TZVP Level Research Articles

Inner reorganization during the radical–biradical transition in a nor-β-lapachone derivative possessing two redox centers

In this work, the electrochemical behaviour of an antitumoral nitro o-quinone derivative obtained from 3-bromo-nor-beta-lapachone was studied. Cyclic voltammetric experiments, in acetonitrile solution, revealed that both quinone and nitro functions are reduced independently as quasi-reversible one-electron transfer processes in this order. Depending on the reduction potential, a radical anion or a biradical dianion is obtained. The formation of these paramagnetic species was confirmed by performing in situ Electrochemical-Electron Spin Resonance (E-ESR) experiments. Analysis of the kinetics of electron transfer associated to those electron uptake processes, in terms of the Marcus-Hush-Levich model, revealed differences in the reorganization energy (lambda((k))) for both steps (lambda((I)): 1.07-1.11 eV; lambda((II)): 1.21-1.30 eV). By evaluating the conformations of the radical and biradical systems by calculations at the BLYP//TZVP level of theory, it was found that the inner component, for the second reduction process (lambda((II))) was approximately 72% of lambda((II)), reflecting modifications in the molecular structure during the radical anion-biradical dianion transition. This change is also reflected in the differences presented by line widths of the ESR signals of both electrogenerated radical and diradical species.

A Combined Experimental and Theoretical Study on the Conformation of Multiarmed Chiral Aryl Ethers

Four series of multiarmed chiral aryl ethers carrying two, three, five, or eight side-chains on a variety of aromatic core molecules (2-5) were prepared. The structure and conformation of 2 and 3 (in the solid state) were determined by the X-ray crystallographic analyses. While a pair of alternated (anti) conformers (i.e, up-down and down-up) were found in the crystal of 2, three side-arms in 3 were aligned in the same direction to give a C(3)-symmetric syn-conformation. Examinations by dispersion-corrected density functional (DFT-D) calculations revealed that two out of six anti- and two out of four syn-conformers of 2 are energetically most important. Two calculated structures of anti-conformers are in good agreement with those found in the solid state by X-ray analysis. Similarly, relevant conformations of syn-3, fully alternated 4, and C(5)-symmetric 5 were optimized at the DFT-D-B-LYP/TZVP level. The structure and conformation of the side-arms in 2-5 in solution were further studied by temperature dependent (1)H NMR and UV-vis spectroscopy. In addition, comparative experimental and theoretical CD spectral studies were carried out in order to elucidate the contribution of the thermodynamically less-stable minor isomers in solution. The CD spectral changes observed for 2 and 3 at varying temperatures were quite different, while the parent chiral arene 1, as well as 4 and 5, only showed an increased intensity of the negative Cotton effect for the (1)L(b) band. The latter behavior is readily accounted for in terms of the conformational freezing of the chiral groups at low temperatures. The unusual CD spectral behavior observed for 2 and 3 was rationalized by the conformational alteration of the side-arms. Because of attractive van der Waals interactions between the aromatic units of the arms in nonpolar solvents, the syn-conformations become gradually more important for 2 at low temperatures, which eventually results in a weak positive Cotton effect for the (1)L(b) band. This was also supported by the SCS-MP2/TZVPP single-point energy calculations for the relevant conformers of 2. For 3, the contribution of the C(3)-symmetrical conformer becomes more important than the less-symmetrical isomers at low temperatures. The conformations of 2 and 3 in their excited states as well as in the oxidized states were also examined.

Rational Design, Synthesis, and Application of a New Receptor for the Molecular Recognition of Tricarboxylate Salts in Aqueous Media

A rational design of a tripodal receptor for the molecular recognition of tricarboxylate salts in aqueous media, based on squaramide, has been performed using high-level DFT calculations (RI-BP86/SVP level of theory) in solution using the COSMO treatment, including some preliminary ab initio calculations at the higher RI-MP2/TZVP level of theory, comparing the ability of squaramide to bind carboxylate salts with two widely used guanidinium salts. The tripodal receptor has been synthesized using a new methodology that has been recently reported by some of us, and its capability of recognizing several mono-, di-, and tricarboxylate salts has been studied experimentally by means of microcalorimetry experiments in a very high competitive media, H(2)O:EtOH 1:3. These experiments give enthalpic and entropic data, which are unfortunately scarce in the literature of molecular recognition of anions. Finally, a fluorimetric ensemble of the receptor with fluorescein has been found to be useful for the fluorimetric determination of zinc citrate in a commercial toothpaste using competition assays.

Dianionic complexes with hexacoordinate silicon(IV) or germanium(IV) and three bidentate ligands of the salicylato(2–) type: Syntheses and structural characterization in the solid state and in solution

