Michał F. Rode

The importance of high-order correlation effects for the CO-CO interaction potential

Abstract The CO–CO interaction energy is calculated for several geometries, both by the supermolecule MP4 and CCSD(T) methods, and by symmetry-adapted perturbation theory. Relatively large differences between the MP4 and CCSD(T) results are explained by means of a diagrammatic analysis of electron correlation effects, supported by quantitative calculations of the fifth-order contributions to the electrostatic interaction energy. It follows from this analysis that the calculation of an accurate intermolecular potential for CO is a particularly difficult problem: even the CCSD(T) method is not sufficiently reliable since it lacks important fifth-order correlation contributions.

A Computational Study on the Mechanism of Intramolecular Oxo−Hydroxy Phototautomerism Driven by Repulsive πσ* State

Potential energy (PE) surfaces of the lowest excited states of the 4(3H)-pyrimidinone/4-hydroxypyrimidine system were investigated with the aid of the CC2 and CASSCF methods of the electronic structure theory. These studies resulted in identification of a low-lying pi sigma* state, which is dissociative with respect to the stretching of the N-H or O-H bonds in the oxo and hydroxy structures of the compound, respectively. After initial excitation to the lowest local n pi* and/or pi pi* singlet states, the system can access the PE surface of the pi sigma* state by crossing a low barrier. It was computationally demonstrated that the system should evolve on the PE surface of the repulsive pi sigma* state toward a broad seam of intersection with the PE surface of the ground state. At the intersection, the nonadiabatic transition to the ground electronic state takes place and the system can either evolve to a minimum of the initially excited tautomer or to the ground-state minimum of the other tautomer. The steps listed above provide a mechanism of photoinduced dissociation-association (PIDA) phototautomerism, experimentally observed for a number of monomeric molecules, structurally similar to 4(3H)-pyrimidinone/4-hydroxypyrimidine. This mechanism describes a new class of intramolecular phototautomeric reactions driven by a repulsive pi sigma* state.

Photophysics of Schiff Bases: Theoretical Study of Salicylidene Methylamine

The proton-transfer reaction in a model aromatic Schiff base, salicylidene methylamine (SMA), in the ground and in the lowest electronically-excited singlet states, is theoretically analyzed with the aid of second-order approximate coupled-cluster model CC2, time-dependent density functional theory (TD-DFT) using the Becke, three-parameter Lee-Yang-Parr (B3LYP) functional, and complete active space perturbation theory CASPT2 electronic structure methods. Computed vertical-absorption spectra for the stable ground-state isomers of SMA fully confirm the photochromism of SMA. The potential-energy profiles of the ground and the lowest excited singlet state are calculated and four photophysically relevant isomeric forms of SMA; α, β, γ, and δ are discussed. The calculations indicate two S(1)/S(0) conical intersections which provide non-adiabatic gates for a radiationless decay to the ground state. The photophysical scheme which emerges from the theoretical study is related to recent experimental results obtained for SMA and its derivatives in the low-temperature argon matrices (J. Grzegorzek, A. Filarowski, Z. Mielke, Phys. Chem. Chem. Phys. 2011, 13, 16596-16605). Our results suggest that aromatic Schiff bases are potential candidates for optically driven molecular switches.

Computational study on the photophysics of protonated benzene.

The reaction paths in the lowest excited electronic states relevant for the photophysics of protonated benzene, C(6)H(7)(+), have been explored by ab initio techniques of electronic structure theory. For this purpose, the first four excited singlet electronic states of C(6)H(7)(+) have been calculated at the CC2/cc-pVTZ level of theory. The CC2 approach has been validated by CASPT2 and TD-DFT calculations. The calculated UV absorption spectrum is in good agreement with the experimental spectrum. It has been found that the out-of-plane and the in-plane ring deformation leads in the excited states in an essential barrierless manner to a low-lying conical intersection between the lowest excited states and with the ground state, providing a mechanism for efficient radiationless deactivation, which is expected to quench luminescence of the isolated molecular ion.

