Standard 6-31G Basis Set Research Articles

Crystallographic and DFT Studies on Pyrrolo[1,2-c]imidazole Scaffolds

The crystal structures of the compounds C15H14N4O2 (1) and C16H16N4O4 (2) are reported and analyzed by single crystal X-ray diffraction technique. Compounds (1) and (2) crystallized in monoclinic space group P21/c and Cc with four molecules in the unit cell, respectively. The unit cell parameters for compound (1) are a = 11.4501(15) Å, b = 9.7869(11) Å, c = 12.3653(15) Å, β = 90.997(11)°, and Volume = 1385.5(3) Å3 and for compound (2) are a = 13.865(2) Å, b = 6.9538(8) Å, c = 16.841(2) Å, β = 98.602(11)°, and Volume = 1605.4(4) Å3. In both compounds (1) and (2), the pyrrolidine ring adopts half-chair conformation. Moreover, both inter- and intramolecular N–H⋯O hydrogen bonds stabilize the crystal structure and play a crucial role in crystal packing. This intermolecular interaction alone constructs C11 chain motif in both compounds. It is also supported by weak intermolecular π-π interaction which is essential for the stability of the crystal packing. Further, the Density Functional Theory (B3LYP) method with standard 6-31G basis set was used in the calculation and calculated geometrical parameter is correlated with the corresponding experimental data. The obtained HOMO and LUMO energies are in negative values indicating that the compounds are in stable state.

Complexation of alkali–metal cations by conformationally rigid, stereoisomeric calix[4]arene crown ethers: A density functional theory study

The quantum mechanical study of the cone, partial-cone, and 1,3-alternate 1,3-diethoxy-p-tert-butylcalix[4]crown-5 and their interaction with the alkali–metal cations (Na +, K +, and Rb +) has been performed. Geometries, binding energies, and binding enthalpies are evaluated at the restricted hybrid Becke’s three parameter exchange functional (B3LYP) level using standard 6-31G basis set and relativistic effective core potentials. The optimized geometric structures are used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal–ligand and cation– π interactions are investigated. The results indicate that intermolecular electrostatic interactions are dominant and the electron-donating oxygen offer lone pair electrons to the contacting RY ∗ (1-center Rydberg) or LP ∗ (1-center valence antibond lone pair) orbitals of M + (Na +, K +, and Rb +). What is more, the cation– π interactions between the metal ion and π-orbitals of the two rotated benzene rings play a certain role (especially in L 3). Our calculations clearly show that solvation effects strongly influence cation selectivity. 1,3-diethoxy-p-tert-butylcalix[4]crown-5 isomers preferentially bind Na +, not K + as found in aqueous environments. However, the calculated results indicate that K + selectivity is recovered when even a few waters of hydration are considered.

Theoretical Study on Interactions of β-cyclodextrin with Trans-dichloro(dipyridine) platinum(II)

To improve the bioavailability of trans-dichloro(dipyridine) platinum(II) (DDP) complexes, an inclusion technique has been developed to modify its physical and chemical properties. In this work, the geometries of the cyclodextrin inclusion complexes with DDP in gas phase are determined by DFT (B3LYP) method at standard 6-31G basis set and LanL2DZ basis set and PM6 semiempirical method. The optimized geometric structures obtained from DFT calculations are used to perform natural bond orbital (NBO) analysis. Intermolecular hydrogen bonds between DDP and β-CD are investigated. The calculated results indicate that DDP entering into the cavity of β-CD from its narrow side is energetic favorable. The calculated results are in good agreement with the experimental results.

An evaluation of the modified 6-31G* basis set for the atoms Ga-Kr using the Gaussian-3 and Gaussian-4 composite methods

The new modified 6-31G* basis set of Mitin and Merz for the elements Ga–Kr is evaluated by applying the Gaussian-3 and Gaussian-4 composite methods to the set of 19 atomisation energies, 15 ionisation energies, 4 electron affinities and 2 proton affinities for atoms and molecules containing these elements. The standard 6-31G basis set with additional diffuse and polarisation functions is replaced by the modified basis set. Results are compared for the calculated geometries and energy values against experimental values, the results using the standard 6-31G basis and other composite methods. The calculated geometries are improved in the Gaussian-3 composite method where MP2 is used to determine the geometries, but are slightly worse in the Gaussian-4 composite method where the density functional B3LYP method is used. The energy values with the new basis set have slightly higher errors against experiment for the Gaussian-3 approach, but the errors for the Gaussian-4 approach are essentially unchanged. If the new basis set is widely accepted after further studies, there appears to be no serious disadvantage in using it for these composite methods.

Comparison of the Standard 6-31G and Binning-Curtiss Basis Sets for Third Row Elements.

Ab initio calculations were carried out for isogyric reactions involving the third row elements, Ga, Ge, As, Se, and Br. Geometries of all the reactants and products were optimized at the HF, MP2, and B3LYP levels of theory using the 6-31G(d) and 6-31G(d,p) basis sets. For molecules containing third row elements geometries, frequencies and thermodynamic properties were calculated using both the standard 6-31G and the Binning-Curtiss (BC6-31G) basis sets. In order to determine the performance of these basis sets, the calculated thermodynamic properties were compared to G3MP2 values and where possible to experimental values. Geometries and frequencies calculated with the standard 6-31G and the BC6-31G basis sets were found to differ significantly. Frequencies calculated with the standard 6-31G basis set were generally in better agreement with the experimental values (MAD=40.1 cm(-1) at B3LYP/6-31G(d,p) and 94.2 cm(-1) at MP2/6-31G(d,p) for unscaled frequencies and 29.6 cm(-1) and 24.4 cm(-1), respectively, for scaled frequencies). For all the reactions investigated, the thermodynamic properties calculated with the standard 6-31G basis set were found to consistently be in better agreement with the G3MP2 and the available experimental results. However, the BC6-31G basis set performs poorly for the reactions involving both second and third row elements. Since, in general, the standard 6-31G basis set performs well for all the reactions, we recommend that the standard 6-31G basis set be used for calculations involving third row elements. Using G3MP2 enthalpies of reaction and available experimental heats of formation (ΔHf), previously unknown ΔHf for CH3SeH, SiH3SeH, CH3AsH2, SiH3AsH2, CH3GeH3, and SiH3GeH3 were found to be 18.3, 18.0, 38.4, 82.4, 41.9, and 117.4 kJ mol(-1), respectively.

