Aromaticity determines the relative stability of kinked vs. straight topologies in polycyclic aromatic hydrocarbons

It is well-known that kinked phenacenes are more stable than their isomeric linear acenes, the archetypal example being phenanthrene that is more stable than anthracene by about 4-8 kcal/mol. In previous studies, the origin of the higher stability of kinked polycyclic aromatic hydrocarbons (PAHs) was found to be better π-bonding interactions, i.e., larger aromaticity, in kinked as compared to linear PAHs. Some years ago, however, Dominikowska and Palusiak (2011) found that dicationic linear anthracene is more stable than the dicationic kinked phenanthrene. Therefore, these authors showed that, in some cases, the linear topology in PAHs can be preferred over the kinked one. Our results using energy decomposition analyses in combination with the turn-upside-down approach show that the origin of the higher stability of dicationic anthracene is the same as in the neutral species, i.e., better π-bonding interactions. A similar result is found for the kinked and straight pyrano-chromenes. We conclude that the aromaticity is the driving force that determines the relative stability of kinked vs. straight topologies in PAHs

Planar vs. three-dimensional X-6(2-), X2Y42-, and X3Y32- (X, Y = B, Al, Ga) metal clusters: an analysis of their relative energies through the turn-upside-down approach

Despite the fact that B and Al belong to the same group 13 elements, the B-6(2-) cluster prefers the planar D-2h geometry, whereas Al-6(2-) favours the Oh structure. In this work, we analyse the origin of the relative stability of D2h and Oh forms in these clusters by means of energy decomposition analysis based on the turn-upside-down approach. Our results show that what causes the different trends observed is the orbital interaction term, which combined with the electrostatic component do (Al-6(2-) and Ga-6(2-)) or do not (B-6(2-)) compensate the higher Pauli repulsion of the Oh form. Analysing the orbital interaction term in more detail, we find that the preference of B-6(2-) for the planar D-2h form has to be attributed to two particular molecular orbital interactions. Our results are in line with a dominant delocalisation force in Al clusters and the preference for more localised bonding in B metal clusters. For mixed clusters, we have found that those with more than two B atoms prefer the planar structure for the same reasons as for B-6(2-)

Exploring the validity of the Glidewell-Lloyd extension of Clar's pi-sextet rule: assessment from polycyclic conjugated hydrocarbons

The Clar pi-sextet rule was formulated as a tool to qualitatively assign the local aromatic character of six-membered rings in benzenoid species. This simple rule has been widely validated both experimentally and theoretically. In 1984, Glidewell and Lloyd reported an extension of this rule to polycyclic conjugated hydrocarbons having rings with any even number of carbon atoms in their structure. In this work, we assess the validity of the Glidewell-Lloyd extension in 69 polycyclic conjugated hydrocarbons composed of different combinations of four-, six-, and eight-membered rings. Our results support the validity of this extension with some exceptions that are discussed. Finally, a minor modification to the rule is proposed

Complexes of adamantane-based group 13 Lewis acids and superacids: Bonding analysis and thermodynamics of hydrogen splitting

The electronic structure and chemical bonding in donor-acceptor complexes formed by group 13 element adamantane and perfluorinated adamantane derivatives EC9R15 (E=B, Al; R=H, F) with Lewis bases XR3 and XC9H15 (X=N, P; R= H, CH3) have been studied using energy decomposition analysis at the BP86/TZ2P level of theory. Larger stability of complexes with perfluorinated adamantane derivatives is mainly due to better electrostatic and orbital interactions. Deformation energies of the fragments and Pauli repulsion are of less importance, with exception for the boron-phosphorus complexes. The MO analysis reveals that LUMO energies of EC9R15 significantly decrease upon fluorination (by 4.7 and 3.6 eV for E=B and Al, respectively) which results in an increase of orbital interaction energies by 27-38 (B) and 15-26 (Al) kcal mol(-1). HOMO energies of XR3 increase in order PH3<NH3<PMe3<PC9H15<NMe3<NC9H15. For the studied complexes, there is a linear correlation between the dissociation energy of the complex and the energy difference between HOMO of the donor and LUMO of the acceptor. The fluorination of the Lewis acid significantly reduces standard enthalpies of the heterolytic hydrogen splitting H-2+D+A=[HD](+)+[HA](-). Analysis of several types of the [HD](+)center dot center dot center dot[HA](-) ion pair formation in the gas phase reveals that structures with additional H center dot center dot center dot F interactions are energetically favorable. Taking into account the ion pair formation, hydrogen splitting is predicted to be highly exothermic in case of the perfluorinated derivatives both in the gas phase and in solution. Thus, fluorinated adamantane-based Lewis superacids are attractive synthetic targets for the construction of the donor-acceptor cryptands

Testing the effectiveness of the isoelectronic substitution principle through the transformation of aromatic osmathiophene derivatives into their inorganic analogues

The objective of the current work is to evaluate the effectiveness of the isoelectronic substitution (IS) principle on a series of complexes with the general formula OsCl2(SX3H3)(PH3)(2), where X-3 represents the moieties CCC, CCB, CCN, CBN, CNB or NCB, formed by substitution of the carbon atoms in CCC by either the isoelectronic B- or N+ separately, or by both. The SX3H3 moiety forms, together with Os, an aromatic five-membered ring (5-MR) called osmathiophene. The preservation of stability and aromaticity in the resulting systems is used to indicate the effectiveness of the IS principle. The aromaticity of the proposed molecules is analyzed according to the magnetic (induced magnetic field (B-ind)) and electronic (through the multicenter index (MCI)) criteria. In addition a chemical bonding analysis on selected species is performed by the adaptive natural density partitioning (AdNDP) method

