Structural Issues in Conjugated Hydrocarbons: High-Resolution Infrared Slit-Jet Spectroscopy of <i>trans</i>-1,3-Butadiene

Abstract

First high-resolution spectroscopic and structural data for jet-cooled trans-1,3-butadiene have been obtained using sub-Doppler, direct absorption tunable difference frequency infrared laser methods in a pulsed slit supersonic expansion. With reduced spectral congestion at Trovib = 15 K, high-resolution vibration−rotation spectra around 3100 cm-1 reveal two hybrid a/b type bands, both of which have been successfully rotationally analyzed and yield first experimental structural information on the ground state of trans-1,3-butadiene. The strong band ν17(bu) at 3100.63 cm-1 is predominantly due to “bright” state excitation of the in-phase asymmetric CH2 stretch, with the weaker band at 3096.14 cm-1 probably arising from a “dark” state anharmonic mixing with ν17. Furthermore, there are many local perturbations in the spectrum that can be successfully explained via anharmonic and Coriolis type coupling between ν17 and an additional dark state centered at 3100.65 cm-1. Intensity analysis for the strong and weak bands permits a- and b-type transition moment ratios to be extracted and compared with results from electronic structure calculations.