Quasi-free Rotation Research Articles

What do far-infrared spectra of solitary water in "water-in-solvent" systems reveal about water's solvation and dynamics?

Classical molecular dynamics simulations of water in ionic and dipolar solvents were used to interpret the far-infrared (FIR) rotation/libration spectra of "solitary water" in terms of water's rotational dynamics and interactions with solvents. Seven solvents represented by nonpolarizable all-atom force fields and a series of idealized variable-charge solvents were used to span the range of solvent polarities (hydrogen bonding) studied experimentally. Simulated spectra capture the solvent dependence observed, as well as the relationship between the frequencies of water libration (νL) and OH stretching bands (νOH). In more strongly interacting solvents, simulated νL are ∼20% higher than those of experiment. In all solvents, the simulated spectra are composites of rotational motions about the two axes perpendicular to water's dipole moment, and the different frequencies of these two motions are responsible for the breadth of the libration band and the bimodal shape observed in halide ionic liquids. Simulations overestimate the separation of these two components in most solvents. The character of water rotational motions changes markedly with solvent polarity, from quasi-free rotation in nonpolar and weakly polar solvents to highly constrained libration in strongly hydrogen bonding environments. The changeover to librational motions dominating the spectrum occurs between solvents such as benzene (νL ∼ 250cm-1) and acetonitrile (νL ∼ 400cm-1). For solvents in the latter category, the mean frequency of the experimental FIR band provides a direct measure of mean-squared torques and, therefore, force constants associated with interactions constraining water's librational motion.

Parallel versus Twisted Pentacenes: Conformational Impact on Singlet Fission.

We placed two pentacene chromophores at the termini of a diacetylene linker to investigate the impact of excitation wavelength, conformational flexibility, and vibronic coupling on singlet fission. Photoexcitation of the low-energy absorption results in a superposed mixture of states, which transform on an ultrafast time-scale into a spin-correlated and vibronically coupled/hot delocalized triplet pair 1(T1T1)deloc. Regardless of temperature, the lifetime for 1(T1T1)deloc is less than 2 ps. In contrast, photoexcitation of the high-energy absorption results in the formation of 1(T1T1)deloc lasting 1.0 ps, which then decays at room temperature within 4 ps via triplet-triplet annihilation. Lowering the temperature enables 1(T1T1)deloc to delocalize and vibronically decouple, in turn affording 1(T1T1)loc. In addition, our results suggest that the quasi-free rotation at the diacetylene spacer may lead to twisted conformations with very low SF quantum yields, highlighting the need of controlling this structural aspect in the design of new singlet fission active molecules.

Influence of solution phase environmental heterogeneity and fluctuations on vibronic spectra: Perylene diimide molecular chromophore complexes in solution.

Ensembles of ab initio parameterized Frenkel-exciton model Hamiltonians for different perylene diimide dimer systems are used, together with various dissipative quantum dynamics approaches, to study the influence of the solvation environment and fluctuations in chromophore relative orientation and packing on the vibronic spectra of two different dimer systems: a π-stacked dimer in aqueous solution in which the relative chromophore geometry is strongly confined by a phosphate bridge and a side-by-side dimer in dichloromethane involving a more flexible alkyne bridge that allows quasi-free rotation of the chromophores relative to one another. These entirely first-principles calculations are found to accurately reproduce the main features of the experimental absorption spectra, providing a detailed mechanistic understanding of how the structural fluctuations and environmental interactions influence the vibronic dynamics and spectroscopy of solutions of these multi-chromophore complexes.

Water clusters in an argon matrix: infrared spectra from molecular dynamics simulations with a self-consistent charge density functional-based tight binding/force-field potential.

