CONFORMATIONS AND BARRIERS TO METHYL GROUP INTERNAL ROTATION IN TWO ASYMMETRIC ETHERS: PROPYL METHYL ETHER AND BUTYL METHYL ETHER.

TC-06: June 19th, 2012

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Brittany E. Long*, Frank DeChiricoǂ, and Stephen A. Cookeǂ

* ǂ

Chirped Pulse Fourier Transform Microwave Spectrometer at Wesleyan

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Novick Lab Picture

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CP-FTMW

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Based upon the work of Pate and coworkers1

Uses a fast passage pulsing technique known as a “chirp”

8-18 GHz range with up to 4 GHz regions at a time

Directly digitized setup shown here TC-04 for more detailed explanation

Vacuum Chamber

1. G. G. Brown, B. C. Dian, K. O. Douglass, S. M. Geyer, B. H. Pate, J. Mol. Spectrosc. 238 (2006) 200.

MOLECULES5

Motivation for Molecules

• To study internal rotation• To study asymmetric aliphatic CHO ethers

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Asymmetric Aliphatic CHO Ethers

• trans-ethyl methyl ether has been studied many different times in the past. • M. Hayashi, et. al., J. Mol. Struct. 28 (1975) 147.• K. Kobayashi, et. al., J. Mol. Spectrosc. 251 (2008) 301.• U. Fuchs, et. al., Astrophys. J. Suppl. Ser. 144 (2003) 277.• Plus several other paper not listed

• trans-trans isomer of propyl methyl ether has been studied• H. Kato, et. al., J. Mol. Spectrosc. 80 (1980) 272.

• Isopropyl methyl ether has been studied• J. Nakagawa, et. al., J. Mol. Struct. 112 (1984) 201.

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Trans-ethyl methyl ether

8*only conformer ever observed

Trans-ethyl methyl ether parameters1

Trans-ethyl methyl ether

A 27991.97(31) MHz

B 4159.48(4) MHz

C 3891.19(4) MHz

ΔJ -1.26(56)* kHz

V3(OCH3) 945 ± 2 cm-1 (893 cm-1 “after kinetic coupling taken into account”)

Iα (OCH3) 3.2063 amu Å2

V3(CCH3) 1154 ± 9 cm-1

Iα (CCH3) 3.1659 amu Å2

F 193.452 GHz

91. M. Hayashi, et. al., J. Mol. Struct. 28 (1975) 147.*value was reported negative in paper

Relative Energies: Propyl methyl ether

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Trans trans- propyl methyl etherEnergy: 83 cm-1 higher

Trans gauche-propyl methyl etherEnergy: 0 cm-1

Potential third conformer-not observedEnergy: 456 cm-1 higher

MP2/6-311G++(2d,2p)

Trans trans-propyl methyl ether

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τ1= 179.6˚

τ2=179.4˚

Trans trans- propyl methyl ether parameters1

Trans trans-propyl methyl ether

A 19755.36(14) MHz

B 2125.45(1) MHz

C 2017.60(1) MHz

ΔJ 3.5(10) kHz

V3(OCH3) “searches for spectra due to the excited state of the OCH3 torsion were unsuccessful.”

Iα (OCH3) --

V3(CCH3) 1154 ± 20 cm-1

Iα (CCH3) 3.1202 amu Å2

F Not reported12

1. H. Kato, et. al., J. Mol. Spectrosc. 80 (1980) 272.

Trans gauche-propyl methyl ether

13a

cb

C1

C4

C3C2

τ1

τ2

Trans gauche-propyl methyl ether

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C2H5

H

CH3

H

H

H

OCH3

CH3

H H

τ1= 179.6˚

τ2=62.9˚ MP2/6-311G++(2d,2p)

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.doi:10.1351/goldbook.

trans gauche

Microwave Spectrum for Propyl Methyl Ether

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404 -312

111-101

212-111

212 - 202

110-101

202-101

211-202

211-110

312-303

404-313

413-404

Trans gauche-propyl methyl ether parameters

State 1 State 2

A /MHz 12231.9909(15) 12232.0581(15)

B /MHz 2622.9483(4) 2622.9433(4)

C /MHz 2379.0418(4) 2379.0368(4)

