Theoretical investigation of the halogen bonded complexes between carbonyl bases and molecular chlorine

The halogen bonded complexes between six carbonyl bases and molecular chlorine are investigated theoretically. The interaction energies calculated at the CCSD(T)/aug‐cc‐pVTZ level range between ?1.61 and ?3.50 kcal mol^?1. These energies are related to the ionization potential, proton affinity, and also to the most negative values (V_s,min) on the electrostatic potential surface of the carbonyl bases. A symmetry adapted perturbation theory decomposition of the energies has been performed. The interaction results in an elongation of the Cl? Cl bond and a contraction of the CF and CH bonds accompanied by a blue shift of the ν(CH) vibrations. The properties of the Cl₂ molecules are discussed as a function of the σ*(Cl? Cl) occupation, the hybridization, and the occupation of the Rydberg orbitals of the two chlorine atoms. Our calculations predict a large enhancement of the infrared and Raman intensities of the ν(Cl? Cl) vibration on going from isolated to complexed Cl₂. © 2015 Wiley Periodicals, Inc.     

Theoretical study of the interaction of fluorinated dimethyl ethers and the ClF and HF molecules. Comparison between halogen and hydrogen bonds

A theoretical study of the halogen‐bonded complexes formed between fluorinated dimethyl ethers (n_F = 0–4) and ClF is carried out using the wB97XD method combined with the 6‐311++G(d,p) basis set. The properties of the complexes are compared with the corresponding properties of the hydrogen‐bonded complexes formed between the same electron donors and HF. The optimized geometries, the interaction energies, relevant natural bonding orbital characteristics along with some vibrational data are calculated. The analyzed properties also include the symmetry adapted perturbation theory decomposition of the energies along with the atoms‐in molecule analysis. For both the halogen and hydrogen bonds, the interaction energies are ruled by the intermolecular hyperconjugation energies. In contrast, the correlations between the binding energies and the basic properties of the ethers or the charge transfer are different for the halogen and hydrogen bonds. The applicability of the Bent's rule to these systems is discussed. © 2016 Wiley Periodicals, Inc.     

The HOX⋯SO2 (X=F,Cl, Br,I) Binary Complexes: Implications for Atmospheric Chemistry

Sulfur dioxide and hypohalous acids (HOX, X=F, Cl, Br, I) are ubiquitous molecules in the atmosphere that are central to important processes like seasonal ozone depletion, acid rain, and cloud nucleation. We present the first theoretical examination of the HOX⋯SO₂ binary complexes and the associated trends due to halogen substitution. Reliable geometries were optimized at the CCSD(T)/aug-cc-pV(T+d)Z level of theory for HOF and HOCl complexes. The HOBr and HOI complexes were optimized at the CCSD(T)/aug-cc-pV(D+d)Z level of theory with the exception of the Br and I atoms which were modeled with an aug-cc-pwCVDZ-PP pseudopotential. 27 HOX⋯SO₂ complexes were characterized and the focal point method was employed to produce CCSDT(Q)/CBS interaction energies. Natural Bond Orbital analysis and Symmetry Adapted Perturbation Theory were used to classify the nature of each principle interaction. The interaction energies of all HOX⋯SO₂ complexes in this study ranged from 1.35 to 3.81 kcal mol⁻¹. The single-interaction hydrogen bonded complexes spanned a range of 2.62 to 3.07 kcal mol⁻¹, while the single-interaction halogen bonded complexes were far more sensitive to halogen substitution ranging from 1.35 to 3.06 kcal mol⁻¹, indicating that the two types of interactions are extremely competitive for heavier halogens. Our results provide insight into the interactions between HOX and SO₂ which may guide further research of related systems.     

