Electronic relaxation effect on sequential a-cleavage bond dissociation energy for carbonyl compounds and the heavy analogues Chiu-Ling Lin, Chin-Hung Lai, San-Yan Chu * Department of Chemistry, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Received 30 July 2001; in final form 20 November 2001 Abstract We made B3LYP/6-311G** theoretical study on the bond dissociation energy (BDE) for the carbonyl compounds and heavy atom analogues (H 2 AB, A ¼ C, Si, Ge; B ¼ O, S, Se), and their halogen substituted compounds. We demonstrate that the second BDE is much weaker than the first one due to the electronic relaxation effect on the AB fragment. The difference in BDE (DBDE) appears to be a property of AB alone correlated well with its value of DE ST (singlet–triplet energy gap), nearly independent of the substituents involved. The normal bond length of the second bond, the weak A–X bond in XAB radical can be interpreted as a result of potential surface avoided-crossing. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction It is interesting to observe that in the stepwise photolysis of acetone in the production of two methyl radicals and carbon monoxide [1–10], CH 3 COCH 3 ! CH 3 þ CH 3 CO ð1Þ CH 3 CO ! CH 3 þ CO ð2Þ the second CC bond dissociation energy (BDE) is only 11 kcal/mol, which is much smaller than the first one of 84 kcal/mol [1]. One can rationalize this interesting result by consideration of the electron relaxation effect on CO. The product, CO, is in a closed-shell ground state ðX 1 R þ Þ. However, the ‘valence state’ of the CO fragment of the acetyl radical CH 3 CO ( 2 A 0 ) can be conceived as having a biradical configuration n 1 p 1 (Scheme 1). After the CC bond is cleaved, the resultant CO (denoted as AB) biradical with open-shell config- uration would presumably relax to a closed-shell ground configuration. The large drop in the sec- ond CC BDE is simply due to such stabilization effect on the CO part. There is some precedent for this stabilization effect. For examples, Huber and Herzberg [11] and Kutzelnigg [12] explained that the difference in the two BDE in BeH 2 and MgH 2 can be identified as ns to np(n ¼ 2; 3) promotion energy. Carter [13] and Chen and coworkers [14] used singlet–triplet 27 June 2002 Chemical Physics Letters 359 (2002) 355–359 www.elsevier.com/locate/cplett * Corresponding author. Fax: +886-35-711082. E-mail address: sychu@mx.nthu.edu.tw (S.-Y. Chu). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)00762-5