Pyrrolizidine alkaloids necine bases: Ab initio,semiempirical, and molecular mechanics approaches to molecular properties

The structural stabilities of endo and exo conformations of retronecine and heliotridine molecules were analyzed using different ab initio, semiempirical, and molecular mechanics methods. All electron and pseudopotential ab initio calculations at the Hartree-Fock level of theory with 6-31G* and CEP-31G* basis sets provided structures in excellent agreement with available experimental results obtained from X-ray crystal structure and ¹H-NMR (nuclear magnetic resonance) studies in D₂O solutions. The exo conformations showed a greater stability for both molecules. The most significant difference between the calculations was found in the ring planarity of heliotridine, whose distortion was associated with the interaction between the O(11)H group and the C(1)-C(2) double bond as well as with a hydrogen bond between O(11)H and N(4). The discrepancy between pseudopotential and all-electron optimized geometries was reduced after inclusion of the innermost electrons of C(1), C(2), and N(4) in the core potential calculation. The MNDO, AM1, and PM3 semiempirical results showed poor agreement with experimental data. The five-membered rings were observed to be planar for AM1 and MNDO calculations. The PM3 calculations for exo-retronecine showed a greater stability than the endo conformer, in agreement with ab initio results. A good agreement was observed between MM3 and ab initio geometries, with small differences probably due to hydrogen bonds. While exo-retronecine was calculated to be more stable than the endo conformer, the MM3 calculations suggested that endo-heliotridine was slightly more stable than the exo form. © 1996 by John Wiley & Sons, Inc.     

Cation influence on the structure and electron density of water in some Men+·H2O complexes

The cation influence on the water molecule in the Li⁺·H₂O, Be²⁺·H₂O, Mg²⁺·H₂O and A1³⁺·H₂O complexes has been studied by means of quantum-mechanical ab initio calculations. A number of general trends are noted. (1) The calculated equilibrium water O-H distances increase with increasing binding energies, i.e. in the order Li⁺, Mg²⁺, Be²⁺, Al³⁺. The H-O-H angles differ by about ±1 ° from the calculated equilibrium angle for the free H₂O molecule; the variation has no systematic trend. (2) The electron density redistribution accompanying the change in the internal H₂O geometry in these complexes is considerably smaller than the redistribution brought about by the direct influence of the external field. (3) The harmonic O-H stretching force constant decreases with increased cation-water bonding. (4) The qualitative features of the density changes are very similar for the four complexes. The magnitudes of the interactions follow the relation Li⁺ < Mg²⁺ < Be²⁺ Al³⁺. An increased polarization of the H₂O molecule occurs with electron migration from the H atoms towards the O atom and an accumulation of electron charge approximately at the centre of the Meⁿ⁺—O bond, especially in Be²⁺·H₂O and A1³⁺·H₂O. An electron deficiency is found in the lone-pair region.     

Ab initio valence-bond calculations of H2O

The first and second bond dissociation energies for H₂O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H₂O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H₂O at this geometry was ?76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was ?75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H₂O → OH + H, D₁=75.38 kcal/mole and OH → O+H, D₂=54.79 kcal/mole.     

Anab initio study of the molecules P2O and P2O+

Summary This paper presents a detailedab initio study of the molecules P₂O and P₂O⁺ at the Hartree-Fock, and Multi-Reference Single and Double Excitation Configuration Interaction levels. An analysis of the geometries and relative stabilities of both molecules is presented, together with a discussion of the dissociation channels and of the electronic spectrum of linear P₂O. The results for the ionic species P₂O⁺ suggest a cyclic geometry for this molecule, as indicated by the calculated vibrational frequencies. The calculations also indicate a surface crossing at a relatively low energy which might lead to P₂O dissociation and may hence be one of the factors contributing to the failure to detect it at room temperatures.     

