Dieter Cremer - Academia.edu (original) (raw)

Papers by Dieter Cremer

Research paper thumbnail of Dimesityldioxirane

Journal of The American Chemical Society, 1997

... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wil... more ... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wilhelm Kappert, † Roland Boese,* ‡ Elfi Kraka,* § Carlos Sosa, and Dieter Cremer §. Contribution from the Lehrstuhl für Organische ...

Research paper thumbnail of An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density

Journal of Physical Chemistry A, 2000

DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyn... more DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those ...

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and .sigma.-aromaticity

Journal of The American Chemical Society, 1986

Research paper thumbnail of Formation of OH radicals in the gas phase ozonolysis of alkenes: the unexpected role of carbonyl oxides

Chemical Physics Letters, 1996

According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and ... more According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and OH radicals is unlikely in view of an activation enthalpy ΔΔHf0(298) of 31 kcal/mol. However, for dimethylcarbonyl oxide there is a low energy rearrangement mode (ΔΔHf0(298): 14.4 kca/mol) which involves a H atom of ghe methyl group and which leads to a hydroperoxy methyl ethene

Research paper thumbnail of Computer Design of Anticancer Drugs. A New Enediyne Warhead

Journal of The American Chemical Society, 2000

Research paper thumbnail of Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane — a CCSD(T), MR-AQCC and DFT investigation

Chemical Physics Letters, 1998

Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c... more Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c in the gas phase were Ž . Ž . Ž . investigated by carrying out CCSD T , MR-AQCC and B3LYP calculations with the 6-31G d, p , 6-311 q G 3df, 3pd and cc-VTZ2Pq f, d basis sets. The inclusion of f functions in the basis set was essential to determine the heat of formation o Ž . Ž . D H 298 of carbonyl oxide 1a and 2a to be 27.0 and y0.3 kcalrmol, respectively. With the latter value, we calculate the f same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of Ž . Ž . 18 kcalrmol to methylenebis oxy 3a , which is calculated to be 1.2 kcalrmol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcalrmol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcalrmol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. q 1998 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Energetics, Kinetics, and Product Distributions of the Reactions of Ozone with Ethene and 2,3Dimethyl2-butene

Journal of Physical Chemistry A, 1997

A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2 H ... more A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2 H 4 and (CH 3 ) 2 CdC(CH 3 ) 2 (TME), respectively, is carried out, using master equations and statistical rate theory. The thermochemical as well as the molecular data required are obtained from CCSD(T)/TZ2P and B3LYP/ DZP calculations. It is shown that the primary ozonides are not collisionally stabilized under atmospheric conditions. In the reaction sequence for O 3 + TME, the same is true for CH 2 dC(CH 3 )OOH formed from (CH 3 ) 2 COO, which completely dissociates to give OH radicals. However, in this system, a pressure dependence is predicted for the relative branching fractions of the reactions of the Criegee intermediate. Under atmospheric conditions, for both examples, the product yields obtained are in reasonable agreement with experimental results.

Research paper thumbnail of Description of conjugation and hyperconjugation in terms of electron distributions

Journal of The American Chemical Society, 1983

... Phys. 1980, 73, 2871-2883. (b) Bader, RF W.; Nguyen-Dang, TT Adu. Quantum Chem. 1981, 14, 63-... more ... Phys. 1980, 73, 2871-2883. (b) Bader, RF W.; Nguyen-Dang, TT Adu. Quantum Chem. 1981, 14, 63-124. ... 7 hu alom I., rhr tmon of an arliacror rrnd ;IC burin. Since a nucleus is a three-dimensional attractor (all three cunaiures of p are negative at a nucleus) iti asrw-iated Page ...

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 15. Three-membered rings: bent bonds, ring strain, and surface delocalization

Journal of The American Chemical Society, 1985

Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradien... more Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradiene, lH-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and n-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6~ (homoaromaticity) or 8~ electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.

Research paper thumbnail of The combination of density functional theory with multi-configuration methods – CAS-DFT

Chemical Physics Letters, 2000

CAS-DFT is presented as a method that allows an economical simultaneous treatment of static and d... more CAS-DFT is presented as a method that allows an economical simultaneous treatment of static and dynamic correlation effects in molecules with multi-reference character. Central problems of CAS-DFT concern the double counting of dynamic correlation effects and the choice of the proper input quantities for the DFT functional. Also, the question of treating both active and inactive orbitals in a consistent way is discussed. Test calculations with CAS-DFT for the ring opening of dioxirane and the excitation energies of methylene prove that the method works reasonably. q 2000 Elsevier Science B.V. All rights reserved. 0009-2614r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Chemische Bindungen ohne Bindungselektronendichte -reicht die Differenzdichteanalyse zur Bindungsbeschreibung aus

Research paper thumbnail of Dimesityldioxirane

Journal of The American Chemical Society, 1997

... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wil... more ... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wilhelm Kappert, † Roland Boese,* ‡ Elfi Kraka,* § Carlos Sosa, and Dieter Cremer §. Contribution from the Lehrstuhl für Organische ...

