Infrared spectroscopy of metal
ion-water complexes
Biswajit Bandyopadhyay, Prosser D. Carnegie and
Michael A. Duncan
Department of Chemistry, University of Georgia, Athens, GA, 30602
www.arches.uga.edu/~maduncan/
U. S. Department of Energy
Introduction
Interaction 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.
n=
Experimental
0
10
5
15
20
+
n=
0
10
5
15
V Arn
20
+
V (H2O)Arn
200
400
600
800
1000
1200
1400
m/z
+
V (H2O)Ar2
photodissociation off
m = 149 amu
photodissociation on
3683 cm
-Ar
-1
-Ar
difference
0
25
50
75
100
m/z
125
150
175
200
Argon “tagging”
Ar elimination
IR Photon
M+(H2O) bond energies are ~ 30-45 kcal/mol ( 1000015000 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 spectra of cation-water systems
M+(H2O) B.E. vs. red shifts
Red shifts in O-H stretches
3756
B. E. (kcal/mol)
3696
+
Cu (H2O)Ar2
46
44
42
40
38
36
34
32
30
28
-1
3623
-1
Asymm OH stretch shift (cm ) Symm OH stretch shift (cm )
3657
Combination
band1
3764
3500
3600
3700
3800
3900
-1
cm
The HOMO of water has partial
bonding character.
Polarization of the electron due to
metal cation removes the electron
density from the O-H bond –
accounts for red shift
1 P.
D. Carnegie, A. B. McCoy, M. A. Duncan
J. Phys. Chem. A 113, 4849 (2009).
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
--
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
--
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
--
80
70
60
50
40
30
100
90
80
70
60
M
+
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
IR spectra of cation-water systems
Intensity pattern switch
The intensity ratio of symmetric and
asymmetric stretch is 1: 18 for free
water
In a metal ion –
water complex
this ratio is ~1:1
3697
+
Ni (H2O)Ar2
3622
Asymmetric stretch-perpendicular
type vibration- less change in
dynamical dipole moment than the
symmetric stretch
3824
Symmetric stretch-parallel type
vibration- Involves greater change in
dynamical dipole moment-gains
greater intensity
3400 3500 3600 3700 3800 3900
-1
cm
Partially resolved rotational structures
(0,1)
Sc+(H2O)Ar
(2,1)
3641
Li+(H2O)Ar
3695
3580
(0,1)
(4,3)
(2,1)
(1,0)
(3,2)
(4,3)
(1,2)
3500
3600
3700
3800
A"
= 13.4 cm-1
B", C" = 0.07, 0.07 cm-1
A'
= 14.3 cm-1
B', C' = 0.07, 0.07 cm-1
B. O.sym = 3629 cm-1
B. O.asym = 3692 cm-1
TJ,K = 15, 40K
3720
3746
(3,2)
3613
(1,2)
3668
(1,0)
0,1
C2
2,1
1,0
1,2
simulation
3,2
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
simulation
3900
4000 3400
-1
cm
• Most of the M+(H2O)Ar complexes have C2v symmetry
• Ar binds to the M+ along the C2 axis. Only light Hatoms are off the axis and contributes to the momentof-inertia along that axis
• Rotational constants are close to 13-14 cm-1
3500
3600
3700
3800
-1
cm
From the partially resolved sub-bands H-O-H
bond angle can be calculated, assuming that
the O-H bond length does not change.
3900
IR spectra of Mn+(H2O)Arn complexes
Different binding sites of argon atoms produce isomers
3644
3557
+
Mn (H2O)Ar3
3614
+
Mn (H2O)Ar4
3586
3554
3648
3665
3215
3644
3594
3524
3586
3554
+
Mn (H2O)Ar3
3665
3215
3540
+
Mn (H2O)Ar2
3549
3643
3584
3638
3662
3218
3584
3660
3659
+
Mn (H2O)Ar
3000
3200
3577
3744
3222
3400
3600
-1
cm
3800
4000
3300
3400
3500
3600
-1
cm
3700
3800
IR spectra of Zn+(H2O)nAr complexes
3425
3662
3687
+
Zn (H2O)4Ar
Appearance of 3425 cm-1 peak
shows that one of the O-H bonds
is interacting with the argon –
Coordination number 4.
3585 3671
+
Zn (H2O)3Ar
3567
3653 3669
3578
+
Zn (H2O)2Ar
Zn+(H2O)2Ar and Zn+(H2O)3Ar
Have similar looking spectra
3546
3644
3567
3727
+
Zn (H2O)Ar
3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000
cm
-1
Argon is off the C2 axis
-s-orbital of the metal ion is
back polarized by water. Argon
does not want to attach
opposite to water.
IR spectrum of Ti+(H2O)Ar complex
Slightly different spectral pattern due to reaction product?
3641
+
Mn (H2O)Ar
3584
3695
3590
3652
+
Ti (H2O)Ar
3720
3613
Sc (H2O)Ar
3668
+
3678
3580
3699
3660
A″, A′=9.0, 11.8 cm-1
B.O =3661 cm-1
T J, K = 10, 20 K.
3652
3678
3699
3590
+
Ti (H2O)Ar
3500
3600
3700
cm
-1
H-Ti2+-OH-
A″, A′=17.5, 15.0 cm-1
B.O =3664 cm-1
?
3800
3900
3500
3600
3700
cm
-1
3800
3900
IR spectrum of V+(H2O)Ar complex
V+(H2O)Ar
3684
3604
3694
3674
3705
3686
3606
V+(H2O)Ne
3500
3550
3676
3600
3650
cm
3700
3750
3800
-1
Nb+(H2O)Ar
3587
3672
3667
3584
Nb+(H2O)Ne
3500
3550
3600
3650
cm
-1
3700
3750
3800
IR spectra of U+(H2O) and Au +(H2O) complexes
3724
3688
U+(H2O)Ar2
3611
3646
3532
Au+(H2O)Ar2
3677
3801
3450 3500 3550 3600 3650 3700 3750 3800 3850 3900
cm
-1
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