5f-element chemistry revealed

by actinide ions in the gas phase

John K. Gibson

Chemical Sciences Division

Lawrence Berkeley National Laboratory

Outline

• Experimental method / actinides

• Molecular thermodynamics

• Exotic oxidation states

• Reaction mechanisms

• Metal-metal bonding

Experimental Approach:

Gas-phase reactions byMass Spectrometry

Bimolecular ion-molecule reactions

I+/- + XY → IX+/- + Y

PuO+ Pa2+ UPt+ U2O6- …

O2 C3H8 CH3OH CD3OH …

Fourier Transform Ion CyclotronResonance Mass Spectrometry

AnnL+/- by laser desorption ionizationof actinide-containing solid targets

Pu+ + O2

2 x 10-7 Torr O2

200 ms

Pu+

PuO+

PuO2+

Pu+ + O2 → PuO+ + O PuO+ + O2 → PuO2+ + O

Pseudo first-order kinetics:d[Pu+]/dt = k[O2][Pu+] = k[Pu+]

5dn

6dn

Actinides:bonding 5f n

3dn

4dn

Lanthanides:localized 4f n

d-block transition elements

f-block transition elements

La

AcTh

5f electrons in molecular bonding ?

Th 6d2 7s2 6d transition metalPa 5f2 6d 7s2 f-bonding (?)U 5f3 6d 7s2

Np 5f4 6d 7s2

Pu 5f6 7s2

Am 5f7 7s2 f-localizedCm 5f7 6d 7s2

Bk 5f9 7s2

Cf 5f10 7s2

Es 5f11 7s2

7                  

6                                     

5                                                        

4                                                                                   

3         

                                                                                                                                

2                                              

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Oxidation States

High Oxidation States / 5f → 6d PromotionDirect 5f participation in chemistry

Alpha decay (4-7 MeV)

U-238 10 / s.mg

Np-237 104 / s.mg

Pu-242 105 / s.mg

Am-243 107 / s.mg

Es-253 109 / s.µg

Experimental Challenges / Hazards

Need good theory!

Gas-phase actinide chemistry:

▪ Fundamental science

▪ Basis for development & validationof theoretical approaches

Molecular thermodynamics

Thermodynamics of PuO2+

PuO+ + O2 → PuO2+ + O

D[OPu+-O] ≥ 498 kJ/mol

If a reaction occurs at low energythen ∆H ≤ 0

∆S undefined, zero?

D[O-O]

D[OPu+-O] =

D[OPu-O] + IE[PuO] – IE[PuO2]+598 +636 -970

= 264 kJ /mol* (<<498 kJ/mol)

*Electron impact of PuO2(g):

F. Capone, et al., J. Phys. Chem. A 1999, 103, 10899

Conflict between experiments: PuO2+

?

15

IE[PuO2] from Electron-Transfer

PuO2+ + DMPT → PuO2 + DMPT +

PuO2+ + DMA → PuO2 + DMA+

No kinetic barrier to electron transfer:

IE[DMPT] ≤ IE[PuO2] ≤ IE[DMA]

IE[PuO2] = 7.03 ± 0.12 eV

vs. IE[PuO2] = 10.1 ± 0.1 eV from Electron Impact

6.93 eV 7.12 eV

X

D[OPu+-O] =

D[OPu-O] + IE[PuO] – IE[PuO2]

+598 +636 -970

= 264 kJ /mol

X

X

672

562 (≥ 498 kJ/mol)

17

IE[PuO2]

New Experimental: 7.02 ± 0.12 eV

Preliminary Theoretical Results

L. Gagliardi, U. Geneva

CASPT2: 6.5 – 7 eV

The Bare Actinyls{O=An=O}2+

AnO2+ + N2O → AnO22+ + N2

UO22+ NpO2

2+ PuO22+

Actinyl Thermodynamics

AnO22+ + X → AnO2

+ + X+

IE[AnO2+] > IE[X] + E*

Barrier from AnO2+ / X+ repulsion

ΔHf[AnO22+] = ΔHf[AnO2

+] + IE[AnO2+]

Actinyl Thermodynamics

ΔHf[AnO22+(g)] ΔHf[AnO2

2+(aq)] ΔHhyd[AnO22+]

UO22+ 1524 -1019 -1665

NpO22+ 1671 -861 -1654

PuO22+ 1727 -822 -1671

ΔHhyd[AnO22+]

AnO22+(g)

AnO22+(aq)

(kJ mol-1)

This Work Calorimetry

Actinyl Hydration / Experiment ↔ DFT

ΔHhyd[AnO22+] ≈ -1660 kJ mol-1*

UO22+, NpO2

2+, PuO22+

ΔHhyd[UO22+ ] = -1676 kJ mol-1

Moskaleva et al. Inorganic Chemistry 43 (2004) 4080

J. Phys. Chem. A 109 (2005) 2768

ΔHhyd[AnO22+] = -1820 ± 10 kJ mol-1

Shamov & Schreckenbach J. Phys. Chem. A 109 (2005) 10961

*Experiment: -1780 with “revised” ΔHhyd[H+(aq)]

Exotic oxidation states

Actinides in High Oxidation States

AnO+ + C2H4O → An(V)O2+ + C2H4

D[OAn+-O] ≥ 354 kJ mol-1

ThO2+ PaO2

+ UO2+

NpO2+ PuO2

+ AmO2+

Electronic structures ?

