Fernando Mota

The C–H⋯π bonds: strength, identification, and hydrogen-bonded nature: a theoretical study

Abstract Using the MP2 method and, in some cases, a basis set-up to aug-cc-pVTZ quality, the properties of C–H⋯π bonds have been investigated in model dimers. Their strength goes from 0.55 up to 2.5 kcal/mol, depending on the C–H carbon hybridization and the π system. The presence of these bonds is identified by the presence of a bond critical point linking the H atom and atoms of the π system. The critical point characteristics and the vibrational shift of the C–H⋯π bonded dimers are similar to those present in C–H⋯O and O–H⋯π hydrogen-bonded dimers, thus indicating a hydrogen-bonded nature.

Comparative analysis of the multicenter, long bond in [TCNE]*- and phenalenyl radical dimers: a unified description of multicenter, long bonds.

The nature of the multicenter, long bond in neutral phenalenyl dimers is analyzed in detail and compared to the multicenter, long bond in [TCNE](2)(2-). These dimers are prototypes of multicenter, long bond in dimers of neutral and anion radicals. This was done by examining the number of electrons (m) and atomic centers (c) involved in the long bond for these dimers, as well as identifying the dominant attractive components of their interaction energy (SOMO-SOMO bonding, dispersion, and the sum of the exchange-repulsion and electrostatic components) in accord with Paulings focus on total bond energies. The long bond in [TCNE](2)(2-) is a 2e(-)/4c bond, the electrostatic component is repulsive, and the dominant attractive component is the dispersion component (-27.7 kcal/mol), about two times larger than the bonding component. In phenalenyl dimers the dispersion component (-31.7 kcal/mol) is about 2.5 times stronger (than the SOMO-SOMO bonding component; hence, the multicenter, long bond in these dimers is closer to a van der Waals bond than to a covalent bond. Consequently, it possesses a two-electrons/fourteen center 2e(-)/14c bond, rather than the 2e(-)/12c bond suggested by the SOMO-SOMO bonding component. The covalent-like properties in phenalenyl dimers result from the dominant dispersion component that enable the fragments to approach each other so that their SOMOs overlap and produce a qualitative MO diagram identical to that found in conventional covalent bonds.

Tunneling versus Hopping in Mixed-Valence Oligo-p-phenylenevinylene Polychlorinated Bis(triphenylmethyl) Radical Anions

Radical anions 1(-•)-5(-•), showing different lengths and incorporating up to five p-phenylenevinylene (PPV) bridges between two polychlorinated triphenylmethyl units, have been prepared by chemical or electrochemical reductions from the corresponding diradicals 1-5 which were prepared using Wittig-Horner-type chemistry. Such radical anions enabled us to study, by means of UV-vis-NIR and variable-temperature electron spin resonance spectroscopies, the long-range intramolecular electron transfer (IET) phenomena in their ground states, probing the influence of increasing the lengths of the bridges without the need of using an external bias to promote IET. The temperature dependence of the IET rate constants of mixed-valence species 1(-•)-5(-•) revealed the presence of two different regimes at low and high temperatures in which the mechanisms of electron tunneling via superexchange and thermally activated hopping are competing. Both mechanisms occur to different extents, depending on the sizes of the radical anions, since the lengths of the oligo-PPV bridges notably influence the tunneling efficiency and the activation energy barriers of the hopping processes, the barriers diminishing when the lengths are increased. The nature of solvents also modifies the IET rates by means of the interactions between the oligo-PPV bridges and the solvents. Finally, in the shortest compounds 1(-•) and 2(-•), the IET induced optically through the superexchange mechanism can also be observed by the exhibited intervalence bands, whose intensities decrease with the length of the PPV bridge.

Dynamics of the N(4Su) + NO(X 2Π) → N2(X 1Σg+) + O(3Pg) atmospheric reaction on the 3A‘ ground potential energy surface. I. Analytical potential energy surface and preliminary quasiclassical trajectory calculations

The N(4Su)+NO(X 2Π)→N2(X 1Σg+)+O(3Pg) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie–Murrell expression for the lowest 3A‘ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. ...

Are non-linear C–H⋯O contacts hydrogen bonds or Van der Waals interactions?: Establishing the limits between hydrogen bonds and Van der Waals interactions

Abstract The hydrogen bond nature of angular C–H⋯O contacts is examined to determine when these contacts are better classified as hydrogen bonds or as Van der Waals bonds. To classify the bond we propose to look at the nature of the intermolecular bond critical point present in the electron density of the complex containing the bond. The physics behind this approach is explained using a qualitative orbital overlap model aimed at describing the main changes in the electronic density of the complex produced by the C–H⋯O bending.

Origin of the Magnetic Bistability in Molecule-Based Magnets: A First-Principles Bottom-Up Study of the TTTA Crystal

The magnetic bistability present in some molecule-based magnets is investigated theoretically at the microscopic level using the purely organic system TTTA (1,3,5-trithia-2,4,6-triazapentalenyl). The TTTA crystal is selected for being one of the best-studied molecule-based systems presenting magnetic bistability. The magnetic properties of the high- and low-temperature structures (HT and LT phases, respectively) are accurately characterized by performing a First-Principles Bottom-Up study of each phase. The changes that the magnetic exchange coupling constants (J(AB)) undergo when the temperature is raised (LT → HT) or lowered (HT → LT) are also fully explored in order to unravel the reasons behind the presence of these two different pathways. The triclinic LT phase is diamagnetic due to the fact that the nearly eclipsed π dimer is effectively magnetically silent and not to formation of a covalent bond between two TTTA molecules. It is also shown that bistability in TTTA results from the coexistence of the monoclinic HT and triclinic LT phases in the temperature range studied.