Al2O3/K2O-containing non-stoichiometric lithium disilicate-based glasses

The crystallisation kinetics of experimental glasses in 3 different systems: (A) Li₂O–SiO₂, (B) Li₂O–Al₂O₃–SiO₂ and (C) Li₂O–K₂O–Al₂O₃–SiO₂ were studied under non-isothermal conditions. The DTA results revealed a stronger tendency to crystallisation of binary compositions in comparison to the ternary and quaternary compositions comprising Al₂O₃ and K₂O which present the lower crystallisation, i.e. the crystallisation propensity follows the trend A > B > C. The devitrification process in the Li₂O–SiO₂ and Li₂O–Al₂O₃–SiO₂ systems began earlier and the rate was higher in comparison to that of glasses in the quaternary Li₂O–K₂O–Al₂O₃–SiO₂ system. Thus, addition of Al₂O₃ and K₂O to glasses of Li₂O–SiO₂ system was demonstrated to promote glass stability against crystallisation. However, the activation energy for crystallisation was shown to depend also on the SiO₂/Li₂O ratio with the binary system showing a decreasing trend with increasing SiO₂/Li₂O ratio, while the opposite tendency was being observed for compositions with added Al₂O₃ and K₂O.     

Zur quantenchemischen Behandlung von Kation-Amid-Solvatkomplexen

A comparison ofab initio calculations employing different basis sets with corresponding CNDO/2 results for the Li⁺/HCONH₂ complex shows that these methods lead to completely different energy surfaces for this system. Reduction of the basis set, even to the minimal size, does not bring about serious changes in the results of theab initio calculations, whereas in the semiempirical treatment some methodical errors seem to occur. When using however, theab initio minimum geometry the CNDO/2 calculations also give a qualitatively correct picture for the influence of the cation on the amide modecule.     

Threshold photoionization behaviour of (Li2O)Lin clusters produced by a laser vaporization source

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify Li_kO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of Li_kO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O^2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li₂O)Li_n clusters, clear shell closure effects are present at n = 8 and n = 20.     

Adsorption behavior of Co and C2H2 on the graphite basal surface: A quantum chemistry study

The adsorption of CO and C₂H₂ molecules on the perfect basal surface of graphite is investigated by adopting cluster models in conjunction with quantum chemical calculations. The noncovalent interaction potential energy curves for three different orientations of CO and C₂H₂ molecules with respect to the inert basal plane of graphite are calculated via semi-empirical and Möller-Plesset ab initio methods. Then, we have considered the effects of interaction energies on the C≡O and C≡C bond lengths by performing the partial geometry optimization procedure on the CO-graphite and C₂H₂-graphite systems in various intermolecular distances. The computational analysis of all physical noncovalent potential energy curves reveals that the relative configurations in which CO and C₂H₂ molecules approach the graphite sheet from out of the plane have stronger interaction energy and so is more favorable from the energetic viewpoint. This means that the graphite layer prefers to increase its thickness via the chemical vapor deposition of CO and C₂H₂ on the graphite.     

Ab initio crystal orbital studies on linear chains of H atoms

Anab initio crystal orbital method is used to calculate the energies of an infinite chain of H atoms and of linear arrangements of H₂ molecules with different interatomic distances. The H₂ arrangements are not stable in respect to isolated molecules. The cohesive energy of an optimized arrangement of H atoms chain is 0.0354 a.u.     

Inhomogeneous electron liquid in the free-space building block Li2C2 plus its dimer and trimer

Quantum-chemical calculations are first reported on the building block Li₂C₂. Two low-lying isomers are predicted, and hence the equilibrium ground-state geometry by the coupled cluster method (CCSD). The ground-state is non-linear, contrasting with the other planar isomer. The dimer (Li₂C₂)₂ is then examined by the Hartree–Fock plus second-order Møller–Plesset (HF+MP2) method. The ground-state geometry is predicted to have C₄ on a linear chain. As to the trimer, the C₆ atoms are predicted at the same ab initio level to lie on a hexagon. But the Li atoms prefer each to bond to two C atoms, rather than reproduce the geometry of benzene. Finally, some contact is made with a recent density functional theory study of solid lithium carbide under pressure.     

