 2–4) and SiCl₄ with dimethylsulphoxide (with or without Mg or Zn);

3. (3) Autodecomposition of organosilicon compounds containing and groups;

4. (4) reaction of R_4−nSiCl_n with metal oxides of high redox potential (with or without the presence of CH₃CN).

Insertions of dialkylsilanones into Si---O---Si and Si---O---C linkages as well as into the Si---Cl bond have been studied.     

Accredited dose measurements for validation of radiation sterilized products

The activities and services of the accredited Risø High Dose Reference Laboratory are described. The laboratory operates according to the European standard EN 45001 regarding Operation of Testing Laboratories, and it fulfills the requirements of being able to deliver traceable dose measurements for control of radiation sterilization. The accredited services include:

1. 1. Irradiation of dosimeters and test samples with cobalt-60 gamma rays.

2. 2. Irradiation of dosimeters and test samples with 10 MeV electrons.

3. 3. Issue of and measurement with calibrated dosimeters.

4. 4. Measurement of the dosimetric parameters of an irradiation facility.

5. 5. Measurement of absorbed dose distribution in irradiated products.

The paper describes these services and the procedures necessary for their execution.     

Adsorption of polyacrylic acid on aluminium oxide: DRIFT spectroscopy and ab initio calculations

Diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was used to study the adsorption process of the water-soluble polyacrylic acid (PAA) polymer on hydrous δ-Al₂O₃. Vibrational assignment of PAA, sodium polyacrylate, (Na–PA) and the PA-oxide surface complex was achieved by comparison of observed band position and intensity in the DRIFT spectra with wavenumbers and intensities from ab initio quantum mechanical calculations. The presented data of polyacrylic acid suggest that IR data calculated ab initio on relatively short oligomers (quantum-mechanical oligomer approach) may provide valuable information regarding the interpretation of polyelectrolyte infrared spectra. Batch adsorption experiments were performed to sorb PAA onto the δ-Al₂O₃ surface. The results obtained from DRIFT studies were compared with adsorption isotherm experiments in order to relate the level of PAA coverage to the nature of the surface complex. Ab initio molecular orbital calculations on PAA/Al₂O₃ clusters were used to model possible surface complexes. Strong correlation were found between theoretical and observed DRIFT frequencies of the antisymmetric R-COO⁻ vibration. A number of possible configurations of the polyacrylic acid/aluminate surface complex were tested via ab initio calculations. These possible configurations included different di-aluminium octahedral Al³⁺ surface models. Results obtained from adsorption isotherm experiments, DRIFT spectra and ab initio calculations indicate that the carboxylate oxygens bridge an Al³⁺-octahedral dimer [Al₂(OH)₂4(H₂O)2(OH)] in a ligand-exchange inner sphere complex.     

XIIth international meeting on radiation processing Avignon 25–30 March 2001 (Polymer irradiation: past–present and future)

Radiations are used efficiently and economically for the production of new or modified polymers. The following processes are considered:

1. Radiation curing;

2. Radiation cross-linking;

3. Radiation grafting.

These processes are commonly used today in industry and provide a broad range of new potential applications in various fields. The history of their development is briefly reported. The chemical reactions underlying these processes are described.

(1) Radiation curing is used commercially on a large scale for the production of improved coatings, lacquers and inks. The process can be conducted at very high speeds. Curing of magnetic formulations leads to particularly stable products, which compete favourably with more conventional materials.

(2) Radiation cross-linking is an established technology in the wire and cable industry. It emparts to the modified insulators improved resistance to solvents, to ageing and to elevated temperatures. The resulting cross-linked network also reduces the migration of fillers and thereby stabilizes in time any message imprinted with magnetic or colored pigments dispersed in a polymer.

(3) Radiation grafting is a powerful method for modifying more profoundly the properties of a polymer and for creating numerous, entirely new materials. The chemical modification can be applied at will into the bulk of the material or limited to a surface zone of any desired depth. This method can be used for instance, for introducing polar groups in the bulk or on the surface of non-polar polymers, for increasing or reducing the wettability of a polymer, for imparting a better compatibility of a polymer to a specific coating and the like.

The irradiation of water-soluble polymers in aqueous solutions, with or without the addition of another monomer gives rise to a variety of cross-linked gels which find useful applications in the biomedical field. Other promising applications will be considered.     