Static polarizabilities of doubly charged polyacetylene oligomers: basis set and electron correlation effects

Abstract Ab initio calculations, carried out with different basis sets, for the static longitudinal linear polarizability, αL, and second order hyperpolarizability, γL, of small doubly charged polyacetylene (PA) chains, are presented. The polarizabilities were calculated using the Hartree–Fock (HF) method while the electron correlation effects were included through the second-order Moller–Plesset perturbation theory (MP2). Positively and negatively charged bipolarons were studied. The results obtained for positive and negative chains show that the ionization state effect decreases more rapidly, as the chain length is increased, for αL than for γL. For both types of charged chains, the incorporation of the electron correlation increases the αL and γL values, as compared to the HF values. A comparison between the results obtained using the standard 6-31G basis set and augmented versions of this set, obtained by the addition of diffuse and polarization functions, shows that 6-31G basis set does not provide a good description of the negative chains studied here and that the addition of extra diffuse functions on the basis set is needed in order to obtain reliable estimates for polarizabilities, specially for γL.

Ab initio polarizabilities calculations of singly charged polyacetylene oligomers

We present results for the static longitudinal linear polarizability and second-order hyperpolarizability of small polyacetylene chains bearing positively and negatively charged solitons, obtained through the second-order Møller–Plesset perturbation theory (MP2) method. Hartree–Fock (HF) calculations for these properties was performed only for negatively charged chains. The standard 6-31G basis set was used in all calculations. Our ab initio calculations showed that, regarding singly charged structures, only the second hyperpolarizability is affected by the ionization state. For both, positive and negative structures, it is shown that the electron correlation effect enhances the linear polarizability, and even more markedly the second hyperpolarizabilities.

Ab initio calculations on the five-membered alumino-silicate framework rings model: implications for dissolution in alkaline solutions

An ab initio restricted Hartree–Fock calculation utilising the standard 6-31G basis set was used to calculate total energies after PM3 calculations of energy-optimised geometries for the five-membered alumino-silicate framework rings cluster for a total of ten T sites. Calculations have shown that in the absence of protons or other ions, the most favourable sites for 1Al, 2Al and 3Al substitution of Si are the T 6, T 1 and T 9 sites respectively. With more Al atoms replacing Si in a cluster, TO bond lengths and TOT angles show lengthening and sharpening trends respectively, which indicates that the structure is distorted to a more relaxed symmetry with O br atoms moving outwards. The calculated bond lengths and angles have been shown to match the values observed in previous studies, including those for a four-membered alumino-silicate single ring cluster. Based on the optimised five-membered alumino-silicate framework rings model, a further ab initio HF calculation has been conducted on ring breakage for releasing Al(Q 3) and Si(Q 3) centres to form T(OH) 4 and HOT(OM) 3 tetrahedra under local and highly alkaline environment. The obtained results suggest that Al(Q 3) compared with Si(Q 3) breaks more readily with the exothermal reaction enthalpy being in excess of −244.4 kJ/mol, while the most reactive Si(Q 3) centre shows an exothermal reaction enthalpy of only −33.8 kJ/mol. This indicates that Al dissolves in preference to Si in local environment. The dissolution mechanism of the five-membered Al–Si framework rings model in highly alkaline solutions has been suggested to be composed of an ion-pairing reaction and an interaction between the remaining broken ring cluster ≡TOH and MOH.

Molecular Mechanics (MM3) Calculations on Oxygen-Containing Phosphorus (Coordination IV) Compounds.

An MM3 force field has been developed for small oxygen-containing phosphorus (coordination IV) compounds. Experimental electron and neutron diffraction, X-ray, and infrared spectral data were utilized in parametrization when available. Results from previous ab initio calculations at both the restricted Hartree-Fock (RHF) and second-order Møller-Plesset (MP2) levels of theory with the standard 6-31G basis set supplemented missing structural data, rotational profiles, and vibrational frequencies. Negative hyperconjugation (the anomeric effect) affects the structures and energies of the molecules under study which contain one or more sp(3)-hybridized oxygen atoms. Natural bond order (NBO) analysis was helpful in identifying the key orbital interactions involved in this effect.

Density Functional Study of the Hexamethyl-(Dewar Benzene) Radical Cation and Some Related Compounds.

A theoretical study has been undertaken of the hexamethyl(Dewar benzene) radical cation (both 2B2 and 2A1 states in C2v symmetry) as well as of two related ions, the pentamethylbenzene and the pentamethylbenzyl trifluoroacetate radical cations, which have been suggested as alternative assignments of the EPR spectrum attributed to the hexamethyl(Dewar benzene) cation. The geometries were optimized at the HF and LDA levels of theory using a standard 6-31G basis set. Single point calculations of the hyperfine coupling constants (hfcc) were then performed at the more accurate MP2, B3PW91, B3LYP and PWP86 levels using larger basis sets. The theoretical results of the present study support the assignment of 2B2 as the ground state of the hexamethyl(Dewar benzene) cation, yielding hydrogen atom hfcc’s in very good agreement with experiment.