A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

An acyclic,dansyl based colorimetric and fluorescent chemosensor for Hg(II) via twisted intramolecular charge transfer (TICT)

An efficient fluorescent chemosensor for Hg²⁺ ion, based on 5-(dimethylamino)-N-(2-mercaptophenyl)naphthalene-1-sulfonamide, has been developed. It exhibits Hg²⁺-selective on–off fluorescence quenching behavior via twisted intramolecular charge transfer (TICT) mechanism, which is rationalized by time dependent density functional theory (TD-DFT) calculations. The system exhibits visible color change from colorless to gray upon Hg²⁺ binding with very high selectivity and sensitivity (as low as 5.0 × 10⁻¹⁰ mol L⁻¹) over other metal ions such as K⁺, Na⁺, Ag⁺, Mn²⁺, Ca²⁺, Ba²⁺, Fe²⁺, Zn²⁺, Pb²⁺, Cu²⁺, Sn²⁺, Cd²⁺, Ni²⁺ and Co²⁺. The present sensing system is also successfully applied for the detection of Hg²⁺ ion in real samples.     

A comparative study of key properties of glycine glycinium picrate (GGP) and glycinium picrate (GP): A combined experimental and quantum chemical approach

Using experimental and computational techniques, a comparative study of electro-optical properties for glycine glycinium picrate (GGP) and glycinium picrate (GP) compounds has been performed. The single crystal of GGP has been grown using slow evaporation technique that was further subjected to experimental characterization of its electro-optical properties. The good optical transparency and mechanical strength at micro level was confirmed from optical and nanoindentation measurements using the Oliver–Pharr method of the grown single crystals. Differential scanning calorimetric (DSC) analysis was done to probe the thermal stability of the grown single crystals. Using the density functional theory (DFT) methods, we have not only investigated the GGP but also proposed GP molecule. Additionally, we have shed light on the molecular geometries, infrared and Raman spectra, linear and nonlinear optical properties of both GGP and GP at molecular level. The time dependent DFT (TD-DFT) approach was adopted to calculate the excitation energies of the molecules in different phases including gas, water, acetone, cyclohexane and chloroform as well. For GGP, its wavelength of maximum absorption is calculated to be ～390 nm at B3LYP/6-31G¹ level of theory. The calculated amplitudes of first hyperpolarizability (β_tot) for GGP and GP are found to be 712 and 970 a. u., respectively, which are about 16 and 23 times larger than that of the urea molecule (a prototype NLO molecule). Thus the present study not only brings to limelight the optical and nonlinear optical properties of GGP but also sheds light on the possible potential of GP as new NLO molecule.     

Photoinduced intramolecular charge transfer reaction in (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester: A fluorescence study in combination with TDDFT calculation

A donor-acceptor substituted aromatic system (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester (MAPAME) has been synthesized, and its photophysical behavior obtained spectroscopically has been compared with the theoretical results. The observed dual fluorescence from MAPAME has been assigned to emission from locally excited and twisted intramolecular charge transfer states. The donor and acceptor angular dependency on the ground and excited states potential energy surfaces have been calculated both in vacuo and in acetonitrile solvent using time dependent density functional theory (TDDFT) and TDDFT polarized continuum model (TDDFT-PCM), respectively. Calculation predicts that a stabilized twisted excited state is responsible for red shifted charge transfer emission.     

几种(C^N)PtIIQ型配合物的电子结构和紫外-可见吸收光谱

在B3LYP/LANL2DZ水平上优化了三种(C^N)PtIIQ 型配合物基态的几何结构, 进行了频率计算, 并采用含时密度泛函(TD-DFT)方法结合极化连续体模型(PCM)计算了目标配合物在CH2Cl2溶液中的电子结构和紫外-可见吸收光谱. 计算值与文献报道值相似. 计算结果表明这三种(C^N)PtIIQ型配合物在可见光区都有强度较大而且宽的吸收峰, 它们的最低能量吸收峰的跃迁具有ILCT(配体内部电荷转移)和部分MLCT(金属向配体的电荷转移)的特征, 不同于PtIIQ2型配合物在多数情况下表现出的ILCT的跃迁性质.     

