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 ChemsitryDepartment of Chemsitry

University of GeorgiaUniversity of Georgiahttp://www.arches.uga.edu/~maduncan

Funding: National Science Foundation

Non-Classical Non-Classical

0.00.51.01.52.02.53.03.5

En

erg

y (

kc

al/

mo

l)

ClassicalVinyl cation

A. R. Sharma, J. Wu, B. J. Braams, S. Carter, R. Schneider, B. Shepler, and J. M. Bowman, J. Chem. Phys 125, 224306 (2006)

• 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

Protonated Acetylene

2200 2400 2600 2800 3000 3200 3400 3600

1

6

Experiment

Pulsed Discharge Cluster Source

0 25 50 75 100 125

m/z

H+(C2H

2)n

n = 1 2 3 4 5

to ~ n=15

300 psi1:10:20:100C2H2 : H2 : Ar : (70/30) Ne/He

39

Ne+

OPO OPA

1 crystalangle tuned

4 crystalsangle tuned

signal (not used)

idler

532 nm

1064 nm

KTPoscillator

KTAdiff. gen.

+ amp of idler beam

LiInS2

diff. gen.

Tunable4.2-10.1 m

2 crystalsangle tuned

Tunable mid-IR2.3-5.0 m

900 V

750 ns

LaserVision (Dean Guyer)

20 30 40 50 60 70 80

H+(C2H

2)-Ar

m/z

H+(C2H

2)

OPO ON - OFF

2200 2400 2600 2800 3000 3200 3400 3600

H+(C2H

2) - Ar (classical)

1000 1500 2000 2500 3000 3500 4000

1.2 1.3 1.4 1.5 1.6 1.7

0

5

10

15

20

25

30

35

40

45

50

55

En

erg

y (

cm

-1)

Acetylene (c.o.m) - H distance (Angstroms)

MP2 / aug-cc-pVTZ

H+(C2H2)-ArProton stretch

2750 3000 3250 3500

H+(C2H

2)

2-Ar

H+(C2H

2)

4

H+(C2H

2)

3

Wavenumber (cm-1)

H+(C2H

2)-Ar

2750 3000 3250 3500

H+(C2H2)2-Ar

H+(C2H2)3

Wavenumber (cm-1)

C4H5+

(C2H2)

H+ (C2H2)3

1500 2000 2500 3000 3500 4000

?

De(Ar) ~1200 cm-1

Protonated Ethylene

900 1200 1500 1800 2100 2400 2700 3000 3300

Protonated Ethylene Dimer ?

t-butyl cation !

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