Theoretical Study of Weakly Bound Dimers between Hydrogen Fluoride and Some Polar Molecules

Weakly bound linear and bent dimers, FH—X (where X = CO, OC, CNH, NCH, N₂O and ON₂), are investigated using the DFT B3LYP and ab initio MP2 methods with the same basis sets (6–311++G(3df,2pd)). The strengths of the H—C or H—N H‐bonds in dimers FH—CO, FH—CNH, and FH—N₂O are compared with those of the H—O or H—N H‐bonds in dimers FH—OC, FH—NCH, and FH—ON₂. The results obtained for the H‐bond distances, the elongation effect of the HF bond, the red shift of the HF stretching frequency, and the energy difference between the dimer and the charge transfer reveal that the H‐bonds of the first group of dimers are stronger than those of the second. The Gibbs energies calculated for the six dimer formations indicate that the weakly bound dimers are unstable at room temperature (T = 298 K) (FH—X's → FH + X's, ΔG < 0).     

Interpreting 2hJ(F,N), 1hJ(H,N) and 1J(F,H) in the hydrogen‐bonded FH–collidine complex

Ab initio EOM‐CCSD calculations were performed to determine ¹⁹F,¹H, ¹⁹F,¹⁵N and ¹H,¹⁵N spin–spin coupling constants in model complexes FH–NH₃ and FH–pyridine as a function of the F—H and F—N distances. The absolute value of ¹J(F,H) decreases and that of ^1hJ(H,N) increases rapidly along the proton‐transfer coordinate, even in the region of the proton‐shared F—H—N hydrogen bond. In contrast, ^2hJ(F,N) remains essentially constant in this region. These results are consistent with the recently reported experimental NMR spectra of FH–collidine which show that ^1hJ(H,N) increases and ¹J(F,H) decreases, while ^2hJ(F,N) remains constant as the temperature of the solution decreases. They suggest that the FH–collidine complex is stabilized by a proton‐shared hydrogen bond over the range of experimental temperatures investigated, being on the traditional side of quasi‐symmetric at high temperatures, and on the ion‐pair side at low temperatures. Copyright © 2002 John Wiley & Sons, Ltd.     

Two Fe3(μ3‐S)2(CO)8 clusters with terminal N‐heterocyclic carbenes

The title compounds with terminal N‐heterocyclic carbenes, namely octacarbonyl(imidazolidinylidene‐κC²)di‐μ₃‐sulfido‐triiron(II)(2 Fe—Fe), [Fe₃(C₃H₆N₂)(μ₃‐S)₂(CO)₈], (I), and octacarbonyl(1‐methylimidazo[1,5‐a]pyridin‐3‐ylidene‐κC³)di‐μ₃‐sulfido‐triiron(II)(2 Fe—Fe), [Fe₃(C₈H₈N₂)(μ₃‐S)₂(CO)₈], (II), have been synthesized. Each compound contains two Fe—Fe bonds and two S atoms above and below a triiron triangle. One of the eight carbonyl ligands deviates significantly from linearity. In (I), dimers generated by an N—H...S hydrogen bond are linked into [001] double chains by a second N—H...S hydrogen bond. These chains are packed by a C—H...O hydrogen bond to yield [101] sheets. In (II), dimers generated by an N—H...S hydrogen bond are linked by C—H...O hydrogen bonds to form [111] double chains.     

Dimers linked by type‐I C—F...F—C contacts in (Z)‐3‐methyl‐4‐[2‐(4‐methylphenyl)hydrazinylidene]‐1‐(pentafluorophenyl)‐1H‐pyrazol‐5(4H)‐one

The title compound, C₁₇H₁₁F₅N₄O, is described and compared with two closely related analogues in the literature. There are two independent molecules in the asymmetric unit, linked by N—H...O hydrogen bonds and π–π interactions into dimeric entities, presenting a noticeable noncrystallographic C₂ symmetry. These dimers are in turn linked by a medium‐strength type‐I C—F...F—C interaction into elongated tetramers. Much weaker C—H...F contacts link the tetramers into broad two‐dimensional substructures parallel to (101).     

