Exocyclic push–pull conjugated compounds. Part 4: rotational barriers in poorly polarized push–pull ethylenes

Electron Density Investigation of a Push–Pull Ethylene (C14H24N2O2⋅H2O) by X‐ray Diffraction atT= 21 K

“…The formally single bonds C2ÀC3 and C3ÀC4 (1.4261(6) ä on average) undergo a significant shortening from the value of 1.464 (18) ä reported [27] for (C=)C sp 2ÀC sp 2(=O) conjugated bonds, and the two C=O bonds (1.2536(7) ä on average) are very elongated with respect to the value for conjugated (C=CÀ)C=O bonds, 1.222(10) ä. The average length of the two NÀC6 bonds, 1.3308(10) ä, is slightly shorter than that reported for (C=)CÀN sp 2 < bonds, 1.355(14) ä.…”

A DFT/TD‐DFT study for the ground and excited states of peramine and some pyrrolopyrazinone compounds

“…2) with increasing solvent polarity due to push-pull character of compounds. [29,30,31,32] Table 2 shows also the dipolar moment values of peramine state I, II, and III. The highest value was observed for flat conformation II, whereas the lowest dipolar moment was observed for TS1 transition state.…”

Correlated MO Study of the Low-Barrier Intramolecular Motions in Donor−Acceptor Ethenes

“…The significant contribution of single-bonded polar electronic configurations D 2 C + -C -A 2 to the ground states of push-pull ethenes 1 has attracted notable theoretical attention. [5][6][7]9,[14][15][16] However, most authors have avoided discussion of possibilities for the indicated intersystem crossing, prudently leaving the problem to multiconfigurational MO calculations. 3,17 Internal rotation around the CdC bond should result in loss of double bond character, and theoretical calculation of acceptable rotational barriers by single-configurational methods as HF theory would be reliable only in the case of favorable cancellation of correlation effects in the relevant ground and transition structures.…”

“…Therefore, contributions of excited singlet and/or triplet electronic configurations to the CdC rotational transition structure of push-pull ethenes are usually considered negligible. However, precise localization of rotational transition structures of push-pull ethenes by Hartree-Fock (HF), second order Møller-Plesset (MP2), and even density functional theory (DFT) methods often fails, 5,14,15 which has been attributed to instability of both DFT and MP2 results with respect to singlet-triplet contamination. 3,19 These circumstances have stimulated our interest to study explicitly the contribution of singlet and triplet electronic configurations to the CdC internal rotation in push-pull ethenes in more detail than has been attempted recently.…”

“…3,19 These circumstances have stimulated our interest to study explicitly the contribution of singlet and triplet electronic configurations to the CdC internal rotation in push-pull ethenes in more detail than has been attempted recently. 3,5,14,15,17 For the sake of comparison with earlier calculations at other levels of theory, as well as with experimental results, our choice of model molecules has been limited to compounds 2-6, as shown in Scheme 1 and Tables 1 and 2. The majority of cyclic model compounds 2 and 3 of the above series have been extensively studied both experimentally and theoretically, 6 showing relatively low CdC rotation barriers.…”