Pople Basis Research Articles

Furan−Formic Acid Dimers: An ab Initio and Matrix Isolation Study

The dimers formed by formic acid (FA) and furan are investigated by ab initio methods and matrix isolation spectroscopy. Nine complexes with binding energies between -3.91 and -0.82 kcal/mol (MP2/6-311++G(d,p) + ZPE + BSSE) are identified. Another five weaker bound complexes are localized at lower level of theory only. The binding in the furan-FA dimers can be described in terms of OH...O, C=O...H, HO...H, CH...O, OH...pi, and CH...pi interactions. Therefore, the furan-FA complexes are classified in two types: (1) the dimers where the OH hydrogen of formic acid interacts with the furan molecule and (2) the dimers where the main interactions of FA with the furan molecule are via the less acidic CH hydrogen. Duning's and Pople's triple and double basis sets were used to study the dependence of the geometries and energies of the complexes from the basis set. BSSE (basis set superposition error) counterpoise corrections (CP) were included during the geometry optimizations of all dimers at the MP2/6-31G(d,p) level of theory. Matrix isolation spectroscopy allowed us to record the IR spectrum of aggregates between FA and furan. By comparison of the experimental IR spectrum with calculated IR spectra of a variety of complexes, it was possible to identify the most stable furan-FA dimer as the major product of the aggregation.

Comparison of large basis set DFT and MP2 calculations in the study of the barrier for internal rotation of 2,3,5,6‐tetrafluoroanisole

AbstractThe barrier for internal rotation around the OCH3 bond in 2,3,5,6‐tetrafluoroanisole was calculated using the density functional theory (DFT) and second‐order Møller–Plesset (MP2) methods with Pople's basis sets up to 6‐311++G(3df,2p) and Jensen basis sets up to pc‐3. The results are converged only if fairly large basis sets are used (at least 6‐311++G(3df,2pd) or pc‐2). Both the DFT and MP2 potential energy curves show internal structure. Two minima and three maxima are observed on the curves, arising from the interplay between lone‐pair delocalization and changes in the hybridization around the oxygen atom, together with the attraction between the positively polarized hydrogens in the methyl group and the negatively polarized fluorine atom at the ortho position. These critical points are somehow ironed out by the addition of zero‐point and thermal corrections to the energy curves. At this level, the MP2 method can describe reasonably well the previously determined single‐well experimental rotational barrier, 2.7 ± 2.0 kcal/mol at 298 K, while all DFT methods yield a much smaller result. As observed experimentally, the OCH3 group is perpendicular to the aryl ring in the equilibrium structure, although two very close minima with an intermediate hump at 90° are still observable. The theoretical free energy barrier of rotation at the MP2(full)/pc‐2 level is 2.0 ± 1.0 kcal/mol, in reasonable agreement with the experimental determination. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Properties of the Halogen−Hydride Interaction: An ab Initio and “Atoms in Molecules” Analysis

X-Cl...H-Y interactions are analyzed by applying ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(2df,2pd) and 6-311++G(3df,3pd)) as well as the Dunning-type bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. For the complexes analyzed here, X-Cl and H-Y may be treated as a Lewis acid and a Lewis base, respectively. The Cl...H interactions are rather weak or at most moderate since, for the strongest interaction of the F3...HLi complex, the binding energy calculated at the MP2/6-311++G(3df,3pd) level of approximation amounts to -3.4 kcal/mol, and the H...Cl distance is equal to 2.65 A, less than the corresponding sum of van der Waals radii. These interactions may be classified as halogen-hydride interactions. However, some of the complexes analyzed, especially F3SiCl...HBeF and F3SiCl...HBeF, are very weakly bound, probably by typical van der Waals interactions.

Density Functional Theory (DFT) Calculations of the Infrared Absorption Spectra of Acetaminophen Complexes Formed with Ethanol and Acetone Species

We have investigated the infrared (IR) vibrational spectra of acetaminophen (N(4-hydroxyphenyl) acetamide or paracetamol) complexes formed with ethanol and acetone in relation to the nature of the specific intermolecular interactions involved in the stabilization of the complexes. The structures and binding energies of the complexes have been determined using Hartree-Fock (HF) and DFT-B3PW91 procedures and different Pople's basis sets as well. The main results are presented and discussed by considering the hydroxyl (OH), amino (NH), and carbonyl (CO) chemical groups of acetaminophen interacting with the acetone or ethanol molecules either separately or in conjunction in the complex formation. The frequency shifts and IR intensity variations associated with the internal modes of acetaminophen (namely nu(OH), nu(NH), and nu(CO)) as well as the most pertinent vibrational probes of ethanol (nu(OH)) and acetone (symmetric nu(CO) and nu(CCC) stretching modes) interacting with acetaminophen have been analyzed. The predicted spectral changes have been critically discussed in comparison with IR absorption measurements of acetaminophen dissolved as a solute in ethanol or acetone CO2 expanded solutions. It is argued that the exchange-correlation contribution taken into account in DFT calculations is likely significant in determining the main IR spectral features of acetaminophen complexes formed with acetone or involving hydrogen-bonded as with ethanol.

