Electronic Spectrum of Dicyanoacetylene. 2. Interpretation of the 2800 Å Transition

Abstract

The 2800 Å forbidden 1 Σ u r 1 Σ g + transition of dicyanoacetylene shows conventional vibronic origins due to the two π g vibrations and a third involving the lower π u vibration. For the latter we argue against a magnetic dipole mechanism and propose that the intensity is electric dipole, involving coupling to a π* r n state of reduced point symmetry (C ∞V or less). The strongest band in the spectrum may be another such origin, but there is a competing assignment and neither meets expectations of intensities in related bands. While the vibrational structure is emphatic that the excited-state remains linear, the rotational constants extracted from the principal bands, all double-headed and violet-degraded (∆B 0 positive), present problems. The apparent contraction or bending of the molecule thus indicated can be countered by large corrections that yield negative values of the equilibrium constant ∆B e . The size of these corrections and further B values extracted from sequence bands indicate that rotational constants are not uniformly linear in the quantum numbers of the bending vibrations. † Part of the special issue "Charles S. Parmenter Festschrift".

Photochemical activity of Titan’s low-altitude condensed haze

“…As illustrated in Fig. 2 (top), the ultraviolet absorption spectrum of C 4 N 2 ice at 100 K has structure at o300 nm in agreement with the gas-phase spectrum [36][37][38][39] , but also a featureless long-wavelength absorption stretching to 4360 nm, beyond where the gas-phase absorption ends. The broad continuum absorption at longer wavelengths than B350 nm could be due to the weak singlet-triplet absorption amplified in the condensed phase, in agreement with detection of phosphorescence from the first triplet (T 1 ) state of C 4 N 2 at B390 nm (ref.…”

Contrasting Dynamics in Isoelectronic Anions Formed by Electron Attachment

“…This has been confirmed later both experimentally 39 and theoretically. 44,46 The lowest excited state of HC 3 N marked in Figure 5 is 1 A″. It is adiabatic, and the vertical excitation energies are 4.77 and 5.1 eV, respectively.…”

“…4 with the unoccupied orbitals ordered as π u π g σ u . 44 The configuration of all excited states marked in Figure 4 is π g 3 π u 1…”

“…. 44 For the configurations that give rise to the Σ u terms, if an extra electron will be added to the σ u unoccupied orbital, the resulting core-excited resonance will have a Σ g symmetry. Since such a resonant state would produce angular distribution in a perfect agreement with the experiment, we ascribe the CN − production in NCCN to be mediated by a core-excited resonance with the configuration π g…”