b. University of Chinese Academy of Sciences, Beijing 100049, China;
c. School of Chemistry, Chemical Engineering and Material, Handan Key Laboratory of Organic Small Molecule Materials, Handan College, Handan 056005, China;
d. Chemistry and Materials Department, Huaibei Normal University, Huaibei 235000, China
Carbonates are widely distributed in nature as inorganic salts,which are formed through environmental chemical reaction of carbon dioxide [1, 2, 3]. At present,carbonates are widely used in glass,food and construction industries ,and in organic synthesis. For example,sodium carbonate and potassium carbonate act as strong bases to catalyze the alkylation of malonate  and the deprotonoation of L-cyanophenol,respectively [6, 7].
Beller and co-workers reported the synthesis of HCOONa using Na2CO3 and MeOH as reactants and ruthenium pincer complexes as homogenous catalysts . There are also some reported organic synthesis reactions between carbon dioxide and aromatic compounds [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19],including the synthesis of aromatic hydrocarbon. Olah and co-workers reported a heterogeneous catalyst Al2Cl6/Al . Nolan and co-workers synthesized homogenous catalysts Cu(NHC)OH (NHC=N-heterocyclic carbene)  and Au(NHC)OH . Those catalysts activate the C-H bonds of aromatic compounds and combine with CO2 for the formation of aromatic carboxylic acids. To the best of our knowledge,the reaction of carbonates and aromatic compounds has not been reported. In this work,we found aqueous sodium carbonate can react with benzene under UV-light exposure and form biphenyl and sodium benzoate,which are distributed in benzene and aqueous solution,respectively (Fig. 1). We also investigated the mechanism of this light-induced reaction using density functional theory (DFT) and time dependent DFT methods [20, 21, 22].Ⅱ. EXPERIMENTS AND CALCULATION A. Materials
Materials including sodium carbonate,sodium hydrogen carbonate,cadmium carbonate,magnesium carbonate,benzene,hydrochloric acid,ethyl acetate,were purchased from Shanghai Chemical Reagent Ltd. and of analytically pure grade without further purification,and carbon dioxide was self-prepared. Deionized water was used throughout this study.B. Analytical methods
The relative absorption intensity was recorrected by the standard-adding method under UV-Vis absorption intensity using an UV-Vis Absorption Spectroscopy (Hitachi UV-3600). GC was measured on an online gas chromatography (Fuli 9790,China) with FID detector using a SE-30 0.53 mm×30 m capillary column using N2 as carrier gas. GC/MS was measured on a TRACE DSQII (Thermol Fisher). 1H NMR and 13C NMR spectra were measured on a Bruker Avance NMR spectrometer (400 MHz) with CDCl3 as solvent and recorded in ppm relative to internal tetramethylsilane standard. High resolution mass spectroscopy data of the product were collected on a Waters Micromass GCT instrument.C. Experimental procedure
Experiments were carried out in a photoreaction apparatus consisting of two parts [29, 30]. The schematic diagram was shown in supplementary materials (Fig.S1). The first part was an annular quartz tube with an empty chamber in which a 375 W medium pressure mercury lamp (Institute of Electric Light Source,Beijing) with a main wavelength of 365 nm was laid. Running water passed through an inner thimble of the annular tube. Owing to continuous cooling,the temperature of the reaction solution was maintained at approximately 30 ℃. The second part was an unsealed beaker with a diameter of 10 cm. At the beginning of the experiment,the reaction solution (25 mL),containing 5 g sodium carbonate,5 mL benzene,and 20 mL deionized water,was put in the unsealed beaker and stirred by a magnetic stirring device. The distance between the light source and the interface of the reaction solution was 11 cm. Before the experiment,the reaction solution had been going through the air for 30 min,and during the experiment,the air was always passed. In order to better disperse two-phase solution prior to exposure of the entire container,the two-phase solution was scattered by ultrasonic oscillation for 20 min. After exposure,the reaction liquid phase samples were extracted,and then characterized using quantitative and qualitative methods. In order to determine the reproducibility of the results,duplicated runs had to be carried out in each condition for averaging the results.D. Computational method
All DFT and TDDFT calculations were performed using the Gaussian 09 suite of programs  for the M06 functional  with the 6-31++G(d,p) basis set [33, 34]. All structures were optimized with solvent effect corrections by the method of integral equation formalism polarizable continuum model (IEFPCM)  for water and benzene as the solvent. Thermal corrections were calculated within the harmonic approximation on optimized structures under T=298.15 K and P=1 atm. Calculating the frequencies for optimized structures and noting the number of imaginary frequencies (IFs) confirmed the nature of intermediates (no IF) and transition states (only one IF). The latter was also confirmed to connect reactants to products by intrinsic reaction coordinate (IRC) calculations. The 3D molecular structures displayed in this work are drawn by using the JIMP2 molecular visualizing and manipulating program .Ⅲ. RESULTS AND DISCUSSION A. Characterization of benzoate and biphenyl
Benzoate: 1H NMR (400 MHz,CDCl3),δ/ppm 8.143-8.119 (m,4H),7.63-7.594 (m,4H),7.495-7.456 (m,3H). 13C NMR (100 MHz,CDCl3),δ 172.5,133.8,130.2,129.3,128.4.
