Chinese Journal of Chemical Physics  2020, Vol. 33 Issue (1): 119-124

#### The article information

Juan Wang, Sven Herbers, Philipp Buschmann, Kevin Lengsfeld, Jens-Uwe Grabow, Gang Feng, Qian Gou

Rotational Spectra and Molecular Structures of Ethylanilines

Chinese Journal of Chemical Physics, 2020, 33(1): 119-124

http://dx.doi.org/10.1063/1674-0068/cjcp1912215

### Article history

Accepted on: December 23, 2019
Rotational Spectra and Molecular Structures of Ethylanilines
Juan Wanga,b , Sven Herbersb , Philipp Buschmannb , Kevin Lengsfeldb , Jens-Uwe Grabowb , Gang Fenga , Qian Goua
Dated: Received on November 28, 2019; Accepted on December 23, 2019
a. School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, China;
b. Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstraße 3A, 30167 Hannover, Germany
Abstract: The molecular structures of three ethylanilines, ortho-, meta- and para-ethylaniline, have been obtained by means of Fourier-transform microwave spectroscopy. Rotational spectra of all three molecules display the nuclear quadrupole hyperfine structures arising from the $^{14}$N nucleus. Comparison of the determined structures allows a direct study of the influence of the position of the ethyl substituent on the structure of the amino group communicated through the phenyl ring.
Key words: Rotational spectroscopy    Molecular structure    Supersonic expansion
Ⅰ. INTRODUCTION

Since Miller first performed the famous experiment showing that amino acids could be synthesized from primoridal molecules (methane, ammonia and water) [1], amines have drawn particular attention because they are believed to be important inventory prebiotic molecules [2]. 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 [6] 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$_2$ group have been spectroscopically identified in the ISM, including formamide [7-9], methylamine [10-12], cyanamide [13-15], acetamide [16], aminoacetonitrile [17-19], urea [20], exclusively due to their rotational spectral features.

As far as the nitrogen bearing any other substituted aromatic molecules are concerned, only benzonitrile has been confidently detected in TMC-1 [21]. The rotational spectroscopic investigation on aniline finds the amino group plane being about 37$^\circ$ out-of-plane with respect to the phenyl ring [22]. The spectrum of 4-methylaniline is rather complicated because of the combination of two large amplitude motions: the inversion of the amino group in the presence of internal rotation of the methyl group [23]. Furthermore, nuclear quadrupole coupling of the $^{14}$N ($I$ = 1) nucleus gives rise to additional splittings. The elongation of -NH$_2$ with -CH$_2$ groups in anline, as in e.g. 2-phenylethylamine [24], leads to the variation of charge distribution in the molecule and, consequently, greatly affects the geometric and electronic structure. It was reported that the observation of four conformers, the two most stable conformers present a gauche disposition of the alkyl-amine chain and are stabilized by a weak NH-$\pi$ interaction between the amino group and the aromatic ring. The other two conformers show an anti-arrangement of the alkyl-amine chain.

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 ($\sim$98%) are brownish liquids used without further purification. The melting points are between $-$44 ℃ and $-$5 ℃, and their boiling points range from 212.3 ℃ to 217.5 ℃.

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 [25] 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 [30], using the CAM-B3LYP (Coulomb-attenuating method, Becke, three-parameter, Lee-Yang-Parr) [31] method with an aug-cc-pVTZ basis set, combined with Grimme's D3 dispersion corrections [32] 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 ($\Delta E$ and $\Delta E_0$ in cm$^{-1}$) of theoretically predicted minima of the EAs are shown in FIG. 1. The global minima (I) of all EAs have similar configurations of the ethyl group tilted out of the phenyl ring plane. Plausible conformers having all heavy atoms within the plane lie much higher in energies. Theoretical spectroscopic parameters, including rotational and quadrupole coupling constants and electric dipole moment components of EAs are reported in Table Ⅰ.

 FIG. 1 Optimized geometrical structures and relative energies with or without zero-point corrections of the three EAs
Table Ⅰ Theoretical spectroscopic parameters (CAM-B3LYP-D3BJ/aug-cc-pVTZ) of the three EAs
B. Experimental results

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 $^{14}$N quadrupole hyperfine pattern ($I$ = 1). No internal rotation splittings were observed in any of the rotational spectra of the EAs, plausibly due to the relatively high barrier to internal rotation of the methyl rotors. Rotational transitions were fitted using the Pickett's SPFIT program [37] with the Watson's S-reduced Hamiltonian in the $I^{\rm{r}}$ representation [38]. The determined spectroscopic constants of all three EAs are reported in the first line of Tables Ⅱ-, and the detailed information of fitted parameters are presented in supplementary materials. Comparing the experimental rotational constants (Tables Ⅱ-) with theoretical ones (Table Ⅰ), the conformational assignments are straightforward to the three global minima. After empirical scaling of rotational constants as observed for the normal species, the spectra of the mono-substituted $^{13}$C and $^{15}$N isotopologues were readily found, measured, and assigned in natural abundance; the determined rotational constants are also reported in Tables Ⅱ-. All the measured transition lines are available in supplementary materials.

