Hydrogen bonding is a key non-covalent interaction in biology, supramolecular chemistry, and organic chemistry. The International Union of Pure and Applied Chemistry (IUPAC) defines hydrogen bonding (HB) as an attractive interaction between a hydrogen atom and an atom or cluster of atoms (Z) from a molecule or molecular fragment Y−H, where Y is more electronegative than H. According to the molecular orbital (MO) theory, there are two reasons for the stability of hydrogen bonds: (i) the electrostatic attraction between proton hydrogen and Z in the Y−Hδ+ fragment; and (ii) covalent donor-acceptor interactions caused by the charge transfer of the highest occupant molecular orbital (HOMO) of Z to the Y−H space-σ antibond orbital. The covalentness of hydrogen bonds is manifested by the elongation of the Y−H bond and the associated decrease in the vibrational frequency of the Y−H bond expansion mode, i.e., the redshift. Hydrogen hydride is also known to be involved in intramolecular interactions that do not meet the IUPAC definition of hydrogen bonding. For example, in the case of the charge-inverted hydrogen bond proposed by Jablonski (CIHB), which involves Y−Hδ-...... Z-type system, where Y is less electronegativity than H and Z is an electron-deficient fragment. From this point of view, hydrogen bonding involves all the interactions involving hydrogen atoms, which can be either protonative to form conventional hydrogen bonds, or hydrogenated to form CIHB. Recently, Civis, Hobza, and colleagues proposed a generalization of the IUPAC definition of hydrogen bonding to cover both proton and hydrogenation forms in the same definition. According to the authors, CIHB still retains many important features of hydrogen bonding, such as the transfer of charge to the σY−Hδ− antibond orbital and the elongation of the Y−Hδ− bond, i.e., the red shift. This means that in CIHB, hydrogen hydride behaves as a Lewis acid, while the electron-deficient Z-fragment behaves as a Lewis base; Therefore, the bonding mechanism of hydrogen bonding and CIHB is the same.
在该工作中，来自荷兰阿姆斯特丹自由大学的Célia Fonseca Guerra教授团队以及南非约翰内斯堡大学的F. Matthias Bickelhaupt教授团队通过详细的Kohn-Sham分子轨道分析,表明了含有氢化性氢的复合物并不形成真正的氢键，而是涉及根本不同的键合机制，其中含氢片段作为路易斯碱，而不是路易斯酸。 该工作以题为““Hydridic Hydrogen-Bond Donors” Are Not Hydrogen-Bond Donors”发表在《Journal of the American Chemical Society》上。
【MemYH fragment】The authors first analyzed the protonability or hydride properties of H atoms in the MemYH fragment of the model, where Y is the element of groups 14, 15 and 16 and periods 2, 3 and 4 in the periodic table (Y = C, Si, Ge, N, P, As, O, S, Se, m=3, 2, 1). The Voronoi deformation density (VDD) charge on the H atom directly bound to the Y atom was analyzed. It is found that Y atoms with electronegativity higher than H lead to the depletion of charge on H, i.e., with a positive VDD charge on H, while Y atoms with lower electronegativity than H lead to the accumulation of charge on H, i.e., with a negative VDD charge on H. Therefore, the properties of the Y atom greatly affect the charge of the H atom in the MemYH monomer, making it exhibit protonic, or hydride properties. According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), hydrogen bonds are formed only when the electronegativity of the Y atom is higher than H. In other words, Me3SiH, Me3GeH, Me2PH, and Me2AsH fragments with hydride hydrogen cannot form hydrogen bonds with electron-rich fragments (i.e., Lewis bases).
