Extensive ab initio study of the OH+HCN reaction: Low lying electronic states of the stationary points on the 2 A′ surface An ab initio derived torsional potential energy surface for (H2O)3. I. Analytical representation and stationary points Erratum: Ab initio configuration interaction study of the lowlying electronic states of MgH [J.The global and local minima, stationary points, and torsional rearrangement processes of cyclic homodromic ͑H 2 O͒ 4 were studied on its four-dimensional torsional intermolecular potential energy surface. Eight different energetically low-lying torsional stationary point structures were found by ab initio theory, and fully structure-optimized at the second-order Mo "ller-Plesset level, using large basis sets. Second-order energies close to the one-particle basis set limit were obtained at these geometries using the explicitly correlated Mo "ller-Plesset method. The effects of higher-order correlation energy terms were investigated by coupled cluster theory, and terms beyond second order were found to cancel in good approximation. The S 4 symmetric global minimum has a square and almost planar O•••O•••O•••O arrangement with free O-H bonds alternating ''up'' and ''down'' relative to this plane, with two isometric versions of this structure. Another torsional conformer with two neighboring up O-H bonds followed by two neighboring down O-H bonds is a local minimum, 0.93 kcal/mol above the global minimum. The four versions of this structure are connected to the global minima via two distinct but almost degenerate first-order torsional saddle points, which occur as two sets of eight isometric versions each, both about 1.24 kcal/mol above the global minimum. Yet another set of eight second-order saddle points lies at 1.38 kcal/mol. The structure with three O-H bonds up and one down is not a stationary point, while the structure with all four O-H bonds on the same side of the plane is a first-order saddle point. The fully planar C 4h symmetric structure is a fourth-order stationary point 2.8 kcal/mol above the minimum. The torsional interconversion paths between this multitude of points are complex, and are discussed in three-dimensional spaces of symmetry-adapted torsional coordinates, and also in a network representation. The torsional normal-mode eigenvectors point fairly directly along the torsional interconversion pathways, but the harmonic frequencies are well below the corresponding barriers. Tunneling interconversion between torsional conformers is, hence, less important than for the water trimer.
Singlet and triplet vertical excitation energies of a series of acceptor parasubstituted N,N-dimethyl–anilines [NC–C6H4–N(CH3)2, NC–C6H4–NH2, OHC–C6H4–N(CH3)2, NC–C6H2(CH3)2–N(CH3)2, (H2N)OC–C6H4–N(CH3)2, (CH3)OC–C6H4–N(CH3)2, O2N–C6H4–N(CH3)2, named, respectively, 4DMAB–CN, 4AB–CN, 4DMAB–CHO, TMAB–CN, 4DMAB–CONH2, 4DMAB–COMe, and 4DMAB–NO2] have been calculated with TDDFT. Geometry optimization and excitation energy calculations have been performed, in most cases, with the B3LYP functional using a 6-31G(d) and a 6-311+G(2d,p) basis set (hereafter referred to as Sm and Bg, respectively). 4DMAB–CN and TMAB–CN have been investigated with particular care since gas-phase absorption spectra exist for those two molecules allowing thus a direct comparison with experimental results. The first and second singlet excited states of 4DMAB–CN, commonly named locally excited (LE) state and charge transfer (CT) state, are 0.1 and 0.04 eV higher than the experimental results at the B3LYP-Bg level, leading to a 0.06 eV underestimation of the gap between the two states. In the case of TMAB–CN, which is twisted in its ground state, B3LYP–(Sm/Bg) results show an error of 0.36 eV for the singlet CT state. Better agreement with experiment is obtained using the MPW1PW91 functional and Bg basis set with an underestimation of 0.17 eV for the singlet CT state and an overestimation of 0.16 eV for the second singlet state. Contrary to DFT/SCI results, the relative order and position of excitation energies of 4AB–CN and 4DMAB–CHO are well reproduced compared to solution spectra results. The singlet CT state using B3LYP and a Bg basis set is calculated 0.1 eV higher in energy than the experimental value obtained in isopentane for 4DMAB–CONH2, while the same excitation energy is predicted 0.08 and 0.28 eV too low compared to the gas-phase values for 4DMAB–COMe and 4DMAB–NO2, respectively. Finally, the CT excitation energy and its relative position to the LE state agrees with the acceptor strength concept.
The goal of this paper is to rationalize the fluorescence activity, experimentally observed for 21 molecules of the same family as the 4-(N,N-dimethyl)anilines, based on their potential energy surfaces calculated within the twisting intramolecular charge transfer model. A classification in four groups is proposed according to the sign of two parameters, ΔEgap, characterizing the energy difference between the vertical locally and charge transfer excited states, and ΔE1S (or ΔẼ1S for pretwisted systems), representing the energy gain of the charge transfer excited state with a perpendicular conformation compared to the first vertical excited state. In this study, the time-dependent density-functional theory has been used to calculate the potential energy surfaces of the ground and excited states along the twisting angle. Computed excitation energies and optimized ground state geometries have been obtained with both B3LYP and MPW1PW91 functionals using a 6-311+G(2d,p), and a 6-31G(d) basis set, respectively. From this study, it follows that ΔEgap and ΔE1S are the main parameters necessary to understand the fluorescence activity of these molecules. The fact that the same fluorescence activity is observed for the members of each group (or subcategory for the particular case of group II), reveals the underlying twisting mechanism as a common process for all the investigated molecules, which explains their experimental dual and nondual emission.
A theoretical concept for setting up 1D magnetic interactions between organic radicals aligned into chains within channels of inclusion compounds is presented by calculating collinear intermolecular binding energies using ab initio methods, followed by a Markov model to predict the characteristic length of chains. Experimentally, inclusion crystals providing radicals in channels of perhydrotriphenylene (PHTP) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were obtained by co-crystallisation or in-diffusion of the 1,3,5-trithia-2,4,6-triazapentalenyl radical (TTTA).
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