 FIG. 2 Left: A section of the broad-band spectrum (red) and simulation (black) by PGOPHER [36]. Right: 616←515 transition of 2-EA recorded with the narrow-band spectrometer, showing the 14N quadrupole coupling hyperfine structure
Table Ⅱ Experimental spectroscopic parameters of 2-EA.
Table Ⅲ Experimental spectroscopic parameters of 3-EA
Table Ⅳ Experimental spectroscopic parameters of 4-EA

By employing Kraitchman's equation [38], the $r_{{ \rm s }}$ coordinates with their Costain-errors are calculated. The $r_{{ \rm s }}$ -structures of the EAs are visualized as foreground spheres in FIG. 3 along with atomic numbering and the inertia principal axes. From the $r_{{ \rm s }}$ coordinates, the bond lengths of the backbone and the orientations of the ethyl group (the dihedral angles) were derived using the program KRA [39] and compiled in Table Ⅴ, where the equilibrium structures ($r_{{ \rm e }}$) are also given for comparison. In order to obtain the effective structure of EAs, the partial $r_0$ structures were reproduced by a least-squares fit taking all sets of rotational constants into account using the STRFIT program [40].

 FIG. 3 CAM-B3LYP-D3BJ/aug-cc-pVTZ calculated geometry, atom numbering and the principal axes of inertia of the three EAs. The experimental $r_{ \rm s }$ position of the isotopically substituted atoms (blue spheres) is included for comparison with the CAM-B3LYP-D3BJ/aug-cc-pVTZ calculated structure (background)
Table Ⅴ The rs, r0 and the re-structure (CAM-B3LYP-D3BJ/aug-cc-pVTZ) of the three EAs

The $r_{{ \rm s }}$ C $-$ N bond gets longer with the ethyl group moving from ortho- to meta- and para- position, meanwhile the C7 atom is not completely in the plane of benzene ring with the dihedral angle $\angle$ (C7-Plen) about 178 $^\circ$, while the ethyl groups dihedral angles [$\angle$ (C $\beta$ C $\alpha$ C7C8)] is trending to be perpendicular to benzene ring (from 75.6 $^\circ$ to 91.3 $^\circ$). The striking differences of 2-EA, compared to the other EAs, can be explained by intramolecular hydrogen bond interaction between N and C $-$ H, as indicated from the independent gradient model (IGM)anaysis [41] presenting in an no-covalent interaction (NCI) plot, which is graphically reported in FIG. 4. The 3D isosurface between the amino and ethyl fragments in all EAs can be identified and quantified from the absolute net electron density gradient attenuation. Two small and sharp spikes (in red), only presenting in the gradient analytical plot of 2-EA, denote the hydrogen bond intramolecular interaction between the amino and ethyl group.

 FIG. 4 The independent gradient model (IGM) analysis of 2-EA, showing the weak hydrogen bond interaction between amino group and ethyl group. The IGM function to plot those pictures: sign($\lambda_2$) $\rho$, $\rho$ represents the electron density and $\lambda_2$ signifies the second eigenvalue of the electron density Hessian matrix
Ⅳ. CONCLUSION

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 $\angle$ (C7-Ben) = 178 $^\circ$ and $\angle$ C $\beta$ C $\alpha$ C7C8 = 75.6(8) $^\circ$, while its orientation is about 90 $^\circ$ in 3-EA and 4-EA, i.e. almost perpendicular to the benzene ring plane. The spectra were readily fitted to a Watson $S$ -reduced Hamiltonian in the $I^{ \rm r }$ representation with no indication of -CH $_3$ internal rotation or the presence of other significantly populated conformers in the supersonic jet.

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
Table S1 Experimental spectroscopic constants of ethyl anilines
Table S2 Transition frequencies of isotopologues of the 2-ehtylaniline in MHz
Table S3 Transition frequencies of isotopologues of the 3-ehtylaniline in MHz
Table S4 Transition frequencies of isotopologues of the 4-ehtylaniline in MHz
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a. 重庆大学化学化工学院，重庆 401331;
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