Recently, Weinhold et al. put forward one new concept of σ̂-type long-bonding and applied it to the comprehension of halogen noble-gas hydrides HNgY (Ng = noble gas; Y = halogen) bonding. The present study extends this new concept into HNgX (X = CN, NC) pseudohalogen molecules. At the B3LYP and CCSD(T) levels of theory, we perform natural bond orbital (NBO) and natural resonance theory (NRT) studies on the HNgX molecules and compare them with the previous results for HNgY molecules. The NBO/NRT results clearly reveal that each of the HNgX, but not the HHeCN, HNeCN, and HNeNC molecules, is composed of three leading resonance structures: two ω-bonding structures H–Ng+:X–, H:–Ng+–X and one long-bonded structure H∧X. This result indicates that the conventional long-bonding exists in these pseudohalogen molecules, like the long-bonding in HNgY molecules. Unexpectedly, we identify a new type of longer long-bonded structure H∧C in HNeNC molecule at the B3LYP level, which disappears at the CCSD level. This misleading prediction at the B3LYP level can be traced back to the singlet diradical character, which induces a low-quality geometry. Therefore, the geometry reoptimization of the noble-gas hydrides is indispensable using CASSCF-based methods, if the noble-gas hydrides fail the “Stable” test because of diradical-type instability.

The bonding between noble gas and noble metal halide like hydrogen bonding (H-bonding) motivates us to investigate the bonding mechanism and the bonding covalency in NgMX (Ng = He, Ne, Ar, Kr, Xe, Rn; M = Cu, Ag, Au; X = F, Cl, Br, I) complexes using natural bond orbital (NBO) and natural resonance theory (NRT) methods. In this study, we introduce the new resonance bonding model in H-bonding into NgMX bonding. We provide strong evidence for resonance bonding involving two important resonance structures: Ng: M–X ↔ Ng+–M :X– in each of NgMX complexes, originating in the nNg → σ*MX hyperconjugative interaction. The covalency of the bonding could be understood by the localized nature of Ng–M bonds in these two resonance structures, and the degree of Ng–M covalency can be quantitatively described by calculated NRT bond orders bNgM. Furthermore, we find that the bond order satisfies conservation of bond order, bNgM + bMX = 1, for all of the studied complexes. On the basis of the conservation of bond order and some statistical correlations, we also reveal that the Ng–M bond (except He–Ag and Ne–Ag bonds) can be tuned by changing the auxiliary ligand X. Overall, the present studies provide new insight into the bonding mechanism and the covalency of the bonding in noble gas–noble metal halides, and develop one resonance bonding model.

Noble-gas hydrides HNgY are frequently described as a single ionic form (H–Ng)+Y−. We apply natural bond orbital (NBO) and natural resonance theory (NRT) analyses to a series of noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I) to gain quantitative insight into the resonance bonding of these hypervalent molecules. We find that each of the studied species should be better represented as a resonance hybrid of three leading resonance structures, namely, H–Ng+ −:Y (I), H:− +Ng–Y (II), and H^Y (III), in which the “ω-bonded” structures I and II arise from the complementary donor–acceptor interactions nY → σ*HNg and nH → σ*NgY, while the “long-bond” (-type) structure III arises from the nNg → *HY/HY interaction. The bonding for all of the studied molecules can be well described in terms of the continuously variable resonance weightings of 3c/4e ω-bonding and -type long-bonding motifs. Furthermore, we find that the calculated bond orders satisfy a generalized form of “conservation of bond order” that incorporates both ω-bonding and long-bonding contributions [viz., (bHNg + bNgY) + bHY = bω-bonding + blong-bonding = 1]. Such “conservation” throughout the title series implies a competitive relationship between ω-bonding and -type long-bonding, whose variations are found to depend in a chemically reasonable manner on the electronegativity of Y and the outer valence-shell character of the central Ng atom. The calculated bond orders are also found to exhibit chemically reasonable correlations with bond lengths, vibrational frequencies, and bond dissociation energies, in accord with Badger's rule and related empirical relationships. Overall, the results provide electronic principles and chemical insight that may prove useful in the rational design of noble-gas hydrides of technological interest.

