Thermochemistry of organic and heteroorganic species. Part XII. Mono- and disubstituted acetylenes and ethynyl free radicals. New electronegativity scale

Abstract

The first important result of the present work is that the enthalpy of formation Δ H° f (298 K) for ethynyl HCC free radical is now firmly established 123±2 kcal mol −1, whereas the most known higher experimental and theoretical values reflect its formation in excited states. For computation of the Δ H° f HCC radical, the most reliable results were obtained by applying isodesmic (formal) reactions CN+RCCH→RCN+ CCH. The isodesmic reaction HCC +RCCH→HCCH+RCC is recommended for computation of the Δ H° f for substituted RCC radicals. With systematic application of the enthalpic shift procedure for more than 20 classes of compounds and functions supplemented by introduction of sub-types and correction terms, this procedure now can be treated as a completed special methodology in molecules’ thermochemistry. Using this methodology, the novel or corrected Δ H° f values for about 120 monosubstituted RCCH acetylenes and 50 other compounds were calculated. For calculating the thermochemical properties of molecules and radicals the new electronegativity (EN) scale was elaborated which comprised for a moment about 210 groups. The general requirements for EN scales were formulated. Two scales EN and EN* were proposed for elements with vacant p- or d-orbitals. For the first time, the polarizability effect (PAZ) of a substituent in relation to EN is considered in detail for diverse types of chemical processes and parameters. It is shown that any EN scale can be applied in practice only for a narrow set of structurally similar substituents rather than as an absolute measure. The enthalpies of formation for 75 diacetylenes XCCX and XCCY are presented. Following the works of McKean on correlation of Δ ν is(CH) values in gas phase IR spectra with ΔBDEs (bond dissociation energies), the literature data on ν RCCH values were applied to calculate the enthalpies of formation for RCC free radicals. For the first time, the variable n cm −1/ kcal −1 for diverse types of C–H bonds directly linked with EN of C-atom was suggested: ∼9 cm −1 for alkane C-atom, ∼11 cm −1 for alkenes RC H CH 2 or RCHC H 2, ∼12 cm −1 for alkynes RCCH and ∼14 cm −1 for RCHO molecules. Analyzing the EN/PAZ interrelationship for particular case of RCC–H bond strengths, the Δ H° f values for 12 RCC radicals were found from literature ν RCC–H values and then calculated for additional 42 radicals together with ν RCCH frequencies. In all these calculations, the value 123 kcal mol −1 for Δ H° f for HCC radical was used. This value was further used to extract from published computed data (after their recalculation), an important thermochemical information on many unstable species (molecules, free radicals, carbenes) valuable for combustion processes. In general, serious inconsistency between our data and literature computed data on thermochemistry of acetylenes and substituted RCC radicals was found.