Transcript Ab Initio and Experimental Studies of the E.ppt

Ab Initio and Experimental
Studies of the E Internal Rotor
State of He-CH3F
Kelly J. Higgins, Zhenhong Yu, and William
Klemperer, Department of Chemistry and
Chemical Biology, Harvard University
Outline
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Introduction
Intermolecular Potential Energy Surface
Bound States
Experimental Methods
A Internal Rotor State
Modified Potential
E Internal Rotor State
Introduction – Helium Complexes
• Sample an appreciable amount of the entire
potential energy surface
• Lower bound states tend to be localized, but
with some overlap
• Intermolecular potentials are qualitatively
similar to heavier rare gas complexes
• Accurate He-molecule potentials useful for
helium nanodroplet spectroscopy
• Excellent test of computational methods
Introduction – He-CH3F
• CH3F exists as two essentially non-interconverting
species
– IHtot= 3/2 and K = 0, 3n (A state He-CH3F)
– IHtot = 1/2 and K = 1, 3n ± 1 (E state He-CH3F)
• Low temperature helium pressure broadening
studies of CH3F by De Lucia and coworkers and
Willey and coworkers (1988-1995)
• One study of CH3F in helium nanodroplets
reported in A. Conjusteau’s Ph.D. thesis at
Princeton (2002)
• SAPT potential by Bussery-Honvault et al (2003)
Study Flow
Calculate ab initio potential and bound states
Observe A-state transitions
Morph ab initio potential to fit observed Astate transitions
Predict E-state bound states and transitions
Observe E-state transitions
He-CH3F Intermolecular Potential
6
-5
-10
-15
4
Å
-20
0
-44.790
2
-46.903
-30.929
C
0
-6
-4
-2
F
0
2
4
6
Å
• MP4/7s5p3d2f(C,F) 6s3p2d(H,He) + 3s3p2d(bond)
• Counterpoise corrected
Bound States
(a)
60
45
18
0
15
0
/
15
12
0
90
60
30
30
0
• Calculated using
Hutson’s BOUND or
Cohen and Saykally’s
collocation program
0
/

(b)
• T-shaped ground state
and linear excited state
60
18
0
15
0
/
15
12
0
90
60
• Nearly free internal
rotation
30
30
0
45
0
/

(c)
60
18
0
15
0
/
15
12
0
90
60
30
30
0
45
0
/

A-State Energy Levels
0
-2
-1
Energy (cm )
-4
-6
J
J
J
J
J
J
J
-8
-10
-12
T-shaped ground state
H-end
F-end
=6
=5
=4
=3
=2
=1
=0
T-shaped
stretch
Experimental Methods
• Fraser-type electric resonance spectrometer
with liquid-He cooled bolometer detector
– Broadband frequency range from 10 MHz to 
– Simple to perform multiple resonance
experiments to find and assign lines
– Fast scanning compared to FTMW
• FTMW in Pat Thaddeus’ Lab
– Higher resolution to resolve hyperfine structure
Observed A-state Transitions
-5
-6
-1
Energy (cm )
-7
-8
-9
J
J
J
J
J
-10
-11
-12
T-shaped ground state
linear
=4
=3
=2
=1
=0
Morphed Potential
Vmorph ( R,  ,  )  E
HF
int
n

MP 4
HF
( R,  ,  )   cl Pl (cos  ) Eint
( R, ,  )  Eint
( R,  ,  )

l 0
• Morph only the correlation energy
– Deepens the well and moves it in radially at the same time
– Calculation is at the basis set limit for SCF but not for
electron correlation
– Requires fewer parameters than morphing the entire
interaction potential
• Morph to best reproduce fitted A-state constants
– Constants: c0 = 1.017727, c1 = 0.017103, c2 = 0.003511,
and c3 = -0.033605 reduce relative rms deviation from
0.42 to 0.022
Morphed Potential
6
-5
-10
-15
4
Å
-20
0
-46.503
2
-50.041
-31.171
C
0
-6
-4
-2
F
0
Å
2
4
6
Predicted E-state Levels
3
-1
Energy (cm )
1
-1
-3
F end
H end
T-shaped
T-shaped
-5
-7
J= 0
1
2
3
4
5
Observed E-state Transitions
3
-1
Energy (cm )
1
-1
-3
F end
H end
T-shaped
T-shaped
-5
-7
J= 0
1
2
3
4
5
E-state 01  11 Transition (FTMW)
12435.5
12435.7
12435.9
Frequency (MHz)
12436.1
E-state 12  01 Transition (FTMW)
21172.2
21172.4
21172.6
Frequency (MHz)
21172.8
E-state 22  11 Transition (MBER)
67475.2
67475.6
67476.0
Frequency (MHz)
67476.4
E-state 13  01 Transition (MBER)
71178.4
71178.8
71179.2
71179.6
Frequency (MHz)
71180.0
Measured E-state Frequencies
Frequency (MHz)
MP4
Morphed
01  11
12435.825(5)
12696
12541
12  11
33608.321(5)
33099
33429
21  11
23588.029(5)
22677
23237
22  11
67476.024(30)
66382
67023
12  01
21172.505(5)
20402
20888
13  01
71179.368(30)
65006
70170
12  21
10020.278(30)
10421
10192
31  21
40485.609(30)
39498
40354
22  21
43887.987(30)
43705
43786
22  12
33867.703(30)
33284
33595
13  12
50006.863(30)
44604
49282
23  12
67616.133(30)
68654
67804
13  22
16139.122(30)
11320
15687
23  22
33748.400(30)
35370
34210
32  22
45497.061(30)
44932
45149
E-state Summary
• 15 transitions corresponding to 9 energy levels
observed, but no T-to-linear yet
• Morphed potential much better at predicting lines
• E-state calculated to be more tightly bound than Astate: 0.249 / 0.304 cm-1 for unmorphed/morphed
potential
• T-to-linear gap greater in E-state by 0.156 / 0.018
cm-1 for unmorphed/morhped potential
• Hyperfine structure needs to be analyzed and
verified for the high frequency lines
Acknowledgements
• National Science Foundation
• Mike McCarthy and Pat Thaddeus
Thank You