High-J rotational spectrum of toluene in |m| ⩽ 3 torsional states

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

“…Based on quantum chemistry calculations described in computational methods section and presented in details in Table S1 of supporting information I, the jet‐cooled MW spectrum (see section 5.1) has been firstly measured and analysed up to J and taking into account both the coupling with internal rotation of the methyl group, the NQC and the spin statistics. The A1/A2 and B1/B2 species were preliminary assigned using the SPFIT/SPCAT Pickett's programs [19] (semi‐rigid rotor model in the representation of A‐reduced Watson's Hamiltonian) and completed with the E1/E2 species arising from internal rotation coupling using Ilyushin's code RAM36hf [20–23] . This code especially developed for the analysis of internal rotation splittings of G 12 point group molecules including high order internal rotation coupling terms for low internal rotation barriers and NQC terms has been firstly used on nitromethane [23] .…”

“…The RAM allows us to eliminate at the zero order certain coupling terms in the kinetic energy between the internal angular momentum p a and the global rotational momentum P, and to simplify the diagonalization of the Hamiltonian. [26,51] This approach is used in the BELGI series of codes, [26,27,54,55] as well as in the RAM36hf code [20,21] of which a version will be used in the 2-NT and 4-NT spectra analyses, respectively. The molecular parameters obtained by those two approaches can be transformed into the PAM by an angle called θ RAM .…”

“…The A1/A2 and B1/B2 species were preliminary assigned using the SPFIT/SPCAT Pickett's programs [19] (semi-rigid rotor model in the I r representation of A-reduced Watson's Hamiltonian) and completed with the E1/ E2 species arising from internal rotation coupling using Ilyushin's code RAM36hf. [20][21][22][23] This code especially developed for the analysis of internal rotation splittings of G 12 point group molecules including high order internal rotation coupling terms for low internal rotation barriers and NQC terms has been firstly used on nitromethane. [23] According to the permanent dipole vector orientation (see Figure 1), only a-type transitions have been observed with a total of 304 individual transitions (83 A1/ A2, 74 B1/B2, 78 E1 and 69 E2 species) assigned to 226 blended lines with a RMS deviation of 1.97 kHz (1.82 kHz for A1/A2, 2.00 kHz for B1/B2, 1.91 kHz for E1 and 2.16 kHz for E2 species).…”

A Comprehensive Spectral Rotational Analysis of the Interstellar Methyl Isocyanate CH3NCO

“…Their values can be precalculated with moderate confidence but, in order to reach spectroscopic accuracy, each such parameter also needs to be supported by an associated centrifugal type expansion, which is much more poorly understood. Furthermore, the J dependence of the observed line sequences also lacks the sharp resonances that allow precise identification of the positions of interacting vibration-rotation energy levels (as has been the case for toluene, Ilyushin et al (2017)). For the time being we prefer to make the extensive laboratory data available for applications and to cite Curl et al (1963), who on being confronted with the CH 3 NCO spectrum stated: There seems to be little value in reporting values of the constants obtained or details of the fits.…”

“…In principle, low barrier methyl group internal rotation is treatable with the RAM36 program of Ilyushin et al (2010). The program has been very successful in treating toluene, which is a molecule with a very similar energy distribution of internal rotor states to that in CH 3 NCO, and also displaying a plethora of perturbations between m = 0 and 1 substates (Ilyushin et al 2017). In fact, RAM36 has already been applied to CH 3 NCO (Halfen et al 2015), albeit to a very small data set, which has later been shown to contain some misassignments (Cernicharo et al 2016).…”