Ab initio SCF MO study of hydrogen bonds between H2S and H2O molecules

The hydrogen bonds between H₂S and H₂O molecules are calculated through anab initio, LCAO MO SCF method using a Gaussian type orbital double-zeta basis set. The capacity of the H₂S molecule to act as an electron acceptor is confirmed. Consultant of the Instituto Mexicano del Petróleo.     

Study of hydrogen bonding interactions relevant to biomolecular structure and function

Ab initio molecular orbital calculations were used to study hydrogen bonding interactions and interatomic distances of a number of hydrogen bonded complexes that are germane to biomolecular structure and function. The calculations were carried out at the STO-3G, 3-21G, 6-31G*, and MP2/6-31G* levels (geometries were fully optimized at each level). For anionic species, 6-31 + G* and MP2/6-31 + G* were also used. In some cases, more sophisticated calculations were also carried out. Whenever possible, the corresponding enthalpy, entropy, and free energy of complexation were calculated. The agreement with the limited quantity of experimental data is good. For comparison, we also carried out semiempirical molecular orbital calculations. In general, AM1 and PM3 give lower interaction enthalpies than the best ab initio results. With regard to structural results, AM1 tends to favor bifurcated structures for O? H-O and N? HO types of hydrogen bonds, but not for hydrogen bonds involving O-H? S and S-H? O, where the usual hydrogen bond patterns are observed. Overall, AM1 geometries are in general in poor agreement with ab initio structural results. On the other hand, PM3 gives geometries similar to the ab initio ones. Hence, from the structural point of view PM3 does show some improvement over AM1. Finally, insights into the formation of cyclic or open formate–water hydrogen bonded complexes are presented. © 1992 by John Wiley & Sons, Inc.     

Interaction of O+ ion with water molecules: A quantum-chemical study

The interaction of O⁺ ion with several (from one to four) water molecules was studied by theab initio (UMP4/4-31G^*) and semiempirical (AM1) quantum-chemical methods. It was found that the energy of binding the O⁺ ion to the first water molecule is appreciably higher than those of binding to the subsequent water molecules. In the complex with a water molecule, whose structure corresponds to that of water oxide, the O⁺ ion retains high reactivity. The barrier to the transfer of O⁺ ion to another water molecule is much lower than the barrier to analogous transfer of O atom from the molecule of water oxide, despite the lower dissociation energy of the H₂O−O bond. Consideration of subsequent interactions with water molecules leads to an increase in the barrier to the transfer of O⁺ ion. The doublet and quadruplet excited states of the O⁺+2 H₂O system were also studied. In these cases, the formation energies are well described by the ion-dipole model. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 981–988, June, 2000.     

Theoretical studies of uracil–(H2O)n (n = 1–7) clusters by ab initio and ABEEMσπ/MM fluctuating charge model

Uracil–(H₂O)_n (n = 1–7) clusters were systemically investigated by ab initio methods and the newly constructed ABEEMσπ/MM fluctuating charge model. Water molecules have been gradually placed in an average plane containing uracil. The geometries of 38 uracil–water complexes were obtained using B3LYP/6-311++G** level optimizations, and the energies were determined at the MP2/6-311++G** level with BSSE corrections. The ABEEMσπ/MM potential model gives reasonable properties of these clusters when comparing with the present ab initio data. For interaction energies, the root mean square deviation is 0.96 kcal/mol, and the linear coefficient reaches 0.997. Furthermore, the ABEEMσπ charges changed when H₂O interacted with the uracil molecule, especially at the sites where the hydrogen bond form. These results show that the ABEEMσπ/MM model is fine giving the overall characteristic hydration properties of uracil–water systems in good agreement with the high-level ab initio calculations.     