Structural effects in electron transfer reactions: comparative interfacial electrochemical kinetics for cis- versus trans-dioxorhenium(V)(bi)pyridine oxidation

The comparative interfacial oxidation kinetics of the approximate structural isomers trans-(O)₂Re^V(py)⁺₄ and cis-(O)₂Re^V(bpy)(py)⁺₂ (py, pyridine; bpy, 2,2′-bipyridine) have been assessed in aqueous solution via conventional cyclic voltammetry at a highly ordered pyrolytic graphite (HOPG) electrode. HOPG was employed because of its known propensity to diminish interfacial electron transfer (ET) rates (by ca. three to four orders of magnitude) and because of a probable lack of importance of kinetic work terms (diffuse double-layer corrections). Measured rates for the trans complex exceed those for the cis by about a factor of 3. Expressed as an effective activation Gibbs energy difference ΔG^*, this corresponds to a cis-trans difference of ca. 3 kJ mol⁻¹. The actual vibrational barriers to ET have determined from a combination of published X-ray structural results (trans complex) and new resonance Raman results (cis complex). The values are 0.6 kJ mol ⁻¹ for the trans oxidation and 4.4 kJ mol⁻¹ for the cis oxidation (i.e. close to the barrier difference inferred from rate measurements). Further analysis shows that most of the barrier difference is associated with displacement of a (predominantly) Re-N(bpy) stretching mode found only in the cis system. Differences in metal-oxo displacements (cis > trans) are also implicated.     

Revisiting H/Pt(111) by a combined experimental study of the H-D exchange reaction and first-principles calculations

We report a detailed investigation of the behavior of chemisorbed hydrogen atoms (Ha) on Pt(111) by a combination of an ex-perimental study of the Ha + Da reaction and first-principles calculations. The coverage-dependent adsorption and desorption behavior of Ha and Da on Pt(111) have been systematically established and can be well interpreted in terms of repulsive inter-actions between adsorbates. Ha adsorbs exclusively on the face-centered cubic (fcc) sites of Pt(111) at coverages not exceeding 1 monolaye...     

Quantum chemical study of the coordination of glycolic acid conformers and their conjugate bases to [Ca(OH2)n] (n=0–4) ions

A detailed exploration of the configurational and conformational space of glycolic acid and their conjugate bases has been carried out with the aid of first principles quantum chemical techniques at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p) levels of theory. The most stable configuration among the eight possible glycolic acid conformers corresponds to the E-s-cis, s-trans configuration, while the highest energy E-s-trans, s-cis conformer was found at 10.88 and 12.17 kcal mol⁻¹ higher in energy at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, respectively. Upon dissociation of glycolic acid the s-cis(syn), and s-trans(anti) configurations of the glycolate anion can be formed. The anti conformer was found to be less stable than the syn one by 14.20 and 16.87 kcal mol⁻¹ at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p)) levels of theory, respectively. The computed B3LYP/6-311+G(d,p) proton affinity of the syn conformer for the protonation process affording the more stable E-s-cis, s-trans conformer, in vacuum was found to be 325.35 kcal mol⁻¹ (ΔG⁰ value). From a methodological point of view, our results confirm the reliability of the integrated computational tool formed by the B3LYP density functional model. This model has subsequently been used to investigate the interaction of Ca²⁺ ions with the glycolic acid conformers and their conjugate bases in vacuum and in the presence of extra water ligands. For the complexes of glycolic acid conformers the η²–O,O–(COOH) coordination, that is the structure that arises from the coordination of the Ca²⁺ to the carboxylic group, is the global minimum of the PES, while the η²–O(OH),O–(COOH) coordination is a local minimum found at only 1.0 and 1.3 kcal mol⁻¹ higher in energy at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, respectively. Moreover, the two isomers exhibit nearly the same binding affinities, which are predicted to be 89 and 85 kcal mol⁻¹ at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, respectively. The same holds also true for the complexes of the glycolate anion. The η²–O,O–(COO⁻) coordination involving the syn conformer of the glycolato ligand, is the global minimum, while the η²–O(OH),O–(COO⁻) one lies at 1.5 and 5.6 kcal mol⁻¹ higher in energy at the B3LYP/6-311+G(d,p) and CCSD(T)/6-31G(d,p) levels of theory, respectively. The other conformer with an η²–O,O–(COO⁻) coordination involving the anti conformer of the glycolato ligand, is less stable by only 0.2 kcal mol⁻¹ at both levels of theory. Noteworthy is the trend seen for the incremental binding energy due to the successive addition of water molecules to [HOCH₂C(O)O]Ca²⁺ species; the computed values are 30.4, 26.8, 22.9 and 16.2 kcal mol⁻¹ at the B3LYP/6-311+G(d,p) level of theory for the mono-, di-, tri- and tetraaqua complexes, respectively. This trend arising from the repulsion of the dipoles between the water ligands and from unfavorable many body interactions is in accordance with those anticipated from electrostatic considerations. The Ca(II)-water interaction weakens with increasing coordination of the metal. Obviously, it is the electrostatic nature of the Ca(II)-water interactions that accounts well for the computed coordination geometries of the cationic (aqua)(glycolato)calcium complexes. Calculated structures, relative stability and bonding properties of the conformers and their complexes with [Ca(OH₂)_n]²⁺ (n=0–4) ions are discussed with respect to computed electronic and spectroscopic properties, such as charge density distribution, harmonic vibrational frequencies and NMR chemical shifts.     