Correlated one‐electron wave functions

Using four basis sets, 6‐311G(d,p), 6‐31+G(d,p), 6‐311++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the acidic H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. By contrast with above the three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen‐bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is a noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD(T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Theoretical study on dimers of 2,6-diamino-3,5-dinitropyridine and its <Emphasis Type="Italic">N</Emphasis>-oxide

First-principle calculations are performed on the dimers of 2,6-diamino-3,5-dinitropyridine (ANPy) and its N-oxide (2,6-diamino-3,5-dinitropyridine-1-oxide, ANPyO). The dimers as well as the monomers are fully optimized by the DFT-B3LYP and HF methods in conjunction with 6-311G**, 6-311++G**, and cc-pVDZ basis sets. The N-O bond length of the pyridine N-oxide moiety decreases in the ANPyO dimer in the dimerization process, which results in a larger deformation energy of the ANPyO submolecule. This deformation prevents the submolecules from further close contact and the formation of strong H-bonds between the nitro and amino groups. The optimized intermolecular distances of the ANPyO dimer are in good agreement with the corresponding experimental values. There is a weak C-H...O hydrogen bond in the ANPyO dimer; the B3LYP method underestimates its binding energy. On the contrary, for the ANPy dimer, the binding energy obtained at the B3LYP level is larger than that obtained at the HF level. The individual O...H strength is stronger in the ANPy dimer than that in ANPyO, which is consistent with the O...H distance. The O...H-C type of the H-bond is stronger in the ANPyO dimer than the ordinary O...H-C bond due to the N-oxide oxygen atom bearing larger negative charges. The corrected binding energy for each hydrogen bond between nitro oxygen and amino hydrogen is about −5 kJ/mol in the ANPy dimer, which is stronger than that in the ANPyO dimer.     

Long‐range π‐type hydrogen bond in dimers CH2?HF,CH2O?H2O,and CH2O?NH3

Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. In contrast with the above three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Oxidation‐Induced C—H Functionalization: A Formal Way for C—H Activation

Cross‐coupling reactions have developed widely and provided a powerful means to synthesize a variety of compounds in each chemical field. The compounds which have C—H bonds are widespread in fossil fuels, chemical raw materials, biologically active molecules, etc. Using these readily‐ available substances as substrates is high atom‐ and step‐economy for cross‐coupling reactions. Over the past decades, our research group focused on finding and developing new strategies for C—H functionalization. Compared with classical C—H activation methods, for example, C—H bonds are deprotonated by strong base or converted into C—M bonds, oxidation‐induced C—H functionalization would be another pathway for C—H bond activation. This perspective shows a brief introduction of our recent works in this oxidation‐induced C—H functionalization. We categorized this approach of these C—H bond activations by the key intermediates, radical cations, radicals and cations.     

Theoretical Comparison of the Linear and Bent Structures for the Weakly Bound CO2—HF Complex

Density‐functional theory method (DFT) B3LYP/6–311++G(3df,2pd) and Moller‐Plesset perturbation method (MP2) MP2/6–311++G(3df,2pd) of Gaussian 03 were selected for the theoretical study of weakly bound CO₂—HF complex. In addition to the well‐known linear structure, the various bent structure complexes were also found in this work. The self‐consistent energy differences were only around 0.02 kJ/mol between the bent structure and linear structure by comparison. From the results of H‐bonding distance, dHF elongation and red shift of VHF vibration frequency, all the evidence shows that the H‐bonding effect in the bent structure is stronger than the linear structure. However, if one compares the Gibbs energy of the complex formation by temperature variation, it is very easily found that the linear form is favored under the thermal conditions of most temperatures whenever T ≥ 40 K. Such a fact is consistent with the former spectroscopic observed result of Klemperer et al.     

Cocrystals of 1,4‐diethynylbenzene with 1,3‐diacetylbenzene and benzene‐1,4‐dicarbaldehyde exhibiting strong nonconventional alkyne–carbonyl C—H…O hydrogen bonds between the components

Weak interactions between organic molecules are important in solid‐state structures where the sum of the weaker interactions support the overall three‐dimensional crystal structure. The sp‐C—H…N hydrogen‐bonding interaction is strong enough to promote the deliberate cocrystallization of a series of diynes with a series of dipyridines. It is also possible that a similar series of cocrystals could be formed between molecules containing a terminal alkyne and molecules which contain carbonyl O atoms as the potential hydrogen‐bond acceptor. I now report the crystal structure of two cocrystals that support this hypothesis. The 1:1 cocrystal of 1,4‐diethynylbenzene with 1,3‐diacetylbenzene, C₁₀H₆·C₁₀H₁₀O₂, (1), and the 1:1 cocrystal of 1,4‐diethynylbenzene with benzene‐1,4‐dicarbaldehyde, C₁₀H₆·C₈H₆O₂, (2), are presented. In both cocrystals, a strong nonconventional ethynyl–carbonyl sp‐C—H…O hydrogen bond is observed between the components. In cocrystal (1), the C—H…O hydrogen‐bond angle is 171.8 (16)° and the H…O and C…O hydrogen‐bond distances are 2.200 (19) and 3.139 (2) Å, respectively. In cocrystal (2), the C—H…O hydrogen‐bond angle is 172.5 (16)° and the H…O and C…O hydrogen‐bond distances are 2.25 (2) and 3.203 (2) Å, respectively.     

