STO-3G Basis Set Research Articles

A theoretical study on vibrational spectra of C 84 fullerenes: results for C 2, D 2, and D 2d isomers

The C 2, D 2, and D 2d isomers of C 84 fullerene were investigated by ab initio molecular orbital calculations. Optimized geometries, relative stabilities, and vibrational spectra of 11 isomers are determined by Hartree–Fock calculations with STO-3G, 3-21G, and D95V basis sets. For the purpose of an assignment on the C 84 structures, we discussed a way to distinguish a specific isomer by comparing the calculated vibrational spectra.

A theoretical study on the vibrational spectra of bis(C- p-carboranyl)(C 2B 10H 11) 2

Bis(C- p-carboranyl)(C 2B 10H 11) 2 has been investigated using ab initio methods to study the structure and vibrational spectra at Hartree–Fock level with sto-3g and 6-31g basis sets. The results obtained show that bis(C- p-carboranyl)(C 2B 10H 11) 2 has D 5 d symmetry, the optimized bond lengths are in agreement with those observed experimentally, as are the quantified vibrational discrepancies. The assignments of the main vibrational features have been made through the analysis of vibrational normal modes. It reveals that the two icosahedral C- p-carboranyl groups in bis(C- p-carboranyl)(C 2B 10H 11) 2 present rigidity and give rise to a stretching mode like a diatomic molecule, whereas the stretching vibration of the C–C bond which links the two groups together appears weak.

Searching for saddle points of potential energy surfaces by following a reduced gradient

The old coordinate driving procedure to find transition structures . in chemical systems is revisited. The well-known gradient criterion, =E x s 0, . which defines the stationary points of the potential energy surface PES , is reduced by one equation corresponding to one search direction. In this manner, abstract curves can be defined connecting stationary points of the PES. Starting at a given minimum, one follows a well-selected coordinate to reach the saddle of interest. Usually, but not necessarily, this coordinate will be related to the . reaction progress. The method, called reduced gradient following RGF , locally has an explicit analytical definition. We present a predictor)corrector method for tracing such curves. RGF uses the gradient and the Hessian matrix or updates of the latter at every curve point. For the purpose of testing a whole surface, the six-dimensional PES of formaldehyde, H CO, was explored by RGF 2 . using the restricted Hartree)Fock RHF method and the STO-3G basis set. Forty-nine minima and saddle points of different indices were found. At least seven stationary points representing bonded structures were detected in addition to those located using another search algorithm on the same level of theory. Further examples are the localization of the saddle for the HCN | CNH . isomerization used for steplength tests and for the ring closure of azidoazo- methine to 1 H-tetrazole. The results show that following the reduced gradient may represent a serious alternative to other methods used to locate saddle points in quantum chemistry. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1087)1100, 1998

Electron Paramagnetic Resonance and ab Initio Structural Studies on Liquid Crystalline Systems

Electron paramagnetic resonance studies, using PD-Tempone as a probe, on the n-hexyl and n-heptyl members of the p-cyanophenyl p-(n-alkyl)benzoate (RCBz) liquid crystals confirm the previously observed directions of the discontinuities in the a and g-factor values on solidification and on melting of the 6CB liquid crystal. Careful analysis of an unusual variation in the g-factor value on cooling and heating in the vicinity of the nematic-to-isotropic transition points to the existence of molecular change alongside the bulk change. Evidence for such changes is provided by the results of differential scanning calorimetry (DSC) measurements and ab initio calculations using the STO-3G basis set. DSC endothermic peaks on cooling (or exothermic peaks on heating) that are independent of cooling (or heating) rates reflect a conformational change. DSC also shows the existence of polymorphic solid that, on heating, undergoes a series of rate-dependent solid−solid modifications until the most stable solid form is re...

