SIMBAD references

Methanimine (CH₂NH) is a simple molecule composed of methylene and imine. The molecule has been detected toward the Galactic center, star-forming regions, circumstellar envelopes, and other galaxies since 1973. In previous studies, the rest frequency of methanimine has been measured for normal species up to the 650-GHz region, but its ¹³CH₂NH, CH₂¹⁵NH, and CH₂ND isotopologs were limited to the 100-GHz region. If a rotational temperature of 100K is assumed for methanimine, the highest intensity falls at approximately 1.5THz. In addition to normal methanimine, the ¹³CH₂NH, CH₂¹⁵NH, and CH₂ND isotopologs in their ground-vibrational states were observed in the frequency range of 120-1600GHz to provide accurate rest frequency information. Based on this study, the calculated rest frequencies below 2THz should be sufficiently precise and support observations using all ALMA and Herschel/HIFI observational bands. Methanimine was generated by pyrolysis of diaminoethane (DAE) vapor at 850°C. ¹³CH₂NH and CH₂¹⁵NH isotopologs were measured with their natural abundance, and deuterization of DAE was performed by mixing normal DAE with deuterated water, D₂O, and then pyrolyzed. This gives the deuterated isotopolog of methanimine, CH₂ND. Spectral measurements were performed by using the 23kHz source-frequency modulated terahertz spectrometer at Toho University. Both a- and b-type transitions up to 1.6 THz for the three isotopologs and the normal species were measured. Rotational and centrifugal distortion constants for the three isotopologs were accurately determined. For normal species (CH₂NH), both electric quadrupole and nuclear spin-rotation coupling constants for nitrogen nucleus were determined, while for the ¹³CH₂NH and CH₂ND species, only electric quadrupole-coupling constants for nitrogen nucleus were determined. Our spectral line frequencies are suitable for a future astronomical search for these isotopologs of methanimine. The 1σ frequency accuracy up to 2THz is lower than 100kHz.