Nicolaas J. R. van Eikema Hommes

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.

Global and Local Aromaticity in Porphyrins: An Analysis Based on Molecular Geometries and Nucleus‐Independent Chemical Shifts

Two pyrrole rings participate in the aromatic structure of porphyrin. Hence, a 22 π-electron description is better than the usual [18]annulene representation. The dianion and the metal complex system favor different electronic structures.

A comparative theoretical study of the allyl alkali metals

Abstract The ab initio structures calculated for the LiCs series of allyl alkali metal compounds prefer symmetrically bridged geometries. Bonding is mainly electrostatic; the natural charges on the metals range from 0.910 (Li) to 0.999 (Cs). Dimerization and solvation, which were studied for allyllithium, result in longer bonds to the metal. Rotational barriers, calculated for the monomers, show the uniform trends to larger values along the series, Cs > Rb > K > 4Na, as is found by experiment. The calculated barrier for monomeric allyllithium, out of line and also too high with regard to experiment, is lowered by dimerization and solvation. The reasons for the abnormally low 1 J (CH) coupling constants (e.g. 131–133 Hz for the central carbon for all alkali metals) have been disputed. Hybridizations given by the Natural Bond Orbital method are in reasonable agreement with those deduced from the usual empirical relationship 0.2 J ( 13 CC) = % s . Model calculations on allyllithium and the allyl anion with imposed structural constraints show that CCC-angle widening is the main cause of the small coupling constants; hydrogen out-of-plane bending and σ-polarization due to the π-charge have smaller influences.

Ab initio models for multiple-hydrogen exchange: comparison of cyclic four- and six-center systems

High‐level ab initio calculations {QCISD(T)/6‐311 +G**//MP2(fu)/6‐31 +G**, with corrections for higher polarization [evaluated at MP2/6‐311 +G(3df,2p)] and ΔZPE//MP2(fu)/6‐31 +G**, i.e., comparable to Gaussian‐2 theory} indicate concerted mechanisms for double‐ and triple‐hydrogen exchange reactions in HF and HCl dimers and trimers, in mixed dimers and trimers containing one NH3, and in mixed dimers of HF, HCl, and NH3 with formic acid. All these reactions proceed via cyclic four‐ or six‐center transition structures, the latter being generally more favorable. Calculated activation barriers (ΔHd̊ at 0 K, kcal/mol) are 42.3 for (HF)2, 20.3 for (HF)3, 41.2 for (HCl)2, 25.6 for (HCl)3, 36.0 for NH3‐HF, 10.6 for NH3(HF)2, 19.9 for NH3‐HCl, 2.3 for NH3(HCl)2, 9.7 for HCO2H‐HF, 7.0 for HCO2H‐HCl, and 11.3, for HCO2H‐NH3. The barriers are lower for the more ionic systems and when more ion pair character is present.

Globale und lokale Aromatizität in Porphyrinen: eine Analyse anhand von Molekülgeometrien und kernunabhängigen chemischen Verschiebungen

Zwei Pyrrolringe sind Bestandteil der aromatischen Struktur von Porphyrin. Daher sollte es eher als 22π-Elektronensystem (siehe rechts) denn als [18]Annulen beschrieben werden. Dagegen bevorzugen sowohl das Dianion als auch die Metallkomplexe grundlegend andere elektronische Strukturen.

Is tetrahedral H 4 2+ a minimum?: anomalous behavior of popular basis sets with the standard p exponents on hydrogen

The nature of the tetrahedral H42+ stationary point (minimum or triply degenerate saddle) depends remarkably upon the theoretical level employed. Harmonic vibrational analyses with, e.g., the 6‐31G** (and 6‐31 + +G**) and Dunnings [4s2p1d;2s1p] [D95(d,p)] basis sets using the standard p exponent suggest (erroneously) that the Td geometry is a minimum at both the HF and MP2 levels. This is not the case at definitive higher levels. The C3H42+ structure with an apical H is another example of the failure of the calculations with the 6‐31G**, 6‐311G**, and D95(d,p) basis sets. Even at MP2/6‐31G** and MP2/ cc‐pVDZ levels, the C3v structure has no negative eigenvalues of the Hessian. Actually, this form is a second‐order saddle point as shown by the MP2/6‐31G** calculation with the optimized exponent. The D4h methane dication structure is also an example of the misleading performance of the 6‐31G** basis set. In all these cases, energy‐optimized hydrogen p exponents give the correct results, i.e., those found with more extended treatments. Optimized values of the hydrogen polarization function exponents eliminate these defects in 6‐31G** calculations. Species with higher coordinate hydrogens may also be calculated reliably by using more than one set of p functions on hydrogen [e.g., the 6‐31G(d,2p) basis set]. Not all cases are critical. A survey of examples, also including some boron compounds, provides calibration.

Are the C4v complexes of cyclobutadiene with CO, NO+, and CS minima ?

Abstract While the C 4v pyramidal structures of the cyclobutadiene-CO and cyclobutadiene-NO + complexes are second-order saddle points rather than minima, the pyramidal C 4v cyclobutadiene-CS complex is a minimum due to stronger interactions of the e π * orbitals of the CS fragment with the cyclobutadiene e g π-orbitals. However, the isomerization barrier of this complex into bicyclo[2.1.0]pententhione, only 0.5 kcal/mol at MP2/6–31G * //MP2/6–31G * +ZPE(HF/6–31G * ), disappears at higher levels. This C 4v C 4 H 4 CS complex is unlikely to be a viable candidate for experimental observation as an isolated entity.

Dilithium diphenylmethanediide; generation, redox relationship with lithium chlorodiphenylmethanide, implication with regard to aggregation

Dilithium diphenylmethanediide, Ph2CLi2, prepared by reaction of dichlorodiphenylmethane with lithium p,p′-di-tert-butylbiphenyl, reacts with organic halides and carbonyl compounds to give lithium Chlorodiphenylmethanide, Ph2CLiCl, as a major product and is assumed to exist in two different states of aggregation.