Hydrogen bonding from molecular orbitals

Valence bond and molecular orbital theory both incorporate the wave description of an atom s electrons into this picture of H2 but m somewhat different ways Both assume that electron waves behave like more familiar waves such as sound and light waves One important property of waves is called interference m physics Constructive interference occurs when two waves combine so as to reinforce each other (m phase) destructive interference occurs when they oppose each other (out of phase) (Figure 2 2) Recall from Section 1 1 that electron waves m atoms are characterized by their wave function which is the same as an orbital For an electron m the most stable state of a hydrogen atom for example this state is defined by the Is wave function and is often called the Is orbital The valence bond model bases the connection between two atoms on the overlap between half filled orbifals of fhe fwo afoms The molecular orbital model assembles a sef of molecular orbifals by combining fhe afomic orbifals of all of fhe atoms m fhe molecule... 

Hydroboration is a stereospecific syn addition that occurs through a four-center TS with simultaneous bonding to boron and hydrogen. The new C—B and C—H bonds are thus both formed from the same face of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene it orbital with the empty p orbital on boron, accompanied by concerted C—H bond formation.158... 

As chemists, much of our intuition concerning chemical bonds is built on simple models introduced in undergraduate chemistry courses. The detailed examination of the H2 molecule via the valence bond and molecular orbital approaches forms the basis of our thinking about bonding when confronted with new systems. Let us examine this model system in further detail to explore the electronic states that arise by occupying two orbitals (derived from the two Is orbitals on the two hydrogen atoms) with two electrons. 

Polarizability-Kow Relationships Molar polarizabilities can be derived from molecular orbital (MO) calculations by the complete neglect of differential overlap (CNDO) method [42]. The following correlation has been found for polar compounds that contain either hydrogen-bond-accepting or hydrogen-bond-donating groups (alkanols, alkanones, dialkyl ethers, alkanenitriles) ... 

The final molecule of this series is methane, the tetrahedral structure of which follows if a fourth unit positive charge is removed from the nucleus in the ammonia lone-pair direction. There are now four equivalent bonding orbitals, which may be represented approximately as linear combinations of carbon s-p hybrid and hydrogen Is functions. The transformation from molecular orbitals into equivalent orbitals or vice versa is exactly the same as for the neon atom. 

K. B. Borisenko, C. W. Bock, I. Hargittai, Intramolecular Hydrogen Bonding and Molecular Geometry of 2-Nitrophenol from a Joint Gas-Phase Electron Diffraction and Ab Initio Molecular Orbital Investigation. J. Phys. Chem. 1994, 98, 1442-1448. 

The formation of a H-bond causes certain changes in the internal geometries of each monomer. For example, the bridging hydrogen moves a little farther away from the donating atom. In addition to the deformations of the nuclear positions, the H-bond formation is accompanied by real redistributions in the electronic structure of each subunit. These polarizations are readily apparent from experimental spectra, as well as from molecular orbital calculations. Taking into account the above perturbations of the two molecules involved in a H-bonded dimer, it is reasonable to presume that the ability of either of these two molecules to form another H-bond is altered by their participation in the first bond . 

The author of [73] deduced a physical model of the hydrogen bond from ah initio molecular orbital wave functions. The characteristic of the model are as follows the dipole moment of the A-H bond /Ta-h ) the difference between the first ionization potential of the electron donor and the noble gas in its row of the Mendeleev Table A / the length R of the hydrogen bonding lone pair. A number of characteristic of the intermolecular strength can be described in terms of these quantities. 

This particular calculation used as a basis set the I5-, 2s-, and three 2p-orbitals of carbon and the l5-orbitals of the four hydrogens. The lowest molecular orbital is principally Is in character. A significant feature of this and other MO calculations on methane is that unlike a picture involving localized bonds derived from 5p -hybrid carbon orbitals, there are not four equivalent orbitals. We can obtain an understanding of this feature of the MO picture by a qualitative analysis of the origin of the methane molecular orbitals. For simplicity, we will consider the orbitals to be derived from the carbon 2s-, 2p -, 2py-, and 2p -orbitals and ignore the carbon Is-orbital. The most convenient frame of reference for the tetrahedral methane molecule is described by a cube with hydrogen atoms at alternate corners and the carbon atom, centered in the cube, as shown below ... 

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