Figure 1. Schematic diagram of MemYH [On the non-existence of hydrogen bonds] To evaluate the ability of proton properties and hydride hydrogen to participate in the formation of hydrogen bonds, the authors investigated the interaction between MemYH fragments and NH3 to form MemYH··· Complexes of the NH3 type. MemYH··· The NH3 complex is only found in MemYH fragments with protonative or near-neutral hydrogen, i.e., Y=C, N, O, S, and Se, of which the most stable and the shortest hydrogen bond is MeOH··· NH3, ΔE = -7.2 kcal mol–1 and rH ··· N = 1.93 Å, the most unstable and the longest hydrogen bond is Me3CH··· NH3, ΔE = -1.0 kcal mol–1 and rH ··· N = 2.68 Å。 The fragments with hydride hydrogen (Y=Si, Ge, P, As) and the slightly protonative Me3CH form a completely different bonding mode, the Me3YH··· HNH2, these bonding modes are less stable from Me2AsH with ΔE = -2.6 kcal mol–1··· Me3CH with HNH2 to ΔE = -1.4 kcal mol–1··· HNH2。 Protonic MemYH··· NH3 with hydride MemYH··· The different bonding modes in HNH2 are accompanied by different bonding mechanisms. ··· at MemYH In NH3, it can be found that the charge on NH3 is depleted after binding, i.e., ΔQNH3 is positive, that is, the charge is transferred from NH3 to the MemYH fragment. The charge transfer mechanism from NH3 to MemYH is a characteristic covalent component of hydrogen bonds, in which the lone pair orbital of NH3 transfers charge to the empty σY–H antibond orbital of MemYH. As the σ*Y–H becomes more crowded, the Y–H bond elongates, resulting in a characteristic redshift of the vibrational frequency associated with the Y–H bond stretching mode. Conversely, in MemYH involving hydride hydrogen··· In the HNH2 bonding mode, there is no charge transfer, i.e., there is no charge depletion or accumulation on NH3, so there are no hydrogen bonds in these complexes. Next, it is necessary to explain why the hydride fragments cannot form MemYH··· NH3 hydrogen bonding complex. The stable hydrogen bonding complex is formed because the electrostatic attraction between Y–Hδ+ and the electron-rich bond acceptor and the covalent charge transfer interaction overcomes the unstable bubble repulsion between the occupied σY–H bonding orbital and the occupied bond acceptor orbital. The electron density of protonic hydrogen is depleted, causing its positively charged nucleus to be exposed to the electron density from the electron-rich fragment, resulting in a stronger electrostatic force of attraction. On the other hand, hydride hydrogen has an excess electron density, which: (i) shields its positively charged nuclei, resulting in a weakening of the electrostatic attraction between Y–Hδ− and hydrogen bond acceptors; and (ii) increasing the size of σY–H on H atoms, resulting in a more erratic Pauli repulsion between σY–H and the bond acceptor orbital being occupied. Therefore, hydrogen bonding complexes involving hydride hydrogen are not formed as the attraction of electrostatic and charge transfer interactions is not sufficient to overcome the repulsive wall created by the Pauli repulsion force. The authors used canonical energy decomposition analysis (EDA) as the H··· N-bond distance (rH··· N) while keeping the fragment geometry frozen into the equilibrium geometry of the complex to analyze representative MeOH··· NH3、Me3CH··· NH3 and Me3SiH ··· Bonding mechanism of NH3 hydrogen bonding complexes. Me3SiH··· NH3 complex is not bound; Therefore, in this case, we let the Me3SiH and NH3 fragments follow rH in their respective equilibrium geometries, respectively··· N are close to each other. Interaction energy curves from MeOH··· Stabilization of NH3 to Me3CH··· Weak stabilization of NH3 to Me3SiH··· The instability of NH3 reflects the trend of hydrogen bond stability in the complex. In this series, the electronegativity of Y is smaller than that of H, which significantly affects the size of the σY–H orbital occupied on the H atom. From MeOH to Me3CH to Me3SiH, the orbital amplitude on the H atom increases significantly, resulting in an increase in the overlap of repulsive orbitals between the σY–H orbital of MemYH and the LPN orbital of NH3, along the MeOH ··· NH3、Me3CH··· NH3 and Me3SiH ··· The orientation of NH3, thus leading to an increase in Pauli repulsion at the time of bond formation. In addition, the electrostatic interaction curve is in the MeOH ··· NH3、Me3CH··· NH3 and Me3SiH ··· NH3 also becomes less stable, especially around the equilibrium hydrogen bond length. This is a direct result of the reduced electronegativity of Y along O, C, and Si, which makes the electrical properties of the providing H atoms less positive (or more negative) in the same series, resulting in a weakening of the electrostatic attraction between MemYH and the electron-rich NH3 fragment.
Figure 2. Equilibrium geometry [Hydrogen hydride as hydrogen bond acceptor] When the hydride hydrogen atom interacts with an electron-deficient halogen bond donor, can hydrogen bonding be the most dominant interaction? To answer this question, the authors investigated the bonding mechanism between MemYH and ICN, where ICN can form hydrogen bonds with the nitrogen terminus and halogen bonds with the iodine terminus. ··· with MemYH NH3 is similar, involving protonation and MemYH fragments (Y = C, N, O, S, Se) that form a conventional Y–Hδ+··· Nδ− hydrogen bonding, in which charge flows to the hydrogen bond donor MemY–H. The strength of this bond is up to MeOH··· 4.6 kcal mol–1 for NCI. On the other hand, the MemYH fragment (Y = Si, Ge, P, As) involving the hydride hydrogen atom forms a conventional Hδ−··· Iδ+–C halogen bonds, in which charge flows out from the halogen bond acceptor MemY–H to the halogen bond donor I–CN. There is an intermediate case, the Me3CH fragment, which exhibits Me3CH at the same time··· NCI and Me3CH··· ICN has two bonding modes. This change in bonding mode is due to the change from protonic MemYH··· NCI hydrogen bonding to MemYH··· ICN halogen bonds, not hydride MemYH··· ICN hydrogen bonding, which is in line with IUPAC's current definition of hydrogen bonding and halogen bonding. MemYH··· The bonding mechanism in NCI involves conventional Y–Hδ+··· Nδ− hydrogen bonding, so with MemYH··· The bonding mechanism of the NH3 complex is similar. The ICN fragment acts as a hydrogen bond acceptor on the N-side, and the charge is transferred through the LPN orbital of the ICN fragment on that side to the empty σ*Y–H orbital of the MemYH fragment containing protonative hydrogen atoms. However, this donor-acceptor interaction is not sufficient to overcome the increase in steric repulsion when it comes to MemYH fragments with hydride hydrogen atoms, and therefore, for Y = Si, Ge, P, and As, MemYH cannot be found··· NCI complex. Through different bonding mechanisms, namely halogen bonding, the authors constructed the hydride MemYH··· The ICN complex is characterized by a covalent giving-acceptance interaction between the occupancy σY–H orbital of MemYH and the empty σI–C antibond orbital of ICN. Thus, upon binding, the charge flows from the MemYH to the ICN fragment. Note that the hydrogen bonding and halogen bonding mechanisms have opposite directions of charge transfer. Therefore, a negative ΔQICN is clear evidence that there is a strong value in MemYH··· In ICN complexes, there is no hydrogen bonding mechanism capable of overcoming halogen bonding. This is because the ICN is a poor hydrogen bonding acceptor on the I side, i.e., a weak Lewis base, because its I lone pair (LPI) orbital is very low.