Using (FH)2 and (FH)3 to Bridge the σ‐Hole and the Lone Pair at P in Complexes with H2XP,for X=CH3, OH,H, CCH,F, Cl,NC, and CN

Ab initio MP2/aug′‐cc‐pVTZ calculations are used to investigate the binary complexes H₂XP:HF, the ternary complexes H₂XP:(FH)₂, and the quaternary complexes H₂XP:(FH)₃, for X=CH₃, OH, H, CCH, F, Cl, NC, and CN. Hydrogen‐bonded (HB) binary complexes are formed between all H₂XP molecules and FH, but only H₂FP, H₂ClP, and H₂(NC)P form pnicogen‐bonded (ZB) complexes with FH. Ternary complexes with (FH)₂ are stabilized by F?H???P and F?H???F hydrogen bonds and F???P pnicogen bonds, except for H₂(CH₃)P:(FH)₂ and H₃P:(FH)₂, which do not have pnicogen bonds. All quaternary complexes H₂XP:(FH)₃ are stabilized by both F?H???P and F?H???F hydrogen bonds and P???F pnicogen bonds. Thus, (FH)₂ with two exceptions, and (FH)₃ can bridge the σ‐hole and the lone pair at P in these complexes. The binding energies of H₂XP:(FH)₃ complexes are significantly greater than the binding energies of H₂XP:(FH)₂ complexes, and nonadditivities are synergistic in both series. Charge transfer occurs across all intermolecular bonds from the lone‐pair donor atom to an antibonding σ* orbital of the acceptor molecule, and stabilizes these complexes. Charge‐transfer energies across the pnicogen bond correlate with the intermolecular P?F distance, while charge‐transfer energies across F?H???P and F?H???F hydrogen bonds correlate with the distance between the lone‐pair donor atom and the hydrogen‐bonded H atom. In binary and quaternary complexes, charge transfer energies also correlate with the distance between the electron‐donor atom and the hydrogen‐bonded F atom. EOM‐CCSD spin‐spin coupling constants ^2hJ(F–P) across F?H???P hydrogen bonds, and ^1pJ(P–F) across pnicogen bonds in binary, ternary, and quaternary complexes exhibit strong correlations with the corresponding intermolecular distances. Hydrogen bonds are better transmitters of F–P coupling data than pnicogen bonds, despite the longer F???P distances in F?H???P hydrogen bonds compared to P???F pnicogen bonds. There is a correlation between the two bond coupling constants ^2hJ(F–F) in the quaternary complexes and the corresponding intermolecular distances, but not in the ternary complexes, a reflection of the distorted geometries of the bridging dimers in ternary complexes.     

Interpreting geometric preferences in π‐stacking interactions through molecular orbital analysis

The preference of π‐stacking interactions for parallel‐displaced (PD) and twisted (TW) conformations over the fully eclipsed sandwich (S) in small π‐stacked dimers of benzene, pyridine, pyrimidine, 1,3,5‐trifluorobenzene, and hexafluorobenzene are examined in terms of enhancement of the inter‐ring density through mixing of the monomer orbitals (MOs). PD and/or TW conformations are consistent with a non‐zero “stack bond order” (SBO), defined in analogy to the bond order of conventional MO theory, as the difference in the occupation of bonding and antibonding π‐type dimer MOs. In the S conformation, the equal number of bonding and antibonding MOs cancel overall stack bonding character between the monomers for an SBO of zero and an overall repulsive interaction. PD from the S shifts the character of at least one antibonding combination of monomer π‐type MOs with nodes perpendicular to the coordinate for PD to bonding, leading to an attractive nonzero SBO. The inter‐ring density measured through the Wiberg bond index analysis shows an enhancement at the PD conformations consistent with greater interpenetration of the monomer densities. This intuitive bonding model for π‐stacking interactions is complementary to highly accurate calculations of π‐stacking energies and allows a predictive understanding of relative stability using cheaper quantum chemical methods.     

Is Electrostatics Sufficient to Describe Hydrogen‐Bonding Interactions?

