Showing papers in "Journal of Molecular Structure-theochem in 1990"
TL;DR: In this article, the stabilities of isolated tautomers of purine, adenine and guanine are discussed by considering their relative internal energies at 0 K predicted by ab initio quantum-mechanical calculations which include relative electronic energies at the SCF level (in a few cases corrected for electron correlation) and zero-point vibrational energies.
Abstract: The stabilities of several isolated tautomers of purine, adenine and guanine are discussed by considering their relative internal energies at 0 K predicted by ab initio quantum-mechanical calculations which include relative electronic energies at the SCF level (in a few cases corrected for electron correlation) and zero-point vibrational energies. The stabilities of the guanine tautomers are compared with the results from similar calculations on isocytosine tautomers. The calculations predict the following tautomers to be most stable: in purine N(9)H; in adenine N(9)H; in guanine three tautomers of similar energy, amino-oxo N(9)H and N(7)H, and amino-hydroxy N(9)H, with the latter slightly predominating; in isocytosine amino-oxo and amino-hydroxy forms with the latter predominating. The predicted relative stabilities of the tautomers in question are in agreement with recent experimental findings for the stabilities of the molecules based on the analysis of their infrared spectra in inert-gas low-temperature matrices. The effect of the environment on the relative stabilities of several tautomers is also discussed briefly.
81 citations
TL;DR: In this article, the relative stabilities of the structural isomers of each compound and their transition states were determined and the internal rotation of the OH group, the harmonic force fields and vibration frequencies, and the molecular electronic properties were compared with each other with reference to those of the monosubstituted analogue, phenol.
Abstract: Results of ab initio SCF calculations of o -, m -, and p -dihydroxybenzenes (catecol, resorcinol and p-hydroquinone) are reported The relative stabilities of the structural isomers of each compound and their transition states were determined The internal rotation of the OH group, the harmonic force fields and vibration frequencies, and the molecular electronic properties were calculated and compared with each other with reference to those of the monosubstituted analogue, phenol The calculated values are also compared with recent results obtained from molecular beam experiments
60 citations
TL;DR: In this article, the effect of solvent on specific properties of two solutes, N -methylformamide (1 ) and methylformate (2 ), was examined making use of a continuous model for solutions in an ab initio quantum-mechanical formulation.
Abstract: The effect of the solvent on some specific properties of two solutes, N -methylformamide ( 1 ) and methylformate ( 2 ), was examined making use of a continuous model for solutions in an ab initio quantum-mechanical formulation. The analysis regards energetic quantities as well as other observables. An estimate of the different contributions for the energy is given, including also the thermal contributions (due to vibrations, rotations and hindered motions inside the solution) for several geometrical configurations of 1 and 2 : the stable cis and trans conformations, and the transition states for internal conversion. An estimate of this kind, based on ab initio calculations, was done here for the first time. Among the properties taken into consideration we quote the results of geometry optimization in solution (for stable as well as for transient conformations), performed with an automated program employed here for the first time. The analysis is extended to solvent effects on the MO energies, on the dipole moment, on the rotational barrier for the CH 3 group (at different conformations), on the atomic populations and bond indexes. An interpretation of all the above-mentioned solvent effects is given. The interpretation is intuitively satisfactory and supported by quantitative correlations of good quality, making use of simple semiclassical concepts, of general applicability, and of the corresponding numerical information derived from the ab initio calculations. The influence of the basis set, ranging from the STO-3G basis set to the 6-31G ∗ one, was also examined.
59 citations
TL;DR: In this paper, the Fukui function was studied for a series of compounds which proved to be adequate model structures for zeolite systems containing bridging hydroxyls and showed that the 3-21G basis may be expected to yield an optimal quality/cost ratio.
