Ab initio calculation of through-space magnetic shielding of linear polycyclic aromatic hydrocarbons (acenes): Extent of aromaticity

Abstract

GIAO-HF within Gaussian 03 was employed to compute the NMR isotropic shielding values of a diatomic hydrogen probe above a series of acenes (linear polycyclic aromatic hydrocarbons). Subtraction of the isotropic shielding of diatomic hydrogen by itself allowed the determination of computed through-space proton NMR shielding increment surfaces for these systems. Shielding was observed above the center of each aromatic ring, but the magnitude of calculated shielding above each ring center depends on the number of fused benzenoid rings. The computed shielding increments above each ring center were correlated to other measures of extent of aromaticity, including geometric, energetic, and magnetic measurements.

Introduction

Since Kekulé introduced the concept of aromaticity over 140 years ago [1] numerous methods have been developed to measure or predict the extent of aromaticity of a substance. Schleyer [2] listed several of these methods in his introduction to a series of review articles on aromaticity. Some of the methods are based on geometry, such as the harmonic oscillator model of aromaticity, HOMA [3], [4], [5], [6]. Others rely on energetics, such as aromatic stabilization energy, ASE [7], [8], [9], [10], [11]. A third category of methods depends on the magnetic properties associated with aromaticity. These include exaltation of magnetic susceptibility, Λ [12], [13], [14], anisotropy of the magnetic susceptibility [15], nuclear magnetic resonance shifts [16], [17], [18], and nucleus-independent chemical shifts, NICS, a measure of the diatropic (for aromatic compounds) or paratropic (for antiaromatic compounds) ring current [19], [20]. NICS or one of its variations, such as aromatic ring current shieldings (ARCS) computed from NICS measurements perpendicular to the plane of aromatic rings [21], Kleinpeter's [22], [23] graphical maps of NICS, named isochemical shielding surfaces (ICSS), or Stanger's partitioned NICS [24] are perhaps the more common methods in use today to measure or predict aromaticity. Cyrañski et al. [25], [26] showed that for a series of 75 five-membered ring π-electron systems and 30 ring-substituted compounds (including aromatic, nonaromatic and antiaromatic systems), loose correlations exist among the four most widely used measures of aromaticity: ASE, Λ, HOMA and NICS. But NICS measurements have limitations in terms of predicting aromaticity vs. antiaromaticity. For instance, the NICS value of the antiaromatic cyclopropenyl anion is negative, indicative of an aromatic structure. Correct assignments of aromaticity and antiaromaticity are obtained if a probe molecule, such as diatomic hydrogen, is used to determine the through-space shielding effect [27]. In contrast, the NICS method involves computing the magnetic shielding at a point in space.

The results of HF-GIAO calculations to calculate through-space NMR shielding effects on a probe molecule, to map the resulting through-space NMR shielding increments, and to develop through-space NMR shielding equations for a number of common organic functional groups, including the benzene ring [28], [29], the carbon–carbon double bond [30], [31], [32], [33], the carbon–carbon triple bond, the carbon–nitrogen triple bond and the nitro group [34], and the carbonyl group [35] have been published. Computed shielding surfaces by functional groups common to peptides [36], simple aromatic and antiaromatic hydrocarbons [27], aryl–cation π-complexes [37] and aryl–aryl π-complexes [38] have been reported. We now report our computational study of the through-space NMR shielding of a diatomic hydrogen probe molecule by some linear polycyclic aromatic hydrocarbons (PAHs).