Myers–Saito and Schmittel cyclizations of enyne-(hetero)-1,2,3-trienes: A DFT study on the structure–reactivity relationship

The Myers–Saito (C²–C⁷) cyclization and the alternative Schmittel (C²–C⁶) cyclization of enyne-1,2,3-triene 1R and its hetero analogues 2R–5R were investigated by using pure density functional theory method (BPW91) in connection with the 6-311G(d, p) basis set. It has been shown that heteroatom-containing reactants lower significantly reaction barriers for both cyclization modes and reduce the difference between the barrier of the C²–C⁷ cyclization mode and that of the C²–C⁶ one. The Myers–Saito cyclization of 4R is associated with the smallest reaction barrier and the highest exothermicity. Whereas the Schmittel cyclization of 4R is exothermic, all the others are predicted to be endothermic.     

Hydrogen-bond structures in poly(2-hydroxyethyl methacrylate): Infrared spectroscopy and quantum chemical calculations with model compounds

Hydrogen-bond structures in poly(2-hydroxyethyl methacrylate) (PHEMA) were investigated by infrared (IR) spectroscopy and quantum chemical calculations (QCC). A monomer of 2-hydroxyethyl methacrylate (HEMA) and model compounds of methyl acetate (MA) and methanol (MeOH) were also used. Evidences for OHOC and OHOH types of hydrogen-bonds were observed in an IR spectrum of a PHEMA solid. It was estimated from the present study that 47.3% of OH groups on the PHEMA side chain terminal are engaged in the OHOC type of hydrogen-bond, while the rest contributes to the OHOH type of hydrogen-bond.     

Understanding the stability, electronic and molecular structure of some copper(III) complexes containing alkyl and non alkyl ligands: Insights from DFT calculations

DFT calculations have been performed for some Cu(III)-alkyl complexes. Complexes 1-19 were optimized to the square planar (sq) geometry and observed no imaginary frequencies. Although formally copper adopts d⁸ configuration (Cu(III)) in all the complexes, the Natural Population Analysis (NPA) revealed that the copper actually in d¹⁰ (Cu(I)) configuration, Bond order calculation suggested that the Cu(III)-Et_trans bond gets more bond order in the presence of poor π-acidic co-ligand (probe ligand). Relatively smaller bond order was calculated for Cu(III)-Me_cis bond than Cu(III)-Et_trans bond and therefore Cu(III)-Et_trans bond is the strongest bond in all the complexes. Calculated less Chemical hardness (η) of complexes 1-19 suggested that all these complexes are less stable in nature. Energy Decomposition Analysis (EDA) revealed that the Cu(III)-Et_trans bond is relatively more stable than the Cu(III)-Me_cis and Cu(III)-L (L = co-ligand/probe ligand) bonds. And also the Cu(III)-alkyl (Cu(III)-Me_cis and Cu(III)-Et_trans) bond in complexes 1-17 is more of ionic in nature. However, Cu(III)-Et_trans bond is relatively more ionic than Cu(III)-Me_cis bond.     

Thermoanalytical Study of Linkage Isomerism in Coordination Compounds. Part 7. A DSC and DFT Investigation of Solid‐state Linkage Isomerization in Bis(thiocyanato)‐bipyridineplatinum(II) Complex

