Molecules

E&R Chapter 12

Outline

• Bonding 12-1– Ionic 12-2– Covalent 12-3

• Molecular Excitations 12-4– Rotation 12-5– Vibration 12-6– Electronic– Electronic + Vibration + Rotation 12-7

Bondinghttp://hyperphysics.phy-astr.gsu.edu/hbase/chemical/chemcon.html#c1

R Nave, Georgia State Univ

Chemical Bondinghttp://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bondcon.html#c1

R Nave, Georgia State Univ

Ionic Bonds

RNave, GSU at http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html#c4

Ionic Bonds

Ionic Bonding

RNave, Georgia State Univ at hyperphysics.phy-astr.gsu.edu/hbase/molecule

Ionic Bonding

a.u. = atomic energy unit = 2*(13.6eV) Brandsen&Joachim

E&R

Covalent Bonds

RNave, GSU at http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/bond.html#c4

Covalent Bonding SYM ASYMspatial spin

ASYM SYMspatial spin

Covalent Bonding

Stot = 1

not really parallel, but spin-symmetric

not really anti, but spin-asym

Stot = 0

Covalent Bonding

Bransden&Joachin

Ionic vs Covalent Bond Properties

• Ionic Characteristics– Crystalline solids

– High melting & boiling point

– Conduct electricity when melted

– Many soluble in water, but not in non-polar liquids

• Covalent Characteristics– Gases, liquids, non-

crystalline solids– Low melting & boiling point

– Poor conductors in all phases

– Many soluble in non-polar liquids but not water

Rotational Spectra

)1(22

1

2

1~

222 rr

IIIKErot

R

rotational A.M.moment of inertia

Rotation

Rotation

Rotational Spectra

Ocean Optics: Nitrogen N2

~ 0.3 eV

~ 0.4 eV

Vibration

Molecule“Spring Const”

( N/m )

HF 970

HCl 480

HBr 410

Hi 320

CO 1860

NO 1530

Vibration (in an Electronic state)

Electronic + Vibration

Electronic + Vibration + Rotation

2.656 eV2.550 eV

electronic excitation gap

vibrational excitation gaps

Electronic + Vibration + Rotation

2.656 eV

electronic excitation gap

vibrational excitation gaps

Vibrational WellVibrational Well

depth ~ 0.063 eV

Electronic, Vibration, RotationElectronic ~ optical & UV

~ 1 – 3 eV

Vibration ~ IR ~ 10ths of eV

Rotation ~ microwave ~ 1000ths of eV

Electronic + Vibration + Rotation

2.656 eV2.550 eV

electronic excitation gap

vibrational excitation gaps

** + Vibration + Rotation

IN A DIATOMIC MOLECULEbecause a photon has one unit of

angular momentum,the rotational quantum number

must change(vib is not angular motion)

r = 0 not allowed

** + Vibration + Rotation

Ocean Optics: Nitrogen N2

~ 0.3 eV

~ 0.4 eV

Some Molecular ConstantsMolecule Equilibrium

Distance

Ro (Å)

Dissociation NRG

Do (eV)

Vibrational freq

v a (cm-1)

Moment of Inertia

Bb (cm-1)

H2+ 1.06 2.65 2297 29.8

H2 0.742 4.48 4395 60.8

O2 1.21 5.08 1580 1.45

N2 1.09 9.75 2360 2.01

CO 1.13 9.60 2170 1.93

NO 1.15 5.3 1904 1.70

HCl 1.28 4.43 2990 10.6

NaCl 2.36 4.22 365 0.190

Notes: a) vibrational frequency in table is given as f / c b) moment of inertia in table is given as hbar2/(2I) / hc

Fluorescence

absorb at one wavelength, emit at another

Phosphorescence

tunneling slows the decay process