Infrared spectroscopy of metal ion-water complexes

Biswajit Bandyopadhyay, Prosser D. Carnegie and Michael A. Duncan

Department of Chemistry, University of Georgia, Athens, GA, 30602www.arches.uga.edu/~maduncan/

U. S. Department of Energy

IntroductionInteraction of water with metal ions is fundamental to understand the chemistry of solvation.

A molecular level understanding is obtained by studying these complexes in the gas phase.

Collision induced dissociation to measure the metal-water binding energies by Armentrout and coworkers.

Electronic spectroscopy of cation- water systems performed by the Brucat, Metz and the Duncan group.

ZEKE spectroscopy by the Blake group and the Duncan group.

Infrared Photodissociation Spectroscopy (IRPD) :

alkali metal cation-water complexes by Lisy and coworkers

alkali earth and main group by Inokuchi, Misaizu and coworkers

Transition metals and alkaline earth metal ions by Williams and coworkers

Transition metal ions by Duncan and coworkers.

Experimental

200 400 600 800 1000 1200 1400

n =

20151050

V+(H2O)Ar

n

m/z

V+Arn

n =

20151050

0 25 50 75 100 125 150 175 200

difference

m/z

photodissociation off

-Ar

V+(H2O)Ar

2

photodissociation on

3683 cm-1

m = 149 amu

-Ar

Argon “tagging”

M+(H2O) bond energies are ~ 30-45 kcal/mol ( 10000-15000 cm-1)

Infrared photon energy ~3000-4000 cm-1

For the M+(H2O)n clusters, water molecules in the second solvent shell have lower binding energies and can be eliminated by a single photon

M+-Ar bonds are weaker and argon falls off when the O-H stretches are excited.

IR Photon

Ar elimination

Red shifts in O-H stretches

3500 3600 3700 3800 3900

Cu+(H2O)Ar

2

cm-1

3623

369637563657

3764

The HOMO of water has partial bonding character.Polarization of the electron due to metal cation removes the electrondensity from the O-H bond –accounts for red shift

Combination band1

IR spectra of cation-water systems

Sc Ti V Cr Mn Fe Co Ni Cu Zn --

28303234363840424446

Sc Ti V Cr Mn Fe Co Ni Cu Zn --

30

40

50

60

70

80

Sc Ti V Cr Mn Fe Co Ni Cu Zn --

60

70

80

90

100

B. E

. (kc

al/m

ol)

Sym

m O

H s

tret

ch s

hif

t (c

m-1)

M+

Asy

mm

OH

str

etch

sh

ift

(cm

-1)

M+(H2O) B.E. vs. red shifts

Red shifts depend on the extent of polarization of water molecule by the metal cation. Closed shell cations or metal ions with fewer d-electrons polarize water the most – more red shift

1 P. D. Carnegie, A. B. McCoy, M. A. DuncanJ. Phys. Chem. A 113, 4849 (2009).

The intensity ratio of symmetric and asymmetric stretch is 1: 18 for free water

Asymmetric stretch-perpendiculartype vibration- less change in dynamical dipole moment than thesymmetric stretch

Symmetric stretch-parallel typevibration- Involves greater change in dynamical dipole moment-gains greater intensity

IR spectra of cation-water systems

Intensity pattern switch

3400 3500 3600 3700 3800 3900

3824

3697

3622

cm-1

Ni+(H2O)Ar

2

In a metal ion –water complex this ratio is ~1:1

Partially resolved rotational structures

3400 3500 3600 3700 3800 3900

3,2

2,1

1,0

0,1

simulation

cm-1

1,2

3641

36

95

37

46

37

20

36

68

36

13

3580

3500 3600 3700 3800 3900 4000

(1,2)

(0,1)

(1,0)

(2,1)

(3,2)(4,3)

cm-1

simulation

(1,2)(4,3)

(3,2)

(1,0)

(2,1)(0,1)

From the partially resolved sub-bands H-O-H bond angle can be calculated, assuming thatthe O-H bond length does not change.

