Spectroscopy of Multiply Charged Metal Ions: Spectroscopy of Multiply Charged Metal Ions: IR Study of MnIR Study of Mn2+2+(18-crown-6 ether)(MeOH)(18-crown-6 ether)(MeOH)1-31-3

Jason D. Rodriguez and James M. Lisy

Department of Chemistry, University of Illinois Urbana-Champaign

Ionophores

• Complex ions in solution selectively– Useful for ion extraction in industrial and environmental processes

• The balance of various non-covalent effects determine selectivity– ion-ionophore– solvent-ionophore– ion-solvent– solvent-solvent

• A common example: Crown Ethers 18-crown-6 ether

Why is It Important to Study Multiply Charged Metal Ions?

• Most gas-phase studies on metal ion solvation have been predominantly on the singly charged state.

• PROBLEM: Many metal ions of chemical and biological interest have common formal charges that are higher than +1.

• If these studies are to be good models for the condensed phase or biological systems, metal ions must be studied in their common oxidation state.

But it is hard to generate multiply charged metal cluster ions by But it is hard to generate multiply charged metal cluster ions by traditional sources………………………………traditional sources………………………………

Generating Ions Via Electrospray

• Developed by J.B. Fenn (Nobel Prize 1992)• Used by P. Kebarle in early 1990s for mass

spectrometric studies of multiply charged metal ion solvation

• Advantage– “Soft” technique—noncovalent interactions present in

solution are preserved in the transfer to the gas phase

• Disadvantage– Ions must be present in solution—solute must be

dissolvable by solvent

Experimental Apparatus

Continuum Surelite II-10 Hz Nd3+:YAG (1064 nm)

Tunable LaserVision OPO/A

Source ChamberDetector ChamberIon Selecting

Quadrupole

Ion Guiding Quadrupole

Ion Analyzing Quadrupole

Octapole Ion guide

ESI Needle

Syringe Pump

Heated Capillary Skimmer

Electrostatic Lenses

Mn2+(18-crown-6 ether)(MeOH)n + hv Mn2+(18-crown-6 ether)(MeOH)n-1 + MeOH

Mass Spectrum of Mn2+(18-crown-6ether)(MeOH)n

150 160 170 180 190 200 210 220 230

m/z

Inte

nsit

y

n=0

n=1n=2

n=3

n=4

IR Summary of Mn2+(18crown6)(MeOH)n

2750 2850 2950 3050 3150 3250 3350 3450 3550 3650

Frequency (cm-1)

Inte

ns

ity

(a

rb.u

nit

s)

n=3

n=2

n=1

IR Summary of Mn2+(18crown6)(MeOH)n

3050 3150 3250 3350 3450 3550 3650Frequency (cm-1)

Inte

ns

ity

(a

rb. u

nit

s)

Gas Phase O-H stretch of MeOH

monomer 3681 cm-1

n=3

n=2

n=1No hydrogen bonding features observed

Hydrogen bonded feature centered at ~3180 cm-1

Free O—H band of methanol located at 3645 cm-1

Broad hydrogen bonded feature centered at ~3150 cm-1

A doublet appears in the Free O—H region, 3645 and 3663 cm-1

Only one feature present in Free O—H region, 3663 cm-1

Ref :Lisy et al. JCP, 95, 1991, 3924.

IR Summary of Mn2+(18crown6)(MeOH)n

2750 2850 2950 3050 3150 3250 3350 3450 3550 3650

Frequency (cm-1)

Inte

ns

ity

(a

rb.

un

its

)

n=3

n=2

n=1

2750 2775 2800 2825 2850 2875 2900 2925 2950 2975 3000 3025 3050

Frequency (cm-1)

Inte

nsi

ty (

arb

. u

nit

s)

Features increase in intensity as solvation increases.

Intensity of these features does not vary much with increase in solvation.

Gas-phase neutral C—H stretches:

MeOHa

(2844,2970, 2999 cm-1)

18-crown-6b

(2870, 2942 cm-1)

a: Serrallach, A. J.Mol.Spectrosc. 1974, 52, 94-129.

b: NIST online Chem. WebBook

n=3

n=2

n=1

IR Summary of Mn2+(18crown6)(MeOH)n

2750 2850 2950 3050 3150 3250 3350 3450 3550 3650

Frequency (cm-1)

Inte

ns

ity

(a

rb.

un

its

)

n=3

n=2

n=1

Calculations done at B3LYP/6-31+G*

Summary of Results

• The ESI technique has been used to generate solvated Mn2+:18-crown-6 ether clusters

• The IR spectra in the Free O—H stretch region of MeOH show changes as number of MeOH solvent molecules increases

• For n=2 and n=3, hydrogen bonding is observed• In the C—H stretch region, there are features

that appear to be due to C—H stretches of both MeOH and 18-crown-6

Future Work

• Computational work is currently in progress to characterize these results.

• This study lays the foundation for further work on solvation studies of multiply charged metal ions.

• The versatility of the ESI will be used to study other multiply charged metal ions and ionophores of chemical and biological interest.

Acknowledgements

• Prof. James M. Lisy• Group Members:

– Ms. Dorothy Miller– Ms. Amy Nicely – Mr. Jordan Beck– Mr. Oscar Rodriguez

• Mrs. Fatima J. Rodriguez• Funding

– NSF– UIUC Graduate College Fellowship– UIUC Chem. Dept. Fellowship