Structure and properties of isolated liquid crystal molecules: jet spectroscopy and ab initio calculations of 4-cyanobiphenyl

We explore the structure, torsional frequencies and electronic properties o f the gas phase 4-cyanobiphenyl molecule, a prototypical liquid crystal cor e fragment, in its ground and first excited singlet electronic states. We e mploy a methodology which combines nb initio quantum mechanical calculation s and fluorescence spectroscopy of laser-desorbed, jet-cooled molecules. Th e aim is to test the predictive power of parameter-free calculations of str ucture and dynamics in an experimental environment which is similar to the computational conditions of low thermal activation and negligible intermole cular interaction. Both spectroscopic and computational results indicate th at very large molecular conformational changes accompany the optical n --> pi* transition. These are found to have a significant influence on the mole cular flexibility and electronic polarity.