Computational thermochemistry of glycolaldehyde

We use a variant of the focal point analysis to refine estimates of the relative energies of the four low‐energy torsional conformers of glycolaldehyde. The most stable form is the cis‐cis structure which enjoys a degree of H‐bonding from hydroxyl H to carbonyl O; here dihedral angles τ₁ (O?C? C? O) and τ₂ (C? C? O? H) both are zero. We optimized structures in both CCSD(T)/aug‐cc‐pVDZ and aug‐cc‐pVTZ; the structures agree within 0.01 Å for bond lengths and 1.0 degrees for valence angles, but the larger basis brings the rotational constants closer to experimental values. According to our extrapolation of CCSD(T) energies evaluated in basis sets ranging to aug‐cc‐pVQZ the trans‐trans form (180°, 180°) has a relative energy of 12.6 kJ/mol. The trans‐gauche conformer (160°, ±75°) is situated at 13.9 kJ/mol and the cis‐trans form (0°, 180°) at 18.9 kJ/mol. Values are corrected for zero point vibrational energy by MP2/aug‐cc‐pVTZ frequencies. Modeling the vibrational spectra is best accomplished by MP2/aug‐cc‐pVTZ with anharmonic corrections. We compute the Watsonian parameters that define the theoretical vibrational‐rotational spectra for the four stable conformers, to assist the search for these species in the interstellar medium. Six transition states are located by G4 and CBS‐QB3 methods as well as extrapolation using energies for structures optimized in CCSD(T)/aug‐cc‐pVDZ structures. We use two isodesmic reactions with two well‐established thermochemical computational schemes G4 and CBS‐QB3 to estimate energy enthalpy and Gibbs energy of formation as well as the entropy of the gas phase system. Our extrapolated electronic energies of species appearing in the isodesmic reactions produce independent values of thermodynamic quantities consistent with G4 and CBS‐QB3. © 2013 Wiley Periodicals, Inc.     

Hydrogen Bonding versus Halogen Bonding: Spectroscopic Investigation of Gas-Phase Complexes Involving Bromide and Chloromethanes

Hydrogen bonding and halogen bonding are important non-covalent interactions that are known to occur in large molecular systems, such as in proteins and crystal structures. Although these interactions are important on a large scale, studying hydrogen and halogen bonding in small, gas-phase chemical species allows for the binding strengths to be determined and compared at a fundamental level. In this study, anion photoelectron spectra are presented for the gas-phase complexes involving bromide and the four chloromethanes, CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄. The stabilisation energy and electron binding energy associated with each complex are determined experimentally, and the spectra are rationalised by high-level CCSD(T) calculations to determine the non-covalent interactions binding the complexes. These calculations involve nucleophilic bromide and electrophilic bromine interactions with chloromethanes, where the binding motifs, dissociation energies and vertical detachment energies are compared in terms of hydrogen bonding and halogen bonding.     

Single‐electron halogen bond: Ab initio study

Ab initio calculations have been performed on single‐electron halogen bonds between methyl radical and bromine‐containing molecules to gain a deeper insight into the nature of such noncovalent interactions. Bader's atoms in molecules (AIM) theory have also been applied to the analysis of the linking of the single‐electron halogen bond. Various characteristics of the R? Br…CH₃ interaction, i.e., binding energies, geometrical parameters and topological properties of the electron density have been determined. The presence of the bond critical points (BCPs) between the bromine atom and methyl radical and the values of electron density and Laplacian of electron density at these BCPs indicate the closed‐shell interactions in the complexes. The single‐electron halogen bonds, which are significantly weaker than the normal halogen bonds, exhibit equally bond strength as compared to the single‐electron hydrogen bond. It has been also found that plotting of the binding energies versus topological properties of the electron density at the BCPs gives two straight lines. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Accurate binding energies of hydrogen,halogen, and dihydrogen bonded complexes and cation enhanced binding strengths

Interaction energy (E_int) values of a variety of hydrogen, halogen, and dihydrogen bonded complexes in the weak, medium, and strong regimes have been computed using W1BD, MP2, M06L density functional theory, and hybrid methods MP4//MP2, MP4//M06L, and CCSD(T)//MP2. W1BD level E_int and CCSD(T) results reported in the literature show very good agreement (mean absolute deviation = 0.19 kcal/mol). MP2 underestimates E_int while M06L shows accurate behavior for all except halogen and charge‐assisted hydrogen bonds. MP4//MP2, MP4//M06L, and CCSD(T)//MP2 yield E_int very close to those obtained from W1BD. The high accuracy energy data at MP4/MP2 is used to study the effect of a cation (Li⁺, NH₄⁺) on the E_int. The cation enhances electron donation from the donor to noncovalent bonding region leading to substantial enhancement in E_int (～141–566% for Li⁺ and ～105–539% for NH₄⁺) and promotes a noncovalent bond in the weak regime to medium regime and that in the medium regime to strong regime. © 2014 Wiley Periodicals, Inc.     

Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X,Y = H,F, Cl) in the ground and lowest excited electronic states,part 1: A test of ab initio methods

Geometrical parameters of tetraatomic carbonyl molecules X₂CO and XYCO (X, Y = H, F, Cl) in the ground (S₀) and lowest excited singlet (S₁) and triplet (T₁) electronic states as well as values of barriers to inversion in S₁ and T₁ states and S₁ ← S₀ and T₁ ← S₀ adiabatic transition energies were systematically investigated by means of various quantum‐chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM‐CCSD, CCSD(T), CR‐EOM‐CCSD(T), CASSCF, MR‐MP2, CASPT2, CASPT3, NEVPT2, MR‐CISD, and MR‐AQCC within cc‐pVTZ and cc‐pVQZ basis sets. The accuracy of quantum‐chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR‐EOM‐CCSD(T), and MR‐AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar in S₁ and T₁ electronic states with CO out‐of‐plane angle ranging from 34° (H₂CO, S₁) to 52° (F₂CO, T₁), and height of barrier to inversion varying from 300 (H₂CO, S₁) to 11,000 (F₂CO, T₁) cm^?1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Modeling halogen bonding with planewave density functional theory: Accuracy and challenges

Inspired by the recent interest of halogen bonding (XB) in the solid state, we detail a comprehensive benchmark study of planewave DFT geometry and interaction energy of lone-pair (LP) type and aromatic (AR) type halogen bonded complexes, using PAW and USPP pseudopotentials. For LP-type XB dimers, PBE-PAW generally agrees with PBE/aug-cc-pVQZ(−pp) geometries but significantly overbinds compared to CCSD(T)/aug-cc-pVQZ(-pp). Grimme's D3 dispersion corrections to PBE-PAW gives better agreement to the MP2/cc-pVTZ(-pp) results for AR-type dimers. For interaction energies, PBE-PAW may overbind or underbind for weaker XBs but clearly overbinds for stronger XBs. D3 dispersion corrections exacerbate the overbinding problem for LP-type complexes but significantly improves agreement for AR-type complexes compared to CCSD(T)/CBS. Finally, for periodic XB crystals, planewave PBE methods slightly underestimate the XB lengths by 0.03 to 0.05 Å. © 2019 Wiley Periodicals, Inc.     

A Halogen‐Bonded Dimeric Resorcinarene Capsule

Iodine (I₂) acts as a bifunctional halogen‐bond donor connecting two macrocyclic molecules of the bowl‐shaped halogen‐bond acceptor, N‐cyclohexyl ammonium resorcinarene chloride 1 , to form the dimeric capsule [(1,4‐dioxane)₃@ 1 ₂(I₂)₂]. The dimeric capsule is constructed solely through halogen bonds and has a single cavity (V=511 ų) large enough to encapsulate three 1,4‐dioxane guest molecules.     

Theoretical study on structures and vibrational spectra of M+(H2O)Ar (M = Cu,Ag, Au)

A theoretical study on the structures and vibrational spectra of M⁺(H₂O)Ar_0‐1 (M = Cu, Ag, Au) complexes was performed using ab initio method. Geometrical structures, binding energies (BEs), OH stretching vibrational frequencies, and infrared (IR) absorption intensities are investigated in detail for various isomers with Ar atom bound to different binding sites of M⁺(H₂O). CCSD(T) calculations predict that BEs are 14.5, 7.5, and 14.4 kcal/mol for Ar atom bound to the noble metal ion in M⁺(H₂O)Ar (M = Cu, Ag, Au) complexes, respectively, and the corresponding values have been computed to be 1.5, 1.3, and 2.1 kcal/mol when Ar atom attaches to a H atom of water molecule. The former structure is predicted to be more stable than the latter structure. Moreover, when compared with the M⁺(H₂O) species, tagging Ar atom to metal cation yields a minor perturbation on the IR spectra, whereas binding Ar atom to an OH site leads to a large redshift in OH stretching vibrations. The relationships between isomers and vibrational spectra are discussed. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Nature of the interaction between ammonia derivatives and carbon disulfide. A theoretical investigation

The interactions between NH₃, its methylated and chlorinated derivatives and CS₂ are investigated by ab initio CCSD(T) and density functional BLYP‐D3 methods. The CCSD(T)/aug‐cc‐pVTZ calculated interaction energies of complexes characterized by the S···N chalcogen bonds range between ?1.71 and ?2.78 kcal mol^?1. The S···N bonds are studied by atoms in molecules, natural bond orbital, and noncovalent interaction methods. The lack of correlation between the interaction energies of methylated amines complexes and the electrostatic potential results from the lone pair effect in aliphatic amines. Different structures of CS₂ complexed with ammonia derivatives, stabilized by other than the S···N chalcogen bonds, are also predicted. These structures are characterized by interaction energies ranging between 1.15 and 3.46 kcal mol^?1. The results show that the complexing ability of CS₂ is not very high but this molecule is able to attack the electrophilic or nucleophilic sites of a guest molecule.     

