Heavy Element π Donation Is Not Less Effective

✍ Scribed by Dipl.-Chem. Jürgen Kapp; Dr. Christian Schade; Dr. Ahmed M. El-Nahasa; Prof. Dr. Paul von Ragué Schleyer

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 372 KB
• Volume: 35
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.199622361

✦ Synopsis

Very strong cation stabilization results from x conjugation, for example, from the interaction between empty carbon 2p orbitals and the lone pairs on nitrogen-or oxygen-based substituents.['-31How do the heavier group 15-17 substituent lone pair donors compare with their lighter analogs? It has been recognized for some time that TC overlap arising from elements of different rows is not reduced due to differences in the p-orbital Conclusions from previous work are contradictory. While the amino group is the most stabilizing x-donor substituent, it is not clear if sulfur or oxygen is the better n do no^-.[^-''^ The 0,' constants, PhO = -0.62, PhS = -0.54, and PhSe = -0.47 are not very different but show a progression down the group." 2, 13] The trend in the calculated hydride affinities of +CH2XH (X = 0, S, Se. Te) is similar, with a sharp difference between oxygen and sulfur." 'I However, earlier ab initio calcuiations gave methyl stabilization energies [Eq. (a)] in the order OH >SeH > SH (52.6, 41.7, and 31.8 kcal mol-', respectively).['41 CH,XH, + CH: 'CH,XH, + CH,

Pioneering mass spectrometric investigations by Taft, Martin, and Lampe,c2] as well as more recent experiment^,'^] gave cl-halomethyl cation stabilization energies that increase down the group: F < C1< Br<I. In contrast, the halogen 0 ; constants (F = -0.07, C1 = 0.11, Br = 0.15, and I = 0.14) reveal the same trend as the chalcogen series." According to the ab initio investigation ten years ago by Bernardi et aI. differences between methyl stabilization energies of second and third period lone pair donors are small within a group but more pronounced along a

We have now extended the previous studies by examining all group 15-17 TC donors from the first five rows at high ab initio levels. Our objective was to compare the methyl-stabilizing effects of the lighter with the heavier elements. Schematic representations of the structures examined are given in Figure 1 : neutral parent systems 15, 15p with planar XH, units (group 15), 16 (group 16), and 17 (group 17) as well as cationic species 15+, 16+, and 17+.

All structures were optimized at the restricted MP2-FC level of theory.[161 Refined energies were obtained in QCISD(T) single point calculations on the MP2-optimized structures. The GAUSSIAN94 program was employed with standard basis sets for molecules containing elements from hydrogen through chlorine during optimizations, frequency runs (6-31 + G*), and single point calculations (6-31 + + G**). Pseudopotentials (PSP) were used to replace the core electrons of systems with heavier elements: a four valence electron (Ve) PSP for carbon and 5-7 Ve PSP for Se-Br and Te-I together with the corre-X = N, P, As, sb [