The INDO/2-AHP method for excited states : Part 2. The adiabatic proton affinities of formaldehyde in ^1,3nπ^∗ and ^1,3ππ^∗ states

Abstract

The gas-phase proton affinities (PA) of formaldehyde in the nπ^∗ and ππ^∗ states have been studied theoretically using the average hole potential (AHP) model at the INDO/2 level of approximation. The PA of H₂CO is predicted to decrease in the sequence PA(¹S_mo PA(nππ^∗) > PA(ππ^∗); moreover, PA(¹nπ^∗) > PA(³nπ^∗) and PA(¹ππ^∗) > PA(³ππ^∗). Structurally, the protonated formaldehyde molecule is planar in the ground and ππ^∗ states with non-linear arrangements of the C---O---H⁺ fragment, but is non-planar in the nπ^∗ state with the proton lying above the molecular plane. Barriers to the internal in-plane and out-of-plane motions of the proton in the ¹S_o, ^1,3nπ and ^1,3ππ^∗ states of H₂COH⊕ have been calculated.