[MOS] May 5, 2015, Patrick Vaccaro, Yale University

[Zina M Queen]

Seminar on

Modern Optics and Spectroscopy

 

Optical rotatory dispersion:  New twists on and old topic

 

Patrick Vaccaro,

Yale University

Tuesday, May 5, 2015

12:00 – 1:00 p.m.

Among the many physiochemical properties that distinguish chiral molecules, perhaps none has had as profound and sustained an impact on chemistry as the characteristic interactions taking place with polarized light. Of special note is the dispersive (non-resonant) phenomenon of circular birefringence (CB), the manifestation of which first was reported over two centuries ago and which still is used routinely (in the more familiar guise of specific optical rotation) to gauge the enantiomeric purity of asymmetric syntheses. Ongoing efforts to probe such electronic optical activity in isolated chiral molecules will be presented, with special emphasis directed towards the marked influence that intramolecular (vibrational/conformational) dynamics and intermolecular (environmental) perturbations can exert upon the intrinsic response. Requisite isolated-molecule measurements have been made possible by our continuing development of cavity ring-down polarimetry (CRDP), an ultrasensitive polarimetric scheme that has permitted the first quantitative analyses of optical rotatory dispersion (or wavelength-resolved CB) to be performed in rarefied media. Various technical aspects of CRDP will be discussed to illustrate the unique capabilities and practical limitations afforded by this approach. Comparison of specific rotation values acquired for diverse rigid and flexible species under complementary isolated and solvated conditions will demonstrate the pronounced, yet oftentimes counterintuitive, effects incurred by the coupling among solute and solvent degrees of freedom. The disparate nature of optical activity in different environments will be highlighted, with quantum-chemical calculations serving to elucidate the structural and electronic provenance of observed behavior. Aside from unraveling processes that mediate chiroptical response in condensed media, the resulting vapor-phase information affords a critical assessment for theoretical predictions of electronic optical activity and for their emerging ability to assist in the determination of absolute stereochemical configuration.
  
Grier Room, MIT Bldg. 34-401

Refreshments served after the lecture
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