Chinese Journal of Chemical Physics  2020, Vol. 33 Issue (1): 125-128

The article information

Hai-hua Zhou, Zeng-kui Liu, Zi-qiu Chen, Ming Sun, Qian Chen, Sheng-wen Duan, Chao Jiao
周海花, 刘增魁, 陈子秋, 孙铭, 陈钱, 段圣文, 焦超
Pure Rotational Spectrum of Dibenzofuran in Range of 2-6 GHz
6 GHz范围内二苯并呋喃的纯旋转光谱
Chinese Journal of Chemical Physics, 2020, 33(1): 125-128
化学物理学报, 2020, 33(1): 125-128

Article history

Received on: December 3, 2019
Accepted on: December 24, 2019
Pure Rotational Spectrum of Dibenzofuran in Range of 2-6 GHz
Hai-hua Zhoua , Zeng-kui Liua , Zi-qiu Chena , Ming Sunb , Qian Chenb , Sheng-wen Duanb , Chao Jiaob     
Dated: Received on December 3, 2019; Accepted on December 24, 2019
a. College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China;
b. School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Abstract: We report the observation and assignment of the rotational spectra of dibenzofuran measured in the range of 2$ - $6 GHz with a newly constructed broadband chirped-pulse Fourier transform microwave (cp-FTMW) spectrometer. An analysis of the microwave spectra led to the assignment of 40 $ b $-type transitions, resulting in the accurate determination of the rotational constants $ A $ = 2278.19770(38) MHz, $ B $ = 601.12248(10) MHz, and $ C $ = 475.753120(98) MHz.
Key words: Dibenzofuran    Polycyclic aromatic hydrocarbons    Broadband rotational spectroscopy    

The origin of ubiquitous unidentified infrared emission (UIR) features seen in the interstellar medium (ISM) remains a mystery. Although polycyclic aromatic hydrocarbons (PAHs) are widely believed to be the carriers for the UIR bands [1, 2], the molecular specific identification has been proven difficult using the commonly employed rotational spectroscopy in radio astronomy due to the often small or zero dipole moment that many PAHs possess. On the other hand, the search for simple aromatic molecules that are likely linked to the formation of PAHs but easier to observe, seems to be a valid approach to constrain models in interstellar chemistry. Dibenzofuran has been proposed to lead to PAH formation at temperatures above 800 ℃ through oxidation pathways [3]. In this context, as a simple oxygenated PAH with considerable dipole moment, it would serve as a candidate for the detection of PAHs in the ISM.

Dibenzofurans have been observed in ambient air [4] and its formation can be attributed to the same source as unsubstituted PAHs such as incomplete combustion in waste incineration [5, 6]. As an oxygenated PAH, dibenzofurans are produced through secondary oxidation of PAHs by chemical and microbiological processes [7, 8]. Higher toxicity such as mutagenicity and carcinogenicity has been shown over the parent PAHs in oxygenated PAHs [9]. Dibenzofuran in particular, undergoes oxidation reactions with OH radicals, NO$ _3 $ radicals and O$ _3 $ in the atmosphere [10], making the information pertaining to its gas-phase structure and spectroscopy of fundamental interest. Dibenzofuran can serve as a prototype of this class of substituted PAHs to be studied by spectroscopy as this tricyclic moiety is present in this family of compounds.

Up to date, dibenzofuran has been investigated by vibrational spectroscopy [11, 12] and rotationally-resolved electronic spectroscopy [13-15]. This work aims to extend the range of laboratory spectroscopy to support the astrophysical observation of potential precursor molecules for multi-ring PAHs through a microwave investigation of its pure rotational spectrum. We report the measurement and analysis of the observed spectra of dibenzofuran in the vibrational ground state in the range of 2$ - $6 GHz.

