Understanding the differences in photochemical properties of substituted aminopyrimidines

The luminescent patterns of several members of the aminopyrimidine family are very different, showing not fluorescence at all, only a fluorescence band, normal or anomalous, or dual fluorescence, depending on the substituents and on the environment (gas phase vs. polar solvents). In this work, we study the lowest excited states of several members of this family that exhibit different fluorescence patterns to try to explain their photochemistry and to understand the effect of the substituents and the environment. We have found that several excited states (local excited (LE), charge transfer (CT) and n _N?C??* states) have minima on the lowest excited potential energy surface (S₁), being their relative energy the determinant factor of the luminescent behavior. If the more stable S₁ minima are of n _N?C??* character, a non-radiative deexcitation channel is the most efficient and the system shows no fluorescence. If the CT and/or LE states are the most stable, the non-radiative deactivation channel is not accessible and the system fluoresces. The relative energies of the CT and LE minima (affected by substituents and by the presence of a polar solvent) and the different magnitude of the oscillator strength for the radiative transition to the ground state determine which emission is more efficient, giving place to normal, anomalous or dual fluorescence. The study has been carried out by CASSCF/CASPT2 computations, including the solvent effect by means of the PCM model.     

Influence of excitation and solvation on the shift of tautomeric equilibrium. A quantum-mechanical study

Two quantum-mechanical models are proposed to described a shift of tautomeric equilibrium as a result of electronic excitation and change of environment. According to the first n PD MEP model which is used to estimate the relative solvation effect on the stability of tautomers in an excited state, the calculation of the interaction energy between a solvent (simulated by a set of n point dipoles, n PD) and an excited solute molecule is based on the molecular electrostatic potential (MEP) of the corresponding excited state. In the second n PDQ model, a solvent represented by a set of n point dipoles and quadrupoles (n PDQ) modifies the solute's hamiltonian via an electrostatic interaction contribution. Comparing the results of the calculation for isolated and solvated tautomers, the n PDQ model is used to estimate the influence of electronic excitation on the change of relative stability of tautomers existing in a solution. An application of both models to 2- and 4-oxopyridine predicts a shift of the tautomeric equilibria in their excited states in accordance with experimental evidence.     

Self-consistent reaction field calculations of the solvent effect on absorption and emission n, π* transitions of diazines

A modified version of the self-consistent reaction field (SCRF) method has been accomplished in order to study the solvent effect on the electronic transition energies. The dielectric relaxation properties both of solvent and solute are directly taken into account in the Hartree—Fock equation. The method has been applied to calculate the solvent shifts, both in absorption and in emission, for n, π^* transitions of the three diazine isomers. It was found that the continuum model is able to describe experimental findings for aprotic solvents; the discrete plus continuum model is needed to account for the solvent shifts observed in water solution.     

On the use of the Bayliss-McRae dispersion model in NMR solvent effects

The use of the Bayliss-McRae theory on the solvent induced electronic frequency shifts for NMR dispersion shifts is criticized. It is suggested that the NMR shifts should actually be proportional to the square of the Bayliss-McRae function. It is shown that the methane gas-to-liquid shifts in eleven halo-methanes as solvents are indeed proportional to this squared function; ?σ_m(CH₄) = 9.62 (n₂²?1)²/(2n₂²+1)² ppm, where n₂ is the refractive index of the solvent. The relation between this solvent factor and several existing continuum models for NMR medium shifts is discussed.     

Cyclometalated rhodium complexes of phenyl- and diphenyl-substituted oxazole and thiazole luminophores

Cyclometaleted rhodium complexes of oxazole and thiazole luminophores [Rh(C??N)₂En]Cl [(C??N)^?, deprotonated forms of 2,5-diphenyloxazole, 2-phenylbenzothiazole, 2-(biphenyl-4-yl)-6-phenylbenzoxazole, and 2-(biphenyl-4-yl)-5-phenyloxazole; En ?? ethylenediamine] were obtained and characterized by the methods of ¹H NMR, IR, and electron absorption and emission spectroscopy. Two cyclometallated ligands in the inner sphere of the complexes are in the cis-C,C positions. Cyclometallating of the luminophores results both in a red shift of intraligand ??-??*-optical transitions (???? ??1.2 kK) as compared to free luminophores and in the appearance of a long-wave band (??_max 376?C392 nm) of a mixed nature: metal-ligand charge transfer/intraligand transition. Alongside with the internal conversion to a low-energy state of the metal-ligand charge transfer/ intraligand transition, the emission degradation of photoexcitation energy results in the intraligand ??-??*-fluorescence of the complexes (??_max 390?C423 nm) at room temperature.     

