NASA/TM-- 1998-208508

Theoretical and Experimental Determination

of the Proton Affinity of (CF3CH2)20

Michael J. Zehe

Lewis Research Center, Cleveland, Ohio

David W. Ball

Cleveland State University, Cleveland, Ohio

December 1998

https://ntrs.nasa.gov/search.jsp?R=19990018827 2018-05-11T17:25:22+00:00Z
The NASA STI Program Office... in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA's scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA's institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA's counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA's

mission.

Specialized services that complement the STI

Program Office's diverse offerings include

creating custom thesauri, building customized

data bases, organizing and publishing research

results.., even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at (301) 621-0134

• Telephone the NASAAccess Help Desk at

(301) 621-0390

Write to:

NASA Access Help Desk

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076
NASA/TM-- 1998-208508

Theoretical and Experimental Determination

of the Proton Affinity of (CF3CH2)20

Michael J. Zehe

Lewis Research Center, Cleveland, Ohio

David W. Ball

Cleveland State University, Cleveland, Ohio

National Aeronautics and

Space Administration

Lewis Research Center

December 1998
Trade names or manufacturers' names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5287 Port Royal Road

Springfield, VA 22100

Price Code: A03
Theoretical and Experimental Determination of the Proton Affinity of (CF3CH2)20

Michael J. Zehe

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

David W. Ball

Department of Chemistry

Cleveland State University

Cleveland, Ohio 44115

ABSTRACT

We report the experimental determination of the proton affinity of the molecule (CF3CH2)20 using

chemical ionization mass spectrometry, and we compare it to the theoretical value obtained for protonation at

the oxygen atom using the calculational methodology (MP2/6-31G**//MP2/3-21G). The proton affinity for this

molecule as measured by bracketing experiments is between 724 kJ/mole and 741 kJ/mole. Ab initio

(MP2/6-31G**//MP2/3-21G) calculations yield a value of about 729 kJ/mole, in agreement with the chemical

ionization experiments. The results of these and related calculations suggest that the (MP2/6-31G**//

MP2/3-21G) methodology is acceptable for estimating the proton affinities of partially- and fully-fluorinated

methyl and ethyl ethers. We submit that any conclusions about the chemistry of fluoroether polymer lubricants

based on their basicity can also be predicted reliably with such calculations.

NASA/TM--1998-208508 1
INTRODUCTION

Thepolymericperfluoroalkyl ethershavebeenthesubjectof intenseinterestin thetechnical

communityrecently,becauseof theirwidespreaduseasliquidlubricantsandtheirchemical behaviorunder

long-termtribologicaluse(Refs.1and2).Ofcontinuinginterestisthechemical degradationwhichcanoccur

whenthesefluidsinteractwithmetalormetaloxidessurfacesinbearingsorslidingcontacts.Thereisagreat

dealof evidencethatthisdegradationinvolvesacid-basechemistrybetweenweaklyacidicsurfacesitesand

basicsitesonthelubricantchain(Refs.3 and4).

Theprotonaffinity(PA)of simplefluoroethermoleculeshasbeenusedtocharacterizetheirreactivityto

surfaceacidsites.Theprimaryexperimental methodsusedformeasurement of gaseousprotonaffinityuseion

cyclotronresonance(ICR)spectroscopy for measurement of theprotontransferto anotherbase,orchemical

ionization(CI)measurements to"bracket"thecompoundbetweentwobasesof knownPA.Thesemeasurements

areexperimentallydifficulttoperformandinterpret(Ref.5).Theiraccuracydependsuponthepresenceof

chemical equilibriumbetweenthespecies,acertaintyin thelocaltemperaturein themeasurement cell,andthe

assumptionthattherotationalandvibrationalcomponentsofASTarezeroorknown.In addition,theremaybe

uncertaintyasto thesiteof protonationonthemolecule.Despitethesedifficulties,thereexistsa scaleofrelative

protonaffinitieswhichis well-established andwell-accepted(Ref.6).

Analternativeto themeasurement ofprotonaffinitiesof smallmoleculesi theirdetermination

throughtheuseof high-levelcomputational chemistry. Moderncomputational chemicalmethodscanyield

veryaccuratevaluesfortheprotonaffinityof moleculescomposedof first-rowelements(Ref.7).Delhalle

etal.(Refs.8and9)performedHartree-Fockcalculationsfor anumberof smallperfluoroether molecules

protonatedattheoxygenatomsandatfluorine;theyreportedthat,relativetoprotonationatoxygen,

protonationatana-fluorinegaveasubstantiallyhigherpredictedPA.Orgel,Ball,andZehe(Ref.10)

includedelectroncorrelationandperformedcalculationsforanumberof symmetrical partiallyandfullyfluorinateddimethylethers.Theirresultsshowedverygoodagreement withexperimental valuesofthePA'sof

dimethylanddiethylether,butthelackofexperimental dataforfluoroetherPA'spreventedcomparison.

