Theoretical characterization of the lowest-energy absorption band of pyrrole

Abstract

The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory ͑CASPT2͒ and its multistate extension ͑MS-CASPT2͒ in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5-6.5 eV and confirm that the bulk of the intensity of the band arises from a * intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence-Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of pyrrole.

Theoretical Modeling of the Doping Process in Polypyrrole by Calculating UV/Vis Absorption Spectra of Neutral and Charged Oligomers

“…Thus the error in the excitation energies of neutral species must be in the TDHF method. A literature search for benchmark calculations revealed that high-level ab initio calculations are published only for pyrrole [93][94][95][96] and not for longer oligomers. The best estimate at the coupledcluster level for the π-π* transition of pyrrole, 6.57 eV, is even higher than our TDHF value of 6.47 eV.…”

Communication: Second-order multireference perturbation theory with explicit correlation: CASPT2-F12

“…Already with the triple-basis set, the CASPT2-F12 dissociation energy is within 0.5 kcal/mol of the CBS limit. In contrast, the basisset truncation error of the conventional CASPT2 method amounts about 3 and 1.3 kcal/mol with the triple-and 36,37 does not appear with this relatively small CAS. In order to describe the Rydberg states properly, the cc-pVXZ-F12 basis sets of C and N have been augmented by one even-tempered diffuse p function 23 ͑denoted as VXZ-F12+ in Table III͒.…”

Femtosecond Multiphoton Ionization of Pyrrole

“…19 According to the calculation result, 17 the only accessible electronic state was 1B 2 (5.99 eV, π(a 2 ) f π*(b 1 )). The contribution of the 1B 2 (ππ*) valence excited state to the lowestenergy absorption band was 1 order of magnitude larger than that of the other excited states, such as 2A 1 (ππ*), 1B 1 (3p y ) and 2B 2 (3p x ).…”

“…The other two states, 1A 2 (3s) and 2A 2 (3p z ), were dipoleforbidden. 19,20 Transition to the 1B 1 (3p y ) and 2B 2 (3p x ) Rydberg state might also contribute to the absorption band. 21 The absorption band to the 1B 2 (ππ*) state displayed a large number of vibrational progressions.…”

“…3,4,12,18 The simultaneous occurrence of many overlapping Rydberg series and valence excited states is responsible for the complex structure of the electronic spectrum of pyrrole. 19 These overlapping and valenceÀRydberg mixing interactions make the theoretical calculations of excited states challenging. 5,17À19 There is another relative weak and sharp absorption band, centered at about 6.8 eV, which has been assigned to Rydberg transition to 4B 1 (1a 2 3d) state (6.78 eV).…”