Tricycanomethanide ion: an infrared, Raman, and tunneling spectroscopy study including isotopic substitution

The tricyanomethanide ion (TCM), C(CN)3-, and its isotopomers, C(CN)2(13CN)- and C(CN)2(C15N)-, were studied experimentally by Raman, IR, and tunneling spectroscopy in the frequency range from about 80 to 4000 cm-1. The primary focus of the present paper is on the ion and its K salt, but Co and Mn complex data are also reported. An 11-parameter empirical force field provided a 2-cm-1 standard deviation fit to all the normal modes of the ion. While many of the assignments agree with those of previous studies (done without isotopic substitution), there are a few differences. In particular, the inactive a2'mode is definitively assigned through the use of tunneling spectroscopy. Ab initio calculations were performed at the 4-31G SCF level to determine the geometry and 11 of the quadratic force constants of the ion. The calculated geometry is in good agreement with that of the KTCM salt, but the calculated force constants are about 25% too large. 36 references, 6 figures, 5 tables