An ab initio study of the stability and electronic structure of univalent magnesium salts

Large basis and well-correlated ab initio electronic structure calculations have been performed on the simple Mg(I) salts, Mg₂F₂ and Mg₂Cl₂. The electron withdrawing power of the halogens gives rise to significant metal-metal bonding and consequently these as yet unobserved, univalent salts are calculated to be 10–12 kcal more stable than if the second magnesium atom were absent from the molecule. With this stability, these species provide an energetically more accessible Mg atom for subsequent reaction than does solid magnesium where the atomization energy is 35 kcal. Thus, formal univalency of magnesium affords a route to activated magnesium.     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

An ab initio study of the structure of the 2-chloroethyl radical

Minimal and split-valence shell basis set calculations, both with and without d orbitais, predict the radical centre to be pyramidal, with the planar radical only 0.3 kcal mol^?1 higher. The barrier to internal rotation is 2 kcal mol^?1. There is no evidence of bridging from chlorine.     

An ab initio LCAO-MO-SCF study of the electronic structure of phosphirane and thiirane

Wave functions have been determined for the C₂H₄PH and C₂H₄S cyclic molecules, using (951/52/3) and (95/52/3) uncontracted Gaussian basis sets for each molecule. From Mulliken population analyses and electron-density plots, it is shown that the valence orbitals of C₂H₄PH and C₂H₄S are closely related and that these are similar to the respective orbitals of cyclopropane.

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Zusammenfassung Für die zyklischen Moleküle C₂H₄PH und C₂H₄S wurden mit den nichtkontrahierten Basissätzen ((951/52/3) und (95/52/3)) von Gaußfunktionen Wellenfunktionen bestimmt. Die Mullikenschen Populationsanalysen sowie Diagramme der Elektronendichte zeigen, daß die Valenzorbitale von C₂H₄PH und C₂H₄S in enger Beziehung stehen und daß diese den entsprechenden Orbitalen des Cyclopropans ähnlich sind.

     

Enthalpy of formation of the cyclopentadienyl radical: photoacoustic calorimetry and ab initio studies

The gas-phase C-H bond dissociation enthalpy (BDE) in 1,3-cyclopentadiene has been determined by time-resolved photoacoustic calorimetry (TR-PAC) as 358 +/- 7 kJ mol(-1). Theoretical results from ab initio complete basis-set approaches, including the composite CBS-Q and CBS-QB3 procedures, and basis-set extrapolated coupled-cluster calculations (CCSD(T)) are reported. The CCSD(T) prediction for the C-H BDE of 1,3-cyclopentadiene (353.3 kJ mol(-1)) is in good agreement with the TR-PAC result. On the basis of the experimental and the theoretical values obtained, we recommend 355 +/- 8 kJ mol(-1) for the C-H BDE of 1,3-cyclopentadiene and 271 +/- 8 kJ mol(-1) for the enthalpy of formation of cyclopentadienyl radical.     

The 1:1 glycine zwitterion-water complex: An ab initio electronic structure study

More than a dozen stationary points on the potential energy surface for the 1:1 glycine zwitterion—water complex have been investigated at Hartree-Fock or MP2 levels of theory with basis sets ranging from split valence (4-31G) to split valence plus polarization and diffuse function (6–31 + + G**) quality. Only one true minimum (GLYZWM, C₁ symmetry) could be located on the potential energy surface. GLYZWM features a bridged water molecule acting as both a hydrogen bond acceptor and donor with the NH₃⁻ and CO₂⁻ units of the glycine zwitterion. The total hydrogen bond energy in GLYZWM is computed as 16 kcal/mol (MP2/6–31 ++ G** // 6–31 ++ G**, including corrections for basis set superpositions errors). The computed vibrational frequencies and normal mode forms of the GLYZWM complex resemble in many cases experimental assignments made for the glycine zwitterion in bulk water on the basis of Raman spectroscopy. © 1996 by John Wiley & Sons, Inc.     

An ab initio study of electronic structure and spectra of 8-bromoguanine: a comparative study with guanine.

