Ab initio quantum-mechanical calculations on trifluoromethylamine

Extensive ab initio molecular-orbital calculations were carried out on trifluoromethylamine (TFM) to elucidate changes in geometry and electronic structure upon fluorination. The calculations show that the decomposition of CF₃NH₂ is slightly endoenergetic, and the heats of atomization of CF₃NH₂ and CH₃NH₂ show decreased stability of the species upon fluorination. Characteristic of CF₃NH₂ is a highly polar, strong, short CN bond. More limited calculations were carried out on CF₃OH and CH₃OH, and the electronic structure of CF₃OH is found to be generally similar to that of CF₃NH₂. The reduced basicity of the fluorinated amine cannot be ascribed to the inductive effect; the enhanced acidity of the fluorinated alcohol reflects the weakening of the OH bond. No evidence leads to a confirmation of the existence of nitrogen–fluorine hyperconjugation in the fluorinated amine.     

Ab initio studies of small AlmFen clusters

The equilibrium geometrical structures of small Al_mFe_n clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies, the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and experiments is reasonable. The ground stable geometrical structures of Fe₄, FeAl₃, Fe₃Al and Fe₂Al₂ clusters favor three-dimension configurations, but Al₄ tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is different from that of bulk alloys.     

Ab initio calculations of silicon-halogen-silicon double bridges

Summary Structure and stability of molecular clusters modelling halogen (F, Cl) double bridges between silicon atoms, H₃SiF₂SiH₃ (1), H₃SiF₂SiF₃ (2), H₃SiCl₂SiH₃ (3), and H₃SiCl₂ SiCl₃ (4), have been investigated by an ab initio pseudopotential method. Asymmetrical bridges Si-X...Si with one strong Si-X bond and one weak Si...X bonding interaction (X=F, Cl) result from the geometry optimization using the LP-31 G basis set. Dissociation energy calculations using the MP2/LP-31G*//LP-31G procedure and considering the basis set superposition error provide a decrease of stability of the structures in the order2>4>3>1. The results are discussed with respect to formation and decomposition of halogenated reaction overlayers formed during the etching of silicon by halogen atoms.

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Ab-Initio-Berechnungen von Silizium-Halogen-Silizium-Doppelbrücken

Zusammenfassung Struktur und Stabilität von molekularen Clustern, die Halogen(F, Cl)-Doppelbrücken zwischen Siliziumatomen modellieren, H₃SiF₂SiH₃ (1), H₃SiF₂SiF₃ (2), H₃SiCl₂SiH₃ (3) und H₃SiCl₂SiCl₃ (4), werden mittels eines Ab-Initio-Pseudopotentialverfahrens untersucht. Bei der Geometrieoptimierung unter Verwendung des LP-31 G-Basissatzes ergeben sich asymmetrische Brücken Si-X...Si mit einer starken Si-X-Bindung und einer schwachen bindenden Si...X-Wechselwirkung (X=F, Cl). Dissoziationsenergieberechnungen durch das MP2/LP-31 G*//LP-31 G-Verfahren unter Berücksichtigung des Basissatzüberlagerungsfehlers liefert eine abnehmende Stabilität der Cluster in der Reihenfolge2>4>3>1. Die Resultate werden im Zusammenhang mit der Bildung und dem Zerfall von halogenierten Reaktionsschichen, welche während des reaktiven Ätzens von Silizium mit Halogenatomen gebildet werden, diskutiert.

     

Ab initio studies on calicene

Optimum geometries of planar and 90°-twisted C_2v calicene are calculated with single-configuration STO-3G and 3-21-G wavefunctions. The barrier to ring–ring rotation is computed. Bond alternation is pronounced in the planar form and decreases in the twisted form, while dipolar character increases on twisting.     

Ab initio study of magnetic interactions of manganese-oxide clusters

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II)₄O₄ model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn₄O₄(OAc)₆ and Mn₃CaO₄(OAc)₆ using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV)₄O₄(OAc)₆ although this model cluster consists of superexchange type of units, Mn(IV)₂O₂ that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O-O bond formation in the OEC.     

