Transcript p208_aiswarya.ppt

Rotational Spectroscopic and Theoretical
Investigations on Benzene-Ethylene Complex
P. Aiswaryalakshmi and E. Arunan
Department of Inorganic and Physical Chemistry,
Indian Institute of Science,
Bangalore. 560012 INDIA
Presented by
Vijayanand Chandrasekaran
Motivation
-stacking or - interaction is important in biology
(DNA), supramolecular chemistry …
Experiments in condensed phase and theoretical works
show evidence of -stacking
However, no gas phase spectroscopic evidence of
clusters of non-ploar molecules showing -stacking
Benzene dimer!!
T-shaped or parallel displaced
-stacking
T-shaped
Parallel Displaced
Gas phase evidence only for T-shaped
Parallel displaced structure has no dipole moment!
Cannot be observed using microwave spectroscopy!
Can we have a non-polar dimer with a dipole
moment?
You can hear more about
benzene dimer tomorrow
Benzene-Ethylene system ??
C6H6-C2H4 complex will have a net dipole moment
Rotational spectroscopic technique can be used to
detect the -stacked structure, if present, in the gas
phase
C-H interactions are also possible
Both benzene and ethylene can act as hydrogen
bond donors
Different possible
structures:
C2H4 as H-donor
-stacked
C6H6 as Hdonor
Three of the possible structures were
considered
-stacking
C2H4 as H-donor
C6H6 as H-donor
All the three can have a net dipole moment
Can be studied using microwave spectroscopy!!
Ab initio Results
-stacking
MP2(full)/
6-311++G**
MP2(full)/
6-311G**
E ( kcal mol-1)
-3.2
-2.5
A (GHz)
2.5453
2.5495
B (GHz)
1.5705
1.6072
C (GHz)
1.5076
1.5418
 (D)
0.2201
0.1738
r (Å)
3.1922
3.2845
C2H4 as H-donor
MP2(full)/
6-311++G**
MP2(full)/
6-311G**
E (kcal mol-1)
-4.2
-2.6
A (GHz)
2.7809
2.7408
B (GHz)
1.2832
1.3287
C (GHz)
1.2723
1.3103
 (D)
0.2777
0.3073
r (Å)
2.4855
2.5664
C6H6 as H-donor
MP2(full)/
6-311G**
E (kcal mol-1)
-0.8
A (GHz)
4.5679
B (GHz)
0.7182
C (GHz)
0.6475
 (D)
0.3337
r (Å)
2.8787
Results from Atoms In Molecules
theoretical calculations
Helps us in understanding ‘bonding’ within the complex!
-stacking
1
2
(BCP)
0.0059
0.0059
L
0.016
0.016
Two bond critical points connecting the C from
Ethylene to C-C bond in benzene (red)
Two ring critical points between ethylene and
Benzene.
C2H4 as H-donor
1
2
(BCP)
0.0059
0.0072
L
0.018
0.022
C6H6 as H-donor
1
(BCP)
0.0057
L
0.014
Bond critical points connect the H from the donor to the acceptor
Criteria for hydrogen bond:
(BCP)  [0.002  0.04] au
L  [-0.15  0.02] au
Numerical values of (BCP) and L shows that both the
geometries with C2H4 and C6H6 as H-donors can form
hydrogen bonded complex
 Pulsed Nozzle Fourier Transform Microwave Spectrometer  the
rotational spectrum of the benzene-ethylene complex
 The search was started on the basis of the prediction made for the
stacked geometry
 The first transition was observed at 7260.6737 MHz
 All the three geometries are nearly prolate top  could get the
corresponding higher J transitions
 Search has been done for ~ 10 GHz (4 GHz- ~ 14GHz)
24 transitions were observed!!!
20 lines were fitted to one of the
optimized structures!!
Experimental results and tentative assignments (MHz):
Transition
Frequency (MHz)
Transition
Frequency (MHz)
212 - 111
4840.4016
515 - 414
12101.3745
202 - 101
4849.8937
505 – 404
12124.1964
202 - 101
4864.5449
505 - 404
12158.6807
211 - 110
4870.3145
12173.1683
313 - 212
7260.6737
514 - 413
14521.7797
303 - 202
7274.7706
616 - 515
14548.6509
303 - 202
7296.4351
606 - 505
312 - 211
7305.0982
606 - 505
14588.7295
414 - 313
9680.9903
615 - 514
14606.1567
404 - 303
9699.5528
Unassigned lines (MHz)
404 - 303
9727.8728
•
7323.7922
413 - 312
9739.4313
•
7321.2977
•
7345.0307
•
9709.4883
Four lines were observed at every J
All the four lines could not be simultaneously fitted
Both extreme lines in each J could be easily assigned to K=1
For K=0, both the middle lines together, independently and an
average of both were tried
Only the average of the middle two lines gave a good fit
K=0 lines show doubling.
The line centre of the K=0 doublet was used
along with K=1 transitions for fitting
Fitted parameters for C6H6-C2H4 complex
Parameters
A (GHz)
5.4(1)
B (MHz)
1221.879(3)
C (MHz)
1206.794(4)
d1(kHz)
-3.84(7)
d2(kHz)
-29(4)
DJ (kHz)
2.31(4)
DJK (MHz)
0.476(2)
Sd (kHz)
7.9
According to the ab initio calculations, this structure is the
global minimum!
Can the other structures be found?
Conclusion:
Rotational spectrum of C6H6 – C2H4 complex has been obtained.
Out of the 24 transitions observed, 20 lines were fitted to the structure with
C2H4 as the hydrogen bond donor.
The unassigned lines could arise from other structures of the dimer or
some unknown complex.
Ab initio and AIM calculations support the observation.
Future Work:
We are planning to study C6H6-C2D4 and C6H5D-C2H4 to
understand the effect of substitution
What is the cause for the doubling of the K=0 lines?
Acknowledgement:
We thank the Indo-French Centre for Promotion of Advanced Research and
Department of Science and Technology (DST) for financial support.
Indian Institute Science for the research fellowship
Thank you all for listening!