Figure 3. MemYH ··· analyzed by geometric configuration and electron density [redshift] of different hydrogen bond types The NH3 hydrogen bonding system is characterized by Y–H bond elongation, which results in a redshift in the Y–H bond expansion frequency. For example, in MeOH··· NH3、MeSH··· NH3 and MeSeH··· In the NH3 series, the Y–H bonds were elongated by 0.016 Å, 0.010 Å and 0.012 Å, respectively, and the stretching frequencies of the symmetrical Y–H bonds in the complex were reduced by -316 cm–1, -125 cm–1 and -134 cm–1, respectively, compared with the isolated MemYH monomer. The elongation of the Y–H bond in the hydrogen bond donor is a manifestation of the covalent nature of the hydrogen bond, i.e., it is caused by the transfer of charge into the empty σY–H orbital by the giving-acceptance interaction. ··· at MeOH NH3、MeSH··· NH3 and MeSeH··· In the NH3 series, the charge of the NH3 fragment is depleted, i.e., ΔQNH3 is positive, while the number of electrons in the empty σY–H orbital increases to 0.04, 0.03, and 0.03 electrons along the same series, respectively. MemYH ··· bonded in halogen bonding In the ICN complex, the Y–H bond is also elongated, resulting in a redshift in the expansion frequency of the Y–H bond. However, due to MemYH··· The hydrogen bonding mechanism in the ICN complex is negligible, so there is no significant change in the electron distribution in the empty σY–H orbital, so the elongation of the Y–H bond in these complexes cannot be explained. Due to halogen bonding, the σY–H bonding orbital loses electrons, and the Y–H bond becomes longer, resulting in a redshift. Therefore, when the hydride Y–Hδ− fragment is used as a Lewis base, the redshift of the expansion frequency of the Y–H bond should also be expected, and is not unique to the hydrogen bond donor. A similar phenomenon was observed when the hydride hydrogen of Me3SiH interacted with various electron-deficient molecules, such as ICF3, BrCN, S(CN)2, P(CN)3, and K+. In these complexes, the Si–H bond elongates and redshifts as the complex forms. However, this is not because Me3SiH acts as a hydrogen bond donor, but as a bond acceptor. The authors found that there was a charge transfer from Me3SiH to electron-deficient fragments and a decrease in the number of electrons occupying the σSi–H bonding orbital, which actually describes different intermolecular interactions, such as halogen, sulfur, and phosphorus bonds. There is a bonding mode in a particular form of dihydrobonding (DHB) in which hydride hydrogen acts as an acceptor for hydrogen bonding. Therefore, the authors provide evidence that the interaction between hydride hydrogen of Me3YH and electron-deficient molecules should not be considered as any type of hydrogen bonding, but rather as a different intermolecular interaction named after the nature of the electron-deficient bond donor.
Figure 4. Energy analysis curves
Figure 5. Summarizing the geometrical configurations of iodide and nitride hydrogen bonds, this paper challenges recent proposals to expand the definition of IUPAC hydrogen bonds to include donors containing hydrogen bonds that are hydrophilic (Y-Hδ+) and hydrophilic (Y-Hδ-). Through detailed Kohn-Sham molecular orbital analysis, the authors showed that complexes containing hydrophilic hydrogen do not form true hydrogen bonds, but involve fundamentally different bonding mechanisms, in which the hydrogen-containing fragments act as Lewis bases, rather than Lewis acids. Characteristic spectral features such as bond elongation and frequency redshift are not unique to hydrogen bonding and may also be due to other types of Lewis acid-base interactions. Therefore, the IUPAC definition of hydrogen bonding should remain limited to the hydrophilic Y-Hδ+ fragment.
--Testing Services--
Source: Frontiers of Polymer Science