Haijun Jiao

NUCLEUS-INDEPENDENT CHEMICAL SHIFTS : A SIMPLE AND EFFICIENT AROMATICITY PROBE

The ability to sustain a diatropic ring current is the defining characteristic of aromatic species.1-7 Cyclic electron delocalization results in enhanced stability, bond length equalization, and special magnetic as well as chemical and physical properties.1 In contrast, antiaromatic compounds sustain paratropic ring currents3 despite their localized, destabilized structures.1-7 We have demonstrated the direct, quantitative relationships among energetic, geometrical, and magnetic criteria of aromaticity in a wide-ranging set of aromatic/antiaromatic fivemembered rings.5a While the diamagnetic susceptibility exaltation (Λ) is uniquely associated with aromaticity, it is highly dependent on the ring size (area2) and requires suitable calibration standards.6 Aromatic stabilization energies (ASEs) of strained and more complicated systems are difficult to evaluate. CC bond length variations in polybenzenoid hydrocarbons can be just as large as those in linear conjugated polyenes.2 The abnormal proton chemical shifts of aromatic molecules are the most commonly employed indicators of ring current effects.1 However, the ca. 2-4 ppm displacements of external protons to lower magnetic fields are relatively modest (e.g., δH ) 7.3 for benzene vs 5.6 for dC-H in cyclohexene). In contrast, the upfield chemical shifts of protons located inside aromatic rings are more unusual. The six inner hydrogens of [18]annulene, for example, resonate at -3.0 ppm vs δ ) 9.28 for the outer protons. This relationship is inverted dramatically in the antiaromatic [18]annulene dianion, C18H18, where δ ) 20.8 and 29.5 (in) vs. -1.1 (out).8 Similar demonstrations of paratropic ring currents in antiaromatic compounds are well documented.3,8,9 Chemical shifts of encapsulated 3He atoms are now employed as experimental and computed measures of aromaticity in fullerenes and fullerene derivatives.10 While the rings of most aromatic systems are too small to accommodate atoms internally, the chemical shifts of hydrogens in bridging positions have long been used as aromaticity and antiaromaticity probes.9 δLi+ can be employed similarly, with the advantage that Li+ complexes with individual rings in polycyclic systems can be computed.4,11 We now propose the use of absolute magnetic shieldings, computed at ring centers (nonweighted mean of the heavy atom coordinates) with available quantum mechanics programs,12 as a new aromaticity/antiaromaticity criterion. To correspond to the familiar NMR chemical shift convention, the signs of the computed values are reversed: Negative “nucleus-independent chemical shifts” (NICSs) denote aromaticity; positive NICSs, antiaromaticity (see Table 1 for selected results). Figure 1, a plot of NICSs vs the ASEs for our set of five-membered ring heterocycles,5a provides calibration. The equally good correlations with magnetic susceptibility exaltations and with structural variations establish NICS as an effective aromaticity criterion. Unlike Λ,6 NICS values for [n]annulenes (Table 1) show only a modest dependence on ring size. The 10 π electron systems give significantly higher values than those with 6 π electrons. The antiaromatic 4n π electron compounds, cyclobutadiene (27.6), pentalene (18.1), heptalene (22.7), and planar D4h cyclooctatetraene (30.1), all show highly positive NICSs. Like the Li+-complex probe,4 