spectroscopic methods in inorganic chemistry
TRANSCRIPT
Spectroscopic Methods in Inorganic Chemistry
Dr.Chris
UP Feb 2016
Overview
(I) IR SPECTROSCOPY
Animation
https://www.youtube.com/watch?v=3RqEIr8NtMI
Polarity and Dipole moments
Dipole moment can be calculated as the product of
the charge (abbreviated Q) times the distance
(abbreviated r) between the charges.
The dipole moment of HCl is 1.03 D, and the bond length is
127 pm. What is the percent ionic character of the HCl bond?
First we will assume that this molecule is 100% ionic. In this
case, the charges are separated by the bond length, and we
can calculate the dipole moment in this extreme case.
The actual dipole moment measured for this molecule is 1.03 D, so the molecule is not completely ionic.
http://wps.prenhall.com/wps/media/objects/4678/4791085/ch10_01.htm
Why do CO2 and CCl4 do not have a dipole moment ?
AM1 calculation “ArgusLab” Freeware
Absorption of energy
Types of motions
Stretch:
symmetric asymmetric
wagging twisting scissoring rocking
Bending:
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/Infrared%3A_Theory
WHAT INFLUENCES THE WAVENUMBER OF A BOND ?
(1) Bond energy
https://www.youtube.com/watch?v=9HfJNnoRMPA
(2) Multiple bonds
(3) Atom weights
VIBRATION MODES FROM SYMMETRY
Normal modes of vibration
Triatomic molecules
Character tables – c2v
Vibration modes
https://www.youtube.com/watch?v=QnnAInt4mlM
Examples
bending stretch stretch
Which peaks belong to stretching or bending ?
Planar BF3 molecules
How many degrees of freedom ? (bond vibrations without rotation and translation)
1
2
3
4
5
Answers
1
2
3
4
5
Final result for BF3
E’ A2’’ E’
Examples
Identify stretching and bending modes !
Unknown anion
IR / Raman Simulation www.molwave.com
ORGANO-METAL COMPOUNDS M(CO)X
CO bonding modes
https://books.google.co.th/books?id=oZeFG6QDNekC&pg=PA382&lpg=PA382&dq=M(CO)2L4&source=bl&ots=u9uyncbsDi&sig=Qr5CRFxT1cPpud5vnjs5PWgkkzc&hl=en&sa=X&ved=0ahUKEwjp84-n5eXKAhVSkY4KHQ5QAiUQ6AEIIDAC#v=onepage&q&f=false
The C–O stretching wavenumbers are shifted to lower values when there are changes in the extent of backbonding in the compound. Removing positive charge from the metal causes the shift of electrons from the metal to the CO π∗ orbitals causes the CO wavenumber values to decrease. The highest excess of negative charge on the metal occurs in the [V(CO)6 ]− complex and so more backbonding occurs than in the other complexes. The next highest excess of electron density is in Cr(CO)6 , and then [Mn(CO)6 ]+.
Character of the M-CO bond
Slightly antibonding HOMO
Electron density on the metal
Higher C-O strength
Lower C-O strength M=C=O character
Ligand donation effects
CO IR vibrations
We compare cis- and trans-ML2(CO)2 complexes in IR:
What are the point groups ?
39
Tetrahedral Td Octahedral Oh Linear: D∞h for A-B-A ( i ) C ∞h for A-B
http://en.wikibooks.org/wiki/Introduction_to_Mathematical_Physics/N_body_problem_in_quantum_mechanics/Molecules 40
Character Tables for cis and trans
41
Representations of 2 C-O groups
Which contains the irreducible representations :
Which contains the irreducible representations :
Conclusion: Number of IR peaks for cis and trans complex:
42
Metal-carbonyl compounds
M(II)hexamine complexes
N-H stretch
N-H bend
M-NH3 bend
M-NH3 rock
The spectra presented in Figure 5.6 show a trend in the wavenumber shifts for the three hexamine complexes; the N–H bands shift to lower wavenumbers from Co to Cr to Ni. This indicates that the N–H bond order (bond strength) decreases as the metal–N bond order increases in the stability order mentioned
FUNCTIONAL GROUPS
Find functional groups https://www.youtube.com/watch?v=ItW6Mj2CQKc
https://www.youtube.com/watch?v=9HfJNnoRMPA
Inorganic Compounds
Spectra Database: sdbs.db.aist.go.jp
http://www.chemicalbook.com/SpectrumEN_144-55-8_IR1.htm
Fe(CN)6 complexes
How can we explain the difference to Fe(III) ?
Raman
IR Spectrum of “Rennie”
Example: identify products in Antacids http://www.ptfarm.pl/pub/File/Acta_Poloniae/2000/2/083.pdf
(1) Carbonate Compound
Ref. spectra
Mg CO3
Ca CO3
O-H Ca-O
Mg-O
(2) Hydroxy Compounds
IR of “Maalox (an)”
IR of “Alusal”
Ref. spectra
Al(OH)3
Mg(OH)2
Al(OH)3
Mg(OH)2
RAMAN
https://www.youtube.com/watch?v=TMLnUmbLwUI
SUPPLEMENTS
How do you distinguish whether the structure of transition metal complex molecule M(CO)4L2 is cis or trans by inspection of the CO stretching region of the IR spectra?
-> determine the symmetry group:
CO substitution pattern
-> Check the character tables:
c2v
d4h
Reducing stretching motions
4 stretching vectors
4 0 0 2
4 0 0 0 0 0 0 4 2 0
Experimental
ATR Method – attenuated total reflection
http://cnx.org/contents/[email protected]:EFEBTsu4@2/Attenuated-Total-Reflectance-F