Ground state geometries of the four tautomeric forms keto-N9H, keto-N7H, enol-N9H and enol-N7H of 8-bromoguanine (8BG) were optimized using the ab initio RHF procedure employing a mixed basis set consisting of the 6-311 + G* basis set for the nitrogen atom of the amino group and the bromine atom, and the 4-31G basis set for all other atoms. These calculations were followed by correlation correction of the total energy at the MP2 level using the same basis set. The different tautomeric forms of 8BG in the ground state were solvated using the isodensity surface polarized continuum model (IPCM) of the SCRF theory both at the RHF and MP2 levels. Excited states were generated employing configuration interaction among singly excited configurations (CIS) obtained using a limited window of filled and empty molecular orbitals. Formation of hydrogen-bonded complexes between 8BG and three water molecules in the ground and excited states was considered in order to account for solvent effects approximately. Excited state geometry was optimized in each case for the lowest singlet excited state which was found to be of pi-pi* type. Vibrational frequency analysis was performed in order to ensure that the stationary points located on the potential energy surfaces by geometry optimization were minima. It is found that 8BG would occur in the ground state dominantly in the keto-N7H form both at the aqueous solution-air interface and inside the bulk liquid. The observed absorption and fluorescence spectra of 8BG can be explained satisfactorily considering only the keto-N7H form of the molecule. The enol tautomers of 8BG do not appear to be important from the point of view of ground state properties or electronic spectra. The observed differences between the behaviors of guanine and 8BG can be easily explained on the basis of the results obtained.     

An ab initio study of the molecular structure and vibration-rotation spectrum of the triplet radical HCCN

The bent triplet cyanocarbene H-C-CN and the linear triplet allene H-C=C=N have been studied by the CASSCF and CI methods, using a DZP basis. Relaxation of all geometrical parameters for the CASSCF energy results in a bent molecule with CCH angle 133° and a barrier to linearity of 6.4 kcal/mol, which was lowered to 2.3 kcal/mol in a subsequent CI calculation. The Davidson correction lowered it further to 1.8 kcal/mol.A 26-term analytical potential energy surface (PES) was fitted to CASSCF, CI, and Davidson corrected CI energies in 94 different geometries. Using these three potentials, the semi-rigid bender model predicts a CCH bending frequency of 782, 505, and 503 cm^–1, resp., which compares favourably with an experimentally observed IR transition line at 458 cm^–1. For the deuterated species, the corresponding frequencies are 610, 407, and 402 cm^–1, to be compared with two possible absorption lines at 405 and 317.5 cm^–1.The PES was then parametrized by adding a variable CCH angle dependence, and a comprehensive vibration-rotation spectrum was calculated variationally, using the exact 4-atom vibration-rotation kinetic Hamiltonian, for a range of barrier heights. Comparison with experiment indicates a barrier in the range 1±0.5 kcal/mol.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

Al2F2分子结构与稳定性的ab initio计算研究

用abinitio方法在UMP2/6-311G(d)水平下计算了Al2F2分子可能的异构体构型和AlF二聚化成Al2F2分子的反应能。UMP2/6-311G(d)和UQCISD(T)/6-311+G(3df)//UMP2/6-311G(d)水平下的能量值均说明具有D2h对称性、1^Ag电子态的异构是Al2F2分子的最稳定构型,其Al-F键长为0.19074nm,键角Al-F-Al和F-Al-F分别为104.62°和75.38°,以及两个强振动,441.27cm^-1和401.93cm^-1,均与实验结果相符合。电子结构分析表明,具有D2h对称性的异构体的活性中心在2个Al原子上,在形成衍生物时是主要的反应加成位置。在UMP2/6-311G(d)和UQCISD(T)6-311+G(3df)//UMP2/6-311G(d)水平下得到了AlF二聚化能量分别为-75.01kJ/mol和-66.07kJ/mol,与文献估计算基本一致,说明AlF二聚化反应能量上是有利的。     

Model of peptide bond-aromatic ring interaction: correlated ab initio quantum chemical study

Aromatic ring-peptide bond interactions (modeled as benzene and formamide, N-methylformamide and N-methylacetamide) are studied by means of advanced computational chemistry methods: second-order M?ller-Plesset (MP2), coupled-cluster single and double excitation model [CCSD(T)], and density functional theory with dispersion (DFT-D). The geometrical preferences of these interactions as well as their interaction energy content, in both parallel and T-shaped arrangements, are investigated. The stabilization energy reaches a value of over 5 kcal/mol for the N-methylformamide-benzene complex at the CCSD(T)/complete basis set (CBS) level. Decomposition of interaction energy by the DFT-symmetry-adapted perturbation treatment (SAPT) technique shows that the parallel and T-shaped arrangements, although similar in their total interaction energies, differ significantly in the proportion of electrostatic and dispersion terms.     

