MNDO Calculations of silicon-containing molecules

A comparison is made of MNDO and MINDO /3 calculations for saturated silicon-containing molecules, and with experimental values, for heats of formation, molecular geometries, charge distributions, and ionization potentials. Except for bond angles, it is found that with the published parameter values the MINDO /3 program gives more reliable results than MNDO . For unsaturated molecules, a comparison of bond lengths and stabilities of Si multiple bonds as given by the two programs and ab initio methods is made, and large discrepancies between predicted structures are pointed out. Some reasons for the dicrepancies are discussed.     

取代基对α-氨基乙腈热解反应影响的研究

用MINDO/3方法洋细研究了取代基对α-氨基乙腈热消除反应的影响. 研究表明, 对于孤立分子的气相反应, 给电子基使活化势垒降低, 吸电子基使活化势垒上升; 而对OH~-催化下的反应, 给电子基和吸电子基均使活化势垒上升.     

MNDO and MINDO/3 study of the reactivity of 3-pyrrolin-2-one tautomers and derivatives

MINDO/3 and MNDO methods have been applied to the study of the reactivity of 3-pyrrolin-2-one tautomers and derivatives. The different parameters that can influence the reactivity from the frontier molecular orbital point of view (atomic charge distribution, frontier orbital energy and frontier orbital reactivity indices) are compared and evaluated with respect to the experimental reactivity already known.     

An excited state paired interacting orbital method

A new method for analyzing and visualizing the molecular excited states, named "excited state paired interacting orbital (EPIO)," is proposed. The method is based both on the paired interacting orbital (PIO) proposed by Fujimoto and Fukui [J. Chem. Phys. 60, 572 (1974)] and the natural transition orbital (NTO) by Martin [J. Chem. Phys. 118, 4775 (2003)]. Within the PIO method, orbital interactions between the two fragmented molecules are represented practically only by a few pairs of fragment orbitals. The NTO method is a means of finding a compact orbital representation for the electronic transitions in the excited states. With the method, electronic transitions are expressed by a few particle-hole orbital pairs and a clear picture on the electronic transitions is obtained. EPIO method is designed to have both properties of the preceding two methods: electronic transitions in composite molecular systems can be expressed with a few pairs of EPIOs which are constructed with fragmented molecular orbitals (MOs). Excited state characters, such as charge transfer and local excitations, are analyzed by using EPIOs with their generation probabilities. Thus, the present method gives us clear information on the composition of MOs which play an important role in the molecular excitation processes, e.g., optical processes.     

Molecular electronic structure of metal phthalocyanines

The molecular–electronic structure of the metal phthalocyanines (Fe, Co, Ni and Cu) has been determined by the molecular orbital treatment. Coulomb integrals of the metal atom occurring in the secular determinants have been approximated equivalent to the valence state ionization energy (VSIE) of a metal orbital for a particular charge configuration. The calculated π-electron charge densities have been found to be higher on the nitrogen atoms as compared to the other atoms in the molecule. This is in agreement with the e.s.r. studies of the metal phthalocyanines. To test the correctness of the molecular orbital calculations, the π-π* transitions (14,000 cm^?1 ? 30000 cm^?1), d-d* transitions (20000 cm^?1 ? 60000 cm^?1) and charge transfer transitions (15000 cm ^?1 ? 30000 cm^?1) have been calculated in the metal phthalocyanine molecules. The calculated frequencies have been compared with the observed ones and found in fair agreement.     

哌嗪二酮的气相Hel光电子能谱及其量化计算

给出了哌嗪二酮的气相HeI紫外光电子能谱(UPS), 并进行了化合物分子的HAM/3, MNDO, MINDO/3, INDO, CNDO/2和EHMO等量子化学计算研究. UPS谱低电离能(<11.00 eV)区的四重峰被指认为分子体系中氧-氧, 氮-氮原子孤对轨道间的通过键相互作用导致的分裂峰. 表明HAM/3和MNDO计算法是预指该化合物实验电离能正确次序、轨道对称性类型以及通过键相互作用导致分裂大小的较好方法.     

Estimation of microscopic, zwitterionic ionization constants, isoelectric point and molecular speciation of organic compounds

Mathematical models based on structure-activity relationships and perturbed molecular orbital theory have been developed to calculate the ionization pK(a)s for a large number of organic molecules. These models include resonance, direct and indirect electrostatic field effects, sigma induction, steric effects, differential solvation and hydrogen bonding. The thermodynamic microscopic ionization constants, pk(i), of molecules with multiple ionization sites and the corresponding complex speciation as a function of pH have been determined using these chemical reactivity models. For a molecule of interest SPARC (SPARC performs automated reasoning in chemistry) calculates all of the microscopic ionization constants and the fraction of each species as a function pH along with the titration (charge) curve. The system has been tested on several biologically and environmentally important compounds.     

