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

• 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 A-state transitions

Predict E-state bound states and transitions

Observe E-state transitions

He-CH3F Intermolecular Potential

• MP4/7s5p3d2f(C,F) 6s3p2d(H,He) + 3s3p2d(bond)• Counterpoise corrected

-5-10-15

-20

0

-6 -4 -2 0 2 4 60

2

4

6Å

Å

-44.790

-30.929-46.903

C F

015

3045

60

015

3045

60

015

3045

60

(a)

(c)

(b)

Bound States

• Calculated using Hutson’s BOUND or Cohen and Saykally’s collocation program

• T-shaped ground state and linear excited state

• Nearly free internal rotation

A-State Energy Levels

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

Morphed Potential

• 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

),,(),,()(cos),,(),,( 4

0

REREPcRERV HFint

MPint

n

lll

HFintmorph

Morphed Potential

-5-10-15

-20

0

-6 -4 -2 0 2 4 60

2

4

6

Å

Å

-46.503

-31.171-50.041

C F

Predicted E-state Levels

-7

-5

-3

-1

1

3

En

erg

y (c

m-1

)

F end

H end

T-shaped

T-shaped

J = 0 1 5432

Observed E-state Transitions

-7

-5

-3

-1

1

3

En

erg

y (c

m-1

)

F end

H end

T-shaped

T-shaped

J = 0 1 5432

12435.5 12435.7 12435.9 12436.1

Frequency (MHz)

E-state 01 11 Transition (FTMW)

E-state 12 01 Transition (FTMW)

21172.2 21172.4 21172.6 21172.8

Frequency (MHz)

E-state 22 11 Transition (MBER)

67475.2 67475.6 67476.0 67476.4

Frequency (MHz)

E-state 13 01 Transition (MBER)

71178.4 71178.8 71179.2 71179.6 71180.0

Frequency (MHz)

Measured E-state FrequenciesFrequency (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 A-state: 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