Ab Initio Self-consistent Field Research Articles

An ab initio study of some nitramine molecules

Ab initio self-consistent field (SCF) molecular orbital calculations have been performed for a series of nitramine molecules. Calculations were conducted on trinitramine, methyldinitramine, methylchloronitramine and trimethylamine using Pople's 6-3IG and 6-3IG ∗ basis sets. Gas phase geometries for these molecules were determined by a minimum energy criterion using gradient techniques. The results are in qualitative agreement with the geometries derived from electron diffraction studies and microwave spectroscopy. The extra-long NN bonds in methyldinitramine and methylchloronitramine were not computed accurately at the SCF level of theory and it was shown that these bonds become longer if electron correlation is included at the second-order Moller-Plesset level. Computations were performed for the heat of formation for trinitramine using Pople's isodesmic reaction concept and compared with a value obtained using an additivity rule.

An ab initio study of H abstraction in halogen-substituted methanes by the -OH radical

The H-abstraction reaction by the OH radical from methane and various halomethanes (CH 3Cl, CH 3F, CH 2Cl 2, CH 2ClF, CH 2F 2, CHCl 2F and CHF 3) was studied using ab initio unrestricted Hartree-Fock (UHF) and multiconfigurational self-consistent field (MCSCF) computational methods. It was found that the structure of the transition state determined at the UHF and MCSCF levels is very similar and that the topology of these reaction surfaces is satisfactorily described using a UHF wavefunction. It is pointed out that to obtain values of the activation energies in good agreement with experiment it is essential to take into account the correlation energy contribution on reactants and transition states. The MP2/6-31G ∗//3-21 G ∗ approach (6–31 G ∗ single-point second-order Møller-Plesset (MP2) computations on the 3-21G s* optimized geometries) is proposed as the minimal computational level needed to obtain a description of these reactions in reasonable agreement with the experimental evidence.

Optimization techniques in energy calculations involving the Hartree-Fock density matrix

Three optimization procedures are examined for their utilization in determining the optimum density matrix for a single determinant wave function. The total energy of a molecular system is written as a function of the density matrix and then optimized subject to the constraints of idempotency and total electron population. This direct calculation of the density matrix (DCDM) method was studied in an attempt to have a formalism which would avoid convergence problems associated with the self-consistent field (SCF) cycle, and which would be applicable to large molecular systems. The optimization procedures studied were the Powell algorithm, Gauss-Jordan reduction, and dynamic programming. Computational factors studied include convergence criteria, stepsize, and weight factors for constraint equation penalty functions. The application considered is for HF. An ab-initio SCF method was used to obtain initial values for the density matrix, and its SCF results were compared to corresponding DCDM predictions. Approximations of the quadratic two-electron energy contributions will be necessary to apply dynamic programming, but this appears to be the method most applicable to large molecular systems if an acceptable approximation can be found. Gauss-Jordan reduction is an applicable technique, but probably not for large molecular systems. BOTM appears to be the method most applicable without introducing approximations, but weight factors for the penalty functions will have to be more efficiently determined as Lagrange multipliers, and this addition would result in a technique probably not applicable to large molecular systems.

The energy band structures of acetylene-methylacetylene copolymers: the influence of chemical substitution

Ab initio self-consistent field linear combination of atomic orbitals (SCF LCAO) crystal orbital band structure calculations for acetylene-methylacetylene copolymers of different acetylene contents (33.3mol.% and 66.7mol.% respectively) in polymers and the Li-doped systems are reported. The copolymers are derived from trans-polyacetylene by replacing some of the hydrogen atoms with methyl groups. The results are compared with those of polyacetylene (pure and Li doped), and the influence of methyl groups on the band structures is investigated. The charge distributions of the systems on the basis of Mulliken population analysis are discussed. The results agree well with experiment.

Electronic effects of peripheral substituents in porphyrins: x-ray photoelectron spectroscopy and ab initio self-consistent field calculations

The electronic effects of a wide variety of peripheral substituents in free base prophyrins have been evaluated using X-ray photoelectron spectroscopy (XPS) and all-electron ab initio calculations. Both methods have identified the 1s energies of the central nitrogens as excellent sensors of the electronic effects of peripheral substituents. Core level photoelectron spectra are reported fornine porphyrins, including unsubtituted prophyrin, octaethylporphyrin, and seven tetraphenylporphyrins

Ab initio study of the structure, cooperativity, and vibrational properties of the H2O: (HF)2 hydrogen bonded complex