In thiscommunication, wereporttheexperimental determinationoftheprotonaffinityof themolecule

(CF3CH2)20usingCI masspectrometry, andwecompareit tothetheoretical valueobtainedforprotonation

attheoxygenatomusingthecalculational methodology(MP2/6-31G**//MP2/3-21G). Thismethodologyhas

NASA/TM--1998-208508 2
beenusedbythecurrentauthors(Refs.10and11)topredictthePAof (CH3)20 and (C2Hs)20 within the

acceptance limits of the more computationally expensive G2 theory, and this work represents in part a test of

its applicability to fluorinated molecules.

EXPERIMENTAL

Chemical ionization mass spectrometry of (CF3CH2)20 was performed on a Finnigan TSQ-45 gas

chromatograph/mass spectrometer using its standard CI sample volume with a filament as its electron source.

The temperature of the sample volume was 100 °C. After the reagent gas (H20, CH20, or CH3OH ) was

introduced and a stable discharge was established, the (CF3CH2)20 was introduced and the mass spectrum was

measured and evaluated for the presence of (CF3CH2)2OH +. The observation of this ion indicates that the PA of

(CF3CH2)20 is higher than that of the reagent gas.

CALCULATIONS

Ab initio calculations were performed using the GAUSSIAN 92 program (Ref. 12) on an IBM RS 6000

workstation or a Cray C-90 supercomputer. Geometry optimizations were first performed using the 3-21G basis

set with an MP2 correction, and the resulting geometry was used for MP2 single point energy calculations with

the 6-31G(d,p) basis set (designated MP2/6-31G**//MP2/3-21G). Initial geometries were chosen as the alltrans configurations (Refs. 13 and 14). The ether was assumed to protonate at the oxygen atom. Thermal

energies were simultaneously computed, and the AH298 for the protonation reaction was determined using:

AH298 = AE 0 + AEthem a + A(PV) (1)

where AE0 is the difference in the total electronic energies of the species at 0 K, and AE,hcrm includes

contributions from zero-point vibrational energy differences, thermal vibrational energy differences, rotational

energy differences, and thermal translational energy differences. A(PV) is the standard conversion from

internal energy to enthalpy and for all proton affinity reactions equals -RT (= -2.48 kJ/mol). All vibrational

frequencies were scaled by a factor of 0.95 for thermochemical calculations.

NASA/TM--1998-208508 3
RESULTSANDDISCUSSION

Themolecularionfor(CF3CH2)20appearsat182ainu,withtheprotonatedformat183ainu.The

strongest peakintheprotonCIspectrappearsat 113ainuandisduetolossof CF3Hfromtheprotonated

ether.Alsopresentisastrongpeakat 163ainu,duetolossof HFfromtheprotonatedmolecule.Theintensity

of theM+I peakat183ainuis<3percentof the strongest peakatmass113.Thisillustratesoneof the

challengesof measuringthemassspectraof protonatedfluorocarbons: thefacilitywithwhichstablefluorine

speciessplitofffromtheenergized, protonatedmolecule.

Theprotonaffinitiesof H20,formaldehydeandmethanol are,respectively, 723.8,741.4,and

773.6kJ/mole(Ref.6).CImasspectrausinganH20/H30+dischargeresultedinastrongpeakatmass183,

indicatingefficientionizationof thebasemoleculebyH30+. For(CF3CH2)20ineitheraformaldehydeor

methanol discharge, however,themass183peakwasabsent.Thustheseexperimentsshowthattheproton

affinityof bis(2,2,2-trifluoroethylether) (g)isbetweenthatof H20(g)andCH20(g), orbetweenabout724

and741U/mole.

All calculationsconvergedonstationarypointsthatweretrueminimaasindicatedbyvibrational

frequencycalculationsthatyieldedpositivefrequenciesonly.Conformational studiestoverifythatthefinal

geometrieswereabsoluteminimawerenotperformed, butit is ourjudgement thatconformational

contributionstotheprotonaffinityareslight.Figures1and2 showthestructuresof theoptimizedmolecule

anditsprotonatedform.A summaryofthestructural parametersisgiveninTablesI andII; weplantopublish

amoredetailedstructural analysisin aforthcomingpaper(Ref.11).

