Ab Initio (ab + initio)

Distribution by Scientific Domains
Distribution within Chemistry

Terms modified by Ab Initio

  • ab initio calculation
  • ab initio computation
  • ab initio determination
  • ab initio investigation
  • ab initio level
  • ab initio method
  • ab initio methods
  • ab initio mo calculation
  • ab initio modeling
  • ab initio molecular dynamics
  • ab initio molecular dynamics simulation
  • ab initio molecular orbital calculation
  • ab initio molecular orbital theory
  • ab initio potential energy surface
  • ab initio prediction
  • ab initio simulation
  • ab initio studies
  • ab initio study
  • ab initio theory

  • Selected Abstracts


    Hetero-,-systems from 2 + 2 cycloreversion, part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH, O, SiH2, PH, S),

    HETEROATOM CHEMISTRY, Issue 7 2007
    Leonid E. Gusel'nikov
    Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3-diheterocyclobutanes, cyclo -(CH2CH2CH2X), and cyclo -(CH2XCH2X), where X = NH, O, SiH2, PH, S, was undertaken by calculating closed-shell singlet molecules at three levels of theory: MP4/6-311G(d)//MP2/6-31G(d)+ZPE, MP4/6-311G(d,p)//MP2/6-31G (d,p)+ZPE, and B3LYP/6-311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3-diazetidine and 1,3-dioxtane. Strain energies of the four-membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one-bond homolysis that results in the formation of the corresponding 1,4-diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH3CH2,CH2XH, CH3CH2,XCH3, and HXCH2,XCH3. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C,H and X,H bond energies in CH3XH calculated at G2 level of theory. Except 1,3-disiletane, in which ring-opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4-diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring-opening enthalpies were negative, the least exothermicity was calculated for , CH2SiH2CH2CH2. The only exception was 1,3-disiletane, which being diradical, CH2SiH2CH2SiH2, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3-disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704,720, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20377 [source]


    Sandwich compounds with central hypercoordinate carbon, nitrogen, and oxygen: A quantum-chemical study

    HETEROATOM CHEMISTRY, Issue 6 2006
    Ruslan M. Minyaev
    Ab initio (MP2(fu)/6-311+G**) and DFT (B3LYP/6-311+G**) calculations predict stable structures of sandwich compounds with the central carbon, nitrogen, and oxygen atoms surrounding lithium counterions between three-membered (BH)3 rings. The lithium counterions play a crucial role in the stabilization of these systems. The topological Bader analysis performed for these sandwich systems demonstrates that the central atom is linked to the ligands by nine-bond paths and, consequently, should be regarded as a nine-coordinated atom. Such a coordination number of carbon is the highest for all known main group organoelement compounds. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:464,474, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20268 [source]


    Revealing the Design Principles of High-Performance Biological Composites Using Ab initio and Multiscale Simulations: The Example of Lobster Cuticle

    ADVANCED MATERIALS, Issue 4 2010
    Svetoslav Nikolov
    Natural materials are hierarchically structured nanocomposites. A bottom-up multiscale approach to model the mechanical response of the chitin-based mineralized cuticle material of Homarus americanus is presented, by combining quantum-mechanical ab initio calculations with hierarchical homogenization. The simulations show how the mechanical properties are transferred from the atomic scale through a sequence of specifically designed microstructures to realize optimal stiffness. [source]


    Electronic spectrum of 2-pyridone+: Ab initio and time-dependent density functional calculations

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2010
    D. Hammoutčne
    Abstract In a comparative study, the doublet and quartet electronic states of the 2-pyridone+ cation are calculated using the PBE0/6-311+G(d,p) technique and the CASSCF and MRCI(+Q) methods in connection with the cc-pVDZ and cc-pVTZ Dunning's basis sets. Our data show that TD-DFT describes quite well the vertical excitation energies of these electronic states, whereas, multiconfiguration methods should be used for the investigation of the fragmentation and the dynamics of this molecular species. This is related to the change of the nature of the wavefunction of these electronic states along the corresponding reactive coordinates not accounted for by TD-DFT methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]


    Ab initio and DFT calculations of benzaldoxime elimination kinetics in the gas phase

