Ioannis D. Petsalakis

MRD CI calculations on the asymmetric stretch potentials of H2O in the ground and the first seven singlet excited states

Abstract Cross sections of the potential energy surfaces of the H 2 O molecules in the first eight singlet states have been generated for the elongation of one of the OH bonds, Keeping the other fixed and for different values of the HOH bond angle. Only the first 1 A″ (Ā 1 B 1 ) and the first excited 1 A′ (B 1 A 1 ) states are found to be dissociative along such cross sections. Several previously unreported conical intersections are noted, one between the 3 1 A′ and 4 1 A′ states and another between the 2 1 A″ and 3 1 A″ species. The OH + H( 2 S) disociation limits for the lowest eight water singlet states are identified, the highest three of which correspond to OH π → 3s and 3p Rydberg excitations.

Bending potentials for H2O in the ground and the first six singlet excited states

Abstract MRD CI calculations are presented for the bending potentials of H 2 O in the ground and first six singlet excited states. The results are discussed with reference to the H( 2 S) + OH(A 2 Σ + ) dissociation path of H 2 O.

Amplified Halogen Bonding in a Small Space

Weak, intermolecular forces are difficult to observe in solution because the molecular encounters are random, short-lived, and overwhelmed by the solvent. In confined spaces such as capsules and the active sites of enzymes or receptors, the encounters are prolonged, prearranged, and isolated from the medium. We report here the application of encapsulation techniques to directly observe halogen bonding. The small volume of the capsule amplifies the concentrations of both donor and acceptor, while the shape of the space permits their proper alignment. The extended lifetime of the encapsulation complex allows the weak interaction to be observed and characterized by conventional NMR methods under conditions in which the interaction would be negligible in bulk solvent.

Nonorthonormal CI for molecular excited states. I. The sudden polarization effect in 90° twisted ethylene

The accurate and efficient calculation of properties of excited states, especially those involved in near‐degeneracies and valence‐Rydberg mixings, may depend crucially on the zeroth order orbitals employed for the description of a few important configurations. When self‐consistent field orbitals specific for the states of interest are employed, one becomes involved with nonorthonormal basis sets, which circumstance has conceptual as well as computational implications. In this work, we have developed a nonorthonormal configuration interaction (NONCI) method and have applied it to the calculation of the ‘‘sudden polarization’’ effect in the zwitterionic excited states of ethylene, in the spirit of a state‐specific theory. A very small NONCI, (6×6), yields similar results to those from previous large CI calculations. In particular, the sudden rise of the dipole moment as a function of the pyramidalization angle of the CH2 group of the 90° twisted Z state is predicted while it is shown that the inclusion of ...

Excited-State Intramolecular Proton Transfer in Hydroxyoxime-Based Chemical Sensors

The electronic structure of a series of β-hydroxy-oximes, with different aromatic cores (naphthalene, pyrene, coumarin, pyridine) between the oxime and the hydroxyl groups, has been investigated by time-dependent density functional theory (TDDFT) and of the naphthalene-based oxime, in addition, by resolution-of-identity second-order perturbative coupled cluster (RICC2) calculations with basis sets up to augmented triple-ζ quality. The particular systems have been proposed as fluorescent sensors of organophosphorus (OP) nerve agents, with enhancement of fluorescence accompanying the sensing of OP agents. It is found that the experimentally observed fluorescence quenching of the oxime sensors in their initial form can be attributed to intramolecular proton transfer upon excitation from the β-hydroxyl group to the nitrogen atom, thus forming a weakly emitting hydroxylaminoquinoid.

Theoretical dipole transition moments for transitions between bound electronic states and non-adiabatic coupling matrix elements between 2Σ+ of HeH

Dipole transition moments and radial non-adiabatic coupling matrix elements between electronic states of HeH are presented. The estimated radiative lifetimes of the excited states of HeH are in good agreement with the available experimental data.

Theoretical dipole transition moments for the transitions to the ground state X2σ+ from the A2σ+, B2Π, C2σ+, D2σ+ and E2Π states and for the B 2?A 2:+ system in HeH

Dipole transition moments have been calculated for the transitions A2+ to X2+, B2 to X2+, C2+ to X2+, D2+ to X2+, E2 to X2+ and B2 to A2+ in HeH. The spectroscopic constants obtained in this work support an experimental assignment for the C 2+ to A 2+ transition.

Adiabatic and quasidiabatic 2Σ+ states of BeH

An approach to the calculation of quasidiabatic states is presented and applied to the rather entangled spectrum of the first five BeH 2Σ+ excited states. The method of transformation from the adiabatic to the diabatic description employs a criterion of maximum nonorthonormal overlap between sequential diabatic states along the coordinate of interest, starting with the dissociation region as a reference point. The computational method is general and capable of separating out diabatic states from a set of adiabatic ones of mixed character, involving valence, Rydberg and ion‐pair contributions.

Conformations and Fluorescence of Encapsulated Stilbene

Absorption and emission spectra of free and encapsulated stilbene in two different capsules were calculated using the DFT and the TDDFT methodology at the B3LYP, CAM-B3LYP, M06-2X, PBE0, and ωB97X-D/6-31G(d,p) levels of theory. The present work is directed toward the theoretical interpretation of recent experimental results on control of stilbene conformation and fluorescence in capsules [Ams, M. R.; et al. Beilstein J. Org. Chem. 2009, 5, 79]. The results of the calculations are in agreement with experiment and show that fluorescence of trans-stilbene persists in the large cage while it is quenched in the small one. It is found that the geometry of trans-stilbene in the ground as well as in the first excited singlet state is unaffected by encapsulation in the large cage, and consequently the absorption and emission spectra are similarly unaffected. In the small cage, the ground state of encapsulated trans-stilbene is distorted, with the two phenyl groups twisted, while the geometry of the excited state, after relaxation, lies at the conical intersection with the ground state. Consequently, there is no emission similar to that of free trans-stilbene, and the state decays nonradiatively to the ground state.

Theory of chemical reactions of vibronically excited H2(B 1Σ+u). I. Prediction of a strongly bound excited state of H4

MRD–CI calculations were carried out on the potential energy surfaces of the ground and first excited singlet of the same symmetry of H4, which predict a bound excited state with respect to H2+H*2 (B 1Σ+u) of 3.1 eV at a geometry of trigonal pyramid. This geometry was predicted theoretically according to a model of chemical bonding for ‘‘maximum ionicity excited states.’’ At the minimum and the three dimensional space surrounding it, there is a near touching of the excited with the ground state hypersurfaces. This fact has implications for the spectroscopy and photochemistry of the H2+H*2 system.