Abstract: The Fukui function, ƒ+, was studied for a series of compounds which proved to be adequate model structures for zeolite systems containing bridging hydroxyls. The structures were derived from the basic structure H3SiOAlH3 in which the hydrogen atoms on Si and Al are gradually replaced by fluorine atoms, thus mimicking changes in the average framework electronegativity upon variation of the Si/Al ratio. A basis-set study of the condensed Fukui function for a series of small molecules (H2CO, NH3 and H2O) indicated that, at least for the larger systems, the 3–21G basis may be expected to yield an optimal quality/cost ratio. Calculations on NH3 show that highly diffuse functions should not be introduced in the evaluation of ƒ+ in order to cope with the anionic metastability problem. The condensed Fukui function, ƒH+ , correlates well with the experimentally observed increase in acidity upon increasing framework electronegativity. Moreover, the function is found to be a more sensitive probe for the acidity of the OH group than is the previously calculated isolated molecule OH group properties, including the dipole-moment derivative. The trends in ƒ+H are duplicated in the contour plots of ƒ+ in the OH region indicating that the “probing” capacity of ƒ+ is not lost by the highly approximate integration performed when condensing ƒ+ via a Mulliken population analysis. A study of ƒ+ along the OH bond axis reveals that its sensitivity, being more difficult to extract than that of the condensed function, is of comparable magnitude to that of ƒH+.
58 citations
TL;DR: In this article, the π-type hydrogen-bond stabilization energies for C 2 H 2 ·HF and C 3 H 6 ·HF were calculated from MP2/6-31G ∗∗ /6- 31G calculations.
Abstract: π-Type hydrogen-bonded systems consisting of acetylene, ethylene, cyclopropane or benzene as proton acceptors and hydrogen fluoride as proton donor have been studied. The calculated π-type hydrogen-bond stabilization energies for C 2 H 2 ·HF, C 2 H 4 ·HF, C 3 H 6 ·HF and C 6 H 6 ·HF are 4.54, 4.71, 4.64 and 4.81 kcal mol −1 , respectively, from MP2/6-31G ∗∗ /6-31G ∗ calculations. The detailed bond connecting descriptions, which are taken from the molecular graphs, have been made on the basis of topological analysis of the electronic density distribution. The stabilization of C 2 H 4 ·HF and C 3 H 6 ·HF π-type hydrogen-bonded systems at the 3-21G level are discussed.
58 citations
TL;DR: In this article, an ab initio SCF molecular orbital approach in conjunction with the isodesmic reaction procedure was used to investigate anomalous energy effects in strained aza systems and some of their nitro derivatives.
Abstract: We have used an ab initio SCF molecular orbital approach in conjunction with the isodesmic reaction procedure to investigate anomalous energy effects in strained aza systems and some of their nitro derivatives. The introduction of nitrogens into strained molecular frameworks is found to confer added degrees of stability. In general this increases with the number of nitrogens in a series of similar molecules. Further stabilization results from N-nitro substitution and the mononitration of secondary carbons; however the polynitration of systems containing highly strained tertiary carbons has a marked destabilizing effect.
49 citations
TL;DR: In this article, an effective procedure is presented for calculating analytical derivatives of the energy in Hartree-Fock-type procedures including configuration interaction in cases where the number of configurations is small.
Abstract: An effective procedure is presented for calculating analytical derivatives of the energy in Hartree-Fock-type procedures including configuration interaction in cases where the number of configurations is small. This situation commonly arises in calculations involving semiempirical models such as MNDO or AM1. Existing numerical procedures are unsatisfactory, having been designed for ab initio applications where large numbers of configurations must be included.
46 citations
TL;DR: A detailed ab initio study of the enol form of acetylacetone is reported in this article, including both an exhaustive conformational analysis with total optimization, and an estimation of the variation of potential energy during interconversion of the two equivalent Cs symmetrical structures of lowest energy by intramolecular proton transfer.
Abstract: A detailed ab initio study of the enol form of acetylacetone is reported including both an exhaustive conformational analysis with total optimization, and an estimation of the variation of potential energy during interconversion of the two equivalent Cs symmetrical structures of lowest energy by intramolecular proton transfer. That the calculated frequency of interconversion, 1.8 × 1011 s−1, is lower than for malonaldehyde, the simplest β-diketone, appears to be largely due to coupling between methyl rotation and the proton-transfer process.
