Paul G. Seybold

Fluorescence Quantum Yields and Their Relation to Lifetimes of Rhodamine 6G and Fluorescein in Nine Solvents: Improved Absolute Standards for Quantum Yields¶

Abstract Absolute fluorescence quantum yields are reported for the rhodamine 6G cation and the fluorescein dianion dyes in nine solvents. This information is combined with previously reported fluorescence lifetimes to deduce radiative and nonradiative decay rates. Along the alcohol series from methanol to octanol, rhodamine 6G displays an increasing radiative rate, in parallel with the square of the refractive index increase, and a slightly decreasing nonradiative rate. Fluorescein is different: the apparent radiative rate actually decreases, suggesting that the emissive species is perturbed in some fashion. For both dyes, fluorescence yields are enhanced in D2O, rising to 0.98, in parallel with a corresponding increase in lifetimes. Protonated solvents invariably give shorter lifetimes and lower quantum yields, contrary to some previous speculation. From this work and an analysis of existing literature values, more precise values have been obtained for two previously proposed absolute quantum yield standards. The yield of fluorescein in 0.1 N NaOH(aq) is 0.925 ± 0.015, and for rhodamine 6G in ethanol, it is 0.950 ± 0.015. In both cases, the solutions are assumed to be in the limit of low concentration, excited close to their long-wave absorption band and at room temperature but may be either air-saturated or free of oxygen.

Absolute pKa Determinations for Substituted Phenols

The CBS-QB3 method was used to calculate the gas-phase free energy difference between 20 phenols and their respective anions, and the CPCM continuum solvation method was applied to calculate the free energy differences of solvation for the phenols and their anions. The CPCM solvation calculations were performed on both gas-phase and solvent-phase optimized structures. Absolute pK(a) calculations with solvated phase optimized structures for the CPCM calculations yielded standard deviations and root-mean-square errors of less than 0.4 pK(a) unit. This study is the most accurate absolute determination of the pK(a) values of phenols, and is among the most accurate of any such calculations for any group of compounds. The ability to make accurate predictions of pK(a) values using a coherent, well-defined approach, without external approximations or fitting to experimental data, is of general importance to the chemical community. The solvated phase optimized structures of the anions are absolutely critical to obtain this level of accuracy, and yield a more realistic charge separation between the negatively charged oxygen and the ring system of the phenoxide anions.

SOLVENT DEPENDENCE OF THE FLUORESCENCE LIFETIMES OF XANTHENE DYES

Fluorescence lifetimes of Ave representative xanthene dye species‐the rhodamine B zwitterion (RB=), the rhoda‐mine B cation (RB+), the rhodamine 6G cation (R6G+), the rhodamine 101 zwitterion (R101) and the fluorescein dianion (F2‐)‐were measured in H2O, D2O and in a series of alcohol solvents ranging from methanol to octanol. The lifetimes of both RB= and RB+ increased markedly as the solvent was varied from water to octanol. In contrast, the lifetimes of R6G+ and R101± decreased slightly over the alcohol series and that of F2‐ increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in D2O were slightly longer than those in H2O. Possible causes for the variations observed are discussed.

Solvatochromism and thermochromism of rhodamine solutions

Abstract Electronic absorption spectra of rhodamine B have been examined in 16 protic and 14 aprotic solvents. Rhodamine B exists in solution as a highly colored zwitterion, a colorless lactone and an intensely colored cation. Protic solvents stabilize the zwitterion: the position of the lactone-zwitterion equilibrium depends upon solvent-dye hydrogen bonding and solvent dielectric/polarizability properties. From its temperature dependence, thermodynamic parameters (ΔG°, ΔH°,ΔS°) for this equilibrium were determined in water, n -propanol and n -octanol. The zwitterion is not stable in aprotic solvents, in which the cation or lactone is favored according to the acidity or basicity of the medium. Absorption spectra of rhodamine 6G, an esterified dye which does not exhibit the above equilibria, are only modestly altered by most variations of solvent and temperature.

Substituent effects on the physical properties and pKa of aniline

The aniline molecule is nonplanar, with its NH2 group lying at an angle of approximately 42 to the plane of the benzene ring. Substituents on the phenyl ring alter this out-of-plane angle as well as other molecular properties such as the ring bond lengths and angles, the barrier to inversion Einv ,a nd the pKa of the amino group. Ab initio 6-311G quantum chemical calculations have been employed to examine these substituent influences and the extent to which they are interrelated. Electron-donating substituents increase the C—N bond length R(C—N),, Einv ,a nd the pKa ,w hereas electron-withdrawing substituents have the opposite effect. Among the molecular parameters that might serve as regression indicators for these changes, Hammett constants, which traditionally have been used to represent substituent electronic effects, yield fair to good correlations for R(C—N) (r 2 D 0.797), (r 2 D 0.804), Einv (r 2 D 0.829), and the amino group pKa (r 2 D 0.931) for aniline and 18 substituted anilines. Of several measures of atomic charge, the Mulliken and electrostatic charges on the amino nitrogen atom show essentially no correlation with these properties. In contrast, the natural charge Qn on the amino nitrogen is well correlated with the bond length R(C—N) (r 2 D 0.889), (r 2 D 0.932), Einv (r 2 D 0.839), and the amino group pKa (r 2 D 0.960). This latter result suggests that the natural charge, rather than either the Mulliken or electrostatic charges, may be the preferred charge descriptor for correlation purposes. Inclusion of electron correlation at the MP2 level increases the correlations of Einv with both (r 2 D 0.951) and Qn (r 2 D 0.892). c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 1107-1115, 2000

Modeling the tissue solubilities and metabolic rate constant (Vmax) of halogenated methanes, ethanes, and ethylenes.

