Ab Initio LCAO MO Research Articles

Tautomerism of xanthine: The second-order MØller-Plesset study

Four 9H and four 7H tautomers of DNA base xanthine were studied by the ab initio LCAO-MO method at the MP2/6-311G**//HF/6-31G** and MP2/6-31G**//HF/6-31G** approximations. All calculated structures are minima at the HF/6-31G** potential energy surface with the dioxo 7H tautomer (A1) being the global minimum. The second most stable tautomer, dioxo-9H (B1) is by 9 kcal/mol less stable. For the A1 → B1 transition the calculated MP2 energy gap corresponds to the equilibrium constant of 2 × 10−7. Therefore, only the major tautomeric form A1 is predicted to be detectable in the gas phase. The 7H and 9H groups of tautomers are discussed separately. Within both groups, the dioxo form (A1-7H, B1-9H) is the most stable one and is succeeded by the 2-dihydroxy (A2, B2) form. However, while the energy difference between A1 and A2 is 10 kcal/mol, the energy difference between B1 a B2 is only 2 kcal/mol. The effect of polar environment was estimated by the SCRF method, using a spherical cavity, at the HF/6-31G** level. These calculations did not change the gas phase stability order of the tautomers. However, the energy difference between A1 and B1 decreased from 9 kcal/mol at the HF/6-31G** level to 4 kcal/mol at the SCRF HF/6-31G** level.

Fractional composition of fly ash from the first unit of the Berezovskaya GRES-1 State regional power plant

The problem of the existence of endohedral analogs of ferrocene is discussed. It is shown that radical CorH5, where Cor is corannulene C20H10 and five atoms H are added to the α-positions relative to the five-membered cycle (pent*) of Cor, can form stable polyhedral dimer D2h-C40H30 (D). The ground state of D is triplet. The energy of dimerization of CorH5 is equal to 175 kcal mol−1. The geometric and electronic structure of hypothetical endohedral complex D5d-2η5[email protected] (1) and its isomer D5d-2η5-Fe(CorH5)2 (1b), which is a sandwich complex, are simulated by the ab initio MO LCAO SCF method (HF/3-21G). It is found that the energy of 1 is less than the energy of 1b by 171 kcal mol−1. The energies of the Fepent* bond in complexes 1 (115 kcal mol−1) and 1b (111 kcal mol−1) are close to those of the FeCp bond in bis(cyclopentadienyl)iron FeCp2 (110 kcal mol−1). Complex 1 and also complex D5d-2η5[email protected]40H10 (2), carbon skeletons of which have an analogous structure to 1 and with ten H added to the α-positions relative to polar pentagons (pent*), can be considered as endohedral analogs of ferrocene. Atom Fe in these complexes interacts with atoms of both polar pentagons. The binding energy of the atom Fe with the polyhedral fragment C40H10 of 2 is a bit less than in 1 (on 6 kcal mol−1).

Guanine, 6-thioguanine and 6-selenoguanine: ab initio HF/DZP and MP2/DZP comparative studies

The molecular geometries of four major tautomers of guanine (Gua), 6-thioguanine (6TG) and 6-selenoguanine (6SeG) were studied using the ab initio LCAO-MO method at the Hartree-Fock level with valence double-zeta basis sets augmented by d and p polarization functions (DZP). Only nonplanar C 1 symmetry species are minimum structures on the HF/DZP potential energy surfaces (PES) of Gua, 6TG and 6SeG as it was concluded from harmonic vibrational frequency calculations carried out at the same level. The similarities between the molecular structures of guanine and its heavier analogs are revealed. The calculated molecular parameters of 6TG and 6SeG tautomers are similar to those of corresponding guanine forms, which suggests the possibility of replacing guanine by its heavier analogs in nucleotides, which could rationalize the observed biological activity of these species. Our best estimation of the relative stabilities of the tautomers includes electron correlation contributions calculated at the MP2/DZP approximation and the zero-pointenergy contributions scaled by 0.9. The global minimum species on the PESs of Gua and 6SeG are those with a hydrogen atom bonded to N7 ( 1), while for 6TG the most stable form at this level is that with a hydrogen atom bonded to N9 ( 3); in the latter, 3 has a relative energy 1.8kJmol −1 lower than that of form 1.