Advantages of the Fourier space RHF band structure approach: Application to polyoxymethylene using a distributed basis set ofs-type Gaussian functions

In this contribution, we outline the Fourier space-restricted Hartree–Fock (FS–RHF) approach to the calculation of the band structure of polyoxymethylene (POM) using a distributed basis set of s-type Gaussian functions (DSGF) to simulate p-type functions. The band structure results are compared to those obtained using minimal STO-3G basis sets, within the conventional RHF direct space (DS) approach, and subminimal floating spherical Gaussian orbital (FSGO) basis sets. While the FSGO basis sets are unable to describe correctly the oxygen lone pairs and their interactions, the DSGF basis set reproduces qualitatively the features of the band structure observed with the minimal basis set. We show that minor differences between the FS and DS results originate in difficulties of lattice summations within the DS approach, illustrating the advantages of the FS method compared to the conventional DS approach. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1045–1054, 1998

Advantages of the Fourier space RHF band structure approach: Application to polyoxymethylene using a distributed basis set of s‐type Gaussian functions

In this contribution, we outline the Fourier space-restricted Hartree–Fock (FS–RHF) approach to the calculation of the band structure of polyoxymethylene (POM) using a distributed basis set of s-type Gaussian functions (DSGF) to simulate p-type functions. The band structure results are compared to those obtained using minimal STO-3G basis sets, within the conventional RHF direct space (DS) approach, and subminimal floating spherical Gaussian orbital (FSGO) basis sets. While the FSGO basis sets are unable to describe correctly the oxygen lone pairs and their interactions, the DSGF basis set reproduces qualitatively the features of the band structure observed with the minimal basis set. We show that minor differences between the FS and DS results originate in difficulties of lattice summations within the DS approach, illustrating the advantages of the FS method compared to the conventional DS approach. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1045–1054, 1998

Synthesis and structure of novel heterocyclic aminotelluranes

A series of novel 6-alkoxy-12H, 14H-[1,2,3]benzoxatellurazino[2,3-b]-[1,2,3]benzoxatellurazines 3 was synthesized from tetraalkoxytelluranes and bis(2-hydroxybenzyl)amines. 1H, 13C and 125Te NMR studies suggest that these aminotelluranes, unlike their orthoamidoester analogs, have a rigid trans-fused conformation. A preferential trans-fused conformation was also deduced from ab initio MO calculations of a model oxatellurazinooxatellurazine 7, using the STO-3G and SBK basis sets, and the Gaussian-92 program, which imply that the trans-fused aminotellurane is more stable than the corresponding cis isomer by 10.3 kcal/mol.

Characteristics of the LinC60 Complexes for n = 1−6 and 12: An ab Initio Study

Ab initio calculations have been performed for LinC60 where n = 1−6 and 12 using STO-3G and 6-31G basis sets. The distance between Li and the center of C60 and geometrical parameters (bond lengths and bond angles) of C60 have been fully optimized. Electronic structures and the HOMO−LUMO energy gap of the complexes have been analyzed. It was found that both basis sets yield results lying with the same trend. Compared to those in a free C60 ligand, C−C and CC bond lengths of C60 in the LinC60 complexes change slightly for increasing n. The complexes with more Li also have an increased of Li−C60 distance, except that for n = 12. In addition, the Li3C60 complex was found to exhibit the lowest energy gap among the investigated clusters. This is in good agreement with experimental observations for KnC60 and RbnC60 complexes and with the natural lattice structure of C60, for which the unit cell contains two tetrahedral and one octahedral sites.

The vibrational spectra and ab initio calculations of trans- and cis-1,3-dibromo-1,3-dimethylcyclobutane

Infrared spectra of the title compounds were recorded as vapours, as solutes in various solvents, and as solids at ambient temperature and at 80 K. Raman spectra of the solids were obtained at various temperatures between ambient and 80 K. While trans-1,3-dibromo-1,3-dimethylcyclobutane exists in one puckered conformer only, the cis compound can be present as a mixture of diequatorial and diaxial bromine conformers. In the trans compound a large majority of the IR and Raman bands of the crystal did not coincide, suggesting C 2h molecular symmetry. Accordingly, this cyclobutane ring appears to be planar or very close to planar in the crystal, as observed for certain cyclobutanes. However, no significant changes between the IR spectra of the vapour, melt and crystal were detected. No additional IR or Raman bands of the cis compound were detected in the vapour phase, in the melt or in saturated solutions compared to the solid state spectra, suggesting the presence of negligible amounts of the diaxial bromine conformer. The IR and Raman spectra of the trans and cis compounds were assigned and compared with the results of ab initio quantum chemical calculations employing the basis sets STO-3G, HF/3-21G∗ and HF/6-31G∗.