Ditopic halogen bonding with bipyrimidines and activated pyrimidines

The potential of pyrimidines to serve as ditopic halogen‐bond acceptors is explored. The halogen‐bonded cocrystals formed from solutions of either 5,5′‐bipyrimidine (C₈H₆N₄) or 1,2‐bis(pyrimidin‐5‐yl)ethyne (C₁₀H₆N₄) and 2 molar equivalents of 1,3‐diiodotetrafluorobenzene (C₆F₄I₂) have a 1:1 composition. Each pyrimidine moiety acts as a single halogen‐bond acceptor and the bipyrimidines act as ditopic halogen‐bond acceptors. In contrast, the activated pyrimidines 2‐ and 5‐{[4‐(dimethylamino)phenyl]ethynyl}pyrimidine (C₁₄H₁₃N₃) are ditopic halogen‐bond acceptors, and 1:1 halogen‐bonded cocrystals are formed from 1:1 mixtures of each of the activated pyrimidines and either 1,2‐ or 1,3‐diiodotetrafluorobenzene. A 1:1 cocrystal was also formed between 2‐{[4‐(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4‐diiodotetrafluorobenzene, while a 2:1 cocrystal was formed between 5‐{[4‐(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4‐diiodotetrafluorobenzene.     

Magnitude and origin of the attraction and directionality of the halogen bonds of the complexes of C6F5X and C6H5X (X = I, Br, Cl and F) with pyridine

The geometries and interaction energies of complexes of pyridine with C₆F₅X, C₆H₅X (X=I, Br, Cl, F and H) and R_FI (R_F=CF₃, C₂F₅ and C₃F₇) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (E_int) for the C₆F₅X–pyridine (X=I, Br, Cl, F and H) complexes at the basis set limit were estimated to be ?5.59, ?4.06, ?2.78, ?0.19 and ?4.37 kcal mol^?1, respectively, whereas the E_int values for the C₆H₅X–pyridine (X=I, Br, Cl and H) complexes were estimated to be ?3.27, ?2.17, ?1.23 and ?1.78 kcal mol^?1, respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C₆F₅X. The values of E_int estimated for the R_FI–pyridine (R_F=CF₃, C₂F₅ and C₃F₇) complexes (?5.14, ?5.38 and ?5.44 kcal mol^?1, respectively) are close to that for the C₆F₅I–pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short‐range (charge‐transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C₆F₅I– and C₂F₅I–pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water–formaldehyde complex. The calculations suggest that the C? I and C? Br halogen bonds play an important role in controlling the structures of molecular assemblies, that the C? Cl bonds play a less important role and that C? F bonds have a negligible impact.     

DLPNO‐CCSD(T) scaled methods for the accurate treatment of large supramolecular complexes

In this work, we present scaled variants of the DLPNO‐CCSD(T) method, dubbed as (LS)DLPNO‐CCSD(T) and (NS)DLPNO‐CCSD(T), to obtain accurate interaction energies in supramolecular complexes governed by noncovalent interactions. The novel scaled schemes are based on the linear combination of the DLPNO‐CCSD(T) correlation energies calculated with the standard (LoosePNO and NormalPNO) and modified (Loose2PNO and Normal2PNO) DLPNO‐CCSD(T) accuracy levels. The scaled DLPNO‐CCSD(T) variants provide nearly TightPNO accuracy, which is essential for the quantification of weak noncovalent interactions, with a noticeable saving in computational cost. Importantly, the accuracy of the proposed schemes is preserved irrespective of the nature and strength of the supramolecular interaction. The (LS)DLPNO‐CCSD(T) and (NS)DLPNO‐CCSD(T) protocols have been used to study in depth the role of the CH–π versus π–π interactions in the supramolecular complex formed by the electron‐donor truxene‐tetrathiafulvalene (truxTTF) and the electron‐acceptor hemifullerene (C₃₀H₁₂). (NS)DLPNO‐CCSD(T)/CBS calculations clearly reveal the higher stability of staggered (dominated by CH–π interactions) versus bowl‐in‐bowl (dominated by π–π interactions) arrangements in the truxTTF•C₃₀H₁₂ heterodimer. Hemifullerene and similar carbon‐based buckybowls are therefore expected to self‐assemble with donor compounds in a richer way other than the typical concave–convex π–π arrangement found in fullerene‐based aggregates. © 2017 Wiley Periodicals, Inc.     

