Ab Initio Simulation of the Ultrafast Circular Dichroism Spectrum of Provitamin D Ring-Opening

We present a methodto simulate ultrafast pump-probe time-resolvedcircular dichroism (TRCD) spectra based on time-dependent densityfunctional theory trajectory surface hopping. The method is appliedto simulate the TRCD spectrum along the photoinduced ring-openingof provitamin D. Simulations reveal that the initial decay of thesignal is due to excited state relaxation, forming the rotationallyflexible previtamin D. We further show that oscillations in the experimentalTRCD spectrum arise from isomerizations between previtamin D rotamerswith different chirality, which are associated with the helical conformationof the triene unit. We give a detailed description of the formationdynamics of different rotamers, playing a key role in the naturalregulation of vitamin D photosynthesis. Going beyond the sole extractionof decay rates, simulations greatly increase the amount of informationthat can be retrieved from ultrafast TRCD, making it a sensitive toolto unravel details in the subpicosecond dynamics of photoinduced chiralitychanges.