Obis Castaño

Organic Thermochemistry at High ab Initio Levels. 1. A G2(MP2) and G2 Study of Cyclic Saturated and Unsaturated Hydrocarbons (Including Aromatics)

With the purpose of exploring the reliability of the enthalpies of formation calculated using G2 methods, we have examined a series of saturated and unsaturated alicyclic hydrocarbons varying the size and the number of formal double bonds in the molecule. Heats of formation have been calculated at the G2(MP2) and G2 levels through both atomization reactions and bond separation isodesmic reactions, and comparison with experimental values has been made. A linear relationship between the differences between experimental and calculated (from atomization reactions) heats of formation and the number of formal double bonds is obtained.

Potential energy surface of cyclooctatetraene

We present a theoretical study of the cyclooctatetraene (COT) molecule. Seven COT structures are located on the singlet ground state potential energy surface. Four of them, which present D2d (tub), Cs (bicyclo[4.2.0]octa-2,4,7-triene or BOT), C2h (chair) and D4 (crown) symmetries are stable species, and the other three are transition state structures showing Cs, D4h, and D8h symmetry. We discuss the symmetry of wave functions for these stationary points. Geometries, energies, and harmonic vibrational frequencies of these structures, and energy gaps between singlet–triplet states and low-lying singlets are presented. For the planar D4h and D8h structures, Jahn–Teller and tunneling effects have also been discussed. Ring inversion, bond shifting and valence isomerization reactive channels from the tub COT conformer are discussed from the point of view of the corresponding transition state structures. Where possible, in order to lend support to this theoretical information comparisons with recent transition s...

Role of bifurcation in the bond shifting of cyclooctatetraene

The present study of the cyclooctatetraene potential energy surface shows the presence of a bifurcation (valley ridge inflection point) in the intrinsic reaction coordinate path between the two transition states of D8h and D4h symmetries. This result is of capital importance for the correct understanding of the bond shifting and ring inversion processes in this compound.

Donor-(π-bridge)-azinium as D-π-A+ one-dimensional and D-π-A+-π-D multidimensional V-shaped chromophores

Heteroaromatic cations reacted with N-heteroarylacetylenes under Sonogashira conditions to allow easy access to potential single donor D-π-A(+) and V-shaped D-π-A(+)-π-D chromophores, where the acceptor moiety A is the π-deficient pyridinium cation and the donor moiety is represented by different π-excessive N-heterocycles. The β hyperpolarizabilities were measured using hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis that combines a variety of computational procedures, including Density Functional Theory (DFT) and correlated Hartree-Fock-based methods (RCIS(D)).

An experimental and theoretical study of the basicity of tetra-tert-butyltetrahedrane.

The gas-phase basicity (GB) of tetra-tert-butyltetrahedrane (tBu4THD) was determined by FT-ICR mass spectrometry and comparison with reference compounds of known basicity. Its GB, 1035+/-10 kJ x mol(-1), makes tetra-tert-butyltetrahedrane one of the strongest bases reported so far. Ab initio calculations [B3LYP/6-31G(d) and B3LYP/6-311 + G(d,p)//6-31G(d)] have been carried out in order to compare the high experimental basicity of tBu4THD with that estimated theoretically. Both B3LYP/6-31G(d) and QCISD(T) calculations were used to determine the reaction path which connects the initial tetrahedrane-ammonium complex with the final products, protonated cyclobutadiene (CBDH+) and ammonia.

Modulating Nitric Oxide Release by S-Nitrosothiol Photocleavage: Mechanism and Substituent Effects

The photochemistry and photophysics of a series of S-nitrosothiols (RSNOs) have been studied computationally. The photocleavage mechanism of the model compound CH(3)SNO to release CH(3)S· and ·NO was studied at the CASPT2 level resulting in a barrierless process when irradiating in the visible region (S(1)), in the near UV region (S(2)) and for photosensitized (T(1)) reaction. The absorption energy required to initiate photocleavage was calculated at the CASPT2 and B3P86 levels showing the possibility of the modulation of NO release by RSNO photoactivation as a function of the substituent R. Good correlations between the wavelengths of the lowest energy (1)(n,π*) and (1)(π,π*) transitions of aryl S-nitrosothiols and the corresponding Hammett constants of the substituents have been obtained.

