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1.
    
The thermo‐solvatochromic behavior of 2,6‐dichloro‐4‐(2,4,6‐triphenylpyridinium‐1‐yl)phenolate (WB), 1‐methylquinolinium‐8‐olate (QB) and 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl]phenolate (MC) was investigated in binary mixtures of water (W) and 2‐alkoxyethanols (ROEtOH) in the temperature ranges from 10 to 60°C (2‐ethoxyethanol and 2‐n‐propoxyethanol) and 10 to 40°C (2‐n‐butoxyethanol). Thermo‐solvatochromic data were treated according to a model that is based on the presence in bulk solution of three solvents, W, ROEtOH and a 1:1 H‐bonded species, ROEtOH–W. Solvation by ROEtOH–W is favored over solvation by either of the two precursor solvents. The present data, and those recently published on thermo‐solvatochromism of the same probes in five alcohols (methanol, ethanol, 1‐propanol, 2‐propanol and 2‐methyl‐2‐propanol) and one ROEtOH (2‐methoxyethanol), were submitted to regression analysis. The results indicate that solvation is more sensitive to solute–solvent hydrophobic interactions than H‐bonding between the probe phenoxy oxygen and the hydroxyl group of the H‐bond donating solvent (HBD). Temperature increase results in gradual desolvation of the probes, due to the concomitant decrease of the structure of all components of the binary solvent mixture. For pure solvents, the temperature‐induced desolvation depends on the structure of the probe (order: WB>MC>QB) and the HBD solvent (order: 2‐ethoxyethanols>aliphatic alcohols, for the same alkyl group; organic solvent>water). The probe solvatochromic response is due to the electronic transition zwitterion→di‐radical; it serves as a model for reactions that are associated with a large polarity difference between the reagents and activated complexes. For WB, for ΔT = 50°C, the desolvation energies range from 2.1 to 3.7 kcal mol−1. The contribution of temperature‐induced desolvation to the activation enthalpies of these reactions is, therefore, important. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

2.
    
We report here how the hydration of complex surfaces can be efficiently studied, thanks to recent advances in classical molecular density functional theory. This is illustrated on the example of the pyrophyllite clay. After presenting the most recent advances, we show that the strength of this implicit method is that: (1) it is in quantitative or semi-quantitative agreement with reference all-atom simulations (molecular dynamics here) for both the solvation structure and energetics, and (2) the computational cost is two to three orders of magnitude less than in explicit methods. The method remains imperfect in that it locally overestimates the polarisation of water close to hydrophylic sites of the clay. The high numerical efficiency of the method is illustrated and exploited to carry out a systematic study of the electrostatic and van der Waals components of the surface–solvent interactions within the most popular force field for clays, CLAYFF. Hydration structure and energetics are found to weakly depend upon the electrostatics. We conclude on the consequences of such findings on future force-field development.  相似文献   

3.
    
The degradation of azo dyes has attracted many research efforts not only due to the resulting environmental problems but also because the azo compounds with various substituents may show different degradation mechanism. It has been computationally found here, for the first time, that the HO• initiated cleavages of C–N and N–N bonds of alizarin yellow R with carboxyl group are kinetically competitive. In view of the formation of HO• adducts, the C–N and N–N bond cleavages of the hydrazone tautomer of alizarin yellow R are also kinetically competitive, but the former is more thermodynamically favorable. This result is different from that previously reported for the hydrozone tautomers of Acid Orange 7 and Acid Orange 8 containing hydroxyl and azo groups in neighboring positions, which are favorable to follow C–N bond cleavage mechanism both kinetically and thermodynamically. The decarboxylation occurs via an attack of HO• to the benzene ring carbon connecting to the carboxyl group rather than a direct attack of HO• to the carboxyl carbon atom. The anion form has higher reactivity than the neutral form in all of the reactions investigated. In addition, a water molecule as a proton relay reagent could significantly reduce the energy barrier for the N–N bond cleavage of alizarin yellow R. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

4.
    