Ⅱ. METHODS A. Experiments

The rotational spectra of dibenzofuran were recorded using a chirp-pulse Fourier Transform microwave (cp-FTMW) spectrometer working between 1 and 18 GHz recently developed at Nanjing University of Science and Technology (NJUST) described in detail [16, 17]. In short, a chirped pulse is produced by mixing linear frequency sweeps generated by an arbitrary wave form generator (Tabor WX1281, 1.25 GS/s, 1 GHz) and the cw output of a microwave synthesizer (Anapico Apsyn420, 1$ - $18 GHz). This broadband pulse is then amplified using a solid state amplifier (BONN, 5W) before being coupled to the custom-built vacuum chamber which contains a pair of high gain horn antenna for broadcasting and receiving the microwave signal. The vacuum chamber is pumped through a molecular pump with a background pressure around 1$ \times $10$ ^{-5} $ Pa. A gas mixture of noble gas seeded with sample molecules was allowed to enter the vacuum chamber through a pulsed solenoid valve with a diameter of 1.0 mm (Parker Series 9). The pulsed valve was designed to heat up to 250 ℃ to obtain sufficient vapor pressure for certain solid chemicals.

The molecular emission is first amplified by a low-noise amplifier (Miteq AFS44 LNA, 1$ - $18 GHz) before being down converted using the microwave synthesizer, which is subsequently digitized using a 2.5 GHz bandwidth oscilloscope (Lecroy WaveRunner 6 Z). Fast Fourier transformation of the resulting free induction decay leads to the broadband spectrum over the full bandwidth of the chirp. The whole sequence can be repeated to produce multiple free induction decays (FIDs) so that the resulting spectrum gives improved signal-to-noise ratio. The microwave synthesizer, the arbitrary wave form generator and the oscilloscope are referenced to a 10 MHz rubidium standard (Stanford Research Systems, FS725) for external stability.

As dibenzofuran is a non-volatile molecule with a vapour pressure of ca. 27 Pa at room temperature, a simple heating unit was included within the pulsed nozzle assembly to increase the temperature to 200 ℃ at which a vapour pressure higher than 120 Pa is expected. A pressure of 0.6 MPa argon was then allowed to pass over the sample as carrier gas for the supersonic jet expansion.

The excitation frequency from the microwave synthesizer was mixed with a 5 $ \mathtt{μ} $s linear frequency sweep, or "chirp", from 0 to 500 MHz, leading to a 1 GHz bandwidth of excitation. The resultant free induction decay (FID) was collected at a sampling rate of 40 GSa/s for 40 $ \mathtt{μ} $s, giving a spectral resolution (full-width-at-half-maximum) of 80 kHz. The data acquisition sequence was repeated and a total of 1, 000, 000 FIDs were averaged to produce the spectrum via fast Fourier transformation. A total of four 1 GHz scans were used to cover the 2$ - $6 GHz region. An overview spectrum of one such 1-GHz window is presented in FIG. 1 with a section showing assigned transitions in FIG. 2.

FIG. 1 An overview of the microwave spectrum of dibenzofuran between 4.5 and 5.5 GHz measured with 1, 000, 000 FIDs
FIG. 2 A zoomed-in spectrum of the microwave spectrum of dibenzofuran with three assigned transitions
B. Computation

Both ab initio and density functional theory (DFT) methods were employed to optimise the ground state structure of dibenzofuran. The ab initio calculation was carried out using the second order Møller-Plesset perturbation theory (MP2) [18] while the DFT calculation was performed using the Becke, 3-parameter, Lee-Yang-Parr (B3LYP) [19, 20], exchange-correlation functional with the correlation consistent polarised valence n-tuple $ \zeta $ (cc-pVnZ, $ n $ = D and T here) basis sets. The augmentation of such basis sets was not used as Treitel and co-workers concluded that the inclusion of diffuse functions does not improve the calculated geometrical parameters for PAH anions significantly [21]. All geometric optimisations were carried out using the Gaussian 09 package [22], and the calculated rotational constants and dipole moment are shown in Table Ⅰ. The Cartesian coordinates of the optimised structures calculated using each level of theory and basis set can be found in supplementary materials.