An approximate ab initio SCFMO method

An approximate SCFMO method of calculation is presented which includes several features which are new to this type of approach. A minimal basis set of Slater type orbitals is used, and all integrals are included, but are evaluated approximately, using a series of analytical formulae which have been chosen to reproduce the rigorous values of the integrals. Results are given for the ground states of the H₂O, CH₄, BH₃, NH₃ and BeH₂ molecules, and are compared with previously calculated results.

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Zusammenfassung Eine Näherungs-SCF-MO-Rechnung mit einigen neuen Abänderungen wird vorgeschlagen. Eine minimale Basis von Slater-Orbitalen wird benutzt; es werden alle Integrale berücksichtigt, jedoch nur näherungsweise berechnet, wobei eine Reihe von analytischen Formeln benutzt wird, welche die exakten Werte der Integrale möglichst reproduzieren sollen. Ergebnisse für den Grundzustand von H₂O, CH₄, BH₃, NH₃ und BeH₂ werden mit früher berechneten Ergebnissen verglichen.

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Résumé Présentation d'une méthode SCF MO approchée comportant certaines caractéristiques nouvelles pour une telle méthode. On emploie une base minimale d'orbitales de Slater en ne négligeant aucune intégrale, mais en évaluant chaque intégrale d'une manière approchée au moyen d'une série de formules analytiques calibrées de manière à reproduire les valeurs exactes des intégrales. Les résultats obtenus pour les états fondamentaux de H₂O, CH₄, BH₃, NH₃ et BeH₂ sont comparés à ceux obtenus auparavant.

     

Ab initio theoretical studies of a novel tungsten dihydrogen complex

Theoretical studies of H₂ bound to W(CO)₃(PH₃)₂ predict a stable η² complex with equal WH bonds (2.15 A) and a slightly lengthened (0.79 A) HH bond. The results are compared with recent experimental studies of the related W(CO)₃(P-i-Pr₃)₂(H₂) complex, and the nature of the bonding is discussed.     

Ab initio Studies on polymers

Ab initio crystal orbital calculations have been performed on the infinite all-trans polyene. A structure with r _c=c= 1.346 Å, r _c-c = 1.446 Å, r _c-h = 1.08 Å, and CCC = 125.3 ° was found to be most stable. The most important force constants, the band structure and the density of states were determined as well.     

An ab initio molecular orbital study of the deprotonation of amines

The geometries of the amines NH₂X and amido anions NHX^?, where X = H, CH₃, NH₂, OH, F, C₂H, CHO, and CN have been optimized using ab initio molecular orbital calculations with a 4-31G basis set. The profiles to rotation about the N? X bonds in CH₃NH^?, NH₂NH^?, and HONH^? are very similar to those for the isoprotic and isoelectronic neutral compounds CH₃OH, NH₂OH, and HOOH. The amines with unsaturated bonds adjacent to the nitrogen atoms undergo considerable skeletal rearrangement on deprotonation such that most of the negative charge of the anion is on the substituent. The computed order of acidity for the amines NH₂X is X = CN > HCO > F ≈ C₂H > OH > NH₂ > CH₃ > H and for the reaction NHX^? + H⁺ → NH₂X the computed energies vary over the range 373–438 kcal/mol.     

Anab initio molecular orbital study of substituted carbonyl compounds

Ab initio SCF calculations with a minimal STO-3G basis set have been performed to determine the equilibrium geometries of two series of carbonyl compounds, RCHO and R₂ CO. For the mono-substituted compounds, R may be CH₃, NH₂, OH, F, CHO, and C₂ H₃. In the disubstituted compounds, R has been restricted to the isoelectronic saturated groups. The computed equilibrium geometries of these compounds are in satisfactory agreement with the experimentally-determined geometries, with an average difference of 0.021 Å between computed and experimental bond lengths, and 1.9 ° in corresponding bond angles. An analysis of the effect of the substituent on the electronic structure of the carbonyl group has also been made. The saturated groups are found to be electron-withdrawing groups relative to H, with the electron withdrawing ability increasing in the order CH₃ 2 2H₂ are also electron withdrawing relative to H, and comparable to CH₃ in acetaldehyde. Vertical ionization potentials andn* transition energies have also been calculated for these molecules at their optimized geometries, experimental geometries, and geometries given by a standard model. The effect of changes in molecular geometry on these computed properties has been analyzed.     

