Benzo[a]pyrene: Standard Thermodynamic Properties from Adiabatic and Combustion Calorimetry and Density Functional Theory

Abstract

The temperature dependence of heat capacity of polycyclic aromatic hydrocarbon, benzo[a]pyrene, was investigated in the temperature range T = 7.5−329.3 K using adiabatic calorimetry. Smoothed C p,m °(T) values between 0 and 329 K are presented along with values for the standard thermodynamic functions:pyrene has been determined using the isothermal combustion calorimeter with a stationary bomb. These new experimental results, together with literature data, are used to calculate the standard enthalpy of formation, Δ f H m °, and standard Gibbs free energy of formation, Δ f G m °, giving Δ f H m °(T = 298.15 K, benzo[a]pyrene, cr) = 162.4 ± 15.5 kJ•mol −1 and Δ f G m °(T = 298.15 K, benzo[a]pyrene, cr) = 347.0 ± 15.7 kJ•mol −1 . Additionally, the quantum chemistry approach was used in order to calculate the electronic structure and thermodynamic properties for benzo[a]pyrene.

Benchmark Thermochemistry of Polycyclic Aromatic Hydrocarbons

“…Of the 24 PAHs for which the experimental enthalpies of formation were recommended in the review by Roux et al, the accuracy of the experimental values was confirmed for only half of the compounds. Among more recent experimental measurements, − the discrepancies from 6 to 34 kJ/mol were obtained for eight compounds. ,,,,− Thus, this study shows that modern quantum chemical methods increase the accuracy of computations to a level that is comparable to, and in some cases exceeds, the accuracy of current thermochemical determinations, particularly for systems that are difficult to study experimentally.…”

“…Comparison with Experimental Data. The recommended gas-phase enthalpies of formation of reference PAHs were taken from the critical review of Roux et al 3 with the exception of indene (2), indane (3), pyrene (13), and benzo[e]pyrene (24). The new experimental data for indene, 25 indane, 26 and pyrene 29 are very close to those recommended earlier, 3 but have smaller uncertainties, whereas the experimental data for benzo[e]pyrene (24) 23 have been obtained recently for the first time.…”

“…ATOMIC­(hc) , ref : atomization reaction. FPD composite approach , ref : generation of reactions preserving the number of carbon atoms in each hybridization state on each side of equations. c When two or more experimental values are given, the first one is used to calculate the difference between the experimental and calculated enthalpies of formation. d Atomization reaction. e Calculated from 10 isodesmic-type reactions where seven PAHs were used as the reference species. f Calculated using experimental value of Δ f H 298 ° (cr) from ref and recommended enthalpy of sublimation from ref . g Calculated using experimental value of Δ f H 298 ° (cr) from ref and enthalpy of sublimation from ref . …”

“…The structures of all compounds studied in this work are shown in Figure . In previous quantum chemical calculations, the estimated Δ f H 298 ° values were compared with the experimental values recommended by Roux et al, while the new experimental data for some of these compounds were obtained later. − These experimental data are also used in this work to make a choice of reference set of PAHs.…”

“…The purpose of this paper is to assess the accuracy of existing experimental data on enthalpies of formation of PAHs, many of which have significant uncertainties, and to recommend benchmark values that can be used for thermochemical calculations and validation of quantum chemical models. To obtain accurate enthalpies of formations for the PAHs, calculations have been carried out using the high-level DLPNO-CCSD­(T 1 ) method that has shown promising results in our previous calculations of the enthalpy of formation of various organic compounds including polycyclic hydrocarbons. − The compounds studied in this work include 24 PAHs for which the experimental Δ f H 298 ° (g) values have been recommended by Roux et al and three compounds (benzo­[ a ]­pyrene, benzo­[ e ]­pyrene, and benzo­[ghi]­perylene) for which the experimental data have only recently been obtained. , In addition, the enthalpies of formation were estimated for three molecules (sumanene, pentalene, and acepentalene) for which the experimental data are not available but their enthalpies of formation were predicted using high-level calculations; ,, these polycyclic hydrocarbons are of interest for estimating the enthalpies of formation of fullerenes. The structures of all compounds studied in this work are shown in Figure .…”