Journal of Chemical Physics

Reversible transitions between the two spectroscopically distinct phases (A and B phases) have been investigated for a new family of polydiacetylenes (PDAs) substituted with side groups of alkyl-urethane [−(CH2)4OCONH(CH2)n−1CH3:n=1−10]. Measurements of optical spectra as well as x-ray and calorimetric studies have revealed the first-order-like phase transitions in a series of PDAs with n=1–10 which are associated with an n-dependent thermal hysteresis ranging from 0 to 60 K in width. It has been found that the thermochromic changes become irreversible in all these PDAs once the hydrogen bond chains in the side groups are interrupted by heating beyond the polymer melting temperature.

The HeI photoelectron spectra of fluoroacetone, 1,1,1-trifluoroacetone, 1,1,1-trichlorotrifluoroacetone, 1,1,3-trichlorotrifluoroacetone, 1,1,3,3-tetrafluoroacetone, 1-chloro-1,1,3,3-tetrafluoroacetone, chloropentafluoroacetone, chloroacetone, and 3-bromo-1,1,1-trifluoroacetone have been determined and the vertical ionization energies measured. They have been compared with ionization energies calculated by CNDO/2 and MO assignments of the bands have been made on this basis. A correlation between the oxygen lone pair ionization energy and the number of fluorines substituted was observed. This paralleled a correlation between the width at half-peak height of the lone pair band and the lone pair ionization energy. The carbonyl stretching frequencies were determined and also found to correlate with the oxygen lone pair ionization energy. The linear plot suggested that the oxygen lone pair ionization energies are related to the sum of the effective electronegativities of the substituents. The first chlorine lone pair ionization energy and the carbonyl π were investigated to see if their ionization energies are related to the sum of the effective electronegativities of the substituents. In all three cases, an empirical relationship, IE= α (XA+XB)+β, was found to hold where α and β are constants determined by least squares fit of the data and whose value depends on the type of orbital considered. The values of α and β for the oxygen lone pairs of halogen substituted acetones were found to be 1.24 and 3.91, respectively. For the chlorine lone pairs, the values were found to be 1.16 and 5.89, while for the carbonyl π, they were found to be 1.98 and 3.17, respectively.

A dynamical approach to the classical decay rates for molecules near a dielectric sphere is presented through the application of the diffraction theory for a dipole antenna established by Van del Pol and Bremmer. This theory is somewhat simpler than but formally equivalent to that established by Ruppin and preserves a feature which is closer to the method of the theory established by Chance, Prock, and Silbey for a flat surface. The results, when compared to those obtained from the static image theory, show that this latter theory can be very inaccurate for large molecule-sphere distances or highly conducting spheres, consistent with previous findings for surfaces with perfect flatness or small roughness.

Two variations of Gaussian-2 (G2) theory are presented. In the first, referred to as G2 (MP2) theory, the basis-set-extension energy corrections are obtained at the 2nd order Mo/ller–Plesset (MP2) level and in the second, referred to as G2(MP3) theory, they are obtained at the MP3 level. The methods are tested out on the set of 125 systems used for validation of G2 theory [J. Chem Phys. 94, 7221 (1991)]. The average absolute deviation of the G2(MP2) and G2(MP3) theories from experiment are 1.58 and 1.52 kcal/mol, respectively, compared to 1.21 kcal/mol for G2 theory. The new methods provide significant savings in computational time and disk storage.

Studies of the effects of Cl2 and SF6 on the production of hydrogen from the x or γ radiolysis of gaseous HCl indicate that 2.5 ± 0.1 electrons per 100 eV form an intermediate Hα, which is not a thermal hydrogen atom. The remainder of the total yield of 4.0 electrons per 100 eV give rise to thermal hydrogen atoms. The results can be explained satisfactorily on the basis of the electron-capture reactions (6a) and (6c) (see text), where the sum of k6a and k6c lies in the range 5 × 10−32 to 2.6 × 10−30 ml2 molecule−2·sec−1. Hα is taken to be a clustered form of HCl− which either produces molecular hydrogen in further reactions with HCl molecules, or is scavenged by Cl2. The rate constant ratio kHα+HCl / kHα+Cl2 was 8 × 10−6 at − 77° ± 2°C and 1.2 ± 0.3 × 10−3 at 1° ± 1°C. Chlorine competes with HCl for the thermal hydrogen atoms formed in Reaction (6a) as well as for Hα, and at − 77° ± 2°C kH+HCl / kH+Cl2 was found to have a value of 2.6 ± 0.7 × 10−3, in good agreement with the data of Klein and Wolfsberg. SF6 cannot scavenge thermal hydrogen atoms and its effects are attributable entirely to a competition with HCl for electrons.

