Hydrogen Bonded Complexes of Acetylacetone and Methanol: HF and DFT level Study

Several possible hydrogen-bonded complexes between the tautomeric forms of acetylacetone and methanol were studied by ab initio methods using 6-311G** and D95** basis functions at the HF and DFT (B3LYP) levels of theory. The calculations were carried out for isolated molecules and solvent assisted complexes by means of the isodensity polarized model (IPCM). The theoretical frequencies were compared with the experimental IR spectrum of an equimolar mixture of acetylacetone and methanol. It was proved that the most stable H-bonded complex acetylacetone–methanol is formed between O-bonded methanol- and the enol molecule.     

Electron and Geometry Structure of Hydrogen-Bonded Complexes of Guanine with One Molecule Methanol. A DFT Level Study

Summary. The geometries, harmonic vibrational frequencies, and energies of eight hydrogen-bonded complexes of guanine with one molecule methanol are computed using the DFT (B3LYP) method together with the 6-31+G* basis functions. In the investigation two stable tautomers of guanine (oxo-amino N9H and oxo-amino N7H) were chosen. They were included in a variety of H-bonded complexes with one molecule methanol. In order to investigate the nature of the intermolecular bonds, the bonding energies and thermodynamic properties of the complexes were calculated.     

Homoleptic Lanthanide Complexes of Chelating Phosphanamides—An Experimental and Theoretical Study

Four chelating ligands are present in the first phosphanamide complexes of Group 3 metals and the lanthanides (see structure shown). However, these ligands coordinate to form a distorted molecular structure. The compounds were characterized by single-crystal X-ray structure analysis and quantum-mechanical investigations with density functional theory and MP2 methods.     

A Conformational Study on Silacyclohexane. Comparison of ab initio (HF,MP2), DFT,and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C₁ symmetry (twist‐C₁), two boat forms with Si in a gunnel position (C₁ symmetry), and one twist form with C₂ symmetry (twist‐C₂). All methods calculate the relative energy to increase in the order chair < twist‐C₂ < twist‐C₁ < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol^–1 for the twist‐C₂, twist‐C₁, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol^–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (C_s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under C_s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C₁ transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.     

Hydrogen Bonding Properties of the Complexes of Formaldehyde and its Derivatives with HF and HCl

Summary.  The complexes of formaldehyde and some of its derivatives with HF and HCl were investigated at HF/6-311 + +G^** and MP2/6-311 + +G^** levels of theory. Interaction energies were corrected for the basis set superposition error (BSSE). The full optimizations of dimers and monomers were performed during calculations. The Bader theory of atoms-in-molecules (AIM) was also applied for the localization of bond critical points (BCP) and for the calculation the electron densities and their Laplacians at these points. The relationships between H-bond energy and parameters obtained from calculations were also studied. Received June 29, 2001. Accepted (revised) October 29, 2001     

Predicting Stable Molecular Structures for (RNC)2AuIX Complexes

Calculations have been performed at the MP2 and DFT levels for investigating the reasons for the difficulties in synthesizing bis(isocyanide)gold(I) halide complexes. Three‐coordinated gold(I) complexes of the type (R₃P)₂Au^IX ( 1 ) can be synthesized, whereas the analogous isocyanide complexes (RNC)₂Au^IX ( 2 ) are not experimentally known. The molecular structures of (R₃P)₂Au^IX (X = Cl, Br, and I) and (RNC)₂Au^IX with X = halide, cyanide, nitrite, methylthiolate, and thiocyanate are compared and structural differences are discussed. Calculations of molecular properties elucidate which factors determine the strength of the gold‐ligand interactions in (RNC)₂Au^IX. The linear bonding mode of RNC favors a T‐shaped geometry instead of the planar Y‐shaped trigonal structure of (R₃P)₂Au^IX complexes that have been synthesized. An increased polarity of the Au–X bond in 2 leads to destabilization of the Y‐shaped structure. Chalcogen‐containing ligands or cyanide appear to be good X‐ligand candidates for synthesis of (RNC)₂Au^IX complexes.     

