Anthony C. Legon

Definition of the hydrogen bond (IUPAC Recommendations 2011)

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Definition of the halogen bond (IUPAC Recommendations 2013)

This recommendation proposes a definition for the term “halogen bond”, which designates a specific subset of the inter- and intramolecular interactions involving a halogen atom in a molecular entity.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Monohydrates of cuprous chloride and argentous chloride: H2O⋅⋅⋅CuCl and H2O⋅⋅⋅AgCl characterized by rotational spectroscopy and ab initio calculations

Pure rotational spectra of the ground vibrational states of ten isotopologues of each of H(2)O⋅⋅⋅CuCl and H(2)O⋅⋅⋅AgCl have been measured and analyzed to determine rotational constants and hyperfine coupling constants for each molecule. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the CCSD(T) level. Both experiment and theory are consistent with structures that are nonplanar at equilibrium. The heavy atoms are collinear while the local C(2) axis of the water molecule intersects the axis defined by the heavy atoms at an angle, φ = 40.9(13)° for Cu and φ = 37.4(16)° for Ag. In the zero-point state, each molecule is effectively planar, undergoing rapid inversion between two equivalent structures where φ has equal magnitude but opposite sign. The equilibrium geometry has C(s) symmetry, however. The ab initio calculations confirm that the timescale of this inversion is at least an order of magnitude faster than that of rotation of the molecule in the lowest rotational energy levels. The molecular geometries are rationalized using simple rules that invoke the electrostatic interactions within the complexes. Centrifugal distortion constants, Δ(J) and Δ(JK), nuclear quadrupole coupling constants, χ(aa)(Cu), χ(aa)(Cl), (χ(bb) - χ(cc))(Cu), and (χ(bb) - χ(cc))(Cl), and the nuclear spin-rotation constant of the copper atom, C(bb)(Cu)+C(cc)(Cu), are also presented.

Characterisation of H2S⋯CuCl and H2S⋯AgCl isolated in the gas phase: A rigidly pyramidal geometry at sulphur revealed by rotational spectroscopy and ab initio calculations

Pure rotational spectra of the ground vibrational states of eight isotopologues of H(2)S···CuCl and twelve isotopologues of H(2)S···AgCl have been analysed allowing rotational constants and hyperfine coupling constants to be determined. The molecular structures have been determined from the measured rotational constants and are presented alongside the results of calculations at the CCSD(T) level. Both molecules have C(s) symmetry at equilibrium and are pyramidal at the sulphur atom. The chlorine, metal, and sulphur atoms are collinear while the local C(2) axis of the hydrogen sulphide molecule intersects the axis defined by the heavy atoms at an angle, φ = 74.46(2)° for Cu and φ = 78.052(6)° for Ag. The molecular geometries are rationalised using simple rules that invoke the electrostatic interactions within the complexes. Centrifugal distortion constants, Δ(J), and nuclear quadrupole coupling constants, χ(aa)(Cu) and χ(aa)(Cl) for H(2)S···CuCl are presented for the first time. The geometry of H(2)S···AgCl is determined with fewer assumptions and greater precision than previously.

A prototype transition-metal olefin complex C2H4⋯AgCl synthesised by laser ablation and characterised by rotational spectroscopy and ab initio methods

C(2)H(4)···Ag-Cl has been synthesised in the gas phase in a pulsed-jet, Fourier-transform microwave spectrometer by the reaction of laser-ablated metallic silver with carbon tetrachloride to give AgCl, which subsequently reacts with ethene to give the complex. The ground-state rotational spectra of six isotopologues (C(2)H(4)···(107)Ag(35)Cl, C(2)H(4)···(109)Ag(35)Cl, C(2)H(4)···(107)Ag(37)Cl, C(2)H(4)···(109)Ag(37)Cl, (13)C(2)H(4)···(107)Ag(35)Cl, and (13)C(2)H(4)···(109)Ag(35)Cl) were recorded and analysed to give rotational constants A(0), B(0), and C(0), centrifugal distortion constants Δ(J) and Δ(JK), and Cl nuclear quadrupole coupling constants χ(aa)(Cl) and χ(bb)(Cl)-χ(cc)(Cl). These spectroscopic constants were interpreted in terms of a geometry for C(2)H(4)···Ag-Cl of C(2V) symmetry in which the AgCl molecule lies along the C(2) axis of ethene that is perpendicular to the C(2)H(4) plane. The Ag atom forms a bond to the midpoint (*) of the ethene π bond. A partial r(s)-geometry and a r(0)-geometry were determined, with the values r(*···Ag) = 2.1719(9) Å, r(C-C) = 1.3518(4) Å, and r(Ag-Cl) = 2.2724(8) Å obtained in the latter case. The C-C bond lengthens on formation of the complex. Detailed ab initio calculations carried out at the CCSD(T)/cc-pVQZ level of theory give results in good agreement with experiment and also reveal that the ethene molecule undergoes a small angular distortion. The distortion is such that the four H atoms move in a direction away from Ag but remain coplanar. The two C atoms are no longer contained in this plane, however. The electric charge redistribution when C(2)H(4)···Ag-Cl is formed and the strength of the π···Ag bond are discussed.

