Assignment of the ~A state in bicyclobutane. The multiphoton ionization spectrum and calculations of transition energies

Non‐empirical calculations of NMR indirect carbon‐carbon coupling constants. Part 5: Bridged bicycloalkanes

“…The agreement between the present B3LYP/6-311G Ł results and microwave experiments is fairly good, and an even better agreement is observed with the corresponding MP2/6-31G Ł data by Walters et al 15 However, one should keep in mind that calculations at this level of theory with a small basis set utilizing orbital-based wavefunctions recovers a rather small proportion of electron correlation and thus any good agreement with experiment may turn out to be fortuitous.…”

“…13 In an early paper by Cox et al, 14 a complete set of structural parameters of bicyclobutane were found from the microwave spectra of its four isotopic species to yield a puckering angle of ca 58°(the corresponding dihedral angle between cyclopropane rings is 121°40 0 ), in very close agreement with the value of 57.3°derived from the IR spectrum by Walters et al 15 and 57.2°from ED data by…”

Theoretical study of spectroscopic properties of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene

“…All the low-lying excitations in BCB and its higher homologs are found to be essentially Rydberg in nature, in full agreement with the CI calculations [24,251. The lowest intravalence HOMO + LUMO transition is calculated to fall at higher energy.…”

“…Also, the oscillator strengths are in substantial accord. The theoretical energy for the HOMO -+ 3s Rydberg transition is somewhat high compared with the spectroscopic value, 5.8 eV [231 or -6.1 [24]. Since the same overestimation has also been encountered by the previous CI calculations, a higher level of inclusion of electron correlation would be needed for a more accurate description of the lowest-excited state of BCB.…”

“…The intensity of many Rydberg transitions of all molecules under TPA is predicted to be large enough to make experimental observation feasible. The lowest-excited state of BCB has been, indeed, investigated via REMPI by Walters et al [24]. Therefore, exploitation of the selection rules in one-photon absorptivity and TPA may aid the identification of the location and nature of the excited states of these small-ring alkanes from polarization studies.…”