Comparison Of Chemical Mechanisms Of Heavier Vs. Lighter Elements
Much attention has been paid to the chemistry of heavier elements in recent years. The phenomena arising from the heavier atoms tend to be explained based on the theory established in the chemistry of lighter elements, especially in the viewpoint of the experimental chemists.
In such cases, one would often mistake superficial causes for the real one, since the mechanisms for the phenomena arising from the heavier atoms are sometimes quite different from those from the lighter atoms, even if the phenomena from the heavier atoms can be well explained, at first glance.
The mechanism for the significantly large indirect one–bond nuclear spin-spin couplings between Pb and C (1Jobsd(Pb, C)) of –1030 Hz for Ph3Pb–, relative to that of Ph4Pb (481 Hz), is a typical example of such a case. The 1J(M, C) values are often used to evaluate the degrees of multiple-bond character between M and C of the M–C bonds since the values are considered to be mainly dependent on degrees of s-character in the bonds.
For example, 1J(Si, C) of about 84 Hz in Brook-type silenes (Si=C) are larger than 1J(Si, C) of 47–48 Hz between sp3-hydridized silicon and attached methyl carbon atoms. The observed results are consistent with the double-bond character in the bonds of the former. However, a similar mechanism seems unlikely for the significantly large 1Jobsd(Pb, C) value of Ph3Pb–, relative to that of Ph4Pb, due to the unfavorable contribution of the Pb–C double-bond character. We felt that this must arise from the large difference in the relativistic effect on 1J(Pb, C) between Ph3Pb– and Ph4Pb.
To elucidate the mechanism for 1J(Pb, C), 1J(M, C) are analyzed for Me4M, Me3M–, Ph4M, and Ph3M–, (M = Pb, Sn, Ge, Si, and/or C) based on the MO theory. The 1J(M, C) values are evaluated under the zeroth-order regular approximation (ZORA) levels of the scalar ZORA relativistic (szr) and the spin-orbit ZORA relativistic (sozr) levels, together with the nonrelativistic (non) level, employing the Slater-type basis sets. Evaluated total values (1JTL(M, C)) reproduced well the observed values. Fermi contact terms (1JFC(Pb, C)) contribute predominantly to 1JTL(Pb, C) (≈ 99%).
Therefore, the mechanisms for the distinct relativistic effect on 1J(Pb, C) are clarified, exemplified by mainly 1JFC:szr(Pb, C: Me4Pb) and 1JFC:szr(Pb, C: Me3Pb–), of which contributions are decomposed into each occupied MO (ψi) and occupied MO to unoccupied MO transition (ψi→ψa). The 1JFC:szr (Pb, C: Me3Pb–) value contributed from ψ55 (HOMO) is –619 Hz (see Figure 1), which is much more negative than the case of 1JFC:non(Pb, C: Me3Pb–) (–214 Hz). Larger contributions are also detected for 1JFC:szr (Pb, C: Me3Pb–) from ψ45 (–136 Hz), y46 (–182 Hz), and ψ54 (–154 Hz), which amount to –472 Hz. As a result, a large negative value of –878 Hz is predicted for 1JFC:szr (Pb, C: Me3Pb–) in total. The ψ55 (HOMO)→ψ67 (LUMO+11) transition contributes much (–464 Hz) in 1JFC:szr (Pb, C: Me3Pb–) (see Figure 1). The large relativistic effect in 1JFC(Pb, C: Me3Pb–) is demonstrated to originate mainly from ψ55 (HOMO), of which character is the s-type lone pair orbital of Me3Pb–.
The small value of 1JFC:szr(Pb, C: Me4Pb) (–14 Hz) is the results of the canceling of two components with the opposite signs due to the symmetry, although the effect is large in magnitudes for the two. The distinct canceling between the contributions is not detected in 1JFC:szr (Pb, C: Me3Pb–), maybe due to typically one MO contributed to the value, corresponding to the s-type lone pair orbital, resulting in the large relativistic effect in 1JFC:szr(Pb, C: Me3Pb–). The mechanism for 1JFC:szr(Pb, C: Me3Pb–) makes us imagine that the s-type lone pair orbital plays a very important role in the distinct relativistic effect on 1J(Pb, C: Ph3Pb–). The predicted results at the sozr level were very similar to those at the szr level, as a whole. Such treatment must be instructive for experimental chemists to analyze their own results, concerning the electronic structures around the M–C bonds for M of heavy atoms through 1J(M, C), where the relationship between the s-character of M–C and 1J(Pb, C) is not always justified.
This study, Relativistic Effect on 1J(M,C) in Me4M, Me3M−, Ph4M, and Ph3M− (M=Pb, Sn, Ge, Si, and/or C): Role of s-Type Lone Pair Orbitals in the Distinct Effect for the Anionic Species was recently published by Hayashi, T. Nishide, W. Nakanishi, and M. Saito in the journal ChemPhysChem.