Observation and Geometry Assignment of Conformations of Styrenes in the Ground and First Excited Singlet State

摘要

Despite considerable experimental and theoretical efforts expended over the last decade, the minimum energy conformations of styrene (1) and substituted styrenes have not been unambiguously determined. Though not proven, the planarity of 1 (i.e., Tau = 0 degrees in 2) appears to be the best explanation for the available data. The barrier to rotation about C(ipso) -C(a) bond for styrene has been estimated to be 2-3 kcal/mol, too low to observe signals for specific conformations by dynamic nuclear magnetic resonance (DNMR). Numerous hindered styrenes and 1-vinyl-naphthalenes have been examined, and for sterically congested systems, nonplanar conformations have been observed and identified by DNMR studies. Herein, we report the use of supersonic molecular jet laser spectroscopy (1) to establish the coplanarity of the vinyl and aromatic ring systems in both S(0) and S(1) in sterically unencumbered styrenes, (2) to observe two minimum energy conformations for an asymmetrically substituted styrene, (3) to demonstrate the nonplanarity (Tau approx. 30 degs.) of a methylstyrene (3) in its ground electronic state S(0), and (4) to demonstrate the planarity (Tau approx. 30 degs.) of 3 in its first excited singlet state S1. Reprints. (aw)