Excited states of the water molecule: Analysis of the valence and Rydberg character

Abstract

The excited states of the water molecule have been analyzed by using the extended quantum-chemical multistate CASPT2 method, namely, MS-CASPT2, in conjunction with large one-electron basis sets of atomic natural orbital type. The study includes 13 singlet and triplet excited states, both valence and 3s-, 3p-, and 3d-members of the Rydberg series converging to the lowest ionization potential and the 3s-and 3p-Rydberg members converging to the second low-lying state of the cation, 1 2 A 1 . The research has been focused on the analysis of the valence or Rydberg character of the low-lying states. The computation of the 1 1 B 1 state of water at different geometries indicates that it has a predominant 3s-Rydberg character at the equilibrium geometry of the molecule but it becomes progressively a valence state described mainly by the one-electron 1b 1 → 4a 1 promotion, as expected from a textbook of general chemistry, upon elongation of the O-H bonds. The described valence-Rydberg mixing is established to be originated by a molecular orbital ͑MO͒ Rydbergization process, as suggested earlier by R. S. Mulliken ͓Acc. Chem. Res. 9, 7 ͑1976͔͒. The same phenomenon occurs also for the 1 1 A 2 state whereas a more complex behavior has been determined for the 2 1 A 1 state, where both MO Rydbergization and configurational mixing take place. Similar conclusions have been obtained for the triplet states of the molecule.

Theoretical study of molecular properties of low-lying electronic excited states of H2O and H2S

“…There are numerous papers in the literature dealing with calculations of excitation energies of water [3,9,[11][12][13][14][15][16][17][18][19][20]. In Table 2 we presented just most recent ones [3,[15][16][17][18]. All of these [6,7,[22][23][24][25][26][27][28][29][30].…”

“…These data together with reference theoretical [3,20,25] and experimental [54] data for the ground states are collected in by all methods which we used. There are numerous papers in the literature dealing with calculations of excitation energies of water [3,9,[11][12][13][14][15][16][17][18][19][20]. In Table 2 we presented just most recent ones [3,[15][16][17][18].…”

Can the Hexagonal Ice-like Model Render the Spectroscopic Fingerprints of Structured Water? Feedback from Quantum-Chemical Computations

“…A u , appearing at 7.42 eV (167 nm), are mainly described by a linear combination of one-electron promotions involving charge reorganization from the π system of the molecule towards the σ region. The states resemble to the lowest excited state of the water molecule itself, both in nature and energetic placement [46,47]. The corresponding vertical electronic transitions are forbidden.…”

Electronic structure of liquid water from polarization-dependent two-photon absorption spectroscopy

“…The 3s / 4a 1 orbital can be described as having either antibonding valence character or dissociative 3s Rydberg character. [19][20][21] In contrast with the broad absorption bands for the two lowest excited states, both of which are repulsive along an O-H bond, the sharp transitions at higher energy populate bound vibronic states with strong Rydberg character. Prominent among these are the 3p z / 5a 1 ← 1b 1 ͑2 1 B 1 ͒ and 3p x / 2b 1 ← 1b 1 ͑3 1 A 1 ͒ transitions at 10.0 and 10.2 eV, respectively.…”

“…The 2b 2 orbital has both valence antibonding and 3p y Rydberg character. 20,21 The linear absorption spectrum of liquid water ͓Fig. 1͑b͔͒ is very different from the gas-phase spectrum because of strong intermolecular interactions in the condensed phase.…”