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Wiktor Zierkiewicz

Bio: Wiktor Zierkiewicz is an academic researcher from Wrocław University of Technology. The author has contributed to research in topics: Natural bond orbital & Hydrogen bond. The author has an hindex of 24, co-authored 99 publications receiving 1941 citations. Previous affiliations of Wiktor Zierkiewicz include Royal Institute of Technology & Academy of Sciences of the Czech Republic.


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TL;DR: In this article, a detailed analysis of the aniline radical cation was performed by using the unrestricted density functional (UB3LYP) and second-order Moller-Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set.
Abstract: Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C6H5NH2+ have been performed by using the unrestricted density functional (UB3LYP) and second-order Moller–Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital (NBO) analysis has revealed that the pπ-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule (an ortho- and para-directing power of the NH2 group in electrophilic substitutions), whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/6-311++G(df,pd) level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear- cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm−1, in the Raman spectrum. The redshift of the N–H stretching frequencies and the blueshift of the C–H stretching frequencies are observed in aniline, upon ionization. As revealed by NBO analysis, the frequency shifts can be correlated with the increase of electron density (ED) on the antibonding orbitals (σNH*) and decrease of ED on σCH*, respectively. These effects are associated with a weakening of N–H bonds and strengthening of C–H bonds in the aniline radical cation. The simulated theoretical Raman and infrared spectra of aniline and its radical cation, reported in this work, can be used in further spectroscopic studies of their van der Waals clusters and hydrogen bonded complexes.

160 citations

Journal ArticleDOI
TL;DR: Details of the study illustrate that the use of TEA results in an active catalyst that has only one ligand bound to the Pd, resulting in a significant lowering of the activation energy for beta-hydride elimination and, therefore, a catalyst that is active at room temperature.
Abstract: The experimental and computational mechanistic details of the Pd(OAc)(2)/TEA-catalyzed aerobic alcohol oxidation system are disclosed. Measurement of various kinetic isotope effects and the activation parameters as well as rate law derivation support rate-limiting deprotonation of the palladium-coordinated alcohol. Rate-limiting deprotonation of the alcohol is contrary to the majority of related kinetic studies for Pd-catalyzed aerobic oxidation of alcohols, which propose rate-limiting beta-hydride elimination. This difference in the rate-limiting step is supported by the computational model, which predicts the activation energy for deprotonation is 3 kcal/mol higher than the activation energy for beta-hydride elimination. The computational features of the similar Pd(OAc)(2)/pyridine system were also elucidated. Details of the study illustrate that the use of TEA results in an active catalyst that has only one ligand bound to the Pd, resulting in a significant lowering of the activation energy for beta-hydride elimination and, therefore, a catalyst that is active at room temperature.

113 citations

Journal ArticleDOI
TL;DR: In this article, the molecular structures of p-chlorophenol and p-bromophenol have been calculated with the MP2, DFT(hybrid), and HF methods using the extended 6-311++G(df, pd) basis set.
Abstract: The molecular structures of p-chlorophenol and p-bromophenol have been calculated with the MP2, DFT(hybrid), and HF methods using the extended 6-311++G(df, pd) basis set. The geometrical parameters of p-ClPh and pBrPh in the gas phase have not been reported as yet. The results show that substitution of phenol with σ-electron-withdrawing groups (Br and Cl) leads to small shortening of the C−C and C−O bonds and small changes in the CCC angles. The structural changes of the phenol ring are governed mainly by the electronegativity of the para-substituent and, to a lesser extend, by resonance factors. The FT-IR spectra of p-ClPh, p-BrPh, and their OD counterparts were measured in CCl4 and cyclohexane solutions in the frequency range of 3700−400 cm-1, and the integrated infrared intensities were determined. The theoretical harmonic frequencies and infrared intensities were calculated for all the molecules using the DFT and HF methods. The best overall agreement between the calculated and experimental spectra ha...

101 citations

Journal ArticleDOI
TL;DR: The symmetry-adapted perturbation theory (SAPT) has been utilized to decompose the total interaction energy into physically meaningful contributions and, contrary to a frequently presented idea, the electric field of the proton acceptor cannot solely explain the different behavior of the H-bonded and improper blue-shifting H- bonded complexes.
Abstract: Twenty two hydrogen-bonded and improper blue-shifting hydrogen-bonded complexes were studied by means of the HF, MP2 and B3LYP methods using the 6-31G(d,p) and 6—311++G(d,p) basis sets. In contrast to the standard H bonding, the origin of the improper blue-shifting H bonding is still not fully understood. Contrary to a frequently presented idea, the electric field of the proton acceptor cannot solely explain the different behavior of the H-bonded and improper blue-shifting H-bonded complexes. Compression of the hydrogen bond due to different attractive forces—dispersion or electrostatics—makes an important contribution as well. The symmetry-adapted perturbation theory (SAPT) has been utilized to decompose the total interaction energy into physically meaningful contributions. In the red-shifting complexes, the induction energy is mostly larger than the dispersion energy while, in the case of blue-shifting complexes, the situation is opposite. Dispersion as an attractive force increases the blue shift in the blue-shifting complexes as it compresses the H bond and, therefore, it increases the Pauli repulsion. On the other hand, dispersion in the red-shifting complexes increases their red shift.

92 citations

Journal ArticleDOI
TL;DR: In this article, the Fourier transform infrared (FT-IR) spectra of phenol and phenol-OD have been measured in carbon tetrachloride and cyclohexane solutions, in the frequency range 3700−400 cm-1, and the experimental integrated infrared intensities are reported.
Abstract: The infrared spectra of phenol and phenol-OD are thoroughly reinvestigated, to resolve the contradictory assignment of some vibrations. The harmonic frequencies, integrated IR intensities, and potential energy distribution (PED) have been calculated by the B3LYP method with the 6-311++G(df,pd) basis set. The Fourier transform infrared (FT-IR) spectra of phenol and phenol-OD have been measured in carbon tetrachloride and cyclohexane solutions, in the frequency range 3700−400 cm-1, and the experimental integrated infrared intensities are reported. On the basis of the results obtained, the detailed assignment of all the fundamental modes of Ph-OH and Ph-OD are presented. The study demonstrates that density functional B3LYP is clearly superior to the ab initio Hartree−Fock (HF) and second-order Moller−Plesset (MP2) methods in reliable prediction of the vibrational spectra of phenol. In particular, it is shown that scaling of the B3LYP-calculated frequencies of the CH and OH(OD) stretching vibrations by the sc...

87 citations


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08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

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TL;DR: In this article, self-assembly is defined as the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds.
Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

2,591 citations