Evaluation of Enthalpies of Benzene—Monosubstituted Benzene interactions by Semi-Empirical MO Calculations. Their Relationship to the Experimental Enthalpies Determined by GLPC

Abstract

Enthalpies of benzene—monosubstituted benzene interactions were evaluated by semi-empirical MO calculations of heats of formation for (i) the benzene (PhH) dimer and (ii) benzene—monosubstituted benzene (PhX) pairs. Calculation methods, initial intermolecular distances (r_I), and so on were investigated for the benzene dimer. The interaction enthalpy and optimized geometry calculated with PM3 were consistent with the previous experimental and theoretical results. As initial arrangements of PhH—PhX systems, one parallel (P) and four vertical arrangements (V_r , V_p , V_m , and V_b) were chosen (Figure 1). In the case of V_p or V_m arrangement where the lowest H atom at the para or meta position to substituent X in PhX was located above the centroid of PhH, calculated enthalpies of the interactions (ΔΔH_f) between PhH and PhX had close correlation with experimental ones (ΔΔH^t) determined by gas-liquid partition chromatography (GLPC), with the correlation coefficient (ρ) as large as 0.94 (Figure 5); differences between the experimental ΔΔH^t values and the ΔΔH^t values calculated from the correlation equation were less than ca. 0.1 kcal mol^-1 . With the other arrangements except for V_p, no appreciable relationship was observed between the ΔΔH_f and ΔΔH^t (Figures 2, 3, and 4).