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  • 1
    Electronic Resource
    Electronic Resource
    Equivalent quantum approach to nuclei and electrons in molecules (1993)
    s.l. : American Chemical Society
    Chemical reviews 93 (1993), S. 2007-2022 
    Details
    ISSN: 1520-6890
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/cr00022a003
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical analysis of electronic absorption spectra of vitamin B12 models (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 7522-7533 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-dependent density-functional theory (TD-DFT) is applied to analyze the electronic absorption spectra of vitamin B12. To accomplish this two model systems were considered: CN-[CoIII-corrin]-CN (dicyanocobinamide, DCC) and imidazole-[CoIII-corrin]-CN (cyanocobalamin, ImCC). For both models 30 lowest excited states were calculated together with transition dipole moments. When the results of TD-DFT calculations were directly compared with experiment it was found that the theoretical values systematically overestimate experimental data by approximately 0.5 eV. The uniform adjustment of the calculated transition energies allowed detailed analysis of electronic absorption spectra of vitamin B12 models. All absorption bands in spectral range 2.0–5.0 eV were readily assigned. In particular, TD-DFT calculations were able to explain the origin of the shift of the lowest absorption band caused by replacement of the–CN axial ligand by imidazole. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1405436
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  • 3
    Electronic Resource
    Electronic Resource
    Potential symmetry breaking, structure and definite vibrational assignment for azulene: Multiconfigurational and density functional results (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5650-5661 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic ab initio determination of the geometry and harmonic force field with inclusion of correlation energy for the ground state of azulene is presented. The calculations were carried out using the unrestricted natural orbital-complete active space (UNO-CAS) method and density functional theory (DFT) using the compound Becke–Lee–Young–Parr (B3-LYP) exchange-correlation functional. The 6-31G* basis set has been used in both methods. The geometry optimized with DFT leads to a structure with approximately equal C–C bond lengths and C2v symmetry, while UNO-CAS results show an alternating bond structure with Cs symmetry and a very low (∼32 cm−1) barrier height. Agreement of the calculated vibrational frequencies with experiment is very good for both methods, except in the B2 symmetry species which contains the bond alternation mode. In this species, the density functional results agree better with experiment than those based on a multiconfigurational wave function, showing that the minimum has C2v symmetry. All vibrations are assigned for azulene and its perdeuterated analog. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470547
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  • 4
    Electronic Resource
    Electronic Resource
    Considerations in constructing a multireference second-order perturbation theory (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3672-3682 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Several possible definitions for a multireference second-order perturbation theory are suggested. These are tested against some standard test problems from the literature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466355
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  • 5
    Electronic Resource
    Electronic Resource
    The inner-hydrogen migration and ground-state structure of porphycene (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5905-5913 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Following on from previous work on free-base porphyrin, we present the results of a comprehensive study on the structure and inner-hydrogen migration in porphycene, a structural isomer of porphyrin. We used density functional theory with the hybrid B3-LYP exchange-correlation functional, and both the 6-31G(d) and a triple-zeta double-polarization (TZ2P) basis set (the latter containing 726 contracted basis functions). Full geometry optimizations were carried out and all stationary points were characterized by vibrational analysis. A scaled quantum mechanical (SQM) treatment of the theoretical force constants shows convincingly that the trans-isomer is the ground state, with trans–trans inner-hydrogen migration taking place—as is the case with porphyrin—in a two-step process via a (highly unstable) cis intermediate. With the TZ2P basis, excluding zero-point effects, the trans–cis barrier height is 4.9 kcal/mol, the cis–trans energy difference is 2.4 kcal/mol and the reverse cis–trans barrier height is only 2.5 kcal/mol. We also map out and fully characterize an alternative, high-energy migration path involving a second, nonplanar cis isomer. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477213
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  • 6
    Electronic Resource
    Electronic Resource
    Implementation of analytical first derivatives for evaluation of the many-body nonadiabatic wave function with explicitly correlated Gaussian functions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 9013-9024 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A nonadiabatic many-particle wave function is generated using an expansion in terms of explicitly correlated Gaussian-type basis functions. In this approach, motions of all particles are correlated at the same time, and electrons and nuclei are distinguished via permutational symmetry. We utilize our newly proposed nonadiabatic variational approach [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)], which does not require the separation of the internal and external motions. The analytical first derivative of the variational functional with respect to the nonlinear parameters appearing in the basis functions are derived and implemented to find the minimum. Numerical examples for the ground state of the hydrogen molecule are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462259
