A b i n i t i o calculations of oscillator and rotatory strengths in the random-phase approximation: Twisted mono-olefins

Abstract

A b initio (STO-nG) computations of ordinary and rotatory intensities of low-lying electronic transitions are presented for twisted ethylene and twisted trans-2-butene in the random-phase approximation (RPA). The intensities are computed in both dipole length and dipole velocity forms, as well as the mixed form for the oscillator strength, and the convergence of these formally equivalent results is examined in the RPA and several other methods for constructing the electronic excitation: the virtual orbital, or single-transition, approximation (STA), the monoexcited configuration-interaction, or Tamm–Dancoff, approximation (TDA), and one version of the higher RPA (HRPA). We show that the RPA has consistent advantages over the TDA for calculation of CD as well as ordinary intensities. Our computations confirm that a localized, ethylenic chromophore is indeed adequate to account for the low-lying CD spectrum in mono-olefins. Further, even with minimal valence-shell basis sets, our RPA rotatory strengths agree essentially completely in both sign and magnitude with the experimental results of Mason and Schnepp on trans-cyclooctene.