Electronic structure of NH+: An ab initio study

We report ab initio self‐consistent field MRSD‐CI electronic structure calculations of the NH⁺ cation. A basis set of DZ + POL quality augmented with Rydberg and bond functions was employed together with an extensive treatment of electron correlation. More than 50 electronic states of NH⁺ are reported, including doublets, quartets, and sextets. Leading configurations, vertical ionization energies of NH, vertical excitation energies of NH⁺, and potential energy curves are reported. Spectroscopic properties calculated for the known bound electronic states of NH⁺ are found in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

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 (B₂O₃) molecule an ab initio molecular orbital study employing a minimal STO-3G basis set with complete geometry optimization is reported. Our results indicate that B₂O₃ 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 B₂O₃ is discussed by means of Walsh diagrams and Mulliken population analysis.     

Comparative study of the electronic structure of pregnanolones by ab initio theory

Pregnanedione (5β‐pregnane, 3,20‐dione), pregnanolone (3β‐hydroxy‐5β‐pregnan‐20‐one), and epipregnanolone (3α‐hydroxy‐5β‐pregnan‐20‐one) result from the 5β‐reduction of progesterone [4‐pregnene, 3‐20‐dione (P)]. These P metabolites induce anesthesia and smooth muscle relaxation (nongenomic actions). In the present study, geometries and electronic structure of these steroids were assessed by ab initio calculations using the 6‐31G* basis set. Consequently, bond distances, valence angles, and dihedral angles were measured. In addition total energy, frontier orbitals, i.e., highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), dipole moment, and electrostatic potentials were calculated. Total energy was higher for P, followed by pregnanedione. Pregnanolones, the hydroxylated progestins, showed the lower energies. Concerning frontier orbitals, P showed the highest HOMO energy and the lowest LUMO energy. Pregnanedione showed lower HOMO and LUMO energy values than pregnanolone and epipregnanolone. P showed both HOMO and LUMO located at the A ring, including the π bond at C4, C5, and the carbonyl at C3. The HOMO in pregnanedione was included mostly in the A ring and the C3 carbonyl group, while the LUMO was shared by the carbonyl groups at C3 and C20. The frontier orbitals of pregnanolone and epipregnanolone were quite similar. The HOMO in both steroids included the B, C, and D rings and the carbonyl at C20. The LUMO was also similar in both pregnanolones including mostly the carbonyl at C20. The dipole moment was shorter for P and pregnanedione and directed toward the acetyl side chain at C17. Pregnanolone and epipregnanolone showed the dipole moment vector larger and directed toward the A ring. The electrostatic potentials were related mostly with the lone pairs of electrons from the oxygens. By the total energy and frontier orbitals energies of the hormones studied, it is concluded that the metabolism of progesterone toward its 5β‐reduced metabolites might be rationalized from the theoretical chemistry point of view. Besides, the importance of the A/B ring cis configuration, dipole moment, and electrostatic potential are highlighted as possible improving elements of molecular interactions to explain the nongenomic biological action of 5β‐reduced progestins. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 433–440, 1999     

Torsion potentials and electronic structure of trifluoromethoxy- and trifluoromethylthiobenzene: an ab initio study

Potential functions of internal rotation about the C_sp²X bonds in molecules C₆H₅XCF₃ (X=O, S) were calculated at the second-order Møller-Plesset perturbation level of theory with 6-31G(d) basis set. The profile of the potential function and the rotation barrier (ΔE^#=3.0 kJ/mol) found for C₆H₅OCF₃ suggest that, depending on experimental conditions, there can be either free rotation about the C_sp²O bond or the conformational equilibrium is shifted to the side of the orthogonal form. The rotational barrier for C₆H₅SCF₃ is 14.7 kJ/mol and the molecule exists in the stable orthogonal conformation. The nature of hybridization, energy and population of lone electron pairs (LPs) on the oxygen and sulfur atoms were considered by using the Natural Bond Orbital (NBO) method. The energy of interactions of the LPs with antibonding π^∗-orbitals of the aromatic moiety were estimated for different conformations. The distribution of electron density in the molecules was discussed. The results were compared with analogous calculations on the molecules C₆H₅XCH₃.     

Molecular and electronic structure of disiloxane,an ab initio MO study

The bond lengths and angels obtained by means of a 4–31G basis agree with electron diffraction data. The calculated SiOSi bending potential, showing a minimum for the linear arrangement, is discussed with regard to available experimental information. Calculated dipole moments and ionization potentials are also in reasonable agreement with experimental data. Comparison is made with STO-3G and INDO results which both overestimate the stability of cyclic structures.     