Ab initio self-consistent field (SCF) and MP2 calculations have been performed in order to investigate the structure, energetics, and spectroscopic properties of H2O: (HF)2. Two conformations of the 1:2 complex are energetically possible. On the one hand is a cyclic complex belonging to the C1 point group and which corresponds to the absolute minimum, and on the other hand is a bifurcated complex with C2v symmetry. In this latter complex the two hydrogen bonds are equivalent whereas in the former there are two strong H-bonds and a weaker one. The energy difference between these two conformers is ∼3 kcal mol−1. From the spectroscopical point of view the bifurcated complex is characterized by νHF frequency shifts smaller than that calculated for the dimer, in the case of the cyclic complex the two different νHF red shifts closely related to the H2O–HF and (HF)2 calculated shifts, respectively, and correspond to those experimentally observed. The cooperative effects on interaction energies and spectroscopic properties have been estimated and analyzed on the basis of the projected induced dipole moment. It is shown that the cyclic complex is favored with respect to both H2O–HF and (HF)2 precursors while the formation of the bifurcated one remains more problematic.

The remarkable enneahydridorhenate dianion: ReH2-9

All-electron, non-relativistic ab initio self-consistent field (SCF) quantum mechanical methods were applied to the unique enneahydridorhenate dianion (ReH2- 9, D 3h ). Three basis sets were constructed and used for rhenium: the (20s14p11d7f/17s11p8d2f) basis is loosely designated DZ; the (23s17p14d9f/18s14p10d3f) is similarly designated TZ; and TZ plus polarization plus diffuse is designated TZP +. The largest basis set, TZP +, has 24s20p15d10f primitive Gaussian functions contracted to 19s17p11d4f. Four basis sets were used for H: the (5s/3s) is designated TZ; (5s2p/3s2p) is designated TZ2P; (6s2p/4s2p) is designated QZ2P; and QZ2P plus one diffuse s function is designated QZ2P +. The well-known experimental D 3h structure was investigated, with the three independent internal coordinates optimized using analytic gradient techniques. The larger basis sets were handled using the direct SCF approach. The largest basis set for which the geometry was optimized was TZP + for Re and QZ2P + for H (with 252 basi...

Intramolecular hydrogen bonding in ribonucleoside sugar hydroxyls. An abinitio study

Abinitio self-consistent field (SCF) calculations were done for a model of a ribonucleoside where only the ring part of the furanoside form of D-ribose is considered. Full geometry optimization and direct location of stationary points were used in order to characterize the geometry of the minimum energy structure and of the transition state for the proton transfer reaction in the anion. Topological analysis of the charge density and its Laplacian for the minimum energy structure allow us to find clear electronic and geometrical evidence of an intramolecular hydrogen bond. On the other hand, our results show a very low energy barrier for the proton transfer so that the proton jumps easily between the two oxygen atoms.

A computational investigation of the effects of substituents on the CP group

Recently, there has been a rapid development in the preparation of multiple bond systems with heavy main group elements phosphorus and carbon. Ab initio self-consistent field (SCF) molecular orbital calculations were performed at the 4-31G ∗ level to determine the effect of substituent groups -F, -CF 3, H-CC-, CH 2CH-, and phenyl, on the CP triple bond. A characteristic feature of phospha-alkynes is the weak sensitivity of the CP bond length to the substituent at the carbon atom. The molecular electrostatic potential analysis clearly reflects the strengths of the electronegativities of these substituent groups and their ability to attract the polarizable charge from the CP triple bond. The CP group deactivates the phenyl ring towards electrophilic attack.

A computational investigation of phosphaalkynes: a new building block

The electronic properties of CP, CP and C-P bonds and their aza analogues, CN, CN and C-N bonds, were investigated using an ab initio self-consistent field molecular orbital approach at the 4-31G ∗ level. The cyclodimerization and cyclotetramerization of the monomer, methylphosphaacetylene (CH 3-CP) were analyzed by the molecular electrostatic potential (MEP) method. Based on the MEP maps, two reactive sites attractive towards an incoming electrophile were identified for phosphaacetylene, but its aza analogue, acetonitrile, showed only a single reactive site. The -CP group, in contrast to the -CN group, tends to form ‘side-on’ bonds with transition metals, and this has been reinforced by MEP maps of phosphaacetylene, indicating a larger negative region near the π-bonds.

