Colin J. Reid

Energies of triplet electronic states of the N2O2+ ion

The energies of triplet electronic states of N2O2+ have been measured by double-charge-transfer spectroscopy using OH+ and F+ as projectile ions. Peaks in the spectra correspond to double-ionization energies of 35.7 ± 0.4, 38.8 ± 0.4, 41.8 ± 0.4 and 43.1 ± 0.4 eV. The lowest energy is identified with the 3Σ– ground state. The energy of this state was calculated using second-order Moller–Plesset perturbation theory with the 3–21G basis set, and found to be 35.84 eV. The other three measured double-ionization energies can be identified with previously computed triplet-state energies.

The dynamics of liquid water: Simulation and submillimeter spectroscopy

Abstract A combination of classical molecular dynamics computer simulation and submillimeter laser and interferometric spectroscopy is used to investigate the dynamical and structural properties of liquid water at 300 K, ϱ = 0.997 gm cm −3 . A new approach to the empirical pair potential, based on the ST2 of Raman and Stillinger has been used to match the low frequency part of the submillimeter spectrum and to generate a range of auto and cross correlation functions. The results from this new atom-atom potential have been compared with those from the work of Stillinger and Rahman and Clementi et al. Broad agreement was obtained and the new potential used to investigate by computer simulation the statistical interdependence of the water molecules rotation and translation in liquid water at 300 K. This investigation has resulted in the detection of a.c.f.s of the classical Coriolis, centripetal, and “non uniform” accelerations of the water molecule, together with some of the numerous cross correlations suggested by the rotating frame theory of Langevin diffusion.

Model predictions of mean librational frequencies for rigid molecules in dilute decalin solution

Abstract The product of the reduced inertia J r of a dipolar molecule and the square of the far infrared absorption maximum wave-number ν max that it displays in dilute solution is shown, using a three-parameter Mori/itinerant oscillator model, to be related to the mean-square action on the molecule and hence by a simple argument to the solute volume of rotation V . Estimates of V have been made for seventeen rigid molecules (mainly substituted phenyls) and their ν max values have been measured in decalin solution at 293 and 110 K. Provided allowance is made for translation—rotation coupling in four cases, the derived relation I r ν 2 max = (8a a 2 c 2 kT ) −1 V 2 P (0) is found to be approximately obeyed at both temperatures with P (0) (the solute-independent mean-square torque acting on a molecule of unit V ) having a value of 4.0 × 10 17 (N m −2 ) 2 at both temperatures. It should now be possible to predict μ max for other solutes in decalin if their structures are known.

Molecular dynamics in Ch2Cl2/decalin glass

Abstract The dielectric loss in a glassy solution of CH 2 Cl 2 in decalin has been measured in the kHz and THz frequency ranges at 107–148 K. The low frequency part of the loss curve exhibits a peak which shifts upwards by about two decades with a 4 K increase in temperature, and at the glass to liquid transition temperature moves almost immediately out of the audio frequency range into the microwave. The far infrared peak in the loss is displaced by 30 cm −1 to 90 cm − in the glass as compared with the liquid solution at 293 K. These results are interpreted with a new version of itinerant libration whereby collective reorientations are treated as essential in explaining the complete loss profile in CH 2 Cl 2 /decalin glass which covers at least twelve decades of frequency. It is argued that in addition to the well documented primary and secondary losses in glasses and polymers, there exists in general a tertiary process at far infrared frequencies analogous to the Poley absorption in liquids. This is part of the dynamical evolution in a wide range of disordered solids.

Solvent shifts in the far i.r.

Abstract Solvent shifts in the far i.r. are rationalized in terms of a mean square torque parameter P (O). Solvent viscosity is not an important factor, although the relative shapes and sizes of solute and solvent are important. To some extent, the polarity of the medium may give rise to a greater mean square torque by dielectric drag, especially in pure dipolar liquids.

Zero-THz absorption profiles in glassy solutions. High frequency γ process and its characterisation

It is established that in addition to the well-documented α and β losses in viscous fluids, there exists a third (γ) process at THz frequencies (far infrared, or submillimetre) which is necessary for the overall time-correlation function of the dipole moment to be well-behaved at short times. The β and γ peaks of the zero-THz loss profile survive in the glassy state as remnants of the microwave and Poley absorptions observable in the liquid at ambient temperature. In order to probe the glassy environment in non-dipolar solvents such as decalin, the behaviour of the following solutes is monitored: dichloromethane (10 % v/v); dibromomethane (3 % v/v); tetrahydrofuran (10 % v/v); toluene (20 % v/v); chloroform (10 % v/v); fluoro-, chloro-, and bromo-benzene (10 % v/v). Some of these solutes are used as probes also into the weakly dipolar glassy environment of o-terphenyl and (10 % pyridine + 90 % toluene). The zero-THz absorption profiles of the solutes display a variety of behaviour above and below the glass transition temperature. For example, CH2Cl2+ decalin has a very pronounced β process in the viscous liquid which is absent from CH2Br2+ decalin. The β process gradually disappears across the halogenobenzene + decalin series from fluoro- to bromo-.The behaviour of the β and γ peaks for asymmetric, planar and intermediate solutes (e.g., CH2Cl2, halogenobenzene and tetrahydrofuran) in dilute solution in glassy decalin, for example, confirm that they are different expressions of the same overall dynamical evolution. For CH2Cl2+ decalin solid solutions the β peak moves very rapidly to high frequencies with increasing temperature. The associated Arrhenius activation enthalpy (ΔHβ) is thus extremely high compared with those of the larger molecules studied in the glass by Johari et al. and by Williams et al. There is a correspondingly dramatic shift in the THz(γ) peak to higher frequencies from its value in the liquid solution at 298 K to that in the glass at 107 K. In tetrahydrofuran + decalin glass ΔHβ is low and the shift in the γ peak much smaller. The same is true for the halogenobenzene + decalin glasses where present data for the β process are in excellent agreement with those of Johari et al., where available.A crude but tractable model capable of reproducing both β and γ processes of the glassy condition is that of itinerant libration.