The linkage isomerization of [Pt(SCN)₂(bipy)] complex in solid‐state was investigated by differential scanning calorimetry (DSC) at diverse heating rates. The conversion of the bis‐thiocyanato isomer (–SCN)₂ to the bis‐isothiocyanato isomer (–NCS)₂ was accompanied by the appearance of an exothermic peak. However, no DSC peak was obtained for isomerization of the (–NCS)₂. The results imply that the (–SCN)₂ isomer is metastable which can convert to the stable isomer (–NCS)₂ at elevated temperatures. Assuming a two‐stage irreversible isomerization, the enthalpy changes of the first stage ΔH _SCN¹ and also second stage ΔH _SCN² isomerization were obtained (–2.99 ± 0.44 and –2.81 ± 0.45 kJ · mol^–1, respectively) using the mathematical resolution of the observed DSC peaks. The thermokinetic parameters of this conversion were determined using Kissinger method. The activation energy values for the first and second stages of isomerization are evaluated, 101.78 ± 7.58 and 106.26 ± 5.87 kJ · mol^–1, respectively. The low values of the activation enthalpy, ΔH ^? and the high negative activation entropy ΔS ^?, obtained by Eyring equation, supported an associative mechanism. A DFT study was employed to detect the electronic structures and the thermodynamic stabilities of the three linkage isomers and the transition states.     

A challenge for density functional theory: the XONO and XNO2 (X=F, Cl, and Br) molecules

The equilibrium geometries, harmonic frequencies, dipole moments, infrared intensities, and relative energies of the cis-XONO, trans-XONO, and XNO₂ (X=F, Cl, and Br) have been investigated using four functionals in common use in Kohn-Sham density functional theory (DFT) calculations. Two of the functionals include non-local or gradient correction terms, while the other two also incorporate some exact Hartree-Fock exchange and are labeled hybrid functionals. The quality of the results obtained from the functionals is determined by comparison to previously published high-level coupled-cluster calculations. The hybrid functionals perform better for prediction of the equilibrium geometries, where the two gradient corrected functionals yield qualitatively incorrect molecular structures for cis-FONO and cis-ClONO. None of the functionals perform well in predicting all six harmonic frequencies, showing that the correlation between equilibrium geometries and harmonic frequencies is not as strong for these DFT methods as it is for conventional wavefunction ab initio methods, such as coupled-cluster theory. Results from the various functionals generally come into better agreement with each other and also with the coupled-cluster results moving down the periodic table. Received: 12 February 1997 / Accepted: 25 March 1997     

Theoretical identification of the lowest-energy structure of (SiC)12 heterocluster: Segregation of C and Si in planar and cage structures

The structures and relative stabilities of various plane and cage isomers of (SiC)₁₂ cluster have been systematically computed using density functional theory at the level of BLYP. A number of starting configurations were generated from the low-energy isomers of C₂₄ cluster via replacing 12 C atoms by Si atoms, which are D_6h planar structure, and the D_6d, D_2h, O_h, and D_4h symmetrical fullerene cages. The heterofullerene cage obeying six isolated squares rules are not the most preferred structural motif for (SiC)₁₂ cluster. The structural candidates with fully alternating Si–C arrangement are energetically unfavorable. Instead, the (SiC)₁₂ cluster tend to adopt plane, bowl, saddle, and highly distorted cage structures. In all cases, segregation of C atoms is a common feature.     

CO2 capture properties of M-C-O-H (M=Li, Na, K) systems: A combined density functional theory and lattice phonon dynamics study

We have computed the phase diagrams for multi-component M-C-O-H (M=Li, Na, K) systems using first-principles density functional theory complemented with lattice phonon calculations. We have identified all CO₂ capture reactions that lie on the Gibbs free energy convex hull as a function of temperature and the partial pressures of CO₂ and H₂O. Our predicted phase diagrams for CO₂ capture reactions are in qualitative and in some instances quantitative agreement with experimental data. The Na₂CO₃/NaHCO₃ and K₂CO₃/KHCO₃ systems were found to be the most promising candidates of all those we investigated for both pre- and post-combustion CO₂ capture. Overall, we show that our calculation approach can be used to screen promising materials for CO₂ capture under different conditions of temperature and pressure.     

Theoretical investigation of C–HM interactions in organometallic complexes: A natural bond orbital (NBO) study

The nature of C–HM agostic interactions in model metal complexes [M²⁺(CH₂CH₃)(PH₃)_nCl] (where M = Sc, Ti, V, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn; n = 1, 2, 3, 4) was studied with the natural bond orbital analysis (NBO) approach using density functional theory (DFT) optimized geometries at the B3LYP/6-31G(d,p) level of theory. The effect of nature of metal, coordination number, oxidation state and ligand field effects on the agostic interaction is examined. A set of 20 crystal structures of organometallic complexes taken from the Cambridge Structural Database (CSD) was studied computationally employing AIM theory and NBO analysis, and the applicability of these methods was critically accessed in demarcating the two types of interaction.     

