Resonance Energy of the Tetrachlorethylene Molecule

Abstract

RECENTLY1, it has been shown that the interpretation of a general potential function determining the modes of vibration for the C2Cl4 molecule supports the hypothesis of resonance among several electronic structures, leading for the C—C bond to a single-bond character of about 15 per cent. This result has been deduced partly from the fact that the C—C force constant lies between 8·0 and 8·5 × 105 dynes/cm., as compared with 9·0 × 105 dynes/cm, for ethylene, and partly from general considerations on the signs of the cross-terms. (From a private communication from Prof. C. Manneback, we adopt 9·0 instead of 9·3 as in our previous publications; in this case, the conclusions are even reinforced.) If so, following Pauling's arguments2, the actual energy of formation of the molecule must be greater than the calculated value by a certain amount, called resonance energy of the system. The definite bond diagram used for our calculation corresponds to the classical distribution of valencies and gives It is to be noted that the value Qcalc.= E (C = C) + 4 E (C—Cl) — 2 Lc is not influenced by the not yet definitely established value of Lc. According to Pauling2, the C=C bond energy is 100 kcal./mole. the the C—Cl bond energy is 66·5 kcal./mole and Lc is 124·3 kcal./mole. Hence the heat of formation Qcalc. is 117·4 kcal./mole.