DFT Calculations of the Molecular Force Field of Vanadyl Nitrate,VO(NO3)3

A structural and vibrational theoretical study for vanadyl nitrate was carried out. The Density Functional Theory (DFT) has been used to study vibrational properties. The structures were fully optimized at the B3LYP/6‐31G*, B3LYP/6‐311G*, and B3LYP/6‐311+G* levels of theory and the harmonic vibrational frequencies were evaluated at the same level. The calculated harmonic vibrational frequencies for vanadyl nitrate are consistent with their experimental IR and Raman spectra in gas and liquid phases. Through these calculations a precise knowledge of the normal modes of vibration was obtained, considering the coordination mode adopted by the nitrate group in the mirror plane as monodentate and bidentate. A total assignment of the observed bands in the vibrational spectra for vanadyl nitrate is proposed in this work. The nature of the V–O and V ← O bonds in the compound was systematically and quantitatively investigated by means of the Natural Bond Order (NBO) analysis. The topological properties of the electronic charge density were analyzed employing Bader's Atoms in Molecules theory (AIM).     

DFT study of the dissociative adsorption of HF on an AlN nanotube

We have investigated the adsorption of hydrogen fluoride (HF) on the AlN nanotube surface using density functional theory in terms of energetic, structural and electronic properties. By overcoming energy barriers of 27.90–52.30 kcal/mol, HF molecule is dissociated into H and F species on the tube surface and its molecular structure is not preserved after the adsorption process. Dissociation energies have been calculated to be −52.57 and −70.10 kcal/mol. The process has negligible effect on the electronic and field emission properties of the AlN nanotube. This process may increase the solubility of AlN nanotubes.     

Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Ab initio and density functional theory calculations of molecular structure and vibrational spectrum of ethyl azidoacetate

Here we report ab initio and density functional results for molecular properties of ethyl azidoacetate (N₃CH₂COOC₂H₅) and for the corresponding singly ionized structure (N₃CH₂COOC₂H₅⁺). Ab initio ionization energies based on Koopmans’ theorem are in excellent agreement with the experimental data from ultraviolet photoelectron spectroscopy. DFT adiabatic energy differences between neutral and ionized structures are very sensitive to electronic correlation effects and are not in very good agreement with experiment. The results for the structure and vibrational frequencies are compared with the experimental data of related molecular structures.     

A Systematic Quantum Chemistry Study on Cyclodextrins

Summary.  AM1 and PM3 modeling of β-hydroxyethyl ether and α-(1→4)-glucobiose indicated that PM3 is advantageous to AM1 in cyclodextrin (CD) chemistry. The conclusion was supported by direct structure optimization of α- and β-CD with AM1 and PM3, in which AM1 gave badly distorted geometries due to unreasonable hydrogen bonding, whereas PM3 reproduced the crystalline structures rather well. Ab initio calculation was for the first time performed on CD, demonstrating the feasibility of this method for future studies concerning CD chemistry. The results also provided valuable insights into the driving forces in CD molecular recognition. Received January 7, 2000. Accepted (revised) March 22, 2000     

A DFT study of the [4+2] cycloadditions of conjugated ketenes (vinylketene, imidoylketene and formylketene) with formaldimine. The pericyclic or pseudopericyclic character from magnetic properties

A comprehensive B3LYP/6-31+G^∗ study of the nature of the [4+2] cycloadditions of conjugated ketenes, vinylketene, imidoylketene and formylketene, with formaldimine was conducted. For each reaction, the complete pathway was determined and changes in different magnetic properties (magnetic susceptibility, χ, magnetic susceptibility anisotropy, χ_anis, and the nucleus-independent chemical shift, NICS) were monitored along the reaction profile with a view to estimate the aromatization associated to the process. We have also applied the ACID (anisotropy of the current-induced density) method with the same purpose. The deep analysis of the results indicates the existence of both disrotatory and conrotatory pericyclic paths for the cyclization step of the cycloaddition of vinylketene with formaldimine and the pseudopericyclic character of reactions with imidoylketene and formylketene.     

High level of ab initio and density functional theory computational study of structural and energetic properties of tetrafluorhydrazine rotamers

The HF, MP2, MP3, MP4, and QCISD ab initio methods were compared with local, hybrid, and gradient-corrected density functional theory (DFT) methods for computing structures and energies of N2F4 rotamers. In all DFT calculations 6-311 + G(2d) basis set was used. The generated structures energies of trans- and gauche-N₂F₄ rotamers, and their dissociation energies to nitrogen difluoride were compared with experimental data. Suitable hybrid and gradient-corrected DFT methods for determining structures and energies for these and similar molecular systems were discussed.     

