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Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity. This method was pioneered by S. W. Benson.〔''Estimation of heats of formation of organic compounds by additivity methods'' N. Cohen, S. W. Benson Chem. Rev.; 1993; 93(7); 2419-2438 (Abstract )〕 == Benson model == (詳細はalkanes and alkenes the method works by collecting a large number of experimental heat of formation data (see: Heat of Formation table) and then divide each molecule up into distinct groups each consisting of a central atom with multiple ligands: : X-(A)i(B)j(C)k(D)l To each group is then assigned an empirical incremental value which is independent on its position inside the molecule and independent of the nature of its neighbors: * P primary C-(C)(H)3 -10.00 * S secondary C-(C)2(H)2 -5.00 * T tertiary C-(C)3(H) -2.40 * Q quaternary C-(C)4 -0.10 * gauche correction +0.80 * 1,5 pentane interference correction +1.60 : in kcal/mol and 298 K The following example illustrates how these values can be derived. The experimental heat of formation of ethane is -20.03 kcal/mol and ethane consists of 2 P groups. Likewise propane (-25.02 kcal/mol) can be written as 2P+S, isobutane (-32.07) as 3P+T and neopentane (-40.18 kcal/mol) as 4P+Q. These four equations and 4 unknowns work out to estimations for P (-10.01 kcal/mol), S (-4.99 kcal/mol), T (-2.03 kcal/mol) and Q (-0.12 kcal/mol). Of course the accuracy will increase when the dataset increases. the data allow the calculation of heat of formation for isomers. For example the pentanes: * n-pentane = 2P + 3S = -35 (exp. -35 kcal/mol) * isopentane = 3P + S + T + 1 gauche correction = -36.6 (exp. -36.7 kcal/mol) * neopentane = 4P + Q = 40.1 (exp. 40.1 kcal/mol) The group additivities for alkenes are: * Cd-(H2) +6.27 * Cd-(C)(D) +8.55 * Cd-(C)2 +10.19 * Cd-(Cd)(H) +6.78 * Cd-(Cd)(C) +8.76 * C-(Cd)(H)3 -10.00 * C-(Cd)(C)(H)2 -4.80 * C-(Cd)(C)2(H) -1.67 * C-(Cd)(C)3 +1.77 * C-(Cd)2(H)2 -4.30 * cis correction +1.10 * alkene gauche correction +0.80 In alkenes the cis isomer is always less stable than the trans isomer by 1.10 kcal/mol. More group additivity tables exist for a wide range of functional groups. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heat of formation group additivity」の詳細全文を読む スポンサード リンク
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