Minimal STO-3G Research Articles

A theoretical study of the interaction of N2 with water molecules. (H2O)n:N2, n=1–8.

A b initio molecular orbital calculations including correlation energy have been carried out on the interaction of a single H2O molecule with N2. The potential energy surface for H2O:N2 is found to have a minimum corresponding to a HOH ⋅⋅⋅ N2 structure with a weak (<2 kcal mol−1) hydrogen bond. A second, less stable, configuration corresponding to a H2O ⋅⋅⋅ N2 structure with N2 bonded side on to the oxygen of H2O was found to be either a minimum or a saddle point in the potential energy surface depending on the level of calculation. The minimal STO-3G basis set was used to investigate the interaction of up to eight H2O molecules with N2. Two types of clusters, one containing only HOH ⋅⋅⋅ N2 interactions and the other containing both HOH ⋅⋅⋅N2 and H2O ⋅⋅⋅ N2 interactions, were investigated for [N2:(H2O)n, n=2–8].

Rotational barriers and stable rotamers in 1,3-butadiene, acrolein and glyoxal

Analytic gradient (Force) methods (ref.1) have been used to optimize completely the geometries of the title compounds at the minima (stable rotamers) and maxima (transition states, TS) in the torsional potential energy curves (TPEC) for rotation about the single C  C bonds. Computations were performed with the minimal STO-3G, split-valence 3–21G and polarized 6–31GX basis sets for 1,3-butadiene and glyoxal, and with only the STO-3G and 3–21G basis sets for acrolein. Whereas complete geometry optimization of 1,3-butadiene along the TPEC with the STO-3G basis led to the prediction of a cis - trans rotamer equilibrium (ref.2), complete optimizations with both the 3–21G and 6–31GX basis sets give a maximum at the cis position and minima at the trans and gauche conformations. Complete geometry optimization of acrolein reveals only cis and trans minima with the TS at 88.2 (STO-3G) or 91.6(3–21G) degrees from the cis. Surprisingly the cis and trans conformers are predicted to be almost equivalent in energy. For glyoxal the TS is revealed to be nearer to the cis than to the trans conformation. Except for 1,3-butadiene, the barrier height to rotation from the trans position, obtained with the STO-3G basis, is much smaller than that obtained with the larger basis sets.

Electron densities and resonance interactions in substituted benzenes, ethylenes and acetylenes

Molecular orbital calculations at the ab initio STO-3G, 4-31G and 6-31G* level have been used to investigate electron populations in monosubstituted benzenes, ethylenes and acetylenes. It is shown that the π-electron transfers to or from the substituent in ethylenes are accurately proportional to those in the corresponding substituted benzenes using either the STO-3G or 4-31G basis sets. However, figures at the split-valence basis levels are not proportional, in either series, to those at the minimal basis STO-3G level. The values for a series of substituted ethylenes and acetylenes are used to investigate the validity of the σ R 0 and σ R 0 (twist) series of substituent constants.

The structure of aminopropenenitrile (CH 2C(NH 2)CN)

Ab initio molecular orbital calculations with minimal STO—3G, split-valence 3-21G, and split-valence plus d-polarization 6-31G* basis sets have been used to obtain optimized structures for aminopropenenitrile. Systematic deficiencies in the geometry predictions with these basis sets have been taken into account with the aid of calculations on the smaller systems, vinylamine and propenenitrile. A complete r o structure for aminopropenenitrile is predicted which yields rotational constants in good agreement with experimental values. Aminopropenenitrile is found to be non-planar with a pyramidal amino group.