“6p hole” ?

“Protactinyl”

PaO2+ + N2O → {O-Pa-O}2+ + N2

D[OPa2+-O] ≥ 167 kJ mol-1

IE[PaO2+] = 16.6 ± 0.4 eV

J. Phys. Chem. A 110 (2006) 5751

Protactinyl: LC-RECP SCF Calculation

PaO2+ PaO2

2+

Pa O Pa Os 2.11 3.67 2.09 3.71p 5.91 8.75 5.75 8.23d 1.66 0.04 1.64 0.05

f 1.86 ----- 1.53 -----

totals 11.54 12.46 11.01 11.99

IE[PaO2+] = 16.61 eV

Pa5.5PaV

Pitzer, Mrozik & Bursten

Why not PaO22+(aq)?

{O-An-O}2+ → An2+ + 2O ΔH / kJ mol-1

UO22+ > PaO2

2+ ≥ NpO22+ > PuO2

2+ > AmO22+

1250 1110 1030 830 600

PaO22+(aq) + ½H2O(l) → PaVO(OH)2+(aq) + ¼O2(g)

ΔG ≈ -110 kJ mol-1

AmO22+ (g) ?

Is bare americyl stable?

AmO22+ Am+ + O2

+

ΔHdissociation ≈ 1 ± 1 eV

?

Reaction mechanisms

• 5f-electron bonding

• “Interfacial” chemistry

• Do 5f electrons participate in molecular bond activation?

• Is 5f electron promotion required: 5f n-1 7s → 5f n-2 6d 7s ?

5fxyz

Carbon-Hydrogen Bond Activation:

5f electrons in Organoactinide Chemistry

Hydrocarbon Activation by An+

Role of the 5f electrons in organoactinide chemistry

SlowAn+- insertion

FastH2-elimination

0

50

100

150

200

250

300

Th Pa U Np Pu Am Cm Bk Cf

Pro

mo

tio

n E

ner

gy

(kJ/

mo

l)

C-An+-Hrequires

5fn-26d7sconfiguration

Reactive

Inert

Intermediate

Beyond Np+, the 5f electrons do notparticipate in C-H bond activation

An+[Ground] → An+[5fn-26d7s]

? ? ?

Hydrocarbon Activation by AnO+

The role of the 5f electrons—early actinides

Employ An valence electrons in An+=O bonds:Do 5f electrons at metal center oxidatively insert ?

MO+ + C2H4 → MOC2H2+ + H2

Dehydrogenation of Ethylene:

TaO+ 0.31

ThO+ <0.001

PaO+ 0.17

UO+ <0.001

NpO+ <0.001 • • •

Organometallics 26 (2007) 3947-3956

Electronic structures of MO+

{Th(7s)O}+ {U(5f3)O}+

Unreactive MO+

Reactive MO+

{Pa(5fx 6dy 7sz)O}+{Ta(5d1 6s1)O}+

SOCISD/RECP: {Pa(5f16d1)O+}

x + y + z = 2

Pitzer, Mrozik & Bursten

Electronic configurations of PaO+

Excitation Energy (eV)0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Orb

ital O

ccup

atio

n

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

5f Orbitals6d Orbitals7s Orbital

High reactivity of PaO+ indicateschemically active 5f electron(s)

PaO+ + H2C=CH2

↓

OPa+-{HC≡CH}

↓

C=CHH

HH

Pa+O

-H2

5f-electrons in organoactinides

C-H activation by:

Pa+(5f26d7s)

Pa2+(5f26d)

Pa(5f6d)O+

5f-participation in σ-typebonding in “C-Pa-H”

UO2(s)

U UU U UU

CH3

O

CH3

O

CH3

O

CH3

O

CH3

O

CH3

O

CH3OH(g)

Gas-Phase Ion Reaction Mechanisms:“Interfacial” Chemistry

Lloyd, Manner & Paffett, Surface Science 1999, 423, 265-275.

“(NH4)+2UO4

2-UO3(s)”

↓ hν

UVO3- UVIO3(OH)- UVIIO4

-

U2V/VIO6

- U3VO8

- U3V/VIO9

-

Uranium Oxide Negative Ions:Molecules & Clusters

UVO3- + CH3OH → UIIIO(OH)2

- + CH2O

k/kCOL = 21%

UVIIO4- + CH3OH → UVO2(OH)2

- + CH2O

k/kCOL = 4%

Molecular Anion Reactions with Methanol

+N2O -N2

+ 2H / -CH2O ↓ 21%

+ OCH2 / -H2 ↓ 21%

+ 2H / -CH2O ↓ 4%

+ CH2 / -H2O ↓ 17%

+ CH2 / -H2O ↓ 18%

UVO3- UVIIO4

-

UIIIO(OH)2- UVO2(OH)2

-

UVO2(OH)(OCH3)-

UVO2(OCH3)2- O=U=O

OCH3

OCH3

-

?

