Complete basis set ab initio computational study of three-dimensional aromaticity in highly symmetric hydrogen clusters

The aromaticity of planar and highly symmetric three-dimensional hydrogen clusters were evaluated with the complete basis set ab initio computational method. The energy of formation of the hydrogen clusters from the hydrogen molecule and hydrogen molecular ions were used in comparison to their relative stabilities. The aromaticity of planar hydrogen clusters, as well as hydrogen clusters in the three dimensions, arranged as highly symmetric regular polyhedra (Plato's polyhedras), were discussed with respect to the Hückel and Möbius aromatic rules.     

The utility of the PM3 method for predicting the reactivities of cyanoethenes in diels-alder reactions with pyrrole

The reliability of PM3 for predicting the reactivity of cyanoethenes in Diels-Alder reactions with pyrrole was tested. Using frontier orbital theory it was predicted that the reaction is LUMO dienophile controlled, and the reaction should be facilitated in comparison to cyclopentadiene additions (pyrrole has higher HOMO energy). The transition structures were generated and compared with similar ones generated with both ab initio and PM3 calculations for cyclopentadiene additions: The geometries of the transition structures are similar, although the new CC forming bonds are 2–6% shorter, and the degree of asynchronicity is higher. The prediction of the activation barriers with the PM3 method failed totally. The preferred exo isomer in each case, indicated that the predicted stereoselectivity was incorrect. The prediction of the relative reactivity was also unreliable. Qualitative analyses of similar series of compounds can be obtained by frontier orbital energy correlations. All conclusions were made on the assumption that the reaction is concerted, which does not exclude the possibility that PM3 might be able to predict reliable reaction barriers if some other mechanisms are considered.     

The computation of the potential energy surface for H2 + OH → H2O + H using ab initio and density functional theory methods

The reaction energy profile for H₂ + OH → H + H₂O was computed using HF, MP2, MP4, QCISD, G1, G2, and G2MP2 ab initio methods. In addition, the B3LYP, B3P86, B3PW91, BLYP, BP291, and SVWN density functional theory (DFT) methods were also used. All the ab initio methods, with the exception of the G series, produced much higher activation barriers and heats of reaction than the experimental values. On the other hand, the DFT methods produced negative forward and reverse barriers which were too low, with the exception of the hybrid DFT methods. The G2 ab initio method generated energies which deviated from the experimental values by ∼ 1 kcal/mol and therefore should be considered a very accurate computational method. The hybrid DFT methods produced positive forward reaction barriers with energies that were 2–4 kcal/mol lower than the experimental values. The geometries of the transition state and energies computed by the ab initio and DFT methods were compared. These results suggest that, in the hybrid exchange functional, the portion of the Slater exchange term should be increased. This may be the reason why the computed energies were too low. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 639–644, 1997     

Preparation of substituted semicarbazides from corresponding amines and hydrazines via phenyl carbamates

A simple synthetic procedure for the conversion of amines and hydrazines into substituted semicarbazides was developed. The initial condensation between the desired amine and phenyl chloroformate into phenyl carbamate is followed by the addition of hydrazine under basic conditions. The reaction is tolerable to a variety of functional groups, with mild conditions and high percent yields.     

AM1 semiempirical searching for suitable thiophene derivatives that will enable thiophenes to act as a diene in the diels-alder reactions

The AM1 semiempirical method was used for theoretical searching of activation of thiophene as a diene for the Diels-Alder reaction. The reactivity of thiophene, electron-withdrawing and electron-donating substituted thiophenes, as well as the S-methylthiophenium ion were studied as the diene for Diels-Alder reactions by evaluating their frontier orbital energies and by calculating reaction barriers with activated and deactivated dienophiles. It was demonstrated that slight activation of the thiophene ring can be obtained with both electron-donating and electron-withdrawing groups attached to the thiophene ring. It was predicted that the actual transformation of thiophenes into the corresponding S-methylthiophenium anions is the best means of activating the thiophenes. The calculated activation energies for normal (non-activated) dienophiles are moderate so mild reaction conditions are predicted. If dienophiles are activated with electron-donating substituents, AM1 calculations predict a two step cycloaddition reaction with a very small activation barrier.     

