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Two States Are Not Enough: Quantitative Evaluation of the Valence‐Bond Intramolecular Charge‐Transfer Model and Its Use in Predicting Bond Length Alternation Effects
Author(s) -
Jarowski Peter D.,
Mo Yirong
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201404978
Subject(s) - intramolecular force , delocalized electron , excited state , bond length , ground state , chemistry , valence bond theory , resonance (particle physics) , valence (chemistry) , dipole , atomic physics , molecular physics , crystallography , physics , molecule , molecular orbital , crystal structure , stereochemistry , organic chemistry
The structural weights of the canonical resonance contributors used in the Two‐state valence‐bond charge‐transfer model, neutral (N, R1) and ionic (VB‐CT, R2), to the ground states and excited states of a series of linear dipolar intramolecular charge‐transfer chromophores containing a buta‐1,3‐dien‐1,4‐diyl bridge have been computed by using the block‐localized wavefunction (BLW) method at the B3LYP/6‐311+G(d) level to provide the first quantitative assessment of this simple model. Ground‐ and excited‐state analysis reveals surprisingly low ground‐state structural weights for the VB‐CT resonance form using either this Two‐state model or an expanded Ten‐state model. The VB‐CT state is found to be more prominent in the excited state. Individual resonance forms were structurally optimized to understand the origins of the bond length alternation (BLA) of the bridging unit. Using a Wheland energy‐based weighting scheme, the weighted average of the optimized bond lengths with the Two‐state model was unable to reproduce the BLA features with values 0.04 to 0.02 Å too large compared to the fully delocalized (FD) structure (BLW: ca. −0.13 to −0.07 Å, FD: ca. −0.09 to −0.05 Å). Instead, an expanded Ten‐state model fit the BLA values of the FD structure to within only 0.001 Å of FD.