•The hexagonal boron cluster B64+ is a good new noble gas trapper.•The NgB bonds in the monocyclic compounds B6Ngn4+(n = 0–6) are covalent bonds and can be represented as the donor-acceptor model.•The NICS analysis shows that the hexagonal boron-noble gas compounds B6Ngn4+(n = 0–6) are typically aromatic systems.The hexagonal boron-noble gas compounds B6Ngn4+ (n = 1–6, Ng = HeRn) have been studied theoretically. The geometry, bond energies and thermodynamics properties are reported. For ArRn, the BNg bond length is close to the sum of the covalent radii of B and Ng, the BNg average bond energy is large up to 59.61–169.43 kcal/mol. There is large charge transfer from Ng to the boron ring, the Ng → boron σ-donation originating from the interaction between the valence p orbital of Ng and the LUMO of the boron ring is the predominant contribution towards the NgB bond stabilization. The NICS analysis shows that B6Ngn4+ have aromaticity.Download high-res image (80KB)Download full-size image

The monocyclic compounds (BRg)3+(D3h), (BRg)42+(D4h), (BRg)53+(D5h) and (BRg)64+(D6h) formed between boron and rare gases Rg (He–Rn) are theoretically predicted to be stable structures and have π-aromaticity with a delocalized nc-2e π-system. For heavier rare gases Ar–Rn, the B–Rg bond energy is quite high and ranges from 15 to 96 kcal mol−1, increasing with the ring size and the atomic number of rare gases; the B–Rg bond length is close to the sum of covalent radii of B and Rg atoms; NBO and AIM analyses show that the B–Rg bonds for Ar–Rn have a typical covalent character. The B–Rg bond is stabilized mainly by σ-donation from the valence p orbital of Rg to the vacant valence orbital of the boron ring. We searched for a large number of isomers for the systems of Ar and found that the titled monocyclic compounds (BAr)3+(D3h), (BAr)42+(D4h) and (BAr)53+(D5h) should be global energy minima. For (BAr)64+ the global energy minimum is an octahedral caged structure, but the titled monocyclic compound is the secondary stable local energy minimum. The energy and thermodynamic stability of the ring BnRgn(n−2)+ cations indicate that these rare gas compounds may be viable species in experiments.

A new series of stable noble gas-Lewis acid compounds NgBeH3BeR, NgBeH3BR+, and NgBH3BR2+ (R = F, H, CH3, Ng = He–Rn) with three 3c-2e H-bridged bonds have been predicted by use of the PBE0 and MP2 methods. The Ng–Be/B bonds are strong and have large binding energies 35–130, 9–38, and 4–13 kcal/mol for the doubly charged cations, singly charged cations, and neutral molecules, respectively. The binding energy and strength of the Ng–Be/B bonds increase largely from He to Rn but are insensitive to electronegativity of the substituent R. The Ng–B bonds in NgBH3BR2+ should be typical covalent bonds and the Ng–Be bonds in NgBeH3BR+ for heavy Ng atoms Kr, Xe, and Rn have some covalent character. The three bridging-H atoms have characteristic infrared vibrational modes with large IR intensity to be detected in spectroscopy experiments.

In this study, the linear halogen bonds Z3CX···Y– (X = Cl, Br; Y, Z = F, Cl, Br) are theoretically investigated. They have large interaction energies—23–160 kJ/mol, and the interactions are closed-shell in nature. In some systems, a blue-shifted halogen bond is formed. Although the electrostatic interaction is important, the intermolecular charge transfer caused by the intermolecular hyperconjugation n(Y−) → σ*(CX) and the intramolecular charge redistribution by the intramolecular hyperconjugation n(Z) → σ*(CX) play important roles in the formation of the halogen bonds.

Density functional theory was applied to study the effects of H-bonds N⋯H–X between pyridine and H2O, HCONH2 and CH3COOH on normal vibrational modes of pyridine at the B3LYP/AUG-cc-pVDZ and B3LYP/AUG-cc-pVTZ levels. The results show that the formation of H-bonds leads to an increase in frequencies of the ring breathing mode v1, the N-para-C stretching mode v6a and the meta-CC stretching mode v8a of pyridine but there was no change in triangle mode v12. The natural bond orbital analysis shows that the frequency blue shift in the ring stretching modes of pyridine is a corporate result of the intermolecular charge transfer caused by the intermolecular hyperconjugation n(N) → σ∗(HX) and the intramolecular charge redistribution caused by intramolecular hyperconjugation n(N) → σ∗(meta-CC) in the pyridine ring. We also found that the magnitude of the frequency blue shift increases with the strength of the hydrogen bonding.