Electronic structure and vibrational spectra of cis-diammine(orotato)platinum(II), a potential cisplatin analogue: DFT and experimental study

Orotic acid (vitamin B₁₃) is a key intermediate in biosynthesis of the pyrimidine nucleotides in living organisms, moreover, it may serve as the biological carrier for some metal ions. cis-Diammine(orotato)platinum(II), cis-[Pt(C₅H₂N₂O₄)(NH₃)₂] can be considered as a new potential cisplatin analogue. The FT-Raman and FT-IR spectra of the title complex are reported, for the first time. The molecular structure, vibrational frequencies, and the theoretical infrared and Raman intensities have been calculated by the density functional mPW1PW91 method. The detailed vibrational assignment has been made on the basis of the calculated potential energy distribution. The theoretically predicted IR and Raman spectra show very good agreement with experiment. Natural bond orbital (NBO) analyses were performed for cisplatin, carboplatin and the title complex. The results provided new data on the nature of platinum–ligand bonding in these compounds. Strong intramolecular hydrogen bond between the orotate ligand and the coordinated ammonia group stabilizes the structure of the platinum(II) complex. Thus, it is suggested that the orotate ligand in the title complex is more inert to the substitution reactions than the chloride ligands in cisplatin.     

Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C7H7)2Cr2(CO)n (n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Binuclear cycloheptatrienylchromium carbonyls of the type (C₇H₇)₂Cr₂(CO)_n (n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η⁷-C₇H₇ rings are not found for (C₇H₇)₂Cr₂(CO)_n structures having two or more carbonyl groups. This result stands in contrast to the related (C_nH_n)₂M₂(CO)_n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C₇H₇)₂Cr₂(CO)_n structures have bent trihapto or pentahapto C₇H₇ rings and CrCr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C₇H₇ ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C₇H₇–Cr bond are predicted for (C₇H₇)₂Cr₂(CO)_n (n = 6, 4, 2). The global minima found for the (C₇H₇)₂Cr₂(CO)_n derivatives for n = 6, 5, and 4 are (η⁵-C₇H₇)(OC)₂CrCr(CO)₄(η¹-C₇H₇), (η³-C₇H₇)(OC)₂CrCr(CO)₃(η^2,1- C₇H₇), and (η⁵-C₇H₇)₂Cr₂(CO)₄, respectively. The global minima for (C₇H₇)₂Cr₂(CO)_n (n = 3, 2) have rearranged C₇H₇ groups. Singlet and triplet structures with heptahapto η⁷-C₇H₇ rings are found for the dimetallocenes (η⁷-C₇H₇)₂Cr₂(CO) and (η⁷-C₇H₇)₂Cr₂, with the singlet structures being of much lower energies in both cases.     

A further breakthrough in biphasic, rhodium-catalyzed hydroformylation: the use of Per(2,6-di-O-methyl)-β-cyclodextrin as inverse phase transfer catalyst

Solvent free biphasic hydroformylalion of various water-insoluble terminal olefins can be achieved in high yields and sclcctivitics by using a water-soluble rhodium/triphenylphosphine trisulfonate catalyst and per(2,6-di-o-mclhyl)-β-cyclodcxtrin as inverse phase transfer catalyst. The catalytic activities were up to ten times higher than those observed without pcr(2,6-di-o-methyl)-β-cyclodextrin.     

密度泛函理论研究DMSO溶剂中邻-二(吡咯-2-甲酰胺基)亚苯与卤素阴离子间的超分子作用

采用混合密度泛函B3LYP方法, 在LANL2DZ水平上优化DMSO溶剂中邻-二(吡咯-2-甲酰胺基)亚苯(PFP)及其卤素阴离子复合物的几何构型, 从几何结构参数、电荷布居、前线轨道、结合能以及热力学参数等角度探讨复合物形成过程中主体分子的构象变化以及主客体间的超分子作用. 研究结果表明, 邻-二(吡咯-2-甲酰胺基)亚苯识别卤素阴离子是一个自发过程. 复合阴离子过程中, 两者的前线轨道发生作用, 电子容易从卤素阴离子的HOMO向主体分子的LUMO转移. 在所形成的四种复合物PFP:X^－(X^－＝F^－, Cl^－, Br^－, I^－)中, 以PFP:F^－最稳定, 与文献报道邻-二(吡咯-2-甲酰胺基)亚苯识别卤素阴离子能力的实验结果相一致. 设计合成含多个吡咯酰胺基团、并能形成合适“杯口”大小的主体化合物, 将增强对卤素阴离子的识别能力.