Synthesis and conformational analysis of furfuryl vinyl ethers

Five furfuryl vinyl ethers were synthesized from the corresponding furfuryl alcohols and acetylene at high and atmospheric pressures in superbasic medium KOH-DMSO. On the example of 2-vinyloxymethylfuran and 2-vinyloxymethyltetrahydrofuran, their conformational structures have been studied by quantum chemistry methods, NMR and IR spectroscopy. In particular, from the results of the quantum-chemical calculations, as well as on the basis of experimental measurement and nonempirical calculations of the high level spin-spin coupling constants ¹³C-¹³C and ¹³C-¹H, a conclusion has been drawn on the s-cis-orientation of the vinyl groups with respect to the C_α-O bond in the predominant conformers of 2-vinyloxymethylfuran and 2-vinyloxymethyltetrahydrofuran.     

Quantum and experimental conformational analysis of model molecules related to collagenic structures

Sequential polytripeptides comprising the repeating sequences (Gly-Pro-X) and (Gly-X-Pro) have been synthesized, and conformational analysis has been carried out using both quantum and experimental methods (X = Pro, Ala, Val, Ile R and Ile S, Leu, Nle, Met and Phe).PCILO computations were performed using appropriate model molecules, with a view to discussing the proline puckering, the cis/trans isomerism of Pro residue and the accessibility to different conformons (δ, R_α, L_α, β sheets and β bends) for the X residues, with the pyrrolidine rings free to adopt the most favorable puckering.Experimental studies show that, whatever sequence is considered in the triplet, the only ordered structures found are the obvious δ collagenlike structure and the β bend.Theoretical studies led to the conclusion that when Pro is in the δ conformation, the β bend is very probably of type II.On this basis, the microheterogeneity in collagen structure and the involvement of β bends in the enzymatic proline hydroxylation are discussed.     

Quantum chemical analysis of the structure and spectroscopic properties of aryl vinyl ethers

A series of aromatic vinyl ethers and some compounds close to them in structure are studied by DFT (B3LYP/6-311+G(2d,p)) and MP2(full)/6-311+G(2d,p) methods. Measurements of Raman spectra are also used. The calculation of vibrational spectra of aryl vinyl ether (AVE) isomers shows that stretching vibrations ν(C=C) are most conformation sensitive. The calculated value of I(C=C) for vinyl phenyl ether more than twice exceeds the corresponding value for vinyl methyl ether. The calculated and experimental values of I(C=C) are consistent with the hypothesis about the presence of a common conjugated π-system in the molecules of substituted AVEs. Here the bridging oxygen atom provides the π,p,π-interaction.     

Everyman's derivation of the theory of atoms in molecules

This paper demonstrates that it is straightforward to develop the theory of an atom in a molecule--the extension of quantum mechanics to an open system--by deriving the necessary equations of motion from Schr?dinger's equation, followed by a comparison of the predicted properties with experiment to determine the correct boundary condition. Although less fundamental than the variational derivation of the quantum theory of atoms in molecules, this heuristic approach makes the quantum mechanics of an atom in a molecule accessible to "everyman" possessing a knowledge of Schr?dinger's equation, aiding its general acceptance by experimental chemists.     

Two-electron integrations in the quantum theory of atoms in molecules

A method to compute two-electron integrals over arbitrary regions of space is introduced and particularized to the basins appearing in the quantum theory of atoms in molecules. The procedure generalizes the conventional multipolar approach to account for overlapping densities. We show that the approach is always convergent and computationally efficient, scaling as N(4) in the worst, two-center case. Several numerical results supporting our claims are also presented.     

Electronic interpretation of conformational preferences in benzyl derivatives and related compounds

Quantum Theory of Atoms in Molecules (QTAIM) analysis on B3LYP/6-311++G(2d,2p) 6d electron densities of five benzyl derivatives (C(6)H(5)-CH(2)X; X = F, Cl, OH, SH, NH(2)) and seven related fluorides of furan, pyrrole, and naphthalene indicates that the preference for perpendicular or gauche conformation exhibited by these compounds is related to the diminution of the steric repulsion between the heteroatom at the substituent and the closest hydrogen in the ring. The electron density reorganization can be satisfactorily explained on the basis of these repulsive interactions, while no evidence of larger hyperconjugative delocalization is observed in the preferred conformations.     

Diatomic interaction energies in the topological theory of atoms in molecules

Summary A partitioning of theab initio total energy into one-center and two-center terms is proposed. The partitioning scheme is developed using the auxiliary function (2, 1; 1, 2) = γ(2, 1)γ(1, 2) and the topological theory of atoms in molecules. It is shown that this scheme can be used at theoretical levels beyond Hartree-Fock. The numerical results indicate that the two-center terms follow the experimental trend of the dissociation energies for a series of related compounds.     