2‐[2‐Nitro‐4‐(trifluoromethyl)benzoyl]cyclohexane‐1,3‐dione (NTBC) is an active component of nitisinone, a medicine against tyrosinemia type I. Using 1H, 13C and 19F NMR spectroscopy it has been found that in the urine of patients treated with nitisinone two compounds possessing CF3 group are always present. They have been isolated by using TLC technique and identified as 4‐hydroxy‐2‐[2‐nitro‐4‐(trifluoromethyl)benzoyl]cyclohexane‐1,3‐dione and 5‐hydroxy‐2‐[2‐nitro‐4‐(trifluoromethyl)benzoyl]cyclohexane‐1,3‐dione, the latter being previously unknown. The constitution, tautomerism and stereochemistry of these compounds have been thoroughly investigated using 1H and 13C NMR spectroscopy supported by theoretical calculations. Molecular structures have been optimized using density functional theory (DFT) with PBE1PBE functional and 6‐31G* basis set. In NMR parameter calculations, the larger 6‐311++G(2d,p) basis set has been used. At both calculation stages, the polarizable continuum model of the solvent has been employed. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
    
Density functional theory [B3LYP/6‐311+G(d,p)] was used in combination with the conductor‐like polarizable continuum model (CPCM) solvation model to investigate the relative stability and site‐specific values of neutral and ionized tautomers of lumazine (LM) and 6‐thienylLM (TLM). Two types of populations should be taken into consideration when calculating the , tautomers, and conformers. The major tautomer of neutral LM in aqueous solution is 13‐LM (the 13 notation refers to the acidic protons being in positions 1 and 3 of LM) TLM has decreased acidity at N8 relative to LM. Further, the trans conformer of TLM is more acidic than cis. Similar to the case of LM, for TLM, N1 is more acidic than N3 in the uracil part. However, N8 is predicted to be a stronger acid than N1 for TLM. This acidity enhancement is essentially because of a specific stabilization of the anion when the thienyl group replaces H. Two factors are responsible for the acidity strength of N8: The thienyl ring upon deprotonation acts inductively as an electron‐withdrawing group, and the excess electron density is dispersed better when the system is trans and contains second‐row atoms. Accurate pKa calculation requires that all conformers/tautomers be included into the calculation. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
The (Z)-4-(phenylamino) pent-3-en-2-one (PAPO) was synthesised applying carbon-based solid acid and described by experimental techniques. Calculated results reveal that its keto-amine form is more stable than its enol-imine form. A relaxed potential energy surface scan has been accomplished based on the optimised geometry of NH tautomeric form to depict the potential energy barrier related to intramolecular proton transfer. The spectroscopic results and theoretical calculations demonstrate that the intramolecular hydrogen bonding strength of PAPO is stronger than that in 4-amino-3-penten-2-one)APO(. In addition, molecular electrostatic potential, total and partial density of stats (TDOS, PDOS) and non-linear optical properties of the compound were studied using same theoretical calculations. Our calculations show that the title molecule has the potential to be used as molecular switch.  相似文献   

7.
    
Vera Deneva 《Molecular physics》2019,117(13):1613-1620
ABSTRACT

The tautomeric optical sensors based on 4-(phenyldiazenyl)naphthalen-1-ol exist in their pure enol tautomeric form as free ligands, while the addition of metal ion fully shifts the equilibrium towards the keto tautomer allowing a red shift in the measured absorbance. This effect is achieved when a side ionophore group is connected to a tautomeric backbone by a spacer in a way that stabilizes the enol form via hydrogen boding. When the ionophore captures the metal ion the keto form is stabilized due to C─O tautomeric group participation in the complex. In the current study, we model theoretically the effect of symmetric tweezer like ionophores (RCOXCOR, where X, being CH or N, is the linker to the tautomeric backbone) on the tautomeric state and complexation ability of 4-(phenyldiazenyl)naphthalen-1-ol containing ligands. It was found that enol form stabilisation is achieved when R?=?NMe2, independing on the linker. Both ligands are unsuitable for capturing alkali metal ions. The calculations predict that the complexation with alkali earth metal ions could lead to a full shift of the tautomeric equilibrium towards keto tautomer.  相似文献   

8.
    
