1CR : UV-Vis, IR 2 3CR on cold windowfaculty.concordia.ca/muchall/chem426/ri-carbe-detect.pdf · 2:...
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82 f) Detection - usually through matrix isolation techniques (as opposed to laser flash methods) 1 CR 2 : UV-Vis, IR 3 CR 2 : UV-Vis, IR, ESR I. Matrix isolation - molecule is immobilized in a cavity, surrounded by layers of inert material - inert gas matrices: deposition of Ar, N 2 on cold window - two techniques: - mix precursor with host gas and deposit, generate RI in the matrix - generate RI in the gas phase, co-deposit with host gas 10-30 K material depends on spectroscopy (organic glasses: frozen organic solvent, 77 K more easily achieved experimentally “reactive” matrix spectroscopic interference) IR thermolysis, photolysis? Na
Transcript of 1CR : UV-Vis, IR 2 3CR on cold windowfaculty.concordia.ca/muchall/chem426/ri-carbe-detect.pdf · 2:...
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f) Detection- usually through matrix isolation techniques
(as opposed to laser flash methods)1CR2: UV-Vis, IR3CR2: UV-Vis, IR, ESR
I. Matrix isolation- molecule is immobilized in a cavity, surrounded by layers of inert material- inert gas matrices: deposition of Ar, N2 on cold window
- two techniques:- mix precursor with host gas and deposit, generate RI in the matrix- generate RI in the gas phase, co-deposit with host gas
10-30 K material dependson spectroscopy
(organic glasses: frozen organic solvent, 77 Kmore easily achieved experimentally“reactive” matrixspectroscopic interference) IR
thermolysis, photolysis?
Na
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- problems:- precursor must be volatile- RI usually only to be generated by hν, Δ- cage effect might render precursor unsuitable (immediate combination
of fragments)- the matrix is a poor heat-sink: excess energy of the RI can give
rearrangements, fragmentations
- identification of the RI:- through its spectrum- through a change in the spectrum upon
- warming of the matrix (to 25-35 K)- reaction with another co-deposit, e.g. O2, CO product studies
Na
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setuphttp://www.astrochem.org/matrix_isol/MI.html
This link is unfortunately expired; the followingis a good general report on matrix isolation, and pp. 24 onward are on photolysis:
Almond, WiltshireAnnu. Rep. Prog. Chem., Sect. C, 97, 3 (2001)
The 3 basic steps in an experiment:
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Example: Baird, Poliakoff et al.J. Am. Chem. Soc. 103, 5190 (1981)N2
hν
N212 K
:
detection by UV
N2
293
296
low chigh c of deposited
20 K
fulvalene
N2 matrix
ON2
C Ohν
- N2
independent generation
product study
12 K
24 K
: + CO
νCNN 2100 cm-1
νCCO 2130 cm-1
Wolff rearrangement
N2 matrix
cyclopentadienyledene
detection by IR30 K12 K
fulvalene
carbene
N2
ketene
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II. ESR of triplet carbenes- two unpaired e-, S=1, MS= –1, 0, +1
singlet tripletin a magnetic field:
no resultingS
resulting S,affected by B
resulting S,not affected by B
resulting S,affected by B
Gerson, HuberElectron Spin Resonance Spectroscopy of Organic Radicals, Wiley, 2003
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II. ESR of triplet carbenes
- three situations in a magnetic field
E
B1 line
+1
-1
0
0
E
B
+1
-1
0
0
zero-field splitting2 lines
relationship between the two e-
E
B
+1
-1
0
02 E
D
in most carbenes
- two allowed transitions: –1 → 0 and 0 → +1- one forbidden transition: –1 → +1
: both spins flipped simultaneously: sometimes observed at g = 4
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- g = 4 explained: for an unpaired electron, usually g = 2
- the transition -1 → +1 needs twice the energy⇒ for a given energy, that’s half the field strength
E
B
+1
-1
0
0
E = hν = gμBB
- the “zero-field parameters” D and E are reported, usually in cm-1
Gerson, HuberElectron Spin Resonance Spectroscopyof Organic Radicals, Wiley, 2003
g = 4
Iiba, Hirai, Tomioka, YoshiokaJ. Am. Chem. Soc. 124, 14308 (2002)
89
E
B
+1
-1
0
02 E
D
D: gives information about the degree of delocalizationsmaller D = more delocalized
E: gives information on the geometryE = 0: states are degenerate
bond angle on C is 180°
CH
... .
CH
H C H
. .hybridization
F3C CH
CH C CH
CPhPh
C
..
..
..
..
E [cm-1]D [cm-1]
0.712
0.628
0.4050
0.3008
0.021
0.000
0.0194
0.0
0.720 0.0263
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III. Photoelectron spectroscopy of (MeO)2C:- He(I) or UV PES- vacuum gas phase technique- measures the kinetic energy of the ejected photoelectrons:
M + hν → M+. + e-
hν is 21 eV: the ionization potential (ionization energy) is21 eV – Ekin = IP
- determines MO energies from the vertical IPs (IEs):Koopmans’ theorem: IPv ≈ -ε
- technique for valence e- (n, π, σ), not for core e-
advantage
band maximum
http://www.chem.arizona.edu/facilities/pes/facility/PES_description.htm
vertical: IPv
adiabatic: IPa
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setup (Werstiuk, McMaster)
SamplePump
e-
AnalyzerHe(I)
CW CO2 26 W ≈ 450 °C
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- stabilized singlet carbene- reasonable lifetime- detect in the gas phase
N N
O OMeOMe
OOMe
OMe- N2
Δ+ :
experiment
O
8 10 12 14
IP (eV)
N2
O
original spectrum
O
OMe
?
OMe
OMe
MeO
MeO
OMe
O
O
MeO- 2 CH3.
?
Muchall, Werstiuk et al.Can. J. Chem. 76, 238 (1998)
final spectrum
CH3-CH3
9.01 eVOMe
OMe:
O
OMe
OMe
MeO
MeO
OMe
O
O
MeO
subtractedspectra:
