An ab initio investigation of zinc chloro complexes

A series of geometry, frequency, and energy calculations of chloroaquazinc(II) complexes were carried out at up to the MP2/6-31+G* level. A thorough examination of all species up to and including hexacoordinate species, and with up to six chlorides, was carried out. The structures of the complexes are compared with experimental data where available. The solution chemistry of zinc(II) in the presence of chloride is discussed, and Raman spectra of zinc perchlorate with increasing amount of chloride are presented.     

Possible gas-phase ion pairs in amine-HCl complexes. An ab initio theoretical study

Ab initio MO calculations were performed for complexes between HCl and NH₃, CH₃NH₂, (CH₃)₂NH and (CH₃) ₃N. SCF geometry optimization for the latter three complexes gives double-minimum potential surfaces, which become single- minimum surfaces when electron correlation is considered. It is proposed that (CH₃)₃NHCl may be an ion pair in the gas phase.     

An ab initio study of the reaction of ammonium ion with hydride ion and lithium hydride

Both the abstraction and substitution mechanisms for the reaction of NH⁺₄ with H^? and the abstraction mechanisms for the reaction with LiH in the gas phase have been investigated by theoretical methods. LiH results to be a better reagent and reactions with and without scrambling are competitive in accordance with experimental findings.     

An ab initio study of the hydration of alkylammonium groups

Ab initio STO 3G calculations of the electronic structure and interaction energies with water of methyl and ethylammonium ions are reported. It is shown that the calculations predict a preferential attack at the -group (relative to the nitrogen), in agreement with experimental facts, and that successive ethylation reduces the favorable energy of hydration.Further calculations show that due to the large positive charges on the hydrogens, direct C–H... O hydrogen bond formation is possible. Thus a water molecule forms such a hydrogen bond of - 10kcal/mole in NMe ₄ ⁺ ; of –9kcal/mole at an position in the ethylated ions and of –6 kcal/mole at a position. The a preference noted above is thus preserved. It seems unlikely that such interactions significantly change the overall hydration described above. Although twelve sites for H-bonding exist in NMe ₄ ⁺ , they approach each other rather closely and after addition of four water molecules further water molecules introduce such intermolecular repulsions as to reduce rather than increase the total hydration energy which remains always slightly lower than that obtained with water in the axial type of site described above [14].     

An ab initio study of halogen—olefin molecular complexes

A systematic computational study, using the 3–21G basis set, of the F₂/CH₂H₄ system shows that several molecular complex configurations can exist and that the axial-perpendicular type is not the most stable. Full geometry optimization results for F₂/C₂H₄ and Cl₂/C₂H₄ in axial-perpendicular configurations are reported and shown to be improvements over previous calculations.     

An ab initio CI investigation of structure and bonding in LiC2H2 complexes

The structures and energies of various LiC₂H₂ complexes have been investigated by means of ab initio molecular orbital calculations. Analytic SCF gradients were employed with a double-ζ basis set to locate and characterize stationary points on the energy surface. Single-point CI calculations using a double-ζ + diffuse and polarization basis set have been carried out at the DZ + P SCF stationary points. With the highest-level theory, the Li—vinylidene complex and the cis bridged adduct are found to be the most favorable arrangements, the former complex being slightly more stable by about 2 kcal mol⁻¹. These molecules are bound respectively by about 5 and 3 kcal mole⁻¹ relative to infinitely separated lithium plus acetylene. Harmonic vibrational frequencies are also reported and confirm the existence of the cis LiC₂H₂ species recently observed in a solid argon matrix.     

Genistein complexes with amines. Part II: ab initio study of the complexes with piperazine and triethylamine

The piperazine and triethylamine complexes of genistein, exhibiting high immunosuppressant activity, were ab initio modeled at RHF/6-31G^** level and results were compared with those obtained for genistein–morpholine complexes by X-ray, NMR, and theoretical methods. The most stable genistein–piperazine complex is formed due to hydrogen bonding of genistein's OH group at position C7 to piperazine's nitrogen atom. In the most stable genistein–triethylamine complex genistein's OH group at position C4′ (position para to phenyl substituent) and trimethylamine nitrogen atom are engaged in hydrogen bond formation. The calculations confirmed our previous NMR conclusion that piperazine is more strongly complexed by genistein than is morpholine or triethylamine. The theoretical ¹³C NMR spectra correlate fairly well with the experimental spectra.     

Gold adatoms and clusters on PPV: An ab initio investigation

We have performed an ab initio investigation of the energetic, structural, electronic, and vibrational properties of Au atoms and clusters adsorbed on poly-p-phenylene vinylene (PPV) chains, Au(n)/PPV (with n =?1, 2, 6, 7, 10, and 12). We find that the Au(n)/PPV systems are energetically stable by 0.5 eV, compared with the isolated systems, viz., PPV chain and Au(n) clusters, thus supporting the formation of Au(n)/PPV nanocomposites. Further support to the formation of Au(n)/PPV has been provided by examining the vibrational properties of pristine PPV and Au(n)/PPV systems. In agreement with experimental measurements, we find a reduction on the in-plane vibrational frequency of C-C bonds of Au(n)/PPV, when compared with the same vibrational modes of pristine PPV. The electronic properties of isolated Au(n) clusters are modified when adsorbed on PPV. The highest occupied states of Au(n)/PPV are mostly concentrated on the Au(n) cluster, while the lowest unoccupied states are mainly localized along the PPV chain. The HOMO-LUMO energy gap of the Au(n)/PPV systems are smaller than the energy gap of the isolated systems, Au(n) clusters, and pristime PPV chains.     