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2008
    Jose R. Mora
    Abstract The mechanism for the gas-phase molecular elimination kinetics of benzaldoxime was examined at MP2/6-31G, MP2/6-31G(d,p), B3LYP/6-31G, B3LYP/6-31G(d,p), MPW1PW91/6-31G, and MPW1PW91/6-31G(d,p) levels of theory. The products of elimination of this oxime are benzonitrile and water. Calculated thermodynamic and kinetic parameters estimated from B3LYP/6-31G was found to be in better agreement with the experimental values. Transition state structure is best described as a four-membered cyclic structure with good approximation to planarity. NBO charges analysis revealed a little greater polarization of the benzylic C,H,+ rather than N,OH,,. Bond indexes and synchronicity parameters are in agreement with a concerted semi-polar type of mechanism with benzylic CH bond breaking as determining step of the reaction. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]


    Erratum for ",-Aminoacrolein: An Ab initio, AIM and NBO Study"

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2008
    Isaac Jimenez-Fabian
    No abstract is available for this article. [source]


    Quantum chemical studies on molecular structural conformations and hydrated forms of salicylamide and O-hydroxybenzoyl cyanide

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2005
    K. Anandan
    Abstract Ab initio and density functional theory (DFT) methods have been employed to study the molecular structural conformations and hydrated forms of both salicylamide (SAM) and O-hydroxybenzoyl cyanide (OHBC). Molecular geometries and energetics have been obtained in the gaseous phase by employing the Mřller,Plesset type 2 MP2/6-311G(2d,2p) and B3LYP/6-311G(2d,2p) levels of theory. The presence of an electron-releasing group (SAM) leads to an increase in the energy of the molecular system, while the presence of an electron-withdrawing group (OHBC) drastically decreases the energy. Chemical reactivity parameters (, and ,) have been calculated using the energy values of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) obtained at the Hartree,Fock (HF)/6-311G(2d,2p) level of theory for all the conformers and the principle of maximum hardness (MHP) has been tested. The condensed Fukui functions have been calculated using the atomic charges obtained through the natural bond orbital (NBO) analysis scheme for all the optimized structures at the B3LYP/6-311G(2d,2p) level of theory, and the most reactive sites of the molecules have been identified. Nuclear magnetic resonance (NMR) studies have been carried out at the B3LYP/6-311G(2d,2p) level of theory for all the conformers in the gaseous phase on the basis of the method of Cheeseman and coworkers. The calculated chemical shift values have been used to discuss the delocalization activity of the electron clouds. The dimeric structures of the most stable conformers of both SAM and OHBC in the gaseous phase have been optimized at the B3LYP/6-311G(2d,2p) level of theory, and the interaction energies have been calculated. The most stable conformers of both compounds bear an intramolecular hydrogen bond, which gives rise to the formation of a pseudo-aromatic ring. These conformers have been allowed to interact with the water molecule. Special emphasis has been given to analysis of the intermolecular hydrogen bonds of the hydrated conformers. Self-consistent reaction field (SCRF) theory has been employed to optimize all the conformers in the aqueous phase (, = 78.39) at the B3LYP/6-311G(2d,2p) level of theory, and the solvent effect has been studied. Vibrational frequency analysis has been performed for all the optimized structures at MP2/6-311G(2d,2p) level of theory, and the stationary points corresponding to local minima without imaginary frequencies have been obtained for all the molecular structures. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]


    Molecular structure, conformational analysis and charge distribution of pralidoxime: Ab initio and DFT studies

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2002
    Alexandre T. Castro
    Abstract A detailed structural study of pralidoxime (2-PAM), the main antidote against organophosphate intoxication, was performed using Hartree-Fock, Möller,Plesset (MP2), and density functional theory (Becke, Lee, Yang, and Paar [B3LYP]) methods. Rotational barriers, equilibrium geometries, and charge distributions were calculated, showing important differences between the two forms available in physiological conditions, namely with the oxime group protonated or unprotonated. For the protonated form, conjugation between the side chain and the pyridinium ring, although present, has little importance, resulting in a flexible structure. On the other hand, the unprotonated form has a more rigid structure and a smaller charge density on the oxime oxygen. Contrary to the common belief, those results strongly suggest that it may be the protonated form of 2-PAM, instead of the unprotonated form, that is responsible for the antidote activity of this compound. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source]


