Interaction between NH2NO and H2O2: A quantum chemistry study

The H‐bonded complexes formed from interaction between NH₂NO (NA) and H₂O₂ (HP) have been investigated by using B3LYP and MP2 methods with a wide range of basis sets. We found six H‐bonded complexes in which three of them have cyclic structure. Calculations carried out at various levels show that the seven‐membered cyclic structure with O···HO and O···HN hydrogen bonding interactions is the most stable complex. The large binding energy obtained for A1 complex probably results from a more linear arrangement of the O···H N and O H···OH‐bonds in the seven‐membered structure A1. The natural bond orbital (NBO) analysis and the Bader's quantum theory of atoms in molecules have been used to elucidate the interaction characteristics of the NA‐HP complexes. The NBO results reveal that the charge transfer energy corresponds to the H‐bond interactions for A1 complex is grater than other complexes. The electrostatic nature of H‐bond interactions is predicted from QTAIM analysis. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Theoretical Investigation on the Reactions of H with (CH3)4‐nSiHn (n = 1–4)

The abstractions of H with (CH₃)_4‐nSiH_n (n = 1–4) have been investigated at high levels of ab initio molecule orbital theory. Geometries have been optimized at the MP‐2 level with 6–31G(d) basis set, and G2MP2 level has been used for the final energy calculations. Theoretical analysis provided conclusive evidence that the main process occurring in each case is the abstraction of H from the Si? H bond leading to the formation of the H₂ and silyl radicals; the abstraction of H from C? H bond has higher barrier and is difficult to react in each case. The kinetics of the title reactions have been calculated with variational transition state theory over the temperature range 200–1000 K, and the theoretical rate constants match well with the experimental values.     

Tautomerism of monochalcogenosilanoic acids CH3Si(O)XH (X = S,Se, Te) in the gas phase and in the polar and aprotic solution: An ab initio computational investigation

Computational investigations by an ab initio molecular orbital method (HF and MP2) with the 6‐311+G(d,p) and 6‐311++G(2df, 2pd) basis sets on the tautomerism of three monochalcogenosilanoic acids CH₃Si(?O)XH (X = S, Se, and Te) in the gas phase and a polar and aprotic solution tetrahydrofuran (THF) was undertaken. Calculated results show that the silanol forms CH₃Si(?X)OH are much more stable than the silanone forms CH₃Si(?O)XH in the gas‐phase, which is different from the monochalcogenocarboxylic acids, where the keto forms CH₃C(?O)XH are dominant. This situation may be attributed to the fact that the Si? O and O? H single bonds in the silanol forms are stronger than the Si? X and X? H single bonds in the silanone forms, respectively, even though the Si?X (X = S, Se, and Te) double bonds are much weaker than the Si?O double bond. These results indicate that the stability of the monochalcogenosilanoic acid tautomers is not determined by the double bond energies, contrary to the earlier explanation based on the incorrect assumption that the Si?S double bond is stronger than the S?O double bond for the tautomeric equilibrium of RSi(?O)SH (R?H, F, Cl, CH₃, OH, NH₂) to shift towards the thione forms [RSi(?S)OH]. The binding with CH₃OCH₃ enhances the preference of the silanol form in the tautomeric equilibrium, and meanwhile significantly lowers the tautomeric barriers by more than 34 kJ/mol in THF solution. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Theoretical study addressing the effects of tautomerism and explicit/implicit water molecules on NQR and NMR parameters of tetrazole‐5‐thione

DFT/B3LYP calculations were employed to study the effects of tautomerism and explicit/implicit water molecules on Nuclear Quadrupole Resonance (NQR) and Nuclear Magnetic Resonance (NMR) tensors of nitrogen nuclei in tetrazole‐5‐thione structure. The obtained results revealed that nuclear quadrupole coupling constant (χ) and isotropic chemical shielding (σ_iso) values of nitrogen nuclei in tetrazole ring of five possible tautomeric forms of tetrazole‐5‐thione, i.e. two thione forms called tautomers A and E and three thiol forms called tautomers B, C, and D, were functions of resonance energy(E₂) values of nitrogen lone pairs. Furthermore, it was observed that by increasing participation of lone pairs of nitrogen atoms in the ring resonance interactions, the σ_iso values around them were increased, while their χ and q_zz values were decreased. However, the results indicated that with exception of tautomer B, the order of q_zz and χ values of nitrogen nuclei in tetrazole ring was exactly opposite of the order of resonance energy values for the same nitrogen nuclei in all tautomers and their mono‐hydrated complexes. In addition, a significant decrease was noticed in χ and q_zz values when a water molecule was put in different positions near the tetrazole ring in tautomers A–E. The mentioned result can be attributed to hydrogen bond formation between nitrogen nuclei and the oxygen of water. In mono‐hydrated complexes, the σ_iso values around nitrogen atoms acting as hydrogen donors in hydrogen bond formation (N―H….OH₂) were decreased, while its values were increased for nitrogen atoms acting as hydrogen acceptors in hydrogen bond formation(N….H―OH). Copyright © 2016 John Wiley & Sons, Ltd.     

