Abstract
Linear triatomic molecules (CO2, N2O, and OCS) are scrutinized for their propensity to form perpendicular tetrel (CO2 and OCS) or pnictogen (N2O) bonds with Lewis bases (dimethyl ether and trimethyl amine) as compared with their tendency to form end-on chalcogen bonds. Comparison of the IR spectra of the complexes with the corresponding monomers in cryogenic solutions in liquid argon enables to determine the stoichiometry and the nature of the complexes. In the present cases, perpendicular tetrel and pnictogen 1:1 complexes are identified mainly on the basis of the lifting of the degenerate ν 2 bending mode with the appearance of both a blue and a red shift. Van ′t Hoff plots of equilibrium constants as a function of temperature lead to complexation enthalpies that, when converted to complexation energies, form the first series of experimental complexation energies on sp1 tetrel bonds in the literature, directly comparable to quantum-chemically obtained values. Their order of magnitude corresponds with what can be expected on the basis of experimental work on halogen and chalcogen bonds and previous computational work on tetrel bonds. Both the order of magnitude and sequence are in fair agreement with both CCSD(T) and DFA calculations, certainly when taking into account the small differences in complexation energies of the different complexes (often not more than a few kJ mol−1) and the experimental error. It should, however, be noted that the OCS chalcogen complexes are not identified experimentally, most probably owing to entropic effects. For a given Lewis base, the stability sequence of the complexes is first successfully interpreted via a classical electrostatic quadrupole–dipole moment model, highlighting the importance of the magnitude and sign of the quadrupole moment of the Lewis acid. This approach is validated by a subsequent analysis of the molecular electrostatic potential, scrutinizing the σ and π holes, as well as the evolution in preference for chalcogen versus tetrel bonds when passing to “higher” chalcogens in agreement with the evolution of the quadrupole moment. The energy decomposition analysis gives further support to the importance/dominance of electrostatic effects, as it turns out to be the largest attractive term in all cases considered, followed by the orbital interaction and the dispersion term. The natural orbitals for chemical valence highlight the sequence of charge transfer in the orbital interaction term, which is dominated by an electron-donating effect of the N or O lone-pair(s) of the base to the central atom of the triatomics, with its value being lower than in the case of comparable halogen bonding situations. The effect is appreciably larger for TMA, in line with its much higher basicity than DME, explaining the comparable complexation energies for DME and TMA despite the much larger dipole moment for DME.
Highlights
Non-covalent interactions (NCIs) have been playing an increasingly important role in chemistry in recent decades, for example, in the interpretation of the structure of biomolecules and the design of macro- and supramolecular entities
Analysis of infrared experimental frequency and intensity data of cryogenic solutions in liquid Ar leads to an unambiguous identification of perpendicular tetrel and pnictogen bonding of three linear triatomics, CO2, one case (OCS), and N2O, with two Lewis bases, dimethyl ether (DME) and TMA
The complexation enthalpies, obtained by Van t Hoff plots, are the first obtained for this type of complex and, after conversion to complexation energies, show the expected order of magnitude when compared with experimental and theoretical literature data on other hydrogen bond congeners such as halogen and chalcogen bonds
Summary
Non-covalent interactions (NCIs) have been playing an increasingly important role in chemistry in recent decades, for example, in the interpretation of the structure of biomolecules and the design of macro- and supramolecular entities. It is sometimes said that, while the 20th century was the century of the covalent bond, the 21st century has become the century of the non-covalent bond. As Schneider formulated it, “with courageous simplification one might assert that the chemistry of the last century was largely the chemistry of the covalent bonding, whereas that of the present century is more likely to be the chemistry of the non-covalent bonding” [1]. The variety of non-covalent interactions (NCIs) invoked in most chemical discussions was limited to permanent dipole–permanent dipole, permanent dipole–induced dipole, and induced dipole–induced dipole interactions, known as the dispersion interaction [2], and, with a unique status, the hydrogen bond. Recent reviews (see, for example, [7,8]) pinpointed that its story is still not at the end
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