Biphenyl:1H NMR (400 MHz,CDCl3),δ/ppm 7.607-7.578 (m,4H),7.462-7.420 (m,4H),7.365-7.332 (m,2H). 13C NMR (100 MHz,CDCl3),δ 141.22,128.7,127.2,127.1.B. Light exposure time effect
The initial amounts of 5 g sodium carbonate and 5 mL benzene were mixed in 20 mL deionized water. The relative absorption intensities at different light exposure time in water are shown in Fig. 2. The relations between the concentration of sodium benzoate and biphenyl at different UV-light exposure time are shown in Fig. 3.
In Fig. 2,the six lines from bottom to top are the absorption spectra measured at 0,1,2,3,4 and 5 h,respectively. As shown in Fig. 2(a),the aqueous solution has absorption peaks at 256 and 360 nm,which indicate the formation of new substances after UV-light exposure. As shown in Fig. 2(b),the absorptions in the range of 205-220 nm come from benzene. The dramatic increase of the absorption intensities with the increase of exposure time in the range of 240-275 nm indicates the formation of new substances in benzene. In Fig. 3,the concentrations of sodium benzoate and biphenyl are 12.2 and 7.4 mmol/L,respectively. Furthermore,we found that the concentrations of benzoates and biphenyl were similar when the fully dissolved carbonates were used,such as potassium carbonate and sodium carbonate. In order to find out possible reactions of benzene and other carbonates under the same condition,we also examined MgCO3,CdCO3,and NaHCO3. The results are shown in Table Ⅰ.
As shown in Table Ⅰ,benzene can react with NaHCO3,but cannot react with insoluble carbonates MgCO3 and CdCO3. After the same 5 h of UV-light exposure,the concentrations of benzoate anion and biphenyl generated from the reaction of benzene with NaHCO3 are significantly lower than the product concentrations of the reaction of benzene with Na2CO3. We believe this is caused by the incomplete ionization of bicarbonate. Since CO2 could form CO32- in water,we also investigated the reaction of carbon dioxide with benzene at the room temperature and 1 atm pressure. After 5 h of UV-light exposure,we only detected trace amounts of benzoate and biphenyl. Therefore,we believe the carbonate anion is the actual reactant in the reaction.C. GC/MS analysis
After UV-light exposure,the products were obtained by post-treatment process. In the aqueous solution,diluted hydrochloric acid was added,then,water was extracted with ethyl acetate. Finally,benzoic acid can be obtained through reduced-pressure distillation. In benzene,biphenyl was obtained after separation by reduced-pressure distillation. The GC/MS data of benzoic acid and biphenyl are shown in Fig. 4. The NMR measured results of benzoic acid and biphenyl are provided in the supplementary material (Fig.S2-S5).
In Fig. 4(a),the m/z of 122,105,and 77 are benzoic acid,C6H5CO+ and C6H5-,respectively. In Fig. 4(b),the m/z of 154 and 76 are the biphenyl and C6H4-. The excess sodium carbonate in water may result in the m/z=44 of carbon dioxide in Fig. 4.
From the above data,we can conclude that the sodium benzoate is the primary product of the reaction between sodium carbonate and benzene,and biphenyl is possibly formed by combining two benzene radicals.D. DFT calculations
The proposed mechanism of the light-induced reaction of benzene and carbonates is shown in Scheme 1. Figure 5 shows the corresponding free energy profile. Figure 6 shows the optimized structures of a key transition state TS1,2 and two important intermediates 2T and 4T.