The stability and geometry of a hydrogen‐bonded dimer is traditionally attributed mainly to the central moiety A?H???B, and is often discussed only in terms of electrostatic interactions. The influence of substituents and of interactions other than electrostatic ones on the stability and geometry of hydrogen‐bonded complexes has seldom been addressed. An analysis of the interaction energy in the water dimer and several alcohol dimers—performed in the present work by using symmetry‐adapted perturbation theory—shows that the size and shape of substituents strongly influence the stabilization of hydrogen‐bonded complexes. The larger and bulkier the substituents are, the more important the attractive dispersion interaction is, which eventually becomes of the same magnitude as the total stabilization energy. Electrostatics alone are a poor predictor of the hydrogen‐bond stability trends in the sequence of dimers investigated, and in fact, dispersion interactions predict these trends better.     

Recent Advances in Organic Electrochemical C—H Functionalization

Organic electrochemistry has a rich history in organic synthesis and has been considered as a promising alternative to traditional chemical oxidants and reductants because it obviates the use of stoichiometric amount of dangerous and toxic reagents. In particular, the electrochemical C—H bonds functionalization is one of the most desirable approaches for the construction of carbon–carbon (C—C) and carbon–heteroatom (C—X) bonds. This review summarizes the substantial progress made in the last few years in C—H functionalization via organic electrochemistry. It is divided into sections on C—C, C—N, C—O, C—S, C–Halogen and C—P bond formation.     

New rac‐XP(O)(OC6H5)(NHC6H4‐p‐CH3) [X = N(CH3)(cyclo‐C6H11) and NH(C3H5)] and rac‐(C6H5CH2NH)P(O)(OC6H5)(NH‐cyclo‐C6H11) mixed‐amide phosphinates

The mixed‐amide phosphinates, rac‐phenyl (N‐methylcyclohexylamido)(p‐tolylamido)phosphinate, C₂₀H₂₇N₂O₂P, (I), and rac‐phenyl (allylamido)(p‐tolylamido)phosphinate, C₁₆H₁₉N₂O₂P, (II), were synthesized from the racemic phosphorus–chlorine compound (R,S)‐(Cl)P(O)(OC₆H₅)(NHC₆H₄‐p‐CH₃). Furthermore, the phosphorus–chlorine compound ClP(O)(OC₆H₅)(NH‐cyclo‐C₆H₁₁) was synthesized for the first time and used for the synthesis of rac‐phenyl (benzylamido)(cyclohexylamido)phosphinate, C₁₉H₂₅N₂O₂P, (III). The strategies for the synthesis of racemic mixed‐amide phosphinates are discussed. The P atom in each compound is in a distorted tetrahedral (N¹)P(=O)(O)(N²) environment. In (I) and (II), the p‐tolylamido substituent makes a longer P—N bond than those involving the N‐methylcyclohexylamido and allylamido substituents. In (III), the differences between the P—N bond lengths involving the cyclohexylamido and benzylamido substituents are not significant. In all three structures, the phosphoryl O atom takes part with the N—H unit in hydrogen‐bonding interactions, viz. an N—H...O=P hydrogen bond for (I) and (N—H)(N—H)...O=P hydrogen bonds for (II) and (III), building linear arrangements along [001] for (I) and along [010] for (III), and a ladder arrangement along [100] for (II).     

A Theoretical Study on Hydrogen‐Bonded Complex of Proflavine Cation and Water: The Site‐dependent Feature of Hydrogen Bond Strengthening and Weakening

The intermolecular hydrogen‐bonds between proflavine cation (PC) and water molecules are investigated by density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods. The ground‐state geometry optimizations, electronic excitation energies and corresponding oscillation strengths of the low‐lying electronically excited states for the isolated proflavine cation, the hydrogen‐bonded PC–H2O dimer and PC–(H2O)2 trimer are calculated. Intermolecular hydrogen bonds at the central site of proflavine molecule are found to be stronger than the peripheral site. The hydrogen bond N–H???O for the hydrogen‐bonded dimer are indicated to be weakened in the excited states, since the excitation energy is increased slightly comparing to the monomer. Hydrogen bonds of PC–(H2O)2 trimer with the same type as the dimer are strengthened in the excited state, which is demonstrated by the decrease of the excited energies. Thus, hydrogen bond strengthening and weakening are observed to reveal site dependent feature in proflavine molecule. Furthermore, the hydrogen bond at central site induces the blue‐shift of the absorption spectrum, while the ones at peripheral site induce red‐shift. Hydrogen bonds with the same type at peripheral and central sites of proflavine molecule provide different effects on the photochemical and photophysical properties of proflavine.     

