Rotational and Torsional Properties of Various Monosubstituted Isotopologues of Acetone (CH₃–CO–CH₃) from Explicitly Correlated Ab Initio Methods

Abstract

Geometric and spectroscopic parameters are determined using explicitly correlated coupled cluster ab initio calculations (CCSD(T)-F12) for five monosubstituted isotopologues of acetone containing 18 O, 13 C, and D. The far-infrared region is explored with a variational procedure of reduced dimensionality that takes the interconversion of the nine minima of the potential energy surface into consideration. The methyl torsional barrier of the main isotopologue, computed to be 245.7 cm −1 , varies slightly with the isotopic substitution. In the monodeuterated species, the two inequivalent internal rotors are hindered by two inequivalent barriers of 244.0 and 244.7 cm −1 . The torsional fundamentals of the main variety are localized at 78.636 cm −1 (ν 12 ) and 128.904 cm −1 (ν 17 ). The differences between splitting components are lower than 0.02 and 0.2 cm −1 in the main isotopologue and in the isotopologues containing 18 O and 13 C, respectively. In the monodeuterated species the subcomponents are separated by ∼15 cm −1 .

Ab Initio Study of the Large Amplitude Motions of Various Monosubstituted Isotopologues of Methylamine (CH3-NH2)

“…This approximation has been corroborated in previous studies of other non-rigid molecules providing really accurate parameters, whose deviations with respect available experimental data, represent few MHz ( Boussesi et al, 2016 ) ( Dalbouha et al, 2016 , 2021 ). In Table 1 , the computed rotational constants of CH 3 NH 2 , 13 CH 3 NH 2, and CH 3 NDH are compared with available experimental parameters ( Ilyushin et al, 2005 ) ( Motiyenko et al, 2016 ) ( Ohashi et al, 1991 ).…”

“…The isotopic substitutions carry out changes in the symmetry. Details concerning the followed procedure can be found in our previous paper devoted to the acetone isotopologues ( Dalbouha et al, 2021 ). The effective surfaces allow to construct Hamiltonians depending on three interacting coordinates, two interacting large amplitude motions, the NH 2 wagging and the CH 3 torsion, and the HNH bending.…”

Large Amplitude Motions of Pyruvic Acid (CH3-CO-COOH)

“…The ground vibrational state rotational constants were computed using the CCSD(T)-F12 equilibrium parameters of Table 2 and the following equation, proposed and verified in previous studies [ 22 , 39 , 40 , 41 ]. B i0 = B ie (CCSD(T)-F12/AVTZ-F12) + ∆B i core (CCSD(T)/CVTZ) + ∆Bi vib (MP2/AVTZ); i = a;.b;.c …”

“…Given the good agreement between ab initio and experimental results for Tc, it can be expected that the MP2/VTZ predicted sets will be useful for future assignments of the remaining conformers. The ground vibrational state rotational constants were computed using the CCSD(T)-F12 equilibrium parameters of Table 2 and the following equation, proposed and verified in previous studies [22,[39][40][41]. Here, ∆B ie core takes into account the core-valence-electron correlation effect on the equilibrium parameters.…”

“…In this paper, we attend to the pyruvic acid from the point of view of ab initio calculations with special attention to the far infrared region (FIR). The work describes structural and spectroscopic properties following the procedures used for molecules such as acetic acid [21], acetone [22] or biacetyl [23], which are properties or functional groups with PA. Ab initio calculations are achieved using explicitly correlated coupled cluster theory [24,25]. The far infrared region, relevant for rotational studies, is explored using a three-dimensional variational procedure [26].…”