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