The present theoretical study aims at investigating the effects of an argon matrix on the structures, energetics, dynamics, and infrared (IR) spectra of small water clusters (H2O)n (n = 1-6). The potential energy surface is obtained from a hybrid self-consistent charge density functional-based tight binding/force-field approach (SCC-DFTB/FF) in which the water clusters are treated at the SCC-DFTB level and the matrix is modeled at the FF level by a cluster consisting of ∼340 Ar atoms with a face centered cubic (fcc) structure, namely (H2O)n/Ar. With respect to a pure FF scheme, this allows a quantum description of the molecular system embedded in the matrix, along with all-atom geometry optimization and molecular dynamics (MD) simulations of the (H2O)n/Ar system. Finite-temperature IR spectra are derived from the MD simulations. The SCC-DFTB/FF scheme is first benchmarked on (H2O)Arn clusters against correlated wave function results and DFT calculations performed in the present work, and against FF data available in the literature. Regarding (H2O)n/Ar systems, the geometries of the water clusters are found to adapt to the fcc environment, possibly leading to intermolecular distortion and matrix perturbation. Several energetical quantities are estimated to characterize the water clusters in the matrix. In the particular case of the water hexamer, substitution and insertion energies for the prism, bag, and cage are found to be lower than that for the 6-member ring isomer. Finite-temperature MD simulations show that the water monomer has a quasifree rotation motion at 13 K, in agreement with experimental data. In the case of the water dimer, the only large-amplitude motion is a distortion-rotation intermolecular motion, whereas only vibration motions around the nuclei equilibrium positions are observed for clusters with larger sizes. Regarding the IR spectra, we find that the matrix environment leads to redshifts of the stretching modes and almost no shift of the bending modes. This is in agreement with experimental data. Furthermore, in the case of the water monomer and dimer, the magnitudes of the computed shifts are in fair agreement with the experimental values. The complex case of the water hexamer, which presents several low-energy isomers, is discussed.

B18(2-): a quasi-planar bowl member of the Wankel motor family.

A quasi-planar member of the so-called 'Wankel motor' family, B18(2-), is found. This boron cluster is an electronically stable dianion and a concentric doubly σ- and π-aromatic system. The inner B6 unit in B18(2-) undergoes quasi-free rotation inside the perimeter of the B12 ring. The absence of any localized σ-bond between the inner ring and the peripheral boron atoms makes the system fluxional.

Time resolved dynamics of phonons and rotons in solid parahydrogen

We use femtosecond optical Kerr effect (OKE) spectroscopy to perform time- and wavelength-resolved pump-probe measurements on the energetics and lifetimes of transverse optical phonons and J = 2 rotons in solid para-hydrogen (pH2). By systematically studying the OKE spectroscopy of pH2 in the gas, liquid, and solid phases for delay times up to 300 ps, we can disentangle homodyne and heterodyne contributions in the solid to the signal that results from the slow phonon (900 fs) and fast roton (94 fs) dynamics. In solid pH2 at 8.5 K, the energies of the J = 2 Raman-active rotons are measured to be 351.98(8) cm(-1), 353.99(8) cm(-1), and 356.00(8) cm(-1) corresponding to the crystal field split MJ = ±1, ±2, and 0 substates. Consistent with the picture of quasi-free molecular rotation within the solid, we observe long-lived roton coherences with T2 lifetimes of 132(5), 114(5), and 82(5) ps for the MJ = ±1, ±2, and 0 substates. In contrast, similar measurements on normal-hydrogen (nH2) solids which nominally contain 75% ortho-hydrogen (oH2) and 25% pH2 molecules display qualitatively different roton dynamics; no persistent roton excitations are observed but rather overdamped librational excitations that decay within 3 ps. The measured low temperature T2 dephasing time of gaseous pH2 implies a collision cross section of 2.08(10) Å(2) which is close to the theoretical value for so-called elastic resonant collisions whereby rotational energy is exchanged between the two colliding partners, but the sum of the rotational energies is preserved. We argue that this same collisional process also determines the T2 dephasing lifetimes in the liquid and solid phases. Finally, OKE spectroscopy on pH2 solids with oH2 concentrations of 1-3% shows evidence for extremely long-lived rotational coherences which likely correspond to J = 2 rotons that are pinned next to single oH2 impurities within the pH2 solid.

Rotational Dynamics in C70: Temperature- and Pressure-Dependent Infrared Studies

We revisit the infrared (IR) spectra of C70 on pure samples as a function of temperature (20−370 K) and pressure (0−10 GPa). Although the rotation of the molecule is not completely free at ambient conditions, the measured spectra are in perfect agreement with previous density functional theory (DFT) calculations on isolated molecules, including the intensities, for 22 out of the possible 31 IR-active modes. We assign the anomalies in the infrared spectra both on decreasing temperature (∼280 K) and increasing pressure (∼0.8 GPa) to the freezing out of uniaxial rotation; the boundary between uniaxial and quasi-free rotation has a much weaker effect. At ∼7.5 GPa an irreversible change of the spectrum occurs, which we attribute to room-temperature dimerization. The pressure at which this dimerization occurs adds a new point to the phase diagram presented by Sundqvist.