The far i.r. absorption of a single crystal of pentachloronitrobenzene

Abstract A far i.r. spectrum for a single crystal of the disordered solid pentachloronitrobenzene (PCNB) is presented and used to characterise the absorptions observed in a pressed disc of the same substance. The spectrum, recorded with the hexad axes of the PCNB molecules aligned parallel to the transmitted beam, enables the four separate absorptions observed in this terahertz frequency range to be assigned to inter or intra-molecular modes of motion. The absorption at lowest frequency is attributed to molecular librations about the hexad axes whereas the high frequency absorptions are considered to be intramolecular arising from torsions of the NO 2 group.

Far infrared absorption of CH2Cl2 in isotropic and cholesteric solvents

The far infrared broad band absorptions of CH2Cl2, a highly dipolar molecule, have been measured carefully in isotropic and cholesteric solvents in order to bring out, by direct comparison, unusual dynamical effects on solute (probe) molecules dissolved in a mesophase. Whereas the CH2Cl2 band maximum (max) shifts by some 30 cm–1 to lower frequency upon dilution in both CCl4 and decalin, very little change occurs in cholesteryl linoleate and cholesteryl oleyl carbonate. This is attributed to an unusual persistence of statistical cross-correlation effects which vanish gradually in isotropic solvents. The observed integrated intensity per molecule (A/N) of CH2Cl2 is decreased significantly in a cholesteric environment, which may be indicative of an unusually large internal field effect. If this were not so then conservation of 〈µ2〉½, the apparent root mean square dipole, demands the enhancement of another rotational type absorption at frequencies other than those of the far infrared. The reorientation of CH2Cl2 in all environments is characterised approximately by itinerant oscillation in two dimensions.

Liquid state dynamics of CH2Cl2. Evaluation of models

Absorption data in the range up to THz frequencies for pure liquid CH2Cl2 and in isotropic solutions in cholesteryl linoleate are used to evaluate critically some recent modelling of the liquid state N-body problem, and to detect some prescience of cholesteric behaviour in monofringent solutions. Adjustable parameters are avoided as far as is necessary to produce the fundamental theoretical absorption contour. The models used can all be expressed as various early approximants of the Mori continued faction expansion of the Liouville equation, and therefore describe the absorption data accurately only at low frequencies, or equivalently at long times when the orientational autocorrelation function decays exponentially. In contrast to the confused and often contradictory evaluations drawn from the available literature on Raman, infrared and n.m.r. spectroscopy, together with depolarised Rayleigh wing and incoherent neutron scattering studies on liquid CH2Cl2 it is shown that the zero to THz absorption profile discriminates clearly between models such as rotational diffusion, extended rotational diffusion, planar itinerant libration and Brownian motion of coupled dipoles. A scheme is suggested for a self-consistent evaluation of the available data from all sources within the context of the Mori continued fraction. This aims at a satisfactory evaluation of such quantities as the mean square torque, and derivatives, so that some statistical assessment may be made of the potential part of the total N-particle hamiltonian. At present, features of observed spectra are reflected in model correlation times which are often physically meaningless and directly contradictory.

Far infrared solution spectra. Volume of rotation and effective torque

Far infrared absorption bands have been measured for about thirty-five dipolar solutes in liquid and glassy decalin. The results are interpreted in terms of a general relation between the volume of molecular rotation and mean square torque. The band frequency maxima (text-decoration:overlineνmax) are dependent at constant temperature only on the root mean square torque divided by the reduced moment of inertia (Ir) so that the data are conveniently classified in terms of the “torque product”Tq= 1038Irtext-decoration:overlineν2max. Denoting by V the volume swept out on molecular rotation the rule Tq∝V2 is obeyed generally both in the liquid and in the glass with a proportionality constant P= 1.38 × 1019 N m–2, the mean force per unit normal area of the rotating molecule. Although P(0) is independent of the temperature it is solvent dependent. For molecules such as C6H5Cl, C6H5Br, C6H5NO2, α-chloro- and α-bromo-naphthalene there are indications that the librational relaxation is strongly coupled to the translation of the centres of mass.