40 citations
TL;DR: The existence of a very long N-N bond in this isomer is calculated to be associated mostly with the nature of the hybridization and the orientation of the lone-pair atomic orbitais on each nitrogen atom as discussed by the authors.
Abstract: The results of some ab initio valence bond calculations, with an STO-5G basis set, are reported for the Ca2v isomer of N2O2. The existence of a very long N-N bond in this isomer is calculated to be associated mostly with the nature of the hybridization and the orientation of the lone-pair atomic orbitais on each nitrogen atom. The optimum forms of the two orbitais have a large 2s component, and overlap appreciably when the N-N internuclear separation is close to that of a normal single bond. The associated strong non-bonded repulsions that exist between the nitrogen lone-pair electrons lengthen the N-N bond. At N-N distances close to the equilibrium value (2.237 A), the orbitals that form the N-N σ bond are almost entirely 2p in character, and oriented at right angles to the N-O bond axes. Although some delocalization of oxygen lone-pair electrons of the Lewis structure does occur, thereby generating a fractional N-N σ bond, this effect produces only a small lengthening of the N-N bond.
38 citations
TL;DR: In this article, the electron densities around N, P, O and S atoms in HCN, HOP, OCH 2 and SCH 2 molecules were calculated at various distances from the nuclei in two or three directions by using the Hartree-Fock method.
Abstract: Electron densities around N, P, O and S atoms in HCN, HOP, OCH 2 and SCH 2 molecules were calculated at various distances from the nuclei in two or three directions by using the Hartree-Fock method with the 6–311G (2 d , p ) and MC-311G (2 d , p ) basis sets. Van der Waals radii (in the isolated molecules) were estimated at the position where the electron density is 0.005 a.u. Theoretical radii are in fairly good agreement with the experimental values in the crystals estimated by Nyburg and Fareman. Smaller polar flattening is obtained in the O atom of OCH 2 and larger polar flattening is confirmed in the S atom of SCH 2 . The electron densities in the minor direction are larger than those in the major direction in both the N atom of HCN and the P atom of HCP at over the wide range of the distance from the nuclei. Negative polar flattening was found for the P atom of the HCP molecule.
36 citations
TL;DR: In this paper, modified MNDO methods for the study of hydrogen-bonded complexes are tested in four simple systems: (H 2 O) 2, NH 4 + · H 2 O, formamide water and formamide-methanol dimers.
Abstract: Recently proposed modified MNDO methods (MNDO/M and MNDO-PM3) for the study of hydrogen-bonded complexes are tested in four simple systems: (H 2 O) 2 , NH 4 + · H 2 O, formamide water and formamide-methanol dimers. A comparison is made with AM1 since this method, intended to be a general improvement of MNDO, was already shown to fail in the former two dimers. Bifurcated hydrogen-bonded structures were always preferred over linear ones. AM1 also shows this artifact in the formamide-water dimer but not in the formamide-methanol one. This is due to the inability of -CH 3 to bind to the carbonyl oxygen. In this case the AM1 optimized global minimum is in excellent agreement with the Hartree-Fock ab initio DZP optimized structure. Moreover, when it is physically impossible to form bifurcated hydrogen bonds, AM1 is in good agreement with ab initio results. MNDO/M and MNDO-PM3 (an improvement on AM1 ) do not give any bifurcated structure either in the former two dimers or in the formamide complexes. MNDO/M and MNDO-PM3 structures are in agreement with ab initio and experimental ones in almost all cases, except for the global minima in both the formamide-water and the formamide- methanol dimers. Singly hydrogen-bonded structures are predicted instead of the experimentally determined and ab initio calculated correct doubly hydrogen-bonded ones. MNDO-PM3, although not entirely correct, gives structures which are more correct than the MNDO/M ones. In conclusion, neither of these semiempirical methods is entirely correct. Overall, MNDO-PM3 seems to be more trustworthy than AM1 or MNDO/M, although these methods may be used when no bifurcated hydrogen bonds (AM1) or cycles (MNDO//M) exist. Failures of MNDO-PM3 indicate that a better semiempirical method is still needed for studies of solvation and hydrogen- bonded clusters.
