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.     

Application of the group-theoretical formulation of the method of diatomics in molecules to the investigation of alkali-metal clusters

The effectiveness and reliability of the method of diatomics in molecules together with its previously proposed group-theoretical formulation has been demonstrated. The calculations have been extended for the first time to cases of hepta- and octaatomic clusters of alkali metals, making it possible to trace the laws governing the transition from small clusters to the solid metal.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 89–94, January–February, 1985.     

Novel method for the investigation of single nanoparticle reactivity

The reactivity of planar surfaces has been well investigated but there are many new unexplored aspects involved in the reactivity of nanoparticle surfaces. Most investigative methods only measure the average properties of the particles present on the surface. In order to investigate the local reactivity of nanoparticles, the STM tip electrode can be used as a local sensor. The feasibility of this approach is demonstrated by detecting some of the hydrogen (through the hydrogen oxidation reaction current at the tip) which is evolved at a single Pd particle on the Au(111) substrate at a constant distance from the tip. In principle, it is thereby possible to determine the reactivities of surfaces on a nanometre scale. To avoid interference from different reactive particles it is essential to use electrodes with a very low particle density. Hence, in this investigation electrodes with single Pd particles which were deposited from the STM tip onto the substrate are used. Results of potential-dependent measurements at single metal nanoparticles will be shown and the possibilities and limitations of the approach will also be discussed.     

三聚氰胺对金属(Ⅱ)卟啉反应活性影响的概念密度泛函研究

选取8个典型的二价金属卟啉MP(M=Ca,Mg,Zn,Cu,Ni,Fe,Co,Mn)与三聚氰胺(L)形成轴向金属配合物(L-MP),应用概念密度泛函工具,系统地计算和比较了L键合前后对其结构和反应性质的影响.结果表明:除钙的特别不稳定物外,L配体对其余6种MP的结构影响较小,它们有较高的化学势指数和较低的总化学硬度而趋向配体的解离;与铁卟啉能形成最稳定的轴向配合物,电子由配体N原子流向铁,中心铁的亲核Fukui指数值大于体系里其他原子的Fukui指数,且发生符号改变.在这些典型的赤道键合配合物中,金属M、配体N之间的二级微扰相互作用能,自然电荷布局以及概念密度泛函指数等方面,存在着一系列线性关系.以上结果可为体内三聚氰胺致结石提供新的启示.     

Excited state properties of sizable molecules in solution: from structure to reactivity

We review some recent advances in quantum mechanical methods devised specifically for the study of excited electronic state of large size molecules in solution. The adopted theoretical/computational framework is rooted in the density functional theory (DFT) and its time-dependent extension (TD-DFT) for the characterization of ground and excited states, in the polarizable continuum model (PCM) for the treatment of bulk solvent effects, and in time-dependent quantum mechanical methods for chemical dynamics. Selected applications to the simulation of absorption spectra, to the interpretation of time-resolved experiments, and to the computation of dissociative electron transfer rates are presented and discussed.     

Ground states of molecules. 56. MNDO calculations for molecules containing sulfur

MNDO has been extended to sulfur, but without inclusion of 3d AO s. Calculations are reported for heats of formation, geometries, dipole moments, and ionization energies of a variety of sulfur-containing molecules. The average discrepancy between calculated and observed heats of formation is larger than for compounds of other elements, a difference probably due, at least partly, to the lower accuracy of the thermochemical data for sulfur compounds. The calculated dipole moments agree well with experiment as do the calculated ionization energies, except for those corresponding to ionization from sulfur “lone-pair” orbitals which are too high by ca. 1 eV, probably as a result of the neglect in NDDO of interactions between inner and valence shell orbitals. As in the case of other third-period elements, the calculated heats of formation of compounds of sulfur in its higher valence states (S^IV, S^VI) were too positive by large amounts, due presumably to the neglect of 3d AO s.     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

Quantum chemical investigation of the electron structure,physical properties and the reactivity of π-electron molecules in excited singlet and triplet states

