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Materials 2015, 8(8), 5385-5397; doi:10.3390/ma8085249

The π-Electron Delocalization in 2-Oxazolines Revisited: Quantification and Comparison with Its Analogue in Esters

1
Polymer Competence Center Leoben, Roseggerstrasse 12, 8700 Leoben, Austria
2
Institute for Chemistry and Technology of Materials, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria
3
Institute of Inorganic Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria
4
MTA-BME Lendület Quantum Chemistry Research Group, Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, P.O. Box 91, 1521 Budapest, Hungary
This publication is dedicated to Professor Hubert Schmidbaur on the occasion of his 80th birthday.
These authors contributed equally to this work.
*
Author to whom correspondence should be addressed.
Academic Editor: Maryam Tabrizian
Received: 24 June 2015 / Accepted: 13 August 2015 / Published: 21 August 2015
View Full-Text   |   Download PDF [1999 KB, uploaded 21 August 2015]   |  

Abstract

The single crystal X-ray analysis of the ester-functionalized 2-oxazoline, methyl 3-(4,5-dihydrooxazol-2-yl)propanoate, revealed Π-electron delocalization along the N–C–O segment in the 2-oxazoline pentacycle to significant extent, which is comparable to its counterpart along the O–C–O segment in the ester. Quantum chemical calculations based on the experimental X-ray geometry of the molecule supported the conjecture that the N–C–O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders were 1.97 and 1.10 for the N=C and C–O bonds, and the computed partial charges for the nitrogen and oxygen atoms of \(-\)0.43 and \(-\)0.44 were almost identical. In the ester group, the bond orders were 1.94 and 1.18 for the C–O bonds, while the partial charges of the oxygen atom are \(-\)0.49 and \(-\)0.41, which demonstrates the similar electronic structure of the N–C–O and O–C–O segments. In 2-oxazolines, despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms oxygen and nitrogen are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen is possible, enabling regioselectivity during the initiation of the cationic ring-opening polymerization of 2-oxazoline monomers, which is a prerequisite for the synthesis of materials with well-defined structures. View Full-Text
Keywords: 2-oxazoline monomers; Π-electron delocalization; regioselectivity of the initiation of cationic ring-opening polymerizations; ester-functionalized 2-oxazoline; hydrolysis of 2-oxazoline; methyl 3-(4,5-dihydrooxazol-2-yl)propanoate 2-oxazoline monomers; Π-electron delocalization; regioselectivity of the initiation of cationic ring-opening polymerizations; ester-functionalized 2-oxazoline; hydrolysis of 2-oxazoline; methyl 3-(4,5-dihydrooxazol-2-yl)propanoate
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Fimberger, M.; Luef, K.P.; Payerl, C.; Fischer, R.C.; Stelzer, F.; Kállay, M.; Wiesbrock, F. The π-Electron Delocalization in 2-Oxazolines Revisited: Quantification and Comparison with Its Analogue in Esters. Materials 2015, 8, 5385-5397.

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