Full-Optimized Reaction Space MCSCF+MP2 Study on Reactions of Diradical Systems: o-C6H4(CH)2, o-C6H4CHN, and o-C6H4N2

Thermal reactivity of the ortho isomers of the titled compounds has been investigated by using the full-optimized reaction space multiconfiguration self-consistent field (MCSCF) method with the STO-3G and 6-31G(d) basis sets, followed by multireference second-order Moller−Plesset perturbation calculation (MCSCF+MP2). The MCSCF+MP2 calculations lead to the prediction that all the ortho isomers have a singlet ground state but the lowest triplet state lies above the singlet ground state by less than 3 kcal/mol. Fortunately, since the spin−orbit coupling between the two states of interest is small and the singlet ground state is more likely to be produced in the photolysis, the thermal behavior of the isomers has been analyzed on the singlet energy surface. In o-phenylenebis(methylene), the E,E stereoisomer undergoes ring-closure reaction with no energy barrier to give a bicyclic compound, while the Z,Z isomer exhibits a situation of competition between the ring-opening reaction and the CH inversion followed by the ring-closure reaction. In (o-nitrenophenyl)methylene, the E isomer exhibits a competitive feature between the ring-opening and ring-closure reactions, whereas the Z isomer reacts into a linear compound with no energy barrier. In o-phenylenebis(nitrene), a spontaneous ring-opening reaction takes place definitely with the formation of a linear compound. These calculated results are in good agreement with the available experimental evidences.

Ab initio effective core potential calculations on lanthanide complexes: basis sets and electron correlation effects in the study of [Gd-(H 2O) 9] 3+

Calculations on [Gd-(H 2O) 9] 3+ at the restricted Hartree-Fock (RHF), density functional theory (DFT) and Møller- Plesset 2 (MP2) levels were performed using an effective core potential (ECP) including 4f electrons in the core for gadolinium. An optimized valence basis set with different contraction schemes for the metal, and the STO-3G, 3-21G, 6-31G ∗, 6-31G ∗∗ and D95 ∗∗ basis sets for water molecules were used. Geometry optimization provided three main conformations: a minimum and two saddle points, each corresponding to a tricapped trigonal prism arrangement of the coordinated water molecules. The minimum energy conformation is of D 3 symmetry and does not correspond to any known experimental structure. The geometries of the two saddle points are of C 3 h and D 3 h symmetry, respectively, and correspond to two different crystallographic structures. At the DFT/D95 ∗∗ (RHF/D95 ∗∗) level these are respectively 4.95 (4.92) and 6.97 (6.26) kcal mol −1 above the global minimum. The results show that the major structural features of the coordination cage observed in the crystallographic structures are mainly due to intramolecular interactions between the coordinated water molecules. Intermolecular interactions, such as crystal packing forces and hydrogen-bond networks, should be responsible for the energy stabilization of the C 3 h and D 3 h structures in the solid state. At the RHF level, the STO-3G basis set is very sensitive to the choice of basis set on the metal center and, even in the best cases, provides unsatisfactory results. On the contrary, the 3-21G basis set provides, independently of the metal basis set, quite reliable geometries and conformational energies in qualitative agreement with those obtained using polarized basis sets. The results from the polarized basis sets themselves are consistent; further, inclusion of the electron correlation causes only a little shortening of the gadolinium-oxygen bond distances with respect to RHF and the conformational energies are substantially unaffected. These results highlight that calculations with the adopted ECP for the metal can provide reliable results in the study of gadolinium complexes also at the RHF level; further, a satisfactory representation of the potential energy surface of these systems can be obtained by 6-31G ∗ single point energy calculations on optimized 3-21G geometries.

An ab initio study of the N8C12 heterofullerene

The result of an ab initio investigation on the hypothetical system N8C12 is presented. This is a fullerene-like structure with Th symmetry. Calculations have been performed at self-consistent field (SCF), second-order Møller-Plesset (MP2), and density functional theory (DFT) level, using three different basis sets: minimal (STO-3G), Valence Double Zeta (VDZ), and (at SCF level only) VDZ plus polarization (VDZP). Second-order analytical derivatives have been computed with the STO-3G basis set and the frequencies obtained are all real. The results indicate that, at this level of description, this hypothetical structure is a local minimum on the potential energy surface. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 83–88, 1997

Quantum chemistry calculations on the effect of electron confinement upon the frontier molecular orbitals of ethylene and benzene in sodalite. Implications on reactivity

Quantum chemistry calculations using HF / STO-3G, CEP-41G, and 3-21G basis sets have been carried out in order to study the effect of the electron confinement of ethylene and benzene in the microporous environment existing in a sodalite cage. The electron confinement produces an increase of 1.06 and 1.31 eV at the 3-21G level in the energy of the HOMO of ethylene and benzene, respectively, when placed in the centre of the cavity with respect to the HOMO energy in the gas phase. There is an increase in the energy of the HOMO when the molecules approach the walls of the zeolite.