Predicting the redox properties of uranyl complexes using electronic structure calculations

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO₂(V)‐UO₂(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO₂F₄]^1‐/2‐ and [UO₂Cl₄]^1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO₂]¹⁺ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.     

Development of eclipsed and staggered forms in some hydrogen bonded complexes

Intermolecular hydrogen bonding in X₃CH···NH₃ (X = H, F, Cl, and Br) complexes has been studied by B3LYP, B3PW91, MP2, MP3, MP4, and CCSD methods using 6‐311++G(d,p) and AUG‐cc‐PVTZ basis sets. These complexes could exist in both eclipsed (EC) and staggered (ST) forms. The differences between binding energies of EC and ST forms are negligible and all EC and ST shapes correspond to minimum stationary states. The order of stabilities of them is in an agreement with the results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. On the basis of low differences between binding energies, ST forms are more stable than EC forms in all complexes with the exception of Br₃CH···NH₃, which behaves just opposite. Although the differences between binding energies are negligible, they are consistent with the results of AIM analysis. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A theoretical evidence for cooperativity effects in fluorine-centered halogen bonds: linear (FCN)2–7 and (FNC)2–7 clusters

A theoretical investigation was performed to study cooperative effects in fluorine-centered halogen bond interactions. We investigated geometry, strength, and origin of the interactions in linear (FCN)_2–7 and (FNC)_2–7 clusters by means of MP2 and CCSD(T) methods. Our results strongly suggest that cooperative effects induced by fluorine-centered halogen bonds are significant in both linear FCN and FNC clusters. CCSD(T)/6-311++G** calculations reveal that for (FCN)_2–7 clusters, the average halogen-bonding energy per monomer increases from ?0.76 kcal/mol in dimer to ?0.92 kcal/mol in heptamer. The results of electron density analysis suggest that the capacity of the linear FCN and FNC clusters to concentrate electrons at the F···N and F···C BCPs enhance considerably with cluster size. The results also indicate that the magnitude of cooperative effects is more important for FCN than for FNC clusters. According to energy decomposition analysis, attractive electrostatic and dispersion components make the major contribution to the F···N and F···C halogen bond interactions. An acceptable correlation is found between different energy terms and total interaction energies, revealing the main role of these interactions for stability of linear (FCN) _n and (FNC) _n clusters.     

Ab initio study of Rg2I− (Rg = Ar,Kr, Xe)

The equilibrium geometries, vibrational frequencies, and dissociation energies of rare gas iodine clusters Rg₂I^?(Rg = Ar, Kr, Xe) were calculated at the Hartree–Fock (HF), second‐order Møller–Plesset (MP2), the coupled cluster method with single and double excitation and a noniterative correction for triple excitations method [CCSD(T)] levels. The title species have bent C_2v structure of about 60° angle. The electron correlation effects and relativistic effects on the geometry and stability were investigated at CCSD(T) level. Both effects stabilize title species. The calculated electron affinities are in good agreement with the experimental results available. The effect of high angular momentum functions (g and h) was studied. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A computational study on the nature of the halogen bond between sulfides and dihalogen molecules

The characteristics and nature of the halogen bonding in a series of B···XY (B = H₂S, H₂CS, (CH₂)₂S; XY = ClF, Cl₂, BrF, BrCl, Br₂) complexes were analyzed by means of the quantum theory of “atoms in molecules” (QTAIM) and “natural bond orbital” (NBO) methodology at the second-order Møller-Plesset (MP2) level. Electrostatic potential, bond length, interaction energy, topological properties of the electron density, the dipole moment, and the charge transfer were investigated systematically. For the same electron donor, the interaction energies follows the B···BrF > B···ClF > B···BrCl > B···Br₂ > B···Cl₂ > B···ClBr order. For the same electron acceptor, the interaction energies increase in the sequence of H₂S, H₂CS, and (CH₂)₂S. Topological analyses show these halogen bonding interactions belong to weak interactions with an electrostatic nature. It was found that the strength of the halogen-bonding interaction correlates well with the electrostatic potential associated with halogen atom and the amount of charge transfer from sulfides to dihalogen molecules, indicating that electrostatic interaction plays an important role in these halogen bonds. Charge transfer is also an important factor in the halogen bonds involved with dihalogen molecules.