Millimeter and submillimeter wave spectra of¹³C methylamine

Abstract

Context. Methylamine (CH 3 NH 2 ) is a light molecule of astrophysical interest, which has an intensive rotational spectrum that extends in the submillimeter wave range and far beyond, even at temperatures characteristic for the interstellar medium. It is likely for 13 C isotopologue of methylamine to be identified in astronomical surveys, but there is no information available for the 13 CH 3 NH 2 millimeter and submillimeter wave spectra. Aims. In this context, to provide reliable predictions of 13 CH 3 NH 2 spectrum in millimeter and submillimeter wave ranges, we have studied rotational spectra of the 13 C methylamine isotopologue in the frequency range from 48 to 945 GHz. Methods. The spectrum of 13 C methylamine was recorded using conventional absorption spectrometers. The analysis of the rotational spectrum of 13 C methylamine in the ground vibrational state was performed on the basis of the group-theoretical high-barrier tunneling Hamiltonian that was developed for methylamine. The available multiple observations of the parent methylamine species toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory were used to make a search for interstellar 13 CH 3 NH 2 .Results. In the recorded spectra, we have assigned 2721 rotational transitions that belong to the ground vibrational state of the 13 CH 3 NH 2 . These measurements were fitted to the Hamiltonian model that uses 75 parameters to achieve an overall weighted rms deviation of 0.73. On the basis of these spectroscopic results, predictions of transition frequencies in the frequency range up to 950 GHz with J ≤ 50 and K a ≤ 20 are presented. The search for interstellar 13 C methylamine in available observational data was not successful and therefore only an upper limit of 6.5 × 10 14 cm −2 can be derived for the column density of 13 CH 3 NH 2 toward Sgr B2(N), assuming the same source size, temperature, linewidth, and systemic velocity as for parent methylamine isotopic species.

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 MP2/AVTZ quartic centrifugal distortion constants corresponding to the asymmetrically reduced Hamiltonian, are shown in Table 2 where they are compared with previous experimental data ( Ilyushin et al, 2005 ) ( Motiyenko et al, 2016 ) ( Ohashi et al, 1991 ).…”

“…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 MP2/AVTZ quartic centrifugal distortion constants corresponding to the asymmetrically reduced Hamiltonian, are shown in Table 2 where they are compared with previous experimental data ( Ilyushin et al, 2005 ) ( Motiyenko et al, 2016 ) ( Ohashi et al, 1991 ). Disagreements between experimental and computed data can be correlated with the level of ab initio calculations used to compute the anharmonic force field.…”

“…Whereas publications about the methylamine main isotopologue are recurrent, less studies attend to other isotopic species. The microwave spectrum of the monosubstituted species CH 3 NHD ( Ohashi et al, 1991 ), CH 2 DNH 2 ( Tamagake & Tsuboi 1974 ), and 13 CH 3 NH 2 ( Motiyenko et al, 2016 ), and the deuterated species, CH 3 ND 2 , CD 3 NH 2 , CD 3 ND 2 ( Lide 1954 ) ( Sastry 1960 ) ( Takagi & Kojima 1971 ) ( Kreglewski et al, 1990a ; 1990b ) were measured and assigned. The infrared absorption spectrum of 15 N-methylamine was inspected in the gas phase ( Hirakawa et al, 1972 ).…”

Isotopic dependence of the hydrogen-transfer-triggered methyl-group rotation in deuterated 5-methyltropolone

“…[1,2]. Previously this formalism was successfully applied to methylamine [6,7,8,9], for which it was originally developed [2]. We also used the same fitting program, i.e., a slightly modified version [3,6] of the program obtained from N. Ohashi [10].…”

Rotational spectroscopy of singly 13C substituted isotopomers of propyne and determination of a semi-empirical equilibrium structure

“…Such rest frequencies are usually based on laboratory spectroscopic investigations and are frequently provided in databases such as the Cologne Database for Molecular Spectroscopy, CDMS 1 [35]. Recent investigations of 13 C-containing isotopologs of molecules detected in space include for example dimethyl ether with one and two 13 C [36], the acetaldehyde isotopomers with one 13 C [37], 13 CH 3 NH 2 [38], isotopic acetone with emphasis on CH 3 13 C(O)CH 3 [39], isotopic c-C 3 H 2 with several doubly substituted species [40], and isotopic methyl cyanide in the 8 = 1 excited state [41].…”