The linear and bifurcated H-bonds in the systems Y⋯H2CZn (n = 1, 2; Z = O, S, Se, F, Cl, Br; Y = Cl−, Br−) are studied at the MP2 level of theory with the basis sets 6-311++G(d,p) and 6-311++G(2df,2p). The linear H-bonds are red-shifted but the bifurcated H-bonds are blue-shifted. The red shifts of the linear H-bonds are caused by direct intermolecular hyperconjugation; the blue shifts of the bifurcated H-bonds are caused by rehybridization, indirect intermolecular hyperconjugation and decrease of intramolecular hyperconjugation. As concerns the topological properties of electron density, both the linear and bifurcated H-bonds satisfy the definitions of H-bonds proposed by both Bader and Popelier. In the bifurcated complexes, there are three intermolecular critical points: one bond critical point between the acceptor atom Y and each hydrogen atom, and a ring critical point inside the tetragon YHCH.

Ab initio quantum chemistry methods were applied to study the bifurcated bent hydrogen bonds Y··· H2CZ (Z = O, S, Se) and Y···H2CZ2 (Z = F, Cl, Br) (Y = Cl−, Br−) at the MP2/6-311++G(d,p) and MP2/6-311++G(2df,2p) levels. The results show that in each complex there are two equivalent blue-shifted H-bonds Y···H-C, and that the interaction energies and blue shifts are large, the energy of each Y···H-C H-bond is 15–27 kJ/mol, and Δr(CH) = −0.1 − −0.5 pm and Δv(CH) = 30 − 80 cm−1. The natural bond orbital analysis shows that these blue-shifted H-bonds are caused by three factors: large rehybridization; small direct intermolecular hyperconjugation and larger indirect intermolecular hyperconjugation; large decrease of intramolecular hyperconjugation. The topological analysis of electron density shows that in each complex there are three intermolecular critical points: there is one bond critical point between the acceptor atom Y and each hydrogen, and there is a ring critical point inside the tetragon YHCH, so these interactions are exactly H-bonding.

H-bonding angle ∠YHX has an important effect on the electronic properties of the H-bond Y⋯HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl− and Br− at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7–12 kcal mol−1, and large blue shifts, Δr(HC) = −0.0025 – −0.006 Å and Δv(HC) = 30–90 cm−1. The natural bond orbital analysis shows that the blue shifts of these H-bonds Y⋯HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) → σ*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to σ*(CH), but to σ*(CZ) of the donor; intramolecular hyperconjugation n(Z) → σ*(CH), in that the electron density in σ*(CH) comes back to n(Z) of the donor such that the occupancy in σ*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y⋯H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y⋯H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.

The hydrogen bonds of HCl and HCCl3 as the proton donors with pyridine as the acceptor were studied at the MP2 level of theory using the five basis sets 6-31G(d,p), 6-311+G(d,p), 6-311++G(d,p), 6-311++G(2df,2p) and AUG-cc-pVDZ. Pyridine and HCl can only form a ClH…N H-bond, which causes a large frequency red shift of 725 cm−1 for the ClH vibration and an elongation 0.0495 Å of this bond using the basis set 6-31G(d,p). Two H-bonds are formed between pyridine and CHCl3: the CH…N hydrogen bond with an elongation 0.0049 Å of the CH bond and a red shift of 80 cm−1 for the CH stretch vibration of CHCl3, and the CH…π interaction with a contraction 0.003 Å of the CH bond and a blue shift of 58 cm−1 for the CH stretch vibration of CHCl3 using the basis set 6-31G(d,p). In these H-bonds, regardless of which are red-shifted or blue-shifted, the IR intensities of the CH and ClH stretch vibrations increase, and the permanent dipole moment derivatives of the proton donors are positive. The natural bond orbital analysis was carried out, and the concepts of hyperconjugation and rehybridization and the theory of Hobza were applied to account for the origin of these hydrogen bonds. A post-Hartree–Fock wavefunction containing electron correlation in the analysis of the natural bond orbital is required for interpreting the CH…N H-bond in pyridineCHCl3.

