Reparameterization of Semiempirical Techniques to Remove Anomalous Energy Minima

Semiempirical molecular orbital calculations have been shown to predict double minima potential surfaces which are not confirmed by further experimental or theoretical evidence.¹ These same techniques predict barriers to internal rotation which are not only in large error but may not even follow the experimental trends.² These discrepancies offer a large decrease in self confidence for the application of these techniques to molecular systems where direct experimental evidence is not available, and these are often the circumstances where one wants a theory to be used. Therefore, our research group is in the process of reparameterizing the procedure INDO³ to yield barriers to internal rotation, bond angles, and bond lengths within reliable confidence limits. During this process of determining new parameters we have observed that some sets of parameters will also remove the anomalous energy minima predicted by the original INDO parameters. To illustrate this we consider in this paper the molecule CO₂.     

Investigation on stabilities and spectroscopy of C80O2 based on C80 (D5d) using density functional theory

The relative stabilities of the 17 possible isomers for C₈₀O₂ based on C₈₀ (D_5d) were studied using Becke three parameters plus Lee, Yang, and Parr's (B3LYP) method and 6‐31G (d) basis set in density functional theory. The most stable geometry of C₈₀O₂ was predicted to be 23,24,27,28‐C₈₀O₂ (A) with annulene‐like structures, where the additive bonds are those between two hexagons (6/6 bonds) near the equatorial belt of C₈₀ (D_5d). Electronic spectra of C₈₀O₂ isomers were calculated based on the optimized geometries using intermediate neglect of differential overlap (INDO) calculation. Compared with those of C₈₀ (D_5d), the first absorptions in the electronic spectra of C₈₀O₂ are blue‐shifted owing to the wide energy gaps. ¹³C nuclear magnetic resonance spectra and nucleus independent chemical shifts of the C₈₀O₂ isomers were computed at B3LYP/6‐31G level. The chemical shifts of the bridged carbon atoms in the epoxy structures of C₈₀O₂ compared with those of the bridged carbon atoms in the annulene‐like structures are changed upfield. Generally, the isomers with the annulene‐like structures of C₈₀O₂ are more aromatic than those with the epoxy structures. The addition of the oxygen atoms near the pole of C₈₀ (D_5d) is favorable to improving the aromaticities of C₈₀O₂. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Calculation of electronic spectra of porphin and its derivatives by a modified INDO/S method

Quantum-chemical calculations of excited electronic states of porphin (H₂P) and Mg-porphin (MgP) have been carried out in the framework of the INDO/S method with varying off-diagonal matrix elements of the one-electron Hamiltonian {ie027-01} and electron-electron interaction integrals {ie027-02} as functions of the internuclear distance. It has been found that a simultaneous increase in the π-type overlap factor for {ie027-03} and decrease in the {ie027-04} integrals, as compared with {ie027-05} calculated by the Nishimoto-Mataga formula, make it possible to reproduce the positions of the Q-and B-transitions in the experimental absorption spectrum with a precision of ∼300 cm⁻¹. In this case, the N_x-transition intensity of the H₂P molecule is halved, which means that only two B-transitions should be related to the Soret band as in the four-orbital model. Using the proposed parameterization (INDO/Sm), electronic spectra have been calculated for a number of porphyrin compounds including chlorine, bacteriochlorin, tetrabenzoporphin, and tetraazaporphin. The results obtained agree with the experimental data to an accuracy of 300–700 cm⁻¹ whereas the accuracy of the standard INDO/S calculation is no better than 3000 cm⁻¹. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 28–35, January–February, 2008.     

C}oS的结构和电子光谱的理论研究

用IND(〕方法研究C,oS的8种构型.表明S原子主要连接在C,。的C,-C}(C,表示C7o的第一种C原子)或C3一 C3键上.且不开环形成三员环状结构.光谱研究表明,C7oS的主要吸收峰与C7。相似,且在500mm以上产生特征吸收峰,可以此作为C,oS形成的依据、     

四苯基卟啉及其系列衍生物电子光谱的INDO/CI研究

运用ＭＮＤＯ计算和群论,优化得到了较满意的中位取代四苯基卟啉（Ｈ２ＴＰＰ）的分子构型。并结合ＩＮＤＯ／ＣＩ方法,对四苯基卟啉及其系列衍生物进行了电子结构的研究和电子光谱主要吸收带的指认。提出五分子轨道模型,揭示了卟啉类化合物电子光谱典型特征的内在原因,计算结果能与实验值较好符合。对卟啉系列衍生物的取代基效应进行了讨论,得到一些有益的结论。     

富勒醇C_(60)(OH)_8的结构和UV谱的理论研究

用 INDO系列方法研究 C60 (OH) 88种异构体的结构和光谱 ,探讨羟基不同加成位置对异构体稳定性的影响 ,表明 8个羟基以 1,2 -加成方式加在 C60 的五元环与六元环相邻棱上所得的异构体最稳定 ,其生成热比次稳定异构体低 2 5 e V,故实验室合成的 C60 (OH) 8主要以这种构型存在。以优化构型为基础 ,计算了 8种异构体的 UV谱 ,对电子跃迁进行理论指认 ,讨论产物 UV谱带红移的原因 ,对反应机理进行探讨 ,并对 C60 R8(R=F,OH,Cl)的某些性质进行了对比。     

Semiempirical INDO/S study on the absorption spectrum of violacein

Violacein is the major pigment produced by Chromobacterium violaceum. We now report the results of semiempirical calculations on the electronic spectrum of violacein. We have used the AM1 and Intermediate Neglect of Differential Overlap (INDO) model Hamiltonians. Solvent effects have been included with the SCRF model for spectroscopy as proposed by Zerner and Karelson. Our results suggest that in the gas phase violacein is almost planar while in ethanolic solution the H-bond interactions force an internal rotation due to sterical factors. The calculated UV-visible spectrum with the inclusion of specific solvent interactions in the Hamiltonian is on good agreement with the experimental spectrum.     

INDO Calculations with inclusion of an effective solvent field. Application to benzosemiquinones

Theoretical Chemistry Accounts - An INDO method extended to include a contribution from the solvent by means of an effective solvent field (ESF) is applied to the three isomeric benzosemiquinones...     

X-Ray Emission Spectra and Electronic Structure of Palladium β-Diketonate Complexes

A series of palladium -diketonate complexes have been studied by X-ray spectroscopy. For interpreting the X-ray emission spectra, electronic structure calculations have been performed by the INDO method for the Pd(acac)₂ and Pd(hfac)₂ complexes in the ground state. The results of these calculations indicate that if one HOMO is excited, the electronic structure of the whole complex undergoes considerable rearrangements changing the sequence of HOMOs.     

Electronic Structures and Spectra of the Bases and Base Pairs of Nucleic Acids

The present paper covers electronic structures and spectra of the bases and the base pairs of nucleic acids calculated by using the INDO/S method. For free bases we give the energy levels of ground states and transition energies of low-lying excited states and discuss the band characters. The results indicate that the calculated spectra are in good agreement with experimental values. On the other hand, our calculations for A-T and G-C pairs are very beneficial to understanding hydrogen bond properties of these pairs.