TL;DR: In this paper, the potential energy surface of β-alanine was investigated with ab initio 4-31G SCF calculations, and the geometric, energies and wavenumbers for all 20 unique local minima were reported together with the potential barriers of the reactions, which interconnect these minima.
Abstract: The potential-energy surface of the neutral form of β-alanine was investigated with ab initio 4-31G SCF calculations. Geometries, energies and wavenumbers for all 20 symmetry unique local minima are reported together with the potential barriers of the reactions, which interconnect these minima. Intramolecular interactions involving the groups CO, OH and NH 2 were deduced from these informations.
TL;DR: In this paper, the effects of the substitution of oxygen by sulphur on molecular properties of thioacetic acid have been investigated using the 3-21G and 4-31G basis sets.
Abstract: Ab initio SCF-MO calculations have been carried out for thioacetic acid using the 3–21G and 4–31G basis sets. Energies and structures of several conformations of the two tautomeric forms of this molecule (the thiol and thion forms) determined by gradient geometry refinement are reported, and some conformationally dependent local geometry trends discussed. The results are compared with available data on acetic and dithioacetic acids in order to assess the effects of the substitution of oxygen by sulphur on molecular properties. It is shown that the O(H)→S(H) substitution originates important changes in the molecular properties, which can be correlated with the greater conjugating ability of the oxygen atom. On the other hand, the calculations indicate that the O=→S=substitution causes minor changes in the relative conformational energies, although other properties may be significantly affected. Charges on atoms, dipole moments and ionization potentials are also presented and compared with the available experimental values.
TL;DR: In this article, it was shown that the free oxalate ion has an orthogonal D2h structure with a variable twist angle between the carboxylate groups.
Abstract: While the oxalate ion is planar in some crystalline oxalates, it is twisted in others, with variable twist angles between the carboxylate groups. Calculations by AM1 and ab initio procedures have now shown free oxalate ion to have an orthogonal D2h structure. Introduction of disembodied charges (“sparkles”) as models of metal cations reduces the twist angle and zinc oxalate is calculated to be planar, like oxalic acid. The hydrogen oxalate anion exists in two isomeric forms with similar energies, a planar form with cis hydroxyl and an orthogonal form with trans hydroxyl.
TL;DR: In this article, MNDO calculations with complete geometry optimization were carried out on cubane and its 21 aza derivatives (from azacubane to octa-azacubaane) using additive models and isodesmic reactions.
Abstract: MNDO calculations with complete geometry optimization were carried out on cubane and its 21 aza derivatives (from azacubane to octa-azacubane). The calculated heats of formation are discussed using additive models and isodesmic reactions. The empirical models show additivity of the Δhf values, but are difficult to rationalize. The isodesmic reactions tend to prove that azacubanes are relatively more stable than cubane itself and are thus reasonable synthetic targets.
TL;DR: In this paper, the authors carried out an electrostatic potential analysis of a series of 1-substituted 2,4-dinitronaphthalenes and some related naphthalene and benzene derivatives using an ab initio selfconsistent field molecular-orbital approach.