C2

• Most of the M+(H2O)Ar complexes have C2v symmetry • Ar binds to the M+ along the C2 axis. Only light H-atoms are off the axis and contributes to the moment-of-inertia along that axis• Rotational constants are close to 13-14 cm-1

A" = 13.4 cm-1

B", C" = 0.07, 0.07 cm-1

A' = 14.3 cm-1B', C' = 0.07, 0.07 cm-1

B. O.sym = 3629 cm-1

B. O.asym = 3692 cm-1

TJ,K = 15, 40K

Li+(H2O)Ar

A'' = 13.7 cm-1

B'', C'' = 0.047 cm-1

A' = 13.4 cm-1

B', C' = 0.047 cm-1

B.O.sym = 3580 cm-1

B.O.asym = 3656 cm-1

T = 50 K

Sc+(H2O)Ar

IR spectra of Mn+(H2O)Arn complexes

3000 3200 3400 3600 3800 4000

Mn+(H2O)Ar

cm-1

35843660

3222 3744

Mn+(H2O)Ar

2 3549

3584

364

3 3662

3218

Mn+(H2O)Ar

3

3554

358

63644

3665

3215

Mn+(H2O)Ar

4

3557

361

4

3648

3215

3300 3400 3500 3600 3700 3800

cm-1

3577

3659

35403638

3524 3594

3554

35

86

3644

3665Mn+(H2O)Ar

3

Different binding sites of argon atoms produce isomers

3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000

Zn+(H2O)Ar

cm-1

3567

3644

3727

Zn+(H2O)

2Ar

3546

3578

3653 3669

Zn+(H2O)

3Ar

3567

3585 3671

Zn+(H2O)

4Ar

3425 3662 3687

IR spectra of Zn+(H2O)nAr complexes

Argon is off the C2 axis-s-orbital of the metal ion isback polarized by water. Argondoes not want to attach opposite to water.

Appearance of 3425 cm-1 peak shows that one of the O-H bondsis interacting with the argon – Coordination number 4.

Zn+(H2O)2Ar and Zn+(H2O)3Ar Have similar looking spectra

Slightly different spectral pattern due to reaction product?

A″, A =17.5, 15.0 cm′ -1

A″, A =9.0, 11.8 cm′ -1

B.O =3664 cm-1

B.O =3661 cm-1

T J, K = 10, 20 K.

H-Ti2+-OH-

3500 3600 3700 3800 3900

37

203

66

8Sc+(H2O)Ar

3580

36

13

3641

36

95

Ti+(H2O)Ar

Mn+(H2O)Ar 3584 3660

36

99

cm-1

3590 3652

36

78

IR spectrum of Ti+(H2O)Ar complex

?3500 3600 3700 3800 3900

Ti+(H2O)Ar

36523590

369936

78

cm-1

3500 3550 3600 3650 3700 3750 3800

3604

3674

3684

3694

3705

cm-1

3606 3686

3676

V+(H2O)Ar

V+(H2O)Ne

IR spectrum of V+(H2O)Ar complex

3500 3550 3600 3650 3700 3750 3800

3587

3672

cm-1

3584 3667

Nb+(H2O)Ar

Nb+(H2O)Ne

3450 3500 3550 3600 3650 3700 3750 3800 3850 3900

3611

3688

3724

3801

3677

3646

3532

cm-1

U+(H2O)Ar2

Au+(H2O)Ar2

IR spectra of U+(H2O) and Au +(H2O) complexes

Conclusions• Red shifts in O-H stretching frequencies• Intensity pattern switch for O-H sym. and asym. stretches• Partially resolved rotational structures• Multiple argons produce isomers• Spectra with multiple waters provide information about coordination

number• Insertion product complicates spectra for early transition metals• Argon tends to go to hydrogen of water molecule in case of Au+- and U+-

water complexes

Acknowledgements

• Prof. Mike Heaven (Emory University) for letting us borrow a uranium rod• U. S. Department of Energy for funding