Benedito J. Costa Cabral

Structure and conformational equilibrium of thiacalix[4]arene by density functional theory

Abstract Density functional theory calculations for the structure and conformational equilibrium of thiacalix[4]arene are reported. The conformational equilibrium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms is compared to the conformational equilibrium of calix[4]arene. Thiacalix[4]arene conformational energies relative to the cone conformer (Δ E s) are reduced in comparison with calix[4]arene. This conformational change is in qualitative agreement with recent NMR spectroscopy measurements of the conformational equilibrium for a tetraethylether of thiacalix[4]arene in a CDCl 3 solution which indicates an enhanced chemical exchange of thiacalixarene conformers in comparison with similar methylene bridged structures. Density functional theory results for the structure of thiacalix[4]arene are in good agreement with recent X-ray diffraction measurements. The electrostatic potentials in the cone conformers of thiacalix[4]arene and calix[4]arene suggest that their complexation or recognition abilities can be significantly different. Dipole moments of the four thiacalix[4]arene conformers are in the order: cone>1,2-alternate>partial-cone>1,3-alternate.

Substituent effects on the O-H bond dissociation enthalpies in phenolic compounds: agreements and controversies

The available experimental data on O-H bond dissociation enthalpies in phenolic compounds indicate that the ring substituent effects on the thermodynamic stability of that bond can be predicted by using a group additivity method. However, the reliability of the estimates is still affected by the uncertainties assigned to many of those experimental results and also by the scarce information on the solvation of phenoxy radicals.

Electronic properties of liquid ammonia: A sequential molecular dynamics/quantum mechanics approach

The electronic properties of liquid ammonia are investigated by a sequential molecular dynamics/quantum mechanics approach. Quantum mechanics calculations for the liquid phase are based on a reparametrized hybrid exchange-correlation functional that reproduces the electronic properties of ammonia clusters [(NH3)n; n=1-5]. For these small clusters, electron binding energies based on Greens function or electron propagator theory, coupled cluster with single, double, and perturbative triple excitations, and density functional theory (DFT) are compared. Reparametrized DFT results for the dipole moment, electron binding energies, and electronic density of states of liquid ammonia are reported. The calculated average dipole moment of liquid ammonia (2.05+/-0.09 D) corresponds to an increase of 27% compared to the gas phase value and it is 0.23 D above a prediction based on a polarizable model of liquid ammonia [Deng et al., J. Chem. Phys. 100, 7590 (1994)]. Our estimate for the ionization potential of liquid ammonia is 9.74+/-0.73 eV, which is approximately 1.0 eV below the gas phase value for the isolated molecule. The theoretical vertical electron affinity of liquid ammonia is predicted as 0.16+/-0.22 eV, in good agreement with the experimental result for the location of the bottom of the conduction band (-V 0=0.2 eV). Vertical ionization potentials and electron affinities correlate with the total dipole moment of ammonia aggregates.

Metastability and weak mixing in classical long-range many-rotator systems.

We perform a molecular dynamical study of the isolated d=1 classical Hamiltonian H=1 / 2 summation operator (N)(i=1)L(2)(i)+ summation operator (i not equal j)[1-cos(theta(i)-theta(j))]/r(alpha)(ij); (alpha> or =0), known to exhibit a second order phase transition, being disordered for u identical with U/NN> or =u(c)(alpha,d) and ordered otherwise [U identical with total energy and N identical with (N(1-alpha/d)-alpha/d)/(1-alpha/d)]. We focus on the nonextensive case alpha/d< or =1 and observe that, for u<u(c), a basin of attraction exists for the initial conditions for which the system quickly relaxes onto a long standing metastable state (whose duration presumably diverges with N-like square root[N]) which eventually crosses over to the microcanonical Boltzmann-Gibbs stable state. It is exhibited that the appropriately scaled maximal Lyapunov exponent lambda(max)(u<u(c))(metastable) proportional, variant N(-kappa(metastable));(N--> infinity ), where, for all values of alpha/d, kappa(metastable) numerically coincides with one third of its value for u>u(c), hence decreases from 1/9 to zero when alpha/d increases from zero to unity, remaining zero thereafter. This simple connection between anomalies above and below the critical point reinforces the nonextensive universality scenario.

A Simple One-Body Approach to the Calculation of the First Electronic Absorption Band of Water.

A one-body decomposition approach for investigating the electronic absorption spectra of molecular systems was proposed and applied to water clusters (H2O)N including up to N = 80 water molecules. Two specific aspects of the present implementation are the inclusion of the coupling between excited states and a simplified representation for the N-body Coulombic effects. For smaller clusters, the results based on the one-body decomposition scheme are in good agreement with full EOM-CCSD calculations. Two different regimes can be identified in the electronic absorption profile of larger water clusters. The first low-energy regime is dominated by local excitonic states on the cluster surface, whereas the higher-energy excitations associated with the second one are of delocalized nature.