the NICS evaluates the aromaticity and antiaromaticity contributions of individual rings in polycyclic systems. Scheme 1 (HF/6-31+G*, data from Table 1) shows NICSs for selected examples. The benzenoid aromatic NICSs provide evidence both for localized and “perimeter” models. The naphthalene (1) NICS (-9.9) resembles that of benzene (-9.7), as do the NICSs for the outer rings of phenanthrene (2) (-10.2) and triphenylene (3); the aromaticity of the central rings of the latter two are reduced. The NICS of the central ring of anthracene (4) (-13.3) exceeds the benzene value in contrast to the outer ring NICS (-8.2). Remarkably, the NICS (-7.0) for the seven-membered ring of azulene (5) is very close to that of the tropylium ion (-7.6 ppm), whereas the azulene five-membered ring NICS (-19.7) is even larger in magnitude than that of the cyclopentadienyl anion (-14.3). The four-membered rings in benzocyclobutadiene (6) (NICS ) 22.5) and in biphenylene (7) (19.0) are antiaromatic, but less so than cyclobutadiene itself (27.6). The six-membered rings in these polycycles are still aromatic, but their NICSs (-2.5 (1) (a) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity; Wiley: New York, 1994. (b) Garratt, P. J. Aromaticity; Wiley: New York, 1986. (c) Eluidge, J. A.; Jackman, L. M. J. Chem. Soc. 1961, 859. (2) Schleyer, P. v. R.; Jiao, H. Pure Appl. Chem. 1996, 28, 209. (3) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811. (4) Jiao, H; Schleyer, P. v. R. AIP Conference Proceedings 330, E.C.C.C.1, Computational Chemistry; Bernardi, F., Rivail, J.-L., Eds.; American Institute of Physics: Woodbury, New York, 1995; p 107. (5) (a) Schleyer, P. v. R.; Freeman, P.; Jiao, H.; Goldfuss, B. Angew. Chem., Int. Ed. Engl. 1995, 34, 337. (b) Jiao, H.; Schleyer, P. v. R. Unpublished IGLO results. (c) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR: Basic Princ. Prog.; Springer: Berlin, 1990; Vol. 23, p 165. (6) Dauben, H. J., Jr.; Wilson, J. D.; Laity, J. L. In Non-Benzenoid Aromatics; Synder, J., Ed.; Academic Press, 1971; Vol. 2, and references cited. The partitioning of ring current or ring current susceptabilitites among various rings in polycyclic syestems were considered earlier, e.g., by Aihara (Aihara, J. J. Am. Chem. Soc. 1985, 207, 298 and refs cited) and by Mallion (Haigh, C. W.; Mallion, J. Chem. Phys. 1982, 76, 1982). (7) Fleischer, U.; Kutzelnigg, W.; Lazzeretti, P.; Mühlenkamp, V. J. Am. Chem. Soc. 1994, 116, 5298. (8) Sondheimer, F. Acc. Chem. Res. 1972, 5, 81. (9) (a) Hunandi, R. J. J. Am. Chem. Soc. 1983, 105, 6889. (b) Pascal, R. A., Jr.; Winans, C. G.; Van Engen, D. J. Am. Chem. Soc. 1989, 111, 3007. (10) (a) Bühl, M.; Thiel, W.; Jiao, H.; Schleyer, P. v. R.; Saunders, M.; Anet, F. A. L. J. Am. Chem. Soc. 1994, 116, 7429 and references cited. (b) Bühl, M.; van Wüllen, C. Chem. Phys. Lett. 1995, 247, 63. The authors have shown that the negative absolute shielding in the center of C60 is nearly the same as δ3He, computed at the same level. (11) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Bühl, M.; Feigel, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 8776. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, ReVision B.2; Gaussian Inc., Pittsburgh, PA, 1995. Figure 1. Plot of NICSs (ppm) vs the aromatic stabilization energies (ASEs, kcal/mol)5a for a set of five-membered ring heterocycles, C4H4X (X ) as shown) (cc ) 0.966). 6317 J. Am. Chem. Soc. 1996, 118, 6317

