Harold Baumann

Density Functional Study of the Oxy-Cope Rearrangement

With the goal of explaining the very large rate acceleration in the anion-assisted Cope rearrangement, the behavior of the prototypes of the Cope rearrangements, namely hexa-1,5-diene (4), hexa-1,5-dien-3-ol (5), and the oxy anion 6 of the latter were compared. For this purpose, two-dimensional DFT (hybrid B3LYP functionals with 6-31G* basis set) potential-energy surfaces (PESs) were computed, based on two interatomic distances. As the reliability of DFT/B3LYP-computed energies can not be taken for granted, we first performed model computations on the experimentally well-studied bridged homotropylidenes 1 – 3. Then, the transition states of the Cope rearrangements of 3-methylhexa-1,5-dien-3-ol (7), (2Z,4Z,7Z)-cyclonona-2,4,7-trien-1-ol (9), 1-methoxy-2-endo-vinylbicyclo[2.2.2]oct-5-en-2-exo-ol (11), and (1S,2R)-2-hydroxy-1-methyl-2-vinylbicyclo[4.4.0]dec-6-en-8-one (arbitrary numbering; 13) and of their oxy anions 8, 10, 12, and 14, respectively, were computed by the same method. These examples were chosen because kinetic data have been measured for most of them (except for 13 and 14) and/or because they furnished already important contributions to the discussion of the character of the Cope rearrangement. The computation of ΔG≠ for a given temperature allowed to calculate the rate constants at that temperature for the different rearrangements and to compare them with the experimental data. In the cases of the neutral and anionic oxy-Cope rearrangements, the equation ΔΔG≠=2.3026⋅RT⋅ΔpKa suggested a correlation between the difference in the pKa values of the pair of reactants and the pair of transition states and the change of the two free energies of activation.

Electronic spectra of biphenylene and diazabiphenylenes

The absorption, linear dichroism (l.d.) and magnetic circular dichroism (m.c.d.) spectra of biphenylene (1), 1,8-diazabiphenylene (2) and 2,7-diazabiphenylene (3) have been measured. The c.d. spectra of the β-cyclodextrin complexes with 1, 2 and 3 have also been measured. The first [(23.0–36.0)× 103 cm–1] and second [(36.0–50.0)× 103 cm–1] absorption bands of 1, 2 and 3 have been assigned by using the observed spectra and the results of CNDO/S CI calculations. It is concluded that the b3u vibration with a frequency of ca. 780 cm–1 takes part in the vibronic coupling between the first and second excited singlet states of 1 and that the second absorption band of 1 is composed of two electronic transitions polarized along the short and long axes.

Electronic spectra of 1,3-diaza-azulene

The linear dichroism and magnetic circular dichroism spectra of 1,3-diaza-azulene (1) have been measured. The circular dichroism spectrum of the β-cyclodextrin complex with 1 is reported. The absorption bands of 1 are assigned by using these observed spectra and the results of molecular-orbital calculations. Our assignment for the first and second absorption bands agrees completely with the previous assignment given by Small and Burke.

Photoelectron spectrum of [14]annulene

The He I photoelectron (PE) spectrum of [14]annulene has been measured and interpreted. UHF-MINDO/3 and UHF-MNDO computations of different molecular structures, characterized by the same π-perimeter (43 perimeter) but differing with respect to their π-bond situations (delocalized or localized) and by the geometry (different distortions imposed on the 43 perimeter in order to accommodate the four protons pointing inside), allowed us to select the energetically most favourable ones (bond delocalized D2-, C2h-, Ci- and Cs structures). The different distortions imposed upon the 43 perimeter in order to accommodate the four protons pointing inside the ring did not dramatically affect their energies. This result is supported by recent abinitio calculations, including electron correlation energy, which favoured a bond delocalized Cs structure. The interpretation of the He I PE spectrum is accomplished by comparing the observed vertical ionization energies with CNDO/S-CI computed energies of the states of the radical cations computed for the different molecular structures. This comparison confirmed that [14]annulene has its π bonds delocalized in accordance with what was deduced from dynamic studies (NMR at different temperatures) and X-ray data. The assignment of the different ionization energies to the relevant states is further substantiated by a correlation of the four first observed ionization energies of [14]annulene with those measured (and assigned by computation) for 15,16-dimethyl-15,16-dihydropyrene and pyrene.

Photoelectron spectra of benzo[b]biphenylene and dibenzo[b,h]biphenylene

The He(I) photoelectron (p.e.) spectra of benzo[b]biphenylene (1) and dibenzo[b,h]biphenylene (2) are reported. The spectra are interpreted by comparison with the results of semi-empirical m.o. calculations. The assignment of the p.e. spectra of both compounds is confirmed by the relationship between the electronic transition energies for the α, p- and β-bands and the ionization potentials.

Potential surface study of a thermally allowed double hydrogen shift in an isomer of [12] annulene

Abstract Trans -4a,10a-dihydrobenzocyclooctene (1) rearranges thermally into 7,8-dihydro-benzocyclooctene ( 3 ). This rearrangement is formally a double 1,5-hydrogen-shift; it may proceed either via two consecutive hydrogen shifts, 4a,7-dihydrobenzocyclooctene ( 2 ) being implied as an intermediate, or the two hydrogens could migrate synchronously. In the latter case, the C 2 -symmetry axes present in the starting, 1 , and final compound, 3 , is preserved along the reaction pathway. In order to determine which is the preferred pathway, a relevant part of the potential surface corresponding to this reaction was investigated using MO-computations. The single determinant UHF-MINDO/3 method, which takes into account electron—electron correlation, was adopted. The two consecutive 1,5-hydrogen-shifts mechanism is shown to be the preferred reaction pathway.

Magnetic circular dichroism spectra and electronic spectra of benzodiazoles

The linear dichroism and magnetic circular dichroism spectra of benzofurazan (BFZ), 2,1,3-benzothiazole (BTD) and 2,1,3-benzoselenadiazole (BSD) have been measured. The circular dichroism spectra of the β-cyclodextrin complexes with BFZ, BTD and BSD are reported. The first absorption bands of BFZ, BTD and BSD are assigned by using the observed spectra and the results of m.o. calculations. Our assignment for the first absorption band is in agreement with previous work.

Electronic spectra of dibenzo[b,h] biphenylene

The u.v., linear dichroism and magnetic circular dichroism spectra of dibenzo[b,h]biphenylene(1) have been measured. The u.v. bands have been assigned with the aid of CNDO/S CI calculations. The applicability of Schmidts relationship between the ionization potentials and electronic transition energies in the case of the biphenylenes [(4n)π-systems] is discussed.

Magnetic circular dichroism and anomalous fluorescence spectra of azulenothiophenes

The magnetic circular dichroism and electronic absorption spectra of azuleno[1,2-b]thiophene and azuleno[2,1-b]thiophene, which are iso-π-electron systems of benz[a]azulene, have been measured. The absorption bands of both azulenothiophenes are interpreted by using these observed spectra and the results of molecular-orbital calculations. The anomalous fluorescence spectra have been observed for both azulenothiophenes and benz[a]azulene. From the assignment of the absorption spectra, it has been concluded that these compounds emit the fluorescence from the second excited singlet state.