Experimental Detection and Properties of H2O⋅⋅⋅AgCl and H2S⋅⋅⋅AgCl by Rotational Spectroscopy

We report the first generation and characterization of two simple compounds formed by the interaction of either H2O or H2S with AgCl, namely H2O···Ag Cl and H2S···Ag Cl. They were observed in the gas phase by rotational spectroscopy. The AgCl is produced by laser ablation of metallic silver in the presence of CCl4 and then picks up an H2O or H2S molecule. AgCl is known, from interpretation of its Cl nuclear quadrupole coupling constant, to have a fractional ionic character of approximately 0.7, so that it has significant ionpair character. The interaction of AgCl with H2Omolecules, and in particular one H2O molecule, is of fundamental chemical interest because the H2O molecules have the opportunity to interact with an incipient Ag ion. How does this interaction differ from those of H2O with the less polar, covalent Lewis acids HCl and ClF (fractional ionic characters of 0.25 and 0.35)? The hydrogenand halogen-bonded complexes H2O···H Cl and H2O···Cl F have each been investigated in the gas phase by rotational spectroscopy as part of an extensive systematic program. H2O···H Cl and H2O···Cl F have equilibrium geometries of Cs symmetry, [2,3] with a pyramidal configuration at O when HCl or ClF forms either a hydrogen or halogen bond to that atom (see Figure 1). In each case, however, there is a low potential-energy barrier to the planar, C2v geometry so that even in the zero-point state the molecule is inverting and is effectively planar. On the other hand, in the analogous pair of complexes H2S···H Cl and H2S···Cl F the potential energy barrier is high enough to ensure that each has a permanently pyramidal configuration at S, with HCl or ClF forming a weak bond to S at approximately 908 to the H2S subunit, as shown in Figure 1. These angular geometries, and those of many other hydrogenand halogen-bonded complexes, can be rationalized by means of some simple empirical rules. For both H2O and H2S acting as Lewis bases, the electrophilic ends H and Cl of the weakly polar molecules HCl and ClF, respectively, are assumed to seek the axis of a nonbonding electron pair carried by the base. A further question is: Are the angular geometries of the resulting molecules H2Y···Ag Cl isomorphic with those of H2Y···H Cl and H2Y···Cl F (Y=O or S), indicating that the empirical rules are also obeyed when AgCl is the Lewis acid? All B···M X (B=CO, M=Cu, Ag, Au) have a linear arrangement similar to those in OC···H Cl and OC···Cl F. Thus, in all these systems, the Lewis acid attaches along the axis of the n-pair on C. A linear geometry was also found in N2···Cu F. Observed rotational transitions of both H2O···Ag Cl and H2S···Ag Cl were characteristic of a nearly prolate asymmetric rotor of large A value having only a-type transitions, which exhibit Cl nuclear quadrupole hyperfine structure. For H2O···Ag Cl, R-branch K 1= 1 transitions of the type (J+ 1)1,J+1 !J1,J and (J+ 1)1,J !J1,J 1 were observed in addition to the (J+ 1)0,J+1 !J0,J series and for a given J were more intense than those having K 1= 0. This observation confirms that the molecule has a pair of equivalent H nuclei exchanged by a rotationC2 about the a axis and therefore that the equilibrium geometry of H2O···Ag Cl is either C2V planar at equilibrium or Cs pyramidal but with a potential-energy barrier to planarity low enough that the v= 0 and 1 states associated with the motion that inverts the configuration at the O atom are well separated. For H2S···Ag Cl only the (J+ 1)0,J+1 !J0,J series could be detected despite a careful search, an observation consistent with a pyramidal configuration at the S atom and no inversion on the microwave timescale. The reason for the different behavior with respect to inversion is presumably that H2S···Ag Cl is more strongly bound than H2O···Ag Cl (see below) and that there is a much larger angle between the n-electron pairs on S than on O. The result is a higher and wider barrier to inversion in H2S···Ag Cl. The usual arguments show that for H2O···Ag Cl in the ground state the K 1= 1 levels occur in combination with the three symmetric proton spin functions while theK 1= 0 levels combine with the single antisymmetric function. Moreover, population transfer from K 1= 1 levels into K 1= 0 levels during the supersonic expansion is hindered by a collisional propensity rule which forbids triplet state (K 1= 1) to Figure 1. Angular geometries of several H2Y M X compounds; d indicates the local C2 axis of the H2O or H2S subunit.

Molecular geometry of OC···AgI determined by broadband rotational spectroscopy and ab initio calculations.