Diatomic interaction energies in the topological theory of atoms in molecules

Summary.  A partitioning of the ab initio total energy into one-center and two-center terms is proposed. The partitioning scheme is developed using the auxiliary function L˜(2, 1; 1, 2)=γ(2, 1)γ(1, 2) and the topological theory of atoms in molecules. It is shown that this scheme can be used at theoretical levels beyond Hartree–Fock. The numerical results indicate that the two-center terms follow the experimental trend of the dissociation energies for a series of related compounds. Received March 5, 1996/Final revision received August 19, 1996/Accepted August 29, 1996     

Performance of the density matrix functional theory in the quantum theory of atoms in molecules

The generalization to arbitrary molecular geometries of the energetic partitioning provided by the atomic virial theorem of the quantum theory of atoms in molecules (QTAIM) leads to an exact and chemically intuitive energy partitioning scheme, the interacting quantum atoms (IQA) approach, that depends on the availability of second-order reduced density matrices (2-RDMs). This work explores the performance of this approach in particular and of the QTAIM in general with approximate 2-RDMs obtained from the density matrix functional theory (DMFT), which rests on the natural expansion (natural orbitals and their corresponding occupation numbers) of the first-order reduced density matrix (1-RDM). A number of these functionals have been implemented in the promolden code and used to perform QTAIM and IQA analyses on several representative molecules and model chemical reactions. Total energies, covalent intra- and interbasin exchange-correlation interactions, as well as localization and delocalization indices have been determined with these functionals from 1-RDMs obtained at different levels of theory. Results are compared to the values computed from the exact 2-RDMs, whenever possible.     

Application of the atoms in molecules theory and computational chemistry in mass spectrometry analysis of 1,4-naphthoquinone derivatives

Mass spectrometry analysis of 2-(acylamino)-1,4-naphthoquinone derivatives was carried out using electrospray ionization ion source in combination with tandem mass spectrometry. Protonated molecules were dissociated by application of the collision-induced dissociation (CID), and the protonation sites were suggested on the basis of the HOMO, molecular electrostatic potential map (MEP), proton affinity, and Fukui functions calculated by B3LYP/6-31+G(d,p). The main fragmentation mechanisms undergone by the protonated ions were elucidated on the basis of energy, geometry, and topology analysis of equilibrium geometries. Compounds exhibiting only aliphatic hydrogens at the lateral chain undergo interesting ketene elimination. On the other hand, only the benzoylium ion formation is detected for 2-benzoylamino-1,4-naphthoquinone. The bonds geometric and atoms in molecules parameters give evidence that acidic hydrogen atoms play an important role in the fragmentation pathways.     

Critical analysis and extension of the Hirshfeld atoms in molecules

The computational approach to the Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] atom in a molecule is critically investigated, and several difficulties are highlighted. It is shown that these difficulties are mitigated by an alternative, iterative version, of the Hirshfeld partitioning procedure. The iterative scheme ensures that the Hirshfeld definition represents a mathematically proper information entropy, allows the Hirshfeld approach to be used for charged molecules, eliminates arbitrariness in the choice of the promolecule, and increases the magnitudes of the charges. The resulting "Hirshfeld-I charges" correlate well with electrostatic potential derived atomic charges.     

Revisiting the variational nature of the quantum theory of atoms in molecules

A simplified derivation of the variational nature of the real space basins used in the quantum theory of atoms in molecules (QTAM) is presented. We focus on pointing out the non-standard characteristics of the variational problem that is solved, and on clarifying some points that tend to be misinterpreted. An explicit discussion of the meaning of the functional minimized is also presented and used to derive both a new form for the atomic virial theorem and to show how basin chemical potentials may be defined.     

On the distribution of dye molecules in stretched poly(vinyl alcohol)

The symmetry of the orientational distribution function of dye molecules in stretched PVA films is studied using polarized fluorescence. The different symmetries are monitored by a simple experimental technique in which angle-resolved fluorescence depolarization ratios are measured. A uniaxial distribution of rhodamine 6G molecules is obtained on stretching the film at temperatures above 60°C. In contrast a biaxial distribution of trypaflavine molecules is found for stretch temperatures in the 50–115°C range. It is suggested that the hydrogen bonding between the PVA chains, and between the dye molecules and the PVA chains, influences the symmetry of the distribution upon stretching.     

An analytical expression for interatomic surfaces in the theory of atoms in molecules

Summary According to the theory of Atoms in Molecules as developed by Bader and coworkers a molecule is partitioned into atoms separated by surfaces of zero flux in the gradient of the charge density. For the first time an accurate and explicit analytical expression is given for these interatomic surfaces. They are generated by a system of differential equations which can in principle be solved by using a series expansion. Unfortunately, this expansion has a small radius of convergence and can therefore not be applied in practice. However, by a combined Chebyshev-Fourier fit to a numerically obtained surface, the interatomic surface is globally described to any given accuracy. Finally, the algorithm is tested on a set of simple molecules and on the amide interatomic surfaces of the glycyl residue |HNCH₂CO|.     

A quantum theory atoms in molecules investigation of Lewis base protonation

This investigation uses atomic properties derived from the quantum theory of atoms in molecules formalism to rationalize the infrared intensity of the stretching vibration that arises as a Lewis base (B) is protonated (B‐H mode). Moreover, the interacting quantum atom (IQA) partition is employed to evaluate the energetics of protonation. All calculations are performed at the CCSD/cc‐pVQZ level except by the IQA analysis, which is carried out by means of the B3LYP/cc‐pVQZ//CCSD/cc‐pVQZ treatment. First, an efficiency scale is established for Lewis bases in terms of the electronic charge transfer potential. Next, this study shows that the intensity of the B‐H stretching depends mostly on the electronic charge amount transferred to the proton. Thus, intensity data provide empirical assessment of Lewis base charge transfer efficiency. Finally, the group separation observed during correlation of proton affinities and electronic charge transfer potential is explained by the interaction energy between fragments of the protonated system.