Extensive ab initio calculations of the phenol⋅H₂O complex were performed at the Hartree–Fock level, using the 6‐31G(d,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 H₂O plane orthogonal to the phenol plane, similar to (H₂O)₂; (b) the lowest‐energy transition state structure, which is nonplanar (C₁ symmetry) and has the H₂O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H₂O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β₂ intermolecular wag. Furthermore, the H₂O torsion τ around the H‐bond axis, and the β₂ mode are strongly anharmonically coupled, and a two‐dimensional τ/β₂ 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⋅H₂O and d‐phenol⋅D₂O employing two‐color resonance‐two‐photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β₁ and β₂ wags of both isotopomers in the S₀ and S₁ electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S₀ state in conjunction with the excitation of the intermolecular σ and β₂ modes.

A combined experimental and theoretical study of the 2‐naphthol⋅H₂O/D₂O system was performed. Two different rotamers of 2‐naphthol (2‐hydroxynaphthalene, 2HN) exist with the O–H bond in cis‐ and trans‐position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6‐31G(d,p) basis set, fully energy‐minimized geometries were computed for both cis‐ and trans‐2HN⋅H₂O of (a) the equilibrium structures with trans‐linear H‐bond arrangement and C_s symmetry and (b) the lowest‐energy transition states for H atom exchange on the H₂O subunit, which have a nonplanar C₁ symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H₂O geometries. The H‐bond stabilization energies with zero point energy corrections included are ≊5.7 kcal/mol for both rotamers, ≊2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H₂O at the same level of theory. Extension of the aromatic π‐system therefore hardly affects the H‐bonding conditions. The barrier height to internal rotation around the H‐bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6‐31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M‐matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a′ intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β₂ wag mode shows the largest anharmonic contributions. For the torsional mode τ (H₂O H‐atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant‐two‐photon ionization and dispersed fluorescence emission spectra of 2HN⋅H₂O and d‐2HN⋅D₂O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Møller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O₃) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O₃ plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C₆ symmetry and a very low barrier V₆=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low‐energy saddle points on the surface are of second and third order; they correspond to crown‐type and planar geometries with C₃ and C_3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry‐adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.

The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.

A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q²³⁰ proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.

Making use of the phenomenon of light-induced spin-crossover in the [Zn1−xFex(ptz)6](BF4)2 spin-crossover system, very high diffraction efficiencies η can be achieved in non-degenerate four-wave-mixing. In the mixed crystal with x=0.1 and at 76 K, i.e. at a temperature where the system is predominantly in the low-spin state, a value for η of 43% was obtained. This is attributed to a phase grating due to the large difference in metal---ligand bondlength between the low-spin ground state and the light-induced high-spin state. The rate constant for the decay of the laser-induced grating as a function of temperature is found to be exactly twice the one of the high-spin→low-spin relaxation, as expected for a dilute system in the absence of cooperative effects.

The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr93- (M=Cr3+, Ho3+) dimers as discrete units, and the magnetic excitations of mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers could thus be observed. The Yb3+-Cr3+ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr3+, has to be included in the exchange Hamiltonian. For the Yb3+-Ho3+ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho3+ ion. Taking into account the full (2J+1) ground-state multiplet of Ho3+, as well as anisotropic exchange, gives a satisfactory description of this dimer.

We present the results of a crystallographic and optical study realized on the system Sr_yBa_1-yFCl_xBr_1-x where x and y vary from zero to one. All mixed crystals studied were of tetragonal symmetry with the parent PbFCl structure. Complete structure determinations were performed for two single crystals with y=0.3 and x=1 respectively 0.7 (nominal composition). This system forms complete solid solutions, in contrast to previously published results.

Spectral hole-burning studies of nile red and cresyl violet in polyvinylbutyral and polyvinylformal films have been performed. From the shape of spectral holes under the influence of an electric field, the dipole moment difference between the ground and excited state of both dyes has been determined. The Stark effect was investigated at different positions in the inhomogeneously broadened absorption band of the guest molecules. The observed dipole moment difference decreases with increasing wavelength. This variation is caused by the matrix induced dipole moment. For nile red, which is a neutral and polar molecule, the distribution of induced dipole moments is strongly correlated with the orientation of its ground state dipole moment. In the case of cresyl violet perchlorate, which is a salt, this distribution is anisotropic for guests absorbing in the blue part of the inhomogeneous band but becomes more isotropic as the absorption wavelength increases. The wavelength dependence of the observed dipole moment is much stronger and is ascribed to the existence of the cresyl violet perchlorate salt in different states of solvation.

Holographic detection of spectral holes is demonstrated in a crystalline host material with signal-to-noise ratios of up to 104. Hole burning occurs in two Pr3+ sites in the Y2SiO5 lattice, in both cases due to population redistribution between the ground-state quadrupole levels. The signal contains contributions due to a resonant hole and several side holes and antiholes, a phenomenon not previously observed using the holographic technique. The diffracted spectrum is modeled in two ways. In the first case the transmission spectrum is used to determine the population gratings and thus the diffraction efficiency. In the second case the transition probabilities between ground- and excited-state Kramer's doublets are used to model the population gratings. The technique is applied to pseudo-Stark-effect measurements from which the crystallographic sites as determined by x-ray analysis are matched to the spectroscopic data presented here. The time decay of the diffracted signal is used to study nuclear spin-lattice relaxation. It is shown that at 1.6 K temperature-dependent phonon-induced processes make no contribution to this decay. The nonexponential time decay of the population upon radio-frequency irradiation resonant with a ground-state quadrupole splitting is attributed to Pr-Pr cross relaxation

A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.

A study of the hole-burning mechanisms of bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (DEAH) and bis[4-(disethylamino)-phenyl]squaraine (DEA) in hydrogen-bonding and non-hydrogen-bonding polymers is presented. Intramolecular H-bonding is only possible for DEAH. In all systems, the spectral holes are not persistent and decay with a distribution of rates ranging from 10-5s-1 to about 1 s-1, the time resolution of the experiment. In H-donating matrices, this distribution varies with the burning wavelength. From the hole-burning efficiencies and the kineticsof the hole refilling, four different types of nonphotochemical hole-burning mechanisms are postulated. The efficiency of these mechanisms depends mainly on the occurrence of processes slowing down the relaxation to the initial product state.

The rotational dynamics of nile red has been studied in polar protic, polar aprotic and non-polar solvents. In the non-polar and the aprotic solvents, with the exception of long alkanenitriles, the rotation dynamics is consistent with the prediction of the Stokes—Einstein—Debye hydrodynamics theory for slip and close to the stick boundary condition, respectively. However in protic solvents, the rotation dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition only if solvent attachment via hydrogen bonding is assumed. The anomalous behaviour observed in longer alkanenitriles has been assigned to the formation of a reverse micelle-like solvation layer around nile red.

The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives

Copper and silver, respectively, were introduced into single crystals of CsCdF₃. Our detailed electron paramagnetic resonance (EPR) study showed that both elements enter the Cd lattice site—copper as Cu²⁺, silver as Ag⁺, which then was converted into Ag²⁺ by x raying the corresponding samples. Cu²⁺ and Ag²⁺ were shown to present in their ground state a pseudostatic Jahn–Teller effect. Motional effects were observed in the respective EPR spectra and studied in some detail for Cu²⁺ as they are seen over a wide temperature range. Predictions of a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature dependent linewidths of the motionally averaged EPR spectrum. A power law (Tⁿ with n≂1.9) was determined for the temperature dependence of the reorientation frequency between 30 and 90 K.