Computed NMR shielding values of unsaturated five-membered-heterocyclic ring compounds and their benzo-analogs as a measure of aromaticity

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Eddie Pittman (Creator)
Institution
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Advisor
Ned Martin

Abstract: The gauge independent atomic orbital (GIAO) Hartree-Fock (HF) technique with a 6-31 G(d, p) basis set was used to calculate the isotropic NMR shielding values of a diatomic hydrogen probe above a set of unsaturated 5-membered heterocyclic aromatic compounds and their benzo- analogs. It has been shown that this technique produced results indicating substantial shielding of the probe over the center of aromatic rings. The current study was conducted to determine if the computed shielding of a diatomic H2 probe is related quantitatively to the extent of aromaticity. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs of electrons or empty orbitals exhibits a stabilization due to conjugation alone. Aromaticity is both a qualitative and quantitative concept. The qualitative aspect, which is the method for identifying a molecule or species as either aromatic, non-aromatic or anti-aromatic, is soundly understood, but the quantitative aspect of aromaticity is less well defined. There are several established methods for measuring aromaticity quantitatively but they are only loosely correlated. This study’s method (?s) for calculations done over the fivemembered rings correlated well with Cyransky’s published data of ASE, ?, NICS(0), NICS(1) and HOMA calculations to yield correlation coefficients of 0.64, 0.49, 0.66, 0.88 and 0.69, respectively. This study’s method (?s) with calculations done over the heterocyclic ring portion of the benzo-analogs yielded a correlation coefficient of 0.67 when matched with Bird’s published ASE data.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2008
Keywords
Nuclear magnetic resonance, Aromaticity (Chemistry), Aromatic compounds, Aromatic compounds -- Magnetic properties
Subjects
Nuclear magnetic resonance
Aromaticity (Chemistry)
Aromatic compounds
Aromatic compounds -- Magnetic properties