MICHAEL O. MCANALLY AND STEPHEN DRUCKER, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702.
We have used several computational methods, including TDDFT and EOM--EE--CCSD, to determine the equilibrium geometry and harmonic vibrational frequencies of 2-cyclohexen-1-one (CHO) in its T1 (n, *) excited state. Atom displacement vectors for the normal modes indicate that the lowest-frequency vibration, 39, is best described as a ring-twisting motion, whereas 38 is a ring-bending vibration consisting mainly of C-5 displacement toward and away from the plane in which the other heavy atoms lie. These updated descriptions are transposed with respect to those in the previous literature.\footnote T.~L.~Smithson and H.~Wieser, J.~Chem.~Phys. 73, 2518 (1980); M.~Z.~M.~Rishard, E.~A.~Brown, L.~K.~Ausman, S.~Drucker, J.~Choo, and J.~Laane, J.~Phys.~Chem.~A 112, 38 (2008). The table below shows that the EOM--EE--CCSD harmonic frequencies generally agree well with fundamentals obtained spectroscopically.b In particular, the 39 frequency determined by EOM--EE--CCSD is more accurate than the TDB3LYP result, with errors of +2% and +20%, respectively. This outcome is traceable in part to a larger 39 reduced mass calculated by EOM--EE--CCSD, stemming from a less planar O=C--C=C dihedral angle (170.4o via EOM--EE--CCSD vs. 177.4o via TDB3LYP).\vspace-.15in
\begincenter Low-frequency fundamentals (cm-1) for CHO in its T1 (n, *) state \vspace-.2in \endcenter \begindisplaymath
\vspace-.05in \enddisplaymath The success of EOM--EE--CCSD in this application could be due to its ability to describe multiconfigurational wavefunctions within a single-reference formalism.