Infrared Resonance Enhanced Photodissociation (IR-REPD) Spectroscopy used to determine solvation and

structure of Ni+(C6H6)n and Ni+(C6H6)nAr complexes

Department of Chemistry, University of Georgia

Athens, GA 30602-2556

www.arches.uga.edu/~maduncan/

J.B. Jaeger, T.D. Jaeger and M.A. Duncan

Introduction

•Dibenzene chromium - E.O. Fischer and W. Hafner in 1955

•aromatic -bonded sandwich structure similar to ferrocene

•Collision Induced Dissociation (Armentrout)

•Equilibrium Mass Spec (Dunbar)

•Ion Mobility (Bowers)

•Photodissociation (Freiser)

•Electronic Spectroscopy/Photodissociation (Duncan)

•Photoelectron Spectroscopy (anions) IR Matrix Deposition, observed multiple decker “sandwiches” (Kaya and Nakajima)

•Magnetic Deflection multiple decker “sandwich” (Nakajima and Knickelbein)

•IR spectra of alkali cation-benzene-water complexes in O-H stretch region (Lisy)

•IR spectra of transition metal cation-benzene complexes- fingerprint region only using free electron laser with ion trap mass spectrometer (Duncan, Meijer)

•Limited to small complexes (two benzene ligands or less) due to mass selection constraints

•Broad laser linewidth (~10 cm-1), multiple photon processes, possibility of thermally and/or electronically excited ions (ArF ionization) yielded wide spectral features (FWHM 25-50 cm-1)

•Comparison to theory was qualitative at best

•More recently IR spectra of V+(C6H6)n and Si+(C6H6)n complexes in the C-H stretch region (2900-3200 cm-1) @ UGA better linewidth and colder ion source

Previous IR Studies on Gas Phase Transition Metal-Cation Benzene Complexes

Production of larger Ni+(benzene)n clusters

Ions direct from jet; pulse extracted

Mass gate – pulsed deflection plates before reflectron give total mass selection

Species intersected by IR output of OPO/OPA (LaserVision; 2000-4400 cm-1; 0.5 cm-1 res; 1-15 mJ/pulse; 5 nsec)

Ability to obtain IR-PD spectra in C-H stretch region by monitoring photofragment yield vs IR wavelength

0 200 400 600 800 1000

75

3

21

0

m/z

Ni+(C6H

6)n

Mass spectrum is dominated by Ni+(C6H6)n ions

Smaller masses in between are Ni+(C6H6)n(H2O), Ni+

(C6H6)nAr, Ni2+(C6H6)n

D0 = 2.52 eV and 1.52 eV (CID,Armentrout)

~ 20,300 cm-1 and 12,200 cm-1

IR active C-H stretch of benzene near 3100 cm-1

At least 7 photons would be needed for benzene elimination!

No dissociation observed for Ni+(C6H6) and Ni+

(C6H6)2

Solution: Rare gas tagging

Ni+Ar binding energy 4436 cm-1

Ni+(benzene) and Ni+(benzene)2 are bound strongly

2800 2900 3000 3100 3200

12

Ni+(C6H

6)Ar

2

cm-1

e1u

Fermi triadbenzene (l)NIST

2 +

13 +

18

12

liq 3093,gas 3101

3037 liq, 3048 gas

13

+ 16

liq 3074, gas 3079

•No Fermi triad observed for the 1,1,2 complex

•The perturbation of the Ni+ is enough to remove the degeneracy of the triad modes

•The recommended frequency for the 12 mode in free benzene is 3063 cm-1 in the absence of the Fermi triad.