At the beginning of the reaction,a benzene molecule is excited to the first excited singlet state 1S1. The similar excitation of benzene has been reported [23, 24, 25, 26, 27, 28]. The TDDFT results of excited orbitals are shown in supplementary material. Then,the benzene molecule can easily relax to triplet states 1T2 and 1T1. In order to form benzoate acid and biphenyl,a C-H bond of benzene must be broken by the carbonate. The lowest transition state for the breaking of a C-H bond is TS1,2,in which a proton is transferred from benzene to CO32-. TS1,2 is 15.7 kcal/mol higher than 1T2 and 43.7 kcal/mol higher than 1T1 in free energy with a spin multiplicity of three. Such barriers indicate that only the benzene molecule at its second excited state 1T2 approaches a carbonate anion in the phase interface of aqueous solution and one transfer a proton to CO32- for the formation of intermediate 2T.
From 2T,the reaction has various pathways to form the final products with spin-crossovers. 2T to 1S0and CO32- (optimized structures in Fig. 6) is 105.9 kcal/mol downhill with the change of spin multiplicity from three to one. Therefore,we believe the ground state 1S0 benzene is unable to react with carbonate anion. At the same time,in the phase interface of aqueous solution and benzene,the intermediate 2T is dissociated to 3T and HCO3- with 12.6 kcal/mol uphill,and then 3T to 3S is 46.5 kcal/mol downhill through spin-crossover. Next,HCO3-approaches 3T and 3S,and forms 4T and 4S through electrophilic reaction,respectively. Then,4T is dissociated to two radicals,6D and ·HCO32- with 1.7 kcal/mol downhill. Then,7S (biphenyl) is formed through the combination of two 6D radicals. Meanwhile,4T and 4S quickly form 5S (benzoate anion) through OH- dissociation,and,·HCO32- is oxidized into CO32-. Finally,the carbonate anion is regenerated with the formation of biphenyl.
From the above analysis,we can conclude that the low yield of product is due to the low concentration of the excited benzene through relaxation process and the spin-crossover,as well as the quick conversion of 2T to 2S. The spin-crossover also affects the reaction pathway. In order to make the reaction proceed continuously,2T needs to stay as triplet. Because the barrier is 17.5 kcal/mol uphill from 2T to 4T,the pathway 2T→3T+HCO3-→4T→6D is thermodynamically less favorable than the pathway 2T→3T+HCO3-→3S+HCO3-→4S. We believe these two pathways are competitive.E. Evaluation of density functionals
In order to examine the dependence of density functionals of this light-induced reaction,other six well-known or recently developed density functionals,including B3LYP [37, 38],CAM-B3LYP ,M06L ,ωB97XD ,PBE0 [42, 43, 44],and HSE06 [45, 46],were selected to calculate the relative free energies of 1T2→TS1,2 and 1S0→4T using the same basis set for the structures optimized by the M06 functional. As shown in Table Ⅱ,the differences of calculated relative free energies using these six density functionals are less than 10 kcal/mol. For 4T,the largest difference between the relative energies calculated by using different functionals is less than 5% compared to the relative energies of 4T. Such a small ratio indicates a weak dependence of density functionals for this light-induced reaction system.Ⅳ. CONCLUSION
In summary,we found benzene can react with carbonates after UV excitation. The DFT method was used to investigate the mechanism of this light-induced reaction. Calculation results indicate that the excited benzene can react with carbonate anion in the phase interface of benzene and aqueous solution. Both the relaxation process of 1S1 and the quick conversion of 2T→2S result in the low yield of products. Since the final products of benzoic acid and biphenyl are distributed in two phases and are easily separated without complicated post-treatment process,our finding may provide new ways for the production of chemicals without difficult separation process.
Supplementary materials: NMR,free energies and atomic coordinates of all optimized structures,TDDFT calculation results and reaction apparatus are given.Ⅴ. ACKNOWLEDGMENTS
This work was supported he 100-Talent Program of Chinese Academy of Sciences,the "One-Three-Five" Strategic Planning of Institute of Chemistry,CAS (No.CMSPY-201305),and the National Natural Science Foundation of China (No.21373228)
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b. 中国科学院大学, 北京 100049;
c. 邯郸学院化学化工与材料学院, 邯郸市有机小分子材料重点实验室, 邯郸 056005;
d. 淮北师范大学化学与材料科学学院, 淮北 235000