b. Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstraße 3A, 30167 Hannover, Germany
Since Miller first performed the famous experiment showing that amino acids could be synthesized from primoridal molecules (methane, ammonia and water) , amines have drawn particular attention because they are believed to be important inventory prebiotic molecules . Despite many efforts for an interstellar detection of glycine [3-5], the simplest and most promising amino acid, is not confirmed yet, its discovery in the comet  implied that such biomolecules could be formed under certain cosmic condition. Spectroscopic characterization in laboratory and subsequent search in the interstellar medium (ISM) of amines thus provide a crucial clue for the chemical evolution toward complex matters and even lives.
Rotational spectroscopy is the most suitable technique for determination of precise molecular gas phase structures, since the rotational constants of a molecule depend directly on the coordinates and masses of the atoms it comprises. While many regions of the electromagnetic spectrum are used for different purposes in astrophysics, rotational spectroscopy, i.e. radio astronomy from cm to the sub-mm wavelength, is one of the most capable technique for an unambiguous identification of molecular species in the interstellar medium. To our best knowledge, only a few molecules containing an -NH
As far as the nitrogen bearing any other substituted aromatic molecules are concerned, only benzonitrile has been confidently detected in TMC-1 . The rotational spectroscopic investigation on aniline finds the amino group plane being about 37
Herein, to better understand how an alkyl chain affects the molecular structure and spectral feature of amines, the rotational spectra of three ethylanilines (EA), namely ortho- (2-EA), meta- (3-EA) and para- (4-EA) ethylaniline have been investigated by using broad- and narrow-band pulsed jet Fourier transform microwave techniques supported by ab initio calculations.Ⅱ. METHODS A. Experiments
The ethylanilines (
Neon as the carrier gas was passed through a heatable reservoir nozzle (70 ℃, General Valve, Series 9, nozzle diameter 0.5 mm), in which the EAs were placed, at a backing pressure of 0.1 MPa, to generate a pulsed supersonic expansion into the measurement chamber. The rotational spectra from 8 GHz to 27 GHz were initially recorded using the I/Q-modulation passage acquired coherence (IMPACT) Fourier-transform microwave (FTMW) broad-band spectrometer in Hannover  utilizing a dual-path off-axis parabolic reflector antenna system. The nuclear quadrupole coupling splittings and rotational spectra of minor isotopologues in natural abundance were recorded using a coaxially oriented beam-resonator arrangement (COBRA) FTMW spectrometer [26-29]. Due to the expansion of the supersonic jet coaxially along the resonator axis, red- and blue-shifted Doppler components can be observed for each rotational transition. All transitions frequencies are determined from the Doppler components arithmetic mean values. The estimated accuracy of the frequency measurements is better than 2 kHz and the resolution is better than 5 kHz.B. Theoretical calculations
Geometry optimizations of EAs were carried out by the Gaussian 16 program , using the CAM-B3LYP (Coulomb-attenuating method, Becke, three-parameter, Lee-Yang-Parr)  method with an aug-cc-pVTZ basis set, combined with Grimme's D3 dispersion corrections  and the Becke-Johnson damping function BJ [33-35]. Harmonic frequency calculations were executed to estimate the zero-point vibrational energies of the molecules.Ⅲ. RESULTS AND DISCUSSION A. Computational results
Two, three, and two stationary points were found to locate on the potential energy surfaces of 2-EA, 3-EA and 4-EA, respectively. The geometries and relative energies (
Spectral searches for the EAs started from broad-band spectra recorded with the IMPACT-FTMW spectrometer. Assignments were carried out by comparing the recorded spectrum with the quantum chemically predicated spectrum. The left part of FIG. 2 shows a panel of the experimental broad-band spectrum, compared to a calculated spectrum from the final fit results of 2-EA with predicted dipole moments. The right panel of FIG. 1 shows the
By employing Kraitchman's equation , the
Rotational spectra of the normal species and minor isotopologues species of 2-EA, 3-EA and 4-EA have been investigated, which allow determinations of their accurate molecular structures. The experimental structures evidence that the ethyl group is tilted out of the benzene ring plane of 2-EA, assuming dihedral angles of
Supplementary materials: All experimental frequency of rotational transitions are listed in the Supplementary materials, including parent species and all mono-substituted isotopologues of three ethylanilines.Ⅴ. ACKNOWLEDGMENTS
This work was supported by Chongqing University under the Program of the Foundation of 100 Young, the Fundamental Research Funds for the Central Universities (No.2018CDQYHG0009), the National Natural Science Foundation of China (No.21703021 and No.U1931104), the Natural Science Foundation of Chongqing, China (No.cstc2017jcyjAX0068 and No.cstc2018jcyjAX0050), and Venture & Innovation Support Program for Chongqing Oversea Returns (No.cx2018064). Juan Wang also thanks the China Scholarship Council (CSC) for the financial support. The Deutsche Forschungsgemeinschaft (DFG) and the Land Niedersachsen aided the work in Hannover.Supplementary material
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b. 德国汉诺威莱布尼兹大学物理化学与电化学研究所，汉诺威 30167