Stabilization of 2,6-Diarylanilinum Cation by Through-Space Cation-pi Interactions

Energetically favorable cation-pi interactions play important roles in numerous molecular recognition processes in chemistry and biology. Herein, we present synergistic experimental and computational physical organic chemistry studies on 2,6-diarylanilines that contain flanking meta/parasubstituted aromatic rings adjacent to the central anilinium ion. A combination of measurements of pK(a) values, structural analyses of 2,6-diarylanilinium cations, and quantum chemical analyses based on the quantitative molecular orbital theory and a canonical energy decomposition analysis (EDA) scheme reveal that through-space cation-pi interactions essentially contribute to observed trends in proton affinities and pK(a) values of 2,6-diarylanilines

Structural preferences in phosphanylthiolato platinum(II) complexes

The transition-metal complexes of heterotopic phosphanylthiolato ligands are useful in various reactions which depend on the stereochemistry of the complexes. Bis-chelate complex [Pt(SCH₂CH₂PPh₂-κ²P,S₂)] (1) was obtained in good yields by direct base-free substitution reaction of the corresponding phosphanylthiol (HSCH₂CH₂PPh₂) with K₂PtCl₄ or by oxidative addition of the same phosphanylthiol to Pt(PPh₃)₄. In agreement with the antisymbiosis rule, complex 1 shows a cis -P,P arrangement in solid state crystallizing in the monoclinic system (C2 /c). Density functional theory (DFT) calculations on 1 reveal the right characteristics for the preferred cis -P,P arrangement, rationalizing its formation. Direct base-free reaction of [PtCl₂(1,5-cyclooctadiene)] with one equivalent of the same phosphanylthiol produce the trinuclear complex [PtCl(μ-SCH₂CH₂PPh₂-κ²P,S)]₃ (2) instead of the binuclear structure common in palladium and nickel derivatives. Crystals of 2 are triclinic (P ) showing a sulfur-bridging edge-sharing cyclic trinuclear complex with square-planar coordination geometry around the platinum atoms and a Pt₃S₃ cycle in skew-boat conformation. This preference for the trinuclear structure was rationalized mechanistically and through conceptual DFT

Mechanism of biomolecular recognition of trimethyllysine by the fluorinated aromatic cage of KDM5A PHD3 finger

The understanding of biomolecular recognition of posttranslationally modified histone proteins is centrally important to the histone code hypothesis. Despite extensive binding and structural studies on the readout of histones, the molecular language by which posttranslational modifications on histone proteins are read remains poorly understood. Here we report physical-organic chemistry studies on the recognition of the positively charged trimethyllysine by the electron-rich aromatic cage containing PHD3 finger of KDM5A. The aromatic character of two tryptophan residues that solely constitute the aromatic cage of KDM5A was fine-tuned by the incorporation of fluorine substituents. Our thermodynamic analyses reveal that the wild-type and fluorinated KDM5A PHD3 fingers associate equally well with trimethyllysine. This work demonstrates that the biomolecular recognition of trimethyllysine by fluorinated aromatic cages is associated with weaker cation-π interactions that are compensated by the energetically more favourable trimethyllysine-mediated release of high-energy water molecules that occupy the aromatic cage

Índexos de localització i deslocalització derivats de la densitat bielectrònica: anàlisi i aplicacions en estructura molecular, reactivitat química i aromaticitat

La present tesi doctoral s'emmarca dins el camp de la química teòrica icomputacional. El primer objectiu, que va ser el de partida, consistia a veure si els índexosbielectrònics derivats de la teoria AIM podien ser útils per estudiar la reorganitzacióelectrònica al llarg d'una reacció. De forma paral·lela es va plantejar un objectiumetodològic, el càlcul dels índexos mencionats a altres nivells de teoria, com podia serDFT o CI. CI. Aquests dos objectius varen portar a molts d'altres, tot i que els principals són:Objectiu 1. Estudi de la reorganització de parells d'electrons al llarg d'unareacció.Objectiu 2. Efecte de la solvatació sobre l'estructura de parells d'electrons d'unamolècula.Objectiu 3. Càlcul dels índexos de localització i deslocalització als nivells de teoriadel funcional de la densitat i post-Hartree-Fock.Objectiu 4. Caracterització de l'enllaç per pont d'hidrogen per mitjà del'estructura dels parells d'electrons.Objectiu 5. L'índex de deslocalització: nou criteri electrònic d'aromaticitat

Huckel's rule of aromaticity categorizes aromatic closo boron hydride clusters

A direct connection is established between three-dimensional aromatic closo boron hydride clusters and planar aromatic [n] annulenes for medium and large boron clusters. In particular, the results prove the existence of a link between the two-dimensional Huckel rule, as followed by aromatic [n] annulenes, and Wade-Mingos' rule of three-dimensional aromaticity, as applied to the aromatic [BnHn](2-) closo boron hydride clusters. The closo boron hydride clusters can be categorized into different series, according to the n value of the Huckel (4n+2) pi rule. The distinct categories studied in this work correspond to n=1, 2, and 3. Each category increases in geometrical difficulty but, more importantly, it is possible to associate each category with the number of pentagonal layers in the structure perpendicular to the main axis. Category 1 has one pentagonal layer, category 2 has two, and category 3 has three