Aktuelle Forschungsergebnisse

 

Latest Research Results, Papers in Print [January 2008]

 

[1]       U. Lottermoser, P. Rademacher, M. Mazik, K. Kowski,
Photoelectron Spectra and Electronic Structures of Substituted Pyrimidines
Eur. J. Org. Chem.
2005, 522-531.
Abstract
The electronic structures of pyrimidine (1) and its substituted derivatives 2-15 have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The ionisation potentials corresponding to the p MOs p1-p3 and the two nN orbitals of the pyrimidine unit could be determined and assigned for 1-15. Multiple linear regression analyses of the IPs related to these orbitals with different substituent constants indicated that Hammett sp values performed well for all these IPs, whereas resonance parameters R and R+ were satisfactory for p and poor for nN ionisations.
Keywords
Photoelectron spectroscopy / Density functional theory calculations / Electronic structure / Ionisation potentials / Linear free energy correlation / Multiple linear regression analysis

 

[2]       Paul Rademacher, Levan Khelashvili and Klaus Kowski
Spectroscopic and Theoretical Studies on Intramolecular OH---
p Hydrogen Bonding in 4-Substituted 2-Allylphenols
Org. Biomol.
Chem., 2005, 3, 2620-2625.
Dedicated to Professor Wolfgang Lüttke, Göttingen, on the occasion of his 85th birthday.
Abstract
2-Allylphenol (1) constitutes a mixture of conformers, in which an OH---
p hydrogen bonded closed (1a) and an open form (1b) can be distinguished. 4-Substituted 2-allyphenols (2-9) have been synthesised and investigated by theoretical and spectroscopic methods. In 1-9, the energy and the structure of the hydrogen bond show distinct variation with the substituent. In the PE spectra of most compounds, two ionisations can be distinguished which are related to the allylic p(C=C) orbitals of the two conformers a and b and differ in energy by DIP(C=C). Alternatively, DIP(C=C) can be determined indirectly from comparison of the PE spectra of the respective phenols and anisoles with the same substituents. DIP(C=C) values between 0.3 and 1.1 eV were found. By IR spectroscopy, frequency shifts Dn(OH) of the O-H vibration in CHCl3 solution were measured. By means of correlation analysis the relationship between the strength of the intramolecular hydrogen bond, DIP(C=C), Dn(OH) values and substituent constants it is established how substituents in 4-position affect the intramolecular OH---p hydrogen bond. The investigations demonstrate that the DIP(C=C) data can be used as descriptors for this intramolecular interaction..
Keywords
4-substituted 2–allylphenols / OH---p hydrogen bonds / photoelectron spectroscopy

 

[3]       P. Rademacher,
Die Isomerie: Ein Grundlagenproblem der Chemie
Chem. Uns. Zeit, 2005, 39, 176-180.
Summary

Studies in the field of isomerism have often created misunderstandings, confusion and controversies, but have also led to new impulses for the development of structure theory, new analytical methods and synthesis of novel compounds. Problems of isomerism are present in nearly all fields of chemistry and have stimulated creation of new concepts and enriched the chemical nomenclature. Presently, probably the most ambitious studies in structure chemistry deal with the elucidation of conformational isomerism of large bio-molecules such as proteins in liquid and solution.

 

[4]       P. Rademacher, L. Khelashvili
Photoelectron Spectra and Electronic Structure of Some 4-Substituted 2-Allylanisoles 
Spectrochim.
Acta A 200665, 249-253.
Abstract
The electronic structures of 4-substituted 2-ally anisoles (1
-9) have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The ionisation potentials corresponding to the p MOs p2 and p3 of the phenyl ring, and the nO orbital of the methoxy group as well as the allylic p(C=C) orbital could be determined and assigned for 1-9. Linear regression analyses of the IPs related to these orbitals with different substituent constants indicated that Hammett sp values performed satisfactorily to fair for p2, p3 and nO but poor for allylic p(C=C). Other substituent constants such as R and R- were fair only for p2 and p3, but poor for nO and p(C=C).
Keywords
Photoelectron spectroscopy / Density functional theory calculations / Electronic structure / Ionisation potentials /  Linear free energy correlation / Substituent effects

 

[5]       P. Rademacher, 
Electronic and Geometrical Structures of Cyclopropanes, Part 5. Fluorocyclopropanes. Linear Correlation of Orbital Energies with C-C Bond Lengths. Further Improvement of the Two-Orbitals Model,
Struct. Chem.
2006, 7, 19-25.
Abstract
The electronic and geometrical structures of fluorocyclopropanes (112) have been analysed using DFT B3LYP calculations. A linear relationship,
Dew = -0.172 Dr - 0.171 (n = 12, R = 0.931), between Dew (in eV), the difference of the energies of the Walsh orbitals wS and wA, and Dr (in pm), the difference of vicinal and distal C–C bond lengths, is established. Correcting the orbital splitting by the basic value at Dr = 0.00 pm, an even better linear correlation Deweff = 0.0720 Dr (n = 12, R = 0.984) is obtained. The results confirm the general applicability of the two-orbitals model for the relationship between geometrical and electronic structures for substituted cyclopropanes.
KEY WORDS:
Density functional theory calculations; electronic structure; molecular structure; correlation analysis

 

[6]       P. Rademacher, P. Choudhary Mohr

            Transannular 1,5-Hydride Shift in 5-Hydroxycyclooctanone: An Experimental and Theoretical Investigation

            Org. Biomol. Chem., 2007, 5, 2699-2703..
Abstract

            As has been found by 1H NMR measurements, eight out of twelve methylene hydrogen atoms of 5-hydroxycyclooctanone (1HK) are exchanged for deuterium atoms under acid and basic conditions. For the reaction in 7.9 M DCl/D2O an activation energy Ea = 19.2 + 0.4 kcal mol‑1 is found. For an explanation of these findings, degenerate transannular 1,5-hydride shift is essential, and this reaction has been analyzed by quantum chemical calculations. Hydride transfer takes place via a tight transition state with a six-membered ring. The activation barrier is lowest in the presence of base. The molecular conformation of the eight-membered ring in the transition state resembles that of the starting structure. It is unlikely that solvent molecules such as water participate in the formation of the transition state. 

          

[7]       P. Choudhary Mohr, P. Rademacher

            Preparation of Keto-ethers of 5-Hydroxycyclooctanone via a Non-classical Protection Method

            Synlett, 2008, 188-190.
Abstract

            Protection of the hydroxy group of the medium-ring hydroxyketone 5-hydroxycyclooctanone (1HK) can be achieved with an alcohol and hydrochloric acid, although the educt predomi­nately exists as the transannular hemiacetal 1HA. The reaction is explained by a mechanism which includes a 1,5-hydride shift as an essential step.

            KEY WORDS:
Medium rings, transannular acetals, protection, transan­nular reactions, hydride shift

 



Letzte Änderung: 24.01.2008

Paul Rademacher, paul.rademacher@uni-duisburg-essen.de