Guanine, 6-thioguanine and 6-selenoguanine: ab initio HF/DZP and MP2/DZP comparative studies

The molecular geometries of four major tautomers of guanine (Gua), 6-thioguanine (6TG) and 6-selenoguanine (6SeG) were studied using the ab initio LCAO-MO method at the Hartree—Fock level with valence double-zeta basis sets augmented by d and p polarization functions (DZP). Only nonplanar C 1 symmetry species are minimum structures on the HF/DZP potential energy surfaces (PES) of Gua, 6TG and 6SeG as it was concluded from harmonic vibrational frequency calculations carried out at the same level. The similarities between the molecular structures of guanine and its heavier analogs are revealed. The calculated molecular parameters of 6TG and 6SeG tautomers are similar to those of corresponding guanine forms, which suggests the possibility of replacing guanine by its heavier analogs in nucleotides, which could rationalize the observed biological activity of these species. Our best estimation of the relative stabilities of the tautomers includes electron correlation contributions calculated at the MP2/DZP approximation and the zero-point-energy contributions scaled by 0.9. The global minimum species on the PESs of Gua and 6SeG are those with a hydrogen atom bonded to N7 ( 1), while for 6TG the most stable form at this level is that with a hydrogen atom bonded to N9 ( 3); in the latter, 3 has a relative energy 1.8 kJ mol −1 lower than that of form 1.

Electronic Structure and Reactivity of Dioxygen-Platinum Complexes: an ab Initio MO-LCAO Study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectronic Structure and Reactivity of Dioxygen-Platinum Complexes: an ab Initio MO-LCAO StudyP. Fantucci, S. Lolli, and M. PizzottiCite this: Inorg. Chem. 1994, 33, 13, 2779–2789Publication Date (Print):June 1, 1994Publication History Published online1 May 2002Published inissue 1 June 1994https://doi.org/10.1021/ic00091a019RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Calibration of angular functions of the?-fluorine isotropic hyperfine interaction based on theab initio LCAO-MO UHF SCF calculations of?-fluorinated ethyl radicals

Ab initio LCAO-MO UHF SCF calculations have been carried out for the s-fluorinated ethyl radicals R-CH2, where R=CH3-nFn (n=0,1,2,3). Parameteres of the s-fluorine angular functions of the isotropic hyperfine interaction have been calculated for the ⇏-radical and radical anion fragments\(R - \mathop C\limits^. \begin{array}{*{20}c} / \\ \backslash \\ \end{array} \) and\(R - \overline {\mathop C\limits^. } \begin{array}{*{20}c} / \\ \backslash \\ \end{array} \)

Tautomers of 6-thioguanine: structures and properties

The molecular geometries of four tautomers of 6-thioguanine (6TG) were studied using ab initio LCAO-MO method at the Hartree-Fock level. All considered species are minimum structures on the 3-21G ≡ 8 potential energy surface (PES) of 6TG as determined from harmonic vibrational frequencies calculations carried out at the same level. The basis set effects on the relative energies of the tautomers and their one-electron properties were studied at the HF/6-31G ≡ 8, HF/6-31G ≡ 8 ≡ 8, and HF/6-311G ≡ 8 ≡ 8 levels. Our best estimation of the relative stabilities of the tautomers includes electron-correlation contributions calculated at the MP2/6-31G ≡ 8 approximation and the zero-point energy contributions scaled by 0.9

Inclusive probability calculations for the K-vacancy transfer in collisions of S15+ on Ar

Using the single-particle amplitudes from a 20-level coupled-channel calculation with ab initio relativistic self consistent LCAO-MO Dirac-Fock-Slater energy eigenvalues and matrix elements the authors calculate within the frame of the inclusive probability formalism impact-parameter-dependent K-hole transfer probabilities. As an example the authors show results for the heavy asymmetric collision system S15+ on Ar for impact energies from 4.7 to 16 MeV. The inclusion probability formalism which reinstates the many-particle aspect of the collision system permits a qualitative and quantitative agreement with the experiment which is not achieved by the single-particle picture.