Investigations by the method of limited configuration interaction (ASMO-CI) upon some classes of nonalternant hydrocarbons show that the excitation of the molecules to the fluorescent and to some higher singlet and triplet states is accompanied with considerable intramolecular charge transfer leading to recharge of the cycles with an odd number of carbon atoms. It is shown that the change of the dipole moments and of some values ¦L¦ characterizing the electron structure in transition from ground to fluorescent state ¦L ^V¦, respectively from ground to phosphorescent state ¦L^T¦, satisfy the inequalities: ¦L ^V¦ > ¦L ^T¦. It follows that the fluorescent state differs more from the ground state in its electron structure and physical properties than does the phosphorescent state.

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Zusammenfassung Die mit der Methode der begrenzten Konfigurationenwechselwirkung (ASMO-CI) für einige Klassen nichtalternierender Kohlenwasserstoffe durchgeführten Untersuchungen zeigen, daß die Anregung der Moleküle in den Fluoreszenz- sowie in einigen Singulett- und Triplettzuständen mit einer beträchtlichen intramolekularen Ladungsübertragung verbunden ist, die zur Umladung der Ringe mit ungerader Anzahl von Kohlenstoffatomen führt. Es wird gezeigt, daß die Veränderung der Dipolmomente und einiger die Elektronenstruktur kennzeichnenden Größen ¦L¦ beim Übergang vom Grund- zum Fluoreszenzzustand ¦L ^V¦ bzw. vom Grund- zum Phosphoreszenzzustand ¦AL ^V¦ folgende Ungleichungen erfüllen: ¦L ^V¦ > ¦L ^T¦ Daraus folgt, daß der Fluoreszenzzustand seiner Elektronenstruktur und seinen physikalischen Eigenschaften nach sich vom Grundzustand mehr unterscheidet als dies beim Phosphoreszenzzustand der Fall ist.

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Résumé Les calculs, effectués par la méthode de l'interaction de configuration limitée (ASMO-CI) pour quelques catégories d'hydrocarbures nonalternants ont montré que l'excitation des molécules jusqu'à l'état de fluorescence et certains états supérieurs singlets et triplets est liée à un transfert intramoléculaire de charges considérable, qui provoque un changement du signe de la charge des anneaus au nombre impair des atomes de carbone. On a montré que les variations des moments dipolaires et de quelques grandeurs ¦L¦ caractérisant la structure électronique, accompagnant les transitions depuis l'état fondamental aux états respectivement de fluorescence ¦L^V¦ et de phosphorescence ¦L^T¦ doivent satisfaire a l'inégalité: ¦L^V¦ > ¦L^T¦. Cela veut dire que la différence dans la structure électronique et les propriétés physiques entre l'état de fluorescence et l'état fondamental est plus grande que celle entre l'état de phosphorescence et l'état fondamental.

     

Theoretical investigation of the electronic structure of graphite + LiNH2 intercalation compounds by the MNDO method

The electronic structures of the clusters C₄₈H₂₄, C₄₈H₂₄Li, and C₄₈H₂₄LiNH₂, which are models of intercalation compounds of graphite, have been calculated by a semiempirical quantum-chemical method in the cluster approximation. It has been shown that an Li atom is strongly polarized in a graphite lattice. When a molecule of LiNH₂ is inserted into a graphite matrix, the charge states of the intercalant molecule and the surrounding carbon atoms change. The cause of the strong increase in the inter planar distances in the intercalation compound of LiNH₂ is the antibonding interaction of the MO's of graphite and the intercalant.Institute of Chemistry, Academy of Sciences of the USSR, Far-Eastern Branch. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 12–16, March–April, 1991.     

