Jinglai Zhang

A CASSCF/CASPT2 insight into excited‐state intramolecular proton transfer of four imidazole derivatives

Excited‐state intramolecular proton transfer (ESIPT) of four imidazole derivatives, 2‐(2′‐hydroxyphenyl)imidazole (HPI), 2‐(2′‐hydroxyphenyl)benzimidazole (HPBI), 2‐(2′‐hydroxyphenyl)‐1H‐phenanthro[9,10‐d]imidazole (HPPI) and 2‐(2′‐hydroxyphenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d]imidazole (HPPPI), were studied by the sophisticated CASSCF/CASPT2 methodology. The state‐averaged SA‐CASSCF method was used to optimize their geometry structures of S0 and S1 electronic states, and the CASPT2 calculations were used for the calibration of all the single‐point energies, including the absorption and emission spectra. A reasonable agreement is found between the theoretical predictions and the available experimental spectral data. The forward ESIPT barriers of four target compounds gradually decrease with the increase of molecular size. On the basis of the present calculations, it is a plausible speculation that the larger the size, the faster is the ESIPT rate, and eventually, HPPPI molecule can undergo a completely barrierless ESIPT to the more stable S1 keto form. Additionally, taking HPI as a representative example, the radiationless decays connecting the S0 and S1/S0 conical intersection structures were also studied by constructing a linearly interpolated internal coordinate (LIIC) reaction path. The qualitative analysis shows that the LIIC barrier of HPI in the keto form is remarkably lower than that of its enol‐form, indicating that the former has a big advantage over the latter in the nonradiative process.

Synthesis, spectroscopic properties and theoretical studies of bis-Schiff bases derived from polyamine and pyrazolones

A series of novel bis-Schiff base were synthesized from 1-aryl-3-methyl-4-benzoyl-5-pyrazolones and diethylenetriamine (or triethylenetetramine) as the starting materials. All of these bis-Schiff bases were characterized by means of NMR, IR, and MS. The UV-vis absorption spectra and fluorescent spectra of these bis-Schiff bases were also measured. Moreover, the B3LYP/6-31G(d) method was used to optimize the ground state geometry of the bis-Schiff bases; and the UV-vis spectroscopic properties of the products were computed and compared with corresponding experimental data based on cc-pVDZ basis set of TD-B3LYP method. It has been found that all of these bis-Schiff bases show a remarkable absorption peak in a wavelength range of 270-340 nm; and their maximum emission peaks are around 348 nm.

Quaternary ammonium-based ionic liquids bearing different numbers of hydroxyl groups as highly efficient catalysts for the fixation of CO2: a theoretical study by QM and MD

The mechanism of coupling reactions of carbon dioxide (CO2) with propylene oxide (PO) catalyzed by a series of hydroxyl-functionalized quaternary ammonium-based ionic liquids (ILs) is investigated by the combination of density functional theory (DFT) and molecular dynamics (MD) methods. The calculated sequence of catalytic activity conflicts with the experimental result if only a single catalyst is considered, which is attributed to the fact that the influence of other hydroxyl groups in the cation is ignored. To include the contribution from multi-hydroxyl groups in the cation, the mechanism is investigated using a model catalyzed by two quaternary ammonium-based ion pairs. Noncovalent interaction (NCI) analysis is utilized as a tool to investigate this phenomenon. Additionally, 256 ion pairs are simulated to further confirm the accuracy of the two-catalyst model. It is expected that this work will open a new pathway to explore the mechanism especially for ionic liquids including multi-functionalized groups.

Insight into the catalytic activity for a series of synthesized and newly designed phosphonium-based ionic liquids on the fixation of carbon dioxide

Abstract Various catalysts have been explored for the coupling reaction of carbon dioxide with propylene oxide in the past decade. However, the search for high-efficiency single-component catalyst in the absence of metal and solvent is still a challenging issue. In this work, the catalytic activity of a series of functionalized phosphonium-based ionic liquids (FPBILs) is investigated by density functional theory. The influences of functional group, chain length of cation, and anion on the catalytic performance have been explored. The carboxyl-functionalized FPBIL with the moderate alkyl chain length presents the best catalytic activity among all investigated catalysts. On the basis of above information, a new sulfonyl hydroxide-functionalized FPBIL is designed. Moreover, its catalytic activity is theoretically examined as compared with that of carboxyl-functionalized FPBIL.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP+) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP+ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP+ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S1 state and triplet T1 state is found to play an important role in the photoisomerization process of BTImP+. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP+ is calculated to be 10.49kcalmol-1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP+.

Crystal structure characterization as well as theoretical study of spectroscopic properties of novel Schiff bases containing pyrazole group

A series of novel Schiff bases containing pyrazole group were synthesized using 1-aryl-3-methyl-4-benzoyl-5-pyrazolone and phenylenediamine as the starting materials. All as-synthesized Schiff bases were characterized by means of NMR, FT-IR, and MS; and the molecular geometries of two Schiff bases as typical examples were determined by means of single crystal X-ray diffraction. In the meantime, the ultraviolet-visible light absorption spectra and fluorescent spectra of various as-synthesized products were also measured. Moreover, the B3LYP/6-1G(d,p) method was used for the optimization of the ground state geometry of the Schiff bases; and the spectroscopic properties of the products were computed and compared with corresponding experimental data based on cc-pVTZ basis set of TD-B3LYP method. It has been found that all as-synthesized Schiff bases show a remarkable absorption peak in a wavelength range of 270-370 nm; and their maximum emission peaks are around 344 nm and 332 nm, respectively.

