This thesis is structured in the following way: In chapter 2 the phenol-water1≤n≤3
clusters are investigated in their ground and lower excited states, and are compared to the existing experimental data. Subsequently in chapter 3 the related 2-napthol-water2 systems are explored in a similar way. Then, in chapter 4, the low lying minima of the aniline di- and trimer cluster potential energy surfaces are located, in order to set up the stage for the investigation of the important structures in the excited states within chapter 5. Finally, the subject of chapter 6 is the structure and binding of adduct compounds of tantalum chlorides and phosphorus sulfide cage molecules.
14
2 Phenol-water 1≤n≤3 clusters
2.1 Introduction
Phenol (Ph) as the simplest aryl alcohol serves as a prototype molecule for struc-turally related subunits of larger aromatic biomolecules (e.g., tyrosine residues in proteins). Therefore the photophysics of this molecule is of particular interest. Fur-thermore, clusters of phenol with simple solvent molecules, especially with water, can be used to study hydrogen bonding, solvation effects, and their influence on the photophysical behavior of the solute, with the phenol again mimicking structurally related chromophores of larger biomolecules. Phenol-water clusters therefore have been extensively investigated by many groups during the last two decades, both experimentally and theoretically. In the context of this work we want to focus on the Ph(H2O)n clusters with 1 ≤ n ≤ 3. Mass-selective absorption spectra ob-tained by one or two-color resonance enhanced multiphoton ionization have been measured by several groups [68–78] in the vicinity of the electronic origin of Ph. It turns out that the absorption spectrum of the n = 2 cluster is strikingly different from those obtained for the n = 1, n = 3, and n = 4 clusters. While on the one hand for all the latter rather narrow line spectra with intense origins have been obtained, the n = 2 cluster, on the other hand, exhibits only a weak, broad, and congested band structure (see Fig. 2.1). Whereas forn= 1 and n= 3 it was read-ily possible to measure well resolved dispersed fluorescence emission [74, 76, 77] or ion dip spectra [73], this has not been possible for n= 2 due to the low quantum yield and anomalously short lifetime of the S1 state. Jacoby et al. postulated in Ref. 77 that a linear geometry might become more stable in the excited state than the cyclic structure corresponding to the ground state vide infra. Due to the high vibrational mode density provided by this linear geometry at the Franck-Condon point, strong vibrational coupling between the modes of the cyclic and the linear structure might occur, explaining the broadening of the band structure. In the same work the authors also stated that the n = 2 cluster may be less stable in the excited state than in the ground state. Yet since the n = 2 features are red-shifted with respect to the Ph origin, this statement appears to be in conflict with experimental evidence.
15
2 Phenol-water1≤n≤3 clusters
Figure 2.1: Mass specific absorption spectra of the Ph(H2O)n, 1≤n ≤3, clusters in the vicinity of the electronic origin of Ph, obtained with two-color resonant two photon ionization spectroscopy in a molecular beam ex-periment. For details see Refs. 74 and 76.
Ab initio electronic structure calculations on these clusters in the electronic ground state S0 (primarily at the level of Hartree-Fock or second-order Møller-Plesset perturbation theory) have been performed by several groups [74, 76, 77, 79–
85], in order to elucidate low-energy geometries and corresponding (harmonic) vi-brational frequencies. Furthermore, diffusion quantum Monte Carlo studies inves-tigating the vibrational ground states on the S0 surfaces of these clusters are also available in the literature [86, 87]. The equilibrium geometry of the binary complex corresponds to a translinear hydrogen bonding arrangement reminiscent of water dimer, with the H2O plane perpendicular to the phenol plane. For the Ph(H2O)2
cluster three cyclic low-energy geometries exist, usually denoted as (Udu), (Uud), and (Udd), indicating, e.g., for the first minimum that the phenyl ring is in the Up, the first free hydrogen in the down, and the second free hydrogen again in the up position with respect to the plane given by the two hydrogen bonds involved (donor-acceptor indicates the direction). The (Udu) is the most stable conformer of Ph(H2O)2 on the S0 surface. The equilibrium geometry of the Ph(H2O)3 cluster is also cyclic and can be described according to the notation above as (Udud).
Only a few calculations on the excited states of these clusters have been pub-lished so far. Energies, geometries, and (harmonic) vibrations of the S1(π∗ ← π) state have been computed by Fang and Liu using the configuration interaction singles (CIS) and complete active space self-consistent field (CASSCF) methods [83, 84]. The equilibrium geometries on the S1(π∗ ← π) surface turned out to be quite similar to those on the S0 surface, i.e., a translinear (TL) hydrogen bond
16
2.1 Introduction and cyclic geometries were obtained for Ph(H2O) and Ph(H2O)2, respectively. In particular, the linear Ph(H2O)2 geometry postulated by Jacoby et al. turned out to lie 5.3 kcal/mol (CASSCF) and 9.2 kcal/mol (single-point CASPT2) above the cyclic minimum-energy geometry on the S1(π∗ ←π) surface. It has to be stressed at this point that neither CIS nor CASSCF (without subsequent treatment of dy-namic correlation) is a reliable method for excited-state calculations, yet analytic energy gradients have not been available for other methods at that time. Re-cently, Sobolewski et al. explored the S1(π∗ ← π) and S2(σ∗ ← π) surfaces of Ph, Ph(H2O), and Ph(H2O)3 (enforcing Cs symmetry) in order to elucidate the mechanism of photoinduced electron and proton transfer in these systems [88, 89].
Geometry optimizations were performed again at the level of CASSCF with ad-ditional single-point energy calculations at the CASPT2 (perturbation theory of second order using a CASSCF reference wave function) level. A conical intersection (CI) between these two surfaces was found, which is responsible for predissociation of the S1(π∗ ←π) and subsequent concerted electron- and protontransfer reactions from the chromophore to the solvent molecules. More recently, the authors also performed quantum-wave-packet dynamics for the case of phenol in order to get a more detailed picture of this predissociation of the S1(π∗ ←π) state. In the light of these results a connection between the observed congested band structure of the absorption spectrum and the anomalously short lifetime of Ph(H2O)2, on the one hand, and the S1(π∗ ← π) / S2(σ∗ ← π) conical intersection, on the other hand, appears to be plausible.
In this work we have explored the S1(π∗ ←π) surface and its conical intersection with S2(σ∗ ←π) at the level of coupled cluster response theory employing the CC2 model [46]. To the best of our knowledge these are the first excited-state geometry optimizations for these systems performed at a theoretical level which includes dynamical electron correlation effects. The Ph(H2O)2 minimum-energy geometries so obtained are qualitatively different from those reported by Fang and Liu at the CASSCF level: Only one stable cyclic ring structure (similar to the one in the S0
state) could be located, which lies about 3 kcal/mol above the global S1(π∗ ← π) minimum. Instead, two distinct conformers were found with the second water molecule acting as a H donor to the aromatic ring system (one of these geometries constitutes the global minimum). Apparently, the oxygen atom and the π-ring system of phenol switch role in acting as H acceptors on going from the S0 to the S1(π∗ ←π) state. Forn= 3 a stable cycle geometry similar to the one obtained for the S0 state was found, yet energetically lower-lying minima featuring bonding to the π-ring system were also located. Additional CASPT2 geometry optimizations, performed for n= 2, confirm the existence of these global minimum structures.
17
2 Phenol-water1≤n≤3 clusters