2020 Vol. 33, No. 1

2020, (1): i-i.
Chinese abstract
Chinese Abstracts
2020, (1): ii-iii.
NO$_3$ and N$_2$O$_5$ are important participants in nocturnal atmospheric chemical processes, and their concentrations are of great significance in the study of the mechanism of nocturnal atmospheric chemical reactions. A two-channel diode laser based cavity ring-down spectroscopy (CRDS) instrument was developed to monitor the concentrations of NO$_3$ and N$_2$O$_5$ in the atmosphere. The effective absorption length ratio and the total loss coefficient of the instrument were calibrated using laboratory standard samples. The effective absorption cross section of NO$_3$ at 662 nm was derived. A detection sensitivity of 1.1 pptv NO$_3$ in air was obtained at a time resolution of 1 s. N$_2$O$_5$ was converted to NO$_3$ and detected online in the second CRDS channel. The instrument was used to monitor the concentrations of NO$_3$ and N$_2$O$_5$ in the atmosphere of winter in Hefei in real time. By comparing the concentration changes of pollutants such as nitrogen oxides, ozone, PM$_{2.5}$ in a rapid air cleaning process, the factors affecting the concentrations of NO$_3$ and N$_2$O$_5$ in the atmosphere were discussed.
CH$_3$ internal rotation is one of the typical large amplitude motions in polyatomic molecules, the spectral analysis and theoretical calculations of which, were developed by Li-Hong Xu and Jon Hougen. We observed a Doppler-free high-resolution and high-precision spectrum of 9-methylanthracene (9MA) by using the collimated supersonic jet and optical frequency comb techniques. The potential energy curve of CH$_3$ internal rotation is expressed by a six-fold symmetric sinusoidal function. It was previously shown that the barrier height ($V_6$) of 9MA-$d_{12}$ was considerably smaller than that of 9MA-$h_{12}$ [M. Baba, et al., J. Phys. Chem. A 113 , 2366 (2009)]. We performed ab initio theoretical calculations of the multi-component molecular orbital method. The barrier reduction by deuterium substitution was partly attributed to the difference between the wave functions of H and D atomic nuclei.
Experimental vibrational spectra of heavy light XH stretching vibrations of simple molecules have been analyzed using the local mode model. In addition, the bond dipole approach, which assumes that the transition dipole moment (TDM) of the XH stretching mode is aligned along the XH bond, has helped analyze experimental spectra. We performed theoretical calculations of the XH stretching vibrations of HOD, HND$^-$, HCD, HSD, HPD$^-$, and HSiD using local mode model and multi-dimensional normal modes. We found that consistent with previous notions, a localized 1D picture to treat the XH stretching vibration is valid even for analyzing the TDM tilt angle. In addition, while the TDM of the OH stretching fundamental transition tilted away from the OH bond in the direction away from the OD bond, that for the XH stretching fundamental of HSD, HND$^-$, HPD$^-$, HCD, and HSiD tilted away from the OH bond but toward the OD bond. This shows that bond dipole approximation may not be a good approximation for the present systems and that the heavy atom X can affect the transition dipole moment direction. The variation of the dipole moment was analyzed using the atoms-in-molecule method.
Direct-comb spectroscopy techniques uses optical frequency combs (OFCs) as spectroscopic light source. They deliver high sensitivity, high frequency resolution and precision in a broad spectral range. Due to these features, the field has burgeoned in recent years. In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the near-infrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar. The highly sensitive measurements spanned the wavenumber range from 6270 cm$^{-1}$ to 6410 cm$^{-1}$, which covered both P and R branch of the second overtone band of CO. The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length, successfully removing ringing and broadening effects caused by instrumental line shape function. The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile. We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC. The results demonstrate that OFC-based cavity-enhanced Fourier-transform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.