Biuret and its sulfur analogs: Structures and energies

AbstractThe calculations for biuret (B) and its two sulfur analogs, thiobiuret (TB) and dithiobiuret (DTB), were carried out using the ab initio LCAO–MO method at the Hartree–Fock level. The molecular structure of B was fully optimized using standard 3‐21G and 6‐31G* basis sets, while geometry optimizations of the TB's and DTB's were performed at the 3‐21G* and 6‐31G* levels. Only trans‐isomers were found to be minimum structures on the HF/3‐21G* potential‐energy surfaces, whereas cis‐forms are the first‐order transition structures. The relative energies of the cis‐conformers at MP 2/6‐31G**//HF/6‐31G* + 0.9 zero point energy (ZPE) level are equal to 47.4, 51.7, and 59.7 kJ mol−1 for TB, B, and DTB, respectively. Because of relatively large dipole moments, cis‐conformers are predicted to be stabilized in polar solvents.

Ab initioMO calculations on the stability of the CF4+, PF5+, SF6+, and MoF6+ ions with the F1s core hole

For the CF, PF, SF, and MoF ions appearing after the F1s photoionization, the possibility of dissociation has been shown by the ab initio MO LCAO method within the Z + 1 core equivalent model. According to the calculations, the decay channel AF AF + F(1s12p6) is energetically open for the ions. So the interpretation of the gas-phase emission FKα spectra, in which the bands are assigned to the discrete transition energies, can be unacceptable for these ions. The conditions and signs of such failure are discussed.

Pseudo Jahn—Teller origin of square-planar configuration instability of main-group-element hydrides

The vibronic origin of the instability of the square-planar configuration of the [AH 4] p hydrides of the second- and third-row elements (A = B, C, N, O, Al, Si, P and S) is revealed by means of ab initio MO—LCAO—RHF calculations of the electronic wavefunctions, vibronic contants and bare force constants using the STO-4G** basis set. The vibronic pseudo Jahn—Teller effect is confirmed as the only cause of instability. It is shown that the main contribution to the instability is due to the vibronic mixing of the ground A 1g state with two excited B 2u states formed by one-electron excitation a 2u → b * 1g (or b 1g → a * 2u) and e u → e * g (in the frozen MO approximation for the excited states). The variation of the vibronic parameters and instability in the above series is discussed.

Are the amino groups in the nucleic acid bases coplanar with the molecular rings? Ab initioHF/6-31G* andMP2/6-31G* studies

The molecular geometries of the amino-tautomers of cytosine, isocytosine, 1-methylcytosine, adenine, and guanine were studied using the ab initio LCAO–MO method at the Hartree–Fock level with the 6-31G* basis set. Also, MP2/6-31G* geometry optimization and vibrational frequency calculations were carried out for planar conformers of cytosine and isocytosine. All planar structures show one imaginary vibration for which the normal mode correspond to pyramidalization of the amino groups, whereas nonplanar conformers possess only real vibrational frequencies. The dependence of the predicted molecular properties upon planarity or lack of planarity of the studied systems as well as biological consequences of a flat potential energy surfaces for the pyramidalization of the NH2 groups are discussed. © 1992 John Wiley & Sons, Inc.

Theoretical evaluation of new nucleic acid bases. II. Ab initio study on tautomerism of pyrazolo[4,3-d]pyrimidine-5,7 (4H,6H)-dione

The four most important tautomers of the title compound were studied by the ab initio LCAO-MO method at the MP2/ 6-31G**//HF/3-21G level. All calculated structures are minima at the HF/3-21G potential energy surface with the dioxo tautomer 1 being the global minimum. Also in a polar environment 1 is predicted to be a predominant form. The second most stable species, oxo—hydroxy tautomer 2 has a relative energy of 38.7 kJ mol −1 MP2/6-31G** + 0.9 ZPE). For the 1→ 2 transition the calculated energy gap corresponds to an equilibrium constant of 10 −1 which is only one order larger than the estimated frequency threshold of spontaneous mutations and it strongly suggests that π is a new purine base with reduced probability of spontaneous mutations.