丝氨酸热分解机理的研究

用热重法(TG)、差热分析(DTA)和红外光谱(IR)方法测试了丝氨酸的热分解过程,用量子化学方法在RHF/6-21G水平上全优化计算了丝氨酸及其热分解中间产物、产物分子的几何构型,得到其总能量和Mulliken集居数等数据.从理论上证明了环氧中间产物的存在.通过对实验结果和计算结果如Mulliken集居数的分析,提出了丝氨酸的热分解反应机理是先失去CO2为主要反应通道,同时伴有先失去NH3而生成环氧中间产物的副反应发生.     

Hydration and stability of nucleic acid bases and base pairs

Empirical, quantum chemical calculations and molecular dynamics simulations of the role of a solvent on tautomerism of nucleic acid bases and structure and properties of nucleic acid base pairs are summarized. Attention was paid to microhydrated (by one and two water molecules) complexes, for which structures found by scanning of empirical potential surfaces were recalculated at a correlated ab initio level. Additionally, isolated as well as mono- and dihydrated H-bonded, T-shaped and stacked structures of all possible nucleic acid base pairs were studied at the same theoretical levels. We demonstrate the strong influence of a solvent on the tautomeric equilibrium between the tautomers of bases and on the spatial arrangement of the bases in a base pair. The results provide clear evidence that the prevalence of either the stacked or hydrogen-bonded structures of the base pairs in the solvent is not determined only by its bulk properties, but rather by specific hydrophilic interactions of the base pair with a small number of solvent molecules.     

Theory and range of modern semiempirical molecular orbital methods

Semiempirical molecular orbital methods have a long history. They serve to tackle large systems and complicated processes beyond the reach of ab initio or density functional methods. Although their setup is derived from Hartree–Fock theory, the design of approximate energy expressions and the empirical parameters are used to achieve higher accuracy than the underlying ab initio theory. In this way the effect of larger basis sets or correlation can be partially simulated. All widely used semiempirical methods establish their accuracy by error statistics for molecular properties with experimental and high-level ab initio or density functional theory calculations as a reference. Their computational efficiency makes them suitable for the study of biochemical systems and solid materials. The present review presents a variety of applications which demonstrate the need for and success of semiempirical methods.     

A quantum-chemical study of para/ortho-toluene alkylation by adsorbed methoxy species on zeolites

MNDO and PM3 semiempirical methods have been employed to study the mechanism of toluene alkylation by methanol on zeolites, by means of the cluster (H)₃SiO(CH₃)T(H)₂OSi(H)₃ in were T = Al, B, Ga. Two possible reaction mechanisms have been found. The first one corresponds to the classical process of aromatic electrophilic substitution, in which the transition state of the reaction shows distances and angles according to the precursor of the Wheland intermediate. The second mechanism proceeds in one concerted step, in which the transfer of the proton from the toluene to the basic oxygen of the zeolite is simultaneous to the transfer of the methyl cation from the zeolite to the toluene. The effect of chemical composition of the cluster on regioselectivity has been studied, and perturbed molecular orbital (PMO) analysis of the system at the ab initio HF/STO-3G level has shown that covalent interactions are important. The energy of the covalent interaction between the molecular orbitals of the toluene and the cluster mainly HOMO-LUMO, are more important in para than in ortho positions.     

A computer program for searching the best model for describing different experimental systems

An algorithm for searching the best polynomial analytical function for describing different experimental systems is presented. It is based

1. (1)on the generation of all possible analytical functions of a given order, with a given number of terms and with a given number of independent variables, and

2. (2)on the calculation of the parameters of all selected functions using the linear regression method.

To show the ability of the program two different examples are given:

1. (1) searching the best univariate polynomial model, and

2. (2) modelling of the stability of a two-component mixture as a function of three factors.

金衬底上阴、阳离子型硫醇分子的量子化学计算

采用量子化学程序Gaussian98从头计算方法，对用做金衬底离子化的2—巯基乙基—二甲基氯化铵和3—巯基丙磺酸钠两种硫醇分子进行了全优化计算，得到了这两种化合物的平衡几何构型、基态能量、分子轨道组成和电荷分布等，并分析讨论了其分子轨道作用性质以及自由硫醇分子和金表面的相互作用．研究发现，表面分子基团带有大部分的净电荷，说明利用金与硫醇分子的自组装可实现金衬底的离子化。     

A quantum chemical investigation of CO, NF, SO, CF2 and their combination products

The combination products of the compounds CO, NF, SO and CF₂ have been investigated by semiempirical and ab initio SCF methods. The semiempirical standard procedures lead to satisfactory molecular geometries except for sulfur compounds. The stabilisation energies, however, are unreasonable. Ab initio calculations with minimal basis sets lead to more satisfactory results, as far as stabilisation energies are concerned.