•The intermolecular hydrogen bonds are strengthened in the excited state S1 and T1 compared to the ground state.•As the complex is excited to the S1 and T1, the FN CO has been electronically excited, while the CH2O remains in the ground state.•The FN CO frequency is largely red shifted. The asymmetry CH2 vibration of CH2O is a large blue shift.Time-dependent density functional method was performed to investigate the intermolecular hydrogen bond between fluorenone and formaldehyde in the electronically excited states. The geometric structures of the hydrogen bonding complex in the ground state and the first singlet and triplet excited states S1 and T1 are optimized respectively by the DFT and TD DFT methods, the vibrational spectra, electronic absorption spectra and fluorescence spectra are calculated. The two intermolecular hydrogen bonds CO⋯HC formed between fluorenone and CH2O in the complex are strengthened in the S1 and T1 states relative to the ground state. The excited states S1 and T1 are locally excited, where fluorenone is excited but CH2O remains in the ground state. The hydrogen bonds cause a frequency blue shift of the involved CH bond in formaldehyde in the ground state, the electron excitations S0 → S1 and S0 → T1 mainly lead to large frequency red shift of the CO bond in fluorenone.

The monocyclic compounds (BRg)3+(D3h), (BRg)42+(D4h), (BRg)53+(D5h) and (BRg)64+(D6h) formed between boron and rare gases Rg (He–Rn) are theoretically predicted to be stable structures and have π-aromaticity with a delocalized nc-2e π-system. For heavier rare gases Ar–Rn, the B–Rg bond energy is quite high and ranges from 15 to 96 kcal mol−1, increasing with the ring size and the atomic number of rare gases; the B–Rg bond length is close to the sum of covalent radii of B and Rg atoms; NBO and AIM analyses show that the B–Rg bonds for Ar–Rn have a typical covalent character. The B–Rg bond is stabilized mainly by σ-donation from the valence p orbital of Rg to the vacant valence orbital of the boron ring. We searched for a large number of isomers for the systems of Ar and found that the titled monocyclic compounds (BAr)3+(D3h), (BAr)42+(D4h) and (BAr)53+(D5h) should be global energy minima. For (BAr)64+ the global energy minimum is an octahedral caged structure, but the titled monocyclic compound is the secondary stable local energy minimum. The energy and thermodynamic stability of the ring BnRgn(n−2)+ cations indicate that these rare gas compounds may be viable species in experiments.

H-bonding angle ∠YHX has an important effect on the electronic properties of the H-bond Y⋯HX, such as intra- and intermolecular hyperconjugations and rehybridization, and topological properties of electron density. We studied the multifurcated bent H-bonds of the proton donors H3CZ (Z = F, Cl, Br), H2CO and H2CF2 with the proton acceptors Cl− and Br− at the four high levels of theory: MP2/6-311++G(d,p), MP2/6-311++G(2df,2p), MP2/6-311++G(3df,3pd) and QCISD/6-311++G(d,p), and found that they are all blue-shifted. These complexes have large interaction energies, 7–12 kcal mol−1, and large blue shifts, Δr(HC) = −0.0025 – −0.006 Å and Δv(HC) = 30–90 cm−1. The natural bond orbital analysis shows that the blue shifts of these H-bonds Y⋯HnCZ are mainly caused by three factors: rehybridization; indirect intermolecular hyperconjugation n(Y) → σ*(CZ), in that the electron density from n(Y) of the proton acceptor is transferred not to σ*(CH), but to σ*(CZ) of the donor; intramolecular hyperconjugation n(Z) → σ*(CH), in that the electron density in σ*(CH) comes back to n(Z) of the donor such that the occupancy in σ*(CH) decreases. The topological properties of the electron density of the bifurcated H-bonds Y⋯H2CZ are similar to those of the usual linear H-bonds, there is a bond critical point between Y and each hydrogen, and a ring critical point inside the tetragon YHCH. However, the topological properties of electron density of the trifurcated H-bonds Y⋯H3CZ are essentially different from those of linear H-bonds, in that the intermolecular bond critical point, which represents a closed-shell interaction, is not between Y and hydrogen, but between Y and carbon.