ΔJ /kHz 1.240(10) 0.883(10)

ΔJK /kHz -8.87(8) -8.73(8)

δJ /kHz 0.2218(15) 0.2216(15)

n 25 25

RMS /kHz 3.9

16*preliminary fit

Problem for trans gauche-propyl methyl ether

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PROBLEM

322-221

500 Shots

Relative Energies: Butyl methyl ether

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Trans trans trans- butyl methyl etherEnergy: 130 cm-1 higher

Trans gauche trans-butyl methyl etherEnergy: 0 cm-1

MP2/6-311G++(2d,2p)

Trans gauche trans-butyl methyl ether

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a

c

b

C1 C4

C3C2

C5

τ1

τ3

τ2

Trans gauche trans- butyl methyl ether

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C2H5

HCH3

H

H

HOCH3

C2H5

H HCH3

HCH2OCH3

H

H H

MP2/6-311G++(2d,2p)

τ1= 179.9˚

τ2= 62.5˚

τ3= 179.0˚

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.doi:10.1351/goldbook.

gauche transtrans

Microwave Spectrum for Butyl Methyl Ether

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Trans gauche trans-butyl methyl ether parametersA State E State XIAM

A /MHz 10259.5099(30) 10259.4285(30) 10259.4592(34)

B /MHz 1445.63599(63) 1445.63479(67) 1445.6470(20)

C /MHz 1356.30672(74) 1356.30572(77) 1356.2945(21)

ΔJ /kHz 0.3063(72) 0.2909(74) 0.299(7)

ΔJK /kHz -7.351(92) -7.383(78) -7.40(10)

ΔK /kHz 98.64(54) 91.11(54) 94.6(7)

δJ /kHz 0.0504(31) 0.0500(30) 0.0515(25)

Da /MHz 0.3870(48)

n 44 40 84

RMS /kHz 6.8 12.9

V3 /cm-1 (OCH3) 780 ± 35

F /GHz 163.2122

Iα /amu Å2 3.3149

κ -0.97993

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Barrier Height ComparisonsMolecule V3 (OCH3) /cm-1 V3(CCH3) /cm-1 Methyl C-O / Å

Trans-ethyl methyl ether1

893 ± 2 1154 ± 9 1.415(6)

Trans trans-propyl methyl ether2

NA 1154 ± 20 1.414(8)

Isopropyl methyl ether3

602 ± 8 NA 1.416

Trans gauche-propyl methyl ether4

Undetermined NA 1.416*

Trans gauche trans-butyl methyl ether5

780 ± 35 NA 1.412*

231. M. Hayashi, et. al., J. Mol. Struct. 28 (1975) 147.2. H. Kato, et. al., J. Mol. Spectrosc. 80 (1980) 272.3. J. Nakagawa, et. al., J. Mol. Struct. 112 (1984) 201.4. This work5. This work* Bond angles from optimized MP2/6-311G++(2d,2p) calculations. Final structure not determined yet.

Conclusions• Rotational constants and centrifugal distortion constants

for trans gauche trans-butyl methyl ether have been determined for the first time.• Work in progress for trans gauche-propyl methyl ether

• Barrier height to internal rotation for trans gauche trans-butyl methyl ether has been determined• XIAM fit for trans gauche-propyl methyl ether to

determine barrier height still a work in progress.• Something unknown seems to be happening with the molecule

• Want to try ERHAM

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Future Work• 13C for both butyl methyl and propyl methyl ethers• Pentyl methyl and hexyl methyl ethers• Laser ablation on lanthanide and actinide molecules. (Th, U,

Lu, Nd)• Chirp room temperature waveguide

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Acknowledgements• Novick, Pringle and Cooke Group• Laboratory Funding from the NSF and US

DOE

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Trans trans- methyl propyl ether

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C2H5

H

CH3

H

τ1= 179.6˚

τ2=179.4˚ MP2/6-311G++(2d,2p)

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.doi:10.1351/goldbook.

CH3

H

OCH3

H

H H

Effective rotation-Coriolis Hamiltonian1

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1. G.S. Grubbs II, et. al., “Methyl Group Internal Rotation and the Choice of Hamiltonian for the Rotational Spectrum of 1,1-Difluoroacetone,” J. Mol. Spectrosc. Submitted