π Bonding in Second and Third Row Molecules: Testing the Strength of Linus's Blanket

Abstract

The flexibility of valence bond (VB) theory provides a new method of calculating pi-bond energies in the double-bonded species H(m)A=BH(n), where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining pi-bond strengths by targeting only the pi bond, while all other factors remain constant. In this manner, a clean separation between sigma- and pi effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row pi bonds conform to the classic statement by Pauling [L. Pauling, The Natiure of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row pi bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying sigma bonds which prevents optimal p(pi)-p(pi) overlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent-ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single sigma bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.

More insight in multiple bonding with valence bond theory

“…about 42% of a total in situ bonding energy of ~ 120 kcal/mol. Such values, albeit being too small to classify the π bonds of N 2 as charge-shift bonds, still show that these π bonds have an important charge-shift character, as was already found in doubly-bonded species in a former study [27]. a Defined as in eq.…”

“…Incidentally, in situ π bonding energies have been calculated in the past [27] for a series a doubly-bonded molecules using eq. 4, and were found to display regular and logical tendencies across the Periodic Table (bond strengthening upon So defined, the in situ bonding energies provide some information that cannot be obtained from experiment.…”

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The Si C bond length was determined as 1.7039(18)Å, and a distance of 1.7045Å was found at CCSD(T)/cc-pV(Q,T)Z level. The C-H and Si-H bond lengths are 1.0819 (12) and 1.4671(9)Å, respectively, and the HCSi and HSiC bond angles are 122.00(4) • and 122. 39(3) • , revealing sp 2 hybridized C and Si atoms.…”

“…39(3) • , revealing sp 2 hybridized C and Si atoms. The strength of the -bond of 6 is calculated to be in the range 37-54 kcal/mol, depending on computational method used [10][11][12]. This is approximately 57-83% of the -bond strength of a regular C C double bond (65 kcal/mol in ethylene [13]).…”

“…Jacobsen and Ziegler used DFT to calculate an intrinsic -bond energy of 54.5 kcal/mol based on the b 2 symmetric orbital interaction [36]. More recently, Shaik, Hiberty and co-workers investigated H m A = BH n molecules with A and B = C, N, O, Si, P, and S, and applied a VB technique which leads to in situ -bond energies with intact frameworks [12]. For H 2 Si CH 2 they find a -bond energy of 47.1 kcal/mol, which is 65% that of ethylene (72.0 kcal/mol) at the same level of theory.…”