In the present study a benchmark set of medium-sized and large aromatic organic molecules with 10-78 atoms is presented. For this test set 0-0 transition energies measured in supersonic jets are compared to those calculated with DFT and the B3LYP functional, ADC(2), CC2 and the spin-scaled CC2 variants SOS-CC2 and SCS-CC2. Geometries of the ground and excited states have been optimized with these methods in polarized triple zeta basis sets. Zero-point vibrational corrections have been calculated with the same methods and basis sets. In addition the energies have been corrected by single point calculations with a triple zeta basis augmented with diffuse functions, aug-cc-pVTZ. The deviations of the theoretical results from experimental electronic origins, which have all been measured in the gas phase with high-resolution techniques, were evaluated. The accuracy of SOS-CC2 is comparable to that of unscaled CC2, whereas ADC(2) has slightly larger errors. The lowest errors were found for SCS-CC2. All correlated wave function methods provide significantly better results than DFT with the B3LYP functional. The effects of the energy corrections from the augmented basis set and the method-consistent calculation of the zero-point vibrational corrections are small. With this benchmark set reliable reference data for 0-0 transition energies for larger organic chromophores are available that can be used to benchmark the accuracy of other quantum chemical methods such as new DFT functionals or semi-empirical methods for excitation energies and structures and thereby augments available benchmark sets augments present benchmark sets which include mainly smaller molecules.

We characterized the entrance channel, reaction threshold, and mechanism of an excited-state H atom transfer reaction along a unidirectionally hydrogen-bonded "wire" -O-H...NH3...NH3...NH3...N. Excitation of supersonically cooled 7-hydroxyquinoline.(NH3)3 to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces H atom transfer with a reaction threshold approximately 200 wave numbers. Further translocation steps along the wire produce the S1 state 7-ketoquinoline.(NH3)3 tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S1 state barriers arise from crossings of a pipi* with a Rydberg-type pisigma* state.

Extensive ab initio calculations of the phenol· H 2 0 complex were performed at the Hartree-Fock level, using the p) and 6-311 + +G(d,p) basis sets. Fully energy-minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H 2 0 plane orthogonal to the phenol plane, similar to (H 2 0h; (b) the lowest-energy transition state structure, which is nonplanar (C 1 symmetry) and has the H 2 0 moiety rotated by ±90°. The calculated MP2/6-311G+ + (d,p) binding energy including basis set superposition error corrections is 6.08 kcallmol; the barrier for internal rotation around the H bond is only 0.4 kcallmol. Intra-and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol' H 2 0. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the {32 intermolecular wag. Furthermore, the H 2 0 torsion T around the H-bond axis, and the {32 mode are strongly anharmonically coupled, and a twodimensional T/{32 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h-phenol . H 2 0 and d-phenol . D 2 0 employing two-color resonance-two-photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The {31 and {32 wags of both isotopomers in the So and SI electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the So state in conjunction with the excitation of the intermolecular a and {32 modes.

To study the electronic interactions in donor–acceptor (D–A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)‐fused dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory.

The S(1)/S(2) state exciton splittings of symmetric doubly hydrogen-bonded gas-phase dimers provide spectroscopic benchmarks for the excited-state electronic couplings between UV chromophores. These have important implications for electronic energy transfer in multichromophoric systems ranging from photosynthetic light-harvesting antennae to photosynthetic reaction centers, conjugated polymers, molecular crystals, and nucleic acids. We provide laser spectroscopic data on the S(1)/S(2) excitonic splitting Δ(exp) of the doubly H-bonded o-cyanophenol (oCP) dimer and compare to the splittings of the dimers of (2-aminopyridine)(2), [(2AP)(2)], (2-pyridone)(2), [(2PY)(2)], (benzoic acid)(2), [(BZA)(2)], and (benzonitrile)(2), [(BN)(2)]. The experimental S(1)/S(2) excitonic splittings are Δ(exp) = 16.4 cm(-1) for (oCP)(2), 11.5 cm(-1) for (2AP)(2), 43.5 cm(-1) for (2PY)(2), and <1 cm(-1) for (BZA)(2). In contrast, the vertical S(1)/S(2) energy gaps Δ(calc) calculated by the approximate second-order coupled cluster (CC2) method for the same dimers are 10-40 times larger than the Δ(exp) values. The qualitative failure of this and other ab initio methods to reproduce the exciton splitting Δ(exp) arises from the Born-Oppenheimer (BO) approximation, which implicitly assumes the strong-coupling case and cannot be employed to evaluate excitonic splittings of systems that are in the weak-coupling limit. Given typical H-bond distances and oscillator strengths, the majority of H-bonded dimers lie in the weak-coupling limit. In this case, the monomer electronic-vibrational coupling upon electronic excitation must be accounted for; the excitonic splittings arise between the vibronic (and not the electronic) transitions. The discrepancy between the BO-based splittings Δ(calc) and the much smaller experimental Δ(exp) values is resolved by taking into account the quenching of the BO splitting by the intramolecular vibronic coupling in the monomer S(1) ← S(0) excitation. The vibrational quenching factors Γ for the five dimers (oCP)(2), (2AP)(2), (2AP)(2), (BN)(2), and (BZA)(2) lie in the range Γ = 0.03-0.2. The quenched excitonic splittings Γ[middle dot]Δ(calc) are found to be in very good agreement with the observed splittings Δ(exp). The vibrational quenching approach predicts reliable Δ(exp) values for the investigated dimers, confirms the importance of vibrational quenching of the electronic Davydov splittings, and provides a sound basis for predicting realistic exciton splittings in multichromophoric systems.

