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  • Kraka, Elfi  (165)
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  • 1
    Language: English
    In: The journal of physical chemistry. A, 10 September 2015, Vol.119(36), pp.9541-56
    Description: Bond anomalies have been investigated for a set of 53 molecules with either N-F, Ti-P, Cr-H, Pb-C, or Pb-F bonds for which reverse rather than inverse bond length-bond strength relationships have been previously claimed. The intrinsic strength of each bond investigated was determined utilizing the associated local stretching force constant obtained at the CCSD(T)/aug-cc-pVTZ level of theory. For the metal containing molecules, LC-ωPBE calculations with the aug-cc-pVTZ (Cr, Pb) and the 6-31++G(d,p) basis set (Ti) were carried out. For bonds containing a metal atom, any bond anomaly could not be confirmed. Previously reported results were due to ill-defined bond strength descriptors or lacking accuracy. In the case of the fluoro amines, methyl fluoro amines, and the fluoro amine oxides, direct or hidden bond anomalies were detected, which result from two or more opposing electronic effects: a dominant bond shortening effect due to electron withdrawal and a bond weakening due to lone pair repulsion or hybridization defects. Bond anomalies can be disguised by a complex interplay of electronic effects. These hidden bond anomalies could be identified in this work for the fluoro amine chalcogenides.
    Keywords: Chemistry;
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 2
    Language: English
    In: The journal of physical chemistry. A, 12 September 2013, Vol.117(36), pp.8981-95
    Description: Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH](+)((1)Σ(+)), [HCO](+)((1)Σ(+)), [HNN](+)((1)Σ(+)), and [HNNH](2+)((1)Σg(+)). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 is found for [HNNH](2+) using scaled CCSD(T)/CBS frequencies determined for CCSD(T)/CBS geometries. Less strong is the NN bond in [FNNH](2+) and [FNNF](2+).
    Keywords: Chemistry;
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 3
    Language: English
    In: Inorganic chemistry, 06 January 2014, Vol.53(1), pp.478-95
    Description: Tolman's electronic parameter (TEP) derived from the A1-symmetrical CO stretching frequency of nickel-phosphine-tricarbonyl complexes, R3PNi(CO)3, is brought to a new, improved level by replacing normal with local vibrational frequencies. CO normal vibrational frequencies are always flawed by mode-mode coupling especially with metal-carbon stretching modes, which leads to coupling frequencies as large as 100 cm(-1) and can become even larger when the transition metal and the number of ligands is changed. Local TEP (LTEP) values, being based on local CO stretching force constants rather than normal mode frequencies, no longer suffer from mode coupling and mass effects. For 42 nickel complexes of the type LNi(CO)3, it is shown that LTEP values provide a different ordering of ligand electronic effects as previously suggested by TEP and CEP values. The general applicability of the LTEP concept is demonstrated.
    Keywords: Chemistry;
    ISSN: 00201669
    E-ISSN: 1520-510X
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  • 4
    Language: English
    In: The Journal of organic chemistry, 21 October 2016, Vol.81(20), pp.9669-9686
    Description: Vibrational frequencies can be measured and calculated with high precision. Therefore, they are excellent tools for analyzing the electronic structure of a molecule. In this connection, the properties of the local vibrational modes of a molecule are best suited. A new procedure is described, which utilizes local CC stretching force constants to derive an aromaticity index (AI) that quantitatively determines the degree of π-delocalization in a cyclic conjugated system. Using Kekulé benzene as a suitable reference, the AIs of 30 mono- and polycyclic conjugated hydrocarbons are calculated. The AI turns out to describe π-delocalization in a balanced way by correctly describing local aromatic units, peripheral, and all-bond delocalization. When comparing the AI with the harmonic oscillator model of AI, the latter is found to exaggerate the antiaromaticity of true and potential 4n π-systems or to wrongly describe local aromaticity. This is a result of a failure of the Badger relationship (the shorter bond is always the stronger bond), which is only a rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI confirms Clar's rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.
