SiLiF: The competition between electronic effects favoring singlet and triplet ground states. A case study

Electronegative substituents such as F favor closed-shell singlet, while electropositive substituents such as Li favor triplet ground electronic states in carbenes and silylenes. Therefore, SiLiF presents a very instructive case study of the competing electronic effects involved. We have examined the minima on the lowest-lying 1A′, 1A″, and 3A″ potential surfaces of SiLiF, using self-consistent-field and configuration interaction methods with a basis set at the double zeta plus polarization level. Two minima, one with a wide and the other with a narrow bond angle θ (LiSiF), have been found for each of the three electronic states, in analogy with SiHLi. The most stable minimum at all levels of approximation is the narrow-angle triplet (3A″ I). It lies below the second most stable minimum, the narrow-angle closed-shell singlet (1A′ I), by 10.5 kcal/mol at the CISD level of theory. This result is similar to the singlet–triplet separation in SiHLi at the same level of theory (10.7 kcal/mol), demonstrating that the electropositive Li atom prevails against the electronegative F. The harmonic vibrational frequencies and the results of a Mulliken population analysis are also presented for each minimum.