Spectroscopic Properties of Formaldehyde in Aqueous Solution: Insights from Car-Parrinello and TDDFT/ CASPT2 Calculations Paola Lupieri, †,‡,§ Emiliano Ippoliti, †,‡ Piero Altoe `, | Marco Garavelli, | M. Mwalaba, ⊥,∇ and Paolo Carloni* ,†,‡,§,# German Research School for Simulation Sciences GmbH, 52425 Ju ¨lich and RWTH Aachen, Germany, SISSA, Via Bonomea 265, 34136 Trieste, Italy, Department of Chemistry “G. Ciamician”, UniVersity of Bologna, Via Selmi 2, I-40126 Bologna, Italy, International Centre for Theoretical Physics (ICTP), Strada Costiera 11, 34151 Trieste, Italy, and Democritos Modeling Center for Research in Atomistic Simulation, Via Bonomea 265, 34136 Trieste, Italy Received July 9, 2010 Abstract: We present Car-Parrinello and Car-Parrinello/molecular mechanics simulations of the structural, vibrational, and electronic properties of formaldehyde in water. The calculated properties of the molecule reproduce experimental values and previous calculations. The n f π* excitation energy, calculated with TDDFT and CASPT2, agrees with experimental data. In particular, it shows a blue shift on going from the gas phase to aqueous solution. Temperature and wave function polarization contributions have been disentangled. Introduction The carbonyl group is a key component of (bio)organic molecules, with interesting optical properties. As we know from textbooks, the oxygen’s electron lone pairs form nonbonding orbitals (n), whose electrons can be promoted to an antibonding π orbital localized over the CdO bond. In the ground states of many carbonyl compounds, the n state is the highest energy occupied molecular orbital (HOMO). 1 In that case, the lowest transition energy is the singlet-singlet transition n f π*. Formaldehyde, the simplest system containing such func- tionality, has been a test case for quantitative prediction of this transition (and recently also of the corresponding emission 2,3 ) in the isolated molecule and aqueous solution. The approaches used range from semiempirical AM1, 4 first- principle DFT, 5-7 ab initio HF, 8,9 and to post-HF techniques such as CISD and CASSCF 6,10-18 for formaldehyde using either molecular mechanics in a QM/MM scheme, 4,6,9-13,17,18 an implicit solvent, 5,19,20 or full quantum representation 7,15,16 for the solvent (see the Supporting Information for details). In the gas phase (at 0 K), the calculated values range between 3.3 and 4.5 eV. This may be compared with the experimental values measured at 330 K, ranging from 3.3 to 5 eV, with a maximum around 3.8 eV. 21,22 In solution, at room temperature (298 or 300 K), these computational approaches predict values in the range 3.5-5.7 eV, indicating a blue shift due to the solvent as well as the temperature between 0.07 and 0.43 eV. Experimentally, formaldehyde undertakes a reaction in water, forming methyleneglycol 23 Since the equilibrium constant of the reaction is 10 4 , the absorption cannot be measured in submolar solutions. However, it can be detected in highly concentrated solutions (10 M or more) at about 4.2 eV, 23 where molecular clusters * Corresponding author e-mail: p.carloni@grs-sim.de. † German Research School for Simulation Sciences. ‡ RWTH Aachen. § SISSA. | University of Bologna. ⊥ ICTP. # Democritos Modeling Center for Research in Atomistic Simulation. ∇ Permanent address: Department of Physics, University of Zambia, School of Natural Sciences, Lusaka 10101, Zambia. H 2 CO + H 2 O f H 2 C(OH) 2 (1) J. Chem. Theory Comput. 2010, 6, 3403–3409 3403 10.1021/ct100384f  2010 American Chemical Society Published on Web 10/21/2010