MNDO study of catenated sulfur: The molecules and ions S3 to S8

Semiempirical molecular orbital calculations by the MNDO method are reported for sulfur rings and chains S₃ through S₈, for the corresponding dications, S through S, and for S. The MNDO method seems quite successful in predicting the geometries of neutral catenated sulfur molecules, even the unusual bond-length alternation and extent of coplanarity in cyclo-S₇. In contrast to hydrocarbon rings, for which its prediction of strain is erratic, MNDO is consistent in its calculated strain energies in small cyclosulfur rings; unfortunately all the strain energies are overestimated by 70%. As a consequence of this error, the method must be considered unreliable in its predictions of structures for the dications S, since many of these ions could potentially exist as strained bicyclic systems. In addition, MNDO appears to have difficulty handling long, partial SS σ bonds, as are found to occur in S. It may be for this reason that MNDO predicts, apparently incorrectly, that the open-chain isomers of S are more stable than are any of the cyclic forms, at least for S to S. With respect to neutral Sⁿ molecules, however, the MNDO predictions appear more reliable than ab initio molecular orbital (MO ) calculations using small basis sets without polarization functions and without configuration interaction (CI ). However, MNDO apparently underestimates by about a factor of two the strength of the three-electron π bonds present in the terminal links of sulfur diradical chains.     

MNDO/3 study of the relative reactivity of ethylene derivatives upon addition to the benzyl radical

The transition states for the addition of a benzyl radical to substituted ethylenes CH₂=CHX were determined by the MNDO/3 method, where X=H, CF₃, CN, CH₃, C₄H₉, C(CH₃)₃, CO₂CH₃, and Si(CH₃)₃. The activation energies of the forward and back reactions were determined.A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1676–1679, July, 1992.     

MNDO method for calculations of magnesium clusters

A new set of parameters for the magnesium atom has been developed within the MNDO method. In contrast to previously published parameters, the new parameters correctly describe molecules with different chemical natures: magnesium halides, organomagnesium compounds and the recently found small magnesium clusters Mg _n (n=2-8). The average errors in the calculated heats of formation and bond lengths of magnesium compounds, including clusters are: 10.7 kcal/mol and 0.167 Å, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1384–1388, August, 1994.     

Parameters of MNDO method for Zn atom

Conclusion The results obtained for zinc molecules and complexes show that the MNDO method can be used in investigating the physicochemical properties of Zn compounds. The calculated data show that the use of the system of parameters proposed in the present work will give a considerably better reproduction of the energetics of zinc-containing systems than can be obtained when using the parameters of [6].Novosibirsk Institute of Bioorganic Chemistry, Siberian Branch of the Academy of Sciences of the USSR. Lenin Komsomol Novosibirsk Order-of-the-Red-Banner-of-Labor State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 28, No. 5, pp. 20–24, September–October, 1987.     

Application of the level-set method to the implicit solvation of nonpolar molecules

A level-set method is developed for numerically capturing the equilibrium solute-solvent interface that is defined by the recently proposed variational implicit solvent model [Dzubiella, Swanson, and McCammon, Phys. Rev. Lett. 104, 527 (2006); J. Chem. Phys. 124, 084905 (2006)]. In the level-set method, a possible solute-solvent interface is represented by the zero level set (i.e., the zero level surface) of a level-set function and is eventually evolved into the equilibrium solute-solvent interface. The evolution law is determined by minimization of a solvation free energy functional that couples both the interfacial energy and the van der Waals type solute-solvent interaction energy. The surface evolution is thus an energy minimizing process, and the equilibrium solute-solvent interface is an output of this process. The method is implemented and applied to the solvation of nonpolar molecules such as two xenon atoms, two parallel paraffin plates, helical alkane chains, and a single fullerence C(60). The level-set solutions show good agreement for the solvation energies when compared to available molecular dynamics simulations. In particular, the method captures solvent dewetting (nanobubble formation) and quantitatively describes the interaction in the strongly hydrophobic plate system.     

MNDO and INDO geometrical dependences of the hydrogen diamagnetic shielding tensor of some small molecules

The structural dependence of the hydrogen diamagnetic shielding tensor in H₂, CH₄, NH₄⁺, H₂O and HCl has been calculated using MNDO and INDO semiempirical methods. The influence of inner shells as well as two-center integrals are taken into account. Results are found to be in good agreement with ab initio predictions, and in the HCl molecule, with experimental data.     

