Ab initio n-electron valence state perturbation theory study of the adiabatic transitions in carbonyl molecules: Formaldehyde, acetaldehyde, and acetone

Abstract

The application of the recently developed second-order n-electron valence state perturbation theory (NEVPT2) to small carbonyl molecules (formaldehyde, acetaldehyde, and acetone) is presented. The adiabatic transition energies are computed for the singlet and triplet n-->pi(*), pi-->pi(*), and sigma-->pi(*) states performing a full geometry optimization of the relevant states at the single state CASSCF level and taking into account the zero point energy correction in the harmonic approximation. The agreement with the known experimental values and with previously published high level calculations confirms that NEVPT2 is an efficient tool to be used for the interpretation of molecular electronic spectra. Moreover, different insight into the nature of the excited states has been obtained. Some of the transitions presented here have never been theoretically computed previously [(3)(pi-->pi(*)) and (3)(sigma-->pi(*)) adiabatic transitions in acetaldehyde and acetone] or have been studied only using moderate level (single reference based) ab initio methods (all adiabatic transitions in acetaldehyde). In the present work a consistent disagreement between NEVPT2 and experiment has been found for the (3)(pi-->pi(*)) adiabatic transition in all molecules: this result is attributed to the low intensity of the transition to the first vibrational levels of the excited state. The n-->pi(*) singlet and triplet vertical transition energies are also reported for all the molecules.

Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X, Y = H, F, Cl) in the ground and lowest excited electronic states, part 1: A test ofab initiomethods

“…Such discrepancies in theoretical geometrical structure for formaldehyde were also reported in other works (see Refs. [49] and [51] for (6-5) and (12-10) AS within ANO basis set, Ref. [52] for (10-9) AS in DZP basis).…”

“…2); this is primarily due to large absolute value of E. Interestingly, SR MP2 method demonstrates the largest E deviation. It should also be noted that NEVPT2 transition energies for formaldehyde calculated using CASSCF (6-5) and (12-10) geometry together with ANO basis [51] slightly agree slightly better with experimental Table VIII; for F 2 CO (S 1 ), experimental barrier of 6000 cm Ϫ1 was assumed. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]…”

On the nature of the π → π* ionic excited states: The V state of ethene as a prototype

“…20 The calculation of the adiabatic π → π * transition energy is also problematic in the small carbonyl molecules. 21 The description of the π → π * ionic excited states presents difficulties also in the aromatic molecules, as has been recognized since the first ab initio studies (see for instance refs. 22 26 ) have confirmed that these states present peculiar problems.…”

Atmospheric photochemical loss of H and H2from formaldehyde: the relevance of ultrafast processes

“…The most recent theoretical studies clearly establish the nature of the excited states. 13,15 In the present study we have employed the TD-B3LYP/6-311+G(3df,2p) method on geometries optimised at the B3LYP/6-31G(d) level whenever possible. The calculated excitation energies are shown in Table 1.…”