Azulene revisited: solid-state structure, invariom modeling and lattice-energy minimization of a classical example of disorder

Abstract

The molecular and solid-state structure of azulene both raise fundamental questions. Therefore, the disordered crystal structure of azulene was re-refined with invariom non-spherical atomic scattering factors from new single-crystal X-ray diffraction data with a resolution of d = 0.45 Å . An unconstrained refinement results in a molecular geometry with C s symmetry. Refinements constrained to fulfill C 2v symmetry, as observed in the gas phase and in highlevel ab initio calculations, lead to similar figures of merit and residual densities as unconstrained ones. Such models are consistent with the structures from microwave spectroscopy and electron diffraction, albeit they are not the same. It is shown that for the disorder present in azulene, the invariom model describes valence electron density as successfully as it does for non-disordered structures, although the disorder still leads to high correlations mainly between positional parameters. Lattice-energy minimizations on a variety of ordered model structures using dispersion-corrected DFT calculations reveal that the local deviations from the average structure are small. Despite the molecular dipole moment there is no significant molecular ordering in any spatial direction. A superposition of all ordered model structures leads to a calculated average structure, which explains not only the experimental determined atomic coordinates, but also the apparently unusual experimental anisotropic displacement parameters.

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…With the exception of one bond to the five-membered ring, which is between 1.37 and 1.39 ,a nd the bond shared with the substituted sixmembered rings,t he remaining five CÀCb onds are significantly longer (between 1.42 and 1.50 ), which is in contrast to pure azulene,where all bond lengths,with exception of the shared one with the cycloptadienyl ring (1.49 ), are between 1.37 and 1.42 . [24] This concludes that the embedded azulene units are of less aromatic character than azulene itself (see also discussion on nuclear independent chemical shifts below).…”

Orientational disorder of monomethyl-quinacridone investigated by Rietveld refinement, structure refinement to the pair distribution function and lattice-energy minimizations

“…A head-to-tail disorder, also called end-to-end disorder (Welberry, 2004), is an orientational disorder of an (elongated) molecule caused by inversion of the entire molecule, or by rotation of the molecule around its short molecular axis by 180 . Examples include carbon monoxide, para-chloronitrobenzene (Thomas et al, 2008), 4-fluoro-biphenyl (Guzei et al, 2017), (CH 3 ) 3 Sn-V(CO) 6 (Englert, 2000) and azulene (Dittrich et al, 2018). This type of disorder is also frequently observed with toluene as solvent molecule in centrosymmetric crystals (cf.…”

“…Criterion (iii) seems to be the weakest one. For example, azulene has a dipole moment of about 0.8 D; nevertheless, it exhibits a statistical head-to-tail disorder with occupancies of 0.5 : 0.5 in the solid state (Dittrich et al, 2018).…”

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…The longest C−C bonds (1.50 Å) are the biaryl axes formed during cycloheptannulation. With the exception of one bond to the five‐membered ring, which is between 1.37 and 1.39 Å, and the bond shared with the substituted six‐membered rings, the remaining five C−C bonds are significantly longer (between 1.42 and 1.50 Å), which is in contrast to pure azulene, where all bond lengths, with exception of the shared one with the cycloptadienyl ring (1.49 Å), are between 1.37 and 1.42 Å . This concludes that the embedded azulene units are of less aromatic character than azulene itself (see also discussion on nuclear independent chemical shifts below).…”