Shielding in and around Oxazole, Imidazole, and Thiazole: How Does the Second Heteroatom Affect Aromaticity and Bonding?

Abstract

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.

Magnetic Shielding Studies of C2 and C2H2 Support Higher than Triple Bond Multiplicity in C2

“…While it was not unexpected that the carbons in C 2 would be more shielded than those in C 2 H 2 , the magnitude of the difference at the full-valence CASSCF-GIAO/cc-pVQZ level, 226.62 ppm vs. 132.87 ppm, comes as a surprise. Another interesting related feature of the isotropic magnetic shielding distribution in C 2 (see Figures 1 and 2) is the absence of deshielded "halos" such as those observed previously around carbon atoms which are sp or sp 2 hybridized [13,[20][21][22][23] (this effect does not occur around sp 3 hybridized carbon atoms [13] ). Clearly, iso .r/ in C 2 shows some depressions along and around the C-C bond, close to each carbon atom, but these depressions are not deep enough to give rise to regions of negative iso .r/ values (or, deshielded "halos") as in C 2 H 2 .…”

“…Isotropic shielding and total electron density contour plots in planes passing through the molecular axes of C 2 and C 2 H 2 .Full-valence CASSCF(8,8)( C 2 )a nd CASSCF(10,10) (C 2 H 2 ), cc-pVQZ basis (GIAOs for the shielding calculations),contour levels at À10(5)80, 90, 100, 120, 140,1 60, 180, 200, 220 ppm for s iso (r), 0.05, 0.1,0 .2, 0.25, 0.3, 0.35. 0.4, 0.5, 1, 2.5,5 , 10, 25, 50, 75 eBohr À3 for 1(r), axesi n. (see Figure 1a nd Figure 2) is the absence of deshielded "halos" such as those observed previously aroundc arbon atoms which are sp or sp 2 hybridized [13,[20][21][22][23] (this effect does not occur around sp 3 hybridized carbon atoms [13] ). Clearly, s iso (r)i nC 2 showss ome depressions along and around the CÀCb ond, closet oe ach carbon atom, but these depressions are not deep enough to give rise to regions of negative s iso (r) values (or,deshielded "halos") as in C 2 H 2 .…”

Eine wasserhaltige Organokaliumverbindung basierend auf Bis(4,6‐tert‐Butylbenzoxazol‐2‐yl)methanid und ihre unerwartete Hydrolysebeständigkeit

“…Obwohl innerhalb dieser Ligandensysteme die Hauptelektronendichte auf den heteroaromatischen Stickstoffatomen lokalisiert ist [23][24][25] und in der Literatur eine Vielzahl an ausschließlich N-gebundenen Strukturen zu finden ist, wird eine O-Koordination an das elektronenarme Metallion bevorzugt. (Abbildung 1).…”

“…Auseiner [23][24][25] und in der Literatur eine Vielzahl an ausschließlich N-gebundenen Strukturen zu finden ist, wird eine O-Koordination an das elektronenarme Metallion bevorzugt. [26][27][28][29][30][31][32][33][34][35] (5) bestimmt.…”

Interpretation of Spectroscopic Markers of Hydrogen Bonds

“…Indeed, an umber of studies in the recentl iterature detail how the amount of this shieldingi sc losely aligned with the degree of aromaticity or even antiaromaticity. [77][78][79][80][81] This sort of reasoning is not limited to aromatic systems. Ar ecent work [82] extended the idea to small hydrocarbons such as butadiene, although it was concerned chiefly with shieldingw ithin the confines of each molecule itself, rather than its neighborhood.…”