An ab initio and molecular mechanical investigation of ureas and amide derivatives

Ab initio molecular orbital theory with the 6-31G* basis set has been used to investigate the geometries and preferred conformations for urea, derivatives of urea, and a few complicated amide derivatives. The results from the ab initio calculations provide insight into the gas-phase rotational barrier about the C? N bond and have been used to generate parameters for the MM2(87) molecular mechanics program. When applicable, theoretical structures are compared with corresponding previously reported experimental geometries. Urea is predicted to be nonplanar with pyramidal amino groups.     

Spin‐orbit ab initio and density functional theory investigation of bismuth monoboronyl,BiBO

The molecular properties of bismuth monoboronyl, BiBO, were investigated using high‐level ab initio and density functional theory calculations by including the effect of spin‐orbit coupling (SOC). SOC does not cause any change in the Bi? B bond length of BiBO, by contrast it causes significant elongation of the Bi? B bond of BiBO^?, by ～0.03 Å. The Bi? B bond length of BiBO^? that is calculated by considering SOC is almost identical to that of BiBO; this result is consistent with a recent experimental study. The term values of excited states of BiBO calculated by including SOC are in good agreement with the experimental results. One excited state which was not assigned in the previous experimental study is the Ω = 0⁺ state generated by strong SOC. In the theoretical calculations on molecules containing 6p‐block elements, including SOC is crucial for obtaining results that are consistent with the corresponding experimental results.     

The α-helix as an ion channel. An ab initio molecular orbital study

Minimal basis set ab initio SCF MO expectation values of the electrostatic potential of an octa-alanine α-helix are reported. The energy profile for a proton moving through the interior of the helix is given. It is suggested that the internal electric field of a helix may be essential in biological ion pumps.     

Temperature‐dependent conformational analysis of cyclononane: An ab initio study

Quantum chemical methods were used for the theoretical determination of the conformational population for the relevant conformers of cyclononane, i.e., TBC, TCB, TCC, and M4 (or C1), which have been previously investigated experimentally through detailed examination of the nuclear magnetic resonance (NMR) spectrum. Our best Gibbs free energy result, evaluated with MP4(SDTQ)/6‐31G(d,p)//MP2/6‐31G(d,p) energy differences and MP2/6‐31G(d,p) thermal corrections, lead to a temperature‐dependent population in excellent agreement with the experimental results based on the analysis of the low temperature ¹³C NMR spectrum. The nice agreement with experiment is achieved using MP2 harmonic frequencies for the evaluation of vibration partition functions within the standard statistic thermodynamics formalism. Theoretical temperature‐dependent infrared (IR) and ¹³C NMR spectra were simulated and compared with experimental data, which confirmed the ab initio conformational population reported here. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Spin-orbit ab initio investigation of the photolysis of bromoiodomethane.

The photodissociation of bromoiodomethane has been investigated by spin-orbit ab initio calculations. The experimentally observed A- and B-bands and the corresponding photoproducts were assigned by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit interaction through complete active space state interaction potential energy curves, vertical excitation energies, and oscillator strengths of low-lying excited states. The present conclusions with respect to the dissociation process in the B-band are different compared with those of previous studies. The reaction between the iso-CH(2)Br-I and iso-CH(2)I-Br species has also been studied. Finally, a set of stable excited states was identified for both isomers. These species might be of importance in the recombination process that follows the photodissociation in a solvent.     

Activation barriers and rate constants for hydration of platinum and palladium square-planar complexes: an ab initio study

In the present work, an ab initio study on hydration (a metal-ligand replacement by water molecule or OH- group) of cis- and transplatin and their palladium analogs was performed within a neutral pseudomolecule approach (e.g., metal-complex+water as reactant complex). Subsequent replacement of the second ligand was considered. Optimizations were performed at the MP2/6-31+G(d) level with single-point energy evaluation using the CCSD(T)/6-31++G(d,p) approach. For the obtained structures of reactants, transition states (TS's), and products, both thermodynamic (reaction energies and Gibbs energies) and kinetic (rate constants) characteristics were estimated. It was found that all the hydration processes are mildly endothermic reactions-in the first step they require 8.7 and 10.2 kcal/mol for ammonium and chloride replacement in cisplatin and 13.8 and 17.8 kcal/mol in the transplatin case, respectively. Corresponding energies for cispalladium amount to 5.2 and 9.8 kcal/mol, and 11.0 and 17.7 kcal/mol for transpalladium. Based on vibrational analyses at MP2/6-31+G(d) level, transition state theory rate constants were computed for all the hydration reactions. A qualitative agreement between the predicted and known experimental data was achieved. It was also found that the close similarities in reaction thermodynamics of both Pd(II) and Pt(II) complexes (average difference for all the hydration reactions are approximately 1.8 kcal/mol) do not correspond to the TS characteristics. The TS energies for examined Pd(II) complexes are about 9.7 kcal/mol lower in comparison with the Pt analogs. This leads to 10(6) times faster reaction course in the Pd cases. This is by 1 or 2 orders of magnitude more than the results based on experimental measurements.     

Blue‐shifted H‐bond in aromatic sulfines: An ab initio calculation

The intramolecular C? H···O?S H‐bond in the aromatic sulfines, HRC?S?O, was analyzed by NBO and QTAIM methods. The results of QTAIM analysis at the MP2/aug‐cc‐pVDZ level of theory show that the C? H···O?S H‐bond meets all the characteristics of an improper, blue shift hydrogen bond. NBO analysis at the MP2/6–31++G(d,p)//MP2/aug‐cc‐pVDZ level predicts a normal relationship between change of bond length and C? H rehybridization. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007