The formate anion: An ab initio study of its equilibrium structure, general harmonic force field and fundamental vibrational frequencies

The general harmonic force field (GHFF) of the formate anion has been calculated by ab initio restricted Hartree—Fock theory using the minimal STO-3G and split valence 4–31G basis sets. This work was carried out in conjunction with a previously reported experimental study in which the ion's force field was determined from its vibrational frequencies in polycrystalline sodium formate by an improved refinement procedure. The ab initio results reported here proved valuable in that study as a guide to the true physical values of the harmonic force constants. The 4–31G basis gives better results than the STO-3G basis. The agreement between the 4–31G and the experimental results suggests that interactions in the crystal are of secondary importance in determining the experimental force field. However, there are discrepancies between the 4–31G values and the experimental values for the force constants which exceed the empirical guidelines for such calculations on neutral molecules. These deviations may be real and due to the different environments of the ion in the two studies or they may be due to experimental and theoretical errors. Conjugative delocalization of an oxygen lone pair in the molecular plane into an antibonding CH bond orbital provides a simple and consistent explanation for the weakness of CH bonds in HCOO − and related species. The inductive effect cannot account for this overall weakness, but it can account for the increase in CH bond strength from H 2CO and HCOO − to HCOOH and HCOOCH 3 and for its decrease from H 2CO to CH 3CHO.

Theoretical studies of C 2H 4O isomers : Part 1. Acetaldehyde ground, triplet n → π* and triplet n → 3 s electronic states

The ground, triplet n → π* and triplet Rydberg n → 3 s states of acetaldehyde have been studied at both the SCF and CI levels. The minimal STO-3G, split valence 3-21G and polarized 6-31G* basis sets have been used in conjunction with both spin unrestricted (UHF) and restricted (RHF) wavefunctions. The CI calculations included all single and double excitations from the SCF reference configuration, excluding three core and the six highest-lying virtual orbitals, using the 3-21G basis set. The Rydberg calculations used the 3-21G basis, augmented by a diffuse s function, plus CI. The agreement between theory and experiment is very good where corresponding data exist. The ground-state methyl rotation barrier is calculated to be 4.3 kJ mol −1 at the 6-31G*/SCF level and 5.2 kJ mol −1 at the 3-21G/CI level, with the syn conformer lowest. The triplet n → π* state is pyramidal, and lies 264 kJ mol −1 above the S 0 state while the vertical excitation energy is 362 kJ mol −1. The main effect of either an RHF versus a UHF wavefunction, a larger basis, or CI, is to increase barrier and excitation energies somewhat. The CO bond length is reduced below the experimentally determined value at the 6-31G* level. The preferred Rydberg state conformation is syn, in agreement with the recent experimental result. The methyl rotation barrier is computed to be 9.0 kJ mol −1 and the excitation energy 595 kJ mol −1 (602 kJ mol −1 vertical).

Theoretical studies of C 2H 4O isomers : Part 1. Acetaldehyde ground, triplet [formula omitted] and triplet n → 3 s electronic states

The ground, triplet n → π ∗ and triplet Rydberg n → 3 s states of acetaldehyde have been studied at both the SCF and CI levels. The minimal STO-3G, split valence 3-21G and polarized 6-31G ∗ basis sets have been used in conjunction with both spin unrestricted (UHF) and restricted (RHF) wavefunctions. The CI calculations included all single and double excitations from the SCF reference configuration, excluding three core and the six highest-lying virtual orbitals, using the 3-21G basis set. The Rydberg calculations used the 3-21G basis, augmented by a diffuse s function, plus CI. The agreement between theory and experiment is very good where corresponding data exist. The ground-state methyl rotation barrier is calculated to be 4.3 kJ mol −1 at the 6-31G ∗/SCF level and 5.2 kJ mol −1 at the 3–21G/CI level, with the syn conformer lowest. The triplet n → π ∗ state is pyramidal, and lies 264 kJ mol −1 above the S 0 state while the vertical excitation energy is 362 kJ mol −1 The main effect of either an RHF versus a UHF wavefunction, a larger basis, or CI, is to increase barrier and excitation energies somewhat. The CO bond length is reduced below the experimentally determined value at the 6-31G ∗ level. The preferred Rydberg state conformation is syn, in agreement with the recent experimental result. The methyl rotation barrier is computed to be 9.0 kJ mol −1 and the excitation energy 595 kJ mol −1 (602 kJ mol −1 vertical).

