Structural and spectroscopic characterization of methyl isocyanate, methyl cyanate, methyl fulminate, and acetonitrile N-oxide using highly correlated ab initio methods

Abstract

Various astrophysical relevant molecules obeying the empirical formula CHNO are characterized using explicitly correlated coupled cluster methods (CCSD(T)-F12). Rotational and rovibrational parameters are provided for four isomers: methyl isocyanate (CHNCO), methyl cyanate (CHOCN), methyl fulminate (CHONC), and acetonitrile N-oxide (CHCNO). A CHCON transition state is inspected. A variational procedure is employed to explore the far infrared region because some species present non-rigidity. Second order perturbation theory is used for the determination of anharmonic frequencies, rovibrational constants, and to predict Fermi resonances. Three species, methyl cyanate, methyl fulminate, and CHCON, show a unique methyl torsion hindered by energy barriers. In methyl isocyanate, the methyl group barrier is so low that the internal top can be considered a free rotor. On the other hand, acetonitrile N-oxide presents a linear skeleton, C symmetry, and free internal rotation. Its equilibrium geometry depends strongly on electron correlation. The remaining isomers present a bend skeleton. Divergences between theoretical rotational constants and previous parameters fitted from observed lines for methyl isocyanate are discussed on the basis of the relevant rovibrational interaction and the quasi-linearity of the molecular skeleton.

Laboratory study of methyl isocyanate ices under astrophysical conditions

“…The splitting in multiple peaks has been attributed to a coupling of the NCO asymmetric stretch with the torsion of the methyl group (Reva et al 2010). In addition, second order perturbation theory calculations (Dalbouha et al 2016; Senent 2017) show that the band maximum is red-shifted by more than 10 cm -1 with respect to the harmonic value due to a Fermi resonance with the δ a -CH 3 + ν-CN combination band. Both the Fermi resonance and the torsion of the methyl group are affected by the H/D substitution, which might explain the change in the band profile between CH 3 NCO and CD 3 NCO observed in the insets of Fig.1…”

“…Electron diffraction (Eyster et al 1940; Anderson et al 1972), microwave (MW) spectra (Curl et al 1963; Lett & Flygare 1967; Koput 1984; Koput 1986; Kasten et al 1986) and theoretical calculations (Koput 1988; Sullivan et al 1994; Zhou & Durig 2009; Reva et al 2010, Dalbouha et al 2016) indicate that the CH 3 NCO molecule is an asymmetric top with a CNC angle of about 140° and a small barrier (≈ 20-30 cm -1 ) for the torsion of the methyl group. Its large amplitude internal motions (CH 3 torsion and CNC bend) complicate the spectral analysis and although the high accuracy of the available experimental data (Cernicharo et al 2016) has allowed the detection of this molecule in the gas phase of the ISM, an appreciable discrepancy persists in the value of the A rotational constant (Halfen et al 2015; Cernicharo et al 2016; Dalbohua et al 2016).…”

“…The bands of the large amplitude motions appear at lower frequencies: 190-170 cm -1 for the CNC in-plane bending, and 20-30 cm -1 for the CH 3 torsion, and have been determined from far IR, Raman and MW spectra (Koput 1986, Sullivan 1994). In spite of their low frequencies, these bands could also have some influence on the mid IR region through combinations with higher frequency modes (Zhou & Durig 2009; Reva et al 2010; Dalbouha et al 2016).…”

Photodissociation of the CH3O and CH3S radical molecules: an ab initio electronic structure study

“…The equilibrium rotational constants of CH 3 O and CH 3 S were computed at the MRCI/CASSCF/AVTZ52 and RCCSD(T)-F12/AVTZ55,56 levels of theory, and compared to the experimental values when available. For the electronic ground state the B e rotational constant was calculated with the equation57,58 Be=Be(RCCSD(T)−F12)+ΔBcore(RCCSD(T)), and with similar equations for the constants A e and C e . In eqn (4) Δ B core is defined as ΔBcore=Be(CV)−Be(V), where B e ( CV ) and B e ( V ) were calculated correlating both core and valence electrons ( CV ) or just the valence electrons ( V ) in the post SCF process.…”

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

“…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.…”