Theoretical study of the excitation spectra of five-membered ring compounds: Cyclopentadiene, furan, and pyrrole

Abstract

Multireference perturbation theory with complete active space self-consistent field ͑CASSCF͒ reference functions was applied to the study of the valence and Rydberg excited states in the range of 5-8 eV of five-membered ring compounds, cyclopentadiene, furan, and pyrrole. The spectra of these molecules have been studied extensively for many years but characterization is far from complete. The present approach can describe all kinds of excited states with the same accuracy. The calculated transition energies are in good agreement with corresponding experimental data. We were able to predict the valence and Rydberg excited states with an accuracy of 0.27 eV or better except for the B 2 ϩ of pyrrole. The valence excited states of five-membered ring compounds were interpreted in terms of the covalent minus states and ionic plus states of the alternate symmetry. The unobserved 1A 1 →A 1 Ϫ transition with very weak intensity, which is hidden under the strong 1A 1 →B 2 ϩ transition, is also discussed. Overall, the present theory supports the assignments recently made by Serrano-Andres et al. based on the CASSCF plus second-order perturbation results. However, there remain some discrepancies in the assignment of the spectrum of pyrrole.

Theoretical characterization of the lowest-energy absorption band of pyrrole

“…Four theoretical studies have been subsequently performed at different levels of theory. Overall, the findings reported by Nakano et al, 9 Trofimov and Schirmer, 10 Palmer et al, 11 and Christiansen et al 12 agree with the assignments made by us in 1993 based on the CASPT2 results. 7 There are, however, certain discrepancies between the CASPT2 and those results, mainly concerning the assignments related to the observed lowest-energy band of the electronic absorption spectrum of pyrrole, worth examining in detail.…”

“…The character of the peak detected at 5.22 eV 13,[15][16][17] has been clearly determined to be of Rydberg nature, corresponding to a transition to the 1 1 A 2 (3s) Rydberg state. The dipole-forbidden character of the lowest electronic transition in pyrrole was firmly established in our previous work 7 and the assignment was supported by most of the previous 7,[20][21][22] and most recent theoretical studies, [9][10][11][12] in agreement with earlier analyses. 15,16 However, for the lowest-energy band recorded in the energy range 5.5-6.5 eV one can find in the theoretical literature ͑not in the experimental one͒ as many arguments in favor of the Rydberg character of the band as about its valence nature ͑for historical details the reader is referred to previous references͒.…”

“…No evidence of any important peak has been found experimentally in the lowenergy tail of the band, which decreases in intensity until the observed weak shoulder near 5. [9][10][11][12] indicates that similar conclusions and assignments are obtained for the 3s and 3p Rydberg states. We restrict the comparison to the most complete results at the CC level estimated to be the best, CCSDR͑3͒ plus basis set correction effects.…”

“…Other multireference treatments such and MRMP placed the state near the MS-CASPT2 results, at 6.01 eV. 9 The discrepancy in the vertical excitation energy of the 1 1 B 2 ( *) valence state is even larger and it will be discussed in the next sections.…”

“…That most of the intensity of the latter can be attributed to the presence of high-lying valence excited states has been supported both experimentally 11,[15][16][17][18] and theoretically. 7,[9][10][11][12] In the same energy range a number of Rydberg transitions have also been identified. There is agreement between theory and experiment regarding the nature and placement of the lowest singlet excited state of pyrrole.…”