Overview of Research in the Drucker Group
In recent years a large sector within the quantum-chemistry community has focused on the efficient calculation of electronic excited states . Knowledge of excited-state potential-energy surfaces is essential for making predictions or developing models in the areas of photochemistry, combustion, atmospheric chemistry, and light emission. In some cases, the potential surfaces available from high-level ab initio techniques, such as CASPT2  or MRCI , have permitted the simulation of multisurface, multichannel reactions with experimental accuracy. Currently such feats are only possible for very small prototype systems like formaldehyde  or O(3P) + C2H4 [5,6].
For larger molecules, it is sometimes possible to obtain reasonable predictions of excited-state properties by using more economical methods, for example TDDFT [7,8,9], CASSCF , or semiempirical approaches . Before applying a less expensive method to a new chemical system, it is critical to have experimental benchmarks to determine the method's general level of accuracy or to establish its applicability to a given class of chemical system [12,13]. Alternatively the benchmarking can be done [14,15,8,16] by comparing a calculated result from the economical method with that obtained  from a highly correlated technique such as CC3 . Even when using such "gold-standard" methods for benchmarking excited-state calculations, it is essential to be cognizant of the gold standard's absolute accuracy limits or pinpoint its vulnerabilities with respect to the type of chemical system being tested. Such information must ultimately come from experimental data.
Our ongoing laboratory research is designed to meet this demand. Through vibronically resolved laser spectroscopy, we are characterizing the low-lying excited states of α,β-unsaturated carbonyl molecules - for example, the prototype acrolein molecule (propenal, CH2=CH-CH=O) [19,20] as well as cyclic enones such as 2-cyclopenten-1-one [21,22] and 2-cyclohexen-1-one [23,24,25]. Beyond their utility in benchmarking computational methods, the enone molecules we study demonstrate an interesting interplay between C=O and C=C moieties [14,12,15] and can serve as models [26,27,28,29,30,31,32,33,34,35,36] for investigating the photochemistry of larger conjugated systems.
The lowest triplet states of these molecules often play a central role in the photochemistry. Following the initial photoexcitation of S1(n,π*), nonradiative relaxation occurs promptly, placing the system on the lowest-energy 3(n,π*) or 3(π,π*) surface [37,30,33,34,36]. An extensive body of computational work shows that these surfaces mediate subsequent enone chemistry, including cycloaddition [27,31,33,32,35], rearrangement , dissociation  or cis-trans isomerization . These reactions are possible because the triplet species are metastable with respect to radiative decay and thus have long enough lifetimes to undergo collision-induced or unimolecular chemical transformation.
For the same reason the triplet excited species are metastable - the spin forbiddenness of radiative transitions to the singlet ground state - the triplet states are challenging to study spectroscopically when starting with a ground-state molecular sample. For enone molecules, the T1(n,π*) ← S0 transition has a molar absorptivity of typically less than 1 M−1 cm−1 , in great contrast to values upwards of 10,000 M−1 cm−1 for spin-allowed transitions. To contend with the extreme weakness of the triplet transitions, we employ the high-sensitivity cavity ringdown (CRD) absorption technique ; alternatively we use phosphorescence excitation spectroscopy with an enhanced light-collection setup [40,41]. Through these approaches, we have been able to measure and analyze the T1(n,π*) ← S0 spectrum of acrolein [19,20], as well as the analogous spectra of a variety of monocyclic enones [21,19,22,42,43,25] under bulk-gas or jet-cooled [22,20] conditions.
These experimental investigations help to fill a serious void in the literature. The monocyclic enone molecules have been part of very few [44,45] excited-state benchmarking studies, in part because of the uniquely complex makeup of the chromophore. Changes upon n → π* excitation are influenced by torsional and angle strain, conjugation, as well as potential hyperconjugative effects involving the oxygen heteroatom. The demand for methods to treat these effects is likely to rise, given the sustained interest in using computational approaches to understand and predict enone photochemistry [33,34,35,36]. Our spectroscopic results can provide critical tests of new methods, stimulate their refinement, and promote a more thorough inclusion of enone species in the excited-state benchmark studies.
Moreover, our work addresses the common predicament of relying on vertical excitation energy [8,13,16] to benchmark excited-state calculations. This quantity is straightforward to calculate, but a precise experimental measurement is not defined. By contrast, we are obtaining fundamental vibrational frequencies (and in some cases overtones) for the triplet excited states from gas-phase absorption or excitation spectra of the enones. Such data rigorously test the shape of computed excited-state potential surfaces and thereby provide a much more complete assessment of the calculations than would be possible from an experimental estimate of vertical excitation energy.
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M. Z. M. Rishard, E. A. Brown, L. K. Ausman, S. Drucker, J. Choo, and J. Laane. Ultraviolet cavity ringdown spectra and the S1(n,π*) ring-inversion potential energy function for 2-cyclohexen-1-one-d0 and its 2,6,6-d3 isotopomer. Journal of Physical Chemistry A, 112:38, 2008.
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L. M. Hoffelt, M. G. Springer, and S. Drucker. Phosphorescence excitation spectrum of the T1(n,π*) ← S0 transition of 4H-pyran-4-one. Journal of Chemical Physics, 128:104312, 2008.
M. G. Springer, N. C. Hlavacek, S. P. Jagusch, A. R. Johnson, and S. Drucker. Cavity ringdown absorption spectrum of the T1(n,π*) ← S0 transition of 4-cyclopenten-1,3-dione. Journal of Physical Chemistry A, 113:13318, 2009.
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