Research paper thumbnail of Chemical Bonds without Bonding Electron Density ? Does the Difference Electron-Density Analysis Suffice for a Description of the Chemical Bond

Angewandte Chemie-international Edition, 1984

... 81.41(4), Rh-CI 1-0 178.2(9), Rh-Asl-C12 104.9(3), Rh-As3-C24 105.7(3), As2-As1 -C 12 99.6(3)... more ... 81.41(4), Rh-CI 1-0 178.2(9), Rh-Asl-C12 104.9(3), Rh-As3-C24 105.7(3), As2-As1 -C 12 99.6(3), As2-As3-C24 97.6(3), As 1 -As2-C 18 98.2(3), As3-As2-Rh-As1 249.0(1), Asl-As2 245.0(2), As2-As3 243.3(1), Rh-As3 247.0(1), Rh-...

Research paper thumbnail of An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density

Journal of Physical Chemistry A, 2000

DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyn... more DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those ...

Research paper thumbnail of Kinetic and Theoretical Investigation of the Gas-Phase Ozonolysis of Isoprene: Carbonyl Oxides as an Important Source for OH Radicals in the Atmosphere

Journal of The American Chemical Society, 1997

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and .sigma.-aromaticity

Journal of The American Chemical Society, 1986

Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradien... more Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradiene, lH-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and n-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6~ (homoaromaticity) or 8~ electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.

Research paper thumbnail of Formation of OH radicals in the gas phase ozonolysis of alkenes: the unexpected role of carbonyl oxides

Chemical Physics Letters, 1996

According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and ... more According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and OH radicals is unlikely in view of an activation enthalpy ΔΔHf0(298) of 31 kcal/mol. However, for dimethylcarbonyl oxide there is a low energy rearrangement mode (ΔΔHf0(298): 14.4 kca/mol) which involves a H atom of ghe methyl group and which leads to a hydroperoxy methyl ethene

Research paper thumbnail of Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals

International Journal of Molecular Sciences, 2002

Unrestricted density functional theory (UDFT) can be used for the description of open-shell singl... more Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two-or multiconfigurational character (but a onedeterminantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective.

Research paper thumbnail of Computer Design of Anticancer Drugs. A New Enediyne Warhead

Journal of The American Chemical Society, 2000

Research paper thumbnail of Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane — a CCSD(T), MR-AQCC and DFT investigation

Chemical Physics Letters, 1998

Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c... more Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c in the gas phase were Ž . Ž . Ž . investigated by carrying out CCSD T , MR-AQCC and B3LYP calculations with the 6-31G d, p , 6-311 q G 3df, 3pd and cc-VTZ2Pq f, d basis sets. The inclusion of f functions in the basis set was essential to determine the heat of formation o Ž . Ž . D H 298 of carbonyl oxide 1a and 2a to be 27.0 and y0.3 kcalrmol, respectively. With the latter value, we calculate the f same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of Ž . Ž . 18 kcalrmol to methylenebis oxy 3a , which is calculated to be 1.2 kcalrmol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcalrmol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcalrmol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. q 1998 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Dimesityldioxirane

Journal of The American Chemical Society, 1997

... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wil... more ... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wilhelm Kappert, † Roland Boese,* ‡ Elfi Kraka,* § Carlos Sosa, and Dieter Cremer §. Contribution from the Lehrstuhl für Organische ...

Research paper thumbnail of An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density

Journal of Physical Chemistry A, 2000

DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyn... more DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those ...

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and .sigma.-aromaticity

Journal of The American Chemical Society, 1986

Research paper thumbnail of Formation of OH radicals in the gas phase ozonolysis of alkenes: the unexpected role of carbonyl oxides

Chemical Physics Letters, 1996

According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and ... more According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and OH radicals is unlikely in view of an activation enthalpy ΔΔHf0(298) of 31 kcal/mol. However, for dimethylcarbonyl oxide there is a low energy rearrangement mode (ΔΔHf0(298): 14.4 kca/mol) which involves a H atom of ghe methyl group and which leads to a hydroperoxy methyl ethene

Research paper thumbnail of Computer Design of Anticancer Drugs. A New Enediyne Warhead

Journal of The American Chemical Society, 2000

Research paper thumbnail of Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane — a CCSD(T), MR-AQCC and DFT investigation