Influence of clustering and molecular orbital shapes on the ionization enhancement in ammonia

The Coulomb explosion of clusters is known to be an efficient source for producing multiply charged ions through an enhanced ionization process. However, the factors responsible for obtaining these high charge states have not been previously explored in detail and remain poorly understood. By comparing intensity-resolved visible laser excitation experiments with semi-classical theory over a range spanning both multiphoton and tunneling ionization regimes, we reveal the mechanism in which extreme ionization proceeds. Under laser conditions that can only singly ionize individual molecules, ammonia clusters generate ions depleted of all valence electrons. The geometries of the molecular orbitals are revealed to be important in driving the ionization, and can be entirely emptied at the energy requirement for removal of the first electron in the orbital. The results are in accord with non-sequential ionization arising from electrons tunneling from three separate molecular orbitals aided through the ionization ignition mechanism.     

Ionization potentials within semiempirical antisymmetrized product of strictly localized geminals approach

The APSLG‐MINDO/3 method is evolved to reproduce the vertical ionization potentials for organic molecules. Two different schemes—one allowing for adjustment of the local electronic structure to the hole in each configuration and another one with a fixed local electronic structure—are developed and implemented. These methods are applied to a series of normal hydrocarbons. The possibility of localization of a hole in the polyethylene cation is discussed. Vertical ionization potentials are also obtained for a set of organic molecules with heteroatoms. Their values together with degeneracy of ionized states are compared with the data of photoelectron spectroscopy. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Ion spectroscopy: Where did it come from; where is it now; and where is it going?

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.     

The asilomar conference on ion spectroscopy,October 16–20, 2009

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.     

Biradicals stabilized by intramolecular charge transfer: properties of heterosubstituted pentalene and cyclooctatetraene biradicals

Intramolecular charge transfer can lead to substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. The formal biradicals 2,5-di-heterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are theoretically predicted to have the potential to be stable, persistent non-Kekulé molecules, as supported by high-level quantum chemical calculations. The singlet-triplet energy gaps and the S(0)-S(1) excitation energies of these molecules are similar to those of aromatic molecules rather than standard biradicals. These formal biradicals have a pronounced zwitterionic character, having a singlet ground state. The marked stabilization of the ground-state singlet for these non-Kekulé molecules is accompanied by a significant destabilization of the highest occupied molecular orbital (HOMO), leading to a low ionization potential (IP). This apparent inconsistency is explained by analyzing the electronic structure of the molecules. In the case of di-aza-pentalene, the energy of the first electronic excited state is only slightly lower than the ionization potential, making it a candidate for molecular autoionization.     

Comparison of semiempirical calculations for silicon compounds

A comparison is made of the performance of the MINDO/3, MNDO, AM1, and PM3 methods in calculating the nature of the dimer reconstruction observed on the silicon (100) crystal surface. Based on this case study we conclude that MINDO/3 gives the most realistic results, with PM3 calculations being quite similar but both MNDO and AM1 missing some key features of this system and giving rather unrealistic charge distributions. Hence use of PM3 is recommended for Si containing molecules where a lack of parameters or other restrictions prevent the use of MINDO/3.     

Similarities in the Intensities of Analogous Rydberg–Rydberg Transitions in the Molecular Series CF x Cl y (x=3, 2, 1; y=1, 2, 3)

In this work, the photoabsorption behaviour of the molecular series CF₃Cl, CF₂Cl₂ and CFCl₃, involving their ground state and two different Rydberg series, has been studied. The discrepancies or similarities in the intensities of homologous transitions in the three CF _x Cl _y molecules have been analysed on account of their electronic structure. Absorption oscillator strengths have been calculated with the Molecular quantum defect orbital (MQDO) approach. Electronic transitions between states belonging to two different unperturbed Rydberg series of the same molecule have been calculated by us for the first time. The quality of the achieved oscillator strengths has been assessed by comparison with, to our knowledge, scarce experimental data available in the literature, through analysis of the discrepancies or similarities in the intensities of homologous transitions in the molecular series CF _x Cl _y when states of different type are involved, and by testing the compliance of regularities by the Rydberg series object of our study.Article for the special issue dedicated to J.-P. Malrieu     

Time-dependent density functional theory study on electronic and spectroscopic properties for Ph<Subscript>2</Subscript>Bq and its complexes