Modeling the 13C chemical‐shift tensor in organic single crystals by quantum mechanical methods: finite basis set effects

The influence of using finite basis sets to calculate (13)C magnetic shieldings were explored using the Hartree-Fock and the B3LYP hybrid density functional methods. The shielding values were compared in a linear least-squares fashion for a test group of 102 (13)C complete chemical-shift tensors determined from 14 organic single crystals. Pople's basis sets allow for the addition of polarization and diffuse functions in a straightforward way, allowing the examination of 81 combinations at the double and triple zeta level. Dunning's correlation-consistent basis sets were explored as well. The errors associated with predicting the shielding values were found to be largely systematic as revealed by the analysis of the determined regression parameters between calculated chemical shieldings and experimental chemical shifts. Expansion of the basis set leads to a convergence of these regression parameters to their ideal values. The random errors, however, do not decrease by employing larger basis sets; therefore, given the appropriate regression parameters, a small basis description such as 3-21G can be adequate in predicting the relative magnetic-shielding values, i.e. the chemical shifts. Furthermore, in certain cases the inclusion of unbalanced diffuse and polarization functions can significantly degrade the predicted shielding rmsd. Unless employed carefully, these functions do not justify their computational expense. The chemical-shift distance is used to evaluate shielding predictions in individual tensor components. The analysis of the chemical-shift's distance between calculated and experimental data indicates an orientational dependence on the magnitude of errors and suggests the use of the shift anisotropy as a useful fiduciary mark to optimize model chemistries for magnetic-shielding calculations.

Scaling factors for vibrational frequencies and zero-point vibrational energies of some recently developed exchange-correlation functionals

The scaling factors for the vibrational frequencies and zero-point vibrational energies evaluated at various combinations of recently developed exchange-correlation functionals and various basis sets are reported. The exchange-correlation functionals considered are B972, B98, HCTH, OLYP, O3LYP, G96LYP, PBE0 and VSXC functionals; the basis sets employed are 3-21G, 6-31G*, 6-31G**, 6-31+G, 6-311G*, 6-311G**, 6-311G(df,p), 6-311+G(df,p), cc-pVDZ and aug-cc-pVDZ. The experimental harmonic frequencies of 122 small molecules and the zero-point vibrational energies of 39 small molecules are used to determine the scaling factors through the least-square fitting procedure. It was found that the scaling factors do not depend significantly on the basis sets considered. The vibrational frequency scaling factors evaluated by using the B98 and PBE0 functionals are recommended on the basis of smallest root mean square error. The zero-point vibrational energy scaling factors evaluated from the B972 functional with Pople's double-zeta basis set and the HCTH functional with Pople's triple-zeta basis set are recommended on the basis of smallest root mean square error.

Properties of the C−H···H Dihydrogen Bond: An ab Initio and Topological Analysis

The dihydrogen-bonded complexes of methane and its fluoro and chloro derivatives with lithium hydride are analyzed using ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(d,p), 6-311++G(2df,2pd), and 6-311++G(3df,3pd)) and the Dunning bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. The results of the calculations show that the binding energy for the analyzed complexes increases with the increase of the number of fluoro or chloro substituents, up to ∼7 kcal/mol. In the same order there is an increase of the electrostatic energy term, showing that for the CF3H···HLi complex the dihydrogen bond interaction is similar in nature as for the water dimer where a conventional O−H···O hydrogen bond exists, while for the CCl3H···HLi dimer the exchange energy term outweighs the electrostatic energy. Hence, the other attractive energy terms are important. A topological analysis based on the Bader theory supports the results of the ab initio cal...

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex.

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order Møller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.

Some theoretical observations on the 1:1 glycine zwitterion–water complex

The structural transformation of three minima on the HF/6-31G ∗ glycine zwitterion–water complex potential energy surface have been investigated at the Hartree-Fock (HF), second order Møller–Plesset perturbation theory (MP2)and the density functional theory using the B3LYP hybrid exchange-correlation functional employing several Pople's standard basis sets. Contrary to the previous computational results, our results clearly confirm that the zwitterionic structure of glycine does not exist in the 1:1 complex with a water molecule in the gas phase. Several above mentioned computational methods were examined. HF calculations performed at various levels (up to 6-311++G(3df,3pd)) demonstrated the difficulty of obtaining reliable results. Generally, the agreement between B3LYP and MP2 calculations was good. With accurate basis sets, they got the proper results. At the same time, the results at different levels from CP-corrected gradient optimization and CP-uncorrected were also compared.

