Chem 24 Lecture 3

Chem 24 Lecture 3 - Today we get into Spectroscopy but...

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1 Today we get into Spectroscopy but first, we return to Molecular Orbitals and briefly discuss what we can learn from them
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2 Properties that can be extracted from Molecular Orbitals Dipole Moments Dipole moment units are charge x length Charges are in esu (electrostatic units) or Coulombs 10 -10 esu = 10 -19 Coulombs molecular distances are on the order of 1 Å, or 10 -8 cm so, molecular dipole moments are on the order of 10 -10 esu x 10 -8 = 10 -18 esu cm or 1 Debye Later we will discuss how dipole moments are measured via measurements of the dielectric constant as a function of Temp (showed by Peter Debye)
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3 Properties that can be extracted from Molecular Orbitals Dipole Moments dV dV x x q qx i i i 2 ) ( two charges separated by a distance x two groups of charges for a continuous charge distribution Using the probability distribution from the wavefunction this last equation describes how dipole moments are calculated using large scale quantum mechanical calculations it also says that dipole moments within a molecule are additive (they add like vectors)
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4 Properties that can be extracted from Molecular Orbitals Dipole Moments Recall this amide bond example we expect a large dipole moment here dipole moment is ~3.7 Debye for this peptide bond but dipole moments are additive water peptide bond glycine glycine dipeptide glycine tripeptide glycine tetrapeptide glycine pentapeptide glycine hexapeptide 1.48 3.7 16.7 28.6 37.2 42.6 49.5 52.0
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5 lowest energy configuration of 2 dipoles This secondary structure results in many ways from dipolar interactions, coupled with steric constraints, and the chirality of the amino acids glycine glycine dipeptide glycine tripeptide glycine tetrapeptide glycine pentapeptide glycine hexapeptide 16.7 28.6 11.9 37.2 8.6 42.6 5.4 49.5 6.9 52.0 2.5  for this structure the dipole moments partially cancel, so one doesn't get the full additive affect Properties that can be extracted from Molecular Orbitals Dipole Moments
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6 E + - * cos * E E U Properties that can be extracted from Molecular Orbitals Dipole Moments There is a potential energy of interaction between an applied electric field and a permanent dipole moment Very important for spectroscopy!! But it means that dipoles will orient within an E-field The E-field applies a torque on a molecule through its dipole moment This has implications for the structure of molecular systems
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7 Properties that can be extracted from Molecular Orbitals Polarizeabilities A molecule exposed to an electric field will distort The distortion can lead to an induced dipole moment (also very important for spectroscopies) * 0 E in  o is the permittivity of vacuum (= 8.8x10 -12 kg -1 m -3 s 4 A 2 ) (unitless) is the constant of proportionality, and is called the molecular polarizeability basically scales as volume – really big, diffuse molecular orbitals are more polarizeable that small MOs induced dipoles
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Chem 24 Lecture 3 - Today we get into Spectroscopy but...

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