Thermo‐solvatochromism of 2,6‐dichloro‐4‐(2,4,6‐triphenylpyridinium‐1‐yl)‐phenolate, 1‐methylquinolinium‐8‐olate and 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl]‐phenolate, in the temperature ranges 10–45°C (methanol) and 10–60°C (1‐ and 2‐propanol) was investigated in binary water–alcohol mixtures. Thermo‐solvatochromic data were treated according to a modified model that explicitly considers the presence of 1:1 water–alcohol species in bulk solution, and its exchange reactions with water and alcohol in the solvation micro‐sphere of the probe employed. Concentrations of these complex species were calculated from density data. Plots of the empirical solvent polarity parameter, ET, versus effective mole fraction of water in the binary mixtures indicate that the probes are preferentially solvated by the alcohol, except for one case. A temperature increase causes gradual desolvation of the probe, due to a decrease in the H‐bonding abilities of all components of the binary solvent mixture. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

9.
    
A novel solvatochromic probe—2,6‐dibromo‐4‐[(E)‐2‐(1‐butylquinolinium‐4‐yl)ethenyl] phenolate, BuQMBr2—has been synthesized and its properties examined. The quinoline‐based probe is soluble in more organic solvents than the parent merocyanine dye, 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl] phenolate, and its pKa is lower by 3.7 units. Its solvatochromic data in binary mixtures of cyclohexane–n‐butanol showed that the deviation from ideal behavior is due to a combination of non‐specific and specific solvent–probe interactions. Its thermo‐solvatochromism has been studied in mixtures of water with methanol, ethanol, 1‐propanol, 2‐propanol and 2‐methyl‐2‐propanol, respectively. The data obtained were analyzed according to a recently introduced model that explicitly considers the presence of 1:1 alcohol–water hydrogen‐bonded species, ROH–W, in bulk solution, and its exchange equilibria with water and alcohol in the probe solvation microsphere. The composition of the latter is given in terms of the appropriate set of solvent fractionation factors. These indicate that the probe is more solvated by alcohol than by water. Additionally, solvation by ROH–W is favored over solvation by either W or ROH. Solvation by alcohols is more affected by probe–ROH hydrophobic interactions than by hydrogen bonding of ROH to the probe phenolate oxygen. Temperature increase results in a gradual desolvation of the probe, due to a decrease in the hydrogen bonding of all components of the binary solvent mixture. The probe has been employed to calculate the effective concentration of interfacial water of sodium dodecyl sulfate micelles, which is 38.9 mol l−1. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

10.
    
The solvation structure of magnesium, zinc(II), and alkaline earth metal ions in N,N‐dimethylformamide (DMF) and N,N‐dimethylacetamide (DMA), and their mixtures has been studied by means of Raman spectroscopy and DFT calculations. The solvation number is revealed to be 6, 7, 8, and 8 for Mg2+, Ca2+, Sr2+, and Ba2+, respectively, in both DMF and DMA. The δ (O C N) vibration of DMF shifts to a higher wavenumber upon binding to the metal ions and the shift Δν(= νbound − νfree) becomes larger, when the ionic radius of the metal ion becomes smaller. The ν (N CH3) vibration of DMA also shifts to a higher wavenumber upon binding to the metal ions. However, the shift Δν saturates for small ions, as well as the transition‐metal (II) ions, implying that steric congestion among solvent molecules takes place in the coordination sphere. It is also indicated that, despite the magnesium ion having practically the same ionic radius as the zinc(II) ion of six‐coordination, their solvation numbers in DMA are significantly different. DFT calculations for these metalsolvate clusters of varying solvation numbers revealed that not only solvent–solvent interaction through space but also the bonding nature of the metal ion plays an essential role in the steric congestion. The individual solvation number and the Raman shift Δν in DMF–DMA mixtures indicate that steric congestion is significant for the magnesium ion, but not appreciable for calcium, strontium, and barium ions, despite the solvation number of these metal ions being large. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

11.
ABSTRACT

In many cases, 5-fluorouracil (5-FU) is the first-line drug used in combined chemotherapy and radiotherapy treatments due to its radio-sensitization properties. It could participate in a tautomerization process similar to that of uracil, where 5-FU may couple to adenine in DNA. At present, we performed structural and spectroscopic studies using quantum chemical methods of neutral and cationic isolated 5-FU anticarcinogenic drug tautomers, either interacting with a water molecule or embedded into an implicit water solvent. Also, we determined the stationary points (both stable structures and transition states) on their ground potential energy surfaces playing a role during the tautomerization processes. For neutral and ionic species in the gas phase and in solvent, the ordering of the tautomers is found to be the same, where the di-keto form of 5-FU is the most stable structure, followed by the keto–enol and di-enol structural forms. The energy barriers for tautomerization are strongly reduced in solvent (< 0.5?eV) compared to isolated species (～2?eV). The patterns of their lowest electronic states are also computed. Our data may help for the identification of these species in vivo and in the laboratory.  相似文献   

12.
    
