ว 30131 เคมีพื้นฐาน พันธะเคมี ว 30131 เคมีพื้นฐาน พันธะเคมี Molecular Orbital Theory นายศราวุทธ แสงอุไร ครูวิชาการสาขาเคมี โรงเรียนมหิดลวิทยานุสรณ์ วันที่ 11 กันยายน 2557 พันธะเคมี อ.ศราวุทธ แสงอุไร

Molecular Orbital Theory The bonding orbital เครื่องหมายของ wave function เหมือนกันมา Overlap กัน พลังงานต่ำลง The antibonding orbital เครื่องหมายของ wave function ต่างกันกันมา Overlap กัน พลังงานสูงขึ้น

Molecular Orbital Theory Anti-bonding orbital energy level diagram ของ The bonding orbital ที่มีอิเล็กตรอนบรรจุจะมีพลังงานต่ำ. the molecule is stable bonding orbital

Molecular Orbital Theory bonding orbital is sometimes given the notation σ1s + - + The signs on the molecular orbitals indicate the sign of the wave function, not ionic charge.

Molecular Orbital Theory anti-bonding orbital is given the σ*1s + - + The signs on the molecular orbitals indicate the sign of the wave function, not ionic charge.

Rules for Combining Atomic Orbitals The number of molecular orbitals = the number of atomic orbitals combined. The strength of the bond depends upon the degree of orbital overlap.

MOs using p orbitals - - + + - + -

σg + - - - - MOs using p orbitals + + For these orbitals, the bonding orbital is gerade, or symmetric around the bond axis.

π Molecular Orbitals + - - + + - side-by-side overlap

Molecular Orbital Diagrams Electrons preferentially occupy molecular orbitals that are lower in energy. Molecular orbitals may be empty, or contain one or two electrons. If two electrons occupy the same molecular orbital, they must be spin paired. When occupying degenerate molecular orbitals, electrons occupy separate orbitals with parallel spins before pairing.

Molecular Orbital Diagrams Although molecular orbitals form from inner (core) electrons as well as valence electrons, many molecular orbital diagrams include only the valence level.

Molecular Orbital Diagrams many molecular orbital diagrams include only the valence level. H2 He2

Molecular Orbital Diagrams N2

Molecular Orbital Diagrams electrons configuration.

Molecular Orbital Diagrams 12 electrons that must be placed in the diagram.

Molecular Orbital Diagrams For O2, there will be a total of 12 valence electrons that must be placed in the diagram.

( e- in bonding orbtls) - ( e- in anti-bonding orbtls) 2 2 Bond Order Bond order is an indicator of the bond strength and length. A bond order of 1 is equivalent to a single bond. Fractional bond orders are possible. The bond order of the molecule = ( e- in bonding orbtls) - ( e- in anti-bonding orbtls) 2 2

This is consistent with a double bond. MO Diagram for O2 The bond order of O2 is: 8-4 = 2 2 This is consistent with a double bond. 2s

Experimental Evidence Oxygen is paramagnetic

MO Diagram for N2

Rules for Combining Atomic Orbitals For heteronuclear molecules: 1. The bonding orbital(s) will reside predominantly on the atom of lower orbital energy (the more electronegative atom). 2. The anti-bonding orbital(s) will reside predominantly on the atom with greater orbital energy (the less electronegative atom).

HF The 2s and 2px orbitals on fluorine interact with the 1s orbital on hydrogen. The py and pz orbitals on fluorine lack proper symmetry to interact with hydrogen, and remain as non-bonding orbitals.

Carbon monoxide In carbon monoxide, the bonding orbitals reside more on the oxygen atom, and the anti-bonding orbitals reside more on the carbon atom.

Carbon monoxide In carbon monoxide, the bonding orbitals reside more on the oxygen atom, and the anti-bonding orbitals reside more on the carbon atom. Example for NO NO+ NO-