Unravelling phenomenon of internal rotation in B13+ through chemical bonding analysis

We describe and explain the fluxionality of B(13)(+). The chemical bonding analysis shows that the inner triangle of B(13)(+) is bound to the peripheral ring by delocalized bonds only, allowing a quasi-free rotation of the inner ring.

Excited-State Dynamics of Donor−Acceptor Bridged Systems Containing a Boron−Dipyrromethene Chromophore: Interplay between Charge Separation and Reorientational Motion

The excited-state dynamics of a series of electron donor-acceptor bridged systems (DABS) consisting of a boron-dipyrromethene chromophore covalently linked to a dinitro-substituted triptycene has been investigated using femtosecond time-resolved spectroscopy. The chromophores differ by the number of bromine atom substituents. The fluorescence lifetime of the DABS without any bromine atom is strongly reduced when going from toluene to polar solvents, this shortening being already present in chloroform. This effect is about 10 times weaker with a single bromine atom and negligible with two bromine atoms on the chromophore. The excited-state lifetime shortening is ascribed to a charge transfer from the excited chromophore to a nitrobenzene moiety, the driving force of this process depending on the number of bromine substituents. The occurrence of this process is further confirmed by the investigation of the excited-state dynamics of the chromophore alone in pure nitrobenzene. Surprisingly, no correlation between the charge separation time constant and the dielectric properties of the solvents could be observed. However, a good correlation between the charge separation time constant and the diffusional reorientation time of the chromophore alone, measured by fluorescence anisotropy, was found. Quantum chemistry calculations suggest that quasi-free rotation about the single bond linking the chromophore to the triptycene moiety permits a sufficient coupling of the donor and the acceptor to ensure efficient charge separation. The charge separation dynamics in these molecules is thus controlled by the reorientational motion of the donor relative to the acceptor.

Synthesis, characterization and nonlinear optical (NLO) properties of a push–pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10−3 a.u. and leads to a β value equal to 49.2 × 10−30 cm5 esu−1. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

Water dimer diffusion on Pd[111] assisted by an H-bond donor-acceptor tunneling exchange.

Based on the results of density functional theory calculations, a novel mechanism for the diffusion of water dimers on metal surfaces is proposed, which relies on the ability of H bonds to rearrange through quantum tunneling. The mechanism involves quasifree rotation of the dimer and exchange of H-bond donor and acceptor molecules. At appropriate temperatures, water dimers diffuse more rapidly than water monomers, thus providing a physical explanation for the experimentally measured high diffusivity of water dimers on Pd[111] [Science 297, 1850 (2002)]].

Second order interference effects in the fundamental band spectra of HCl and DCl in liquid Xe

Second order interference effects have been incorporated in the calculation of the quasifree rotation absorption coefficient of diatomic molecules in non polar solvents. These effects are analysed in the calculation of the fundamental band infrared spectra of HCl and DCl in liquid Xe at T = 205 K, and the results are compared with the experimental data of Knudtson and Weitz [J. Chem. Phys. 83, 927 (1985)]. Second order interference effects show relevant contributions both in the spectral regions near to the maximum of the P and R branches, and in the PR inter-branch region.

Structural and physical properties of pressure polymerized C 60

We discuss the structural and dynamic properties of C 60 polymerized under low- P, low- T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under these conditions arise from nucleation of molecular chains in random directions because of the quasi-free molecular rotation under standard reaction conditions in the fcc phase of C 60. Polymerization in He gives results qualitatively different from those obtained in other media.

Dynamics of N(CH 3) 4GeCl 3

The molecular and lattice dynamics of N(CH 3) 4GeCl 3 have been investigated by temperature-dependent inelastic and quasielastic neutron scattering, and by Raman spectroscopy. The observed temperature-dependent quasielastic scattering indicates a quasi-free rotation of the CH 3-groups at temperatures above 85 K and a reorientational motion of the whole N(CH 3) 4-groups at temperatures above 300 K. The N(CH 3) 4-groups interact only weakly with the GeCl 3-groups. The structural phase transitions at 170, 200 and 424 K seem to be unrelated to the temperature-induced changes of the molecular dynamics.

