The Search for Bishomoaromatic Semibullvalenes and Barbaralanes:  Computational Evidence of Their Identification by UV/Vis and IR Spectroscopy and Prediction of the Existence of a Blue Bishomoaromatic Semibullvalene

Abstract

Time-dependent B3LYP/6-31G calculations have been performed at the optimized C(2) or C(2v) geometries of several substituted semibullvalenes (1(deloc)) and barbaralanes (2(deloc)), to compare the computed vertical electronic excitation energies with the temperature-dependent, long-wavelength absorptions that have been observed in the UV/vis spectra of some of these compounds by Quast and co-workers. The excellent agreement between the calculated vertical excitation energies and the observed values of lambda(max) provides strong support for the identification of the bishomoaromatic species 1(deloc) and 2(deloc) as the source of these absorptions. Furthermore, the CN stretching frequencies, computed for the C(2) geometry of 1,5-dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene (1f(deloc)), fit the low-frequency absorptions seen in the IR spectrum of 1f, thus furnishing independent evidence that bishomoaromatic geometries of semibullvalenes have, in fact, been observed spectroscopically. B3LYP/6-31G calculations predict that 2,6-dicyano-4,8-diphenylsemibullvalene 1c has a C(2) equilibrium geometry (1c(deloc)) and that the long-wavelength UV/vis absorption (lambda(max) = 585 nm) and CN stretching frequencies (2192 and 2194 cm(-1)) computed for 1c(deloc) should serve to identify this bishomoaromatic semibullvalene when it is synthesized.

The Experimental Realization of a Neutral Homoaromatic Carbocycle

“…[24] Recently, Quast and co-workers confirmed the presence of the delocalized form 17 c in equilibrium with the localized forms of dinitrile 17 using DFT calculations and experimental IR spectroscopy. [25] They identified nitrile bands in the IR spectra for both the localized and delocalized species. In contrast, the ambient temperature 1 H and 13 C NMR spectra of 17 are time averaged and the 13 C signals show no line broadening down to the limit of accessible low temperatures.…”

“…For exceptionally rapid chemical processes (on the order of the femto-to picosecond timescale) dynamic infrared spectra, characterized by infrared-band coalescence, may be obtained. [29] If there were an equilibration of the localized and delocalized species, with the delocalized form either at a stationary point or as a vibrationally excited state, [25,30,31] it is doubtful that our observed vapor-phase spectra would result. Additionally, it is extremely unlikely that there would be no change in the vapor-phase spectra over the experimentally investigated 60 8C temperature range should any such equilibration be occurring.…”

“…In addition, if these spectra were the result of ultrafast dynamics, some temperature dependence of the line shapes (more than would be caused by the higher rotational states) is anticipated. Considering Model 2 and assuming an energy difference of 1 kcal mol À1 between 12 a,b and 12 c, [34] for simple Boltzmann distributions at 503 and 563 K there would only be approximately 27 or 29 % respectively of 12 c. The IR spectra resulting from this distribution would be dominated by the spectra of 12 a,b, but would also contain, at lower intensity, the spectra of 12 c. [35] Given that DFT calculations tend to overestimate the stability of the delocalized forms of semibullvalenes and barbaralanes by up to 3 kcal mol À1 , [25] temperature alone cannot be responsible for the dramatic change in the IR spectra and structure of 12. Assuming an IR detection limit of 5 % of the major component under the vapor-phase conditions, for Model 3, 12 c must be at least 3.3 kcal mol À1 more stable than 12 a,b (a simple Boltzmann distribution calculated at 563 K gives ca.…”

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

“…Over the last few decades, researchers have discovered and explored a wealth of structures based on 1 ,3, 4 the intriguing fundamental goal being the experimental identification of a semibullvalene having a delocalized bishomoaromatic ground state. Strategies towards this goal have mainly been twofold: (a) introduction of substituents5–15 or coordinating entities16, 17 aiming at electronic stabilization of 1b relative to 1a and (b) annelation of small rings7, 18–24 which by strain destabilizes 1a relative to 1b . Both strategies have been pursued by experimental and computational means, the latter proving to be a valuable tool in the search for homoaromatic structures 5–8, 11–17, 19–21, 23.…”

Theoretical design of tetra(arenediyl)bis(allyl) derivatives as model compounds for Cope rearrangement transition states

“…5 As an application, drugs can be designed on the basis of their similitude to the convenient TS geometries. Although some of such model compounds have been characterised through UV-vis 7,12 and IR 10 spectroscopies, no X-ray diffraction data have been reported for such species up to now. Although some of such model compounds have been characterised through UV-vis 7,12 and IR 10 spectroscopies, no X-ray diffraction data have been reported for such species up to now.…”