Electronic excitation in a saturated chain: An MS-CASPT2 treatment of the anti conformer of n-tetrasilane

Citation
R. Crespo et al., Electronic excitation in a saturated chain: An MS-CASPT2 treatment of the anti conformer of n-tetrasilane, J PHYS CH A, 104(37), 2000, pp. 8593-8599
Citations number
32
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY A
ISSN journal
1089-5639 → ACNP
Volume
104
Issue
37
Year of publication
2000
Pages
8593 - 8599
Database
ISI
SICI code
1089-5639(20000921)104:37<8593:EEIASC>2.0.ZU;2-V
Abstract
The singlet-singlet electronic spectrum of the anti conformer of n-tetrasil ane has been studied using multiconfigurational wave functions (CASSCF), se cond-order perturbation theory (CASPT2), and its multistate extension (MS-C ASPT2), in conjunction with large ANO-type basis sets including Rydberg fun ctions. The calculations include the 4s, 4p, and 3d members of the Rydberg series converging on the first ionization. Mixing of valence and Rydberg st ates observed in the CASSCF wave functions is not fully rectified by single -reference CASPT2 theory, whereas the MS-CASPT2 method separates the valenc e and Rydberg states effectively. At the MS-CASPT2 level, six valence excit ed states have been found below the lowest Rydberg transition, predicted at 7.40 eV. In terms of natural orbitals, they correspond to single electron promotions from the a-symmetry HOMO to four sigma* and two pi* valence orbi tals. Vertical dipole-allowed valence transition energies (oscillator stren gths) are computed at 6.33 eV (1.12), 6.68 eV (0.01), and 6.96 eV (0.15), f or excitation to 1(1)B(u), 1(1)A(u), and 2(1)B(u), states, respectively. Th ree vertical dipole-forbidden valence transitions are predicted to be inter leaved among them at 6.55 eV (2(1)A(g)), 6.87 eV (3(1)A(g)), and 7.10 eV (1 (1)B(g)). The results are consistent with the available experimental data a nd are easily rationalized in terms of a simple Huckel model of sigma deloc alization.