Dissected Nucleus-Independent Chemical Shift Analysis of π-Aromaticity and Antiaromaticity

Analysis of the basic π-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.

Aromaticity of pericyclic reaction transition structures: magnetic evidence

The transition states of thermally allowed pericyclic reactions are aromatic. They not only have highly delocalized structures and large resonance stabilizations (energies of concert), but also strongly enhanced magnetic susceptibilities (Λ) and appreciable NICS (nucleus-independent chemical shifts) values arising from the diatropic ring currents. Aromaticity is the consequence of cyclic electron delocalization, which can have σ and hybrid, and not just π character.

Facts and artifacts about aromatic stability estimation

Abstract The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of Cue5fbC or Cue5fbX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗.

Monocyclic (CH)9+—A Heilbronner Möbius Aromatic System Revealed

Probably first generated as a reactive intermediate in the early 1970s, the C2-symmetric (CH)9+ cation (1) has now been recognized to be the first representative of the aromatic Mobius [4n]annulenes predicted by Heilbronner in 1964.

Structure and stability of B+13 clusters

The structures and energies of B+13, observed experimentally to be an unusually abundant species among cationic boron clusters, have been studied systematically with B3LYP/6–31G* density functional theory. The most thermodynamically stable B+12 and B+13 clusters are confirmed to have planar or quasiplanar rather than globular structures. However, the computed dissociation energies of the 3‐dimensional B+13 clusters are much closer to the experimental values than those of the planar or quasiplanar structures. Hence, planar and 3‐dimensional B+13 may both exist.u2003© 1998 John Wiley & Sons, Inc.u2003J Comput Chem 19: 203–214, 1998

Introductory lecture. Electrostatic acceleration of the 1,5-H shifts in cyclopentadiene and in penta-1,3-diene by Li+ complexation: aromaticity of the transition structures

High-level ab initio molecular orbital calculations reproduce the experimental activation parameters for the 1,5-H shifts in cyclopentadiene and in penta-1,3-diene excellently and predict a remarkable electrostatic acceleration of both 1,5-H shifts by Li+ complexation. These catalytic effects (8.0 and 5.2 kcal mol–1, respectively) are electrostatic in origin: the transition states are more stabilized by Li+ than the ground states. Replacement of Li+ by a positive charge gives similar results. The aromaticity of the transition states was evidenced by various criteria: by the large energies of concert, by C—C bond-length equalization and by the ring-current effects: upfield δLi+ shifts, the deshielding of δH as well as the exalted magnetic susceptibilities and magnetic anisotropies computed using the IGLO method.

Monocyclisches (CH)9+ – ein Heilbronner-Möbius-Aren

Fast dreisig Jahre unerkannt blieb das wahrscheinlich schon Anfang der siebziger Jahre als reaktive Zwischenstufe erzeugte monocyclische, C2-symmetrische Kation (CH)9+1: Es ist das erste der bereits 1964 von Heilbronner vorhergesagten Mobius-aromatischen [4n]Annulene.

The structure and stability of Si60 and Ge60 cages: A computational study

Structural studies of fullerene‐like Si60 and Ge60 cages using ab initio methods were augmented by density functional tight‐binding molecular dynamics (DFTB‐MD) simulations of finite temperature effects. Neither the perfect Ih symmetry nor the distorted Th structures are true minima. The energies of both are high relative to distorted, lower symmetry minima, Ci and T, respectively, which still preserve C60‐type connectivity. Both Si60 and Ge60 favor Ci symmetry cages in which Si and Ge vertexes exhibit either near‐trigonal or pyramidal geometries. These structural variations imply significant reactivity differences between different positions. The small magnetic shielding effects (NICS) indicate that aromaticity is not important in these systems. The inorganic fullerene cages have lower stabilities compared with their carbon analogs. Si60 is stable towards spontaneous disintegration up to 700 K according to DFTB‐MD simulations, and thus has potential for experimental observation. In contrast, Ge60 preserves its cage structure only up to 200 K.

Evidence for the Möbius aromatic character of eight π electron conrotatory transition structure. Magnetic criteria

The Mobius aromatic character of the 8π electron transition structure for the conrotatory electrocyclic ring closure of (Z,Z)-octa-1,3,5,7-tetraene is demonstrated by downfield and upfield 1H chemical shifts as well as the enhanced magnetic susceptibility computed by the IGLO method. H1,8 (outer) are deshielded and H1,8 (inner) are shielded in the transition structure.