The formation of negative ions by resonance capture processes of the type XY+e=X+Y—, has been studied for the hydrogen halides. A monoenergetic electron source has been used and it has been possible to determine the true shape of the resonance capture peaks in these cases. It has been found that the negative ions appear at energies which are in agreement with recent values for the dissociation energies of the H—X bonds, and the electron affinities of the halogen atoms.

The ion-pair yield for the decomposition of gaseous HBr by 120-kV x rays has been determined from measurements of hydrogen yields and saturation ionization currents in an ionization chamber. At HBr pressures in the range 200 to 305 torr, M-HBr / N is 4.8 ± 0.1. Up to one-half of this yield may be due to the excitation of HBr molecules to neutral excited states, and their subsequent dissociation to Br and H atoms. Under the conditions employed, all H atoms are expected to undergo Reaction (7):H + HBr→H2+Br. (7) A study of the effects of SF6 indicated that the yield of scavengeable thermal electrons, which form hydrogen, is 3.1 per 100 eV. The results of competition experiments with this scavenger were incompatible with electron-positive-ion combination and the dissociative capture reaction, e + HBr→H + Br−, (3) as sources of hydrogen molecules or atoms. They may be explained in terms of thermal electron capture reactions which are second order in HBr. Physical evidence for at least one such process, viz., e + 2HBr→H + Br — H — Br−, (14a) has recently been obtained.

The swarm-beam technique has been employed to study the electron-attachment processes in HX and DX (X = halogen) molecules. Attachment cross sections as a function of electron energy σc(ε), corrected for the finite width of the electron beam, are reported for the direct dissociative attachment, i.e., HX(or DX) + e→H (or D) + X−. There is a strong increase in σc(ε) in going from HCl to HBr to HI, i.e., with decreasing energy of the dissociative-attachment peak. The ratio of σc(ε) for isotopic species is found to be very nearly equal to the square root of the inverse ratio of the reduced masses of the products. In addition to the direct dissociative electron attachment to HX and DX molecules, a separate attachment process occurs at thermal electron energies.

An infrared double resonance laser spectroscopic technique is used to study state-resolved rotational and vibrational energy transfer in the isotopically substituted methane molecule,13CD4 . Molecules are prepared in a selected rovibrational state by CO2 laser pumping, with the quantum numbers v, J, and Cn completely specified. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v′, J′, Cn′ ) are carried out using time-resolved tunable diode laser absorption spectroscopy. The depopulation rates due to collisions between methane and the rare gases are on the order of the Lennard-Jones collision frequencies. Self-relaxation is slightly more efficient than the Lennard-Jones estimate. The rather small relaxation rates are characteristic of a short-range potential, or ‘‘strong-collision’’ regime, expected for a molecule without a dipole moment. The state-to-state energy transfer measurements reveal a dramatic selectivity of rotational energy transfer pathways with respect to the fine-structure rotational states Cn . Relaxation occurs through a surprisingly small subset of the energetically accessible pathways. Also suggested is a preference for population transfer to occur within the initial vibrational angular momentum sublevel of the ν4 (F2 ) vibrational state, which has three sublevels in consequence of Coriolis interaction. This preference can be formulated as a propensity for Δ(R−J)=0 transitions. We deduce that large changes of J(ΔJ∼5) can occur in single collisions between methane molecules, based on a simple kinetic model of the data. This is also characteristic of

short-range collisions in which it is likely that no single multipolar interaction dominates. Collisional relaxation between the ν4 and ν2 bending vibrations proceeds more slowly than rotational relaxation, but as fast as transfer among the closely grouped stretching and bend-overtone levels, measured previously in CH4 . No evidence for rotationally specific V–V transfer is found. We discuss an exhaustive spectroscopic analysis of 13CD4 that provides unambiguous spectral assignments for use in detecting vibrationally excited molecules (v4 =1) in specific rotational states.

The shift in chemical equilibria due to isotope substitution is frequently exploited to obtain insight into a wide variety of chemical and physical processes. It is a purely quantum mechanical effect, which can be computed exactly using simulations based on the path integral formalism. Here we discuss how these techniques can be made dramatically more efficient, and how they ultimately outperform quasi-harmonic approximations to treat quantum liquids not only in terms of accuracy, but also in terms of computational cost. To achieve this goal we introduce path integral quantum mechanics estimators based on free energy perturbation, which enable the evaluation of isotope effects using only a single path integral molecular dynamics trajectory of the naturally abundant isotope. We use as an example the calculation of the free energy change associated with H/D and 16O/18O substitutions in liquid water, and of the fractionation of those isotopes between the liquid and the vapor phase. In doing so, we demonstrate and discuss quantitatively the relative benefits of each approach, thereby providing a set of guidelines that should facilitate the choice of the most appropriate method in different, commonly encountered scenarios. The efficiency of the estimators we introduce and the analysis that we perform should in particular facilitate accurate ab initio calculation of isotope effects in condensed phase systems.