Radiative cooling of C7⁻.

The spontaneous and photo-induced neutralization of C7⁻ produced in a laser ablation source was measured in an electrostatic storage ring. The measurements provide three independent determinations of the radiative cooling of the ions, based on the short time spontaneous decay and on the integrated amplitude and the shape of the photo-induced neutralization signal. The amplitude of the photo-induced signal was measured between 0.5 ms and 35 ms and found to depend on photon wavelength and ion storage time. All three signals can be reproduced with identical thermal IR radiative cooling rates with oscillator strengths equal to theoretical predictions. In addition, the measurements provide the excitation energy distribution.

Are oxocarbon dianions aromatic?

Assessment of the cyclic electron delocalization of the oxocarbon dianions, C(n)()O(n)()(2)(-) and their neutral counterparts C(n)()O(n)() (n = 3-6), by means of structural, energetic, and magnetic criteria, shows that C(3)O(3)(2)(-) is doubly aromatic (both sigma and pi cyclic electron delocalization), C(4)O(4)(2)(-) is moderately aromatic, but C(5)O(5)(2)(-), as well as C(6)O(6)(2)(-), are less so. Localized orbital contributions, computed by the individual gauge for localized orbitals method (IGLO), to the nucleus-independent chemical shifts (NICS) allow pi effects to be disected from the sigma single bonds and other influences. The C-C(pi) contribution to (NICS(0,pi) (i.e., at the center of the ring) in oxocarbon dianions decreases with ring size but shows little ring size effect at points 1.0 A above the ring. On the basis of the same criteria, C(4)O(4) exhibits cyclic electron delocalization due to partial occupancy of the sigma CC bonds. However, the dissociation energies of all the neutral oxocarbons, C(n)()O(n)(), are highly exothermic.