Abstract : Highly correlated ab-initio wave functions within the UCCSD(T)-F12 approach have been used to map the Potential Energy Surface (PES) and to study the stability of the first excited
triplet states of the NCS−/CNS− and NCO−/CNO− anions. These a3Π states for linear ge-
ometries or their 3A′ and 3A” bent components correlate with the lowest dissociation asymptote
of NCX− (X = S and O) along the NC-X coordinates. The X1Σ+ linear ground state of these
anions is known to be stable with respect to the X2Π linear ground state of the corresponding
neutral molecule with a rather large Electron Affinity (EA), calculated 3.57 eV and 3.70 eV
for NCS and NCO respectively, in good agreement with previous experimental and theoretical
data. The a3Π state of the NCS− anion is positioned below the X2Π state of the neutral at long
NC-S distances and on contrary to the linear X2Π state of the neutral, the minimum of energy
of the a3Π state is found for bent geometries with an angle close to 140 degree : the stability
of its two components, 3A′ and 3A”, in bent geometries, has been discussed and it is found
that some anionic forms are stable with respect to the X2Π state of the neutral. The absolute
minima of the triplet states have been determined. Only for NCS−, it is found that the absolute
minimum of the 3A′ triplet state is below the minimum of the neutral X2Π state. The linear
CNS− and CNO− isomers present a minimum at long distance, located below the minimum of
their corresponding neutral CNX ground states.