Theoretical study of three-body nonadditive interactions for the H2S-(H2O)2 system

An ab initio SCF MO study of the hydrogen-bonded system formed by an H₂S molecule interacting with two water molecules is presented. The nonadditive contributions to the interaction energies are obtained using the 4-31G basis sets which tend to overestimate the dipole moments of the H₂S and H₂O molecules. Thus possibly too large interactions, including upper-limit values for the nonadditive three-body contributions, were obtained. The nonadditive corrections were found to be comparable in magnitude to the cases of other first-row hydride trimers. On this basis one can infer that they probably do not affect the proton and deuterium exchange in hydrated H₂S (as occurs in the G-S heavy water enrichment process) and that they do not play an important role in the formation of clathrates involving hydrogen sulphide.     

A fullyab initio potential energy surface for ClH2 reactive system

Anab initio analytical potential energy surface called BW3 for the ClH₂ reactive system is presented. The fit of this surface is based on about 1 200ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H₂ and HCl, and the endothermicity of the Cl + H₂ abstraction reaction. For the Cl + H₂ abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

Wellenmechanische Absolutrechnungen an Molekülen und Atomsystemen mit der SCF?MO?LC(LCGO) Methode. VIII. Das System (Li2H)+

The (Li₂H)⁺ has been investigated ab initio in the linear configuration, with the H atom in the middle of the system, for five different distances R_LiH, taking all six electrons into account, using the Allgemeines Programmsystem/SCF? MO? LC(LCGO) Verfahren. A bond distance R_LiH of 3.14 a.u., a total energy of ?15.289 a.u., and an ionization energy of 15.1 eV were found. Comparing the results of SCF investigations, the formation energy of (Li₂H)⁺ from LiH and Li⁺ was computed to be 59.7 kcal/mole (2.58 eV). Using the energy curve near the minimum, a force constant for the symmetric vibration of k = 0.13777 × 10⁶ dyn/cm and a frequency ω = 577.9 cm^?1 were found.     

Improvement of the hydrogen bonding correction to MNDO for calculations of biochemical interest

Recent suggestions for correcting H? Acceptor interactions within MNDO, together with results of crystallographic analysis, were used to modify this SCF semiempirical calculation for multiple hydrogen bonded associations. Thermodynamic profiles for model systems of biochemical interest such as H₂O? H₂O, hydration of neutral and charged molecules, dimerizations and proton transfers show the advantages of this method. Its treatment of charges, bonding, geometries, energetics and vibrational frequencies is shown to be comparable to ab initio calculations with various basis sets. However, basic MNDO deficiencies and criteria applied for H-bonding result in some too high barriers for proton transfers.     

Theoretical studies of O2−:(H2O)n clusters

The interaction of superoxide ion O₂^? with up to four water molecules [O₂^?: (H₂O)_n, n = 1, 2, 4] has been investigated using ab initio molecular orbital theory. The binding energy of O₂^?: H₂O is calculated to be ?20.6 kcal/mol in good agreement with gas phase experimental data. At the MP3/6-31G^* level the O₂^?:H₂O complex has a C_2v structure with a double (cyclic) hydrogen bond between O₂^? and H₂O. A C_s structure with a single hydrogen bond is only 0.7 kcal/mol less stable. Interaction of H₂O with the doubly occupied π^* orbital of O₂^? is preferred slightly over interaction with the singly occupied π^* orbital. Natural bond orbital analysis suggests that both electrostatic and charge transfer interactions are important in anionic complexes. The charge transfer occurs predominantly in the O₂^? → H₂O direction and is important in determining the relative stabilities of the different structures and states. Singly and doubly hydrogen-bonded structures for the O₂^?: (H₂O)₂ and O₂^?: (H₂O)₄ clusters were found to be similar in stability and the increase in binding of the cluster becomes smaller as each additional water molecule is added to the cluster.