Conjugation in phosphabutadienes: ab initio investigation and NMR spectral manifestation
The 2,3-diphospha- and 2-aza-3-phosphabuta- 1,3-dienes (H3Si)(2)C=P-X=CYY' (1 X= P, Y, Y' = NH2; 2 X = P, Y, Y' = H; 7 X = N, Y, Y' = NH2; 8 X = N, Y, Y' = H; 9 X = N, Y = NH2 (E-), Y' = H (Z-); 10 X = N, Y = H (E-), Y' = NH2 (Z-)) as well as the isostructural butadienes (H3Si)(2)C=CH-CH=CY2 (5 Y = NH2; 6 Y = H), the symmetrical diphosphabutadienes (H3Si)(2)C=P-P=C(SiH3), (3) and (H2N)(2)C=P-P=C(NH2)(2) (4), were explored with ab initio calculations. The analysis of the geometrical parameters, natural atomic charges and Wiberg bond indices, rotational energy surfaces and energies of the isodesmic reactions shows an essential contribution of pi-electron delocalization for the model butadienes 1, 5, 7, 9, and 10. On the contrary, in 2, 3 and 4 the stabilization due to the conjugation effects is negligible or in general absent. The NMR chemical shielding values, calculated for 1-10 with the GIAO procedure at the RHF/6-311 + G(d,p) level, as well as those published for the electron-rich butadienes (Me3Si)(2)C=P-X=C(NMe2)(2) (X = N, P), agree qualitatively with the global conception of Variation of the structural parameters and charge distribution due to the pi-electron delocalization in the molecules. (C) 2000 Elsevier Science B.V. All rights reserved.
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