Thermodynamic and ab initio analysis of the controversial enthalpy of formation of formaldehyde

There are two values, -26.0 and -27.7 mol(-1), that are routinely reported in literature evaluations for the standard enthalpy of formation, Delta H-f degrees(298), of formaldehyde (CH2=O), where error limits are less than the difference in values. In this study, we summarize the reported literature for formaldehyde enthalpy values based on evaluated measurements and on computational studies. Using experimental reaction enthalpies for a series of reactions involving formaldehyde, in conjunction with known enthalpies of formation, its enthalpy is determined to be -26.05 +/- 0.42 kcal mol(-1), which we believe is the most accurate enthalpy currently available. For the same reaction series, the reaction enthalpies are evaluated using six computational methods: CBS-Q, CBS-Q//B3, CBS-APNO, G2, G3, and G3B3 yield Delta H-f degrees(298) = -25.90 +/- 1.17 kcal mol(-1), which is in good agreement to our experimentally derived result. Furthermore, the computational chemistry methods G3, G3MP2B3, CCSD/6-311 + G(2df,p)//B3LYP/6-31G(d), CCSD(T)/6-311 + G(2df,p)//B3LYP/6-31G(d), and CBS-APNO in conjunction with isodesmic and homodesmic reactions are used to determine Delta H-f degrees(298). Results from a series of five work reactions at the higher levels of calculation are -26.30 +/- 0.39 kcal mol(-1) with G3, -26.45 +/- 0.38 kcal mol-1 with G3MP2B3, -26.09 +/- 0.37 kcal mol(-1) with CBS-APNO, -26.19 +/- 0.48 kcal mol(-1) with CCSD, and -26.16 +/- 0.58 kcal mol(-1) with CCSD(T). Results from heat of atomization calculations using seven accurate ab initio methods yields an enthalpy value of -26.82 +/- 0.99 kcal mol(-1). The results using isodesmic reactions are found to give enthalpies more accurate than both other computational approaches and are of similar accuracy to atomization enthalpy calculations derived from computationally intensive W1 and CBS-APNO methods. Overall, our most accurate calculations provide an enthalpy of formation in the range of -26.2 to -26.7 kcal mol(-1), which is within computational error of the suggested experimental value. The relative merits of each of the three computational methods are discussed and depend upon the accuracy of experimental enthalpies of formation required in the calculations and the importance of systematic computational errors in the work reaction. Our results also calculate Delta H-f degrees(298) for the formyl anion (HCO-) as 1.28 +/- 0.43 kcal mol(-1).