Chemical Physics Letters, 1998

Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c... more Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c in the gas phase were Ž . Ž . Ž . investigated by carrying out CCSD T , MR-AQCC and B3LYP calculations with the 6-31G d, p , 6-311 q G 3df, 3pd and cc-VTZ2Pq f, d basis sets. The inclusion of f functions in the basis set was essential to determine the heat of formation o Ž . Ž . D H 298 of carbonyl oxide 1a and 2a to be 27.0 and y0.3 kcalrmol, respectively. With the latter value, we calculate the f same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of Ž . Ž . 18 kcalrmol to methylenebis oxy 3a , which is calculated to be 1.2 kcalrmol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcalrmol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcalrmol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. q 1998 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Energetics, Kinetics, and Product Distributions of the Reactions of Ozone with Ethene and 2,3Dimethyl2-butene

Journal of Physical Chemistry A, 1997

A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2 H ... more A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C 2 H 4 and (CH 3 ) 2 CdC(CH 3 ) 2 (TME), respectively, is carried out, using master equations and statistical rate theory. The thermochemical as well as the molecular data required are obtained from CCSD(T)/TZ2P and B3LYP/ DZP calculations. It is shown that the primary ozonides are not collisionally stabilized under atmospheric conditions. In the reaction sequence for O 3 + TME, the same is true for CH 2 dC(CH 3 )OOH formed from (CH 3 ) 2 COO, which completely dissociates to give OH radicals. However, in this system, a pressure dependence is predicted for the relative branching fractions of the reactions of the Criegee intermediate. Under atmospheric conditions, for both examples, the product yields obtained are in reasonable agreement with experimental results.

Research paper thumbnail of Description of conjugation and hyperconjugation in terms of electron distributions

Journal of The American Chemical Society, 1983

... Phys. 1980, 73, 2871-2883. (b) Bader, RF W.; Nguyen-Dang, TT Adu. Quantum Chem. 1981, 14, 63-... more ... Phys. 1980, 73, 2871-2883. (b) Bader, RF W.; Nguyen-Dang, TT Adu. Quantum Chem. 1981, 14, 63-124. ... 7 hu alom I., rhr tmon of an arliacror rrnd ;IC burin. Since a nucleus is a three-dimensional attractor (all three cunaiures of p are negative at a nucleus) iti asrw-iated Page ...

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 15. Three-membered rings: bent bonds, ring strain, and surface delocalization

Journal of The American Chemical Society, 1985

Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradien... more Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradiene, lH-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and n-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6~ (homoaromaticity) or 8~ electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.

Research paper thumbnail of The combination of density functional theory with multi-configuration methods – CAS-DFT

Chemical Physics Letters, 2000

CAS-DFT is presented as a method that allows an economical simultaneous treatment of static and d... more CAS-DFT is presented as a method that allows an economical simultaneous treatment of static and dynamic correlation effects in molecules with multi-reference character. Central problems of CAS-DFT concern the double counting of dynamic correlation effects and the choice of the proper input quantities for the DFT functional. Also, the question of treating both active and inactive orbitals in a consistent way is discussed. Test calculations with CAS-DFT for the ring opening of dioxirane and the excitation energies of methylene prove that the method works reasonably. q 2000 Elsevier Science B.V. All rights reserved. 0009-2614r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Chemische Bindungen ohne Bindungselektronendichte -reicht die Differenzdichteanalyse zur Bindungsbeschreibung aus

Research paper thumbnail of Dimesityldioxirane

Journal of The American Chemical Society, 1997

... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wil... more ... Wolfram Sander,* † Kerstin Schroeder, † Sengodagounder Muthusamy, † Andreas Kirschfeld, † Wilhelm Kappert, † Roland Boese,* ‡ Elfi Kraka,* § Carlos Sosa, and Dieter Cremer §. Contribution from the Lehrstuhl für Organische ...

Research paper thumbnail of Chemical Bonds without Bonding Electron Density ? Does the Difference Electron-Density Analysis Suffice for a Description of the Chemical Bond

Angewandte Chemie-international Edition, 1984

... 81.41(4), Rh-CI 1-0 178.2(9), Rh-Asl-C12 104.9(3), Rh-As3-C24 105.7(3), As2-As1 -C 12 99.6(3)... more ... 81.41(4), Rh-CI 1-0 178.2(9), Rh-Asl-C12 104.9(3), Rh-As3-C24 105.7(3), As2-As1 -C 12 99.6(3), As2-As3-C24 97.6(3), As 1 -As2-C 18 98.2(3), As3-As2-Rh-As1 249.0(1), Asl-As2 245.0(2), As2-As3 243.3(1), Rh-As3 247.0(1), Rh-...

