Thorium Monosilicide, ThSi: An Experimental and Theoretical Study

The present theoretical
and experimental combination study investigates
the ThSi molecule in detail. Computationally, we utilized high-level
multireference and coupled-cluster levels of theory conjoined with
large correlation consistent basis sets to study a series of electronic
and spin–orbit states of ThSi. We report potential energy curves
(PECs), electron configurations at equilibrium distances, spectroscopic
constants, energetics, and spin–orbit coupling effects for
16 electronic states of ThSi. The studied 16 electronic states are
arranged tightly within 0.9 eV, highlighting the complexity of the
electronic spectrum of ThSi. The ground electronic state of ThSi is
a single-reference 11Σ+ state that derives
from the 1σ22σ21π4 electronic configuration. The Ω = 0+ spin–orbit
ground state of ThSi is composed of 11Σ+ (47%) and 13Π (44%) electronic states. Our measured
bond energy (D0) of ThSi, obtained using resonant two-photon
ionization (R2PI) spectroscopy is 3.146(4) eV, where the assigned
error limit is given in parentheses in units of the last quoted digits.
The computed D0 of ThSi (Ω = 0+) at the
CBS-C-CCSD(T)-δT(Q)-δDK-δSO level (3.181 eV) is
in good agreement with the experimental value. Our derived enthalpy
of formation for ThSi, ΔfH0Ko(ThSi(g)), is
971.8(6.0) kJ/mol. Finally, we have performed density functional theory
(DFT) calculations for ThSi(11Σ+) using
16 exchange correlation functionals that span multiple rungs of “Jacob’s
ladder” of density functional approximation (DFA) to assess
the DFT errors on D0, re, and
ωe of ThSi with respect to experimental and ab initio
coupled-cluster values.