Study on the behavior of energy convergence in ab initio crystal orbital calculations

This article studies the dependence on the cutoff scheme of ab initio crystal orbital calculations with no long-range correction. We have thoroughly studied the Namur cutoff and cell-wise cutoff schemes through calculations of polyethylene and LiH chains. The Namur cutoff gives the fastest energy convergence with respect to the number of neighbors (N0). The energy convergence behavior with respect to N0 depends on the basis set. The Namur cutoff shows the fastest convergence with the STO-3G basis set, intermediate convergence with the MINI basis set, and the slowest convergence with the (7s4p/3s) basis set. The cell-wise cutoff shows exactly the reverse order of the Namur cutoff. The Namur cutoff destroys the translational symmetry. Both the Namur cutoff and cell-wise cutoff schemes introduce slight asymmetry on the two equivalent C-C bonds of polyethylene when calculating with a C2H4 unit cell. The asymmetry with the Namur cutoff can be made to disappear by increasing N0 a little. The calculations on two different unit-cell structures of trans-polyacetylene show the effect of the cutoff scheme on the total energy. Only the symmetric cutoff energies are the same. Disagreement related to the Namur cutoff disappears at N0 = 20, however, that related to the cell-wise and modified symmetric cutoff schemes remains at N0 ⩽ 20. The optimized geometry and vibrational frequency are not as sensitive to the cutoff method except with the symmetric cutoff. A compilation of all results shows that the Namur cutoff is the superior cutoff scheme when calculating the insulator using the minimal basis set, especially the STO-3G basis set.

Basis sets for ab initio periodic Hartree?Fock studies of zeolite/adsorbate interactions: He, Ne, and Ar in silica sodalite

Silica sodalite is an ideal model system to establish base-line computer requirements of ab initio periodic Hartree-Fock (PHF) calculations of zeolites. In this article, the authors investigate the effect of various basis sets on the structural and electronic properties of bulk silica sodalite. They also study the interaction of He, Ne, and Ar with the sodalite cage. This work shows that basis-set superposition errors (BSSE) in calculations using STO-3G and 6-21G(*) basis sets are as large as the interaction energies, leading to poor confidence in the results. To cure this problem, the authors present high-quality basis sets for si, O, He, Ne, and Ar, optimized for use with PHF methods, and demonstrate that the new basis set greatly reduces BSSE. The theoretical barriers for transfer of the rare gases between sodalite cages are 5.6, 13.2, and 62.1 kcal/mol for He, Ne, and Ar. 27 refs., 6 figs., 8 tabs.

A linear relation between the ionization potentials of HeI photoelectron spectroscopy and NMR chemical shifts. A PES study of the electronic structure of alkyl isocyanides

HeI photoelectron spectra (PES) of alkyl isocyanides RNC, where R is -CH 3, -CH 2CH 3, -CH 2CH 2CH 3, -CH(CH 3) 2, -C(CH 3) 3 and -cyclo-C 6H 11, are reported. The assignment of the bands is made with the aid of bandshapes, fine vibrational structure and ab initio SCF MO calculations with STO-6G basis sets for each of the molecules studied. The first sharp peak with the lowest ionization potential and with different vibrational spacings is assigned to electron ionization of the HOMO which is composed of a dominant contribution of the carbon atom, similar to the ‘lone-pair’ orbital of the carbon atom in the isocyano group. A linear relationship between the lowest PES experimental ionization potentials and the 13C NMR chemical shifts of the carbon in the isocyano group is found.