The structures, energetics, and infrared (IR) spectra of the cationic monomethylamine-water clusters, [(CH$_3$NH$_2$)(H$_2$O)$_n$]$^+$ ($n$=1$-$5), have been studied using quantum chemical calculations at the MP2/6-311+G(2d,p) level. The results reveal that the formation of proton-transferred CH$_2$NH$_3$$^+$ ion core structure is preferred via the intramolecular proton transfer from the methyl group to the nitrogen atom and the water molecules act as the acceptor for the O$\cdots$HN hydrogen bonds with the positively charged NH$_3$$^+$ moiety of CH$_2$NH$_3$$^+$, whose motif is retained in the larger clusters. The CH$_3$NH$_2$$^+$ ion core structure is predicted to be less energetically favorable. Vibrational frequencies of CH stretches, hydrogen-bonded and free NH stretches, and hydrogen-bonded OH stretches in the calculated IR spectra of the CH$_2$NH$_3$$^+$ and CH$_3$NH$_2$$^+$ type structures are different from each other, which would afford the sensitive probes for fundamental understanding of hydrogen bonding networks generated from the radiation-induced chemical processes in the [(CH$_3$NH$_2$)(H$_2$O)$_n$]$^+$ complexes.
We report the development of a static magnetic field Faraday rotation spectrometer for NO detection. A 5.33 μm continuous-wave quantum cascade laser was used as the probing laser. Line absorption at 1875.81 cm$^{-1}$ ($^2\Pi_{3/2}$Q(3/2), $v$=1$\leftarrow$0) was chosen for the detection. By using a Chernin type multipass cell, a detection precision of 1.15 ppbv (1$\sigma$, 1s) was achieved with an absorption pathlength of 108 m. This value was reduced to 0.43 ppbv by increasing the data-acquisition time to 15 s.
Pyrrolidine, a five membered heterocyclic molecule, is widely existing in organism. Herein, infrared spectra of pyrrolidine monomer in neutral and cationic states were obtained by vacuum ultraviolet ionization, infrared photodissociation and time of flight mass spectrometry. Both in neutral and cationic states, it is found that their CH stretching vibration bands are red shifted. In the IR spectrum of neutral pyrrolidine, because the electric dipole moment of NH is small, we have not observed the NH stretching vibration bands. However, the NH stretching vibration band of pyrrolidine is greatly enhanced after ionization, and this band red-shifts compared with the previous experiment. The red shifts of CH stretching vibrations in neutral and cationic states are caused by the negative and positive hyperconjugation, respectively. The enhancement and red shift of the NH stretching band are owing to the ejection of the electrons on the N atom after ionization. Through the calculations, it is found that the acidity of the CH bond is a little stronger than that of NH bond. These kinds of studies would be helpful to understand the intrinsic properties of biomolecules in neutral and cationic states, and to provide reference for the further study of living organic macromolecules.
The ground state rotational spectrum of 2, 3, 6-trifluoropyridine has been investigated in the 2.0$ - $20.0 GHz region by pulsed jet Fourier transform microwave spectroscopy. The experimental rotational constants are $ A $ = 3134.4479(2) MHz, $ B $ = 1346.79372(7) MHz, and $ C $ = 941.99495(6) MHz. The transitions are so intense that rotational transitions of all mono-$ ^{13} $C and $ ^{15} $N isotopologues are measured in natural abundance. The semi-experimental equilibrium rotational constants of the 7 isotopologues were derived by taking account of the anharmonic vibrational corrections, which allowed a semi-experimental determination of the equilibrium structure of 2, 3, 6-trifluoropyridine.
The high resolution rotational spectrum of 2-(trifluoromethyl)pyridine in 2$ - $20 GHz was recorded and analyzed. Spectroscopic parameters including rotational constants, nuclear quadrupole coupling constants of $ ^{14} $N as well as the centrifugal distortion constants were determined. The rotational spectra of five mono-substituted $ ^{13} $C and one $ ^{15} $N isotopologues were also measured and assigned in natural abundance. Experimental results complemented by ab initio calculations lead to an accurate determination of the skeleton structure. The values of the planar moment inertia $ P_{cc} $ were determined to be 44.46 uÅ$ ^2 $ for all the measured isotopologues, indicating a C$ _ \rm{s} $ symmetry of this molecule. The molecular electrostatic surface potential was calculated to illustrate the trifluoromethyl substitution effects on the electron distribution.
Reducing sizes of precious metals and utilization of the mixed small clusters of them as catalysts in reactions are important methods due to more active sites for higher catalytic efficiency. Based on first-principles calculations in this work, we found that the platinum-based clusters of Pt$ _3 $X (X = Al, Si, Cu) which have the magic number 4 can effectively catalyze the water decomposition and hydrogen production in just one-step reaction process. The adsorbates of the H$ _2 $O@Pt$ _3 $X clusters have strong absorption in the ultraviolet and visible regions with wavelength from 300 nm to 760 nm, indicating the sunlight can be used to drive catalytic hydrolysis for producing clean hydrogen. In addition, the O atom remains on the clusters after hydrolysis and can react with CO to form CO$ _2 $ in activation barrier of 0.34$ - $0.58 eV, showing the recycling ability of the products after hydrolysis for eliminating the "poisoning'' CO by oxidation. Moreover, the formed CO$ _2 $ molecule can be detached from the Pt$ _3 $X clusters at 323 K. Our results provide interesting guidance for practical designing the useful photocatalysts.