1,2,4,6-Cycloheptatetraenes racemizations: substituent effects via ab initio

Racemizations of three series of non-planar 1-, 4- and 5-X-C₇H₅ cyclic conjugated allenes (1_x, 2_x, and 3_x to 1′_x, 2′_x and 3′_x, respectively) are investigated using ab initio HF, B3LYB and MP2 methods with a 6-31G* basis set (X=H, F, Cl, Br, I, OH, NH₂, CN and NO₂). These three topologically different series of allenes are found to go through their corresponding planar 2-, 3- and 4-X-C₇H₅ singlet carbenic transition states, 1_x–TS, 2_x–TS and 3_x–TS (Series 1: 1_x1_x–TS1′_x; Series 2: 2_x2_x–TS2′_x; Series 3: 3_x3_x–TS3′_x).Frequency calculations show negative force constants for all planar and singlet carbeneic transition states: 1_x–TS, 2_x–TS and 3_x–TS.Based on HF/6-31G*, B3LYP/6-31G* and MP2/6-31g* calculations, the order of substituent effects on energy barrier (ΔG^#) of racemization, increase in the order of: NO₂>CN>H>I>ClBr>OH>F>NH₂ for all three Series 1–3. The height of the energy barrier related to each substituent (X) is highest in Series 3 and lowest in Series 1.Hammet free energy relationships (ρ) found via B3LYP, for Series 1, 2 and 3, are −5.8, −3.4 and −4.1, respectively. Rather similar ρ values are found using HF (−4.3, −4.4 and −4.3). These data indicate that electron-releasing groups highly increase the rate of enantiomeric interconversions.Linearity of the free energy relationships is found to be a function of steric effects. Hence, higher correlation coefficients (R) are found for Hammet free energy relationships for Series 3. This is followed by Series 2, which in turn has a more linear free energy relationship than Series 1 (Series 3>Series 2>Series 1).     

Ab initio potential energy surface for Ar 3 +

The potential energy surface (PES) of linear Ar ₃ ⁺ is calculated at the MP4/6-31G* level including all single, double, triple and quadruple excitations. The results show that the PES of the linear Ar ₃ ⁺ has a very flat valley along the asymmetric stretching vibration normal mode, ν₃. A higher level quadratic configuration interaction calculation including single, double and triple substitutions QCISD (T) along this flat valley suggests that an asymmetric geometry energy minimum reported earlier based on MP2 [1] is due to symmetry breaking in UHF. The global minimum of the PES is found to be for the symmetric geometry atR _ab =R _bc =2.66±0.01 Å, which is in good agreement with the MRD-CI calculation [2] and expectations from our earlier photodissociation experiments [3]. The calculational results are compared with other theoretical calculations, and are discussed in the context of the photodissociation and dynamics of dissociation experiments conducted on Ar ₃ ⁺ .     

AB initio calculations and matrix ftir studies of the sulfur trioxide radical anion

The structures and vibrational frequencies of SO⁻₃ (C_3v) and SO⁻₂ (C_2v) have been calculated at the UHF SCF/3-21 + G¹ level. By cocondensation of Cs atoms and SO₃ in an Ar matrix the FTIR spectrum of Cs₄SO₃ has been measured. The molecule is proposed to have C_s symmetry with SO₃ binding to Cs in a bidentate fashion.     

Ab initio study of hydrazoic acid

SCF and CI calculations were carried out on the ground^1A state of HN₃. The equilibrium geometry and vibration frequencies were computed. The results point to a planar structure (groupC _s) but to a non-linear (170 °) N-N-N conformation. The calculated vibration frequencies are in fair agreement with experimental assignments.The dissociation path of the molecule to NH and N₂ products was investigated and compared to the isoelectronic reaction of diazomethane. The dissociation energy of hydrazoic acid is estimated to be about –8 kcal/mole, with a potential barrier to dissociation of about 30 kcal/mole.Boursier IRSIA     

Anab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines

Summary The equilibrium geometry and barrier to pyramidal inversion of triisopropylamine, N(CH(CH₃)₂)₃, is computed at SCF level of theory. For comparison, results for ammonia NH₃ (including a near HF calculation), trimethylamine N(CH₃)₃ and the three analogous phosphine compounds PH₃, P(CH₃)₃ and P(CH(CH₃)₂)₃ are presented as well.     

