hydrogen bond

(noun)

A weak bond in which a hydrogen atom in one molecule is attracted to an electronegative atom (usually nitrogen or oxygen) in the same or different molecule.

Examples of hydrogen bond in the following topics:

  • Biology: DNA Structure and Replication

    • Hydrogen bonding, brought on by electrostatic interactions, is critical to holding together strands of DNA.
    • Electrostatic interactions, in this case otherwise known as hydrogen bonds, are what hold these bonds together.
    • For adenine and thymine, there are two such hydrogen bonds.
    • These enzymes break the electrostatic hydrogen bonds between the two strands.
    • Describe effect of the DNA replication process on the hydrogen bonds

  • Friction: Static

    • When surface area is below the micrometer range, Van der Waals' forces, electrostatic interactions and hydrogen bonding can cause two materials to adhere to one another.

  • Special Properties of Water

    • Water has this unique characteristic because of the particular nature of the hydrogen bond in H2O.

  • Spherical Distribution of Charge

    • Generally, as the distance between ions increases, the energy of attraction approaches zero and ionic bonding is less favorable.
    • As the magnitude of opposing charges increases, energy increases and ionic bonding is more favorable.
    • The charge distribution of the oxygen molecule is negative, and attracts the two positive hydrogen molecules.
    • It is a polar molecule because there is still a permanent charge separation because the electrons spend more time near the oxygen than the hydrogens.
    • The electrons spend more time near the oxygen than the hydrogens, giving a permanent charge separation as shown.

  • Multielectron Atoms

    • Hydrogen is the only atom in the periodic table that has one electron in the orbitals under ground state.
    • In hydrogen-like atoms (those with only one electron), the net force on the electron is just as large as the electric attraction from the nucleus.
    • Therefore, these electrons are not as strongly bonded to the nucleus as electrons closer to the nucleus.

  • Energy of a Bohr Orbit

    • Based on his assumptions, Bohr derived several important properties of the hydrogen atom from the classical physics.
    • So, if a nucleus has $Z$ protons ($Z=1$ for hydrogen, $Z=2$ for helium, etc.) and only one electron, that atom is called a hydrogen-like atom.
    • The spectra of hydrogen-like ions are similar to hydrogen, but shifted to higher energy by the greater attractive force between the electron and nucleus.
    • Using this equation, the energy of a photon emitted by a hydrogen atom is given by the difference of two hydrogen energy levels:
    • Bohr's model predicted experimental hydrogen spectrum extremely well.

  • The H_2^+ Molecular Ion

    • An example which illustrates much of the physics of diatomic molecules is the hydrogen molecular ion $H_2^+$.
    • For the case in point, we will guess that the wavefunction of the electron is a linear combination of atomic orbitals (LCAO), specifically the $E_j({\bf R})$ state of hydrogen.
    • The states with even parity (gerade) tend to bond.

  • Problems

    • Derive the lifetime of the $n=2, l=1, m=0$ state of hydrogen to emit a photon and end up in the $n=1, l=0, m=0$ state.
    • Consider that the mass fraction of the different atoms are hydrogen (0.7), helium (0.27), carbon (0.008), oxygen (0.016) and iron (0.004).

  • The Discovery of the Parts of the Atom

    • (The hydrogen-1 atom, however, has no neutrons, and a positive hydrogen ion has no electrons. )
    • In 1917 (in experiments reported in 1919), Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of the proton.
    • Earlier, Rutherford learned to create hydrogen nuclei as a type of radiation produced as a yield of the impact of alpha particles on hydrogen gas; these nuclei were recognized by their unique penetration signature in air and their appearance in scintillation detectors.
    • Rutherford determined that the only possible source of this hydrogen was the nitrogen, and therefore nitrogen must contain hydrogen nuclei.
    • One hydrogen nucleus was knocked off by the impact of the alpha particle, producing oxygen-17 in the process.

  • NMR and MRIs

    • Each water molecule has two hydrogen nuclei or protons.
    • When a person is inside the scanner's powerful magnetic field, the hydrogen protons in their body align with the direction of the field.
    • After the electromagnetic field is turned off, the rotations of the hydrogen protons return to thermodynamic equilibrium, and then realign with the static magnetic field.
    • Hydrogen protons in different tissues return to their equilibrium state at different relaxation rates.
    • Images are then constructed by performing a complex mathematical analysis of the signals emitted by the hydrogen protons.