Criegee intermediates are of significance in the atmospheric chemistry. In this work, the ro-vibrational spectra of the simplest deuterated Criegee intermediate, CD$ _2 $OO, were studied by a vibrational self-consistent field/virtual configuration interaction (VSCF/VCI) method based on a nine-dimensional accurate potential energy surface and dipole surface for its ground electronic state. The calculated fundamental vibrational frequencies and rotational constants are in excellent agreement with the available experimental results. These data are useful for further spectroscopic studies of CD$ _2 $OO. Especially, the rotational constants for excited vibrational levels are essential for experimental spectral assignments. However, the infrared intensities from different resources, including the current computation, the experiment, and previous calculations at the NEVPT2 and B3LYP levels, deviate significantly.
As one of the biological endogenous pigments, biliverdin (BV) and its dimethyl ester (BVE) have extremely weak fluorescence in solution with quantum yield less than 0.01%. However, the situation reverses with the addition of zinc ions. The strength for fluorescence of BVE-Zn$ ^{2+} $ complex is greatly enhanced and fluorescence quantum yield can increase to $ \sim $5%. Herein, we studied ultrafast excited state dynamics of BVE-Zn$ ^{2+} $ complex in ethanol, $ n $-propanol, and DMSO solutions in order to reveal the mechanism of fluorescence quantum yield enhancement. The results show that BVE can form a stable coordination complex with zinc with 1:1 stoichiometry in solution. BVE is structurally and energetically more stable in the complex. Using picosecond time-resolve fluorescence and femtosecond transient absorption spectroscopy, we show that smaller non-radiative rate constant of BVE-Zn$ ^{2+} $ complex in DMSO is the key to increasing its fluorescence quantum yield and the excited state decay mechanism is also revealed. These results provide valuable information about the fluorescence property change after BVE binding to metal ions and may provide a guidance for the study of phytochromes or other fluorescence proteins in which BV/BVE acts as chromophores.
The $ C^2\Pi $-$ X^2\Pi $(0, 0) band of AgO has been reinvestigated by laser induced fluorescence spectroscopy with a spectral resolution of $ \sim $0.02 cm$ ^{-1} $. The AgO molecules are produced by discharging a gas mixture of O$ _2 $/Ar with silver needle electrodes in a supersonic jet expansion. By employing a home-made narrowband single longitude mode optical parametric oscillator (SLM-OPO) as the laser source, high-resolution spectra of the $ C^2\Pi $-$ X^2\Pi $(0, 0) band have been recorded for both $ ^{107} $Ag$ ^{16} $O and $ ^{109} $Ag$ ^{16} $O isotopologues. The spectroscopic constants of the $ C^2\Pi $ state are consequently determined, with the $ ^{109} $Ag$ ^{16} $O one being reported for the first time. The nature of the spin-orbit coupling effect in the $ C^2\Pi $ state is proposed to be due to state mixing with the nearby repulsive $ ^{4}\Sigma^{-} $ and $ ^{4}\Pi $ states.
Astrochemistry has made great progress in recent years. Especially the grain surface chemistry played important roles in the explanation of the formation of the interstellar molecules. In this review, we will discuss the progress, including the different numerical methods to simulate the ice mantles in the astrochemical models. We will also introduce the laboratory astrochemical experimental results, and their contributions to the grain surface chemistry in the review.
Newly born stars are surrounded by gas and dust with a flattened axisymmetric distribution termed protoplanetary disk, in which planets are formed. Observations of these objects are necessary for understanding the formation and early evolution of stars and planets, and for revealing the composition of the raw material from which planets are made. Numerical models can extract important parameters from the observational data, including the gas and dust mass of the disk. These parameters are used as input for further modeling, e.g., to calculate the chemical composition of the disk. A consistent thermochemical model should be able to reproduce the abundances of different species in the disk. However, this good wish has been challenged for many disks: models over-predict the emission line intensity of some species; namely, they are depleted (with respect to expectations from canonical models). In this review we show how this disparity indicates that dust evolution has significant effects on gas chemistry, and may indicate the earliest stages of planet formation.