Ammonium Ion Complexes of Tetraethyl Resorcarene: An Ab Initio Study

The complexation of various ammonium ions with a resorcarene host was evaluated by ab initio calculations. The approximations of the binding locations and the interaction energies for each guest are reported. The supramolecular complex formation also affects the conformation of the resorcarene host.     

p-Benzyne

A noble gas matrix at low temperature was used to investigate the photochemical behavior of diacetyl terephthaloyl diperoxide and dipropionyl terephthaloyl diperoxide as well as 1,4-diiodobenzene. All three photoreactions formed small quantities of a compound with IR absorption bands at 725 and 980 cm⁻¹, which disappeared when the matrix was annealed. These bands correspond to the most intense of the calculated bands (B3LYP) for 1,4-didehydrobenzene (p-benzyne) ( 1 ). That decomposition of the peroxides in fact leads to 1 is confirmed by vapor-phase pyrolysis experiments in which (Z)- 2 was obtained in high yield.     

Theoretical DFT and experimental Raman and NMR studies on thiophene, 3-methylthiophene and selenophene

The results of extended MO calculations using density functional theory (DFT) approximation supported by experimental Raman, ¹H and ¹³C NMR studies on thiophene are reported. Raman spectra of liquid thiophene were re-examined and the performance of a hybrid B3PW91 density functional was compared with the ab initio restricted Hartree–Fock (RHF) method. With the basis sets of the 6-311++G^** quality, the DFT calculated bond lengths, dipole moments and harmonic vibrations were predicted in a very good agreement with available experimental data.

Additionally, the results on thiophene were extended by calculations on 3-methylthiophene and selenophene. In this case, a significant change in geometry and charge distribution in thiophene ring due to a methyl group substituent or replacement of sulphur by selene atom was observed.

A linear correlation between the predicted harmonic vibrational frequencies (scaled using SQM method) and experimental ones for thiophene, selenophene and 3-methylthiophene was shown. The theoretically calculated spectra have satisfactorily reproduced the available experimental spectra for thiophene and selenophene.     

Matrix-isolation FT-IR study and theoretical calculations of the vibrational, tautomeric and H-bonding properties of hypoxanthine

The neutral compound hypoxanthine is investigated using the technique of matrix-isolation FT-IR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3-LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9-r1) are the three most stable forms. The experimental FT-IR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the H-bonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect.     

Theoretical study of the walk rearrangement in perfluorotetramethyl (Dewar thiophene) exo-S-oxide

A theoretical study of the ‘walk’ rearrangement in bicyclo[2.1.0]pentene and perfluorotetramethyl (Dewar thiophene) exo-S-oxide has been carried out. Despite the differences between them, the results for both reactions show an enhancement of aromaticity in the transition state, which is consistent with a pericyclic behavior. NBO calculations show that the small activation energy for the second reaction can be interpreted in terms of a strong stabilization of the transition state by the exo-oxide substituent. So, the mechanism proposed in the past should be revised.     

A DFT Investigation on Hydrogen Adsorption Based on Alkali-metal Organic Complexes

Using density functional theory calculation based on the B3LYP method,we have studied the interactions of H2 molecules with alkali-metal organic complexes C6H6-nLin(n = 1～3),C6H5Na and C6H5K.A significant part of the electronic charge of M s orbital(Li 2s,Na 3s,K 4s) is donated to phenyl and is accommodated by H2 bonding orbital.For all the complexes considered,each bonded alkali-metal atom can adsorb up to five H2 in molecular form with the mean binding energy of 0.59,0.55 and 0.56 eV/H2 molecule for C6H6-nLin(n = 1～3),C6H5Na and C6H5K,respectively.The kinetic stability of these hydrogen-covered organometallic complexes is discussed in terms of energy gap between HOMO and LUMO.It is remarkable that these alkali-metal organic complexes can store up to 23.80 wt% hydrogen.Therefore,the complexes studied may be used as hydrogen storage materials.