Abstract: We have carried out an electrostatic potential ( V ( r )) analysis of a series of 1-substituted 2,4-dinitronaphthalenes and some related naphthalene and benzene derivatives using an ab initio selfconsistent-field molecular-orbital approach. Our aim was to investigate the nature of V ( r ) above the planes of these systems and possibly to relate it to the formation of π complexes. Negative potentials of varying strengths are observed above the planes of naphthalene and its 1-halo derivatives and are interpreted as being due to the π electrons of the corresponding aromatic rings. The magnitudes of V ( r ) in these regions in the 1-halonaphthalenes decrease in the order 1-fluoro > 1-bromo > 1-chloro, indicating that chlorine is the most effective of these halogens in withdrawing electronic charge from the remainder of the naphthalene molecule. We attribute this trend to two opposing factors: electronegativity, which decreases in the order F > Cl > Br; and charge capacity, which increases in going from fluorine to bromine. The strong electron-withdrawing power of even one nitro group in the naphthalene derivatives suffices to eliminate the negative ring potentials. Positive potential regions are thus observed above the planes of all the nitro containing systems. These show a build up of positive potential above the C-NO 2 bond regions and, in some cases, above the aromatic portion of the molecule. In general, the V ( r ) maxima for the 1-substituted dinitronaphthalenes and trinitrobenzene increase in the same order as do experimentally observed equilibrium constants for the formation of their respective π-complexes with OH − . This is consistent with the interpretation of the potential maxima as reflecting relative susceptibilities for nucleophilic attack.
TL;DR: In this article, the contributions from localized molecular orbitais within the polarization propagator approach (CLOPPA) were used to obtain experimental information on molecular structures from high-resolution NMR spectroscopy.
Abstract: The quantum-chemical analysis of factors defining the stereospecific behaviour of nuclear spin-spin coupling constants is shown to be an important aid in obtaining experimental information on molecular structures from high-resolution NMR spectroscopy. The analyses presented in this paper are based on the contributions from localized molecular orbitais within the polarization propagator approach (CLOPPA) method and examples are given where factors defining heteroatom lone-pair orientational effects on spin-spin coupling constants are discussed.
TL;DR: In this paper, the MNDO effective charge model in which the solvent effect is introduced through the dielectric constant of solvent was applied to the zwitterionic and neutral forms of glycine, alanine and serine.
Abstract: The MNDO effective charge model in which the solvent effect is introduced through the dielectric constant of solvent was applied to the zwitterionic and neutral forms of glycine, alanine and serine, and the conformational properties of these species in vacuo and in solution were examined. The solvent effect on the conformational potential-energy surfaces described by two rotational angles of C-C and C-N bonds is clarified. The solvation energy of the zwitterionic and neutral forms and the relative stability of these two forms are discussed.
TL;DR: The structural features of benzocyclobutenes were studied using several semi-empirical and ab initio techniques as mentioned in this paper, and it was concluded that the experimental X-ray structure of the perfluoro derivative of the latter compound is seriously in error.
Abstract: The structural features of benzocyclobutenes were studied using several semiempirical and ab initio techniques. Qualitative hybridization arguments and actual 6–31 G Hartree-Fock calculations show conclusively that benzocyclobutenes exhibit a typical Mills-Nixon effect which is most pronounced in benzo[1,2:3,4:5,6]tricyclobutene. It is concluded that the experimental X-ray structure of the perfluoro derivative of the latter compound is seriously in error.
TL;DR: The structures of a variety of nitroalkanes, and nitrocyclopropane, nitroethylene, and Nitrobenzene have been studied using molecular mechanics methods as discussed by the authors.
Abstract: The structures of a variety of nitroalkanes, and nitrocyclopropane, nitroethylene, and nitrobenzene have been studied using molecular mechanics methods. Heats of formation fit well and the vibrational spectra moderately well.