Reaction of tetramethoxysilane or tetramethoxygermane with salicylic acid and morpholine (molar ratio 1:3:2) in tetrahydrofuran yielded morpholiniummer-tris[salicylato(2–)-O1,O3]silicate(mer-5) and morpholiniummer-tris[salicylato(2–)-O1,O3]germanate (mer-8), respectively. Treatment of tetramethoxysilane with 5-chlorosalicylic acid and piperidine (molar ratio 1:3:2) in tetrahydrofuran afforded piperidinium mer-tris[5-chlorosalicylato(2–)-O1,O3]silicate–ditetrahydrofuran (mer-6·2THF). Triethylammonium mer-tris[3-methylsalicylato(2–)-O1,O3]silicate (mer-7) was obtained analogously by reaction of tetramethoxysilane with three molar equivalents of 3-methylsalicylic acid and two molar equivalents of triethylamine in dichloromethane/diethyl ether. The racemic compounds mer-5, mer-6· 2THF,mer-7, and mer-8 were characterized by elemental analyses (C, H, N), single-crystal X-ray diffraction, as well as solid-state (29Si) and solution(1H, 13C, 29Si) NMR studies. The structural characterizationwas complemented by computational studies (HF studies, TZVP level) of thefac- and mer-tris[salicylato(2–)-O1,O3]silicatedianion. In addition, the behavior of mer-7 in solution was studied by VT 1HNMR experiments.

Syntheses, Structures, and Sensory Characteristics of the Perfume Ingredient Majantol and Its Analogs Sila-majantol and Germa-majantol: A Study on C/Si/Ge Bioisosterism

The perfume ingredient majantol [2,2-dimethyl-3-(3-methylphenyl)propan-1-ol (la)] and its silicon- and germanium-containing analogs sila-majantol [(hydroxymethyl)dimethyl(3-methylbenzyl)silane (1b)] and germa-majantol [(hydroxymethyl)dimethyl(3-methylbenzyl)-germane (1c)], respectively, were synthesized from i-PrC(O)Ph (four steps, 1a), Me 2 Si(CH 2 Cl)Cl (three steps, → 1b), or Cl 3 GeCH 2 Cl (four steps, → 1c). The structures of the C/Si/Ge analogs 1a/1b/1c were determined by crystal structure analyses and computational studies [RI-MP2 calculations (TZVP level)]. The odor of majantol (la) was found to be strong fresh-floral, aqueous-aldehydic, and characteristic of lily-of-the-valley flowers, whereas that of sila-majantol (lb) was considerably less intense than that of la. It lacked the aqueous-aldehydic freshness, but possessed nevertheless the characteristic lily-of-the-valley odor, but it was more terpineol-like. The odor of germa-majantol (lc) was very weak and not characteristic; the majantol impression could be recognized only as a shade.

Ab initio calculations of vibrational properties and conformer stabilities in 1,2-Si 2H 4Cl 2, 1,1,2-Si 2H 3Cl 3 and 1,1,1-Si 2H 3Cl 3, and comparisons with observed infrared and Raman spectra

Ab initio calculations of vibration frequencies and intensities have been made for 1,2-Si 2H 4Cl 2 (conformers 12G, 12T), 1,1,2-Si 2H 3Cl 3 (conformers 112C S , 112C 1 ) and 1,1,1-Si 2H 3Cl 3 ( 111), at an HF 6–31 G ∗ level. Energies and abundancies of both conformers have been obtained for 12 and 112 at this level, and for 12 also at HF tzvp , MP2 6–31 G ∗ MP2 tzvp levels. Similar energies are also reported for 1,2-Si 2H 4F 2 at HF 6–31 G ∗ and HF tzvp levels. A simple method of scaling the HF 6–31 G ∗ frequencies permits assignment of a number of bands observed in the infrared and Raman spectra of 1,2-Si 2H 4Cl 2, and of a few in infrared spectra of 112 and 111. Positions of unobserved bands are predicted to within an expected accuracy of ≈10 cm −1. The T conformer is slightly more stable than the G one in both 1,2-dichloro and 1,2-difluoro compounds, unlike the situation in the corresponding haloethanes. However, the relative stability of the 12G and 12T conformers is somewhat sensitive to basis set and level of treatment. The best estimate for the abundance X(G) of 12G is ≈0.6, in rough agreement with observed spectra. For the 112C 1 conformer, an HF 6–31 G ∗ calculation gives X(C 1) ≈ 0.9, a likely upper limit.

SiH stretching frequencies in chlorodisilanes and ab initio calculations of geometry, stretching frequency, vibrational intensity and Mulliken and King effective atomic charges

Infrared frequencies in the νSiH region observed in mixtures of chlorodisilanes are assigned by the joint use of substituent effects, determined earlier in Si 2H 5Cl and 1,1-Si 2H 4Cl 2, and ab initio calculations for 1,2-Si 2H 4Cl 2, 1,1,2-Si 2H 3Cl 3 and SiH 3SiCl 3, which together with 1,1,2,2-Si 2H 2Cl 4 were identified as being present. νSiH frequencies for all the staggered conformers are predicted using harmonic local mode theory. Ab initio geometries were obtained for the above three compounds at the HF 6–31 G ∗ level, and for both conformers of 1,2-Si 2H 4Cl 2 at HF tzvp and MP2 tzvp levels. Alpha or gauche chlorine substitution shortens both SiH and SiCl bonds, the former reflecting νSiH changes. Trans chlorine substitution shortens SiCl bonds slightly, but has no effect on SiH bonds. The local mode behaviour of νSiH vibrations extends to both infrared and Raman intensities. Ab initio-based atomic polar tensors are used to obtain dipole derivatives, dμ dr , for the SiH and SiCl bonds and King effective atomic charges, ξ, for all atoms. The changes in the latter are compared with changes in the Mulliken atomic charges and a close connection is found in the case of gauche chlorine substitution.