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Available experimental data on the collisional energy transfer per collision (A@ between the reactant molecules benzene, toluene, azulene, and cycloheptatriene and various medium molecules are analyzed in terms of the concept of energy transfer efficiency ,& . This quantity is the ratio of the experimental and the statistical limit value for (AE) as obtained by the assumption of a microcanonical energy redistribution. Although the statistical limit is typically more than 1 order of magnitude too high, it correlates very strongly with the experimental values with respect to the dependence on molecular complexity, reactant energy, and medium temperature. We focus here on molecular complexity and show that the experimental data are very well represented by a simple geometrical correlation model wherein each of the two colliding molecules is represented by its own efficiency factor. The model may be used to extrapolate experimental data. It also gives physical insight into the factors determining energy transfer efficiency and can be extended to incorporate more accurate statistical theories.

Serum triazolam levels were determined in eight geriatric patients (average age 80 years) on Days 1 and 7 of administration of triazolam 0.25 mg once daily, 1 h after a standard breakfast. Triazolam was rapidly absorbed reaching average peak concentrations of 2.0 and 2.04 ng/ml, 1.5 and 1.38 h after administration on Days 1 and 7, respectively. The mean apparent elimination half-life was 1.41 h (range 0.73-4.13 h) on Day 1 and 1.37 h (range 0.69-3.36 h) on Day 7. There was no significant difference between mean serum triazolam concentrations or pharmacokinetic parameters on Days 1 and 7 of the treatment. Serum samples were also assayed for alpha-hydroxytriazolam, an active metabolite of triazolam, but none could be detected in any of the samples from Days 1 or 7, assay sensitivity 0.09 ng/2 ml serum. The range of half-lives of triazolam in the patients in the present study is in close agreement with that previously reported in elderly subjects. The study provides further evidence of the lack of change in pharmacokinetic parameters on multiple dosing and that drug accumulation did not occur.

Words: Computer Experiments / Elementary Reactions / Energy Transfer / Molecular Collisions Statistical TheoryThe RRKM theory of unimolecular reaction rates is a statistical mechanical theory based on an assumption of microcanonical equilibrium in the reactant phase space. The energy transfer in reactant medium collisions was originally described by a canonical strong collision assumption, i.e., an assumption of full thermal equilibration in each collision. In our work we first introduce a microcanonical strong collision assumption which gives the RRKM theory a consistent form. we then introduce parametrizations of the degree of weakness (nonergodicity) of the collisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active reactant and medium degrees of freedom. The corresponding partially ergodic collision theory (PECT) yields physical functional forms of the collisional energy transfer kernel P ( E ' , E ) . In order to resolve the energy and temperature dependence and the dependence on interaction strength a multiple encounter theory is introduced (PEMET). Initially each encounter may be described by a semiempirical PECT model. Eventually the encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach/quantum encounter) type. Simple statistical collision models only distinguish between "hits and misses". In reality the energy transfer efficiency exhibits characteristic fall off with increasing impact parameter b. This b-dependence can be explicitly accounted for in the master equation for the reaction rate coefficient.

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