TL;DR: In this paper, energy calculations at the SCF level with full geometry optimization and constraint to C 2v symmetry were done on hydrogen malonate using the MINI-1, 4-31G, 4 -31 (+)G, 6-31 + + + G, 6 −31G ∗∗ and 6 -31 + ∗ ∗ basis sets, and the energy differences between both forms, indicative of the adiabatic potential barrier, are in the region of 2 kcal mol −1 and thus in favour of the asymmetrically placed pro
Abstract: Energy calculations at the SCF level with full geometry optimization and constraint to C 2v symmetry were done on hydrogen malonate using the MINI-1, 4-31G, 4-31 (+)G, 6-31 + + G, 6-31G ∗∗ and 6–31 + + G ∗∗ basis sets. The energy differences between both forms, indicative of the adiabatic potential barrier, are in the region of 2 kcal mol −1 and thus in favour of the asymmetrically placed proton, except for the calculation using the minimal base. Correction for correlation at the MP2 level, applied to the SCF calculated geometries, lowers the potential barrier. Nonadiabatic barriers at the HF level are substantially higher. Lithium ions placed 3 A and 5 A from the carboxyl groups induce a difference of ca. 19 kcal mol −1 between the two proton positions. The potential barrier is raised by the effect of the solvent cavity. This effect stabilizes by 7.75 kcal mol −1 the conformation with one carboxylic group perpendicular to the other relative to one with the intramolecular hydrogen bond.
TL;DR: In this paper, the force fields and dipole-moment derivatives of the L-alanyl dipeptide, CHsCONHCHCHCHs CONHCH, in the C5, Cp, and Cy conformations with intramolecular NH* * *OC hydrogen bonding were derived using the 4-21 Gaussian basis set.
Abstract: We have previously calculated the force fields and dipole-moment derivatives of the L-alanyl dipeptide, CHsCONHCHCHsCONHCH, in the C5, Cp, and Cy conformations with intramolecular NH* * *OC hydrogen bonding. We have now extended these studies with similar ones on four open, non-hydrogen-bonded conformers optimized by Scarsdale et al. - the/&, on, (Y,, and a!’ structures. The force constants were derived using the 4-21 Gaussian basis set and were scaled as before. The present results help to isolate the effects of conformation on the force fields and dipole derivatives, and also provide further insight into the effects of hydrogen bonding by comparison with the previous results. Additional effects can also be seen, such as those of the non-planarity of the peptide group, and the influence on the CO bond when the adjacent NH forms a hydrogen bond, and vice versa.
TL;DR: In this article, a comparative ab initio study of the internal rotations in ethylamine and n-propylamine using a 3-21G(N * ) basis set and fully optimized geometries is presented.
Abstract: A comparative ab initio study of the internal rotations in ethylamine and n-propylamine using a 3-21G(N * ) basis set and fully optimized geometries is presented. For n-propylamine, the NH 2 rotation about the :N-C bond, for a trans skeleton, and the rotations about the C1-C2 bond, for trans and gauche orientations of the nitrogen lone pair, are considered.
TL;DR: In this paper, the structures of the cis and trans conformations of fluoroacetaldehyde were calculated at the RHF/3-21G, MP2/6-31G∗ basis set levels.
Abstract: The structures of the cis (the fluorine atom and the aldehydic hydrogen eclipse one another) and trans conformations of fluoroacetaldehyde were calculated at the RHF/3-21G, RHF/6-31G∗ and MP2/6-31G∗ basis set levels. In each case, the cis form was determined to be more stable than the trans conformer and the energy difference was calculated to be 1193, 871 and 591 cm−1 at the RHF/3-21G, RHF/6-31G∗ and MP2/6-31G∗ basis set levels, respectively. Full optimization was also performed at the cis/trans transitional state and the cis to trans barrier was calculated to be 2284, 1992 and 1701 cm−1 at the RHF/3-21G, RHF/6-31G∗ and MP2/6-31G∗ basis set levels, respectively. The structural parameters of both the cis and trans conformations, barriers to internal rotation, conformational stability, and calculated vibrational frequencies are compared with the corresponding quantities of some similar molecules.
TL;DR: In this article, the molecular structures of valine and threonine were studied by ab initio gradient optimization at the 4-21G level, and conformational effects associated with the χ1 [C(α)-C(β)] torsion were determined in valine.