The hydrogen bond donor and acceptor ability of thioformic acid

The molecular orbital (MP2) and density functional theory calculations (B3LYP) using 6-311++G(2d,2p) basis set have been carried out on adducts of the cis and trans conformers of two tautomeric forms of thioformic acid (TFA) with water. Thirteen adducts of TFA with water have been optimized. Similar calculations have been carried out on adducts of formic acid with water and seven energy minima have been obtained for them. Our findings indicate that the specific interactions with water play an important role in the conformational stability as thiol form of TFA is the most stable form in gas phase, while it is the thione form which is the most stable in 1:1 adduct. The H-bond acceptor ability of S and O at the thiocarbonyl and carbonyl positions in TFA and FA, respectively, has been compared and observed to be only slightly lower in the former. However, the H-bond donor ability of S–H has been observed to be nearly half to that of O–H. The contributors to stabilization energies of adducts are explored by analyzing geometrical variations, atomic charges, and electron delocalizations.     

Tautomeric equilibria of 5-fluorouracil anionic species in water

It has long been postulated that rare tautomeric or ionized forms of nucleic acid bases may play a role in mispair formation. Therefore, ab initio quantum chemical investigations on the tautomeric equilibrium in 5-fluorouracil (5FU) and its anions (deprotonated from N1, AN1, and from N3, AN3) and their tautomeric forms in water were performed. The effect of the water as solvent was introduced using solute-solvent clusters (four water molecules). The influence of the water molecules on the tautomeric reactions between different forms was considered by multiple proton transfer mechanisms. We show that when a water dimer is located in the reaction site between the two pairs of N-H and C═O groups, the assistive effect of the water molecules is strengthened. All calculations of the solute-water complexes were carried out at an MP2 level of theory and supplemented with correction for higher order correlation terms at CCSD(T) level, using the 6-31+G(d,p) basis set. The ab initio calculated frequencies and Raman intensities of 5FU and its anions AN1, AN3, and dianion are in good agreement with the experimental Raman frequencies in aqueous solution at different pH. In order to establish the pH-induced structural transformation in the molecule of 5FU, further (1)H, (19)F, and (13)C NMR spectra in water solution for pH = 6.9-13.8 were acquired and the chemical shift alterations were determined as a function of pH. On the basis of NMR spectroscopic data obtained for 5FU in aqueous solution at alkaline pH, we suggest the existence of a mixture of the anionic tautomeric forms predicted by our theoretical calculations.     

(E)‐2‐[(4‐Chlorophenyl)iminomethyl]‐5‐methoxyphenol and (E)‐2‐[(2‐chlorophenyl)iminomethyl]‐5‐methoxyphenol: X‐ray and DFT‐calculated structures

The crystal structures of the title 4‐chlorophenyl, (I), and 2‐chlorophenyl, (II), compounds, both C₁₄H₁₂ClNO₂, have been determined using X‐ray diffraction techniques and the molecular structures have also been optimized at the B3LYP/6‐31 G(d,p) level using density functional theory (DFT). The X‐ray study shows that the title compounds both have strong intramolecular O—H...N hydrogen bonds and that the crystal networks are primarily determined by weak C—H...π and van der Waals interactions. The strong intramolecular O—H...N hydrogen bond is evidence of the preference for the phenol–imine tautomeric form in the solid state. The IR spectra of the compounds were recorded experimentally and also calculated for comparison. The results from both the experiment and theoretical calculations are compared in this study.     

Theoretical Study on the Hydrogen Bonding Interactions in Complexes of 5‐Hydroxytryptamine with Water

The energies, geometries and harmonic vibrational frequencies of 1:1 5‐hydroxytryptamine‐water (5‐HT‐H₂O) complexes are studied at the MP2/6‐311++G(d,p) level. Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM) analyses and the localized molecular orbital energy decomposition analysis (LMO‐EDA) were performed to explore the nature of the hydrogen‐bonding interactions in these complexes. Various types of hydrogen bonds (H‐bonds) are formed in these 5‐HT‐H₂O complexes. The intermolecular C4H5^5‐HT···O^w H‐bond in HTW3 is strengthened due to the cooperativity, whereas no such cooperativity is found in the other 5‐HT‐H₂O complexes. H‐bond in which nitrogen atom of amino in 5‐HT acted as proton donors was stronger than other H‐bonds. Our researches show that the hydrogen bonding interaction plays a vital role on the relative stabilities of 5‐HT‐H₂O complexes.     

