Gas‐Phase Electron‐Diffraction Investigation and Quantum‐Chemical Calculations of the Structure of 1,5‐Dimethylsemibullvalene‐2,4,6,8‐tetracarboxylic Dianhydride

Abstract

Dedicated to Professor Jack D. Dunitz. K. H. has known Jack for about 55 of his 80 years beginning when we were both post-doctoral fellows at Caltech. Those were fun times!The bridged homotropilidines have been of interest for decades because their molecules offer the potential for homoaromaticity. Although many of these have been shown not to be homoaromatic, the energy differences of the delocalized (homoaromatic) forms and the localized (nonhomoaromatic) ones, and the barriers to the interconversion of the localized forms via a Cope rearrangement, have been found to vary greatly. The title compound is a strong candidate for homoaromaticity, and, since the structures of the possible localized and delocalized forms could differ significantly, we have carried out an electron-diffraction investigation of it augmented by quantum-mechanical calculations with different basis sets at several levels of theory. Three models were explored: one representing a localized form of C s symmetry, one a delocalized form of C 2v symmetry, and one a 2 : 1 mixture of the localized/delocalized forms. Although none of the models could be ruled out, the experimental evidence slightly favors the C s form. These results are consistent with those from the DFT B3PW91 calculations with basis sets ranging from 6-31G(d) to cc-pVTZ, which, surprisingly, predict essentially equal thermally corrected free energies for each. The results are discussed.Introduction. ± The molecular structures of the bridged homotropilidines, of which bullvalene and semibullvalene (1) are important examples, have been of great interest for well over three decades. One reason for this interest is that the molecules might be homoaromatic [1] as a consequence of a −negative× barrier to a Cope rearrangement [2] as illustrated below for the skeleton of semibullvalene (1). The homoaromatic, or delocalized, form of the molecule, 1b, is expected to have C 2v symmetry, and the nonhomoaromatic, or localized, forms 1a and 1c should have C s symmetry. The most obvious structural differences between the homoaromatic and localized forms lie in the nearest-neighbor distances between atoms in the six-membered ring of 1b, and the corresponding atoms in 1a and 1c: in 1b the C 2 ÀC 3 , C 3 ÀC 4 , C 6 ÀC 7 , and C 7 ÀC 8 distances should be characteristic of C,C aromatic bonds (ca. 1.4 ä), while the C 2 ÀC 8 and C 4 ÀC 6 distances are expected to be elongated (ca. 2 ä), but in 1a (1c) their values as double, single, and nonbonds are anticipated [3].There is abundant experimental and theoretical evidence that the ground state of semibullvalene itself is not homoaromatic (for a summary of current information about

The Experimental Realization of a Neutral Homoaromatic Carbocycle

“…Our early gas-phase experiments, photoelectron spectroscopy on 15 (in collaboration with Professor Rolf Gleiter, University of Heidelberg), gas-phase electron diffraction, [12] gasphase NMR spectroscopy (in collaboration with Dr. Alexander Marchione, DuPont Corporate Center for Analytical Sciences) and fluorescence excitation in a supersonic beam coupled with double resonance (in collaboration with Professor Bernhard Dick, University of Regensburg), were either unsuccessful or inconclusive. Despite these disappointments, the potential of using vapor-phase infrared (IR) spectroscopy to determine whether 12 aQ12 b or, as predicted, 12 c is the ground state remained.…”

“…The results from our semiempirical, density functional theory (DFT), and ab initio calculations, suggest that the annelated semibullvalenes 10, 11, and 12 (amongst others) should be homoaromatic. [11,12] Dannenberg et al predicted, from semiempirical calculations, that 13 should be homoaromatic. [13] More recent higher level calculations by Bordens group lead them to predict that 13 is not homoaromatic but that 14 should be.…”

“…To test our predictions, we synthesized 1,5-dimethyl-2,8:4,6-semibullvalenetetracarboxylic acid dianhydride (12). A detailed analysis of anhydride 12 and its unsymmetrical analogue 15, by variable-temperature (VT) solution and solid state NMR including modified Saunders isotopic perturbation, VT X-ray structure (293-15 K) and electron density (15 K), and VT UV/Vis and solvatochromism studies, showed that this species is not homoaromatic in the solid or solution states.…”

“…At all correlated levels of theory, except for B3PW91/6-31G*, the only stationary point found is the delocalized 12 c indicating that 12 resides on a single-minimum potential energy surface (PES) with a homoaromatic ground state. [1,9,12,18] This discrepancy leads directly to two very interesting possibilities: The level of theory, which does an excellent job of modeling a wide range of systems including other semibullvalenes and a vast array of pericyclic reactions, is not adequate for these complex molecules. It should be noted that there is a tendency for these calculations to overestimate the stability of the delocalized forms.…”

“…[9] Earlier, Paquette and Chamot proposed that in the semibullvalene 9 the bisannelation would inhibit the "breathing motion" associated with the Cope rearrangement and thus perhaps favor the delocalized form. [10] In essence, the focus of our work is to use strain-induced elevation of the energy of equilibrating localized ground states of the parent semibullvalene (8 aQ8 b) to Keywords: Cope rearrangement · density functional calculations · homoaromaticity · IR spectroscopy · semibullvalenes Abstract: Infrared spectra recorded for 1,5-dimethyl-2,8:4,6-semibullvalenetetracarboxylic acid dianhydride (12) in the condensed and vapor phase clearly prove that in the vapor phase the dianhydride 12 is a homoaromatic ground state semibullvalene.…”