The Prominence of Facilitator π-Holes: The Classic N←N Pnictogen Bonding in Nitrobenzene-Ammonia Dimer with its Structural Elucidation and Experimental Characterization at Low Temperatures

The nitrogen of nitro group is a paradigmatic pnictogen due the presence of a π-hole and a number of studies have been performed recently on prototypical nitromethane (NM). Homodimers and heterodimers of NM are sustained by π-hole driven pnictogen bonds hosted by nitrogen. To understand the effect of substitution on this π-hole and thus the pnictogen bond, heterodimers of nitrobenzene (NB; phenyl substitution in place of methyl) with ammonia (AM) have been probed, as a test case, using matrix isolation infrared spectroscopy and ab initio computations. Of the four structures optimized on the potential energy surface the energetically dominant global minimum, stabilized by π-hole driven O=N←N pnictogen bonding with co-operative N-H←O hydrogen bonding, was experimentally identified at low temperatures. A comparison of the pnictogen bonding of NB-AM dimers with NM counterpart (NM-AM dimers) divulged the dominance of electrostatic origin of pnictogen bonding in both the class of dimers. The reduced strength of pnictogen bonding in NB-AM dimers in comparison to NM-AM dimers was discerned, which has been established to be a consequence of the reduced electrostatic potential at the π-hole of NB relative to that in NM. The strength of π-hole driven pnictogen bond was directly correlated with the binding energy and the infrared shifts in the signature vibrational bands of the NB, NM and AM submolecules due to dimerization under matrix isolated conditions at low temperatures.

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.

Conformational topography of tris(2-methylbutyl) phosphate and the influence of methyl branching at the non-hyperconjugative carbon on the conformational landscape: insights from matrix isolation infrared spectroscopy and DFT computations

The branching of a methyl group in a linear chain has a profound influence on the conformational morphology as it wields a strong control in reducing a large number of conformations. To unravel the effect of branching on the second non-hyperconjugative carbon atom on the conformational landscape, the conformations of tris(2-methylbutyl)phosphate (T2MBP) were studied using Density Functional Theory (DFT) computations and matrix isolation infrared spectroscopy. Experimentally, T2MBP along with N₂/Ar/Kr/Xe gases was effusively expanded and deposited at a low temperature of 12 K, which was subsequently probed using infrared spectroscopy. The computations of all the conformations were accomplished using the B3LYP level of theory with the 6-311++G(d,p) basis set. A dimethyl(2-methylbutyl) phosphate (DM2MBP) prototype, a molecule containing a single 2-methylbutyl moiety, was examined for its conformations. Computations predicted 18 and 9 conformations each for the 'gauche' and 'trans' families, respectively, in which the third branched carbon completely influences the orientation of the fourth carbon, which simplifies the conformational problem of DM2MBP. Of the 18 and 9 bunches each in the 'gauche' and 'trans' families, only 7 and 3 conformations, respectively, became energetically important, which when extrapolated to T2MBP resulted in 343 and 147 conformational possibilities. The factor of degeneracy further reduced these numbers and a total of 168 conformations effectively contribute to the conformational composition of T2MBP in the gas phase. The role of stereo electronic and steric factors prevalent in the conformational clusters of T2MBP was unravelled respectively using natural bond orbital and non-covalent interaction analyses.

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.

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

Strong proton-shared hydrogen bonding in a methyl imidazole⋯HCl complex: evidence from matrix isolation infrared spectroscopy and ab initio computations

Evidence for proton-shared hydrogen bonding is provided in a methyl imidazole⋯HCl complex using matrix isolation infrared spectroscopy and ab initio computations.

Prototypical cyclohexane dimers: spectroscopic evidence for σ stacking at low temperatures

In this work, the first unambiguous spectroscopic evidence for the existence of σ stacking interactions in cyclohexane dimers has been provided using matrix isolation infrared spectroscopy.

Conformations of diethyl ether and its interaction with pyrrole at low temperatures

Conformations of diethyl ether (DEE) were studied at low temperatures in N₂ and Ar matrixes. Computations performed at B3LYP/aug-cc-pVDZ level of theory yielded three minima corresponding to tt, tg^± and g^±g^± conformers of DEE. Of the three, the tt and tg^± conformers of DEE were experimentally identified in N₂ and Ar matrixes. Furthermore, hydrogen bonded complexes of pyrrole (py) with DEE have been investigated using Density Functional Theory (DFT) and matrix isolation infrared spectroscopy. Computations performed at B3LYP level of theory using aug-cc-pVDZ basis set on pyrrole with tt and tg^± conformers of DEE gave py-DEE-tt and py-DEE-tg^± complexes, both characterized by NH⋯O interaction. Experimental evidence for the formation of py-DEE-tt and py-DEE-tg^± complexes was affirmed from the shifts in the NH stretching, NH bending regions of pyrrole and COC and CH stretching regions of DEE. NBO analysis was carried out to understand the charge-transfer delocalization interactions in the conformers of DEE and its hydrogen bonded complexes.

Elusive hypervalent phosphorus⋯π interactions: evidence for paradigm transformation from hydrogen to phosphorus bonding at low temperatures

A paradigm transformation from hydrogen to phosphorus bonding is found to depend on the proton affinity of the interacting π-systems.