    Fundamental vibrational frequencies and dominant resonances in methylamine isotopologues by ab initio and density functional theory methods,

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2008
    Chen Levi
    Abstract Ab initio and density functional theory (DFT) calculations were performed for obtaining fundamental vibrational frequencies of methylamine, CH3NH2, and its deuterated variants CH3ND2, CD3NH2, and CD3ND2. The calculations were carried out using the CCSD(T) coupled cluster approximation with cc-pVTZ and cc-pVQZ basis sets, and by the DFT method with the semiempirical hybrid functional B97-1 with polarization consistent pc-2 and pc-3 basis sets. Reasonable performance of the DFT harmonic and ab initio harmonic calculations was found, which improved considerably upon combination of the harmonic fundamental frequencies with anharmonic corrections from the smaller, pc-2, basis. The computed anharmonic fundamental frequencies of methylamine isotopologues agree very well with the experimental values and represent a useful tool for assignment and analysis of the dominant resonances. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source]


    Ab initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexes

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2002
    H. Valdés
    Abstract Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm,1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 444,455, 2002; DOI 10.1002/jcc.10041 [source]


    Molecular mechanics (MM4) calculations on carbonyl compounds part I: aldehydes

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2001
    Charles H. Langley
    Abstract Aliphatic aldehydes have been studied with the aid of the MM4 force field. The structures, moments of inertia, vibrational spectra, conformational energies, barriers to internal rotation, and dipole moments have been examined for six compounds (nine conformations). MM4 parameters have been developed to fit the indicated quantities to the wide variety of experimental data. Ab initio (MP2) and density functional theory (B3LYP) calculations have been used to augment and/or replace experimental data, as appropriate. Because more, and to some extent, better, data have become available since MM3 was developed, it was anticipated that the overall accuracy of the information calculated with MM4 would be better than with MM3. The best single measure of the overall accuracy of a force field is the accuracy to which the moments of inertia of a set of compounds (from microwave spectroscopy) can be reproduced. For all of the 20 moments (seven conformations) experimentally known for the aldehyde compounds, the MM4 rms error is 0.30%, while with MM3, the most accurate force field presently available, the rms error over the same set is 1.01%. The calculation of the vibrational spectra was also improved overall. For the four aldehydes that were fully analyzed (over a total of 78 frequencies), the rms errors with MM4 and MM3 are 18 and 38 cm,1, respectively. These improvements came from several sources, but the major ones were separate parameters involving the carbonyl carbon for formaldehyde, the alkyl aldehydes and the ketones, and new crossterms featured in the MM4 force field that are not present in the MM3 version. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1396,1425, 2001 [source]


    Benzdiynes revisited: ab initio and density functional theory

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2001
    Sundaram Arulmozhiraja
    Abstract Ab initio and density functional theory (DFT) studies were performed on three isomers of tetradehydrobenzene (benzdiynes). Four different density functionals (BPW91, BLYP, B3LYP, and B1LYP) and two higher levels of theory [QCISD and CCSD(T)] incorporating basis sets up to Dunning's correlation-consistent polarized valence triple-, (cc-pVTZ) were utilized for this purpose. Stability tests showed that more stable solutions were available for 1,4-benzdiyne with unrestricted than with restricted DFT, while solutions obtained with later descriptions of 1,3-benzdiyne and 1,2,3,5-tetradehydrobenzene were stable. UB3LYP provided better geometry for 1,4-benzdiyne. Unlike in an earlier study, the energy difference between 1,2,3,5-tetradehydrobenzene and 1,4-benzdiyne calculated with the B3LYP functional coincided well with that calculated at CCSD(T): 13.07 and 14.32 kcal/mol at the B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels, respectively. 1,2,3,5-Tetradehydrobenzene was 8 kcal/mol more stable than 1,3-benzdiyne at the CCSD(T)/cc-pVTZ level. The heats of formation of benzdiynes were obtained by using the G2, CBS-Q, and CBS-QB3 methodologies. The heats of formation calculated for 1,3-benzdiyne and 1,2,3,5-tetradehydrobenzene were 208.6 and 197.9 kcal/mol, respectively, at the CBS-QB3 level of theory. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 923,930, 2001 [source]