Theoretical study of the interaction between LiNH2 and HMgH

The lithium bond between HMgH and LiNH₂ has been predicted and characterized with quantum chemical calculations at the MP2/6‐311++G(d,p) level. Upon formation of the lithium bond, both the Mg? H and Li? N bonds are stretched. The Li? N bond undergoes a red shift, whereas the Mg? H bond exhibits a blue shift. The lithium‐bonded complex is controlled mainly by electrostatic and polarization interactions. The binding energy of HMgH with LiNH₂ is computed to be 12.47 kcal/mol. The binding of the two molecules is enhanced by the substitution with the methyl group in the Li acceptor, whereas it is weakened by the replacement with whether the electron‐withdrawing group such as F, Cl, CN, NC, or the electron‐donating group (OH and HN₂). A negative cooperativity is present in the ternary system of 2LiNH₂ and HMgH. The polarization interaction plays an important role in the negative cooperativity. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

DFT Study of Hydrogen-Bonded 1,3,5-Triazine-Water Complexes

The 1,3,5-triazine-water hydrogen bonding interactions have been investigated using the density functional theory B3LYP method and 6-31 ＋＋G^＊＊ basis, obtaining one, two and seven energy minima of the ground states for the 1,3,5-triazine-water, 1,3,5-triazine-（water）2 and 1,3,5-triazine-（water）3 complexes respectively. The fully optimized geometries and binding energies were reported for the various stationary points. The global minima of 1,3,5-triazine-（water）2 and 1,3,5-triazine-（water）3 complexes have a hydrogen bond N…H-O and a chain of water molecules, terminated by a hydrogen bond O…H-C. The binding energies are 13.38, 39.52 and 67.79 kJ/mol for the most stable 1,3,5-triazine-water, 1,3,5-triazine-（water）2 and 1,3,5-triazine-（water）3 complexes respectively, after the basis set superposition error and zero point energy corrections. The H-O symmetric stretching modes of water in the complexes are red-shifted relative to those of the monomer water. In addition, the NBO analysis indicates that inter-molecule charge transfer is 0.02145 e, 0.02501 e and 0.02777 e for the most stable 1 ： 1, 1 ： 2 and 1 ： 3 complexes between 1,3,5-triazine and water, respectively.     

Hydrogen Bonding in Phosphine Oxide/Phosphate–Phenol Complexes

To develop a new solvent‐impregnated resin (SIR) system for the removal of phenols and thiophenols from water, complex formation by hydrogen bonding of phosphine oxides and phosphates is studied using isothermal titration calorimetry (ITC) and quantum chemical modeling. Six different computational methods are used: B3LYP, M06‐2X, MP2, spin component‐scaled (SCS) MP2 [all four with 6‐311+G(d,p) basis set], a complete basis set extrapolation at the MP2 level (MP2/CBS), and the composite CBS‐Q model. This reveals a range of binding enthalpies (ΔH) for phenol–phosphine oxide and phenol–phosphate complexes and their thio analogues. Both structural (bond lengths/angles) and electronic elements (charges, bond orders) are studied. Furthermore, solvent effects are investigated theoretically by the PCM solvent model and experimentally via ITC. From our calculations, a trialkylphosphine oxide is found to be the most promising extractant for phenol in SIRs, yielding ΔH=?14.5 and ?9.8 kcal mol^?1 with phenol and thiophenol, respectively (MP2/CBS), without dimer formation that would hamper the phenol complexation. In ITC measurements, the ΔH of this complex was most negative in the noncoordinating solvent cyclohexane, and slightly less so in π–π interacting solvents such as benzene. The strongest binding is found for the dimethyl phosphate–phenol complex [?15.1 kcal mol^?1 (MP2/CBS)], due to the formation of two H‐bonds (P?O???H‐O‐ and P‐O‐H???O‐H); however, dimer formation of these phosphates competes with complexation of phenol, and would thus hamper their use in industrial extractions. CBS‐Q calculations display erroneous trends for sulfur compounds, and are found to be unsuitable. Computationally relatively cheap SCS‐MP2 and M06‐2X calculations did accurately agree with the much more elaborate MP2/CBS method, with an average deviation of less than 1 kcal mol^?1.     

