Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion.

We have quantum chemically investigated the origin of the atypical blueshift of the H-C bond stretching frequency in the hydrogen-bonded complex X- •••H3 C-Y (X, Y=F, Cl, Br, I), as compared to the corresponding redshift occurring in Cl- •••H3 N and Cl- •••H3 C-H, using relativistic density functional theory (DFT) at ZORA-BLYP-D3(BJ)/QZ4P. Previously, this blueshift was attributed, among others, to the contraction of the H-C bonds as the H3 C moiety becomes less pyramidal. Herein, we provide quantitative evidence that, instead, the blueshift arises from a direct and strong X- •••C interaction of the HOMO of A- with the backside lobe on carbon of the low-lying C-Y antibonding σ* LUMO of the H3 C-Y fragment. This X- •••C bond, in essence a tetrel bond, pushes the H atoms towards a shorter H-C distance and makes the H3 C moiety more planar. The blueshift may, therefore, serve as a diagnostic for tetrel bonding.

Blueshift in Trifurcated Hydrogen Bonds: A Tradeoff between Tetrel Bonding and Steric Repulsion.

The front cover artwork is provided by the TheoCheM group at the Vrije Universiteit Amsterdam. The image shows how, in X- •••H3 C-Y complexes, the Lewis base X- tetrel-binds to the central C while sterically pushing the H atoms towards C; hence, the compression and blueshift of the H-C bonds. Read the full text of the Research Article at 10.1002/cphc.202300480.

A sum rule for Inelastic Electron Tunneling Spectroscopy: an ab initio study of a donor (TTF) and acceptors (TCNE, TCNQ and DCNQI) parallelly oriented on Cu(100).

We performed first-principles simulations of Inelastic Electron Tunneling Spectroscopy (IETS) for horizontally lying individual molecules that form popular donor-acceptor pairs (the TTF donor and its possible partner acceptors TCNE, TCNQ and DCNQI) on Cu(100). We find that the highest frequency C-H stretching modes are highly active for the (electron-rich) donor molecule but inactive for the (electron-poor) acceptors. We explain this contrasting response by the spatial extension of sp(3) rehybridization upon adsorption: the donor molecule entirely deforms into sp(3) while the acceptors rehybridize only at their outer ends leaving the central spacer unaffected. The sp(3)-induced buckling permits in-plane vibration modes to overlap with the π-type tunneling states and hence to be detected in IETS. In addition, the IET-spectra of a family of cyano-group acceptors, TCNE, TCNQ and DCNQI, show a recurring pattern of signals from vibrations involving their common CN outer ends plus a set of compound-dependent signals arising from the spacing moiety. The IET-response of individual chemical groups thus adds up for these flat-lying acceptor molecules, evidencing a sum rule that may facilitate their identification.

Probing nitrosyl ligation of surface-confined metalloporphyrins by inelastic electron tunneling spectroscopy.

Complexes obtained by the ligation of nitric oxide (NO) to metalloporphyrins represent important model systems with biological relevance. Herein we report a molecular-level investigation of surface-confined cobalt tetraphenyl porphyrin (Co-TPP) species and their interaction with NO under ultrahigh vacuum conditions. It is demonstrated that individual NO adducts can be desorbed using the atomically sharp tip of a scanning tunneling microscope, whereby a writing process is implemented for fully saturated regular metalloporphyrin arrays. The low-energy vibrational characteristics of individual Co-TPP-nitrosyl complexes probed by inelastic electron tunneling spectroscopy (IETS) reveal a prominent signature at an energy of ~/=31 meV. Using density functional theory-based IETS simulations-the first to be performed on such an extensive interfacial nanosystem-we succeed to reproduce the low-frequency spectrum for the NO-ligated complex and explain the absence of IETS activity for bare Co-TPP. Moreover, we can conclusively assign the IETS peak of NO-Co-TPP to a unique vibration mode involving the NO complexation site, namely, the in-plane Co-N-O rocking mode. In addition, we verify that the propensity rules previously designed on small aromatic systems and molecular fragments hold true for a metal-organic entity. This work notably permits one to envisage IETS spectroscopy as a sensitive tool to chemically characterize hybrid interfaces formed by complex metal-organic units and gaseous adducts.

Resonance Charges to Encode Selection Rules in Inelastic Electron Tunneling Spectroscopy.

From extensive simulations of a set of covalently grafted phenyl derivatives onto Cu(111), we derive a simplistic rule that selectively predicts the onset of stretching vibrations in inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope. Indeed the rise (extinction) of the highest-frequency modes is found to correlate to the accumulation (depletion) of π electron density at the metal-organic contact point. This π electron density can be fine-tuned by the usage of (de) activating aromatic substituent at different ring positions. This finding provides a simple analysis tool that can be used to reveal structural characteristics on the atomic scale by IETS.