    Vibrational spectroscopic studies, conformations and quantum chemical calculations of 3,3,3-trifluoropropyl- silane and 3,3,3-trifluoropropylsilane- d3,

    JOURNAL OF RAMAN SPECTROSCOPY, Issue 1-3 2006
    Peter Klaeboe
    Abstract Infrared spectra of 3,3,3-trifluoropropylsilane (CF3CH2CH2SiH3) and 3,3,3-trifluoropropylsilane- d3 (CF3 CH2CH2SiD3) were obtained in the vapour, liquid, and crystalline solid phases in the range 4000,50 cm,1. Additional spectra in argon matrices at 5 K were recorded before and after annealing to temperatures 20,34 K. Raman spectra of the compounds as liquids were recorded at various temperatures between 296 and 183 K and spectra of the amorphous and crystalline solids were obtained. The spectra revealed the existence of two conformers (anti and gauche) in the fluid phases and in the matrices. When the two vapours were shock-frozen on a cold finger at 78 K, they turned partly crystalline immediately. After subsequent annealing to 140,150 K, ca 7,9 Raman bands of both molecules present in the liquids vanished in the crystal. Similar variations in intensity were observed in the corresponding infrared spectra before and after annealing. The spectra revealed the existence of one conformer (anti) in the crystal. From Raman intensity variations of three independent pairs of anti and gauche bands between 298 and 173 K for the parent compound, and 298 and 183 K for the deuterated analogue, the values ,confHo(gauche,anti) = 4.1 ± 0.3 kJ mol,1 for the parent compound and the same value for the deuterated species were obtained in the liquid state. Annealing experiments in the matrices show that the gauche bands vanish after annealing, demonstrating that the anti conformer also has the lower energy here and that the barrier to gauche , anti inter-conversion is around 5,6 kJ mol,1. The spectra of both conformers have been interpreted in detail. Ab initio and DFT calculations at the HF/6,311G**, B3LYP/6,311 G** and MP2/6,311 G** levels gave optimized geometries, infrared and Raman intensities and vibrational wavenumbers for the anti and gauche conformers. The conformational enthalpy difference derived from the calculations was between 6.0 and 4.1 kJ mol,1 with anti being the low energy conformer. Copyright © 2006 John Wiley & Sons, Ltd. [source]


    Conformations and properties of the L -tryptophyl-containing peptides in solution, depending on the pH,Theoretical study vs. experiments

    BIOPOLYMERS, Issue 8 2010
    Bojidarka B. Ivanova
    Abstract The conformational preference and electronic properties of three L -tryptophyl-containing dipeptides, i.e., glycyl- L -tryptophane (H-Gly-Trp-OH), L -alanyl- L -tryptophane (H-Ala-Trp-OH), and L -methionyl- L -tryptophane (L -Met-Trp-OH) in solution depending on the pH of the media are studied both theoretically and experimentally. The effect of the protonation of the COO, and deprotonation of the NH as well as the alkaline hydrolysis of the amide fragment in a strong basic media on the electronic spectra are discussed. Ab initio and density functional theory (DFT) methods as well as the time-dependent DFT (TD-DFT) method as a function of the basis set are performed with a view to obtain the geometry and electronic properties of all of the species as well as the intermediate, obtained in the alkaline hydrolysis mechanism. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 727,734, 2010. This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source]


    Ab initio and AIM Theoretical Analysis of Hydrogen-Bond Radius of HD (D: F, Cl, Br, CN, HO, HS and CCH) Donors and Some Acceptors.

    CHEMINFORM, Issue 7 2007
    B. Raghavendra
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]


    Ab initio and Density Functional Study of Thionitroso XNS and Thiazyl Isomers XSN, X: H, F, Cl, Br, OH, SH, NH2, CH3, CF3, and SiF3.

    CHEMINFORM, Issue 33 2004
    Pablo A. Denis
    No abstract is available for this article. [source]


    Ab initio and DFT Study of the Molecular Mechanisms of SO3 and SOCl2 Reactions with Water in the Gas Phase.