X

Preliminary Theoretical Results / UO3H2-

M. Michelini & N. Russo, U. Calabria

PW91/ZORA B3LYP/SDD

UO(OH)2-

UO2(OH)H-

H I

O=U=O I

OH

X

Structures & Mechanismsfrom Isotopic Labeling

UO3- + CD3OH → UO3HD- (+ CD2O)

UO3HD- + CD3OH → UO4HCD3- (+ HD)

UO4HCD3- + CD3OH → UO4C2D6

- (+ H2O)

No Isotopic Scrambling

O=U

UO3HD- + CD3O-H → UO4HCD3- (+ HD)

O-H

O-D

HH & HD

O-U-OO-

H D

UO3HD- + CD3O-H → UO4HCD3- (+ HD)

O=U=O

D

O

D

HH

H

O

O=U=O

H

HD

Cluster Anion Reactions with Methanol

UV/VI2O6

-

+ OCH3, H ↓ 11%

UV/VI2O5(OCH3)(OH)-

+ CH2 / -H2O ↓ 27%

UV/VI2O5(OCH3)2

-

•••

UV/VI2O3(OCH3)6

-

↓

Samesequence

for

UV3O8

-

&

UV/VI3O9

-

Cluster Anion Reactions with Methanol

UV,VI2O6

- + 6CD3OH → UV,VI2O3(OCD3)6

- + 3H2O

U U

O

O

O

OCD3

OCD3

OCD3

D3CO

D3CO

D3CO

-

Analogous to methoxidation of UOx(s) surfaces

?

Metal-metal bonding

Actinide – Transition MetalCovalent Bonding

J. M. Ritchey, et al., J. Am. Chem. Soc. 1985, 107, 501-503.

Bimetallic Ions by LDI ofActinide-Transition Metal Alloys

ThPt+

PaPt+

UPt+

NpPt+

PuPt+

AmPt+

CmPt+

UIr+

UAu+

20% U / 80% AuNd-YAG 1064 nm

P. Pyykkö et al., Chem. Phys. Lett. 381 (2003) 45

“Autogenic Isolobality”

2Au (5d106s1) ~ 2H (1s1)

3Pt (5d96s1) ~ 3O (2s22p4)

4Ir (5d76s2) ~ 4N (2s22p3)

Actinide-Transition Metal Bonding:“Autogenic Isolobality”

Gagliardi & Pyykkö, Angewandte Chem. 43 (2004) 1573

{U-Au}+ ↔ {U-H}+

{U=Pt}+ ↔ {U=O}+

{U≡Ir}+ ↔ {U≡N}+

U+ UO+ 0.34 NR <0.001

Ir+ IrCH2+ 0.21 IrC2H4

+ 0.23

Pt+ PtCH2+ 0.16 PtC2H4

+ 0.24

Au+ AuCH2+ 0.11 AuC2H4

+ 0.19

UIr+ OUIr+ 0.24 NR <0.001

UPt+ OUPt+ 0.21 NR <0.001

UAu+ OUAu+ 0.28 NR <0.001

Reactivities of UM5d+ Bimetallic Ions

C2H4O C2H6

Reactivities of UM5d+

{AuI-UII}+ {PtII=UIII}+ {IrIII≡UIV}+

• Reactivities of M5d are “shut off”, consistent with covalent bonding & bonding saturation: AuI; PtII; IrIII

• Reactivities of the UM5d+ are similar

to bare U+; oxidation consistent with oxidation states up to U(VI) in {Ir≡U=O}+

{U=O}2+ + N2O → {O=U=O}2+ + N2

{U≡N}+ + O2 → {O=U≡N}+ + O

Pseudoactinyls

1.65 Å

1.72 Å 1.63 Å

Pyykkö et al., 1994

“Metalloactinyls”:{OUIr}+ as an Analogue to Uranyl

{U≡Ir}+ + N2O → {O=U≡Ir}+ + N2

1.75 Å 2.15 Å

Gagliardi & Pyykkö, 2004

Gas-Phase Actinide Ion Chemistry

A probe of fundamental aspectsof actinide chemistry:

molecular thermodynamics to “surface chemistry” of clusters.

Theory ↔ Experiment

Experiment

Instituto Tecnológico e Nuclear, Portugal

J. Marçalo, A. Pires de Matos, M. Santos

Oak Ridge National Laboratory, USA

R.G. Haire

Theory

Ohio State University, USA

R.M. Pitzer, M.K. Mrozik, R. Tyagi

University of Tennessee, USA

B.E. Bursten

University of Calabria, Italy

N. Russo, M. Michelini

University of Geneva, Switzerland

L. Gagliardi

Research at Lawrence Berkeley National Lab supported byOffice of Basic Energy Sciences, U.S. Department of Energy