Solvent effect on (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO): a RISM-SCF-SEDD study

The solvent dependence of several properties of (2,2,6,6-Tetramethylpiperidine-1-yl)oxyl (TEMPO) is investigated by the reference interaction site model self-consistent field (RISM-SCF) theory. Time-dependent density functional theory (TDDFT) coupled with RISM-SCF-SEDD (spatial electron density distribution) is used to evaluate the n ?? ??* transition energies and the results are compared to the reported experimental values.     

Solvent effect on the singlet excited-state lifetimes of nucleic acid bases: A computational study of 5-fluorouracil and uracil in acetonitrile and water

The first comprehensive quantum mechanical study of solvent effects on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and aqueous solution have been computed at the time-dependent density-functional theory level, using the polarizable continuum model (PCM) to take into account bulk solvent effects. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. The S0/S1 conical intersection, located in the presence of hydrogen-bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground-state recovery, involving out-of-plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited-state surfaces in the Franck-Condon (FC) region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/pi* excited state. This finding provides the first unifying explanation for the experimental trend of 5-fluorouracil excited-state lifetime in different solvents. The microscopic mechanisms underlying solvent effects on the excited-state behavior of nucleobases are discussed.     

Coordination chemistry of bis(2-pyridylimine) ligands with Ag(I): formation of two structurally different coordination polymers and one metallocycle controlled by linker and the solvent system

Reaction of equimolar amounts of AgClO₄ and bis[4-(2-pyridylmethyleneamino)phenyl] methane (L¹) or bis[4-(2-pyridylmethyleneamino)phenyl] ether (L²) in a 1:1 solvent mixture of CH₃CN and CH₂Cl₂ leads to the formation of two infinite coordination polymers of the composition {[Ag(L¹)]ClO₄·CH₃CN}_n (1) and {[Ag(L²)]ClO₄·CH₂Cl₂}_n (2). Whereas 1 represents a homochiral single-stranded helicate the related complex 2 shows a typical zigzag chain arrangement. Both structures are characterized by a distorted tetrahedral coordination environment of the Ag(I) centres each based on a N₄ coordination pattern of two ligand molecules. The resulting strands are connected by a hydrogen bonding network including ClO₄ ^? anions and solvent molecules forming 2-D layers. Additional ?ШC?? and CH?C?? interactions between the aromatic parts of the ligand molecules give a 3-D arrangement of the packing. In contrast, a discrete dinuclear metallocycle, [Ag₂(L²)₂](ClO₄)₂·CH₃OH (3), has been formed by reaction of AgClO₄ with L² when CH₂Cl₂ in the solvent mixture was replaced by CH₃OH. Again each Ag(I) has a distorted tetrahedral geometry and is coordinated to two pyridylimine units of two ligand molecules. Additional weak hydrogen bonds involving perchlorate and solvent molecules as well as edge-to-face and face-to-face ?ШC?? interactions allow a 3-D packing arrangement.     

Thermochemical investigations of nearly idela binary solvents. II. Standard heats of solution in systems of nonspecific interactions

Standard heats of solution at 25°C are reported for squalane in mixtures ofn-heptane + iso-octane and in binary mixtures of chloroform, carbon tetrachloride, and cyclohexane; for cyclohexane in chloroform + carbon tetrachloride; and for chloroform in carbon tetrachloride + cyclohexane. General equations based on simple mixing models are developed for the excess thermodynamic properties of a solute at infinite dilution in a binary solvent. For the systems studied, mixing equations based on volume fractions give better agreement than those based on mole fractions, and the agreement is further improved by the use of weighting factors based on the properties of the solute + solvent binary systems.     

GC�CMS Determination of PAHs in Fish Samples Following Salting-out-Assisted Solvent Extraction-Gel Permeation Chromatography

A fast and effective sample cleanup procedure for the quantification of polycyclic aromatic hydrocarbons (PAHs) in fish samples is presented. The procedure involved extraction of fish samples using acetonitrile and cleanup by an automated gel-permeation chromatography (GPC) following liquid?Cliquid partition into n-hexane. The extracted samples were analyzed by gas chromatography-mass spectrometry (GC?CMS). Electron ionization was employed in a single analysis for the determination of PAHs in the selected ion monitoring mode. Three different solvents were studied for the extraction step: acetonitrile/n-hexane, methanol/n-hexane and acetone/n-hexane. The best solvent was found to be acetonitrile/n-hexane. The cleanup technique resulted in a good separation of analytes from co-extractive matrix compounds.     