    CHEMINFORM, Issue 25 2004
    Stanislav K. Ignatov
    No abstract is available for this article. [source]


    ChemInform Abstract: Ab initio and Density Functional Calculations on the Ring,Chain Tautomerism of ,-Oxocarboxylic Acids (I),(IV).

    CHEMINFORM, Issue 4 2002
    Walter M. F. Fabian
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]


    Ab initio and density functional theory studies of the structure, gas-phase acidity and aromaticity of tetraselenosquaric acid

    CHINESE JOURNAL OF CHEMISTRY, Issue 6 2000
    Li-Xin Zhou
    Abstract Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ,G*) and aromaticity of tetraselenosquaric acid (3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2 C4 Se4)are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6,311+G** /RHF/6 - 311 + G** ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G** //B3LYP/6 -311 + G**). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) -19.13 (CSGT(IGAIM)-RHF/6,311 + G**// RHF/6,311 + G** and -32.91 (4,·10,6 m,3/mol)(CSGT(I-GAIM)-B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is ,G1*(298k) = 1257.7 and ,G*2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4-dihydroxy-3-cyclobutene-1, 2-dione, H2 C4 3,4-dithiohydroxy-3-cyclobutene-1,2-dithione, H2C4 S4, 3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2C4Se4). [source]


    Calculated Enthalpies for Dimerisation of Binary, Unsaturated, Main-Group Element Hydrides as a Means to Analyse Their Potential for Multiple Bonding

    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2003
    Hans-Jörg Himmel
    Abstract Herein, the dimerisation of subvalent, binary, main-group element hydrides with the potential for multiple bonding is studied using both hybrid DFT (B3LYP) and ab initio [MP2 and CCSD(T)] methods. The [2+2] cycloaddition is an important and characteristic reaction of derivatives of ethylene. A comparison of dimerisation reactions for several compounds with the potential for multiple bonding should, therefore, shed light on the properties of these species. Our study includes the hydrides E2H2 (E = B, Al, Ga, N P or As), E2H4 (E = C, Si or Ge) and ENH4 (E = B, Al or Ga) and their dimers. Several isomeric forms of the monomers and dimers have to be considered. The trends within a group and a period are established and the factors responsible for them are discussed. It turns out that, generally, the enthalpies for dimerisation increase for heavier homologues, reflecting that the most important factor is the reduced strength of the E,E bonds in the monomers prior to dimerisation and, to some degree, also the reduced ring strain in the cyclic dimers. The exceptions are the dimerisations of B2H2 and Al2H2, both of which lead to the tetrahedral E4H4 species (E = B or Al). Dimerisation of Al2H2 is associated with a smaller enthalpy than that for the dimerisation of B2H2. Comparisons and analyses are made complicated because of the changes in the structures of the isomeric global minima between homologues. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]


    Cycloadditions and Methylene Transfer in Reactions of Substituted Thiocarbonyl S -Methylides with Thiobenzophenone: A Computational Study

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2005
    Reiner Sustmann
    Abstract Regiochemistry and methylene transfer reactions in cycloadditions of aliphatic thiocarbonyl S -methylides and thiobenzophenone are analyzed by ab initio [(U)HF/3-21G*] and DFT calculations [(U)B3LYP/6-31G*//(U)HF/3-21G* and (U)B3LYP/6-31G*]. The formation of regioisomeric 1,3-dithiolanes is explained by the competition of concerted (2,4-substituted 1,3-dithiolane) and stepwise cycloaddition via C,C -biradicals (4,5-substituted 1,3-dithiolane). Aliphatic thiocarbonyl S -methylides with sterically demanding substituents undergo substantial methylene transfer in the reaction with thiobenzophenone. This process involves dissociation of the C,C -biradical intermediate with liberation of thiobenzophenone S -methylide which, in turn, combines with a second molecule of thiobenzophenone. Calculated activation parameters for the different processes are in agreement with the experimental observations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]


    A Theoretical ab initio Study of the Capacity of Several Binding Units for the Molecular Recognition of Anions

