Calculating Flash Point Numbers from Molecular Structure: An Improved Method for Predicting the Flash Points of Acyclic Alkanes

We report a novel method for calculating flash points of acyclic alkanes from flash point numbers, <i>N</i>_FP, which can be calculated from experimental or calculated boiling point numbers (<i>Y</i>_BP) with the equationNFP=1.020YBP−1.083Flash points (FP) are then determined from the relationshipFP(K)=23.369NFP2/3+20.010NFP1/3+31.901For a data set of 102 linear and branched alkanes, the correlation of literature and predicted flash points has <i>R</i>² = 0.985 and an average absolute deviation of 3.38 K. <i>N</i>_FP values can also be estimated directly from molecular structure to produce an even closer correspondence of literature and predicted FP values. Furthermore, <i>N</i>_FP values provide a new method to evaluate the reliability of literature flash point data

Improved Prediction of Hydrocarbon Flash Points from Boiling Point Data

Flash points (<i>T</i>_FP) of hydrocarbons are calculated from their flash point numbers, <i>N</i>_FP, with the relationshipTFP(K)=23.369NFP2/3+20.010NFP1/3+31.901In turn, the <i>N</i>_FP values can be predicted from experimental boiling point numbers (<i>Y</i>_BP) and molecular structure with the equationNFP=0.987YBP+0.176D+0.687T+0.712B−0.176where <i>D</i> is the number of olefinic double bonds in the structure, <i>T</i> is the number of triple bonds, and <i>B</i> is the number of aromatic rings. For a data set consisting of 300 diverse hydrocarbons, the average absolute deviation between the literature and predicted flash points was 2.9 K

Femtosecond and Temperature-Dependent Picosecond Dynamics of Ultrafast Excited-State Proton Transfer in Water–Dioxane Mixtures

Synthetic flavylium salts like the 7-hydroxy-4-methylflavylium (HMF) cation have been used as prototypes to study the chemistry and photochemistry of anthocyanins, the major group of water-soluble pigments in the plant kingdom. In this work, a combination of fluorescence upconversion with femtosecond time resolution and time-correlated single photon counting (TCSPC) with picosecond time resolution have been employed to investigate in details the excited-state proton transfer (ESPT) of HMF in water and in binary water/1,4-dioxane mixtures. TCSPC measurements as a function of temperature provide activation parameters for all of the individual rate constants involved in the proton transfer, including those for dissociation and recombination of the geminate excited base–proton pair (A*···H⁺) that can be detected in the water/dioxane mixtures (but not in water). Unlike the other rate constants, the deprotonation rate constant <i>k</i>_d shows a non-Arrhenius dependence on temperature in both water and water/dioxane mixtures. At low temperatures <i>k</i>_d is close to the dielectric relaxation rate of the solvent with a barrier of ca. 8 kJ mol^–1, suggesting that the solvent reorganization is the rate-limiting step. At higher temperatures (>30 °C) the proton transfer process is nearly barrierless and solvent-dependent. Fluorescence upconversion results in H₂O, D₂O, and water/dioxane mixtures confirm the two-step model for the ESPT of HMF and provide additional details of the early events prior to the onset of proton transfer, attributed to conformational relaxation and solvent reaccommodation around the initially formed excited state. The results are consistent with DFT calculations that indicate that charge redistribution occurs after rather than prior to the onset of the ESPT process

Group Contribution Method To Predict Boiling Points and Flash Points of Alkylbenzenes

Boiling point numbers (<i>Y</i>_BP) of alkylbenzenes are predicted directly from the molecular structure with the relationship <i>Y</i>_BP = <i>Ar</i>_<i>i</i> + 1.726 + 2.779<i>C</i> + 1.716<i>M</i>₃ + 1.564<i>M</i> + 4.204<i>E</i>₃ + 3.905<i>E</i> – 0.329<i>D</i> + 0.241<i>G</i> + 0.479<i>V</i> + 0.967<i>T</i> + 0.574<i>S</i>. Here, <i>Ar</i>_<i>i</i> is a parameter that depends upon the substitution pattern of the aromatic ring, while the remainder of the equation is the same as that reported earlier for calculating the <i>Y</i>_BP values of alkanes. The boiling points (<i>T</i>_B) of the alkylbenzenes are then calculated from the relationship <i>T</i>_B (K) = −16.802<i>Y</i>_BP^2/3 + 337.377<i>Y</i>_BP^1/3 – 437.883. For a data set consisting of 130 alkylbenzenes having 7–40 carbon atoms, the average absolute deviation between the literature and predicted <i>T</i>_B values was 1.67 K and the <i>R</i>² of the correlation was 0.999. In addition, <i>Y</i>_BP values calculated with this method can be used to predict the flash points of the alkylbenzenes

Prediction of Crude Oil Properties and Chemical Composition by Means of Steady-State and Time-Resolved Fluorescence

Steady-state and time-resolved fluorescence measurements are reported for several crude oils and their saturates, aromatics, resins, and asphaltenes (SARA) fractions (saturates, aromatics and resins), isolated from maltene after pentane precipitation of the asphaltenes. There is a clear relationship between the American Petroleum Institute (API) grade of the crude oils and their fluorescence emission intensity and maxima. Dilution of the crude oil samples with cyclohexane results in a significant increase of emission intensity and a blue shift, which is a clear indication of the presence of energy-transfer processes between the emissive chromophores present in the crude oil. Both the fluorescence spectra and the mean fluorescence lifetimes of the three SARA fractions and their mixtures indicate that the aromatics and resins are the major contributors to the emission of crude oils. Total synchronous fluorescence scan (TSFS) spectral maps are preferable to steady-state fluorescence spectra for discriminating between the fractions, making TSFS maps a particularly interesting choice for the development of fluorescence-based methods for the characterization and classification of crude oils. More detailed studies, using a much wider range of excitation and emission wavelengths, are necessary to determine the utility of time-resolved fluorescence (TRF) data for this purpose. Preliminary models constructed using TSFS spectra from 21 crude oil samples show a very good correlation (<i>R</i>² > 0.88) between the calculated and measured values of API and the SARA fraction concentrations. The use of models based on a fast fluorescence measurement may thus be an alternative to tedious and time-consuming chemical analysis in refineries

Improved Synthesis of Analogues of Red Wine Pyranoanthocyanin Pigments

An improved procedure is described for the preparation of pyranoflavylium cations from the reaction of 5,7-dihydroxy-4-methylflavylium cation with aromatic aldehydes. Modifications of the procedure of Chassaing et al. (<i>Tetrahedron Lett</i>. <b>2008</b>, <i>49</i>, 6999–7004; <i>Tetrahedron</i> <b>2015</b>, <i>71</i>, 3066–3078) circumvent the reported restriction to electron-rich benzaldehydes and provide access to a wide variety of substituted pyranoflavylium cations, including those with electron-withdrawing substituents or an attached heterocyclic or polycyclic aromatic ring. This opens the way for studies of substituent and structural effects on the ground and excited states of these pyranoanthocyanin analogues, the behavior of which should mirror fundamental aspects of the chemistry and photophysics of the pyranoanthocyanin chromophores present in mature red wines