Trimethylene Sulfide···(HCl)n(n= 1, 2) Complexes: A Theoretical Study

The potential energy surfaces (PESs) of the trimethylene sulfide···(HCl)n (n = 1, 2) complexes were explored at the MP2 level of theory by using Pople's and Dunning's basis sets including diffuse functions. Two different axial and equatorial dimer hydrogen-bonded complexes were located on the PESs. The computed geometries are in good agreement with the microwave spectroscopic determination of these two structures. MP4SDTQ//MP2 and QCISD(T)//MP2 single-point calculations were carried out on the significative points on the PESs (minima and transition structures). Comparison among supermolecule uncorrected, counterpoise procedure (CP)-corrected, and BSSE-free complete basis set (CBS) interaction energies suggests that the CP algorithm overcorrects the basis set superposition error (BSSE). However, the supermolecule CP-corrected interaction energies agree rather well with the corresponding BSSE-free symmetry adapted perturbation theory (SAPT) values. The energetic analysis shows that the axial conformer is slightly more stable than the equatorial one, in agreement with the experimental findings. The nature of the interactions in the complexes is analyzed using different theoretical tools: Bader's, NEDA's, Fukui's, and SAPT results are presented and discussed. The interconversion between the two conformers takes place through ring puckering motion involving an energetic barrier of 214−266 cm-1 (QCISD//MP2 level). This small barrier is consistent with the experimental observation that only the axial conformer is detected when argon is used as a carrier gas. The possibility for a trimer structure to be present in the molecular beam was analyzed from the theoretical viewpoint. Structural and energetic information on the trimethylene sulfide···(HCl)2 complex from ab initio calculations can be helpful to plan further experimental work to detect such a trimer structure.

Investigation of the quality of standard sulfur basis sets for molecules using a new generator coordinate method approach: Application to 2,2′‐bithiophene

AbstractIn this work we have applied the generator coordinate method (GCM) to analyze the quality of the standard 6‐311G** and 6‐311++G** Pople's basis sets for the sulfur atom at the Hartree–Fock level of theory. When using the GCM theory, it is necessary to uncontract the basis set to obtain information about the quality of the basis set. To study large molecules such as bithiophene, for which it is difficult to use uncontracted basis sets at the ab initio level, we have developed a technique that makes possible the use of contracted basis sets within the GCM theory. To test such an approach, we have studied small molecules containing the sulfur atom, such as H2S and SCl2. The results show the suitability of using contracted basis sets for these systems with the GCM theory. In the study of contracted basis sets for the bithiophene molecule, we proposed and tested a modified basis set containing additional diffuse functions on the sulfur atom in the light of the GCM approach. The current study enables for the first time the assessment of the quality of atomic basis sets inside a molecular environment. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 194–204, 2003

The 1:1 glycine–water complex: some theoretical observations

The structures, interaction energies, electronic properties for different conformers of glycine(Ip)–water complex have been determined employing density functional theory using the B3LYP hybrid exchange-correlation functional with the Pople's standard basis sets. Ab initio MP2 calculations were carried out to verify the appropriateness of the B3LYP methods for glycine(Ip)–water system. The basis set superposition error has been eliminated by using the full counterpoise correction method. The results from CP-corrected gradient optimization and the ones from CP-uncorrected were also compared. At last, the vibrational frequencies of the most stable conformer A( C 1) were discussed.

Density Functional Computational Thermochemistry: Isomerization of Sulfine and Its Enthalpy of Formation

Density functional (DFT), second-order perturbation theory (MP2), and coupled-cluster [(CCSD(T)] calculations using Pople's basis sets up to 6-311++G(3df,2pd) and Dunning's correlation consistent basis sets have been employed to determine the enthalpy of formation of sulfine, CH2SO, 1, using the isodesmic reaction CH2S + SO2 ⇌ CH2SO + SO. Previous calculations showed an inconsistency between the enthalpy of formation obtained using this methodology, ΔfHo298.15(1) = −52 ± 10 kJ/mol, and the value obtained employing the isomerization reaction CH2SO (1) ⇌ HC(O)SH (2) if Benson's estimate for the enthalpy of formation of isomer 2 (thioformic acid) was employed. This result was particularly vexing, since the computed enthalpy of formation of 1 was reasonably in agreement with Benson's own estimate. In this paper we extended our previous study using the reactions HC(O)−XH + RH ⇌ H2CO + R−XH with R = H, Me, Et, Pr, and i-Pr. X was either sulfur, to obtain the enthalpy of formation of 2, or oxygen, to assess the errors to be expected in the use of these reactions for the evaluation of ΔfH°. The result, ΔfHo298.15(2) = −121 ± 8 kJ/mol, arrived at after a critical assessment of B3LYP, MP2, and CCDSD(T) results, is in complete agreement with the value of −126 ± 4 kJ/mol estimated by Benson. This implies that the isomerization reaction cannot be employed for the determination of the enthalpy of formation of sulfine. We ascribe this inadequacy to the errors introduced due to the change in the oxidation state of sulfur.

