The crystallographically determined bond length, valence angle, and torsion angle information in the Cambridge Structural Database (CSD) has many uses. However, accessing it by means of conventional substructure searching requires nontrivial user intervention. In consequence, these valuable data have been underutilized and have not been directly accessible to client applications. The situation has been remedied by development of a new program (Mogul) for automated retrieval of molecular geometry data from the CSD. The program uses a system of keys to encode the chemical environments of fragments (bonds, valence angles, and acyclic torsions) from CSD structures. Fragments with identical keys are deemed to be chemically identical and are grouped together, and the distribution of the appropriate geometrical parameter (bond length, valence angle, or torsion angle) is computed and stored. Use of a search tree indexed on key values, together with a novel similarity calculation, then enables the distribution matching any given query fragment (or the distributions most closely matching, if an adequate exact match is unavailable) to be found easily and with no user intervention. Validation experiments indicate that, with rare exceptions, search results afford precise and unbiased estimates of molecular geometrical preferences. Such estimates may be used, for example, to validate the geometries of libraries of modeled molecules or of newly determined crystal structures or to assist structure solution from low-resolution (e.g. powder diffraction) X-ray data.

The enCIFer program permits the location, reporting and correction of syntax and format violations in single-or multi-block crystallographic information ®les (CIFs). The program also permits the editing of existing individual or looped data items and the addition of new data in these categories, and provides dataentry wizards for the addition of two types of standard information for smallmolecule structural studies, namely publication data and chemical and physical property information. Facilities for the graphical visualization and manipulation of structure(s) in a CIF are also provided.

An algorithm for the computation of initial relaxation directions (IRD) from the tip of a conical intersection is discussed. The steepest descent paths that can be computed starting from these IRD provide a description of the ground state relaxation of the "cold" excited state species that occur in organic photochemistry where slow motion and/or thermal equilibration is possible (such as in cool jet, in matrices, and in solution). Under such conditions we show that the central conclusions drawn from a search for IRD and those obtained from semiclassical trajectory computations are the same. In this paper, IRD computations are used to investigate the mechanism of photoproduct formation and distribution in the photolysis of cyclohexadiene (CHD) and cZc-hexatriene (cZc-HT). A systematic search for the IRD in the region of the 2A 1 /1A 1 conical intersection (see Celani, P.; Ottani, S.; Olivucci, M.; Bernardi, F.; Robb, M. A. J. Am. Chem. Soc. 1994, 116, 10141-10151) located on the 2A 1 potential energy surface of these systems yields three relaxation paths. The first two paths, which start in the strict vicinity of the intersection, are nearly equivalent energetically and lead to production of CHD and cZc-HT, respectively. The third path, which begins at a much larger distance, lies higher in energy and ends at a methylenecyclopentene diradical (MCPD) minimum. Further, while the first two paths define directions that form a 60°angle with the excited state entry channel (i.e. the direction along where the conical intersection region is entered), the third path is orthogonal. It is shown that these findings are consistent with the experimental observations which show nearly equivalent quantum yields for CHD and cZc-HT and no production of MCPD. The results of the IRD computations have been validated by investigating the decay dynamics of trajectories starting from a "circle" of points around the conical intersection, with the initial kinetic energy distributed in randomly sampled vibrational modes. These computations have been carried out using a trajectory-surface-hopping (TSH) method and a hybrid molecular mechanics valence bond (MM-VB) force field to model the ab initio potentials.

CASSCF and CASMP2 calculations show that the minimum on the fulvene S 1 potential energy surface is an S 0 /S 1 conical intersection with a 90°twisted methylene group. We have also located a distinct planar azulenelike crossing point at higher energy, where the methylene is free to twist. The fulvene intersectionswhich exists for all twist anglessleads to efficient, irreversible radiationless decay and explains the lack of observed S 1 fluorescence. We have modeled the femtosecond excited state motion leading to ultrafast decay that would be initiated by exciting the 0-0 and higher vibrational levels using semiclassical molecular dynamics with a hybrid quantum-mechanical/ force-field potential (MMVB). Our simulation suggests that, with increased vibrational energy, decay occurs in the planar crossing region before relaxation by twisting can take place, and that isomerization might only be seen by pumping the 0-0 transition in laser studies.