Structure and electronic properties of a benzene-water solution

Electronic properties of benzene in water were investigated by a sequential quantum mechanical/molecular dynamics approach. Emphasis was placed on the analysis of the structure, polarization effects, and ionization spectrum. By adopting a polarizable model for both benzene and water the structure of the benzene-water solution is in good agreement with data from first principles molecular dynamics. Further, strong evidence that water molecules acquire enhanced orientational order near the benzene molecule is found. Upon hydration, the quadrupole moment of benzene is not significantly changed in comparison with the gas-phase value. We are also reporting results for the dynamic polarizability of benzene in water. Our results indicate that the low energy behaviour of the dynamic polarizability of gas-phase and hydrated benzene is quite similar. Outer valence Greens function calculations for benzene in liquid water show a splitting of the gas-phase energy levels associated with the 1e(1g)(π), 2e(2g), and 2e(1u) orbitals upon hydration. Lifting of the orbitals degeneracy and redshift of the outer valence bands is related to symmetry breaking of the benzene structure in solution and polarization effects from the surrounding water molecules.

Ab initio study of the conformational equilibrium of ethylene glycol

SummaryThe conformational equilibrium of ethylene glycol (CH2OHCH2OH) has been examined by performing geometry optimizations at the 6-31G*, MP2/6-31G* and 6-31G** levels. Final energies have been calculated at the MP3 level with the optimized geometries. The two most stable conformers are atGg′ andgGg′ but it is verified that the inclusion of electronic correlations reduces their energy difference of 0.6 kcal/mol at the HF level to less than 0.2 kcal/mol. The possible coexistence of the two most stable conformers is in agreement with some previous studies of Frei et al. For thetXg′ conformer a detailed analysis of the intramolecular potential as a function of rotation around the C-C bond is also reported.

Hydrogen bonding and the dipole moment of hydrofluorocarbons by density functional theory

Recent measurements of the dielectric permittivity of hydrofluorocarbons in the liquid phase have allowed calculation of the dipole moments in a liquid environment. These values were based on Kirkwood theory, and were significantly greater than the corresponding gas phase dipole moments. In order to understand some features suggesting possible hindered rotation of the molecules in the liquid, density functional and self-consistent-reaction-field calculations for a series of HFC molecules including CHF2CF3 (HFC-125), CH2FCF3 (HFC-134a), CH3CF3 (HFC-143a), CH2F2 (HFC-32) and CHF2CH3 (HFC-152a) are reported. Particular emphasis has been given to the calculation of dimerisation energies, rotational potentials, polarisabilities and dipole moments. We discuss hydrogen bonding in hydrofluorocarbon dimers and the relationship between the structure and charge distribution of the dimers and the dipole moment in the liquid predicted by relative permittivity measurements. For HFC-32 we have calculated the average dipole moment in small clusters (n = 2–10). The structure of the clusters has been determined by density functional theory optimisations (n = 2–6) and Monte Carlo simulations (n = 2–10). The average dipole moment of the HFC-32 decamer is 2.35 D, which represents a 17% increase relative to the free monomer (2.0 D). We find that the enhancement of the monomer dipole induced by hydrogen bonding in HFC-32 clusters is much less pronounced in comparison with the considerable increase (50%) observed in water clusters.

Dynamic polarizability, Cauchy moments, and the optical absorption spectrum of liquid water: A sequential molecular dynamics/quantum mechanical approach

The dynamic polarizability and optical absorption spectrum of liquid water in the 6-15 eV energy range are investigated by a sequential molecular dynamics (MD)/quantum mechanical approach. The MD simulations are based on a polarizable model for liquid water. Calculation of electronic properties relies on time-dependent density functional and equation-of-motion coupled-cluster theories. Results for the dynamic polarizability, Cauchy moments, S(-2), S(-4), S(-6), and dielectric properties of liquid water are reported. The theoretical predictions for the optical absorption spectrum of liquid water are in good agreement with experimental information.

Energetic Differences between the Five-and Six-Membered Ring Hydrocarbons : Strain Energies in the Parent and Radical Molecules

The C-H bond dissociation enthalpies (BDEs) for the five- and six-membered ring alkanes, alkenes, and dienes were investigated and discussed in terms of conventional strain energies (SEs). New determinations are reported for cyclopentane and cyclohexane by time-resolved photoacoustic calorimetry and quantum chemistry methods. The C-H BDEs for the alkenes yielding the alkyl radicals cyclopenten-4-yl and cyclohexen-4-yl and the alpha-C-H BDE in cyclopentene were also calculated. The s-homodesmotic model was used to determine SEs for both the parent molecules and the radicals. When the appropriate s-homodesmotic model is chosen, the obtained SEs are in good agreement with the ones derived from group additivity schemes. The different BDEs in the title molecules are explained by the calculated SEs in the parent molecules and their radicals: (1) BDEs leading to alkyl radicals are ca. 10 kJ mol (-1) lower in cyclopentane and cyclopentene than in cyclohexane and cyclohexene, due to a smaller eclipsing strain in the five-membered radicals relative to the parent molecules (six-membered hydrocarbons and their radicals are essentially strain free). (2) C-H BDEs in cyclopentene and cyclohexene leading to the allyl radicals are similar because cyclopenten-3-yl has almost as much strain as its parent molecule, due to a synperiplanar configuration. (3) The C-H BDE in 1,3-cyclopentadiene is 27 kJ mol (-1) higher than in 1,4-cyclohexadiene due to the stabilizing effect of the conjugated double bond in 1,3-cyclopentadiene and not to a destabilization of the cyclopentadienyl radical. The chemical insight afforded by group additivity methods in choosing the correct model for SE estimation is highlighted.