Experimental solvent:air and tissue:air partition coefficients for 25 halogenated methanes, ethanes, and ethylenes in saline solution; olive oil; and rat blood, muscle, liver, and fat tissues have been examined using theoretical molecular modeling techniques. The metabolic rate constant, Vmax, was also investigated by these techniques for 19 chlorinated compounds in this group. Two graph theoretical approaches (the distance method of Wiener and the connectivity index method of Randic, Kier, and Hall) and an approach utilizing ad hoc molecular descriptors were employed. Satisfactory regression models for solubility were obtained with both the Randic-Kier-Hall approach and the ad hoc descriptors approach. Fluorine substituents decrease tissue solubilities, whereas both clorine and bromine substituents increase tissue solubilities, with the relative influence being Cl less than Br. Tissue solubilities can also be represented conveniently in terms of contributions from oil and saline solubilities, a procedure reinforced by factor analysis of the data. Equations derived by these methods adequately estimated the solubilities for eight additional compounds. No approach could successfully model Vmax for all 19 compounds, but a subset of 16 compounds was modeled using the connectivity indices. The equation is limited in its use but indicated future modeling directions for Vmax.

A spectroscopic/thermodynamic study of the rhodamine B lactone ⇌ zwitterion equilibrium

Abstract Temperature dependences of the visible absorption spectra of rhodamine B (RB) in water and 18 alcohol solvents have been examined and utilized to estimate thermodynamic parameters (Δ G °, Δ H °, Δ S °) for the RB lactone ⇌ zwitterion (L ⇌ Z) equilibria in these sol From the L ⇌ Z equilibrium in the strong hydrogen-bonding solvent trifluoroethanol an intrinsic RB zwitterion molar absorptivity of e = 1.30 × 1O 5 dm 3 /mol·cm is estimated. The thermodynamic results suggest that the position of the RB L ⇌ Z equilibrium is strongly influence by self-association of the solvent. The position of the equilibrium in aliphatic alcohols can be predicted from simple features of the solvent molecules. Consequences of the results regarding calculation of the natural radiative lifetime of the fluorescent state from the absorption spectrum are discussed.

Intuitive and counterintuitive noncovalent interactions of aromatic π regions with the hydrogen and the nitrogen of HCN

Abstract We have investigated intuitive and counterintuitive complex formation between eight aromatic molecules and HCN. In four of the former, the π regions had negative electrostatic potentials; in the other four, the π regions had positive potentials. Each aromatic molecule was allowed to interact through its π region with both the hydrogen (positive potential) of HCN and also the nitrogen (negative potential). In eight cases, therefore, interaction was intuitively favorable (positive/negative) while in the other eight, attractive interaction would be counterintuitive on the basis of the ground state electrostatic potentials (positive/positive or negative/negative). The intuitive interactions all led to bound complexes, and five of the counterintuitive did as well. The Hellmann–Feynman theorem was invoked to help explain the formation of the five counterintuitive complexes in terms of polarization/dispersion. Very good correlations were obtained, for the intuitive and also the counterintuitive complexes, between the computed interaction energies and values predicted solely on the basis of the most positive and the most negative electrostatic potentials in the π regions and on the HCN.

Computational Approaches for the Prediction of pKa Values

Introduction Absolute pKa Calculations Thermodynamic Cycles Gas Phase Gibbs Free Energy Calculations Solvation Gibbs Free Energy Calculations Pitfalls and Lessons from the Literature Concluding Remarks on Absolute pKa Calculations Relative pKa Calculations Quantitative Structure-Acidity Relationships (QSARs) Basic Principles of the QSAR approach Hammett and Taft Constants The Search for Useful Quantum Chemical Descriptors Alternative Approaches Commercial and Free Programs Oxyacids and Related Compounds Alcohols, Phenols, and Carboxylic Acids Phosphonic Acids Hydroxamic Acids and Oximes Silanols Thiols Nitrogen Acids Aliphatic Amines Anilines Azoles and Some Other Heterocyclics Amino Acids Pyridines and Related Heterocyclics Purines and Pyrimidines Additional Types of Acids Carbon Acids Inorganic Acids Polyprotic Acids Superacids Excited State Acids Acids in Non-aqueous Solvents Deuterium Oxide Dimethyl Sulfoxide Acetonitrile Tetrahydrofuran 1,2-Dichloroethane Other Solvents and Commentary Additional Factors Influencing Acidity and Basicity Thermodynamics Temperature Effects on Acidity Steric Effects and Hydrogen Bonding Isotope Effects Conclusions

Computational estimation of pKa values

The pKa of a compound is one of its most important properties as it defines the specific molecular forms that will prevail under different pH conditions. Accordingly, accurate means for computational estimation of this property are of particular interest. Two main techniques for this purpose have emerged: (1) a first principles approach that relies on basic physical concepts and requires high computational resources, but is independent of experimental input and (2) a linear free energy or quantitative structure–activity relationship (QSAR) approach that combines molecular structural and energetic descriptors with available experimental pKa data to reduce computational demand and yield good accuracy. In this overview, these methods are described and their advantages and limitations are noted. WIREs Comput Mol Sci 2015, 5:290–297. doi: 10.1002/wcms.1218