Table Ⅰ Computational results for dibenzofuran (∆a = 0)
Ⅲ. ASSIGNMENT AND RESULTS A. Effective Hamiltonian

The analyses of the pure rotational spectra in the GHz and THz regions were carried out using Watson's $ A $-reduced effective Hamiltonian [23] in the $ I^{\rm{r}} $ representation, which is shown here including up to sextic centrifugal distortion constants:

$ \begin{array}{l} \hat H_{{\rm{rot}}}^{v,v} = {A_v}\hat J_z^2 + {B_v}\hat J_x^2 + {C_v}\hat J_y^2 - \Delta _J^v{{\hat J}^4} - \Delta _{JK}^v{{\hat J}^2}\hat J_z^2 + \\ \;\;\;\;\;\;\;\;\;\Delta _K^v\hat J_z^4 - \frac{1}{2}{\left[ {\left( {\delta _J^v{{\hat J}^2} + \delta _K^v\hat J_z^2} \right),\left( {\hat J_ + ^2 + \hat J_ - ^2} \right)} \right]_ + } + \\ \;\;\;\;\;\;\;\;\;\phi _J^v{{\hat J}^6} + \phi _{JK}^v{{\hat J}^4}\hat J_z^2 + \phi _{KJ}^v{{\hat J}^2}\hat J_z^4 + \phi _K^v\hat J_z^6 + \\ \;\;\;\;\;\;\;\;\;\frac{1}{2}{\left[ {\left( {\eta _J^v{{\hat J}^4} + \eta _{JK}^v{{\hat J}^2}\hat J_z^2 + \eta _K^v\hat J_z^4} \right),\left( {\hat J_ + ^2 + \hat J_ - ^2} \right)} \right]_ + }\quad \end{array} $ (1)

where with i = $ \sqrt{(-1)} $, we have the angular momentum operators:

$ \begin{eqnarray} {\hat J^2} = \hat J_x^2 + \hat J_y^2 + \hat J_z^2, \quad {\hat J_ \pm } = {\hat J_x} \pm \rm{i}{\hat J_y} \end{eqnarray} $

All observed transitions were fitted using the $ I^{\rm{r}} $ representation in the Pickette's SPFIT/SPCAT spectral fitting programme [24].

B. Assignment of the pure rotational spectra in the ground state

Dibenzofuran is an asymmetric rotor of $ C_{\rm{2v}} $ symmetry. Its permanent electric dipole moment is expected to lie along the $ b $-axis allowing transitions to obey $ b $-type selection rules (eo$ \leftrightarrow $oe and oe$ \leftrightarrow $eo, e for even and o for odd values of the quantum numbers $ K_{\rm{a}} $ and $ K_{\rm{c}} $) [25]. The principal axis system for dibenzofuran is given in FIG. 3.

FIG. 3 Structure of dibenzofuran (carbon atoms are labeled in grey and oxygen in red). The $ C_2 $ symmetry axis coincides with the $ b $-axis. Axes definition in the principal inertial axis system is shown at the top. The $ c $-axis is perpendicular to the $ ab $ plane

The assignment of the observed spectra in the range of 2$ - $6 GHz was assisted by a simulation of the spectrum using the calculated rotational constants at MP2/cc-pVTZ level. A total of 40 $ b $-type transitions were assigned using the PGOPHER programme [26, 27] in the range of 2$ - $6 GHz with $ J_\max $ = 19 and $ K_{\rm{a}}\ _\max $ = 5. The assigned transitions were fitted and the resulting spectroscopic parameters are given in Table Ⅱ. The root-mean-square deviation $ d_{\rm{rms}} $ of the fit is 4 kHz, with the typical linewidths (full-width at half-maximum) of about 80 kHz. A full list of the assigned transitions is provided in supplementary materials.