Hydrogen bonding in acetylacetaldehyde: Theoretical insights from the theory of atoms in molecules

All the possible conformations of tautomeric structures (keto and enol) of acetylacetaldehyde (AAD) were fully optimized at HF, B3LYP, and MP2 levels with 6‐31G(d,p) and 6‐311++G(d,p) basis sets to determine the conformational equilibrium. Theoretical results show that two chelated enol forms have extra stability with respect to the other conformers, but identification of global minimum is very difficult. The high level ab initio calculations G2(MP2) and CBS‐QB3) also support the HF conclusion. It seems that the chelated enol forms have equal stability, and the energy gap between them is probably lies in the computational error range. Finally, the analysis of hydrogen bond in these molecules by quantum theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical investigation of gas phase ethanol–(water)n (n = 1–5) clusters and comparison with gas phase pure water clusters (water)n (n = 2–6)

Various properties (such as optimal structures, structural parameters, hydrogen bonds, natural bond orbital charge distributions, binding energies, electron densities at hydrogen bond critical points, cooperative effects, and so on) of gas phase ethanol–(water)_n (n = 1–5) clusters with the change in the number of water molecules have been systematically explored at the MP2/aug‐cc‐pVTZ//MP2/6‐311++G(d,p) computational level. The study of optimal structures shows that the most stable ethanol‐water heterodimer is the one where exists one primary hydrogen bond (O? H…O) and one secondary hydrogen bond (C? H …O) simultaneously. The cyclic geometric pattern formed by the primary hydrogen bonds, where all the molecules are proton acceptor and proton donor simultaneously, is the most stable configuration for ethanol–(water)_n (n = 2–4) clusters, and a transition from two‐dimensional cyclic to three‐dimensional structures occurs at n = 5. At the same time, the cluster stability seems to correlate with the number of primary hydrogen bonds, because the secondary hydrogen bond was extremely weaker than the primary hydrogen bond. Furthermore, the comparison of cooperative effects between ethanol–water clusters and gas phase pure water clusters has been analyzed from two aspects. First of all, for the cyclic structure, the cooperative effect in the former is slightly stronger than that of the latter with the increasing of water molecules. Second, for the ethanol–(water)₅ and (water)₆ structure, the cooperative effect in the former is also correspondingly stronger than that of the latter except for the ethanol–(water)₅ book structure. © 2012 Wiley Periodicals, Inc.     

An ab initio study of excited states of guanine in the gas phase and aqueous media: Electronic transitions and mechanism of spectral oscillations

Ground state geometries of the four tautomeric forms keto‐N9H, keto‐N7H, enol‐N9H, and enol‐N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4‐31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6‐311+G* basis set was used. These calculations were also extended to hydrogen‐bonded complexes of three water molecules with each of the keto‐N9H (G9‐3W) and keto‐N7H (G7‐3W) forms of guanine. Relative stabilities of the four above‐mentioned tautomers of guanine as well as those of G9‐3W and G7‐3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self‐consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6‐31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto‐N9H and keto‐N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto‐N7H form is most stable in the gas phase, while the keto‐N9H form is most stable in aqueous solution. It appears that both the keto‐N9H and keto‐N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto‐N9H form but the keto‐N7H form of the same would also make some contribution to it. The enol‐N9H and enol‐N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto‐N7H form of the molecule while the fluorescence of oxygen‐rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto‐N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000     

Dimer Mechanism for the Tautomeric Conversion of 4-Amino-2-oxopyrimidine

Values of the enthalpy (ΔH), atomic charges (q_i), and bond orders (P_ij) for six tautomeric forms of 4-amino-2-oxopyrimidine have been calculated by the semiempirical AM1 quantum-chemical method. It was found that the most stable of these is 4-amino-2-oxo-1H-pyrimidine (cytosine). By analogy with complementary pairs of nucleotide bases, the tautomeric conversion of cytosine and other oxo forms can come about via a dimeric intermolecular mechanism. A novel interpretation is proposed for the conversion of cytosine to 4-amino-2-oxo-3H-pyrimidine as a result of a 1H-3H two stage proton transfer.__________Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 564–568, April, 2005.     

Borane–Lewis Base Complexes as Homolytic Hydrogen Atom Donors

Radical stabilization energies (RSE)s have been calculated for a variety of boryl radicals complexed to Lewis bases at the G3(MP2)‐RAD level of theory. These are referenced to the B? H bond dissociation energy (BDE) in BH₃ determined at W4.3 level. High RSE values (and thus low BDE(B? H) values) have been found for borane complexes of a variety of five‐ and six‐membered ring heterocycles. Variations of RSE values have been correlated with the strength of Lewis acid–Lewis base complex formation at the boryl radical stage. The analysis of charge‐ and spin‐density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.     