A torsional potential energy surface for the cyclic water trimer was calculated at the level of second-order Mo/ller–Plesset perturbation theory. For the construction of this ab initio surface, the first-order wave function was expanded in a many-electron basis which linearly depends on the interelectronic coordinates r12. The one-electron basis of Gaussian orbitals was calibrated on the water monomer and dimer to ensure that the ab initio surface computed represents the (near- ) basis set limit for the level of theory applied. The positions of the free O—H bonds are described by three torsional angles. The respective three-dimensional torsional space was investigated by 70 counterpoise corrected single-point calculations for various values of these angles, providing a grid to fit an analytical representation of the potential energy surface. The four symmetry unique stationary points previously found at the Hartree–Fock and conventional Mo/ller–Plesset levels [Schütz et al., J. Chem. Phys. 99, 5228 (1993)] were studied in detail: Relative energies of the structures were calculated by applying second-order Mo/ller–Plesset and coupled cluster methods; harmonic vibrational frequencies were calculated at the second-order Mo/ller–Plesset level with a 6-311++G(d,p) basis set at these stationary points. It is expected that the present torsional potential energy surface for the water trimer will play an important role in the understanding of the vibrational transitions observed by far-infrared vibration–rotation–tunneling spectroscopy in terms of a nearly free pseudorotational interconversion on a cyclic vibrational–tunneling path.

Excited-state proton transfer from 1-naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1-naphthol⋅(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster-size-specific laser-spectroscopic techniques: resonant two-photon ionization (R2PI) and laser-induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited-state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1-naphthol; (2) the medium-size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1-naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm−1, characteristic of the emission of the excited-state 1-naphtholate anion. Excited-state proton transfer appears to occur in the largest observed clusters (n≥30), yet the fluorescence spectra do not converge fully to that of 1-naphtholate anion in bulk water. These three behaviors are discussed in terms of a model based on three distinct excited states connected by two electronic and geometric rearrangement processes. The model accounts in a unified way for the complete range of aqueous solvation behavior observed here as well as in many other solvent systems studied previously. The extent of proton transfer reaction is largely solvent controlled, the major determinants being the proton affinity of the solvent or solvent cluster, and its ability to resolvate the nascent ion pair on a subnanosecond time scale. In bulk ice, the slow solvent relaxation results in complete absence of excited-state proton transfer from both 1- and 2-naphthol.

Fully optimized structures were calculated for (H2O)n, n=5 and 8, at the SCF (self-consistent field) level using the 4–31G and, for n=5, also 6–31G* basis sets. The n=5 cluster was found to have a cyclic structure with five H bonded and five free hydrogens. The n=8 minimum energy structure has almost D2d symmetry, with an approximately cubical oxygen framework and four tetrahedrally arranged free hydrogens; four of the water molecules are single- and four are double-hydrogen donors. Harmonic vibrational frequencies, IR and Raman intensities were calculated for n=5 and 8, as well as for the previously optimized n=2–4 clusters. The band positions and intensities in the 3000–3800 cm−1 region correlate well with IR predissociation spectra of (H2O)n clusters. The O–H stretching frequencies of single- and double-hydrogen donor water molecules are relatively well separated from each other, and both from the frequency region of the free O–H stretches, suggesting a new interpretation for some of the data. The low-frequency translational/librational modes of both n=5 and 8 show strong mixing with intramolecular stretching and bending. The stretch–stretch coupling constants for OH oscillators on different molecules kij(OH,OH) show a strong increase, and those for intramolecular coupling kii(OH,OH) a rapid decrease with increasing cluster size. For n≥5, kij(OH,OH)≫kii(OH,OH), implying that the cluster can be viewed as a supermolecule of strongly coupled O–H oscillators. The n=8 spectra show significant similarity to those of ice.