Substituent effects in intermolecular C(sp3)-H ⋯ O(sp3) contacts: how strong can a C(sp3)-H ⋯ O(sp3) hydrogen bond be?

Abstract The dependence of the strength of the C(sp 3 )-H ⋯ interaction, with functional groups attached to the carbon atom, has been investigated for various families of R 1 R 2 R 3 CH ⋯ OH 2 model complexes, totalling 14 model complexes, the simplest of which is CH 4 ⋯ OH 2 . The study has ben carried out using the Hartree-Fock and second-order Moller-Plesset methods, and also with the Becke-Lee-Yang-Parr (B-LYP) functional. The substituent effect can be important. Thus, the C(sp 3 )-H ⋯ O(sp 3 ) interaction becomes eight times stronger (−2.41 kcal/mol) when R 1 = R 2 = R 3 = F than when R 1 = R 2 = R 3 = H. The B-LYP functional reproduces the trends but the energies are always 0.7–1.2 kcal/mol weaker.

Interanionic (-)O-H...O(-) interactions: a solid-state and computational study of the ring and chain motifs.

The (-)O-H...O(-) interaction formed by the anions HCO3-, HCO4, HC4O4 and HC5O5- (HA-), obtained upon monodeprotonation of the corresponding carbonic, oxalic, squaric and croconic acids (H2A), has been investigated theoretically and experimentally. The ring (RING) and chain (CHAIN) hydrogen bond motifs established between these anions have been analysed in terms of geometry and energy and their occurrence in crystalline salts investigated by searching the Cambridge Structural Database (CSD) and the Inorganic Chemistry Structural Database (ICSD). It has been shown that hydrogen carbonates form RINGs, with the notable exception of NaHCO3, while only CHAINs are known for hydrogen oxalates. Hydrogen squarates and hydrogen croconates can form both RINGs and CHAINs. The structures of Rb- and Cs- hydrogen croconates, which present the two alternative motifs, have been discussed together with that of the hydrated salt NaHC5O5.H2O. The relationship between RING and CHAIN has been examined in the light of ab initio calculations. A rigorous quantum chemical study of the nature of the interanionic (-)O-H...O(-) interaction in both vacuum and condensed phase has shown that the interaction energy is dominated by the electrostatic component which becomes attractive at short O...O distances (less than 2.5 A) if the net ionic charge on the anion is delocalised away from the -OH group. It has been demonstrated that the RING motif is slightly metastable with respect to dissociation in the gas phase, but becomes stable in the crystal owing to the influence of the Madelung field. However, the CHAIN motif is unstable both in the gas phase and in the crystal. It is argued that interanionic (-)O-H...O(-) interactions ought to be regarded as stabilising bonding interactions rather than proper intermolecular hydrogen bonds because the RING and CHAIN aggregates are not energetically stable on an absolute scale of bonding energy (i.e., in the absence of counterions). The presence of very short non-hydrogen-bridged O...O contacts resulting from charge compression of polyatomic anions bridged by alkali cations is also discussed.

The origin of the two-electron/four-centers CC bond in π-TCNE22− dimers: Electrostatic or dispersion?

The structure and stability of the π‐TCNE22− dimers in K2TCNE2 aggregates is revisited trying to find if the origin of their two‐electron/four‐centers CC bond are the electrostatic K+‐TCNE− interactions or the dispersion interactions between the anions. The study is done at the HF, B3LYP, CASSCF (2,2), and MCQDPT/CASSCF (2,2) levels using the 6‐31+G(d) basis set. Our results show that the only minima of this aggregate that preserves the π‐TCNE22− structure has the two K+ atoms placed in equatorial positions in between the two TCNE− planes. When the K+ atoms are placed along the D2h axis of the anions the structure is not a minimum. The main energetic component responsible for the stability of these aggregates comes from the cation–anion interactions. However, a proper accounting of the dispersion component (as done in the MCQDPT/CASSCF (2,2) calculations) is needed to make the closed‐shell singlet more stable than the open‐shell singlet. Thus, the bond results from the combination of the electrostatic and dispersion components, being the first the dominant one. The optimum geometry of the closed‐shell singlet is very similar to the experimental one found in crystals.

The key role of vibrational entropy in the phase transitions of dithiazolyl-based bistable magnetic materials

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature diamagnetic and high-temperature paramagnetic phases, with a large hysteresis loop that encompasses room temperature. Here, based on ab initio molecular dynamics simulations and new X-ray measurements, we uncover that the regular stacking motif of the high-temperature polymorph is the result of a fast intra-stack pair-exchange dynamics, whereby TTTA radicals continually exchange the adjacent TTTA neighbour (upper or lower) with which they form an eclipsed dimer. Such unique dynamics, observed in the paramagnetic phase within the whole hysteresis loop, is the origin of a significant vibrational entropic gain in the low-temperature to high-temperature transition and thereby it plays a key role in driving the phase transition. This finding provides a new key concept that needs to be explored for the rational design of novel molecule-based bistable magnetic materials.