Cavities in molecular liquids and the theory of hydrophobic solubilities

Thermal configurational data on neat liquids are used to obtain the work of formation of hard spherical cavities of atomic size in six molecular solvents: n-hexane, n-dodecane, n-undecyl alcohol, chloroform, carbon tetrachloride, and water. These results are used to test a recent suggestion that the differences between nonaqueous solvents and liquid water in solvation of inert gases are not principally due to the hydrogen-bonded structure of liquid water but rather to the comparatively small size of the water molecule. The frequencies of occurrence of cavities in liquid water can be meaningfully distinguished from those in the organic solvents. Liquid water has a larger fractional free volume, but that free volume is distributed in smaller packets. With respect to cavity work, water is compared to a solvent of the same molecular density and composed of hard spheres of the same size as the water molecule. That comparison indicates that the hard-sphere liquid finds more ways to configure its free volume in order to accommodate an atomic solute of substantial size and thus, would be more favorable solvent for inert gases. The scaled particle model of inert gas solubility in liquid water predicts cavity works 20% below the numerical data for TIP4P water at 300 K and 1.0 g/cm3 for cavity radii near 2.0 angstroms. It is argued that the sign of this difference is just the sign that ought to be expected and that the magnitude of this difference measures structural differences between water and the directly comparable hard-sphere liquid. In conjunction with previous data, these results indicate that atomic sized cavities should be considered submacroscopic.     

Low-energy dissociative recombination in small polyatomic molecules

Indirect dissociative recombination of low-energy electrons and molecular ions often occurs through capture into vibrationally excited Rydberg states. Properties of vibrational autoionization, the inverse of this capture mechanism, are used to develop some general ideas about the indirect recombination process, and these ideas are illustrated by examples from the literature. In particular, the Δv = -1 propensity rule for vibrational autoionization, i.e., that vibrational autoionization occurs by the minimum energetically allowed change in vibrational quantum numbers, leads to the prediction of thresholds in the dissociative recombination cross sections and rates at the corresponding vibrational thresholds. Capture into rotationally excited Rydberg states is also discussed in terms of recent low-temperature studies of the dissociative recombination of H(3)(+).     

A letter signed: the very beginnings of Dalton's atomic theory

This paper explores the provenance and content of a previously unknown personal letter by John Dalton (1766-1844), which is dated 12 April 1803. It relates to a startling breakthrough in Dalton's research, which pre-dates by five months the earliest date in his laboratory notebook, namely, 6 September 1803. The author acquired the letter about thirty years ago, and now offers it to the public. He makes no attempt to explain how it contributes to--or even changes--our understanding of Dalton, but leaves that privilege to Dalton scholars.     

Spin dynamics in conducting polymers studied by μSR

We report studies of spin dynamics in the conducting polymers polyaniline and polypyrrole using both μ⁺SR and μ^-SR techniques. These measurements reveal characteristic field dependences and cutoff frequencies for the muon spin relaxation which can be related to the spin diffusion process. Clear evidence is seen for increased spin localisation at low temperatures where a crossover occurs from two or three dimensional spin diffusion to a one dimensional diffusion regime. This revised version was published online in August 2006 with corrections to the Cover Date.     

Microwave and UV excitation spectra of 4-fluorobenzyl alcohol at high resolution. S0 and S1 structures and tunneling motions along the low frequency -CH2OH torsional coordinate in both electronic states

Rotationally resolved electronic spectra of several low frequency vibrational bands that appear in the S(1) ← S(0) transition of 4-fluorobenzyl alcohol (4FBA) in the collision-free environment of a molecular beam have been observed and assigned. Each transition is split into two or more components by the tunneling motion of the attached -CH(2)OH group. A similar splitting is observed in the microwave spectrum of 4FBA. Analyses of these data show that 4FBA has a gauche structure in both electronic states, but that the ground state C(1)C(2)-C(7)O dihedral angle of ～60° changes by ～30° when the photon is absorbed. The barriers to the torsional motion of the attached -CH(2)OH group are also quite different in the two electronic states; V(2) ～ 300 cm(-1) high and ～60° wide in the S(0) state, and V(2) ～ 300 cm(-1) high and ～120° wide (or V(2) ～ 1200 cm(-1) high and ～60° wide) in the S(1) state. Possible reasons for these behaviors are discussed.     

Cer

Ohne Zusammenfassung     

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.     

A microwave-excited emissive detector for gas chromatography Further studies with sulphur compounds

Improvements in the design and operation of the microwave excited detector for gas chromatography have led to an increase in the sensitivity and a lowering of detection limits for sulphur compounds.     

Gamma-ray spectroscopy of87Y

The level structure of⁸⁷Y has been studied using in-beam γ-ray spectroscopic methods. Many previously unknown states have been observed. Members of theπ(g _9/2 )ν(g _9/2)^?2 multiplet are identified. The results are discussed in terms of the weak-coupling model.