Pure rotational spectra of the ground vibrational states of six isotopologues of OC···AgI have been measured by chirped-pulse Fourier transform microwave spectroscopy. The spectra are assigned to determine the rotational constant, B(0), centrifugal distortion constant, D(J), and nuclear quadrupole coupling constant of the iodine atom, χ(aa)(I). The complex is linear. Isotopic substitutions at the silver, carbon, and oxygen atoms allow bond lengths to be established by the r(0), r(s), and r(m)((1)) methods of structure determination. The length of the C-O bond, r(CO), in the r(0) geometry for OC···AgI is 0.008 Å shorter than that found in the free CO molecule. The length of the Ag-I bond, r(AgI), is 0.013 Å shorter than in free AgI. χ(aa)(I) is determined to be -769.84(22) MHz for OC···(107)AgI implying an ionic character of 0.66 for the metal halide bond. Attachment of carbon monoxide to the isolated AgI molecule results in an increase of the ionic character of AgI of 0.12. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the explicitly correlated coupled-cluster singles, doubles and perturbative triples level. Vibrational frequencies, the electric dipole moment, the nuclear quadrupole coupling constant, and the dissociation energy of the molecule have been calculated.

Changes in the Geometries of C2H2 and C2H4 on Coordination to CuCl Revealed by Broadband Rotational Spectroscopy and ab-Initio Calculations

The molecular geometries of isolated complexes in which a single molecule of C2H4 or C2H2 is bound to CuCl have been determined through pure rotational spectroscopy and ab-initio calculations. The C2H2···CuCl and C2H4···CuCl complexes are generated through laser vaporization of a copper rod in the presence of a gas sample undergoing supersonic expansion and containing C2H2 (or C2H4), CCl4, and Ar. Results are presented for five isotopologues of C2H2···CuCl and six isotopologues of C2H4···CuCl. Both of these complexes adopt C(2v), T-shaped geometries in which the hydrocarbon binds to the copper atom through its π electrons such that the metal is equidistant from all H atoms. The linear and planar geometries of free C2H2 and C2H4, respectively, are observed to distort significantly on attachment to the CuCl unit, and the various changes are quantified. The ∠(*-C-H) parameter in C2H2 (where * indicates the midpoint of the C≡C bond) is measured to be 192.4(7)° in the r0 geometry of the complex representing a significant change from the linear geometry of the free molecule. This distortion of the linear geometry of C2H2 involves the hydrogen atoms moving away from the copper atom within the complex. Ab-initio calculations at the CCSD(T)(F12*)/AVTZ level predict a dihedral ∠(HCCCu) angle of 96.05° in C2H4···CuCl, and the experimental results are consistent with such a distortion from planarity. The bonds connecting the carbon atoms within each of C2H2 and C2H4, respectively, extend by 0.027 and 0.029 Å relative to the bond lengths in the isolated molecules. Force constants, k(σ), and nuclear quadrupole coupling constants, χ(aa)(Cu), [χ(bb)(Cu) - χ(cc)(Cu)], χ(aa)(Cl), and [χ(bb)(Cl) - χ(cc)(Cl)], are independently determined for all isotopologues of C2H2···CuCl studied and for four isotopologues of C2H4···CuCl.

Distortion of ethyne on formation of a π complex with silver chloride: C2H2⋯Ag–Cl characterised by rotational spectroscopy and ab initio calculations

C(2)H(2)⋯Ag-Cl was formed from ethyne and AgCl in the gas phase and its rotational spectrum observed by both the chirped-pulse and Fabry-Perot cavity versions of Fourier-transform microwave spectroscopy. Reaction of laser-ablated silver metal with CCl(4) gave AgCl which then reacted with ethyne to give the complex. Ground-state rotational spectra of the six isotopologues (12)C(2)H(2)⋯(107)Ag(35)Cl, (12)C(2)H(2)⋯(109)Ag(35)Cl, (12)C(2)H(2)⋯(107)Ag(37)Cl, (12)C(2)H(2)⋯(109)Ag(37)Cl, (13)C(2)H(2)⋯(107)Ag(35)Cl, and (13)C(2)H(2)⋯(109)Ag(35)Cl were analysed to yield rotational constants A(0), B(0), and C(0), centrifugal distortion constants Δ(J), Δ(JK), and δ(J), and Cl nuclear quadrupole coupling constants χ(aa)(Cl) and χ(bb)(Cl)-χ(cc)(Cl). A less complete analysis was possible for (12)C(2)D(2)⋯(107)Ag(35)Cl and (12)C(2)D(2)⋯(109)Ag(35)Cl. Observed principal moments of inertia were interpreted in terms of a planar, T-shaped geometry of C(2v) symmetry in which the AgCl molecule lies along a C(2) axis of ethyne and the Ag atom forms a bond to the midpoint (∗) of the ethyne π bond. r(0) and r(m)(1) geometries and an almost complete r(s)-geometry were established. The ethyne molecule distorts on complex formation by lengthening of the C≡C bond and movement of the two H atoms away from the C≡C internuclear line and the Ag atom. The r(m)(1) bond lengths and angles are as follows: r(∗⋯Ag) = 2.1800(3) Å, r(C-C) = 1.2220(20) Å, r(Ag-Cl) = 2.2658(3) Å and the angle H-C≡∗ has the value 187.79(1)°. Ab initio calculations at the coupled-cluster singles and doubles level of theory with a perturbative treatment of triples (F12∗)∕cc-pVTZ yield a r(e) geometry in excellent agreement with the experimental r(m)(1) version, including the ethyne angular distortion.