DeLin Shen

Stimulation of diesel-fuel ignition by benzyl radicals☆

Benzyl radicals are often regarded as being inert, and not capable of initiating chain reactions. However, during the preignition phase in a diesel engine, the benzylperoxy radical, formed by the addition of O{sub 2} to the benzyl radical can be present in significant proportions. Using the most recent data, the equilibrium constant for the formation of the adduct is about 6.6 atm{sup {minus}1} at 600 K, and about 0.066 atm{sup {minus}1} at 800 K; thus, if the partial pressure of O{sub 2} is 5 atm, 97% of the benzyl radicals are in the peroxy form at 600 K, 25% at 800 K. One possible fate of the benzylperoxy radical, proposed to explain the formation of benzaldehyde in the low-temperature oxidation of toluene between 723 and 788 K, is the release of an OH radical. In this Note, the authors demonstrate that at low preignition temperature, dibenzyl mercury and {omega},{omega}{prime}-azo-toluene are more effective ignition promoters than is the usual diesel-fuel ignition improver, iso-octyl nitrate. They also show, by using ab initio molecular orbital calculations, that there is a plausible mechanism by which thermal decomposition of the benzylperoxy radical could lead to the production of OH radicals.

Sensitivity of molecular dynamics unimolecular rate calculations to defects in the potential-energy surface

We report a detailed examination of the reasons why classical trajectory calculations of the rate of isomerisation of methyl isocyanide to methyl cyanide fail, by at least an order of magnitude, to match the observed results. We conclude that the internal motions in the CH3NC molecule are essentially chaotic, and the discrepancy stems from an inadequate degree of coupling between the vibrational motions on the assumed potential-energy surface and the reaction coordinate. Some consequences of these flaws are quite unacceptable: that the rate of decay is lower the greater the average potential energy; or that there is a severe departure from the random-gap law for the first few thousand wavenumbers above threshold.The difficulties impeding a full and satisfactory calculation of the rate of a unimolecular reaction of a polyatomic molecule by classical trajectory methods are outlined.

Randomization and isomerization rate constants for isocyanogen

Classical trajectory calculations have been performed on the isomerization of isocyanogen to cyanogen using an ab initio MP2/6-311G* potential-energy surface. By running two parallel sets of calculations at each value of the energy, one in which the trajectories were started with the atoms in a standard configuration but having random momenta, and the other in which they were started with the atoms in random positions but having no momenta, it is possible to deduce both the randomization and the isomerization rates. The randomization rates are directly proportional to the density of states at the energies concerned, i.e., analogous to the Fermis ‘golden rule’ normally applied to continuum state mixing. The isomerization rate, after allowance for the exclusion of zero-point energy pooling, is approximately one-third of the transition-state rate, calculated with the same potential-energy parameters, over the temperature range 400–600 K.

Molecular rotation and isomerisation in hydrogen isocyanide

Molecular rotation appears to accelerate the randomisation of energy in the HNC molecule, but in the limit of very high rotational energy, it reduces the overall rate of isomerisation. Since the rotational motion is highly regular, but the vibrational motion is only partly so, various qualitative indicators of chaotic behaviour give conflicting diagnoses.Parallel calculations on the isomerisation of isocyanogen (CN—CN) to cyanogen (NC—CN) reveal that isocyanogen behaves more like the small molecule HNC than the large molecule CH3NC.

THEORETICAL CALCULATION OF INTRAMOLECULAR REACTIONS IN METHYLPEROXYL AND ETHYLPEROXYL RADICALS

Ab initio molecular orbital calculations on the intramolecular rearrangements of the methylperoxyl and ethylperoxyl radicals are reported, together with transition-state structures and vibration frequencies for the ethylperoxyl reactions. Estimated equilibrium constants for the formation of two methylperoxyl isomers, CH(OH)2 and CH2(OH)O, are also reported.

Effect of rotation on molecular shape

The moment of inertia of CH3NC about the z axis is very small, and the rotational spacings for increasing quantum number K are large. In the T-shaped transition state for isomerization to CH3CN, on the other hand, both the N and C atoms lie off the axis, and the moment of inertia for rotation in this direction becomes large; thus, for a given amount of angular momentum, the energies of the stable molecules are raised very much more than the energy of the transition state. In the limit of very high K, triangular C-N-C configurations become the most stable, and the two linear arrangements become transition states. As a result, the equilibrium arrangements of the heavy atoms in CH3NC and, to a lesser extent in CH3CN, become nonlinear at quite moderate rotational energies, and the possibility of detecting this distortion experimentally is discussed.

Zero-point energy problem in molecular dynamics calculations, and an improved method for sampling trajectories

Methods proposed recently to overcome the zero-point energy problem in classical trajectory calculations are unsuitable for use with non-RRKM systems because they can provide an efficient route to energy randomisation. A new method of sampling, which virtually eliminates the problems associated with direct trajectories, is also described.

Classical dynamics of intramolecular energy flow in ethane

Classical trajectory calculations on an ab initio potential surface are reported for the ethane molecule. Most calculations were performed for molecules possessing high rotational energy, with the initial conditions chosen to be consistent with rotational quantum numbers in the range J⩽ 280 and 0 ⩽K≲ 60. The initial energies were usually chosen to be above the dissociation limit for the process C2H6→ 2CH3, and the conditions under which separation could occur, even when classically forbidden, were explored. During the trajectory, instantaneous values of the rotational quantum numbers, J, K, and of the vibrational angular momentum quantum number J(v), vary with time. These variations are small, but it is shown that they can be sufficient, at some energies, for the height of the centrifugal barrier to fall below the value of the total energy of the molecule, allowing it to dissociate when the initial conditions were such that it could not.

Distribution of vibrational potential energy in molecular systems

It is shown that for a collection of n classical harmonic oscillators, the long-time distribution of potential energies P is approximated by sinm(πP) for n⩾4, where m=(8n/π2−1/√2) and P is scaled to lie between 0 and 1. As n→∞, the distribution tends to a δ-function centered about P=0.5. When coupling is present between the oscillators, the effective value of m is reduced, so that the breadth of the potential energy distribution reflects the degree of randomization in the system.

Faraday communications. Rotational disintegration of polyatomic molecules

Classical trajectory calculations on the relative nuclear motions in the J= 280 rotational state of ethane show that the molecule is stable when the quantum number K is less than ca. 50. For higher values of K, the molecule breaks up into two methyl radicals, with a lifetime which decreases as K increases: at K= 120, the lifetime is ca. one rotational period, even though the total energy is below the top of the effective centrifugal barrier.