Infrared Photodissociation Spectroscopy of C4N-, C6N- and C8N-

The gas-phase vibrational spectroscopy of cold C2nN- (n=2-4) anions is investigated in the CC and CN multiple bond stretching region (1700-2250cm(-1)) by means of infrared photodissociation (IRPD) spectroscopy in a cryogenically cooled ion trap of the corresponding messenger-tagged complexes. The IRPD spectra are assigned to N-terminated linear structures with triplet ground states ((3)sigma(-)) based on a comparison with harmonic vibrational frequencies and intensities from density functional theory computations. In contrast to the polyacetylenic C2n+1N- anions, the linear C-C chains investigated here exhibit cumulenic character, which is most pronounced in C4N- and decreases with chain length. Additional intense transitions are observed for C6N- above 3000cm(-1) and are attributed to overtone and combination bands involving the CC stretching modes, based on anharmonic computations. The influence of a D-2 tag on the vibrational features of C2nN- anions is shown to be small.