TheneutralmoleculehasC2vsymmetry,withacoplanararrangement of all fiveetherbackboneatoms

andtwoof theterminalfuorineatoms.Protonationto(CF3CH2)2OH +changestheC-O-Cbonddistancesand

anglesonlyslightly,butnowtheterminalcarbonatomshaverotatedoutof theC-O-Cplaneby42degrees,

andtheCF 3 groups have themselves rotated by a few degrees. The result is an ion with Cs symmetry, with an

(O)H--F distance of about 2.12A. This distance is well within the limits that define hydrogen bonding,

(Ref. 15) and we suggest that intramolecular hydrogen bonding is contributing to the overall stability of the

protonated ether. It is difficult to tell the extent to which the intramolecular hydrogen bonding is contributing

to the overall stability, but it may be substantial, based upon common H-bond energies of 10 to

35 kJ/mole (Ref. 15).

NASA/TM--1998-208508 4
TableIII listsasummaryof thecalculatedsingle-point energiesof theunprotonatedandprotonated

ethersusingdifferentlevelsof theory,withanindicationof thepredictedprotonaffinity.TheMP2/6-

31G**//MP2/3-21Gcalculationspredictaprotonaffinityof about729U/mole,in agreement withthechemical

ionizationexperiments. Weconcludethatcalculationsatthislevelcanbeconsideredreliableforpredicting

theprotonaffinitiesofrelatedcompounds.

Earlierwork(Ref.10and11)showedthatMP2/6-31G**//MP2/3-21G calculationsdidaverygoodjob

of predictingprotonaffinitiesof dimethyletheranditsfluorinatedderivatives. Inseveralcases,the

calculational resultsapproachedthelevelof performanceexpectedfortheG2levelof theory(Ref.16).We

suggest, therefore, thatthecurrentmethodologyisacceptablefor estimatingtheprotonaffinitiesofpartiallyandfully-fluorinatedmethyl(Ref.10)andethyl(Ref.11)ethers.Wefurthersubmitthatanyconclusionsabout

thechemistryof fluoroetherpolymerlubricantsbasedontheirbasicitycanalsobepredictedreliablywithsuch

calculations. Specifically, theapproximately80to 100kJ/mole(excludingintramolecular H-bonding)

decreasein theprotonaffinitywhichresultswhendiethyletheris fullyfluorinated, indicatesthatcautionmust

beexercisedwhenattributingperfluoroether lubricantdecompositiontoacid-basechemistryatoxygen.

REFERENCES

1. P.Herrera-Fierro, W.R.Jones,S.V.Pepper,J. Vac. Sci. Tech. A 1993, 11,354.

2. J. Pacansky, M. Miller, W. Hutton, V. Liu, A. Scheiner, J. Am. Chem. Soc. 1991,113, 329.

3. M.J. Zehe, O. Faut, STLE Trans. 1990, 33,634.

4. P. Kasai, Macromolecules 1992, 25, 6791.

5. M. Meot-Ner, L.W. Sieck, JACS 1991,113, 4460.

6. S.G. Lias, J.F. Liebman, R.D. Levin, J. Phys. Chem. Ref. Data 1984, 13,695.

7. B.J. Smith, L. Radom, JACS 1993,115,4885.

8. M. Fontaine, J. Delhalle, M. Defranceschi, G. Lecayon, Int. J. Quant. Chem. 1993, 46, 171.

9. M. Fontaine, J. Delhalle, M. Defranceschi, J.M.M. Bourin, J. Mol. Struct. 1993, 300, 10.

10. V.B. Orgel, D.W. Ball, M.J. Zehe, Journal of Molecular Structure(Theochem) 1997, 417, 195.

NASA/TM--1998-208508 5
11. M.J.Zehe,D.Ball,"OptimizedStructuresandProtonAffinitiesofFluorinatedDiethylEthers:AnAb

InitioStudy,"manuscript in preparation.

12. GAUSSIAN92.RevisionsBandD.2.M.J.Frisch,G.W.Trucks,M.Head-Gordon, P.M.W.Gill,M.W.

Wong,J.B.Foresman, B.G.Johnson, H.B.Schlegel, M.A.Robb,E.S.Replogle, R.Gomperts, J.L.

Andres,K.Raghavachari, J.S.Binkley,C.Gonzales, R.L.Martin,D.J.Fox,D.J.Defrees, J.Baker,

J.J.P.Stewart,andJ.A.Pople,Gaussian, Inc.,Pittsburgh, PA,1992.

