Thomas W. Dingle

Benzannelated annulenes. 8. Toward the understanding of benzannelated annulenes: synthesis and properties of [a,h]- and [a,i]-ring dibenzannelated dihydropyrenes

The benzannelated dihydropyrenes 3 and 4 were synthesized from 1,3-bis(bromomethyl)naphthalene (16) and 1,3-bis(bromomethyl)-2-methylnaphthalene (22) (the latter obtained in 18% yield in seven steps from 2,3-dimethylnaphthalene) in 7.4% and 4.8% overall yields, respectively, using Stevens or Wittig rearrangement-Hofmann elimination sequences on the dithiacyclophanes 13 and 14, followed by valence tautomerization of the resulting cyclophanedienes 11 and 12. The dihydrobenzopyrene 3 rapidly dehydrogenates to benzo[a]pyrene (21), whereas the dimethyl derivative 4 is relatively stable. The internal protons of 3 and 4 show only about 50% of the shielding of the corresponding protons in the parent hydrocarbons 8 and 1, respectively, and this effect is ascribed to bond localization caused by the benzannelating ring. The residual diatropicity of 3 or 4 is larger than that for the related less rigid 40 or 41. Unlike the parent 1, no evidence was found for a reversible valence isomerization of 4 to 12. Coupling constants of the external protons of 4 were used to compare bond orders to calculated ones, and it was shown that both rings of 4 considerably perturb the delocalization of the other, in accordance with Giinther’s calculations. A comparison of the IH NMR spectra of 4 and benzo[a]pyrene (21) was made. While many annulenes are now known,3 few are as well suited for the study of effects associated with aromaticity as Boekelheide’s4 trans1 5 1 6-dimethyldihydr~pyrene~ (1). This annulene is stable and has a planar6 14a-electron periphery, but most important it has its internal bridges and substituents close to the “center of the ring current”, which make them extremely sensitive’ probes for changes in ring current and hence aromaticity.* Thus study of a phenomenon which changes the aromaticity of an annulene should have a much more pronounced magnetic resonance effect on internal groups, e.g., the internal methyl protons of 1, which normally appear a t 6 -4.25, than for external groups, e.g., the external ring protons of 1, or indeed any other annulene.

Anisotropic repulsive potential energy surfaces from Hartree–Fock calculations for HeCO2 and HeOCS

Hartree–Fock calculations are presented for the repulsive interactions of He with CO2 and OCS. The results are well described by parametrizing the anisotropic potential energy surface as a sum over interactions between He and each atom of the molecule. The interaction of He with the oxygen ends of both molecules is almost identical, thereby reducing the number of potential fitting parameters required. The analytic surfaces obtained yield good agreement with pressure broadening measurements, which probe the anisotropy while being independent of the van der Waals attraction. It is suggested that the sum‐over‐sites parametrization may be useful in constructing semiempirical surfaces that do include the van der Waals attraction. The sum‐over‐sites parametrization is also particularly well suited to describing the vibrational dependence of the repulsive anisotropy.

The HeNe interatomic potential from multiproperty fits and Hartree–Fock calculations

New high‐resolution differential scattering cross sections are reported for the HeNe interaction. These experimental results are combined with Hartree–Fock calculations in constructing a highly accurate interatomic potential. The new potential is capable of reproducing all available experimental data judged to be sufficiently reliable. This includes properties that are highly sensitive to the very weak attractive well and its outer bowl, in addition to the weakly repulsive wall. The potential is compared to those previously proposed for HeNe, particularly to one obtained by direct inversion of differential cross section data of similarly high quality. The potential crosses through zero at σ=2.699 A; its minimum occurs at rm=3.029 A with a depth of e=1.83 meV.

An experimental NMR method to estimate resonance energies of 4N+2 π-electron systems relative to that of benzene.☆

Abstract The change in chemical shift (Δδ) of the internal methyl protons of 1 on annelation with various aromatics, is proportional to RE*, the resonance energy of the annelating aromatic less any resonance energy of a residual aromatic ring in the Kekule structure(s) which has the 14π ring of 1 delocalized. This correlation is linear between RE* values of 0 and 1.5 times the resonance energy of benzene, and hence, can be used to predict resonance energies of other aromatics relative to benzene, simply by measurement of chemical shift.

The 275-nm absorption system of phenyl isocyanate

Abstract The vapor-phase absorption spectrum of phenyl isocyanate was photographed in the 285- to 250-nm region. The π ∗ ← π system origin lies at 36 345 cm−1. While considerable vibrational structure is apparent in the spectrum, all the observed features are diffuse. A vibrational analysis is presented. Theoretical calculations based on π-SCF and HAM 3 methods give results in good agreement with observations in the electronic and photoelectron spectrum.

Quantum defects from atomic spectra

Graphical method for using measured atomic level energies to determine ionization potential and quantum defects.