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 1 2005
    Carolina Garau
    Abstract Noncovalent bonding between anions and the ,-cloud of electron-deficient aromatic rings has recently attracted considerable attention. Complexes of bromide with different electron-deficient aromatic rings and with other binding units based on hydrogen bonding (urea, thiourea and squaramide) have been studied and compared using high level B3LYP/6-31++G** ab initio and Molecular Interaction Potential with polarization (MIPp) calculations. Our findings reveal that electron-deficient aromatic rings are excellent candidates for constructing anion receptors and that the interaction strength can be modulated by the nature of the ring. Supporting experimental evidence has been obtained from X-ray structures retrieved from the Cambridge Structural Database. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]


    A Mechanistic Rationale for the Mode Selectivity in the Intramolecular Cyclization of Ethylene-Tethered Iminoketenimines: [2+2] versus [4+2] Stepwise Cycloadditions

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2004
    Mateo Alajarín
    Abstract A stepwise mechanism, via a zwitterionic intermediate, has been established by ab initio and DFT calculations for the intramolecular cyclization of N -(3-azabut-3-enyl)ketenimine into its corresponding [2+2] cycloadduct. The control of the mode selectivity ([2+2] versus [4+2] cycloaddition) in the intramolecular cyclization of C -vinyl- N -(iminoethylene)ketenimines, which favors the [4+2] cycloadducts, also has been rationalized by comparing the energies calculated for both reaction pathways; the results have been confirmed by a quantitative kinetic analysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]


    Retrospective selection bias (or the benefit of hindsight)

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001
    Francesco Mulargia
    SUMMARY The complexity of geophysical systems makes modelling them a formidable task, and in many cases research studies are still in the phenomenological stage. In earthquake physics, long timescales and the lack of any natural laboratory restrict research to retrospective analysis of data. Such ,fishing expedition' approaches lead to optimal selection of data, albeit not always consciously. This introduces significant biases, which are capable of falsely representing simple statistical fluctuations as significant anomalies requiring fundamental explanations. This paper identifies three different strategies for discriminating real issues from artefacts generated retrospectively. The first attempts to identify ab initio each optimal choice and account for it. Unfortunately, a satisfactory solution can only be achieved in particular cases. The second strategy acknowledges this difficulty as well as the unavoidable existence of bias, and classifies all ,anomalous' observations as artefacts unless their retrospective probability of occurrence is exceedingly low (for instance, beyond six standard deviations). However, such a strategy is also likely to reject some scientifically important anomalies. The third strategy relies on two separate steps with learning and validation performed on effectively independent sets of data. This approach appears to be preferable in the case of small samples, such as are frequently encountered in geophysics, but the requirement for forward validation implies long waiting times before credible conclusions can be reached. A practical application to pattern recognition, which is the prototype of retrospective ,fishing expeditions', is presented, illustrating that valid conclusions are hard to find. [source]


    Reactivity and Selectivity of Captodative Olefins as Dienes in Hetero -Diels,Alder Reactions

    HELVETICA CHIMICA ACTA, Issue 10 2008
    Rubén Sanabria
    Abstract The reactivity and selectivity of the the captodative olefins 1-acylvinyl benzoates 1a,1f and 3a as heterodienes in hetero -Diels,Alder reactions in the presence of electron-rich dienophiles is described. Heterodienes 1 undergo regioselective cycloaddition with the alkyl vinyl etherdienophiles 6a,b and 9 to give the corresponding dihydro-2H -pyrans 7, 8, and 10 under thermal conditions. The reactivity of these cycloadditions depends, to a large extent, on the electronic demand of the substituent in the aroyloxy group of the heterodiene. Frontier-molecular-orbital (FMO; ab initio) and density-functional-theory (DFT) calculations of the ground and transition states account for the reactivity and regioselectivity observed in these processes. [source]


    Structure Investigation of Bridgehead Aziridine: Synthesis, Theoretical, and Crystallographic Study of 2,4,6-Triphenyl-1,3-diazabicyclo[3.1.0]hex-3-ene

