Determining molecular structures and conformations directly from electron diffraction using a genetic algorithm.

A global optimization strategy, based upon application of a genetic algorithm (GA), is demonstrated as an approach for determining the structures of molecules possessing significant conformational flexibility directly from gas-phase electron diffraction data. In contrast to the common approach to molecular structure determination, based on trial-and-error assessment of structures available from quantum chemical calculations, the GA approach described here does not require expensive quantum mechanical calculations or manual searching of the potential energy surface of the sample molecule, relying instead upon simple comparison between the experimental and calculated diffraction pattern derived from a proposed trial molecular structure. Structures as complex as all-trans retinal and p-coumaric acid, both important chromophores in photosensing processes, may be determined by this approach. In the examples presented here, we find that the GA approach can determine the correct conformation of a flexible molecule described by 11 independent torsion angles. We also demonstrate applications to samples comprising a mixture of two distinct molecular conformations. With these results we conclude that applications of this approach are very promising in elucidating the structures of large molecules directly from electron diffraction data.     

Cover Picture

The cover picture shows the experimental principles of femtochemistry, the field concerned with the real-time observation of physical, chemical, and biological changes on the femtosecond time scale. The clocking of the femtosecond events is made using laser pulses, one to initiate the change and others to take snapshots. Studies of the motion at atomic-scale resolution provide a telescopic view of the molecular world. More information about this fascinating topic is described by A. H. Zewail in his Nobel Lecture on page 2586 ff. It is quite fitting that at the back of this issue is the inaugural issue of ChemPhysChem, announced in earlier editorials.     

Magnetic sensitivity of the 3330.8 Å state of s-triazine

We report more precise measurements of the magnetic field effect on the supposed first singlet state of s-triazine at 3330.8 Å, and find a g-factor of 0.11 ± 0.08 assuming the observed effects are caused by a splitting. We also report new results that are in agreement with previous work on the magnetic dipole nature of the transition, but a concern about the assumed cylindrical symmetry of the 4.2°K s-triazine crystal is raised.     

Measurements of molecular dephasing and radiationless decay by laser-acoustic diffraction spectroscopy

In this paper we present a method for the measurements of molecular dephasing and radiationless decay in gases and solids. The technique utilizes a single-mode of a tunable dye laser and an extracavity acousto-optic element. The latter diffracts the laser beam into or out of the sampe for finite times when a train of radio frequency pulses is fed into the transducer. Using this laser-acoustic diffraction spectroscopy (LADS) we report some new results on iodine gas, and pentacene in a p-terphenyl host at 1.8 K. Combining these new results of LADS on pentacene and our earlier findings using the frequency switching method we present an analysis for the effect of laser bandwidth on the decay of the prepared excited states.