Colin Steel

Collisional activation of cyclobutene by hexafluorobenzene: A chemical probe for highly energetic collisions in reactive systems

A new technique for studying reactive collisions is reported. A vibrationally hot donor molecule excites a cold reactant molecule via a single supercollision, transporting it above its threshold energy for reaction. In addition, this technique provides a means by which the functional form of transition probabilites can be found experimentally. As a specific example, vibrationally excited (110 kcal mol−1) electronic ground state hexafluorobenzene, produced by UV excitation followed by internal conversion, is the donor of vibrational energy in the collisional excitation of two acceptors, cyclobutene and cyclopropane. For cyclobutene φsens, the quantum yield of 1,3‐butadiene formation in terms of light absorbed by the donor, increased markedly as the total pressure decreased. At the lowest pressures employed, 20 mTorr, φsens=(9.1±0.6)×10−4. The data are discussed in terms of highly energetic (strong) collisions which transfer enough energy to the cyclobutene to bring molecules above the critical energy for r...

High Frequency Factors in Unimolecular Reactions

The high frequency factors which are associated with certain unimolecular reactions are discussed in terms of an extended Kassel‐type model, in which the critical energy is localized in more than one oscillator.

Chemical thermometers in megawatt infrared laser chemistry: The decomposition of cyclobutanone sensitized by ammonia

Abstract Ammonia excited by CO 2 laser radiation may be used to sensitize unimolecular reactions of compounds which do not absorb the radiation directly at the chosen frequency. If cyclobutanone is present as a minor component then analysis of the products formed yields information as to the effective temperature reached by the system and the time the latter remains at this temperature.

Exchange-induced resonance energy transfer

Data for singlet-singlet resonance energy transfer from benzene and naphthalene to some azo compounds and biacetyl are explained in terms of electron exchange. An estimated value of 30 cm−1 for the exchange interaction leads to satisfactory agreement with experiment.

Triplet-sensitized photobleaching of crystal violet

Abstract Crystal violet (CV + ) and five related triarylmethane dyes quench triplet benzophenone ( 3 K) in acetonitrile at diffusion-controlled rates ( k q ∼ 1 × 10 10 M −1 s −1 ). The interaction with CV + results in bleaching of CV + with a quantum yield, K Φ (−CV + ), dependent on the concentration of CV + with a limiting value of 0.2 at high [CV + . Benzopinacol (K 2 H 2 ) is also formed in this reaction but its yield decreases as [CV + ] increases. In contrast, although both triplet anthracene ( 3 An) and triplet naphthalene ( 3 Nap) are also efficiently quenched by CV + at diffusion-controlled rates, there is only very inefficient fading, An Φ (−CV + ) = 1.1 × 10 −5 . These results are interpreted in terms of a mechanism in which energy transfer to form triplet crystal violet, 3 CV + , is the only major channel in the case of anthracene and naphthalene, i.e. 3 CV + (like 1 CV + ) is unreactive. However, in the case of benzophenone, reduction of 3 K by CV + with a rate constant k h = 1.4 × 10 9 M −1 s −1 to form ketyl radicals (KH . ) can compete (about 14%) with this energy transfer. The ketyl radicals so formed can either reduce CV + to form leuco crystal violet (CVH) or form benzopinacol.

Laser induced excitation of ammonia at 1076 cm−1 in the megawatt region

Absorption of high‐power CO2‐laser radiation by 5–244 Torr of ammonia has been studied. The incident infrared radiation energy was varied in the range 0.04–0.70 J cm−2 pulse−1 corresponding to average power levels of 0.2–3.5 MW cm−2, respectively. It is shown that experimental measurements can conveniently be summarized using an empirical absorption law, which is a modified Lambert–Beer law, written in terms of fluences or doses. A kinetic model is analyzed in relation to the data thus presented.

The effects of temperature and pressure on the lifetime of benzophenone emission

Abstract The variation in the lifetime of flash-excited gaseous benzophenone with pressure and temperature indicates that (1) self-quenching is a relatively inefficient process for the long-lived emission, k sq = 9 × 10 5 M −1 s −1 (estimated from solution data) at 25°C and 1.2 × 10 7 M −1 s −1 at 170°C and (2) the lifetime decreases with increasing temperature as a result of photochemical and photophysical decay pathways which have significant activation energies. The importance of diffusion to the walls on lifetime measurements is discussed.

Inverted region in intermolecular electronic energy transfer

Abstract A critique of some recent publications (concerning the so-called inverted region in exchange-induced electronic energy transfer) is presented. It is first pointed out that the phenomena reported therein have in fact been known for nearly two decades; next, a derivation of the expression (used in the aforementioned communications) for k EnT, the transfer rate constant, is presented; finally, it is stressed that this formula cannot provide reliable quantitative predictions.

A thermodynamic analysis of the first solvation shells of alkali and halide ions in liquid water and in the gas phase

Abstract Let n denote the number of water molecules in the nearest-neighbour shell (NS) of an ion J± in liquid water, and denote J± ·nH2O in the gas phase by J± · NSG (g). The standard free energy of hydration ΔG°hyd (J± ·NSG) can then be deduced by a thermodynamic cycle involving ΔG°n for the formation of J± · NSG (g) and ΔG°hyd for the transfer of J± (g) to water. Values of ΔG°hyd (J± ·NSG) for alkali and halide ions are substantial, ranging from 48 % to 86 % of ΔG0 hyd. The values of ΔG0 hyd(J± ·NSG) can be accounted for largely by the calculated work—electrostatic (ΔWelec) and surface (ΔWsurt)—in the process J± ·NSG (g) → J± (aq). ΔWelec is the major contributor. ΔWsurf depends on whether (i) J± ·NSG (g) can be represented by a cluster consisting of the ion and n separate water molecules, or (ii) there is some molecular complex formation within that cluster. In fact, ΔWsurt < 20 kJ mol−1 for the alkali ions and of the order of 100 kJ mol−1 for the halide ions. A reasonable case can be built that the a...

Collisional Effects in IRMPD of Medium .Sized Molecules

Abstract Absorption and dissociation measurements for the infrared multi-photon decomposition (IRMPD) of 1,3,5-trioxane are reported. Although at a given fluence the energy absorbed per molecule is independent of pressure, the dissociation yield per pulse is markedly pressure dependent. A simple quantitative model is developed to account for this in terms of a combination of (1) de-Boltzmannization of the population in the “active” levels from which reaction originates (2) pressure dependence in the rate of cooling of the excited gas by thermal conduction. Data for cyclobutanone, cis-3,4-dichlorocyclobutene and ethyl acetate are similarly explained.