A b i n i t i o study of the electronic structure of polymeric quasi-one-dimensional superlattices of furan and thiophene

A b initio band structure results of polyfuran (PFU) and polythiophene (PTP) obtained with a double zeta basis set are reported. The electronic density of states (DOS) of the various quasi-one-dimensional copolymers (superlattices) of the type (AmBn)x (A=furan; B=thiophene) have been calculated numerically within the ab initio self-consistent field (SCF) tight binding approximation. These copolymers on the basis of the band positions of PFU and PTP are found to belong to the class of type I superlattices. The trends in their electronic structure and conduction properties as a function of (i) composition (m/n); (ii) block sizes m and n; and (iii) arrangement of the blocks in the copolymer chain are discussed.

Structural studies of bitetrahedryl molecule C 8H 6, coupled tricyclo[3.1.0.0]hexyl molecule C 12H 14, and coupled bicyclo[1.1.0]butane derivatives

The equilibrium geometries of the bitetrahedryl molecule C 8H 6, the coupled tricyclo[3.1.0.0]hexyl molecule C 12H 14, the 1,1′-bi(tricyclo[3.1.0.0]hexane-6,6′-dicarboxylic acid dimethyl ester), and the 1,1′-bi(tricyclo[4.1.0.0]heptane-7,7′-dicarboxylic acid dimethyl ester) have been optimized by the AM1 semi-empirical quantum-mechanical method. As suggested by Ermer, Bell, Schäfer and Szeimies, the central CC bond distance for the coupled tricyclo[3.1.0.0]hexyl molecule is predicted to be longer than that for the bitetrahedryl molecule. Our AM1 calculations predict the central CC distance in the bitetrahedryl molecule to be 0.023 Å less than in the coupled tricyclo[3.1.0.0]hexyl molecule, which is in good agreement with ab initio self-consistent field (SCF) calculations. However, AM1 calculations predict that the shortest CC bond in tricyclo[3.1.0.0]hexyl molecule is the central one, while ab initio SCF calculations predict the adjacent CC bond to be the shortest. X-ray crystal structures of the esters indicate that the shortest CC bond is the central one. AM1 calculations on the esters also predict that the central CC bond is the shortest such bond ever found.

Structure of the transition state for hydrogen molecule elimination from 1,4-cyclohexadiene

Reactant, product, and transition-state regions of the potential energy surface for the dissociation of 1,4-cyclohexadiene (1,4-CHDN) to benzene and H{sub 2} have been studied by using ab initio multiconfiguration self-consistent field (MCSCF) wave functions with STO-3G, 6-31G, and 6-31G** basis sets. At all computed levels of theory, the carbon framework of the reactant 1,4-CHDN is planar, and the transition state has a C-symmetry boat-like conformation. The transition state is fairly tight and is centrally located; the C-H and H-H distances are each extended by about 35-40% from their respective equilibrium values in 1,4-CHDN and H{sub 2}. At the best level of theory, the computed values for the activation barrier and reaction exothermicity for H{sub 2} elimination are 60.1 and 9.3 kcal/mol, respectively, compared to an experimental activation energy of 43.8 and an exothermicity of 6.5 kcal/mol. 28 refs., 3 fig., 5 tab.

Stability and energetics of metastable molecules: tetraazatetrahedrane (N4), hexaazabenzene (N6), and octaazacubane (N8)

By use of ab initio self-consistent field (SCF), coupled-cluster, and many-body perturbation theory (MBPT) methods, the potential nitrogen molecules, tetrahedral N 4 and octahedral N 8 , are found to be metastable, corresponding to local minima on their respective potential energy surfaces. Barriers to unimolecular dissociation appear to be large enough that the unknown molecules could be formed. Three additional N, structures (C s , C 2v , and D 2h ) are also found

From butadiene to polyacetylene: An a b i n i t i o study on the vibrational spectra of polyenes

We report the results of a systematic ab initio self-consistent field (SCF) study on the structure and vibrational spectra of all-trans polyenes ranging from C4H6 to C18H20. We include the limiting case of the infinite all-trans polyacetylene. Trends in the evolution of the vibrational frequencies upon increasing the size of the polyenes and the convergence of edge effects and of bulk properties are monitored. We point out the inherent limitations of any SCF-type approach for the description of the vibrational spectra of polyenes and discuss difficulties encountered in the extrapolation to infinite chain length. On the basis of a suite of calculations on the smaller members of the polyene series trans-1,3-butadiene and all-trans-1,3,5-hexatriene carried out with a high-level electron correlation method, a new scaled force field has been suggested recently by us [M. Kofranek, A. Karpfen, and H. Lischka, Int. J. Quantum Chem. Symp. 24, 721 (1990)] which reproduces most of the experimentally known trends in these oligoenes, in particular, the vibrational frequencies in the region of the carbon–carbon double and single bond stretches. The performance of this scaled force field for the prediction of the vibrational spectra of larger all-trans polyenes is investigated in detail. We also apply it to compute the harmonic phonon dispersion curves of polyacetylene and compare with available experimental data.