    HELVETICA CHIMICA ACTA, Issue 2 2006
    Giuseppe Bruno
    Abstract A one-pot three-component procedure to efficiently create the 1,3-diazabicyclo[3.1.0]hex-3-ene system is reported. The molecular structure of 2,4,6-triphenyl-1,3-diazabicyclo[3.1.0]hex-3-ene (3) was studied by X-ray diffraction and compared to ab initio and density-functional-theory (DFT) calculations restricted to the core moiety. Geometry optimizations for structural isomers and tautomeric forms of this aziridine fragment, taken as simplified models, were carried out at high calculation levels. Moreover, the same methods were utilized to evaluate the proton affinity of two crucial aziridine tautomers. [source]


    Experimental and theoretical studies on some new pyrrol-2,3-diones formation

    HETEROATOM CHEMISTRY, Issue 1 2004
    Ismail Yildirim
    4-Benzoyl-5-phenyl-2,3-furandione (1) reacts with asymmetric disubstituted urea derivatives like 1,1-dimethylurea (2a) and 1,1-diethylurea (2b) by the elimination of a H2O molecule to give the 4-benzoyl-1-(N,N -dialkylcarbamyl)-5-phenyl-2,3-pyrroldiones 3a and 3b. The structures of 3a,b were determined by the 13C NMR, 1H NMR, IR spectroscopic data and elemental analyses. The electronic structures of the reactants, their transition states, intermediate states, and final products of the reactions were investigated on the basis of AM1 and ab initio (DFT) methods. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 15:9,14, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10204 [source]


    Toward Site-Specific Stamping of Graphene

    ADVANCED MATERIALS, Issue 12 2009
    Dongsheng Li
    A physical hypothesis based on theory and ab initio modeling for the stamping of graphene and few-layers graphene (FLG) on silica is presented, and the feasibility of site-specific stamping of FLG patterns is demonstrated experimentally (figure shows ten-layer FLG pattern stamped on silica). The site-specific nature of the patterned stamps could enable high-throughput fabrication of future graphene-based integrated devices. [source]


    Measurements of the kinetics of the OH + ,-pinene and OH + ,-pinene reactions at low pressure

    INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2002
    B. Chuong
    The rate constants for the OH + ,-pinene and OH + ,-pinene reactions have been measured in 5 Torr of He using discharge-flow systems coupled with resonance fluorescence and laser-induced fluorescence detection of the OH radical. At room temperature, the measured effective bimolecular rate constant for the OH + ,-pinene reaction was (6.08 ± 0.24) × 10,11 cm3 molecule,1 s,1. These results are in excellent agreement with previous absolute measurements of this rate constant, but are approximately 13% greater than the value currently recommended for atmospheric modeling. The measured effective bimolecular rate constant for the OH + ,-pinene reaction at room temperature was (7.72 ± 0.44) × 10,11 cm3 molecule,1 s,1, in excellent agreement with previous measurements and current recommendations. Above 300 K, the effective bimolecular rate constants for these reactions display a negative temperature dependence suggesting that OH addition dominates the reaction mechanisms under these conditions. This negative temperature dependence is larger than that observed at higher pressures. The measured rate constants for the OH + ,-pinene and OH + ,-pinene reactions are in good agreement with established reactivity trends relating the rate constant for OH + alkene reactions with the ionization potential of the alkene when ab initio calculated energies for the highest occupied molecular orbital are used as surrogates for the ionization potentials for ,- and ,-pinene. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 300,308, 2002 [source]


    Kinetics and mechanism for the H-for-X exchange process in the H + C6H5X reactions: A computational study

    INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2001
    I. V. Tokmakov
    The addition of H atoms to benzene and toluene and subsequent transformations were investigated using high level ab initio and density functional theory methods. Molecular structures and vibrational frequencies calculated at the B3LYP/6-311++G(d,p) level of theory were used in combination with adjusted G2M energetic parameters for RRKM rate constant calculations. Standard heats of formation for cyclohexadienyl and cyclohexadienyl, 6-methyl radicals calculated through isodesmic reactions amounted to 49.5 ± 2 and 42.9 ± 3 kcal/mol, respectively. Rate constants for various elementary reactions involved in the H-for-X exchange (X = D, CH3) were calculated and closely correlated with the available experimental kinetic data. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 633,653, 2001 [source]