Internal dynamics contributions to the CH stretching overtone spectra of gaseous nitromethane NO2CH3

The methyl CH stretching overtone spectra of gaseous nitromethane NO2CH3 have been recorded with Fourier transform infrared conventional near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. They all exhibit a complex structure with, at ΔvCH=1 and 2, a characteristic asymmetric top vibration-rotation profile which vanishes as vibrational energy increases. In addition, in these two lower energy spectra, the perpendicular stretching vibrations exhibit a widely spaced fine structure profile resulting from a Coriolis coupling induced by the methyl internal rotation. These excited spectra have been analyzed with a theoretical model which takes into account, in the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by a Morse potential, with the quasi-free internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Till Δv=3, the theoretical treatment yields normal modes. From the third overtone, the vibrational energy is seen to be localized and thus the calculations proceed in a local mode basis from Δv=4–6. Most of the parameters of this model and their variation with the internal rotation coordinate θ are the same as those used to account for the overtone spectra of the monohydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak IVR localized at the second overtone where only the two first tiers are effective and at the third overtone where three tiers must be considered to give a good reproduction of the experimental spectral features. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.

Phenylene vinylene oligomers studied by theoretical methods: Joint analysis of computational and x‐ray results of the configurational isomers of 1,4‐bis[2‐(3,4,5‐trimethoxyphenyl)ethenyl]benzene

The configurational isomers of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene have been investigated by ab initio and MOPAC-AM1 semiempirical methods. The calculations were guided by and compared with single crystal X-ray results of the trans, trans-isomer (taken from the literature) and of the cis,cis-isomer (reported here). Using 4-21G-based ab initio calculations, free state geometries, deviations from coplanarity, and barriers to rotation of the central and peripheral rings were evaluated. Such barriers were also enumerated for the solid state of the cis,cis- and trans,trans-isomers. A single-molecule cluster surrounded by point charges sufficed to rationalize observed solid state properties in the trans,trans-isomer, including the quasi-free rotation of the central ring. A multimolecule cluster, however, was required to rationalize the restricted rotation of the rings in the cis,cis-isomer. MOPAC-AM1 methods were used to calculate geometries and energies of rotameric forms on the singlet photoisomerization path cis,cis → cis,trans → trans,trans. Finally, UV absorption wavelengths and oscillator strengths were calculated and the electronic structure of the states discussed. © 1996 by John Wiley & Sons, Inc.

Structural phase transitions in ‘hcp’ C70 single-crystals

The thermal expansion of vapor-grownC70 single crystals ahs been investigated using high-resolution capacitance dilatometry from 5–380 K. Measurements were made both parallel and perpendicular to the hexagonalc-axis. Three first-order phase transitions which we associate with the consecutive disordering of theC70 molecules are observed upon heating at ≈280 K (long-axis spinning), ≈300 K (long-axis precession) and ≈355 K (quasi-free rotation), respectively. The highest-temperature transition exhibits a very large (≈50 K) thermal hysteresis. Powder and single-crystal X-ray diffraction show that the crystals are predominantly hexagonal-close-packed (HCP) with an idealc/a≈1.63 above 360 K andc/a≈1.84 at 295 K.

Ab initio study of the deconjugation in tri-substituted BD 3 boron systems (D=OH, NH 2, BH 4)

Ab initio calculations have been performed on BD 3 systems with D=OH, NH 2, and BH 4. The energy curves for the synchronous rotation of the π-donor substituents about the B—D linkage have been computed in each case. For D=OH and D=NH 2, the lowest energy is found for θ=0° whereas the minimum is obtained for θ=27.5° for D=BH 4. In all cases, the energy curves are flat in the range 0°–30°. These results are discussed in the light of experimental data. A rationale of the quasi-free rotation is given through an orbital analysis.

Molecular Dynamics of the Muonium-C60Radical in SolidC60

The molecular dynamics and electronic structure of the μ+-C60 radical in crystalline C60 have been studied using muon spin rotation and relaxation. At room temperature μ+-C60 appears to be in a state of quasifree rotation. At the critical temperature TS=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on C60, suggesting that the molecular dynamics of μ+-C60 are strongly coupled to those of its C60 neighbors.

Molecular dynamics ofμ+-C60radical in solidC60

The molecular dynamics and electronic structure of the ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ radical in crystalline ${\mathrm{C}}_{60}$ have been studied using muon spin rotation and relaxation. At room temperature ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ appears to be in a state of quasifree rotation. At the critical temperature ${\mathit{T}}_{\mathit{S}}$=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on ${\mathrm{C}}_{60}$, suggesting that the molecular dynamics of ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ are strongly coupled to those of its ${\mathrm{C}}_{60}$ neighbors.