Ultrafast diffraction and structural dynamics: The nature of complex molecules far from equilibrium

• DOI: 10.1073/pnas.131192898
• Publication Date: 2002-07-26T14:37:36Z
• Authors: Chong-Yu Ruan; Vladimir A. Lobastov; Ramesh Srinivasan; Boyd M. Goodson; Hyotcherl Ihee; Ahmed H. Zewail
• Keywords: 1,3,5-hexatriene; di-s-cis-Z-hexatriene; di-s-trans-Z-hexatriene; 540; 541; 530; 01 natural sciences; 0103 physical sciences; 1,3,5-cycloheptatriene; 1,3- cyclohexadiene; mono-s-cis-Z-hexatriene; ultrafast electron diffraction; Caltech Library Services

Studies of molecular structures at or near their equilibrium configurations have long provided information on geometry in terms bond distances and angles. Far-from-equilibrium are relatively unknown—especially for complex systems—and generally, neither dynamics nor average geometries can be extrapolated from values. For such nonequilibrium structures, vibrational amplitudes play a central role phenomena as energy redistribution chemical reactivity. Ultrafast electron diffraction, which was developed to study transient provides direct method probing the nature molecules far equilibrium. Here we present our ultrafast diffraction observations two cyclic hydrocarbons. At high internal energies ≈4 eV, these display markedly different behavior. 1,3,5-cycloheptatriene, excitation results formation hot ground-state with similar those initial structure, but nearly three times amplitude. Energy is redistributed within 5 ps, negative temperature characterizing population. In contrast, ring-opening reaction 1,3-cyclohexadiene shown result C—C distance over 1.7 Å, 0.2 Å away any expected value. Even up 400 remains trapped large-amplitude motions comprised torsion asymmetric stretching. These studies promise new direction studying structural systems.