Density functional computational thermochemistry: determination of the enthalpy of formation of sulfine, CH 2SO, at room temperature

Density functional and coupled-cluster calculations using Pople's basis sets up to 6-311++G(3df,2pd) have been employed to determine the heat of formation of sulfine, CH 2SO, 1, using the isodesmic reaction CH 2 S+ SO 2 ⇌ CH 2 SO+ SO . Other reactions, employed previously to determine the enthalpy of formation of sulfine at the CAS-SDCI/ CASSCF ab initio level, were used as well. The analysis of the results shows that: (a) the errors in the calculation of the enthalpies for the individual molecules do cancel reasonably only for the isodesmic reaction, and not for those used previously; (b) density functional methods produce smaller errors than CCSDT in the calculation of the enthalpies of formation of the molecules involved in this reaction; (c) the actual heat of formation of sulfine is determined as Δ f H o 298.15( 1 )=−52±10 kJ/mol, more in agreement with the prediction of Benson than with the ab initio value derived by Ruttink et al.; (d) the proton affinity of sulfine, calculated at the density functional level (792.0 kJ/mol) agrees reasonably well with the experimental result, 787.6±2.6 kJ/mol, but the enthalpy of formation of 1 derived from this proton affinity using the assumptions of Ruttink or Bouchoux is in disagreement with the value determined previously.

Validity of the Minimum Polarizability Principle in Molecular Vibrations and Internal Rotations: An ab Initio SCF Study

Molecular vibrations in ammonia (NH3) and hydrogen sulfide (H2S), and internal rotations in hydrogen peroxide (HOOH), hydrogen thioperoxide (HSOH), hydrogen persulfide (HSSH), and ethylene (C2H4) are studied using ab initio SCF methods at the Hartree−Fock level using a standard Pople 6-311G** basis set. Polarizability values are calculated using both Pople's and Sadlej's basis sets. Any nontotally symmetric distortion in bond length or bond angle along the vibrational symmetry coordinates of a molecule around its equilibrium geometry decreases the equilibrium hardness value and increases the equilibrium polarizability value. During rotational isomerization the minimum energy conformation corresponds to the maximum hardness and minimum polarizability values and the maximum energy conformation corresponds to the minimum hardness and maximum polarizability values. Density functional calculations confirm these observed trends. In general we have found that the conditions of maximum hardness and minimum polarizability complement the minimum energy criterion for molecular stability.

Ab initio Hartree- Fock calculations of the interaction energy of bimolecular complexes

A systematic study of basis set superposition error (BSSE)-free Hartree-Fock interaction energies for bimolecular complexes has been carried out. The BSSE corrections were performed using Mayer's chemical Hamiltonian approach with conventional energy (CHA/CE) and compared with the Boys-Bernardi counterpoise correction scheme. A total of 46 different basis sets were used, ranging from minimal to large polarized (13s8p2dlf/8s2pld)/ [8s5p2dlf/5s2pld] basis sets, on a total of eight systems containing hydrogen and first-row atoms only. The convergence of the BSSE-free interaction energy is studied as a function of basis set size. Corrected interaction energies are generally converged once polarization functions are added. Systems could be classified into two types depending on the presence or absence of multiple bonds. The limiting behavior of the interaction energy and the BSSE content as N → ∞ for Pople's STO-NG, N-21 G and N-31G basis sets, was predicted by Wynn's epsilon method. A full study of p, d, and f polarization functions was carried out using the fixed (13s8p/8s)/[8s5p/5s] basis as a starting point.

Ab initio quantum organic chemistry—I : STO-NG calculations of methane and methyl anion

We report ab initio SCF calculations on methane and Me anion using Pople's STO-4G basis set. Optimization of exponents and geometry gives good agreement with previous calculations and with experiment for bond distances and force constants. The carbanion is more difficult to treat and various isotropic and anisotropic basis sets are compared with extended basis set calculations. The minimum basis set calculations with optimized exponents produce abnormally long C—H bonds. Inversion barriers and population analyses are compared and a series of electron density and MO contours and 3D diagrams is presented. We conclude that minimum basis set calculations of carbanions may be useful in comparing related sets of compounds but it is better to assume geometry in such treatments.