Recently, Legon et al. reported the first generation and characterization of H2O/H2S···AgCl complexes by rotational spectroscopy and proposed whether there is a silver bond analogous to the more familiar hydrogen and halogen bonds. In this study, a theoretical investigation was performed to answer this question and to deepen the nature of intermolecular interactions for H2O/H2S···M–Cl (M = Cu, Ag, and Au) complexes. NBO analyses reveal that two types of delocalization interactions coexist in these complexes. Apart from the expected σ-donation interaction, the hyperconjugation interaction between H2O/H2S and M–Cl also takes part in the bonding. On the basis of such a dual-bonding mechanism, one class of bond, termed Cu/Ag bond, was defined in this study. In addition, the topological properties at a bond critical point, binding energies, and stretching frequency shifts studied here support that Cu/Ag bond is a sister bond to Au bond put forward previously by Sadlej et al. The Cu/Ag/Au bond is partially covalent and partially electrostatic in nature. Finally, the dual-bonding mechanism of Cu/Ag/Au bond was further discussed. This dual-bonding scheme may be considered a new synergistic bonding model for coordination compounds.

We report a theoretical study on the chiral discrimination of different chiral formers of hydrogen-bonded complexes of butan-2-ol (“ga”, “ag”, and “gg”) with hydrogen peroxide. Complexes formed between two isolated chiral hydrogen peroxide (M and P) and chiral butan-2-ol (S) molecules have been investigated by second-order Møller–Plesset theory (MP2). Altogether, twelve minimum structures were located, and they are bound by intermolecular hydrogen bonds. Among the complexes, HOOH (M and P) and hydrogen atom of chiral carbon on the same side are named SM and SP, respectively; HOOH (M and P) and hydrogen atom of chiral carbon on the two sides are named SM-2 and SP-2, respectively. The largest chirodiastaltic energy of the two most stable complexes was found for SM–SP of “gg”, at −0.223 kcal mol−1 in favor of the SM complex in the “gg” configuration. The largest diastereofacial energy was found for (SM-2)-SM of “gg”, at 3.491 kcal mol−1 in favor of the SM complex in the “gg” configuration. Moreover, the diastereofacial interactions lead to a preference for the SM and SP over the SM-2 and SP-2 for all the butan-2-ol···HOOH complexes. The optimized structures, interaction energies and chirodiastaltic energies for various isomers were estimated. The harmonic frequencies, IR intensities, rotational constants and dipole moments were also reported.

The complexes of formamide with four small molecules (HF, H2O, H2S, NH3) are studied by MP2 and B3LYP methods with 6-311++G(d,p) basis set. The optimized geometric parameters and intermolecular interaction energies for all complexes are estimated. The origin of blue and red shifts of C–H and C–N bonds not participating in H-bonding are analyzed by using natural bond orbital (NBO) theory. Large electron density transfer from electron lone pair of the electron donor to electron acceptor leads to the formation of a new electronic structure and results in changes in geometry and vibrational frequency. These changes are attributed to two major factors: electron density redistribution and rehybridization. Our results suggest that Hobza’s and Weinhold’s theories can be also used to explain the stretching vibrational shifts of some bonds not directly participating in hydrogen bond formation.

Noble-gas hydrides HNgY are frequently described as a single ionic form (H–Ng)+Y−. We apply natural bond orbital (NBO) and natural resonance theory (NRT) analyses to a series of noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I) to gain quantitative insight into the resonance bonding of these hypervalent molecules. We find that each of the studied species should be better represented as a resonance hybrid of three leading resonance structures, namely, H–Ng+ −:Y (I), H:− +Ng–Y (II), and H^Y (III), in which the “ω-bonded” structures I and II arise from the complementary donor–acceptor interactions nY → σ*HNg and nH → σ*NgY, while the “long-bond” (-type) structure III arises from the nNg → *HY/HY interaction. The bonding for all of the studied molecules can be well described in terms of the continuously variable resonance weightings of 3c/4e ω-bonding and -type long-bonding motifs. Furthermore, we find that the calculated bond orders satisfy a generalized form of “conservation of bond order” that incorporates both ω-bonding and long-bonding contributions [viz., (bHNg + bNgY) + bHY = bω-bonding + blong-bonding = 1]. Such “conservation” throughout the title series implies a competitive relationship between ω-bonding and -type long-bonding, whose variations are found to depend in a chemically reasonable manner on the electronegativity of Y and the outer valence-shell character of the central Ng atom. The calculated bond orders are also found to exhibit chemically reasonable correlations with bond lengths, vibrational frequencies, and bond dissociation energies, in accord with Badger's rule and related empirical relationships. Overall, the results provide electronic principles and chemical insight that may prove useful in the rational design of noble-gas hydrides of technological interest.