High level ab initio and density functional theory study of bond selective dissociation of CH3SH and CH3CH2SH radical cations

Selective bond dissociation energies for CH₃SH and CH₃CH₂SH radical cations were evaluated with G1, G2, G2MP2, B3LYP, BLYP, and SVWN computational methods. It was determined that both G2 and CBSQ evaluate very accurate bond dissociation energies for thiol radical cations, while gradient-corrected BLYP computes the best energies of three employed DFT methods. For the CH₃CH₂SH radical cation, new, higher than previously estimated selective bond dissociation energies were suggested. Received: 10 September 1997 / Accepted: 9 September 1998 / Published online: 11 November 1998     

Thermodynamics of Chromium(VI) Anionic Species Sorption onto Surfactant-Modified Montmorillonite Clay

Batch sorption experiments performed on Cr(VI) species sorption showed a significantly enhanced removal of inorganic hexavalent chromium anionic species from aqueous solution by montmorillonite clays modified with quaternary amine, hexadecyltrimethylammonium (HDTMA) bromide. Unmodified clay had no affinity for chromium(VI) species. The sorption of Cr(VI) species has been carried out as a function of pH, contact time, adsorbate concentration (4.14x10(-5) to 8.62x10(-3) M), and temperature (5-45 degrees C). The surfactant-modified clay surface was stable when exposed to extremes in pH. The optimum pH for maximum sorption of Cr(VI) species was found to be at pH 1 and was constant between pH 2 and pH 6. The sorption data obtained was well described by DKR and Langmuir sorption isotherms. Sorption energy (E) for (i) surfactant sorption by montmorillonite clay and (ii) sorption of chromium(VI) species by surfactant modified clay have been computed from the DKR equation. Sorption energy evaluated for the sorption of both surfactant and Cr(VI) species showed that an ion-exchange mechanism was operative. The mechanism of retention appears to be replacement of counterion of the surfactant by Cr(VI) anionic species. Adsorbent capacity for the sorption of Cr(VI) species has been evaluated from the Langmuir sorption isotherm data. Thermodynamic parameters (Delta H degrees, Delta S degrees and Delta G degrees ) for surfactant sorption on montmorillonite clay and Cr(VI) sorption by modified clay have been evaluated. The specific rate constant for sorption of Cr(VI) species on modified montmorillonite was rapid during the first 10 min and equilibrium was found to be attained within 30 min. The sorption of Cr(VI) species onto modified montmorillonite clay followed first-order rate kinetics. Copyright 2000 Academic Press.     

AM1 computational study of cycloaddition reaction of vinyl sulfene generated from thiete 1,1-dioxide

Theoretical studies of the cycloaddition reaction between vinyl sulfene, norbornene and related systems are presented. Since the parameterized AMI semiempirical method was used, the generated data are only of qualitative value. The vinyl sulfene reactivity was assessed by using frontier molecular orbital (FMO) energy gaps, bond orders, and the charge distribution of the reactants. To obtain more information, as far as the reaction outcome and its feasibility, the transition state structures and activation barriers for competitive pathways are computed.     

Theoretical study of azide anion addition to nonpolar and polar double and triple bonds

Transition structures for the 1,3-polar addition of azide anion to hydrogen cyanide, formaldimine, nitrogen, cis- andtrans -diazene, ethylene and acetylene were obtained at the MP2/6-31+G^* theoretical model. The additions can be divided into two groups: addition to a triple bond, giving rise to an aromatic heterocyclic product, and addition to a double bond, forming a non-aromatic product. All transition structures correspond to a concerted mechanism for the polar cycloaddition. Symmetrical dienophiles, apart from cis-diazene, give rise to synchronous transition structures. The anomaly is explained in terms of strong n-n repulsion of the reactants in the transition structure. The reactivity of the compounds can be rationalized in terms of the bond orders of the newly forming bonds, from the frontier orbital energy differences and from the charge transfer from the azide anion to the dienophile. The quantitative correlation of the reactivity has been judged on the basis of the activation energies of the reactions calculated at MP2/6-31+G^* and MP3/6-31+G^*. It is predicted that the addition of azide to nitrogen is the slowest and that the additions to hydrogen cyanide and acetylene have the lowest barriers, in agreement with literature data.     

Preparation of 5,5′‐pyrilidene and 5,5′‐quinolidene bis‐barbituric acid derivatives

NMR reaction following experiments were used to find optimal conditions for the barbituric acid double addition to aromatic and heteroaromatic carboxaldehydes. It was established that aromatic aldehydes with electron‐donating substituents such as hydroxy, methoxy, and dimethylamino produce only the single addition barbituric acid adduct (barbituric acid benzylidenes). If these electron‐donating substituents are transformed into electron‐withdrawing substituents by virtue of protonation (NMe₂ to NHMe₂⁺) then the double barbituric acid adduct becomes the sole product of the reaction. This is also true regardless of the reaction media if strong electron‐withdrawing substituents (such as a nitro group) are present. Considering that the reactive species for nitrogen containing aromatic heterocycles are actually the conjugated acids (electron deficient molecule) only the double barbituric acid adducts are isolated. All synthetic procedures presented are applicable to multi‐gram scale preparations of double barbituric acid adducts.