Determination and characterization of a transition structure for water–formaldehyde addition

Quantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas‐phase water–formaldehyde addition (FW1‡) are described, and a new transition‐structure search algorithm is presented. Basis‐set‐dependent effects are assessed by comparisons of computed properties obtained from self‐consistent field (SCF) molecular orbital (MO) calculations with STO‐3G, 4‐31G, and 6‐31G^** basis sets in the absence of electron correlation. The results obtained suggest that STO‐3G‐level calculations may be sufficiently reliable for the prediction of the transition structure of FW1‡ and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FW1‡ from water and formaldehyde (−44 kcal mol⁻¹) is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO‐3G (˜ −45 kcal mol⁻¹) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO‐3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4‐31G force constants being slightly superior to scaled STO‐3G force constants.