Quantum-chemical investigation of mechanisms of reactions of nucleophilic addition to acetylene. 7. Evaluation of possible interactions in C2H2/MOH/DMSO system

Within the framework of the ab initio method SCF MO LCAO, using a model potential, the influence of alkali metal cations on the reactivity of the acetylene molecule has been investigated. Activation of acetylene with respect to a nucleophile is favored by a lowering of energy of the vacant π* orbital upon coordination, a change in its form as a consequence of sufficiently free motion of the cation along the triple bond, and stabilization of the transdistorted form of acetylene in the complex. In the acetylene/alkali/DMSO system, the activating capability of cations is related to the ease of replacement of a solvent molecule in the solvate complex of the acetylene molecule, this capability increasing in the series Li < Na< K.

The ab initio surfaces of the potential energy and dipole moment of 1Σ g+ CO 2. Stretching vibrational states

An ab initio SCF MO LCAO calculation of the potential energy surface (PES) of 1Σ g + CO 2 with a Huzinaga—Dunning (10s6p2d)/4s3p2d basis set is presented. A PES up to ≈ 10000 cm −1 has been fitted by the fourth-order polynomial with analogous fitting of the components of the dipole moment by third-order polynomials. Stretching vibrational states were obtained by direct numerical integration of a one-dimension Schrödinger equation and compared with that obtained by diagonalisation in the basis of harmonic oscillator functions.

Ab initio mo calculation of nuclear quadrupole coupling constant of 2H and 17O nuclei in water molecule

A large number of ab initio LCAO MO calculations have been done using Cartesian Gaussian functions of mainly double zeta (DZ) quality. The effect of the augmentation of the DZ basis with polarization function (DZ+Pf), with bond centred functions (DZ+Bf) and with polarization plus bond centred functions (DZ+Bf+Pf) have been studied for quadrupole coupling constants of 2H and 17O nuclei. The variations of nuclear quadrupole coupling with exponent of s and p type bond centred functions were also studied. It is found that the augmentation by s and p bond function with exponent s (1.0) and p (1.2) gives values of 2H and 17O quadrupole coupling constants very close to the experimental ones.

An ab initio study of anharmonic potential energy surfaces for bound water molecules

One-dimensional potential energy expansions in stretching, bending and rigid-body translational and rotational coordinates up to fourth order have been calculated for bound H2O molecules by abinitio MO LCAO SCF methods. Water in crystalline ice and LiOH·H2O have been simulated by calculations on (H2O)4·H2O and (Li+)2(OH−)2H2O clusters, each surrounded by point charges. The force constants ƒ 2, ƒ 3 and ƒ 4 in the potential energy expression V=ƒ 2Δr3+ƒ 3Δr3+ƒ 4Δr4 were calculated, where Δr is a simultaneous stretching of the OH bonds in the water molecule. The force constants were found to decrease linearly with the equilibrium OH distance. Anharmonicity constants X were calculated for rigid-body vibrations of H2O in LiOH·H2O and found to be 14 cm−1 for the rotational vibration and less than 1 cm−1 for the translational vibration.

Ab initio LCAO-MO calculations on α-D-glucopyranose, β-D-fructopyranose, and their thiopyranoid-ring analogues. Application to a theory of sweetness

Ab initio SCF LCAO-MO calculations at the STO-3G level have been performed on the title compounds and the results are discussed in terms of net atomic charges and Mulliken population analysis, with a view to providing an explanation for the greater sweetness observed in the case of the thio analogues.

Three-dimensional structure of phosphorus-containing heterocycles

By means of dynamic PMR spectrometry, UV spectra, dipole moments, and ab-initio SCF MO LCAO calculations of model compounds, it has been shown that for 2-hydro-2-oxo-4,5,7,8-dibenzo-4H-1,3,2-dioxaphosphocins there is characteristically a three-component equilibrium with the participation of both of the boat-chair forms and one boat form with equatorial position of the P-H bond. The thermodynamic characteristics of the equilibrium have been determined.

Ab initio calculations on H transfer in the HF trimer

AbstractSystematic ab initio LCAO MO calculations with basis sets of increasing size have been performed to analyze the changes in relative stabilization energies of the two cyclic (D3h and C3h symmetry) and the noncyclic HF trimers. The results form the basis for a discussion of cyclic H‐transfer reactions.