Research paper thumbnail of An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT: Stability Test, Spin Density, and On-Top Pair Density

Journal of Physical Chemistry A, 2000

DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyn... more DFT calculations provide a reliable description of the Bergman reaction of (Z)-hex-3-ene-1,5-diyne 1 provided the following are considered. (a) Restricted DFT (RDFT) calculations along the reaction path have to be replaced by unrestricted DFT (UDFT) calculations at those ...

Research paper thumbnail of Kinetic and Theoretical Investigation of the Gas-Phase Ozonolysis of Isoprene: Carbonyl Oxides as an Important Source for OH Radicals in the Atmosphere

Journal of The American Chemical Society, 1997

Research paper thumbnail of Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane carbon-carbon and carbon-hydrogen bond energies, 1,3 interactions, and .sigma.-aromaticity

Journal of The American Chemical Society, 1986

Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradien... more Restricted Hartree-Fock calculations have been carried out for 1,3,&cycloheptatriene, norcaradiene, lH-azepine, benzenimine (azanorcardiene), oxepine and benzene oxide (oxanorcaradiene) employing the STO-3G, 4-31G and 6-31G* basis sets. Theoretical geometries, conformations and barriers to ring inversion have been obtained and compared with the available experimental structural data. It has been found that the cyclotrienes possess a boat conformation with a constant admixture of 22% chair character leading to a flattening of the triene part and n-electron delocalization typical for a planar polyene. The structural data obtained in this work suggest that cyclic delocalization of either 6~ (homoaromaticity) or 8~ electrons (antiaromaticity) is not present in the three cyclotrienes. The former possibility, however, cannot be excluded in the case of the three norcaradienes.

Research paper thumbnail of Formation of OH radicals in the gas phase ozonolysis of alkenes: the unexpected role of carbonyl oxides

Chemical Physics Letters, 1996

According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and ... more According to CCSD(T)/TZ + 2P calculations, the decomposition of carbonyl oxide, H2COO to HCO and OH radicals is unlikely in view of an activation enthalpy ΔΔHf0(298) of 31 kcal/mol. However, for dimethylcarbonyl oxide there is a low energy rearrangement mode (ΔΔHf0(298): 14.4 kca/mol) which involves a H atom of ghe methyl group and which leads to a hydroperoxy methyl ethene

Research paper thumbnail of Can Unrestricted Density-Functional Theory Describe Open Shell Singlet Biradicals

International Journal of Molecular Sciences, 2002

Unrestricted density functional theory (UDFT) can be used for the description of open-shell singl... more Unrestricted density functional theory (UDFT) can be used for the description of open-shell singlet (OSS) biradicals provided a number of precautions are considered. Biradicals that require a two-determinantal wave function (e.g. OSS state of carbenes) cannot be described by UDFT for principal reasons. However, if the overlap between the open-shell orbitals is small (the single electrons are located at different atomic centers) errors become small and, then, the principal failure of UDFT in these cases is not apparent and may even be disguised by the fact that UDFT has the advantage of describing spin polarization better than any restricted open shell DFT method. In the case of OSS biradicals with two-or multiconfigurational character (but a onedeterminantal form of the leading configuration), reasonable results can be obtained by broken-symmetry (BS)-UDFT, however in each case this has to be checked. In no case is it reasonable to lower the symmetry of a molecule to get a suitable UDFT description. Hybrid functionals such as B3LYP perform better than pure DFT functionals in BS-UDFT calculations because the former reduce the self-interaction error of DFT exchange functionals, which mimics unspecified static electron correlation effects, so that the inclusion of specific static electron correlation effects via the form of the wavefunction becomes more effective.

Research paper thumbnail of Computer Design of Anticancer Drugs. A New Enediyne Warhead

Journal of The American Chemical Society, 2000

Research paper thumbnail of Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane — a CCSD(T), MR-AQCC and DFT investigation

Chemical Physics Letters, 1998

Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c... more Ž . Ž . Formation and decomposition of dioxirane 2a , methyldioxirane 2b and dimethyldioxirane 2c in the gas phase were Ž . Ž . Ž . investigated by carrying out CCSD T , MR-AQCC and B3LYP calculations with the 6-31G d, p , 6-311 q G 3df, 3pd and cc-VTZ2Pq f, d basis sets. The inclusion of f functions in the basis set was essential to determine the heat of formation o Ž . Ž . D H 298 of carbonyl oxide 1a and 2a to be 27.0 and y0.3 kcalrmol, respectively. With the latter value, we calculate the f same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of Ž . Ž . 18 kcalrmol to methylenebis oxy 3a , which is calculated to be 1.2 kcalrmol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcalrmol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcalrmol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. q 1998 Elsevier Science B.V. All rights reserved.