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

The first important result of the present work is that the enthalpy of formation DH8f (298 K) for ethynyl HCxCz free radical is now firmly established 123 ^ 2 kcal mol21, whereas the most known higher experimental and theoretical values reflect its formation in excited states. For computation of the DH8f HCxCz radical, the most reliable results were obtained by applying isodesmic (formal) reactions zCxN þ RCxCH ! RCxN þ zCxCH. The isodesmic reaction HCxCz þ RCxCH ! HCxCH þ RCxCz is recommended for computation of the DH8f for substituted RCxCz 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 DH8f values for about 120 monosubstituted RCxCH 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 XCxCX and XCxCY are presented. Following the works of McKean on correlation of DnisðCHÞ values in gas phase IR spectra with DBDEs (bond dissociation energies), the literature data on n RCxCH values were applied to calculate the enthalpies of formation for RCxCz free radicals. For the first time, the variable n cm21/ kcal21 for diverse types of C–H bonds directly linked with EN of C-atom was suggested: ,9 cm21 for alkane C-atom, ,11 cm21 for alkenes RCHyCH2 or RCHyCH2, ,12 cm21 for alkynes RCxCH and ,14 cm21 for RCHyO molecules. Analyzing the EN/PAZ interrelationship for particular case of RCxC–H bond strengths, the DH8f values for 12 RCxCz radicals were found from literature n RCxC–H values and then calculated for additional 42 radicals together with n RCxCH frequencies. In all these calculations, the value 123 kcal mol21 for DH8f for HCxCz 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 RCxCz radicals was found. q 2004 Published by Elsevier B.V.