Steric effects on alkyl cation affinities of maingroup‐element hydrides

Abstract

We have carried out an extensive exploration of gas-phase alkyl cation affinities (ACA) of archetypal anionic and neutral bases across the periodic system using zeroth order regular approximation-relativistic density functional theory at BP86/QZ4P//BP86/TZ2P. ACA values were computed for the methyl, ethyl, i-propyl and t-butyl cations and compared with the corresponding proton affinities (PA). One purpose of this work is to provide an intrinsically consistent set of values of the 298 K ACA of all anionic (XH (n-1)(-)) and neutral bases (XH(n)) constituted by maingroup-element hydrides of groups 14-17 and the noble gases (group 18) along the periods 1-6. Another purpose is to determine and rationalize the trend in affinity for a cation as the latter varies from proton to t-butyl cation. This undertaking is supported by quantitative bond energy decomposition analyses. Correlations are established between PA and ACA values.

Generalized Anomeric Effect on N-Alkyl-amino Cation Affinities: A G2(+)M Investigation

“…As in the previous comparisons between methyl cation affinities and PAs, the ACAs are also consistently lower than the corresponding PAs due to the complete absence of any steric repulsion in the latter cases, but the most important difference is that there is a reasonable linear correlation, R 2 = 0.939 for MCA vs PA, or R 2 = 0.987 for i -PCA vs PA, as shown in Figure , similar to previous studies on the correlation between alkyl cation affinities and PAs, − but there is no general and straightforward relationship between the NAACAs and PAs. Instead, discernible linear correlations only exist for the bases within Group 17 ( R 2 = 1.000), the second Period ( R 2 = 0.996 for ACA vs PA; R 2 = 0.988 for NDMACA vs PA), and within the third and fourth Periods ( R 2 = 0.960 for ACA vs PA; R 2 = 0.982 for NDMACA vs PA), as displayed in Figures and .…”

“…These trends are also found in alkyl cation affinities. 14,15 The largest reduction in ACA occurs from 328.1 (H 3 Si − ) to 307.6 (H 3 Ge − ) kcal mol −1 , whereas the smallest difference, only 0.8 kcal/mol, is found from 306.5 (H 2 N − ) to 305.7 (H 2 P − ) kcal mol −1 . These results can be attributed to the stronger negative hyperconjugation found in H 2 NSiH 3 and H 2 NPH 2 .…”

tert-Butyl Cation Affinities of Maingroup-Element Hydrides: Effect of Methyl Substituents at the Protophilic Center

“…However, for a given number of methyl substituents, trends in t -BCA of methyl-substituted bases across the periodic table (with variation of protophilic center X) are similar to trends in t -BCA of the unsubstituted bases. The latter, in turn, were shown previously to correlate nicely linearly with the corresponding proton affinities , and methyl, ethyl, and isopropyl cation affinities. Thus, the t -BCA values decrease along the second-period bases Me m CH n –1– m – , Me m NH n –1– m – , and Me m OH n –1– m – as valence 2p atomic orbitals of the basic atom become more compact and stable.…”

“…Finally, we have estimated the extent to which the computed cation affinities are affected by dispersion effects, using Grimme’s DFT-D3 method . This undertaking was prompted by the fact that in this work, we analyze systems with maximal steric bulk in the series of model systems investigated so far in this study and previous studies. − More steric contact not only leads to more steric (Pauli) repulsion (vide supra) but can also be expected to result in the weaker but stabilizing dispersion forces becoming more significant and to increase the cation affinities of interest. Thus, we have computed dispersion corrections for a selection of small through bulky cations (i.e., H + , Me + , and t -Bu + ) in combination with small through bulky bases (i.e., without and with methyl substituents).…”

G2(+)M Study on N-Alkylamino Cation Affinities of Neutral Main-Group Element Hydrides: Trends Across the Periodic Table

“…7 These correlations were also observed by Bickelhaupt et al in their DFT studies on the gas-phase ACA, including MCA, ECA, i-PCA, and t-BCA, of H n−1 X − and H n X, constituted by main-group element hydrides (MEH) of groups 14−17 and the noble gases (group 18) along the periods 1− 6. 9,10 It is also found that these ACA values are systematically lower than the corresponding PA values because of the increased Pauli repulsion if one goes from proton to alkyl cation. Moreover, the ACA values of H n−1 X − and H n X decrease systematically and monotonically in the order MCA > ECA and anionic bases with different methods, including high-level quantum chemistry composite methods, including W1, G3, G3B3, and G2, conventional ab initio methods, such as CCSD(T) and MP2, as well as a selection of density functional theory (DFT) methods.…”