Excited-State Intramolecular Proton Transfer: Photoswitching in Salicylidene Methylamine Derivatives

The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012, 13, 4287-4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π-electron-donating and π-electron-withdrawing substituents results in an electron-density push-and-pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization.

Reply to the Comment on “The importance of high-order correlation effects for the CO–CO interaction potential” [Chem. Phys. Lett. 314 (1999) 326]

Abstract In an earlier Letter [Chem. Phys. Lett. 314 (1999) 326] we pointed out that the CCSD(T) method lacks some terms necessary for an accurate computation of the CO–CO interaction. In a Comment by Pedersen et al. [Chem. Phys. Lett. 304 (2000) 419] this finding is contested. In the present Reply we show that Pedersen et al. missed the main point of our argument.

Effect of chemical substitutions on photo-switching properties of 3-hydroxy-picolinic acid studied by ab initio methods

Effect of chemical substitutions to the molecular structure of 3-hydroxy-picolinic acid on photo-switching properties of the system operating on excited-state intramolecular double proton transfer (d-ESIPT) process [M. F. Rode and A. L. Sobolewski, Chem. Phys. 409, 41 (2012)] was studied with the aid of electronic structure theory methods. It was shown that simultaneous application of electron-donating and electron-withdrawing substitutions at certain positions of the molecular frame increases the height of the S0-state tautomerization barrier (ensuring thermal stability of isomers) and facilitates a barrierless access to the S1/S0 conical intersection from the Franck-Condon region of the S1 potential-energy surface. Results of study point to the conclusion that the most challenging issue for practical design of a fast molecular photoswitch based on d-ESIPT phenomenon are to ensure a selectivity of optical excitation of a given tautomeric form of the system.

Photophysics of indole-2-carboxylic acid in an aqueous environment studied by fluorescence spectroscopy in combination with ab initio calculations

The photo-physics and -chemistry of indoles are known to be highly complex and strongly dependent on their precise molecular structure and environment. Combination of spectroscopic analysis with quantum chemical calculations should be a powerful tool to unravel precise excited state deactivation mechanisms. At the same time, combined studies are seldom and likely far from trivial. In this work we explore the feasibility of combining spectroscopic and quantum-chemical data into one consistent model. The molecule of choice is indole-2-carboxylic acid (ICA) in aqueous media. Excited state dynamics are determined by time-resolved fluorescence experiments, while excited state reaction pathways of ICA-H(2)O clusters are explored by ab initio calculations.

The (H2O)2CO ternary complex: cyclic or linear?

Abstract Optimal structures, interaction energies, and harmonic vibrational frequencies of the (H2O)2CO ternary complex have been determined from the supermolecular calculations with the aug-cc-pVDZ and the aug-cc-pVTZ basis sets. Energetic properties of the complex have been calculated at MP4 and CCSD(T) levels. We located three low-energy configurations on the potential energy surface corresponding to two isomeric H-bonded cyclic complexes and one linear structure. The tunneling motion of the water molecule across the plane of the heavy atoms ring was studied. The calculations of the vibrational frequencies and infrared intensities for this complex are presented to facilitate the frequency assignments of future experimental spectra.

Electronic Spectra and Reversible Photoisomerization of Protonated Naphthalenes in Solid Neon

Alpha- and beta-protonated naphthalenes (α- and β-HN(+)) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C(10)H(9)(+) ion beam. The absorption spectra reveal S(1)/S(2) ← S(0) transitions with onsets at 502.1 and 396.1 nm for α-HN(+), and 534.5 and 322.3 nm in the case of β-HN(+). Wavelength-dispersed fluorescence was detected for α-HN(+), starting at 504.4 nm. Light-induced α-HN(+) → β-HN(+) isomerization was observed upon S(2) ← S(0) excitation of α-HN(+), whereas β-HN(+) relaxed back into the more stable alpha form either upon excitation to S(1) or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN(+) ↔ β-HN(+) photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.