Fast Vibrational Coupling

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Science  23 Sep 2005:
Vol. 309, Issue 5743, pp. 1967
DOI: 10.1126/science.309.5743.1967c

Multipulse nuclear magnetic resonance (NMR) spectroscopy is useful for determining the conformations of proteins and other large molecules in solution, but its temporal resolution is limited to microseconds. Recently synthesized nanoscale switches and motors operate on a picosecond time scale, and so require a faster method to gauge their operation. In principle, two-dimensional infrared (2D IR) spectroscopy offers the necessary increase in resolution because it measures coupling between atomic vibrations, rather than nuclear spins.

Larsen et al. have taken the preliminary step of showing that a 2D IR pulse sequence effectively reveals the static structure of a rotaxane in solution. This common molecular switch motif consists of a macrocycle that is suspended on an axle via hydrogen bonding; elaborations of this basic structure allow the ring to move when light, current, or chemical reagents are applied. The spectroscopic study quantified coupling between vibrating carbonyl (C=O) groups on the ring and those on the shaft. Analysis of the data through modeling yielded the distance (r= 6.9 Å) and angle (θ= 48°) between these groups, opening the door to a real-time dynamics study of switch and motor operations. — JSY

Proc. Natl. Acad. Sci. U.S.A.10.1073/pnas.0505313102 (2005).

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