The photoabsorption and photodissociation of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region is one of the most important photochemical processes in the interstellar medium, thus it has attracted numerous experimental and theoretical studies. Here, we employed the two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method to measure the relative branching ratios [C($ ^3 $P$ _0 $)+O($ ^1 $D)]{[C($ ^3 $P$ _0 $)+O($ ^3 $P)]+ [C($ ^3 $P$ _0 $)+O($ ^1 $D)]} and [C($ ^3 $P$ _2 $)+O($ ^1 $D)]{[C($ ^3 $P$ _2 $)+O($ ^3 $P)]+[C($ ^3 $P$ _2 $)+O($ ^1 $D)]} in the VUV photoexcitation energy range of 108000$ - $113200 cm$ ^{-1} $. Here, one tunable VUV laser beam is used to excite CO to specific rovibronic states, and a second independently tunable VUV laser beam is used to state-selectively ionize C($ ^3 $P$ _0 $) and C($ ^3 $P$ _2 $) for detection. State-selective photoionization through the 1VUV+1UV/visible resonance-enhanced multiphoton ionization scheme has greatly enhanced the detection sensitivity, which makes many new weak absorption bands observable in the current study. The branching ratio measurement shows that the spin-forbidden channels C($ ^3 $P$ _0 $)+O($ ^1 $D) and C($ ^3 $P$ _2 $)+O($ ^1 $D) only open at several discrete narrow energy windows. This might be caused by certain accidental resonance-enhanced spin-orbit interactions between the directly excited Rydberg states and valence states of triplet type which finally dissociate into the spin-forbidden channels.
Recent detections of C$ _{60} $, C$ _{70} $, and C$ _{60} $$ ^+ $ in space induced extensive studies of fullerene derivatives in circumstellar environments. As the promising fullerene sources, protoplanetary nebulae (PPNe) shows a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at $ \sim $21 $ \mathtt{μ} $m. Hydrogenation converts fullerenes into fulleranes, which breaks the symmetry of fullerene molecules and produces new infrared bands. In this work, we investigate the possibility of fulleranes (C$ _{60} $H$ _m $) as the carrier of the 21 $ \mathtt{μ} $m feature in terms of theoretical vibrational spectra of fulleranes. The evidences favoring and disfavoring the fullerane hypothesis are presented. We made an initial guess for the hydrogen coverage of C$ _{60} $H$ _m $ that may contribute to the 21 $ \mathtt{μ} $m feature.
Molecular oxygen (O$ _2 $) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O$ _2 $ is rare in the interstellar medium. It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O$ _2 $ and H$ _2 $O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O$ _2 $ abundances are in good agreement with observations at temperatures rising after 10$ ^5 $ yr. In both isothermal and warm-up models, the steady-state O$ _2 $ fractional abundance is independent of the hydrogen density, as long as the temperature is high enough ($ > $30 K), at which O$ _2 $ is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O$ _2 $, whether molecular oxygen is under cold conditions or in warm regions.
Here we present the study on chemical properties of massive star forming clumps using N$ _2 $H$ ^+ $(1-0), H$ ^{13} $CO$ ^+ $(1-0), HCN(1-0) and HN$ ^{13} $C(1-0) data from the literature [Astron. Astrophys. 563 , A97 (2014)]. We found that abundances of H$ ^{13} $CO$ ^+ $ and HN$ ^{13} $C are affected by H$ _2 $ column densities. As the median values of these two abundances increase by nearly 10 times from stages A to B, H$ ^{13} $CO$ ^+ $ and HN$ ^{13} $C are suitable for tracing the evolution of massive star forming clumps. The order of rapidity in growth of abundances of all the four studied molecules from stages A to B, is H$ ^{13} $CO$ ^+ $, HCN, HN$ ^{13} $C, and N$ _2 $H$ ^+ $, from the highest to the lowest. Our results suggest that the observing optically thin molecular lines with high angular resolution are necessary to study the chemical evolution of massive star forming clumps.
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} $N nucleus. Comparison of the determined structures allows a direct study of the influence of the position of the ethyl substituent on the structure of the amino group communicated through the phenyl ring.
We report the observation and assignment of the rotational spectra of dibenzofuran measured in the range of 2$ - $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.