The molecular structures of the ground state and the first singlet excited state for diphenylboron analogs of Alq₃ [Ph₂Bq where q is 8-hydroxyquinoline (QH)] and its three derivatives were optimized with the Density Functional Theory and ab initio “configuration interaction with single excitations” method, respectively. The frontier molecular orbital characteristics of Ph₂Bq were analyzed systematically in order to study the electronic transition mechanism. Electronic and spectroscopic properties of complexes have been investigated with Time-Dependent Density Functional Theory, which indicates that the emissions of Ph₂Bq and its derivatives originate from the electronic π → π* transitions within the QH ligands. That means that one might tune the emission wavelengths and improve charge transfer properties through the effect of substituent on the 8-hydroxyquinoline ligand. Similar calculations were carried out for isolated QH and its three derivatives for comparison. We found that the highest occupied molecular orbital and the lowest unoccupied molecular orbital of Ph₂Bq are similar to those of QH and their spectroscopic properties change similarly when they are substituted by the same group, which suggests that one can search possibility of a red or blue emission from Ph₂Bq derivatives by analyzing QH and its derivatives.     

Electronic absorption spectrum of radiation-produced pentane and hexane radical cations in saturated chlorofluorocarbon and perfluorocarbon matrices at 77 K.

Pentane and hexane radical cations are produced by irradiation, stabilized in saturated chlorofluorocarbon and perfluorocarbon matrices at 77 K and studied by electronic spectrometry. Special emphasis is laid on the obtainment of pure spectra by suitable deconvolution techniques. Electronic absorption spectra of these cations consist of a broad structureless absorption band with maximum at respectively 400 nm (pentane) and 480 nm (hexane). The absorption is intrinsic and is not to a considerable extent affected by the matrix. It is due to transitions from fully-occupied orbitals to the half occupied molecular orbital where the unpaired electron resides. A comparison is made of the electronic absorption spectra with photodissociation spectra obtained by ICR techniques and with theoretical spectra obtained by a conformationally-averaged MINDO/3 approach that includes transition-moment weighting.     

Bond lengthening and through-bond interactions in p,p'-dibenzene and related molecules

The structure of p,p'-dibenzene (PDB) has been investigated by full geometry optimizations using the empirical force field (EFF) and MINDO/3 methods. While other structural parameters are in good agreement, the central bond length calculated by MINDO/3 (1.595 Å), as confirmed by an ab initio (STO-3G basis set) optimization (1.596 Å), is in striking contrast to the corresponding length calculated by EFF (1.543 Å). A detailed analysis of the electronic structure of PDB based on a quantitative perturbational molecular orbital treatment reveals that through-bond coupling of the four π systems is responsible for an elongation of the σ bond which mediates this interaction. Further studies using the EFF and MINDO/3 approaches demonstrate that extended C-C single bonds can arise even in structures with fewer than four π systems. The effect of substituents on the central bond length in PDB has been briefly investigated. (MINDO/3). A variety of other structures have been identified in which bond lengthening may result from through-bond coupling.     

Electronic and Chemical Characterization of Aluminum–Nitrogen (AlN) Substituted Fullerenes: C58AlN to C24Al12N12

Density functional theory calculations (B3LYP/6-311G*) are applied to devise a series of AlN-substituted C₆₀ fullerenes, avoiding weak homonuclear Al–Al and N–N bonds. The substitutional structures, energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, ionization potentials, binding energies, as well as dipole moments have been systematically investigated. The band gap (HOMO–LUMO gap) is larger for all the AlN-substituted fullerenes than C₆₀. The properties of heterofullerenes, especially, the HOMO–LUMO strongly depend on the number of AlN units. Natural charge analyses indicate that doping of fullerene with AlN units exerts electronic environment diversity to the cage. High charge transfer on the surfaces of our heterofullerenes provokes more studies on their possible application for hydrogen storage.     

8-羟基喹啉铍及其衍生物电子光谱性质的含时密度泛函理论研究

运用密度泛函理论(DFT)B3LYP方法和abinitioHF单激发组态相互作用(CIS)法分别优化了有机金属配合物8-羟基喹啉铍(BeQ₂)及其3种衍生物分子的基态及最低激发单重态几何结构.系统分析了分子结构、前线分子轨道特征和能级分布规律以探索电子跃迁机理.应用含时密度泛函理论(TD-DFT)计算分子的电子光谱,揭示了BeQ₂及其衍生物的发光源于配体中π→π^*电子跃迁,指出通过配体修饰可以有效地影响配合物前线分子轨道分布,调整发光波段,并有效提高电荷转移量.