IR-induced photorotamerization of 2-aminoethanol in low-temperature matrices. AB initio optimized geometries of conformers

The νOH and πOH regions of 2-aminoethanol have been studied in low-temperature matrices. In the vapour phase and in Ar, Kr, Xe and N2 matrices this compound exists mostly as an intramolecularly hydrogen bonded OH…N conformer (g′Gg′). On IR irradiation in matrices this conformer converts to NH…O “bonded” conformers. Photorotamerization occurs most efficiently after absorption of radiation corresponding to the νOH frequency. Radiation in the near-IR region has also been found to induce the same conformer changes. The IR-induced rotamerization is reversible, since selective irradiation at the νOH frequencies of the NH…O “bonded” conformers produces the intramolecularly hydrogen bonded OH…N conformer g′Gg′. The reverse reaction is 10–15 times slower than the forward reaction. A rapid temperature and irradiation dependent subprocess has been found in the nitrogen matrix. The conformers have been assigned with the aid of results obtained from ab initio calculations (minimal STO-3G basis) of fully optimized geometries of all the conformers. The possible photorotamerization paths are discussed with the aid of the calculated potential energy surface. On the basis of the calculations a trans-angle rule is presented for primary alcohols and amines.

A theoretical investigation on the role of solvent in solvolytic reactions

The dissociation curve of fluoromethane in hydrofluoric acid was calculated by anab initio SCF MO technique using the minimal STO-3G basis set. The model of the solvent includes eleven HF molecules, simulating the first solvation shell. The position of these molecules was borrowed from a previous semiempirical calculation. A preliminaryab initio study of solvation of CH 3 + and F+ in H2O and HF justifies this assumption.

The structure of vinylamine

Theoretical structures for vinylamine have been obtained by ab initio molecular orbital calculations, using the minimal STO—3G, split-valence 3—21G, and split-valence plus d-polarization 6—31G* basis sets. The theoretical structures were corrected for systematic deficiencies in the basis sets and by use of experimental rotational constants to predict a complete r 0 structure for vinylamine. Vinylamine is predicted to be non-planar with a pyramidal amino group.

Ab initio study of rotational isomerism in vinylcyclopropane

The geometry of vinylcyclopropane has been completely optimized at each critical point by analytic gradient (force) methods at the minimal STO-3G and the split-valence 3-21G basis set levels. The geometries obtained for the various critical points have been used to generate potential energy curves for vinyl group rotation within the rigid rotor approximation. Comparison of these curves clearly demonstrates the importance of complete geometry optimization. The potential energy curve for vinyl group rotation, generated with the s-trans STO-3G optimized geometry, predicts secondary gauche minima which are an artifact of the rigid rotor approximation. With complete geometry optimization along the curve, the STO- 3G basis set computations predict only s-trans and s-cis minima. In contrast, the complete optimizations with the 3-21G basis set, in agreement with experiment, predict a three-fold rotational contour with two equivalent gauche minima. These minima lie 6.86 kJ mol −1 above th e s-trans minimum. The computed barrier to rotation for the s-trans → gauche interconversion is 13.3 kJ mol −1. The electric dipole moment computed with the 3-21G basis for the s-trans 3-21G optimized geometry is 0.446 D or about 10% less than the experimental value.

Structures of C 3H 6O species in the triplet state: Methyloxirane, propanal, acetone, methyl vinyl ether and propenol

The geometries of thirteen triplet C 3H 6O species, plus methyloxirane and propene in the ground state, have been completely optimized by analytic gradient (force) techniques at both the minimal STO-3G and split valence 3-21G basis set levels. Single calculations using the 6-31G* polarized basis set were performed for each 3-21G optimum structure. With the exception of bond angles at oxygen and the pyramidality at carbon radical centers, the geometries obtained with the two basis sets differ little. Except for the propene +O( 3 P) atom, the addition of d orbitals has little effect on the relative energies compared to the 3-21G energy differences (Δ E), but both were markedly different from the STO-3G results. Whereas the latter Δ E values covered a range of almost 40 kcal mol −1, this range was reduced to 24.5 and 22.4 kcal mol −1 at the 3-21G and 6-31G −* levels, respectively.

