Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1-Propanol Radicals.

The potential energy surface involved in the thermal decomposition of 1-propanol radicals was investigated in detail using automated codes (tsscds2018 and Q2DTor). From the predicted elementary reactions, a relevant reaction network was constructed to study the decomposition at temperatures in the range 1000-2000 K. Specifically, this relevant network comprises 18 conformational reaction channels (CRCs), which in general exhibit a large wealth of conformers of reactants and transition states. Rate constants for all the CRCs were calculated using two approaches within the formulation of variational transition-state theory (VTST), as incorporated in the TheRa program. The simplest, one-well (1W) approach considers only the most stable conformer of the reactant and that of the transition state. In the second, more accurate approach, contributions from all the reactant and transition-state conformers are taken into account using the multipath (MP) formulation of VTST. In addition, kinetic Monte Carlo (KMC) simulations were performed to compute product branching ratios. The results show significant differences between the values of the rate constants calculated with the two VTST approaches. In addition, the KMC simulations carried out with the two sets of rate constants indicate that, depending on the radical considered as reactant, the 1W and the MP approaches may display different qualitative pictures of the whole decomposition process.

Theoretical design of molecular grippers for anion recognition based on subporphyrazines and subphthalocyanines.

Subphthalocyanines (SubPcs) are 14 π-electron aromatic macrocycles containing three 1,3-diiminoisoindol units N-fused around a central boron atom. Their particular concave π-extended structure makes them potentially useful in fields related to optoelectronics and molecular electronics. Also, their structure makes them potential receptors of complementary convex-shaped molecules. Inspired by the properties of subphthalocyanines and by previous studies about the possibility of obtaining neutral receptors composed of aromatic rings that can bind anions, we performed a theoretical study to analyze the ability of subphthalocyanines and some SubPc analogues to capture anions by non-covalent interactions. This may be relevant, since they can produce a variety of chromogenic reagents for anion detection that are not destroyed in the detection process, since the non-covalent interactions are weak enough to reverse the procedure. We characterized complexes formed between several SubPc derivatives and different anions, and also studied the influence of this process on the optical properties of these molecules. The stabilization trend reflected by the energetic results and the changes shown in the absorption wavelengths of the host molecule under complexation confirm our hypothesis about the possibility of these kinds of molecules being used as chromogenic reagents for anion capture, even in solvents with a large dielectric constant like water.

A computational study of the protonation of simple amines in water clusters.

The microsolvation study of a group of amines with a variable number of water molecules was performed by conducting a theoretical analysis of the properties of the clusters formed by the amines with up to seven molecules of water. We describe the microsolvation of several amines focusing on the dissociation of a water molecule that transfers a proton to the amine and forms a hydroxide ion. Ab initio calculations were performed on these clusters employing the DFT/B3LYP and MP2 methods with the 6-311++G(2d,p) basis set. Several stationary points for each cluster were thus located and characterized as minima from frequency calculations. Intermolecular BSSE corrected interaction energies were obtained. The protonation mechanism of the amines was examined in terms of some parameters that include the lengths of the bonds involved in the process of proton transfer and the frequencies associated with certain O-H and N-H stretching modes. On the basis of the calculations, all studied amines present similar behavior but trimethylamine, whose limitations to be integrated in the water hydrogen bond network cause the instability of some of their complexes. The cyclic configurations are the most stable structures up to five water molecules due to the presence of cooperative effects associated with the hydrogen bonds of water molecules. However, when the number of water molecules increases the spatial forms become the most stable configurations. The dissociated forms were not found to have the most stable configuration in any of the studied systems but energetic differences between the dissociated and non-dissociated forms decrease with the number of water molecules.

Anion-π aromatic neutral tweezers complexes: are they stable in polar solvents?

The impact of the solvent environment on the stabilization of the complexes formed by fluorine (T-F) and cyanide (T-CN) substituted tweezers with halide anions has been investigated theoretically. The study was carried out using computational methodologies based on density functional theory (DFT) and symmetry adapted perturbation theory (SAPT). Interaction energies were obtained at the M05-2X/6-31+G* level. The obtained results show a large stability of the complexes in solvents with large dielectric constant and prove the suitability of these molecular tweezers as potential hosts for anion recognition in solution. A detailed analysis of the effects of the solvent on the electron withdrawing ability of the substituents and its influence on the complex stability has been performed. In particular, the interaction energy in solution was split up into intermonomer and solvent-complex terms. In turn, the intermonomer interaction energy was partitioned into electrostatic, exchange, and polarization terms. Polar resonance structures in T-CN complexes are favored by polar solvents, giving rise to a stabilization of the intermonomer interaction, the opposite is found for T-F complexes. The solvent-complex energy increases with the polarity of the solvent in T-CN complexes, nonetheless the energy reaches a maximum and then decreases slowly in T-F complexes. An electron density analysis was also performed before and after complexation, providing an explanation to the trends followed by the interaction energies and their different components in solution.

Enhancing the interactions between neutral molecular tweezers and anions.