Quantum chemical studies on prototautomerism of 1H‐imidazo[4,5‐c]pyridine

The structural features of the 1H‐imidazo[4,5‐c]pyridine (ICPY) tautomers and homodimers of the most stable tautomers have been studied by quantum chemical methods. FTIR and Raman spectra of the ICPY were recorded in the range of 4000–60 cm^?1 and 3500–5 cm^?1. The predominant tautomer among four possible isomers of ICPY were determined. The optimized geometries and vibrational frequencies of possible ICPY tautomers and dimers were computed by B3LYP/DFT method with 6‐311++G(d,p) and 6‐31G(d) basis sets. All vibrational frequencies assigned in detail with the help of total energy distribution (TED) and isotopic shifts. ICPY dimeric forms were also characterized according to their hydrogen bonding interactions, and it has been found that the most stable ICPY homodimer establishes moderate strong N ? H …N type hydrogen bond. ¹H NMR, ¹³C NMR, and ¹⁵N NMR properties have been calculated for all tautomeric forms using the gauge independent atomic orbital (GIAO) method. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Matrix-isolation FT-IR study and theoretical calculations of the vibrational, tautomeric and H-bonding properties of hypoxanthine

The neutral compound hypoxanthine is investigated using the technique of matrix-isolation FT-IR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3-LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9-r1) are the three most stable forms. The experimental FT-IR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the H-bonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect.     

Molecular Self‐Recognition: Rotational Spectra of the Dimeric 2‐Fluoroethanol Conformers

Fluoroalcohols show competitive formation of intra‐ and intermolecular hydrogen bonds, a property that may be crucial for the protein‐altering process in a fluoroalcohol/water solution. In this study, we examine the intra‐ and intermolecular interactions of 2‐fluoroethanol (FE) in its dimeric conformers by using rotational spectroscopy and ab initio calculations. Three pairs of homo‐ and heterochiral dimeric FE conformers are predicted to be local minima at the MP2/6‐311++G(d,p) level of theory. They are solely made of the slightly distorted most stable G+g?/G?g+ FE monomer units. Jet‐cooled rotational spectra of four out of the six predicted dimeric conformers were observed and unambiguously assigned for the first time. All four observed dimeric conformers have compact geometries in which the fluoromethyl group of the acceptor tilts towards the donor and ensures a large contact area. Experimentally, the insertion of the O? H group of one FE subunit into the intramolecular O? H???F bond of the other was found to lead to a higher stabilisation than the pure association through an intermolecular O? H???O? H link. The hetero‐ and homochiral combinations were observed to be preferred in the inserted and the associated dimeric conformers, respectively. The experimental rotational constants and the stability ordering are compared with the ab initio calculations at the MP2 level with the 6‐311++G(d,p) and aug‐cc‐pVTZ basis sets. The effects of fluorination and the competing inter‐ and intramolecular hydrogen bonds on the stability of the dimeric FE conformers are discussed.     

An electronic structure theory investigation of the physical chemistry of the intermolecular complexes of cyclopropenylidene with hydrogen halides

The proton accepting and donating abilities of cyclopropenylidene (c‐C₃H₂) on its complexation with hydrogen halides H? X (X = F, Cl, Br) are analyzed using density‐functional theory with three functionals (PBE0, B3LYP, and B3LYP‐D) and benchmarked against second‐order Møller–Plesset (MP2) theory. Standard signatures including, inter alia, dipole moment enhancement, charge transfer from the carbenic lone pair to the antibonding σ*(H? X) orbital, and H? X bond elongation are examined to ascertain the presence of hydrogen bonding in these complexes. The latter property is found to be accompanied with a pronounced red shift in the bond stretching frequency and with a substantial increase in the infrared intensity of the band on complex formation. The MP2/aug‐cc‐pVTZ c‐C₃H₂···H? F complex potential energy surface turns out to be an asymmetric deep single well, while asymmetric double wells are found for the c‐C₃H₂···H? Cl and c‐C₃H₂···H? Br complexes, with an energy barrier of 4.1 kcal mol^?1 for proton transfer along the hydrogen bond in the latter complex. Hydrogen‐bond energy decomposition, with the reduced variational space self‐consistent field approach, indicates that there are large polarization and charge‐transfer interactions between the interacting partners in c‐C₃H₂···H? Br compared to the other two complexes. The C···H bonds are found to be predominantly ionic with partial covalent character, unveiled by the quantum theory of atoms in molecules. The present results reveal that the c‐C₃H₂ carbene divalent carbon can act as a proton acceptor and is responsible for the formation of hydrogen bonds in the complexes investigated. © 2012 Wiley Periodicals, Inc.