13. G.D.Smith,R.L.Jaffe,D.Y.Yoon,Macromolecules 1994, 27(12), 3166.

14. G.D. Smith, R.L. Jaffe, D.Y. Yoon, Macromolecules 1995, 28(17), 5804.

15. Pimentel, George C., "The Hydrogen Bond," Reinhold Pub. Corp., New York, 1960.

16. L.A. Curtiss, K. Raghvachari, G.W. Trucks, J.A. Pople, J. Chem. Phys. 94 (1991), 7221.

NASA/TM--1998-208508 6
F 2

F 3

F 6

F 5

F 1

C 3 0

01 0 2

0 4

F 4

H H H H

Figure 1.--MP2/3-21G Optimized Geometry for (CF3CH2)2) (C2v).

TABLE I.--MP2/3-21G OPTIMIZED

STRUCTURAL PARAMETERS

FOR (CF3CH2)20 (C2v)

Bond Value

C1-O-C2 116.0 °

C-O 1.46

C1-C3 1.51

C3-C1-O-C2 145.5 °

C3-F1 1.38

C3-F2 1.37

F1-C3-C1-O 180 °

NASA/TM--1998-208508 7
F F F

F 1

C3

H

C 1

H

H

O

H

C2

H

F

F

Figure 2.--MP2/3-21G Optimized Geometry for (CF3CH2)2OH + (Cs).

TABLE II.--MP2/3-21G OPTIMIZED

STRUCTURAL PARAMETERS FOR

(CF3CH2)2OH +(Cs)

Bond Value

C1-O-C2 121.9 °

C-O 1.52

c1-c3 1.53

C3-C1-O-C2 169.1 °

O-H 1.01 X

H-O-C1-C3 35.6 °

F-H (min) 1.91

NASA/TM--1998-208508 8
TABLEIII.--CALCULATEDENERGIES(HARTREES) ANDESTIMATEDPROTON

AFFINITYFOR(CF3CH2)2 O+BASEDONTHESTRUCTUREFROMMP2/3-21G

OPTIMIZATIONS

HF/3-21G6-31G//MP2/3-21G6-31G*//MP2/3-21G6-31G**//MP2/3-21G

(CF3CH2)20-820.82442 -826.21700 -827.00468 -827.03589

(CF3CH2)2OH + -821.11574 -826.49085 -827.28370 -827.32412

PA,kJ/mole 736.6 690.7 704.3 728.5

aCalculationincludeschangesinenergydueto thermalvibrationalandrotationalenergyand

APV; see text for details.

NASA/TM--1998-208508 9
Form Approved REPORT DOCUMENTATION PAGE

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Theoretical and Experimental Determination of the Proton

Affinity of (CF3CH2)20

6. AUTHOR(S)

Michael J. Zehe and David W. Ball

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135- 3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546- 0001

WU-523-22-13-00

8. PERFORMING ORGANIZATION

REPORTNUMBER

E-11358

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1998-208508

11. SUPPLEMENTARY NOTES

Michael J. Zehe, NASA Lewis Research Center, and David W. Ball, Cleveland State University, Department of Chemistry,

Cleveland, Ohio 44115. Responsible person, Michael J. Zehe, organization code 5140, (216) 433-5833.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Categories: 05, 19, 07 and 25 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 6214)390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

We report the experimental determination of the proton affinity of the molecule (CF3CH2)20 using chemical ionization

mass spectrometry, and we compare it to the theoretical value obtained for protonation at the oxygen atom using the

calculational methodology (MP2/6-31G**//MP2/3-21G). The proton affinity for this molecule as measured by bracketing

experiments was between 724 k J/mole and 741 kJ/mole. Ab initio (MP2/6-31G**//MP2/3-21 G) calculations yield a value

of about 729 U/mole, in agreement with the chemical ionization experiments. The results of these and related calculations suggest that the (MP2/6-31G**//MP2/3-21G) methodology is acceptable for estimating the proton affinities of

partially- and fully-fluorinated methyl and ethyl ethers. We submit that any conclusions about the chemistry of

fluoroether polymer lubricants based on their basicity can also be predicted reliably with such calculations.

14. SUBJECT TERMS

Quantum chemical; Lubricants; Modelling; Semiempirical; Lubrication; Perfluoroethers

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

15. NUMBER OF PAGES

15

16. PRICE CODE

A03

18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF THIS PAGE OF ABSTRACT

Unclassified Unclassified

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-1B

298-102