Ab initio study of low-lying electronic states of the FNO2 molecule

Ab initio electronic structure calculations are reported for low-lying electronic states, ¹A₁, ¹A₂, ³A₂, ¹B₁, ³B₁, ¹B₂, and ³B₂ of the FNO₂ molecule. Geometric parameters for the ground state ¹A₁ are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.     

Anomeric effect in phosphate moieties of carbohydrates: An Ab initio study

Ab initio SCF MO calculations at the STO -3G level have been carried out on CH₂OHOPO₃^?H and CH₂OHOPO₃^2?, which have been considered as model systems for the Cl-phosphate moiety of sugars. The results predict higher anomeric energy for the phosphate moiety at Cl atom of pyranosides. Also a trans arrangement of the exocyclic O—P bond is preferred rather than a gauche arrangement, thus exhibiting a reverse exo-anomeric effect. A complete potential energy map has been constructed for CH₃OPO₃^?H, a model system for C6-phosphate moiety. It is seen that the bond angle optimization brings down the relative energies of various conformations. The effect of the phosphate group on the preferred conformation of phosphate containing polysaccharides is also discussed.     

Ab initio study of a purine nucleoside: Adenosine

The ab initio SCF LCAO-MO method is used to compute the main electronic properties of a purine nucleoside, adenosine, in two specific conformational arrangements (3′-endo conformation of the ribose, gt orientation of the extracyclic CH₂OH group, anti orientation of the base with respect to the sugar and 3′-endo conformation of the ribose, gg orientation of the extra-cyclic CH₂OH group, syn orientation of the base with respect to the sugar). The results are compared with those performed for the isolated component fragments, adenine and 3′-endo riboses.     

Ab initio calculation on the H3 activated complex

A full configuration interaction treatment has been carried out for linear, symmetrical H₃ activated complex, with a limited basis set of Slater orbitals. A similar calculation is performed on H₂ in order to obtain an estimate of the activation energy of the reaction H+H₂=H₂+H. The variation of nonlinear parameters in the basis set is studied and the different behaviour of the H₃ and H₂ resulting wave functions examined. — A larger basis set is needed.

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Zusammenfassung Eine vollstÄndige CI-Behandlung wurde für einen linearen, symmetrischen, aktivierten H₃-Komplex mit einem begrenzten Basissatz von Slaterorbitalen durchgeführt. Eine Ähnliche Rechnung wird für H₂ ausgeführt, um eine AbschÄtzung der Aktivierungsenergie der Reaktion H+H₂=H₂+H zu erhalten. Die Variation von nichtlinearen Parametern im Basissatz wird untersucht und das verschiedenartige Verhalten der resultierenden H₃ und H₂ Wellenfunktion überprüft. — Ein grö\erer Basissatz wird benötigt!

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Résumé Interaction de configurations totale pour le complexe activé H₃ linéaire et symétrique, dans une base limitée d'orbitales de Slater. Un calcul analogue a été effectué sur H₂ afin d'évaluer l'énergie d'activation de la réaction H+H₂=H₂+H. La variation des paramétres non linéaires de la base est étudiée, et l'on examine le comportement différent des fonctions d'onde de H₃ et de H₂. Une base plus étendue serait nécessaire.

     

Ab initio MRD-CI study of the rydberg states of methylene

Potential curves have been calculated for the low-lying Rydberg states of CH₂ as well as for a number of its valence-shell species by employing the ab initio MRD-CI method. The first Rydberg transition is found to occur with a vertical energy of 6.38 eV (1b₁ → 3s), but the corresponding upper state is believed to be strongly predissociated since it correlates directly with the CH(²II) + H(²S_g) ground state fragments at lower energy. The assignment of the first observed Rydberg transition at 8.757 eV by Herzberg as 1b₁ → 3dπ is confirmed almost quantitatively in the calculations, while the corresponding minimum 1P value is computed to be 10.21 eV compared to the experimental result of 10.3 ± 0.1 eV. The dissociation energy of methylene in its ground state is calculated to be 4.47 eV, and this result also fits in well with experimental evidence, which determines a lower limit for this quantity of D₀ > 4.23 eV. Finally, it is found that none of the Rydberg states nor any of the higher-lying valence-shell species of methylene are of sufficiently low energy to play a significant role in the experimental determination of the ¹A₁-³B₁ splitting of this system.