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  • 7
    Electronic Resource
    Electronic Resource
    Newton–Raphson optimization of the many-body nonadiabatic wave function expressed in terms of explicitly correlated Gaussian functions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5063-5073 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A nonadiabatic many-body wave function is represented in terms of explicitly correlated Gaussian-type basis functions. Motions of all particles (nuclei and electrons) are treated equally and particles are distinguished via permutational symmetry. The nonadiabatic wave function is determined in a variational calculation with the use of the method proposed recently [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)]. In this approach no direct separation of the center-of-mass motion from the internal motion is required. The theory of analytical first and second derivatives of the variational functional with respect to the Gaussian exponents and its computational implementation in conjunction with the Newton–Raphson optimization technique is described. Finally, some numerical examples are shown.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463827
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  • 8
    Electronic Resource
    Electronic Resource
    An effective method for generating nonadiabatic many-body wave function using explicitly correlated Gaussian-type functions (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6681-6698 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: General formalism for the application of explicitly correlated Gaussian-type basis functions for nonadiabatic calculations on many-body systems is presented. In this approach the motions of all particles are correlated in the same time. The energy associated with the external degrees of freedom, i.e., the motion of the center of mass, is eliminated in an effective way from the total energy of the system. In order to achieve this, methodology for construction of the many-body nonadiabatic wave function and algorithms for evaluation of the multicenter and multiparticle integrals involving explicitly correlated Gaussian cluster functions are derived. Next the computational implementation of the method is discussed. Finally, variational calculations for a model three-body system are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461538
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  • 9
    Electronic Resource
    Electronic Resource
    Will the real FeCO please stand up? (1997)
    Springer
    JBIC 2 (1997), S. 516-520 
    Details
    ISSN: 1432-1327
    Keywords: Key words Myoglobin-CO ; Bond distortion ; Bond energy ; CO poisoning ; Hydrogen bond ; Infrared ; Raman
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract  The paradigm that nature protects us from CO poisoning by forcing the bound CO to bend over in heme proteins, thereby reducing its binding affinity, is now in textbooks, but is nevertheless problematic. Results from vibrational spectroscopy give no evidence for bent CO, although X-ray crystallography continues to indicate appreciable distortions in myoglobin. However, the energetic significance of the discrepancy is doubtful, since new Density Functional Theory calculations indicate that much less energy is required to distort the CO than had been thought, perhaps 2 kcal/mol or less. Binding studies on site-directed mutants of myoglobin suggest that steric hindrance by the distal histidine is worth ca. 1 kcal/mol. While the distal histidine does account for the discrimination by Mb against CO and in favor of O2, most of the effect is due to its H-bond with bound O2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s007750050164
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  • 10
    Electronic Resource
    Electronic Resource
    The inner-hydrogen migration in free base porphyrin (1997)
    Springer
    Theoretical chemistry accounts 97 (1997), S. 59-66 
    Details
    ISSN: 1432-2234
    Keywords: Key words: DFT ; Dynamics ; Barrier height ; Isomerization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract. We present the results of a comprehensive study on the inner-hydrogen migration in free base porphyrin, using density functional theory with the hybrid B3-LYP exchange-correlation functional, and both the 6-31G(d ) and a triple-zeta double-polarization (TZ2P) basis set. The latter computations, involving 726 contracted functions, are the largest calculations on this system to date. Full geometry optimization was carried out for the cis and trans minima, the transition state for trans-cis isomerization, and the symmetric stationary point for the synchronous trans-trans isomerization. All stationary points were characterized by vibrational analysis. Our results strongly support the conclusion, reached by earlier workers, that trans-trans hydrogen transfer occurs in a two-step process via a cis intermediate. With the TZ2P basis and including zero-point effects for the -h 2 isotopomer, the trans-cis barrier height is 13.1 kcal/mol, the cis-trans energy difference is 8.1 kcal/mol and the reverse cis-trans barrier height is 5.0 kcal/mol. The trans-cis barrier height agrees well with the value of Braun et al. (J Am Chem Soc (1996) 118: 7231) obtained from NMR line shapes and a modified Bell tunneling model, but our cis-trans energy difference is higher, and the reverse barrier is lower, than the values of Braun et al. Tunneling precludes the existence of -h 2 cis-porphyrin as an observable species, but the -d 2 and, especially, -t 2 isotopomers might be observable at low temperatures if the reverse barrier is higher than our calculated value. We predict the theoretical vibrational spectrum of cis-porphyrin and suggest that IR active modes at 566 cm−1 and 2333 cm−1 in the -d 2 isotopomer may be used to detect the presence of the cis intermediate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002140050237
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