    Keywords: Chemistry;
    ISSN: 00223263
    E-ISSN: 1520-6904
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  • 5
    Language: English
    In: The journal of physical chemistry. A, 05 March 2015, Vol.119(9), pp.1443-5
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 6
    Language: English
    In: The Journal of Chemical Physics, 28 February 2014, Vol.140(8)
    Description: The formic acid dimer in its C 2h -symmetrical cyclic form is stabilized by two equivalent H-bonds. The currently accepted interaction energy is 18.75 kcal/mol whereas the experimental binding energy D 0 value is only 14.22 ±0.12 kcal/mol [ F. Kollipost, R. W. Larsen, A. V. Domanskaya, M. Nörenberg, and M. A. Suhm, J. Chem. Phys. 136 , 151101 ( 2012 ) ]. Calculation of the binding energies D e and D 0 at the CCSD(T) (Coupled Cluster with Single and Double excitations and perturbative Triple excitations)/CBS (Complete Basis Set) level of theory, utilizing CCSD(T)/CBS geometries and the frequencies of the dimer and monomer, reveals that there is a 3.2 kcal/mol difference between interaction energy and binding energy D e , which results from (i) not relaxing the geometry of the monomers upon dissociation of the dimer and (ii) approximating CCSD(T) correlation effects with MP2. The most accurate CCSD(T)/CBS values obtained in this work are D e = 15.55 and D 0 = 14.32 kcal/mol where the latter binding energy differs from the experimental value by 0.1 kcal/mol. The necessity of employing augmented VQZ and VPZ calculations and relaxing monomer geometries of H-bonded complexes upon dissociation to obtain reliable binding energies is emphasized.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
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  • 7
    Language: English
    In: The journal of physical chemistry. A, 05 March 2015, Vol.119(9), pp.1642-56
    Description: A set of 36 pnicogen homo- and heterodimers, R3E···ER3 and R3E···E′R′3, involving differently substituted group Va elements E = N, P, and As has been investigated at the ωB97X-D/aug-cc-pVTZ level of theory to determine the strength of the pnicogen bond with the help of the local E···E′ stretching force constants k(a). The latter are directly related to the amount of charge transferred from an E donor lone pair orbital to an E′ acceptor σ* orbital, in the sense of a through-space anomeric effect. This leads to a buildup of electron density in the intermonomer region and a distinct pnicogen bond strength order quantitatively assessed via k(a). However, the complex binding energy ΔE depends only partly on the pnicogen bond strength as H,E-attractions, H-bonding, dipole-dipole, or multipole-multipole attractions also contribute to the stability of pnicogen bonded dimers. A variation from through-space anomeric to second order hyperonjugative, and skewed π,π interactions is observed. Charge transfer into a π* substituent orbital of the acceptor increases the absolute value of ΔE by electrostatic effects but has a smaller impact on the pnicogen bond strength. A set of 10 dimers obtains its stability from covalent pnicogen bonding whereas all other dimers are stabilized by electrostatic interactions. The latter are quantified by the magnitude of the local intermonomer bending force constants XE···E′. Analysis of the frontier orbitals of monomer and dimer in connection with the investigation of electron difference densities, and atomic charges lead to a simple rationalization of the various facets of pnicogen bonding. The temperature at which a given dimer is observable under experimental conditions is provided.
    Keywords: Arsenic -- Chemistry ; Fluorine -- Chemistry
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 8
    Language: English
    In: The Journal of Chemical Physics, 28 August 2012, Vol.137(8)
    Description: Information on the electronic structure of a molecule and its chemical bonds is encoded in the molecular normal vibrational modes. However, normal vibrational modes result from a coupling of local vibrational modes, which means that only the latter can provide detailed insight into bonding and other structural features. In this work, it is proven that the adiabatic internal coordinate vibrational modes of Konkoli and Cremer [ Int. J. Quantum Chem. 67 , 29 ( 1998 )] 10.1002/(SICI)1097-461X(1998)67:1〈29::AID-QUA3〉3.0.CO;2-0 represent a unique set of local modes that is directly related to the normal vibrational modes. The missing link between these two sets of modes are the compliance constants of Decius, which turn out to be the reciprocals of the local mode force constants of Konkoli and Cremer. Using the compliance constants matrix, the local mode frequencies of any molecule can be converted into its normal mode frequencies with the help of an adiabatic connection scheme that defines the coupling of the local modes in terms of coupling frequencies and reveals how avoided crossings between the local modes lead to changes in the character of the normal modes.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
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  • 9
    Language: English
    In: The journal of physical chemistry. A, 25 February 2016, Vol.120(7), pp.1097-111
    Description: The Diels-Alder reaction between 1,3-butadiene and ethene is investigated from far-out in the entrance channel to the very last step in the exit channel thus passing two bifurcation points and extending the range of the reaction valley studied with URVA (Unified Reaction Valley Approach) by 300% compared to previous studies. For the first time, the pre- and postchemical steps of the reaction are analyzed at the same level of theory as the actual chemical processes utilizing the path curvature and its decomposition into internal coordinate or curvilinear coordinate components. A first smaller charge transfer to the dienophile facilitates the rotation of gauche butadiene into its cis form. The actual chemical processes are initiated by a second larger charge transfer to the dienophile that facilitates pyramidalization of the reacting carbon centers, bond equalization, and biradicaloid formation of the reactants. The transition state is aromatically stabilized and moved by five path units into the entrance channel in line with the Hammond-Leffler postulate. The pseudorotation of the boat form into the halfchair of cyclohexene is analyzed. Predictions are made for the Diels-Alder reaction based on a 11-phase mechanism obtained by the URVA analysis.
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 10
    Language: English
    In: The journal of physical chemistry. A, 14 September 2017, Vol.121(36), pp.6845-6862
    Description: A diverse set of 100 chalcogen-bonded complexes comprising neutral, cationic, anionic, divalent, and double bonded chalcogens has been investigated using ωB97X-D/aug-cc-pVTZ to determine geometries, binding energies, electron and energy density distributions, difference density distributions, vibrational frequencies, local stretching force constants, and associated bond strength orders. The accuracy of ωB97X-D was accessed by CCSD(T)/aug-cc-pVTZ calculations of a subset of 12 complexes and by the CCSD(T)/aug-cc-pVTZ //ωB97X-D binding energies of 95 complexes. Most of the weak chalcogen bonds can be rationalized on the basis of electrostatic contributions, but as the bond becomes stronger, covalent contributions can assume a primary role in the strength and geometry of the complexes. Covalency in chalcogen bonds involves the charge transfer from a lone pair orbital of a Lewis base into the σ* orbital of a divalent chalcogen or a π* orbital of a double bonded chalcogen. We describe for the first time a symmetric chalcogen-bonded homodimer stabilized by a charge transfer from a lone pair orbital into a π* orbital. New polymeric materials based on chalcogen bonds should take advantage of the extra stabilization granted by multiple chalcogen bonds, as is shown for 1,2,5-telluradiazole dimers.
    Keywords: Chemistry;
    ISSN: 10895639
    E-ISSN: 1520-5215
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