Pablo Sanz

Bonding in Tropolone, 2‐Aminotropone, and Aminotroponimine: No Evidence of Resonance‐Assisted Hydrogen‐Bond Effects

The properties of the intramolecular hydrogen bond (IMHB) in tropolone, aminotropone, and aminotroponimine have been compared with those in the corresponding saturated analogues at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. In general, all those compounds in which the seven-membered ring is unsaturated exhibit a stronger IMHB than their saturated counterparts. Nevertheless, this enhanced strength is not primarily due to resonance-assisted hydrogen-bond effects, but to the much higher intrinsic basicity and acidity of the hydrogen-bond acceptor and donor groups, respectively, in the unsaturated compounds. These acidity and basicity enhancements have a double origin: 1) the unsaturated nature of the moiety to which the hydrogen-bond donor and acceptor are attached and 2) the cyclic nature of the compounds under scrutiny. As has been found for hydroxymethylene and aminomethylene cyclobutanones, and cyclobutenones and their nitrogen-containing analogues, the IMHB strength follows the [donor, acceptor] trend: [OH, C=NH]>[OH, C=O]>[NH(2), C=NH]>[NH(2), C=O] and fulfills a Steiner-Limbach correlation similar to that followed by intermolecular hydrogen bonds.

Characterization of intramolecular hydrogen bonds and competitive chalcogen-chalcogen interactions on the basis of the topology of the charge density

Density functional calculations, at the B3LYP/6-311 + G(3df,2p) level, have been carried out for the complete series of β-chalcogenvinylaldehydes to investigate whether the topology of the charge density of these systems can provide some useful information on the strength of X–H⋯Y or X⋯H–Y intramolecular hydrogen bonds or on the strength of H–X⋯Y or X⋯Y–H intramolecular chalcogen–chalcogen interactions. We have shown that, in general, there exist good linear correlations between the charge density at the ring critical point and (a) the relative strength of these interactions as measured by appropriate homodesmotic reactions, (b) the X⋯Y chalcogen–chalcogen distance. It must be emphasized, however, that while the latter correlation is systematically fulfilled, the former is only observed when the relative strength of the intramolecular interaction, either a IHB or a chalcogen–chalcogen interaction, varies inversely with the chalcogen–chalcogen distance. Also importantly, the variation of the charge density at the ring critical point correlates both, with the change in the stability of the system and with the change in the chalcogen–chalcogen distance, when the IHB is replaced by the chalcogen–chalcogen interaction, through an internal rotation of the X–H or the Y–H group.

The Role of Chalcogen–Chalcogen Interactions in the Intrinsic Basicity and Acidity of -Chalcogenovinyl(thio)aldehydes HC(X)CHCHCYH (X=O, S; Y=Se, Te)

The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.

1,8-Chalcogen-bridged naphthalenes. Strong carbon bases in the gas phase

High-level density functional theory computations have been used to estimate the gas-phase proton affinities of the complete series of 1,8-chalcogen-bridged naphthalene derivatives. Our estimates show the 1,8-naphthalenediylbis(oxy) to be an oxygen base as strong as the so-called proton sponges. All the remaining chalcogen-bridged naphthalene derivatives are predicted to be among the strongest known carbon bases in the gas phase, the para carbon with respect to the most electronegative chalcogen atom being the most basic site. This enhanced basicity reflects the electron-donating ability of sulfur, selenium and tellurium, and the aromatization of the five-membered ring in the protonated form. Protonation at the heteroatoms is always disfavored with respect to ring protonation, but the energy gap between the carbon-protonated and chalcogen-protonated species decreases significantly on going from sulfur to tellurium derivatives. When the two chalcogen atoms involved are different the basicity of the system is controlled by the amount of charge transfer from the less electronegative chalcogen atom to the most electronegative one through the X–Y bond. This effect is maximum in OTe, which accordingly becomes the strongest base of the whole series.

Intramolecular magnesium bonds in malonaldehyde-like systems: a critical view of the resonance-assisted phenomena

Abstract Through the use of high-level G4-theory calculations, we have investigated the structure, stability, and bonding of a set of Mg derivatives formed by replacing the –OH group of malonaldehyde or only the hydrogen atom of this group by a –MgH group. To give insight into the resonance-assisted phenomenon, which might be involved in the stabilization of these compounds, we also included the corresponding saturated analogs in our survey. The effect of the rigidity of the molecular framework was considered by analyzing the Mg derivatives of (Z)-4-(hydroxymethylene)cyclobut-2-enone, obtained through the same substitutions mentioned above. The effect of replacing the carbonyl group by an imino group was also contemplated. In all cases, the global minimum is a cyclic conformer stabilized through the formation of rather strong intramolecular magnesium bonds. The strength of these interactions is directly related with the intrinsic basicity of the carbonyl group (or the imino group) and the intrinsic acidity of the –MgH group, rather than with a resonance-assisted phenomenon. As a matter of fact, for all the investigated systems, the conclusion is that resonance in the cyclic conformer is directly correlated with the strength of the intramolecular magnesium bond, and not vice versa. Interestingly, the strength and characteristics of these interactions for these Mg-containing derivatives are very similar to those of the corresponding Be-containing analogs.