TY - JOUR
T1 - Shielding in and around Oxazole, Imidazole, and Thiazole
T2 - How Does the Second Heteroatom Affect Aromaticity and Bonding?
AU - Horner, Kate E.
AU - Karadakov, Peter B.
PY - 2015/7/17
Y1 - 2015/7/17
N2 - Isotropic magnetic shielding distributions in the regions of space surrounding oxazole, imidazole, and thiazole are used to investigate aromaticity and bonding in these five-membered heterocycles with two heteroatoms. This is achieved by constructing HF-GIAO and MP2-GIAO (Hartree-Fock and second-order Møller-Plesset perturbation theory with gauge-including atomic orbitals) isotropic shielding plots, within the 6-311++G(d,p) basis, using regular two-dimensional 0.05 Å grids in the molecular plane and in planes 0.5 and 1 Å above it. The extent of isotropic shielding delocalization in the contour plots in planes 1 Å above the molecular plane, which is a new sensitive two-dimensional aromaticity criterion, indicates that aromaticity decreases in the order thiazole > imidazole > oxazole; in combination with previous results on furan, pyrrole, and thiophene (J. Org. Chem. 2013, 78, 8037-9043), the aromaticity ordering in the six five-membered heterocycles becomes thiophene > thiazole > pyrrole > imidazole > furan > oxazole. The results suggest that the inclusion of a second heteroatom in a five-membered heterocycle has a detrimental effect on its aromaticity, which is very minor in oxazole, when compared to furan, and small but noticeable in imidazole and pyrrole and in thiazole and thiophene.
AB - Isotropic magnetic shielding distributions in the regions of space surrounding oxazole, imidazole, and thiazole are used to investigate aromaticity and bonding in these five-membered heterocycles with two heteroatoms. This is achieved by constructing HF-GIAO and MP2-GIAO (Hartree-Fock and second-order Møller-Plesset perturbation theory with gauge-including atomic orbitals) isotropic shielding plots, within the 6-311++G(d,p) basis, using regular two-dimensional 0.05 Å grids in the molecular plane and in planes 0.5 and 1 Å above it. The extent of isotropic shielding delocalization in the contour plots in planes 1 Å above the molecular plane, which is a new sensitive two-dimensional aromaticity criterion, indicates that aromaticity decreases in the order thiazole > imidazole > oxazole; in combination with previous results on furan, pyrrole, and thiophene (J. Org. Chem. 2013, 78, 8037-9043), the aromaticity ordering in the six five-membered heterocycles becomes thiophene > thiazole > pyrrole > imidazole > furan > oxazole. The results suggest that the inclusion of a second heteroatom in a five-membered heterocycle has a detrimental effect on its aromaticity, which is very minor in oxazole, when compared to furan, and small but noticeable in imidazole and pyrrole and in thiazole and thiophene.
UR - http://www.scopus.com/inward/record.url?scp=84937598692&partnerID=8YFLogxK
U2 - 10.1021/acs.joc.5b01010
DO - 10.1021/acs.joc.5b01010
M3 - Article
AN - SCOPUS:84937598692
SN - 0022-3263
VL - 80
SP - 7150
EP - 7157
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 14
ER -