Infrared Spectroscopy of H+(C2H2)nAr
and H+(C2H4)nAr (n=1,2)
Gary E. Douberly, Allen M. Ricks, Brian W. Ticknor, and
Michael A. Duncan
Department of Chemsitry
University of Georgia
http://www.arches.uga.edu/~maduncan
Funding: National Science Foundation
Protonated Acetylene
Plasma Chemistry, Astrophysics, Quantum Chemistry, Spectroscopy
•
T. Oka (1989, 1995) High Resolution Spectroscopy of 6 (asymmetric CH
stretch)
Bridged, Non-Classical, Carbonium ion
•
Prototype of Carbonium ions, Reaction Intermediates in Organic Chemistry
Energy (kcal/mol)
•
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Classical
Vinyl cation
Non-Classical
Non-Classical
A. R. Sharma, J. Wu, B. J. Braams, S. Carter, R. Schneider, B. Shepler, and J. M. Bowman, J. Chem. Phys 125, 224306 (2006)
MP2 / aug-cc-pVTZ
6
2
1
2200
2400
2600
2800
3000
3200
-1
Wavenumber (cm )
3400
3600
Experiment
Pulsed Discharge
Cluster Source
n = 1
2
3
4
5
+
H (C2H2)n
39
Ne+
0
25
300 psi
1:10:20:100
C2H2 : H2 : Ar : (70/30) Ne/He
to ~ n=15
50
75
m/z
100
125
+
H (C2H2)
900 V
+
H (C2H2)-Ar
750 ns
OPO ON - OFF
20
30
40
50
m/z
60
Tunable mid-IR
2.3-5.0 m
OPA
OPO
KTP
oscillator
532 nm
1 crystal
angle tuned
1064 nm
idler
signal
(not used)
KTA
diff. gen.
+ amp of idler beam
4 crystals
angle tuned
70
80
Tunable
4.2-10.1 m
LiInS2
diff. gen.
2 crystals
angle tuned
LaserVision (Dean Guyer)
6
2
+
H (C2H2) - Ar
+
H (C2H2) - Ar
2200
2400
(non-classical)
(classical)
2600
2800
3000
3200
-1
Wavenumber (cm )
3400
3600
55
50
-1
Energy (cm )
45
40
35
30
25
20
15
10
5
H+(C2H2)2-Ar
0
1.2
1.3
1.4
1.5
1.6
1.7
m/z=93
losing 40
Acetylene (c.o.m) - H distance (Angstroms)
MP2 / aug-cc-pVTZ
H+(C2H2)-Ar
Proton stretch
x50
1000
1500
2000
2500
3000
-1
Wavenumbers (cm )
3500
4000
+
H (C2H2)-Ar
+
H (C2H2)2-Ar
+
H (C2H2)3
+
H (C2H2)4
2750
3000
3250
3500
-1
Wavenumber (cm )
H+(C2H2)2-Ar
H+(C2H2)3
2750
3000
3250
Wavenumber (cm-1)
3500
H+ (C2H2)3
C4H5+ (C2H2)
Protonated Ethylene
+
H (C2H4)-Ar
m/z=69
losing 40
?
De(Ar) ~1200 cm-1
1500
2000
2500
3000
3500
-1
Wavenumber (cm )
4000
Protonated Ethylene Dimer ?
t-butyl cation !
m/z=97
loss of 40
+
C (CH3)3-Ar
+
H (C2H4)2-Ar
900
1200
1500
1800
2100
2400
2700
-1
Wavenumber (cm )
3000
3300
Summary
• Confirmation of Non-Classical Bridged Structure of Protonated
Acetylene
• First observation of Proton Stretch 2 in H+(C2H2)-Ar
• 900 cm-1 red-shift of Proton Stretch upon formation of Bridged Dimer
• Higher order clusters consist of 4 membered ring C4H5+(C2H2)n
• Formation of various Carbocations in acetylene discharge!
• Infrared Spectroscopy of mass-selected Carbocations!
C+(CH3)3 t-butyl cation