ChemInform Abstract: THE GEOMETRIC AND ELECTRONIC STRUCTURE OF C6O64‐. SPIN MULTIPLICITY OF THE GROUND STATE

Abstract Ab initio molecular orbital theory with the minimal STO—3G basis set has been used to examine the geometric and electronic structure of C 6 O 6 4− . Two-determinant wavefunctions were used to provide a qualitatively adequate description of some of the electronic states. Under D 6h Symmetry the ground state is a triplet. However, Jahn—Teller distortion to D 2h structures leads to singlet states with energy lower than the D 6h triplet, i.e. the ground state of C 6 O 6 4− is predicted to be a D 2h singlet. This is consistent with the observed diamagnetism of K 4 C 6 O 6 and with its infrared spectrum.

Ab initio molecular orbital study of dimer CHF 3

Various dimer configurations of fluoroform have been studied by means of the GAUSSIAN-70 algorithm using the minimal STO-3G basis set. The largest dimerization energy, 0.68 kcal mol , occurs in an end-to-end linear geometry with a H...F distance of 2.3 Å and the electron donor molecule tilted at 45° with respect to the C-H...F axis.

The geometric and electronic structure of C 6O 64−. Spin multiplicity of the ground state

Ab initio molecular orbital theory with the minimal STO—3G basis set has been used to examine the geometric and electronic structure of C 6O 6 4−. Two-determinant wavefunctions were used to provide a qualitatively adequate description of some of the electronic states. Under D 6h Symmetry the ground state is a triplet. However, Jahn—Teller distortion to D 2h structures leads to singlet states with energy lower than the D 6h triplet, i.e. the ground state of C 6O 6 4− is predicted to be a D 2h singlet. This is consistent with the observed diamagnetism of K 4C 6O 6 and with its infrared spectrum.

Ground state structures of sulfur dicyanide and its cation

A b initoio LCAO-MO SCF calculation on S(CN) and S(CN)2t have been carried out, using minimal STO-3G basis sets. The calculations predict a bent SCN linkage for S(CN)2 in its ground electronic state.(AIP)

Cis-trans isomerism in infinite polyacetylenes: an Ab initio study

Ab initio crystal orbital calculations have been performed on several cis-trans isomers of infinite polyacetylenes (polyenes) using a minimal STO-3G basis set. Bond distances and bond angles have been optimized to obtain the equilibrium structure of these polymers. The following relative order of stabilities was obtained (energies are given in kcal mol -1 per C 2H 2-unit relative to the most stable all-trans isomer): all-trans: 0.0, cis-transoid: 1.9, trans-cisoid: 2.1, regular trans (bond order bond alternation solution): 5.2, regular cis-trans: 7.2, regular trans (metallic solution): 7.3.

An ab initio reinvestigation of the geometric and electronic structure of boron trioxide

In an attempt to resolve the controversy over the structure of the boron trioxide (B 2O 3) molecule an ab initio molecular orbital study employing a minimal STO-3G basis set with complete geometry optimization is reported. Our results indicate that B 2O 3 is a planar “w” shaped molecule with a rather small inversion barrier around the central atom and a quite important coupling between BOB and OBO angles. The computed bond distances are consistent with previous electron diffraction results, whereas the apex angle is in better agreement with the most recent IR study. The results obtained by the MNDO method are in good agreement with the ab initio ones. The electronic structure of B 2O 3 is discussed by means of Walsh diagrams and Mulliken population analysis.

Molecular structure of the boron(III) oxide molecule in the SCF approximation

The potential surface for the boron(III) oxide (B 2O 3) ground state has been calculated in restricted HF approximation with a minimal STO—3G basis set. The equilibrium geometry has C 2v symmetry; the corresponding structural parameters are as follows: r(O i—B 1)= 1.241 Å; r(B i—O)= 1.341 Å ; ∠B 1QB 2 = 142° and ǒO 1B 1O = 177°. Calculations were also carried out using the SCF-Xα approach for two configurations of B 2O 3 with C 2v and D ∞h symmetry.

The role of internal H-bonding in the conformational preferences of some molecules of the formula CH 3CHYCH 2X

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti. The other H-bonded conformer, where the methyl and amino groups are gauche, is predicted to be ca. 1.2 kcal mol −1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol −1, with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol −1) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol −1should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.