3190
J. Phys. Chem. 1985,89, 3190-3193
Electronic Absorption Spectra of NEC-CEN', Neon Matrices
H-CS-CsN',
and CH,-Cs-CEN+
in
Jan Fulara, Samuel Leutwyler, John P. Maier,* and Uta Spittel Institut fur Physikalische Chemie der Universitat Basel, CH-4056, Basel, Switzerland (Received: April 30, 1985)
The optical absorption spectra of electronic transitions of cyanogen, cyanoacetylene, and methylcyanoacetylenecations have been obtained in 4.5 K neon matrices. A vibrational interpretation of the spectra is given and leads to some fundamental vibrational frequencies of these cations in their excited electronic states. These observations provide the first new means of spectroscopicallycharacterizing the cyanogen and cyanoacetylene cations since their study by photoelectron spectroscopy, because they do not relax by a radiative pathway.
Introduction Cyanoacetylenes have attracted special attention in recent times due to the detection of the nonsymmetric cyanoacetylenes H(C=C)n-C=N with n = 1-5 in interstellar medium by means of their microwave transitions.' While the nonsymmetric cyanoacetylenes are detectable due to their huge dipole moments, the lack of dipole moment of the symmetric dicyano analogues N=C-(C=C),-C=N precludes their study and detection by means of microwave spectroscopy. We have been concerned with spectroscopic studies of the open-shell cations of such species, initially by exciting emission by electron impact and then by laser excitation of fluorescence. Thus the cations R-(C=C),-C=N+ could be studied in emission by using effusive, R = H , n = 2, and R = CN, n = 1, 2,2 and later supersonic sources as well as by laser excitation R = CN, n = 1,3R = CH3, n = 2.4 However, for a number of the closely related cyanoacetylene cations no gas-phase emission spectra were d e t ~ t e d . Thus, ~ while the diacetylene cation emission has been of central importance in the study of open-shell cations by optical s p e c t r o ~ c o p y radiative ,~~~ decay could not be detected for the isoelectronic cyanogen and cyanoacetylene cations! As the lowest fragment appearance potentials of these cations are higher than the respective lowest excited electronic states, the lack of emission indicates dominant nonradiative with a quantum yield > intramolecular decay pathways for their excited electronic states. However, the rates of these processes (
