Transcript RAPCol05_submitted.ppt

Inversion Motions in the Dimethyl EtherCarbon Disulfide Dimer
Rebecca A. Peebles, Josh J. Newby and Sean A. Peebles
Department of Chemistry
Eastern Illinois University
Charleston, IL 61920
Introduction
• Increasing interest in C-H and other weak
hydrogen bonds
• Dimethyl ether (DME) complexes
-Relatively acidic hydrogen atoms
-Oxygen atom with lone pairs
• DME-HCCH,1 DME-CO2,2 DME-OCS3
• DME-HF,4 DME-HCl,5 DME-Rg6
1Newby,
J.J.; Serafin, M.M.; Peebles, R.A.; Peebles, S.A. J. Phys. Chem. A. 2005, 109, 5316.
J.J.; Peebles, R.A.; Peebles, S.A. J. Phys. Chem. A 2004, 108, 7372.
3Newby, J.J.; Peebles, R.A.; Peebles, S.A. J. Phys. Chem. A 2004, 108, 11234.
4Ottaviani, P.; Caminati, W.; Velino, B.; Blanco, S.; Lessarri, A.; López, J.; Alonso, J. ChemPhysChem. 2004, 5, 336.
5Ottaviani, P.; Caminati, W.; Velino, B.; López, J.C. Chem. Phys. Lett. 2004, 394, 262.
6Multiple references
2Newby,
F
C
l
Experimental Methods
• Balle-Flygare Fourier transform microwave
spectrometer, 4.5 – 16 GHz
• ~1.5% each DME and CS2 pressurized to
2 atm with He/Ne
• Stark effects with voltages up to 5 kV
– Calibrated with J = 1  0 of OCS
Spectrum
Arb. intensity
• c-type transitions doubled by ~180 MHz
Q-branch series from J = 1 to J = 7
R-branch series from J = 1 – 0 to J = 6 – 5
• a-type transitions doubled by 50 – 500 kHz
R-branch series from J = 4 – 3 to J = 9 – 8
• No resolved splittings
707-606
from internal rotation
of DME methyl
groups
8306
8306.5
8307
Frequency / MHz
8307.5
8308
16000
616 - 606
212 - 202
14000
313 - 303
12000
717 - 707
625-524, 624-523
616-515
606-505
200000
Arbitrary Intensity
10000
Frequency/MHz
414 - 404
8000
515 - 505
6000
615-514
4000
111 - 101
a-type
c-type, lower component
c-type, upper component
0
7000
7200
7400
Frequency / MHz
7600
Spectroscopic Constants
0–
0+
A (average) / MHz
8099.5906(12)
B / MHz
604.67510(18)
604.65215(18)
C / MHz
582.50781(23)
582.55142(23)
DJ / kHz
0.4804(12)
0.4819(12)
DJK / kHz
1.28(7)
1.536(7)
0.0191(11)
0.0150(11)
d1 / kHz
d2 / Hz
–3.49(64)
HJKK / kHz
–0.057(15)
DE / MHz
90.3411(14)
N
78
Dnrms / kHz
2.7
Structural Considerations
Monomer
Dimer
Paa or Pbb (u Å2) 47.04660(3)
47.07653(5)
Pcc (u Å2)
15.31910(4)
3.207326(3)
ma = 0.924(8) D, mc = 1.101(21) D,
mtotal = 1.438(17) D
• CS2 must lie in the plane bisecting the COC angle
of the monomer
MP2/6-311++G(2d,2p) Results
S
S
C
S
I
II
S
IV
III
I
II
III
IV
Expt.
A/MHz
8068
2621
10100
9546
8099.6
B/MHz
617
1151
544
558
604.7
C/MHz
595
939
520
534
582.5
DE/kJ mol-1
-13.025
-12.425
-11.958
-11.622
–
DEcorrected/kJ mol-1
-6.955
-5.620
-7.075
-7.328
–
Barrier to Inversion
• Meyer one-dimensional model1
2 2
• Potential function of the form: V ( )  B2 1    e  
• Also varied a according to: a ( )    a e
a

1Meyer,
R., J. Mol. Spectrosc. 1979, 76, 266.
 e 
S
C
ae
ae = 12.5°
S
Rcm = 4.83Å
e
e
e = 37.9°
B2 = 78cm-1
-
-37.9
0
+37.9
+
ae
Intermolecular Interaction
• Force constant, ks, for intermolecular bond:

16 4 mRcm  4 B 4  4C 4  B  C  ( B  C ) 2
ks 
hD J
2
2
• Binding energy, EB, estimated from:
 1 
2
E B    k s Rcm
 72 
• ks = 2.57(6) N m-1, EB = 5.0(2) kJ mol-1
– Similar to DME-rare gas complexes

Comparison of Force Constants
Complex
DME-Ne
DME-Ar
DME-CS2
DME-Kr
DME-Xe
(DME)2
DME-OCS
DME-CO2
DME-HF
ks (N m-1)
1.0
2.3
2.57(6)
2.6
3.0
4.7
6.7(2)
10.9(2)
12(2)
EB (kJ mol-1)
1.0
2.5
5.0(2)
2.9
3.7
5.7
7.1(2)
9.7(2)
9(2)
Conclusions
• Theory and experiment consistent with Cs
symmetry; CS2 along lone pair of DME
• Tunneling of CS2 between DME lone pairs
– Barrier ~78(1) cm-1
– Tunneling frequency 90.3411(14) MHz
• MP2/6-311++G(2d,2p) calculations provide
useful structural predictions
• BSSE and ZPE corrections are essential for
determining energy ordering
Acknowledgements
• The American Chemical Society Petroleum
Research Fund, #39752-GB6
• Prof. Walther Caminati
• Prof. Robert Kuczkowski
• Michal Serafin
Evidence for/against Structure I
• Planar moments of I seem to agree better
with experiment (but IV is also close)
• Dipole moment components of I agree
better with experiment
• Tunneling pathway and barrier calculation
make more sense in light of IV
• Both I and IV can be brought into good
agreement with experiment via minor
structural modifications
• I is similar to other linear molecules; IV is
similar to noble gases
Dipole Moment Components
I
II
III
IV
Expt.
A/MHz
8068
2621
10100
9546
8099.6
B/MHz
617
1151
544
558
604.7
C/MHz
595
939
520
534
582.5
DE/kJ mol-1
-13.025
-12.425
-11.958
-11.622
–
DEcorrected/kJ mol-1
-6.955
-5.620
-7.075
-7.328
–
ma/D
0.902
0.201
2.266
1.933
0.924(8)
mb/D
0
0.994
0
0
0
mc/D
1.294
0
0
0.757
1.101(21)
mtotal/D
1.577
1.014
2.266
2.076
1.438(17)
Paa/u Å2
802
392
925
900
820
Pbb/u Å2
46
146
46
47
47
Pcc/u Å2
16
47
4
6
15