New J. Chem, 39, 6791, 2015

DOI:10.1039/C5NJ00600G

C. Trujillo, G. Sánchez-Sanz, I. Alkorta, J. Elguero

Halogen, Chalcogen and Pnictogen Interactions in (XNO2)2 Homodimers (X = F, Cl, Br, I)

A theoretical study of the XNO2 homodimers (X = F, Cl, Br and I) has been carried out by means of the Møller-Plesset (MP2) methodology. Twenty-two different minimum structures have been found, involving pnictogen, chalcogen and halogen bonds. MP2 interaction energies range between –0.4 to –17.5 kJ•mol–1. Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) approaches have been used to analyse the nature of the interaction within both monomers, obtaining good correlations between Laplacian values and bond distances. NBO E(2) orbital interaction energies are found to be up to 39.0 kJ•mol–1. Charge transfer between monomers are in agreement with those in AIM and NBO findings, showing the highest charge transferred in those asymmetric dimers which involve pure halogen bonds. Symmetry Adapted Perturbation Theory (SAPT-DFT) results show that the interactions are driven by the dispersion term, followed by the electrostatic one. The induction term present the lowest contribution with the exception of complexes 1 and 5 of the iodine derivative in which Ei(2) shows the maximum contribution to the total forces.

Phys. Chem. Chem. Phys, 17, 14961, 2015

DOI:10.1039/C5CP00876J

G. Sánchez-Sanz, C. Trujillo, I. Rozas, I. Alkorta

Influence of fluoro and cyano substituents in the aromatic and antiaromatic characteristics of cyclooctatetraene

An exhaustive and systematic study of the structural and electronic properties of cyclooctatetraene (COT) upon substitution of hydrogen atoms by fluoro and cyano groups has been carried out in order to analyse the influence of both substituents on the aromaticity. We found that C-C distances decrease with fluoro substitution while in cyano derivatives the opposite happens. All the compounds retain their original structural type, with the exception of the cyano derivatives; thus, compounds 25CN6T, 27CN6T and 30CN8T show boat-like structure, whereas compounds 20CN5T, 26CN6T, and 29CN7T present twisted structures. Regarding the relative energies of those compounds with the same number of substitutions, it was found that compounds where the X groups were more separated among them were the most stable ones. Inversion barriers (ΔETS) were found to increase with the number of substitutions; in the case of fluoro derivatives these barriers have a two-fold, increase compared to the parent compound while in the cyano ones a three-fold increase was observed. The aromatic character based on the NICS values, was found to increase in the ground singlet states and in the transition states of both fluoro and cyano derivatives. For triplet states, a decrease of the aromatic behaviour was found upon substitution. NICS profiles and 3D NICS isosurfaces confirm such findings. Finally, HOMA indexes corroborate the aromatic changes described by the NICS values, although, no good correlations between both quantities were found.

ChemPhysChem,16, 2140, 2015

DOI:10.1002/cphc.201500317

C. Trujillo, G. Sánchez-Sanz, I. Alkorta, J. Elguero

Computational study of proton transfer in 3(5)-hydroxypyrazole tautomers assisted by water

The tautomerism of 3(5)-hydroxypyrazole has been studied at the B3LYP, CCSD and G3B3 computational levels, including gas phase, PCM-Water solvent effects, and proton transfer assisted by water molecules. To understand the propensity of tautomerization, hydrogen-bonded acidity and basicity of neutral species have been approached by means of Platts equations. B3LYP data reveals acidity alpha-values within 0.33 and 0.71, while basicity beta-values lay between 0.22-0.58. Tautomerism processes are highly dependent on the solvent environment, and we have found a significant reduction of the transition barriers upon solvation. In addition, the inclusion of a single water molecule to assist the proton transfer considerably decreases the barriers between tautomers. While the second water molecule reduces those barriers, its effect is less appreciable than in the first one. Neutral species present more stable minima than anionic and cationic species, but relatively similar transition barriers than anionic tautomers.

Tetrahedron, 71, 826, 2015

DOI:10.1016/j.tet.2014.12.058

G. Sánchez-Sanz

Aromatic behaviour of benzene and naphthalene upon pnictogen substitution

A thorough study of the structural and aromatic properties of derivatives of benzene and naphthalene where one or more -CH- groups have been substituted by a nitrogen, phosphorous or arsenic atom has been carried out at B3LYP/6-311++G(d,p) level. Relative energies between isomers range from 5.0 to 294 kJ mol-1 finding the largest relative energies in compounds with nitrogen substitutions. In general, most of the compounds show to be planar with a few exceptions which exhibit twisted structures. Wiberg bond indexes as well as bond distances indicate that in almost all the cases the bond nature is benzene-like. Aromatic characteristics have been addressed calculating NICS values, profiles, isosurfaces and HOMA indexes. NICS(1) and (2) present values close to those of benzene showing aromatic behaviour, confirmed by NICS profiles and 3D NICS isosurfaces. HOMA indexes obtained for those compounds with reported parameterized bonds are in agreement with their corresponding aromatic nature.

Comput. Theor. Chem. 1053, 305, 2015

 DOI:10.1016/j.comptc.2014.07.009

G. Sánchez-Sanz, C. TrujilloI. Alkorta, J. Elguero

Theoretical study of cyanophosphines: Pnicogen vs. dipole-dipole interactions

Cyanophosphine derivative dimers, [HXP(CN)]2 with X = H, F and Cl,  have been characterized by means of CCSD(T)/aug’-cc-pVTZ//MP2/aug’-cc-pVTZ computational level calculations. Different interactions have been found upon complexation, such as hydrogen bonds, pnicogen bonds and dipole···dipole interactions. The intermolecular distances range between 2.84 and 3.53 Å and the binding energies between –34.7 and –3.6 kJ•mol–1. Compounds with dipole···dipole interactions present shorter contact distances and larger (more negative) binding energies than those with pure P···P pnicogen bonds. Electron density shift maps show larger variations in compounds with dipole···dipole interactions than in those with pure pnicogen ones, in line with the energetic results. However, NBO analysis suggests that the complexes with P···P pnicogen bonds, in special those with XP···PX (X = F, Cl) show E(2) orbital interaction energies much larger than the dipole···dipole ones. 

J. Heterocycl. Chem. 52, 336, 2015

DOI: 10.1002/jhet.1955

R. Aggarwal, V. Kumar, G. Singh, D. Sanz, R. M. Claramunt,  I. Alkorta, G. Sánchez-Sanz, J. Elguero

An NMR and Computational Study of Azolo[a]pyrimidines with Special Emphasis on Pyrazolo[1,5-a]pyrimidines

The reaction of 3(5)-amino-5(3)-hydrazinopyrazole, a bifunctional compound, with  3-oxo-3-phenylpropanenitrile and two of its p-substituted derivatives affords 2,5-diaryl-7-aminopyrazolo[1,5-a]pyrimidines. A mechanism for this unexpected reaction involving the formation of hydrazine is proposed. The position of the aryl substituents on the bicyclic ring has been established by the combined use of NMR and DFT calculations. Moreover, the chemical shifts have been calculated, and some general rules have been withdrawn.

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