Electronic structure of the cysteine thiyl radical: a DFT and correlated ab initio study

The electronic structure and the unusual EPR parameters of sulfur-centered alkyl thiyl radical from cysteine are investigated by density functional theory (DFT) and correlated ab initio calculations. Three geometry-optimized, staggered conformations of the radical are found that lie within 630 cm(-1) in energy. The EPR g-values are sensitive to the energy difference between the nearly-degenerate singly occupied orbital and one of the lone-pair orbitals (excitation energies of 1732, 1083, and 3429 cm(-1) from Multireference Configuration Interaction calculations for the structures corresponding to the three minima), both of which are almost pure sulfur 3p orbitals. Because of the near degeneracy, the second order correction to the g tensor, which is widely used to analyze g-values of paramagnetic systems, is insufficient to obtain accurate g-values of the cysteine thiyl radical. Instead, an expression for the g tensor must be used in which third order corrections are taken into account. The near-degeneracy can be affected to roughly equal extents by changes in the structure of the radical and by hydrogen bonds to the sulfur. The magnitude of the hyperfine coupling constants for the beta protons of the cysteine thiyl radical is found to depend on the structure of the radical. On the basis of a detailed comparison between experimental and calculated g-values and hyperfine coupling constants an attempt is made to identify the structure of thiyl radicals and the number of hydrogen bonds to the sulfur.     

An ab initio study of the electronic spectrum of dichlorocarbene CCl2

The lower-lying electronic states of dichlorocarbene have been studied by ab initio methods at different levels of accuracy. For the ¹B₁ ← ¹A₁ transition, the calculated transition energy (1.95 eV) is in good agreement with the gas-phase value (2.10 eV) of Predmore, Murray and Harmony. The discrepancy with the previous CI study by Ha. Gremlich and Bühler is commented on. The vibrational frequencies of CCl₂ in both ¹A₁ and ¹B₁ states were calculated and compared with experiment. The first four triplet and four cationic states have also been examined.     

An ab initio analysis of the electronic structure and harmonic frequencies of nickel porphyrin

Ab initio calculations are performed to understand the geometry, electronic structure, and vibrational frequencies of nickel porphyrin (NiP). Hartree-Fock (HF) and second-order perturbation (MP2) theories are applied with polarized basis sets. The calculated geometrical parameters are in very good agreement with the crystal structure determination. The electronic structure and bonding are analyzed in terms of complexation and correlation effects. Not unexpectedly, the HF depiction of the metal-porphyrin interaction is rather ionic while ligand σ donation is dominant at the MP2 level. Scaled HF frequencies of NiP and its isotopomers are in very good agreement with observed infrared and resonance Raman data. Received: 7 January 1997 / Accepted: 6 May 1997     

An ab initio MO calculation of the electronic structure and geometry of protonated methanol

Protonated methanol, CH₃ OH₂⁺, has been studied using the LCAO—MO—SCF method with a 7, 3 and 9, 5, 1 Gaussian orbital basis set on the heavy atoms and 4s on hydrogen. It is found that the ground state is non-planar around oxygen, in contrast with previous calculations, with an inversion barrier of 3 kcal mol^?1. The changes in electron distribution in the reacting systemCH₃⁺ + H₂O → CF₃OH₂⁺is also examined.     

An ab initio study of the low-lying electronic states of S3

Accurate calculations of the low-lying singlet and triplet electronic states of thiozone, S(3), have been carried out using large multireference configuration interaction wave functions. Cuts of the full potential energy surfaces along the stretching and bending coordinates have been presented, together with the vertical excitation spectra. The strong experimentally observed absorption around 395 nm is assigned to the 1 (1)B(2) state, which correlates to ground state products. Absorption at wavelengths shorter than 260 nm is predicted to lead to singlet excited state products, S(2) (a (1)Delta(g))+S((1)D). The spectroscopic properties of the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) electronic states of the S(2) radical have also been accurately characterized in this work. The investigations of the low-lying electronic states were accompanied by accurate ground state coupled cluster calculations of the thermochemistry of both S(2) and S(3) using large correlation consistent basis sets with corrections for core-valence correlation, scalar relativity, and atomic spin-orbit effects. Resulting values for D(0)(S(2)+S) and SigmaD(0) for S(3) are predicted to be 61.3 and 162.7 kcal/mol, respectively, with conservative uncertainties of +/-1 kcal/mol. Analogous calculations predict the C(2v)-D(3h) (open-cyclic) isomerization energy of S(3) to be 4.4+/-0.5 kcal/mol.