•Blue shift of 30 and 36 cm-1 respectively for doublet

12

V+(C6H

6)Ar

2800 2900 3000 3100 3200 3300

Ni+(C6H

6)Ar

2

cm-1

30 and 36 cm-1

25 cm-1 •Similar blue shift for V+

(C6H6)Ar complexes

•The perturbation of the cation

is enough to remove the degeneracy of the triad modes

•Dopfer and coworkers found blue shift of ~30 cm-1 C6H6

+(L) complexes. Due to stiffening of C-H bonds upon electron removal from the HOMO e1g

orbital

3000 3025 3050 3075 3100 3125 3150 3175

cm-1

Ni+(C6H

6) 2B

2 C

2v

•DFT calculated by S. Klippenstein using B3LYP functional with 6-311++G(d,p) basis set

•Theory calculated without argon

•The benzene ligand has two types of nonequivalent hydrogens yielding two main IR active modes 3089 & 3093 cm-1

•Good agreement between predicted vibrational spectrum and the observed doublet

•The angle of the benzene ring distortion is exaggerated for demonstration, actually ~1-2 degrees

•Smaller band not evident within our sensitivity

Ni+(C6H

6)

2 2B

2g D

2hNi+(C

6H

6)

2 2A C

1

Ni+(C6H

6)

2Ar

3000 3025 3050 3075 3100 3125 3150 3175

•The 1,2,1 spectrum shows a single mode with red shoulder shading centered at about the same position as 1,1,2

•DFT predicts two doublet spin states close in energy

•DFT incorrectly predicted the lowest energy ground state to be 2A C1

•The 2B2g D2h structure appears to be a much better match for our spectrum

Ni+(C6H

6)

2Ar

Ni+(C6H

6)

3Ar

2700 2800 2900 3000 3100 3200 3300

Ni+(C6H

6)Ar

2

cm-1

•Spectrum changes dramatically at the 1,3 cluster size

•S/N increases from 1,2,1 to 1,3. This indicates the efficiency of photodissociation due to external ligands not bound directly to the Ni+

•The “core” mode is still evident in the 1,3 complex blue line

•New multiplet of modes are first evidence for the presence of the Fermi triad modes

2700 2800 2900 3000 3100 3200 3300

Ni+(C6H

6)3

cm-1

Ni+(C6H

6)3Ar

•The 1,3,1 spectrum is a little sharper than the 1,3 spectrum, however the spectral position is relatively unchanged

•The high S/N for the 1,3 spectrum suggests efficient photodissociation is achieved without Ar tagging

•This means that the loss of the third benzene occurs easily; it must be bound externally

•Benzene dimer is bound by ~800-1000 cm-1

•The appearance of Fermi triad (occurs when benzene is unperturbed) and the photodissociation efficiency both suggest that benzene ligands are beginning to pile up on the outside of the complex

2700 2800 2900 3000 3100 3200 3300

Ni+(C6H6)3

cm-1

Ni+(C6H6)5

Ni+(C6H

6)

6

2

13+

18

13

16

C6H

6 (l)

Ni+(C6H

6)

4

12

•Ligands not bound directly to Ni+

•Appearance of Fermi triad suggest unperturbed benzene ligands

•High s/n indicates IR photodissociation spectra occurs efficiently

•Benzene dimer bound by ~800-1000 cm-1

•The larger complexes have essentially the same bands

•The similarity between the NIST spectrum and our spectra suggest solvation following the third benzene ligand

Conclusions

•First IR-REPD spectra produced for Ni+(C6H6)n and Ni+(C6H6)nAr complexes

•The perturbation of the benzene ring by the Ni+ removes the degeneracies of the Fermi triad

•The blue shifted modes observed in the smaller complexes’ spectra are most likely from a stiffening of the C-H bonds due to charge removal from the benzene ligand

•The 1,1,2 spectrum agrees with calculated 2B2 ground state structure where the benzene ring is distorted in a C2v manner

•DFT incorrectly predicts the ground state of the 1,2,1 complex as a 2A state with C1 symmetry, but the observed spectrum instead matches better with the distorted 2B2g D2h structure

•The larger clusters show spectra similar to liquid benzene, showing a Fermi triad of frequencies and therefore solvation after the addition of three benzenes