The solvation effects observed in water‐organic solutions were studied by combining data for reaction kinetics and dissolution equilibria by means of a linear free‐energy (similarity) analysis. Kinetic data for the pH‐independent hydrolysis of (4‐methoxyphenyl)‐2,2‐dichloroacetate measured in this work and solubility data for naphthalene, and other substrates of low polarity, in aqueous binary mixtures of methanol, ethanol, acetonitrile, dimethyl sulfoxide (DMSO), and 1,4‐dioxane were used. Linear similarity relationships were discovered for these data over the full range of solvent compositions studied. To gain insight into the similarities observed between these different phenomena, molecular dynamics simulations were carried out for naphthalene and an ester in water–acetonitrile solutions. The results revealed considerable preferential solvation of these substrates by the co‐solvent. Linear relationships between the experimental data and the mole fractions of acetonitrile in the solvation shells of substrates were found. Surprisingly, a linear relationship was found between the mole fractions of acetonitrile in the solvation shells of the ester and naphthalene. This linearity indicated that a similar solvation mechanism governs even such different phenomena as dissolution and reaction kinetics. The relationships between the experimental data and the results of the molecular dynamics calculations found in this work explained the solvent effect observed in water‐organic solutions on the molecular level. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

13.
The solvation of infinitely dilute solutes in supercritical solvents is illustrated by integral equation calculations, according to a recently proposed molecular-based formalism that characterizes the solvent environment around individual species and connects it to the resulting macroscopic solvation behavior. The formalism is applied to the analysis of the solubility enhancement of nonelectrolyte species, the solvent effect in kinetic rate constants, and the solvation of ionic species. Finally, some relevant theoretical implications are discussed regarding the modeling of high-temperature solutions.  相似文献   

14.
有机分子的互变异构现象在溶液中较为常见.而对于有机固体而言,存在互变异构可能的分子常以单一的能量最稳定的异构体形式存在.2-吡啶甲酸（PCA）是一个较为罕见的案例,它的晶体结构中同时存在有中性分子和两性离子两种互变异构体.由于超长的质子纵向弛豫时间,PCA的固体13C核磁共振（NMR）实验的化学位移归属存在困难.密度泛函理论（DFT）计算,特别是基于周期性模型的方式是一种可以准确快捷归属其化学位移的方案.然而,由于PCA结构中活泼质子的占位无序,其结构并不能直接或经简单处理后递交计算.本文中,我们通过晶体学手段构建了一个虚拟晶体结构.基于该虚拟结构的13C化学位移计算值可与实验值准确吻合.此外,不同互变异构状态的PCA表现出不同的特征化学位移,这一信息可被用来分析PCA在其晶体复合物中的分子状态.  相似文献   

15.
    
The molecular geometries of the possible conformations of formic, oxalic, glyoxylic and pyruvic acids have been fully optimized at DFT B3LYP/6‐311++G(d,p) levels of calculation in vacuum as well as in water and acetone solution. Solutions were treated according to the SCRF PCM approach but some formic acid–water and formic acid–acetone clusters as well as adducts of oxalic acid with two or four water molecules were also taken into account for testing the importance of specific solute–solvent effects. All the most stable isomers of the title compounds are characterized by weak intramolecular hydrogen bonds, whose strengths (EHB) cannot be correctly estimated as stability difference between the open and chelate forms since the energy of the former isomer is, in turn, stabilized by a weak hydrogen bridge due to the formic acid moiety. Following the Rotation Barrier Method (RBM), proposed some years ago, EHB in the examined molecules (gas phase) falls in the range of 18–22 kJ/mol for oxalic acid (9.6 kJ/mol for the c‐C‐t isomer), 16.8 kJ/mol for glyoxylic acid and 19.8 kJ/mol for pyruvic acid. Most of them disappear at all, or nearly at all, both in acetone and aqueous solution, in consequence of the solvent effect. The frequencies of the OH and CO stretching modes, calculated according to the anharmonic oscillator model, are in very good agreement with the experimental literature data, where available. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

16.
    