Several structural modifications to the original molecular tweezers of the Klärner's group were made with the aim of improving its binding capacity towards anions. The proposed modifications raise the molecular electrostatic potential inside the cavity and provide more conformational flexibility. The complexes of these new molecules with the halide anions Cl(-), Br(-), I(-) were optimized at the MPW1B95/6-31+G* level of theory. The molecular interactions were analyzed by single point density fitted local second-order Møller-Plesset perturbation theory (DF-LMP2) and DF-LMP2 spin-component-scaled MP2 (SCS-MP2), calculations were performed with the cc-pVTZ basis set. In view of the large magnitude of the interaction energies computed and the stability of the complexes in different solvents, this kind of molecule is a good candidate as molecular host for anion recognition.

Hydrogen transfer vs proton transfer in 7-hydroxy-quinoline.(NH3)3: a CASSCF/CASPT2 study.

Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.

Towards the design of neutral molecular tweezers for anion recognition.

Molecular tweezers are simple molecular receptors that can be characterized by the presence of two flat pincers separated by a more or less rigid tether. They have the ability to form complexes with a substrate molecule by gripping the substrate between the tips of the tweezers in a similar manner to that of mechanical tweezers. Klärner et al. synthesized one of the structurally simplest molecular tweezers, which is reported to bind electrodeficient aromatic and aliphatic substrates as well as organic cations. Complexes between these molecular tweezers and electron-rich aromatic, aliphatic, or anionic substrates have not been observed. Inspired by several recent reports that describe the interaction of hexafluorobenzene with electron-rich sites of molecules, we conducted a theoretical study to show the possibility of building molecular tweezers, based on those synthesized by Klärner, which were able to bind to anions and thus increase their potential as molecular receptors. We characterized complexes formed between several fluorinated derivatives of simple tweezers and an iodine anion, and analyzed the nature of the intermolecular interactions as well as the energetics for the process of complexation. The stabilization trend reflected by the energetic results when fluorine substituents were added to benzene rings confirms our hypothesis about the possibility of obtaining neutral tweezers composed of aromatic rings that can bind anions.

Chemical behavior of the biradicaloid (HO...ONO) singlet states of peroxynitrous acid. The oxidation of hydrocarbons, sulfides, and selenides.

Various high levels of theory have been applied to the characterization of two higher lying biradicaloid metastable singlet states of peroxynitrous acid. A singlet minimum (cis-2) was located that had an elongated O-O distance (2.17 A) and was only 12.2 kcal/mol [UB3LYP/6-311+G(3df,2p)+ZPVE] higher in energy than its ground-state precursor. A trans-metastable singlet (trans-2) was 10.9 kcal/mol higher in energy than ground-state HO-ONO. CASSCF(12,10)/6-311+G(d,p) calculations predict the optimized geometries of these cis- and trans-metastable singlets to be close to those obtained with DFT. Optimization of cis- and trans-2 within the COSMO solvent model suggests that both exist as energy minima in polar media. Both cis- and trans-2 exist as hydrogen bonded complexes with several water molecules. These collective data suggest that solvated forms of cis-2.3H(2)O and trans-2.3H(2)O represent the elusive higher lying biradicaloid minima that were recently (J. Am. Chem. Soc. 2003, 125, 16204) advocated as the metastable forms of peroxynitrous acid (HOONO). The involvement of metastable trans-2 in the gas phase oxidation of methane and isobutane is firmly established to take place on the unrestricted [UB3LYP/6-311+G(d,p)] potential energy surface (PES) with classical activations barriers for the hydrogen abstraction step that are 15.7 and 5.9 kcal/mol lower than the corresponding activation energies for producing products methanol and tert-butyl alcohol formed on the restricted PES. The oxidation of dimethyl sulfide and dimethyl selenide, two-electron oxidations, proceeds by an S(N)2-like attack of the heteroatom lone pair on the O-O bond of ground-state peroxynitrous acid. No involvement of metastable forms of HO-ONO was discernible.

Theoretical analysis of peroxynitrous acid: characterization of its elusive biradicaloid (HO...ONO) singlet states.

Various high levels of theory (DFT, QCISD, BD(TQ), and CASSCF) have been applied to the characterization of two higher-lying biradicaloid singlet states of peroxynitrous acid. A singlet minimum (cis-2) was located that had an elongated O-O distance of 2.17 A and was only 14.4 kcal/mol [UB3LYP/6-311+G(3df,2p)] higher in energy than its cis-peroxynitrous acid ground-state precursor. A trans metastable higher-lying singlet (trans-2) was 12.8 kcal/mol higher in energy than ground-state HO-ONO. Complete active space calculations [CAS(12,10)/6-311+G(d,p)] predicted the optimized geometries of these cis and trans metastable singlets to be quite close to those obtained with the DFT method. Geometry optimization of both cis- and trans-2 within the COSMO solvent model suggest that both exist as energy minima in polar media with elongated O-O distances of 2.14 and 2.09 A. Both cis- and trans-2 exist as hydrogen-bonded complexes with several water molecules. These collective data suggest that solvated forms of cis-2.3H2O and trans-2.3H2O represent the elusive higher-lying biradicaloid minima that have been previously advocated (J. Am. Chem. Soc. 1996, 118, 3125) as the metastable forms of peroxynitrous acid (HOONO*).