Experimental analysis and modified rotor description of the infrared fundamental band of HCl in Ar, Kr, and Xe solutions

We report an experimental study of the rotovibrational fundamental PQR-band shapes in the IR absorption spectra of HCl dissolved in condensed rare gases in a wide range of temperatures. The effective vibrational frequencies are determined from analysis of the fine rotational structure partially resolved in the band wings. The central Q-branch components appear redshifted with respect to the effective vibrational frequencies, their shifts in different solvents found to match the HCl stretching mode shifts in binary Rg...HCl van der Waals heterodimers. Theoretical quasi-free rotor and modified rotor models are applied to describe evolution of the band profiles at changing thermodynamic conditions. Both models are shown to reproduce equally well the observed spectral density distributions in the band wings. However, the modified rotor formalism that accounts for depopulation of the lower-energy rotational solute states provides better agreement with the experiment in the range of the P- and R-branch maxima. We surmise that the Q branches separated from the measured spectral profiles are formed by transitions between rotationally hindered states of diatomic molecules coupled to the solvent by the local anisotropy of the interaction potential.     

丙酮分子n→π%*跃迁光谱的平均溶剂静电势/分子动力学方法研究

采用量子力学/分子动力学方法研究了具体溶剂分子结构对溶质光谱行为的静电影响. 通过拟合溶质所处外电场和引入溶剂分子极化率, 考虑了溶质溶剂分子之间的相互极化效应, 得到合理的溶质和溶剂分子的电荷分布. 经过严格推导发现, 在传统的显溶剂模型中, 平衡和非平衡溶剂化能表达式均未考虑溶剂分子永久偶极弹簧能的贡献. 因此, 在正确计算永久偶极弹簧能的基础上, 重新建立了溶剂化能的表达式和新的吸收/发射光谱移动公式. 采用修改后的ASEP/MD程序, 计算得到了与实验值比较吻合的丙酮在水溶液中n→π*跃迁的光谱移动值, 验证了新公式的合理性.     

Solvation dynamics in acetonitrile: a study incorporating solute electronic response and nuclear relaxation

The solvent reorganization process after electronic excitation of a polar solute in a polar solvent such as acetonitrile is related mainly to the time evolution of the solute-solvent electrostatic interaction. Modern laser-based techniques have sufficient time resolution to follow this decay in real time, providing information to be confirmed and interpreted by theories and models. We present here a study aimed at the investigation of the different steps involved in the process taking place after a vertical S(0) --> S(1) excitation of a large size chromophore, coumarin 153 (C153), in acetonitrile, from both the solute and the solvent points of view. To do this, we use accurate quantum mechanical calculations for the solute properties within the polarizable continuum model (PCM) and classical molecular dynamics (MD) simulations, both equilibrium and nonequilibrium, for C153 in the presence of the solvent. The geometry of the solute is allowed to change in order to study the role of internal motions in the time-dependent solvation process. The solvent response function has been obtained from the simulation data and compared to experiment, while the comparison between equilibrium and nonequilibrium MD results for the solvation response confirms the validity of the linear response approximation in the C153-acetonitrile system. The MD trajectories have also been used to monitor the structure of the solvation shell and to determine its change in response to the change in the solute partial charges.     

Rate and thermodynamic studies of the interaction between water and iso-butyl cellosolve by ultrasonic methods

Ultrasonic absorption and velocity measurements in aqueous solution of iso-butyl cellosolve (ethylene glycol iso-butyl ether) as a function of the concentration are reported. The two relaxational absorptions have been attributed to the perturbation of the equilibria expressed by AB?A+B and Aα(1/n)A_n where A is the solute, B is the solvent, AB is the complex and A _n is the solute aggregate. The rate constants for each step have been determined. From the concentration dependence of the maximum excess absorption per wave length, the enthalpy change and the volume change for the reaction between the solute and the solvent have been determined for aqueous solutions of butyl cellosolve (ethylene glycol n-butyl ether), iso-butyl cellosolve and propyl cellosolve (ethylene glycol n-propyl ether). The results are consistent with a hydrogen bonding reaction. The effect of the ethers on water structure are considered and it is clear that the fraction of water molecules which can hydrogen bond to the solute decreases with the increasing hydrophobicity of the solute.     

Prediction of the 1H and 13C NMR spectra of alpha-D-glucose in water by DFT methods and MD simulations

We have applied computational protocols based on DFT and molecular dynamics simulations to the prediction of the alkyl 1H and 13C chemical shifts of alpha-d-glucose in water. Computed data have been compared with accurate experimental chemical shifts obtained in our laboratory. 13C chemical shifts do not show a marked solvent effect. In contrast, the results for 1H chemical shifts provided by structures optimized in the gas phase are only fair and point out that it is necessary to take into account both the flexibility of the glucose structure and the strong effect exerted by solvent water thereupon. Thus, molecular dynamics simulations were carried out to model both the internal geometry as well as the influence of solvent molecules on the conformational distribution of the solute. Snapshots from the simulation were used as input to DFT NMR calculations with varying degrees of sophistication. The most important factor that affects the accuracy of computed 1H chemical shifts is the solute geometry; the effect of the solvent on the shielding constants can be reasonably accounted for by self-consistent reaction field models without the need of explicitly including solvent molecules in the NMR property calculation.