Abstract
The 2800 Å forbidden 1Σu- ← 1Σg+ transition of dicyanoacetylene shows conventional vibronic origins due to the two πg vibrations and a third involving the lower πu vibration. For the latter we argue against a magnetic dipole mechanism and propose that the intensity is electric dipole, involving coupling to a π* ← 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 (ΔB0 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 ΔBe. 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.
Original language | English |
---|---|
Pages (from-to) | 10637-10641 |
Number of pages | 5 |
Journal | Journal of Physical Chemistry A |
Volume | 107 |
Issue number | 49 |
DOIs | |
Publication status | Published - 11 Dec 2003 |