Ab initio Characterization of Size Dependence of Electronic Spectra for Linear Anionic Carbon Clusters C-n(-) (n=4-17)

In this article, we determine the ground‐state equilibrium geometries of the linear anionic carbon clusters C  n− (n = 4–17) by means of the density functional theory B3LYP, CAM‐B3LYP, and coupled cluster CCSD(T) calculations, as well as their electronic spectra obtained by the multireference second‐order perturbation theory CASPT2 method. These studies indicate that these linear anions possess doublet 2∏g or 2∏u ground state, and the even‐numbered clusters are generally acetylenic, whereas the odd‐numbered ones are essentially cumulenic. The energy differences, electron affinities, and incremental binding energies of C  n− chains all exhibit a notable tread of parity alternation, with n‐even chains being more stable than n‐odd ones. In addition, the predicted vertical excitation energies from the ground state to four low‐lying excited states are in reasonably good agreement with the available experimental observations, and the calculations for the higher excited electronic transitions can provide accurate information for the experimentalists and spectroscopists. Interestingly, the absorption wavelengths of the 12∏u/g ← X2∏g/u transitions of the n‐even clusters show a nonlinear trend of exponential growth, whereas those of the n‐odd counterparts are found to obey a linear relationship as a function of the chain size, as shown experimentally. Moreover, the absorption wavelengths of the transitions to the higher excited states of C  n− series have the similar linear size dependence as well.

Catalytic performance of a series of guanidinium-based ionic liquids in the coupling reaction of carbon dioxide with epoxides

In order to gain more insight into the high catalytic activity of N′′-(2-aminoethyl)-N,N,N′,N′-tetramethylguanidine bromide ([TMGC2H4NH2]Br) and to select a more suitable catalyst for the synthesis of propylene carbonate (PC), the cycloaddition reaction of carbon dioxide (CO2) into the epoxide (EO), catalyzed by a series of functional guanidinium-based ionic liquids (FGBILs), is schematically studied by the Density Functional Theory (DFT). The calculated results indicate that the formation of carbamic acid is a common pathway with a lower barrier height when the NH2-functionalized IL encounters CO2. What is more, the formation of carbamic acid is helpful in decreasing the barrier height of the ring-opening step. Two ILs are involved in the process of forming carbamic acid. Sparked by this, the mechanism catalyzed by two ILs is explored for comparison with the model catalyzed by one IL. In addition, the catalytic activities of other functionalized guanidinium-based ionic liquids are investigated, the results indicate that the task-specified ILs have better catalytic activity than those without functional groups because of the increased acidity. Besides the cation, the influence of different anions and substrates is also investigated.

Insight into the activity of efficient acid–base bifunctional catalysts for the coupling reaction of CO2

The fixation of carbon dioxide with epoxide catalysed by a series of carboxylic acidic functionalised ionic liquids (ILs) catalysts is investigated by density functional theory method. Except for the catalysts reported by the experiment, the catalytic activity of a new designed catalyst (4c see Scheme 1) is also explored. These ILs are categorised into four groups according to the different cation structures and number of functional groups. The effects of different chain length, anion, and cation structure on the catalytic activity are explored. The elongation of alkyl chain length in cation will increase the product yield, while increasing the chain bulk has almost negligible effect on the enhancement of catalytic activity. Utilisation of imidazole group as the cation is better than pyridine group. And the cation with two functional groups will have a better catalytic activity than that with one functional group.

Theoretical insight into electronic spectra of carbon chain carbenes H2Cn (n = 3−10)

Ground-state geometries of carbenes H2Cn (n = 3-10) have been fully optimized with the C2ν-symmetry constraint at the density functional theory and restricted-spin coupled-cluster single-double plus perturbative triple excitation levels of theory, respectively. Comparison of structures corresponding to the X(1)A1 and B(1)B1 electronic states has been made by the complete active space self-consistent field calculations. Parity alternation effect on various properties of the ground-state geometries has been discovered in the present study, which generally gives illustration for the relative stabilities of the titled carbon chains. Further calculations on their electronic spectra have been carried out by means of the complete active space second-order perturbation theory method along with the cc-pVTZ basis set. It is found that the vertical excitation energies of the dipole-allowed B(1)B1 ← X(1)A1 transition in the gas phase are 2.28, 4.75, 1.69, 3.66, 1.30, 2.94, 1.12, and 2.49 eV, respectively, which agree very well with the available experimental result for H2C3 (2.27 eV). In addition, the vertical excitation energies for both transitions B(1)B1 ← X(1)A1 and A(1)A2 ← X(1)A1 are found to obey a nonlinear ΔE-n relationship as a function of chain size by performing curves fitting.