The standard enthalpies of formation of 1- and 2-Adamantyl cations and radicals. An ab initio study

Abstract The results of a G2(MP2) computational study involving 1- and 2-Adamantyl cations ( 1 + , 2 + ) as well as 1- and 2-Adamantyl radicals ( 1 , 2 ) are presented. They provide purely computational thermodynamic data for the following processes: (i) Ionization of 1 and 2 , (ii) Exchange of hydrogen atoms or hydride anions between Adamantyl radicals or cations and alkyl radicals or cations, respectively. These data, once combined with the experimental enthalpies of formation of iso -C 3 H 7 ,iso -C 3 H 7 + ,tert -C 4 H 9 and tert -C 4 H 9 + , allowed us to screen the available experimental data and to define a self-consistent set of experimentally-based standard enthalpies of formation, Δ f H 0 m , for Adamantyl species, namely: Δ f H 0 m ( 1 + , g )=162.0±2.0 ; Δ f H 0 m ( 2 + , g )=171.9±2.0 ; Δ f H 0 m ( 1 , g )=17.9±2.1 , Δ f H 0 m ( 2 , g )=16.6±2.0 kcal mol −1 .

Superacid chemistry in the gas phase: Dissociative proton attachment to halomethanes

The standard Gibbs energy changes for the halide transfer processes in the gas phase have been determined by means of Fourier transform ion cyclotron resonance mass spectrometry (FT ICR): 1-Ad-X(g)+R+(g)→1-Ad+(g)+R-X(g) X = Halogen; l-Ad = 1-Adamantyl; R-X = CCl4 (1), CBr4 (2), CI4 (3) and CBr3F (4). To this end, systematic use was made of the dissociative proton attachment method (DPA) [J-L.M. Abboud et al., J. Am. Chem. Soc. 116 (1994) 2486–2492.], which allowed one to experimentally determine the onsets for reactions: R-X(g)+BH+(g)→R+(g)+HX(g)+B(g) R = CCl3, CBr3, CI3, CBr2F, 1-Ad. In the case of 4, two different onsets were determined, respectively, corresponding to the formation of CBr2F+(g) and CBr3+ (g). The thermodynamics of several reactions (see the first equation above) were studied at the MP2/6–31G(d) level. The results agree very satisfactorily with the experimental results.

The intrinsic basicity of urea. An experimental (FT ICR) and theoretical (G2) study

Abstract The gas phase basicity of urea (1), GB(1), [i.e. the standard Gibbs energy change for the reaction 1H + (g) ⇄ 1 (g) + H + (g) ] has been determined experimentally for the first time. This was achieved by means of Fourier Transform Ion Cyclotron Resonance Spectroscopy (FT ICR). 1 and 1H+ have been treated at the G2 level. This study suggests that 1(g) is very nearly a 1:1 mixture of two conformers of C2 and Cs symmetries. 1(g) is an oxygen base. The calculated and experimental values of GB(1) are in excellent agreement. An equally excellent agreement also exists between the experimental and calculated standard enthalpies of formation of 1(g). The vibrational frequencies of the conformers of 1 and of 1H+ have been calculated. This information, as well as the structures optimized at the MP2(FULL)/6-31G(d) level have been examined and compared to the available experimental data. The results thus obtained shed light on the mechanism of the stabilizing effect of 1H+(g) provided by the amino groups.

Calculations of interaction energies of ellipticine derivatives with DNA base pairs

Properties of various derivatives of ellipticine and their stacking interactions with adenine–thymine and guanine–cytosine base pairs were investigated with an ab initio correlated method. The dependence of the interaction energy on the distance between an intercalator and a base-pair and on the twist angle between them was studied. The results were compared with three empirical potentials, two of which being based on the AMBER force field and the other one based on the Lifson–Hagler empirical potential. The global interaction energy minima geometries of three systems were searched. The performance of the empirical potentials is discussed.