Pentavalent P…π phosphorus bonding with associated Cl…π halogen bonding in influencing the geometry of POCl3-Phenylacetylene heterodimers: Evidence from matrix isolation infrared spectroscopy and ab initio computations

Matrix-isolation IR spectra of iodotrifluoroethylene (C2F3I)

The infrared spectra of iodotrifluoroethylene (ITFE) recorded under matrix-isolation (MI) conditions in para-hydrogen, neon and argon were investigated. The experimental spectra were analyzed by comparison with computed anharmonic spectra obtained in the second-order vibrational perturbation theory (VPT2) framework at the MP2 and revDSD-PBEP86-D3BJ levels of theory. In para-hydrogen and neon matrices, the experimentally observable bands in the range of 1800-650 cm-1 could be assigned to vibrational transitions of monomeric ITFE. The spectral resolution even allowed assignments of transitions arising from 13C-isotopologues and the observation of various higher-order resonances in the range up to ∼3550 cm-1. A comprehensive series of MI experiments in argon obtained by varying several experimental parameters revealed a dependence of the spectra on the deposition temperature. The spectra generally showed strong site-splitting effects due to the existence of different local environments around the ITFE molecule. Detailed analysis of the experimental spectra resulted in the identification of bands which are differently affected by matrix annealing. This observation led to the conclusion that ITFE occupies two major matrix sites of different stability. Calculations on ITFE dimers confirmed that spectral changes during annealing are due to the formation of dimers, which are stabilized through π-π interactions.

Structural Elucidation of N2O Clusters at Low Temperatures: Exemplary Framework Stabilized by π-Hole-Driven N···O and N···N Pnicogen Bonding Interactions

N2O is a classic prototype, in which central nitrogen is sufficiently electropositive with a positive potential of 20 kcal mol-1 in magnitude to qualify it as a possible pnicogen. This was applied to a test with N2O clusters using ab initio calculations in association with various molecular topographic tools. The structure of the energetically dominant and N2O dimer was in favor of a perpendicular geometry, where the central nitrogen atom of the N2O submolecule assumed a near 90° angle with the adjacent N═O and/or N═N moiety, which provides the affirmation of central nitrogen as a possible π-hole-driven pnicogen. The terminal nitrogen and oxygen atoms of N2O continue to act as conventional electron donors (Lewis bases) with a negative potential. Overall, predominant π-hole-driven N···O and N···N pnicogen bonding interactions were observed to stabilize N2O clusters. Furthermore, N2O clusters (dimers and trimers) were synthesized at low temperatures in an Ar matrix using molecular beam (effusive and supersonic expansion) experiments. The geometries of these clusters were characterized by probing infrared spectroscopy with corroboration from ab initio computational methods. In addition to our previously investigated nitromethane and nitrobenzene systems, N2O also makes it to a pnicogen bonder's club with the central nitrogen as a π-hole-driven pnicogen.

Unique Dispersion-Induced Tetrel Bond with Co-operative σ-hole-Induced Pnicogen Bond in the POCl3-Acetone Heterodimer: Experimental Confirmation at Low Temperatures

Both tetrel and pnicogen bonds are known to be induced through σ-/π-holes. This work reports computational and experimental evidence of the carbonyl carbon of acetone hosting a tetrel bond by dispersion rather electrostatic forces, for the first time, while phosphorus of POCl₃ sustains pnicogen bonding via the σ-hole. Heterodimers of POCl₃ with acetone (CH₃COCH₃) have been isolated within inert gas matrixes of Ar and N₂ at 12 K. Characteristic vibrational bands at P═O stretching of POCl₃ and C═O stretching of CH₃COCH₃ have been obtained in support of the computations. The potential energy surface has been traced computationally using ab initio and density functional methods. CH₃COCH₃ harboring such a tetrel bond, in itself, is quite intriguing. The interplay of these interactions has been comprehended by the quantum theory of atoms in molecules, natural bond orbital, energy decomposition, electrostatic potential mapping, and noncovalent interaction analyses.

Nitrogen as a pnicogen?: evidence for π-hole driven novel pnicogen bonding interactions in nitromethane-ammonia aggregates using matrix isolation infrared spectroscopy and ab initio computations

The role of nitrogen, the first member of the pnicogen group, as an electron donor in hypervalent non-covalent interactions has been established long ago, while observation of its electron accepting capability is still elusive experimentally, and remains quite intriguing, conceptually. In the light of minimal computational exploration of this novel class of pnicogen bonding so far, the present work provides experimental proof with unprecedented clarity, for the existence of N(acceptor)N(donor) interaction using the model nitromethane (NM) molecule with ammonia (AM) as a Lewis base in NM-AM aggregates. The NM-AM dimer, in which the nitrogen atom of NM (as a unique pnicogen) accepts electrons from AM (the traditional electron donor), was synthesized at low temperatures under isolated conditions within inert gas matrixes and was characterized using infrared spectroscopy. The experimental generation of the NM-AM dimer stabilized via NN interaction has strong corroboration from ab initio calculations. Furthermore, confirmation regarding the directional prevalence of this NN interaction over C-HN and N-HO hydrogen bonding is elucidated quantitatively by quantum theory of atoms in molecules (QTAIM), electrostatic potential mapping (ESP), natural bond orbital (NBO), non-covalent interaction (NCI) and energy decomposition (ED) analyses. The present study also allows the extension of σ-hole/π-hole driven interactions to the atoms of the second period, in spite of their low polarizability.

Unusual blue to red shifting of C-H stretching frequency of CHCl3 in co-operatively P⋯Cl phosphorus bonded POCl3-CHCl3 heterodimers at low temperature inert matrixes

Heterodimers of POCl₃-CHCl₃ were generated in Ne, Ar, and Kr matrixes at low temperatures and were studied using infrared spectroscopy. The remarkable role of co-operative pentavalent phosphorus bonding in the stabilization of the structure dictated by hydrogen bonding is deciphered. The complete potential energy surface of the heterodimer was scanned by ab initio and density functional theory computational methodologies. The hydrogen bond between the phosphoryl oxygen of POCl₃ and C-H group of CHCl₃ in heterodimers induces a blue-shift in the C-H stretching frequency within the Ne matrix. However, in Ar and Kr matrixes, the C-H stretching frequency is exceptionally red-shifted in stark contrast with Ne. The plausibility of the Fermi resonance by the C-H stretching vibrational mode with higher order modes in the heterodimers has been eliminated as a possible cause within Ar and Kr matrixes by isotopic substitution (CDCl₃) experiments. To evaluate the influence of matrixes as a possible cause of red-shift, self-consistent Iso-density polarized continuum reaction field model was applied. This conveyed the important role of the dielectric matrixes in inducing the fascinating vibrational shift from blue (Ne) to red (Ar and Kr) due to the matrix specific transmutation of the POCl₃-CHCl₃ structure. The heterodimer produced in the Ne matrix possesses a cyclic structure stabilized by hydrogen bonding with co-operative phosphorus bonding, while in Ar and Kr the generation of an acyclic open structure stabilized solely by hydrogen bonding is promoted. Compelling justification regarding the dispersion force based influence of matrix environments in addition to the well-known dielectric influence is presented.