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Two unrelated computational projects were proposed for this 2-year grant.  The first project involved molecular dynamics simulations of solvation, solvation dynamics, and solvatochromism in gas-expanded liquids (GXLs).  The work was completed prior to the last yearly report, and 2 papers describing this work are now published.

The second project involves modeling the isomerization dynamics of malononitriles.  Motivation for this work was explained in the previous progress report.  Over the past year, we have undertaken extensive experimental studies to characterize two such molecules, DMN and JDMN, shown at the left.  This experimental work was supported by DOE.  PRF support was used to perform parallel computational studies on these two molecules (and related species).  We have calculated properties of the ground electronic states of DMN and JDMN molecules using various levels of theory such as B3LYP, LC-B3LYP, and RI-CC2 with good quality triple zeta basis sets and compared predicted structures and electronic properties with experimental values.  We found that no single method provides completely accurate predictions for all properties, in particular the dipole moments of these molecules are over-estimated, but all ground-state predictions are reasonable.  Calculations of the S₀ ® S₁ transition were also calculated using these methods.  Whereas all methods provided surprisingly accurate predictions of their gas-phase transition frequencies, TD-B3LYP calculations badly underestimated the changes in dipole moment they undergo upon electronic excitation.  Long-range corrections were did not correct this deficiency of the DFT method.  We explored the excited-state potential surface using TD-B3LYP, RI-CC2, and SA2-CASSCF calculations.  These calculations revealed the deactivation mechanism to be a double-bond isomerization that leads to a conical intersection between S₁ and S₀.  Using the best S₁ potential surfaces we generated, we then performed classical molecular dynamics simulations of the S₁ isomerization of DMN in acetonitrile solution.  Time-resolved spectra calculated from these simulations showed that the time scale for this process was close to the ~1 ps fluorescence lifetime observed experimentally for this system.  These simulations therefore helped validate the proposed isomerization mechanism of internal conversion.  A paper reporting this work is currently under revision.  We intend to continue computational work in order to refine our understanding of these systems, in particular using simulations to help explain the environmental sensitivity of these “molecular rotor” probes.