The photochemistry of suprofen (SPF) was investigated by femtosecond transient absorption (fs‐TA), resonance Raman (RR) and nanosecond time‐resolved resonance Raman (ns‐TR3) spectroscopic methods to gain additional information so as to better elucidate the possible photochemical reaction mechanism of suprofen in several different solvents. In neat acetonitrile (MeCN), the fs‐TA and ns‐TR3 experimental data indicated that the lowest lying excited singlet state S1 (nπ*) underwent an efficient intersystem crossing process (ISC) to the excited triplet state T3 (ππ*), followed by an internal conversion (IC) process to T1 (ππ*). In the aqueous solution, a triplet biradical species (3ETK‐1) was obtained as the product of a decarboxylation process from triplet suprofen anion (3SPF−) and the reaction rate of the decarboxylation process was determined by the concentration of H2O. A protonation process for 3ETK‐1 leads to formation of a neutral species (3ETK‐3) that was directly observed by ns‐TR3 spectra, then this 3ETK‐3 species decayed via ISC process to generate final product. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.
The kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate were investigated with and without ultrasonic irradiation in acetonitrile–water binary mixtures containing 0.008 to 35 wt.% of acetonitrile and the kinetic sonication effects (kson/knon) were calculated. Molecular dynamics (MD) simulations of the structure of the solutions were performed with ethyl acetate as the model ester. The ester is preferentially solvated by acetonitrile. The excess of acetonitrile over water in the solvation shell grows fast with an increase in the co-solvent content in the bulk solution. In parallel, the formation of a second solvation shell rich in acetonitrile takes place. Significant kinetic sonication effects for the hydrolysis were explained with facile destruction of the diffuse second solvation shell followed by a rearrangement of the remaining solvent layer under sonication. The rate levelling effect of ultrasound was discussed. In an aqueous-organic binary solvent, independent of the solvent composition, the ultrasonic irradiation evokes changes in the reaction medium which result in an almost identical solvation state of the reagent thus leading to the reaction rate levelling.  相似文献   

18.
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19.
    
Static and dynamic electronic and vibrational first-order hyperpolarisabilities (β) of the lowest energy neutral adenine tautomers (amine forms A7 and A9) were obtained in gaseous and aqueous phases by using Hartree–Fock, Møller–Plesset second-order and fourth-order perturbation theory (MP2 and MP4-SDQ) and conventional and long-range corrected density functional theory methods with the Dunning's correlation-consistent cc-pVDZ, aug-cc-pVDZ, aug-cc-pVTZ and d-aug-cc-pVDZ basis sets. Frequency-dependent properties were calculated at the characteristic wavelength of the Nd:YAG laser (1064 nm) for the second harmonic generation and electro-optical Pockels effect nonlinear optical processes. Solvent effects were introduced under the polarised continuum model approximation. The electronic βe values of the investigated isomers are noticeably affected by the theoretical level, basis set and solvation. In vacuum, the static and dynamic βe values of A9 are greater than the corresponding data of A7, whereas the contribution of the solvent significantly enhances the hyperpolarisabilities of the A7 tautomer, resulting in βe(A9)/βe(A7) ratios between 0.5 and 0.6. The vibrational hyperpolarisabilities of the adenine tautomers are quite close to each other.  相似文献   

20.
    
The vibrational spectra of chloranilic acid (2,5‐dihydroxy‐3,6‐dichloro‐[1,4]‐benzoquinone) in the solid state were studied by using inelastic neutron scattering (INS), infrared (IR) and Raman (R) spectroscopy. The spectra were compared with simulated spectra using the Gaussian and Climax programs. Sufficiently good agreement between the experimental and theoretical (DFT) spectra is observed although the calculations show that in the crystalline state a bifurcation of hydrogen bonds takes place. Relatively strong intermolecular interactions are noticeable when the experimental (x‐ray) and calculated bond lengths and angles with participation of OH groups are compared. The studies of the deuterium isotope effect in the IR and R spectra enabled us to analyse the low‐frequency out‐of‐plane vibrations and particularly the ν(OH) and ν(OD) modes. In the case of the ν(OH) and ν(OD) vibrations, one observes a strong asymmetry of the bands (low‐frequency wings), which can be interpreted in terms of a coupling of the ν(OH) mode with low‐frequency ones damped by the lattice phonons and no fine structure of the ν(OH) and ν(OD) bands is observed. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

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