A novel interpretation of Hund's rule for two-electron molecular systems

Two approximate equality formulations for the energy-component differences between a triplet and a singlet state of a two-electron molecular system with the same geometry are proposed: i.e. Δ V ee = Δϵ a, Δ E HF = Δϵ b. These equalities concerning the interpretation of Hund's multiplicity rule indicate a novel theoretical background. The validity of these relationships in different basis sets and under different nuclear separations was also investigated. No basis-set or bond-length dependence of these equality formulations were confirmed by the ab initio self-consistent field (SCF) results. The reasons for the above formulations and the possibility of extending them to many-electron systems are discussed.

Applications of the many-body perturbation theory in the localized representation: structural effects in the correlation energy of normal saturated hyd

An ab initio self-consistent field (SCF) method with standard STO-3G and 6-31G ∗ basis sets was used to study some normal saturated hydrocarbons. The occupied and virtual canonical orbitals were localized separately by Boys' procedure and the correlation energy corrections were calculated in the localized representation of the many-body perturbation theory. Introducing the concept of “neighbourhood order” the local and non-local effects can be separated and the correlation energy corrections can be partitioned accordingly. It was found that the correlation energy corrections in these molecules are predominantly of local origin. The contributions to the pair correlation energies are transferable to a good approximation. In analysing the contributions of the localized virtual orbitals, some interesting structural features were identified.

Protonation and hydration of hydroxyketene

The protonation and hydration reactions of hydroxyketene have been examined by ab initio self-consistent field (SCF) calculations at the 3-21G and 6-31G ∗ basis-set levels. This ketene serves as a model compound for phenylhydroxyketene whose hydration has been studied experimentally by laser flash photolysis. Results of the calculations on the protonation reaction reveal that the operative mechanism involves protonation at C β of the ketene in accordance with previous experimental results and theoretical calculations. Calculations on the hydration reaction involving the water dimer predict that the ketene CO bond is hydrated, yielding the enol of α-hydroxyacetic acid as the intermediate product. This is in agreement with the observation of mandelic acid enol (Ph(HO)CC(OH) 2) as an intermediate in the hydration of phenylhydroxyketene. The negative logarithm of the equilibrium constant for the gas phase keto-enol tautomerization, p K E=17.7 at 298 K (3-21G//3-21G), determined for α-hydroxyacetic acid, indicates that the enol content of this acid is 13 times greater than for the parent acetic acid in the gas phase.

Intramolecular hydrogen bonding in 2-aminoethanol, 3-aminopropanol and 4-aminobutanol

Two different intramolecular hydrogen-bonds are formed by 2-aminoethanol, 3-aminopropanol and 4-aminobutanol, both of which lead to cyclic conformations. Results based on ab initio self-consistent field (SCF) calculations for the stronger interaction -N⋯H-O-are presented. The influence of ring size on this interaction is discussed.

Carboxylic acid enols from the hydration of ketenes: a theoretical investigation

Ab initio self-consistent field (SCF) calculations were used to explain the observation of mandelic acid enol and 6,6-fulvenediol as intermediates in the hydration of hydroxyphenylketene and cyclopentadienylidenemethanone, respectively, in acidic and neutral aqueous solutions. The protonation of ketene and the model compounds cyclopropylidenemethanone and cyclopropenyli-denemethanone were investigated at the 3-21G basis-set level. Single-point energy determinations at the 6-31G ∗ level on the 3-21G optimized geometries were also carried out. A mechanism involving oxygen protonation was ruled out for all three ketenes. The reactions of ketene and cyclo-propenylidenemethanone with the water dimer were also investigated at the 3-21G and 6-31G ∗ basis-set levels. For ketene, the addition of a water dimer across CO is favoured by 13.9 kcal mol −1 (6-31G ∗//3-21G), whereas for cyclopropenylidenemethanone addition across CC is favoured by 4.6 kcal mol −1 (6-31G ∗//3-21G). The p K a values 18.8, 28.3 and 30.2 at 298 K (3-21G//3-21G) were calculated for the gas phase keto-enol tautomerizations of acetic acid, 1-cyclopropanecarboxylic acid and 3-cyclopropenecarboxylic acid, respectively. These results indicate that a carboxylic acid enol arises only from water addition across the CO bond of the corresponding ketene precursor, and not via a protonation mechanism.