The strongest bond in the universe? Accurate calculation of compliance matrices for the ions N2H+, HCO+, and HOC+

Abstract

Compliance matrices of protonated CO and ${\mathrm{N}}_2$ are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 Å/mdyn) to ${\mathrm{CO–H}}^{+}$ the C–O bond is weakened (0.062 Å/mdyn), while the C–O bond in ${\mathrm{H–CO}}^{+}$ is getting stronger (0.045 Å/mdyn). After protonation, the N–N bond strength is getting stronger (from 0.043 to 0.042 Å/mdyn), too. The invariance of compliance matrix elements $C_{\mathrm{ij}}$ under completion of ${\mathrm{(x}}_i{\mathrm{,x}}_j)$ to a complete set ${\mathrm{(\dots ,x}}_i{\mathrm{,\dots ,x}}_j\mathrm{,\dots )}$ of internal coordinates is demonstrated.