Semilocalized approach to investigation of chemical reactivity

Application of the power series for the one‐electron density matrix 36 to the case of two interacting molecules is shown to yield a semilocalized approach to investigate chemical reactivity, which is characterized by the following distinctive features: (1) Electron density (ED) redistributions embracing orbitals of the reaction centers of both molecules and of their neighboring fragments are studied instead of the total intermolecular interaction energy; (2) the ED redistributions are expressed directly in the basis of fragmental orbitals (FOs) without passing to the basis of delocalized molecular orbitals (MOs) of initial molecules; (3) terms describing the ED redistributions due to an intermolecular contact arise as additive corrections to the purely monomolecular terms and thereby may be analyzed independently; (4) local ED redistributions only between orbitals of the reaction centers of both molecules are described by lower‐order terms of the power series, whereas those embracing both the reaction centers and their neighborhoods are represented by higher‐order terms. As opposed to the standard perturbative methods based on invoking the delocalized (canonical) MOs of isolated molecules, the results of the approach suggested are in‐line with the well‐known intuition‐based concepts of the classic chemistry concerning reactivity, namely, with the assumption about different roles of the reaction center and of its neighborhood in a chemical process, with the expectation about extinction of the indirect influence of a certain fragment (substituent) when its distance from the reaction center grows, etc. Such a parallelism yields quantum chemical analogs for the classic concepts and thereby gives an additional insight into their nature. The scope of validity of these concepts also is discussed. Applicability of the approach suggested to specific chemical problems is illustrated by a brief consideration of the S_N2 and Ad_E2 reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 302–316, 2003     

Application of the computationally efficient self-consistent-charge density-functional tight-binding method to magnesium-containing molecules

The geometric properties, ionization potentials, heats of formation, incremental binding energies, and protonation energies for up to 75 magnesium-containing compounds have been studied using the self-consistent-charge density-functional tight-binding method (SCC-DFTB), the complete-basis set (CBS-QB3) method, traditional B3LYP density-functional theory, and a number of modern semiempirical methods such as Austin Model 1 (AM1), modified neglect of diatomic overlap without and with inclusion of d functions (MNDO, MNDO/d), and the Parametric Method 3 (PM3) and its modification (PM5). The test set contains some widely varying chemical motifs including ionic or covalent, closed-shell or radical compounds, and many biologically relevant complexes. Geometric data are compared to experiment, if available, and otherwise to previous high-level ab initio calculations or the present B3LYP results. SCC-DFTB is found to predict bond lengths to high accuracy, with the root-mean-square (RMS) error being less than half that found for the other semiempirical methods. However, SCC-DFTB performs very poorly for absolute heats of formation, giving an RMS error of 29 kcal mol(-1), but for this property B3LYP and the other semiempirical methods also yield poor but useful results with errors of 12-22 kcal mol(-1). Nevertheless, SCC-DFTB does provide useful results for biologically relevant chemical-process energies such as protonation energies (RMS error 10 kcal mol(-1), with the range 6-19 kcal mol(-1) found for the other semiempirical methods) and ligation energies (RMS error 9 kcal mol(-1), less than the errors of 12-23 kcal mol(-1) found for the other semiempirical methods). SCC-DFTB is shown to provide a computationally expedient means of calculating properties of magnesium compounds, providing results with at most double the inaccuracy of the high-quality but dramatically more-expensive B3LYP method.     

Chromatoscopy — The new method of investigation of the structure of molecules

Summary The paper illustrates the two possibilities of physicochemical measurements: prediction of separation on the basis of the study of intermolecular interactions between a sample component and the adsorbents, and investigation of the structure of molecules, based on gas chromatographic measurements. A new name, chromatoscopy, is suggested for the latter technique. Based on recent studies, examples are given for the new method.