Bond contraction

AuH and Au2 serve as benchmark molecules to test the performance of various relativistic approximations. Figure 4.7 shows predictions for relativistic bond contractions of Au2 from various quantum chemical calculations over more than a decade. In the early years of relativistic quantum chemistry these predictions varied significantly (between 0.2 and 0.3 A), but as the methods and algorithms became more refined, and the computers more powerful, the relativistic bond contraction for Au2 converged and is now at 0.26 A. 

Figure4.7 Relativistic bond contractions A re for Au2 calculated in the years from 1989 to 2001 using different quantum chemical methods. Electron correlation effects Acte = te(corn) — /"e(HF) at the relativistic level are shown on the right hand side of each bar if available. From the left to the right in chronological order Hartree-Fock-Slater results from Ziegler et al. [147] AIMP coupled pair functional results from Stbmberg and Wahlgren [148] EC-ARPP results from Schwerdtfeger [5] EDA results from Haberlen and Rdsch [149] Dirac-Fock-Slater...

Figure 4.10 Calculated relativistic bond contractions ARte in A (circles and solid line, axis on the left-hand side) and relativistic change in the dissociation energy contractions (triangles and dashed line, axis on the right-hand side) for various diatomic compounds as a function ofthe electronegativity of the ligand.

For gold compounds in the high oxidation state (e.g., AuCU , AuFe , AuFs etc.,) we And rather small bond contractions as the electronegative ligands reduce the 6s... 

Remarkably, the Hg—C bonds are shorter than the Cd—C bonds in the respective cyano-spinel 109 the probable reason is relativistic bond contraction with the Hg atom. 

The Rh dimer after H2 adsorption exhibited similar EXAFS oscillation and Fourier transform to those for the fresh imprinted catalyst Detailed analysis of the EXAFS data confirmed retention of the local conformation of the Rh dimer with a Rh-Rh bond (CN = 1.3 0.4), two Rh-O bonds (CN = 1.7 0.5) and a Rh-P bond (CN = 1.2 0.2). No formation of Rh metallic particles was observed. However the Rh-Rh bond contracted from 0.268 0.001 to 0.265 0.001 nm with the hydride dimer, indicating stabilizahon of the dimer structure by electronic rearrangement due to monohydride coordination on both Rh atoms in the dimer. After reaction of the Rh-dimer hydride species with 3-methyl-2-pentene, the shrunken Rh-Rh bond of the monohydride species expanded again to recover the... 

Zincblende structure with top Ga and As atoms rotated into, resp. out of surface, (keeping about constant mutual bond length, rotated by projected angle of 27°) Ga and As back bonds contracted by —2.5% and -3.6%, resp. 

Table 15 Imbalance parameter n, progress in C-Y bond contraction and planarization of the a-carbon at the transition state in the CH3Y/CH2=Y and NCCH2Y/NCCH=Y systems... 

C-Y bond contractions and of the planarization of the a-carbon as measured by the % progress in the change of the pyramidal angle a. The pyramidal angle is defined as shown in 56 where the dashed lines are the projection... 

There is a strong inverse correlation between n and the progress in the planarization as well as the progress in the C-Y bond contraction. The relatively small Aa /Aa° values indicate substantial retention of the sp3 character of the a-carbon that is particularly pronounced for CH3C=CH which has the largest n value, but still appreciable for CH3NO and CH3CH=S which have the smallest n values. 

Relativistic effects on calculated NMR shieldings and chemical shifts have sometimes been divided into "direct" and "indirect" effects. According to this point of view, indirect effects are those that result from relativistic changes of the molecular geometry (the well-known relativistic bond contraction (55) in particular) whereas direct effects refer to a fixed geometry. 

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