Table Ⅱ Spectroscopic constants for the ground state of dibenzofuran

Pure rotational transitions were measured and assigned from 2 GHz to 6 GHz, providing the first study in the microwave range. This allowed for an accurate determination of the rotational constants. Comparing the rotational constants with values obtained from reported high resolution electronic spectroscopy studies [14, 15], the accuracy of our values is improved by at least two orders of magnitude as shown in Table Ⅱ, reflecting the high resolution of Fourier transform microwave (FTMW) over techniques in the optical range. The root-mean-square deviations ($ \sigma_{\rm{rms}} $) of the fit of 4.6 kHz is considerably less than one-tenth of the $ \sim $80 kHz observed linewidths, suggesting the effective Hamiltonian employed here provides an accurate description of observed spectral features in this range. As a result, the spectroscopic parameters reproduce the experimental spectrum very well. In comparison, the $ \sigma_{\rm{rms}} $ values in Ref.[14] and Ref.[15] are a few MHz. It is worth mentioning that such an agreement was achieved by including only three rotational constants in the fit while centrifugal distortion constants were found not sensitive to the observed transitions in this study.

All calculations in this study show a planar structure for dibenzofuran with $ C_{\rm{2v}} $ symmetry, indicating the only non-zero dipole moment being along the $ C_2 $ axis which coincides with the $ b $-axis through the oxygen atom as shown in FIG. 3. This is consistent with the experimental findings that only $ b $-type transitions were observed. Although the experimentally obtained rotational constants alone were not enough for accurate structural determination, one can determine the inertial defect to verify the expected planar cyclic structure. Dibenzofuran with a $ C_{\rm{2v}} $ is expected to have a zero inertial defect as it is planar (see Table Ⅱ). However, our value of the inertial defect calculated using the experimental rotational constants is $ - $0.285652. As it is a slightly negative number, we can attribute it to the averaging of vibrational levels of the out-of-plane, low-lying, large amplitude motions that are often associated with PAHs. In particular, the "butterfly" motions are ubiquitous in tricyclic molecules and the fundamental of such motion in dibenzofuran is at only $ \sim $100 cm$ ^{-1} $ [28].


In the current study, we presented the first spectroscopic study of dibenzofuran in the microwave region. It has been demonstrated that cp-FTMW is capable of measuring such a non-volatile molecule in a supersonic jet. The results from this work have the potential to guide the astrophysical and atmospheric searches for this molecule via remote sensing. It has been observed that the equilibrium structure of this tricyclic molecule has coupled with the low-lying, out-of-plane modes and our accurately determined rotational constants can provide the basis for the rovibrational studies of such modes in the far-infrared region.

Supplementary materials: The line list of assigned transitions as well as the theoretical geometric parameters of dibenzofuran is included in the electronic supplementary data.


This work was supported by the National Natural Science Foundation of China (No.61627802 and No.U1531107), the Fundamental Research Funds for Chinese Central Universities (No.lzujbky-2018-k08, No.lzujbky-2019-65, and No.lzujbky-2019-ct05).

Supplementary Materials for

The following content is included as the Supplementary Materials of this publication:

Table S1 Observed pure rotational transitions (in MHz) of the ground state of dibenzofuran
Table S2 Dibenzofuran's B3LYP/cc-PVDZ Optimized Geometry
Table S3 Dibenzofuran's B3LYP/cc-PVTZ Optimized Geometry
Table S4 Dibenzofuran's MP2/cc-PVDZ Optimized Geometry
Table S5 Dibenzofuran's MP2/cc-PVTZ Optimized Geometry
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6 GHz范围内二苯并呋喃的纯旋转光谱
周海花a , 刘增魁a , 陈子秋a , 孙铭b , 陈钱b , 段圣文b , 焦超b     
a. 兰州大学化学化学工程学院,兰州 730000;
b. 南京理工大学电子工程与光电技术学院,南京 210094
摘要: 本文使用自己搭建的宽带啁啾脉冲傅里叶变换微波谱仪对二苯并呋喃在2$\sim $6 GHz 范围内的转动光谱的测量和归属. 对微波光谱的分析获得了40个b型跃迁的归属,精确地确定了旋转常数$A$=\mbox{2278.19770(38) MHz}、\mbox{$B$=601.12248(10) MHz}和$C$=475.753 120(98) MHz.
关键词: 二苯并呋喃    多环芳烃    宽带转动光谱