Abstract: The molecular structures of valine and threonine were studied by ab initio gradient optimization at the 4-21G level. The conformational effects associated with the χ1 [C(α)-C(β)] torsion were determined in valine. Conformationally dependent changes of several hundredths of an angstrom were found for bond distances, and up to 5 ° for bond angles, and the χ1 potential was found to be asymmetric. Ten structures were optimized for threonine, displaying different types of hydrogen bonding, including polycyclic networks established by hydrogen bonds in some forms. Quantitative differences in bond distances and angles are given for diastereoisomers of threonine which differ in chirality at the β-carbon.
TL;DR: In this article, high level ab initio studies have been carried out on hydroxy-, dihydroxy- and methylhydroxy carbene and two conformers were found in the electronic ground state.
Abstract: High level ab initio studies have been carried out on hydroxy-, dihydroxy- and methylhydroxycarbene. For hydroxy- and methylhydroxy-carbene two conformers were found in the electronic ground state. For dihydroxycarbene, calculations predict three singlet conformers. Reliable results for the triplet species were found to require very high-level calculations. For hydroxy- and methylhydroxy-carbene the T 1 surface crosses the S o surface and for dihydroxycarbene the S o – T 1 gap is at least 100 kJ mol −1 at the MP2/6-31G** level. Carefully optimized geometries of the carbene species are given, as well as their potential-energy surfaces. The ab initio calculated vibrational spectra and infrared intensities of the singlet conformers are also given.
TL;DR: In this paper, semi-empirical AM1, MNDO-PM3 and MP n/4-31G (n = 2-4) calculations were performed on the hydrogen-bonded intermediate complexes and the transition state for the unassisted addition of one water molecule to formaldehyde in order to assess the similarity of the three models in the study of chemical reactions.
Abstract: Semiempirical AM1 and MNDO-PM3 as well as MP n /4-31G ( n =2–4) calculations were done on the hydrogen-bonded intermediate complexes and the transition state for the unassisted addition of one water molecule to formaldehyde in order to assess the similarity of the three models in the study of chemical reactions. The errors inherent in the low-level ab initio calculation were estimated both by extension of the basis set and by correction of the basis-set-superposition error (BSSE). Basis-set extension was performed by using MP n /6-31G( d , p ) calculations. BSSE was corrected both at the Hartree-Fock and the post-Hartree-Fock level using the counterpoise method and accounting for deformation of the fragments. Drawbacks of the semiempirical methods are reported and analysed in relation to the ab initio results.
TL;DR: In this paper, AM1, 4-21G, 6-31G and 6 -31G * calculations were carried out on tetrahedrane, its four azaderivatives, cyclobutadiene and its six azadervatives.
Abstract: AM1, 4–21G, 6–31G and 6–31G * calculations were carried out on tetrahedrane, its four azaderivatives, cyclobutadiene and its six azaderivatives. Examination of the effect on the geometries of the type of calculation leads to the conclusion that the 6-31G basis set is the most convenient method of those studied. Heats of formation are discussed using different isodesmic equations. Additive models have been found that relate different properties of these molecules.
TL;DR: In this article, the numerical evaluation of two-centre nuclear-attraction integrals with exponentially declining basis functions is discussed, and some new sequence transformations are derived recently which allow a more economical and more reliable evaluation of some of the most troublesome series expansions.
Abstract: The numerical evaluation of some molecular integrals with exponentially declining basis functions is discussed. Compact expressions containing only a finite number of terms can be derived for such two-centre nuclear-attraction and Coulomb integrals. These expressions allow a very economical evaluation of these two-centre integrals. However, these expressions cannot be used if the two centres are close together because they contain terms which then become singular. In such a situation, it is necessary to use alternative infinite-series representations which do not contain singular terms. Unfortunately, these infinite series often converge so slowly that an evaluation by simply adding up their terms would not work. Consequently, alternative techniques must be used for the evaluation of these series. The use of nonlinear sequence transformations for the acceleration of the convergence of these series has been described previously. Some new sequence transformations were derived recently which allow a more economical and more reliable evaluation of some of the most troublesome series expansions. Some further tests and their comparisons are discussed.