Paramagnetic spin catalysis of a radical recombination reaction

Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H2 and O2 molecules is calculatedab initio in STO-6G and 6–31 G* basis sets with complete active space configuration interaction. Calculations are done at a fixed O−H distance (≃3 A), scanning the H−H bond length from 0.6 till 12 A at the linear geometry of collision. The mixture of the triplet (‘T’)3∑ u + and singlet (‘S’)1∑ g + states of the hydrogen moiety is possible because both states have the same triplet symmetry in the ‘collision’ complex with O2 (3∑ g − ). A strong mixture of the ‘S’ (1∑ g + , H2 +3∑ g − , O2) and ‘T’) and ‘T’ (3∑ u + , H2 +3∑ g − , O2) states is actually obtained even at large H−H distances. The quintet and singlet states5,1(3∑ u + , H2 +3∑ g − , O2) are also considered for comparison of the exchange potentials. Atr(H−H)≥4.4 A the ‘S-T’ splitting is approximately constant (12 cm-1 in the STO-6G basis set; 55.5 cm-1 in the 6–31 G* basis set) and is determined by the exchange interaction between O2 and the nearest hydrogen atom in the O−O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the ‘triplet-singlet’ nonadiabatic transition to the lowest ‘S’ reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the ‘T-S’ transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O2 perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.

Biproton transfer in associates of the formic acid molecule with H2O...YH systems (YH=H2O, HOF and HOOH)

An ab initio (STO-3G basis set) method is used to calculate the pathways and activation barriers of cooperative biproton migrations in the associates of the formic acid molecule with water and YH (YH=H2O, HOF, and HOOH). The associate with YH=H2O exhibits a cooperative triproton transfer with an activation barrier of 4.46 (Cs symmetry) or 3.08 kcal/mole (C2 symmetry); in the systems with Y=HOF and HOOH, the biproton transfer is accompanied by synchronous shifts of two hydroxyl groups and has the activation barriers of 75.03 (C2, Y=HOF), 75.74 (Cs, Y=HOF), and 89.62 kcal/mole (Y=HOOH). The hypothesis of stereochemical similarity for low-barrier cooperative processes in molecular associates is proposed.

Molecular and electronic structure of B10H12L2 compounds, where L is a lewis base

The geometrical characteristics of the B10H12L2 complexes of the borane nido-cluster |B10H12| with Lewis base molecules L=NH3, CH3CN, Me2S, Py, which are typical electron pair donors, are analyzed. The complexes are calculated by the MNDO quantum chemical method. Chemical bonding between |B10H12| and L is characterized using, the fragment analysis of molecular orbitals (MOs). It is shown that both occupied and unoccupied MOs of the |B10H12| fragment make important contributions to the bonding. The higher MOs of the system as well as its lower MOs with orbital energies up to 20 eV and higher involve the unoccupied orbitals of the cluster. The electron density is transferred from one L orbital to two unoccupied MOs of |B10H12|, 9a1 and 6b2. There is a large group of HOMOs defined as lone electron pairs of |B10H12| (boron AOs), whose energies change almost symbatically due to bonding to L. The localization of the electron paris by an ab initio calculation in the STO-3G basis set revealed ten three-center bonds and one four-center bond, B(1)-B(2)-B(3)-B(4). The charge transferred from L to |B10H12| is localized mainly on the B(6) and B(9) atoms. The B-L bond is of σ type; a significant θ component is observed for L=CH3CN and Py. It is shown that the simplest two-orbital model of a donor-acceptor bond is inapplicable in the case of the B10H12L2 compounds.

Ab. initio calculations of Cl2SO �nAICl3 complexes (n = 1, 2)

Fragments' of the potential energy surfaces (PES) for the SOCl2 ·nAICl3 (n = 1, 2) complexes were calculated by theab initio MO LCAO method using the RHF approximation for the STO-3G basis set and adding a 3d-AO for the S atom, as well as by the semiempirical MNDO method. Two local minima, assigned to the donor-acceptor complex Cl2SO→+AlCl3 (la) and to Cl3SOAlCl2 (1b) were located on the PES atn = 1. Two local minima corresponding to two donor-acceptor complexes Cl2SO→Al2Cl6 and , were also located on the PES atn = 2. An analysis of the enthalpies of cation formation in the SOC12 +nAICl3 (n = 1-3) systems calculated by theab initio method shows that the enthalpy of formation of the SOCl+ cation atn = 2 is 17 kcal mol−1 less than that atn = 1; the structure of the Cl2SOAlCl2 + cation with two strong electrophilic centers at the Al and S atoms becomes more favorable energetically atn = 3. The results of calculations for Cl2SO ·nAICl3 complexes by the MNDO method are in agreement with those obtained by theab initio method except for the geometry of complexes containing the Cl3SO fragment and the charge values on the S atoms.