Ab initio spectroscopic characterization of the radical CH3OCH2 at low temperatures

Abstract

Spectroscopic and structural properties of methoxymethyl radical (CH 3 OCH 2 , RDME) are determined using explicitly correlated ab initio methods. This radical of astrophysical and atmospheric relevance has not been fully characterized at low temperatures, which has delayed the astrophysical searches. We provide rovibrational parameters, excitations to the low energy electronic states, torsional and inversion barriers and low vibrational energy levels. In the electronic ground state (X 2 A), which appears "clean" from non-adiabatic effects, the minimum energy structure is an asymmetric geometry which rotational constants and dipole moment have been determined to be A 0 =46718.6745 MHz, B 0 =10748.4182 MHz, and C 0 =9272.5105 MHz, and 1.432 D (μ A =0.6952D, μ B =1.215D, μ C =0.3016D), respectively. A variational procedure has been applied to determine torsion-inversion energy levels. Each level splits into 3 subcomponents (A 1 /A 2 and E) corresponding to the three methyl torsion minima. Although the potential energy surface presents 12 minima, at low temperatures, the infrared band shapes correspond to a surface with only three minima because the top of the inversion V α barrier at α=0º (109 cm -1 ) stands below the zero point vibrational energy and the CH 2 torsional barrier is relatively high (~2000 cm -1 ). The methyl torsion barrier was computed to be ~500 cm -1 and produces a splitting of 0.01 cm -1 of the ground vibrational state.

Competitive Gas Phase Reactions for the Production of Isomers C2O2H4. Spectroscopic Constants of Methyl Formate

“…The validity of this approximation has been corroborated in previous studies of other molecules providing really accurate parameters, the deviations of which are of few MHz. 59 In the case of cis-methyl formate the computed parameters are in a very good agreement with previous experimental data (A 0 = 19982.19530(56) MHz, B 0 = 6914.03971(17) MHz, C 0 = 5304.49791(17) MHz 32 ). The parameters for the trans conformer, for which the agreement with the laboratory data 34 is modest, are also provided.…”

“…It is well-known that VPT2 is not the proper treatment of the internal motion responsible for the nonrigidity as some unrealistic anharmonic contributions can be noted . For the LAMs, it is difficult to predict good fundamentals.…”

“…For both conformers, Table collects the CCSD­(T)-F12/AVTZ structural parameters and the equilibrium rotational constants as well as the MP2 dipole moment components (μ A c = 1.7621D, μ B c = 0.9729D) and the centrifugal distortion constants corresponding to the I r representation of the asymmetrically reduced Hamiltonian where they are compared with available experimental data. ,, The computed A e , B e , and C e parameters were employed to estimate the ground vibrational state rotational constants using the following equation: where Δ B vib represents the vibrational contribution derived from the α r i vibration–rotation interaction parameters determined with second order perturbation theory (VPT2) and the MP2/AVTZ cubic force field . Δ B e core was computed from the CCSD­(T)/cc-pVCTZ parameters, B e (CV) and B e (V), which were calculated correlating both core and valence electrons (CV) and just only considering the valence electrons (V) in the post-SCF process: The validity of this approximation has been corroborated in previous studies of other molecules providing really accurate parameters, the deviations of which are of few MHz . In the case of ci s-methyl formate the computed parameters are in a very good agreement with previous experimental data ( A 0 = 19982.19530(56) MHz, B 0 = 6914.03971(17) MHz, C 0 = 5304.49791(17) MHz).…”

Theoretical spectroscopic study of acetyl (CH3CO), vinoxy (CH2CHO), and 1-methylvinoxy (CH3COCH2) radicals. Barrierless formation processes of acetone in the gas phase

“…A complex distribution of the torsional energy levels can be expected. With respect to the CH 2 torsion, the barrier in CH 3 COCH 2 is ~0.70 V 2 CH2-CHO and of the same order of magnitude as in CH 3 OCH 2 43 . Excited electronic states.…”

“…The next section presents the results and the discussion about the barrierless formation processes of acetone and the corresponding kinetic rates; the spectroscopic characterization of the radicals using two different procedures (one of them suitable for species with large amplitude vibrations and the far infrared region), is explored. Our recent studies of acetone 42 and the CH 3 OCH 2 radical 43 are examples of this last procedure. Finally, the conclusions are drawn.…”