The first crystal structures of six‐ and seven‐membered tellurium‐ and nitrogen‐containing (Te—N) heterocycles: 2H‐1,4‐benzotellurazin‐3(4H)‐one and 2,3‐dihydro‐1,5‐benzotellurazepin‐4(5H)‐one

There is a paucity of data concerning the structures of six‐ and seven‐membered tellurium‐ and nitrogen‐containing (Te—N) heterocycles. The title compounds, C₈H₇NOTe, (I), and C₉H₉NOTe, (II), represent the first structurally characterized members of their respective classes. Both crystallize with two independent molecules in the asymmetric unit. When compared to their sulfur analogs, they exhibit slightly greater deviations from planarity to accommodate the larger chalcogenide atom, with (II) adopting a pronounced twist‐boat conformation. The C—Te—C angles of 85.49 (15) and 85.89 (15)° for the two independent molecules of (I) were found to be somewhat smaller than those of 97.4 (2) and 97.77 (19)° for the two independent molecules of (II). The C—Te bond lengths [2.109 (4)–2.158 (5) Å] are in good agreement with those predicted by the covalent radii. Intermolecular N—H...O hydrogen bonding in (I) forms centrosymmetric R₂²(8) dimers, while that in (II) forms chains. In addition, intermolecular Te...O contacts [3.159 (3)–3.200 (3) Å] exist in (I).     

Quantum chemical investigation of linear hydrogen bonding in ONCCN···HX (X = F,Cl, Br) dimers

Linear hydrogen bonding formed between the nitrogen end of cyanogen‐N‐oxide (ONCCN) and hydrogen halides HX (X = F, Cl, Br) has been observed in their ground Σ states. The order of agreement of energetic stabilities between the correlated functionals used in this calculation is: B3LYP < PBE0 < PBE < PW91 in conjunction with the 6–311++G(3df,3pd) basis set. Analysis of various parameters describing the existence of H‐bonds in these dimers follows the conventional trend: ONCCN···HF > ONCCN···HCl > ONCCN···HBr in the series, except H‐bond lengths and static dipole polarizabilities which are in reverse order. The atomic charges obtained from the Mulliken and natural population analysis is used to assess the charge transfer effects that accompany the dimer formation. It is found from the investigation that the dimers having highest binding energy are accompanied by the highest transfer of charge. The ¹⁴N nuclear quadrupole coupling constants of the monomer ON¹CCN² are found to be decreased upon complection and in the series it increases from F through Br. We observed enhancements in the values of the dimer dipole moment and intrinsic dipole polarizabilities compared with the sum of the monomer values by intermolecular electrical interaction. Investigation reveals vibrational spectral shifts of HX and CN stretching modes similar to the conventional red‐shifted H‐bonded dimers; for the former case, the infrared band intensity increases significantly. Finally, the new vibrational modes originated from the intermolecular interaction are outlined. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Density Functional Theory Study of Hydrogen Bonds of Bipyridine with 1,3,5-Benzenetricarboxylic Acid

The hydrogen‐bonded dimer and trimer formed between 1,3,5‐benzenetricarboxylic acid and bipyridine have been investigated using a density functional theory (DFT) method and 6‐31++G** basis set. The interaction energies are ?45.783 and ?89.998 kJ·mol^?1 for the most stable dimer and trimer, respectively, after the basis set superposition error and zero‐point corrections. The formation of O–H...N hydrogen bonds makes O–H symmetric stretching modes in the dimer and trimer red‐shifted relative to those of the 1,3,5‐benzenetricarboxylic acid monomer. The natural bond orbit analysis shows that the inter‐molecular charge transfers are 0.60475e and 1.20225e for the dimer and trimer, respectively. Thermodynamic analysis indicates that the formation of trimer is an exothermic and spontaneous process at low and room temperature. A supramolecule can be constructed through the strong N···H–O intermolecular hydrogen bonds between bipyridine and 1,3,5‐benzenetricarboxylic acid, which is in good agreement with the experimental results.     

Nonempirical Atom-Atom Potentials for Main Components of Intermolecular Interaction Energy

Atom-atom potentials representing separate contributions to the nonempirical interaction energy have been derived in the SCF decomposition scheme corrected for basis set superposition error by the counterpoise method. The nontransferable long-range electrostatic multipole and classical induction terms have been evaluated directly from cumulative atomic multipole expansions, whereas the short-range exchange, charge-transfer, and electrostatic penetration contributions have been represented by simplified potentials of the form (β + δR^?1) exp(?δR) fitted to the corresponding ab initio results for 336 dimer configurations formed by HF, H₂O, NH₃, CH₄, CO, and CO₂. The dominant anisotropic character of electrostatic multipole atom-atom potentials and much more isotropic nature of the potentials representing short-range terms is illustrated in the Appendix for head-on interactions in CO ‥ OC and HF ‥ FH dimers.