Biswajit Bandyopadhyay, Tim Cheng and Michael A. Duncan

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

Infrared Spectroscopy of Protonated Benzene-Water Complexes

Motivation

• Proton transfer is ubiquitous in chemistry and biology.

• Bridged proton systems are key intermediates in proton transfer reactions.

• Protonated benzene-water complexes provide a simple system to study hydrophobic-hydrophilic interface.

• Protonated benzene complex and protonated water clusters have been studied by our group previously.

J. R. Roscioli, L. R. McCunn, M. A. Johnson, Science, 2007, 316, 249

PA of water: 693.8 kJ/molPA of Benzene: 753.6 kJ/mol∆PA( Bz-water ): -59.8 kJ/mol

Experimental

Arc Discharge(1-2kV) to create ions

Mass Selection

IR Excitation

2000-4500 cm-1

600-2200 cm-1

(Bz)H+(H2O)Ar→(Bz)H+(H2O)+Ar

1000 1500 2000 2500 3000 3500

cm-1

x10

2050

35993525

1530

1435

1000 2000 3000

x25

14293381

3554

2235

1000 1500 2000 2500 3000 3500

3393

35493091

1957

1652

B3LYP/6-311+G(d, p)

Ar B.E.= 2.0 kcal/mol

B.E.= 25 kcal/mol

B.E.= 26 kcal/mol

Infrared spectrum of protonated benzene-water complex

2000 2500 3000 3500 4000

cm-1

37343639

2649

2601

2000 2500 3000 3500 4000

35603670

x10

29332160

2000 2500 3000 3500 4000

3656

3696

3522

2000 2500 3000 3500 4000

3509

37013553

37943020

∆E= 0 kcal/mol

∆E= 13.7 kcal/mol

B3LYP/6-311+G(d, p)

(Bz)H+(H2O)2→(Bz)+(H2O)2H+

H+(H2O)2Ar→H+(H2O)+Ar

Looks like protonatedWater dimer

Infrared spectrum of protonated benzene-(water)2 complex

2000 2500 3000 3500

cm-1

37113532

2000 2500 3000 3500

x53185 3661

3571

2000 2500 3000 3500

35773638

3722

3811/3

825

36433734

3804

3192

2000 2500 3000 3500

∆E= 0 kcal/mol

∆E= 20.9 kcal/mol

(Bz)H+(H2O)3→(Bz)+(H2O)3H+

H+(H2O)3Ar→H+(H3O)+Ar

Infrared spectrum of protonated benzene-(water)3 complex

Looks like protonatedWater trimer

∆E= 24.9 kcal/mol

∆E= 0 kcal/mol

2000 2500 3000 3500 4000

cm-1

2565

2432

3343

3738

3462

2000 2500 3000 3500 4000

x5

2904

2958

35113735

3645

2492

2000 2500 3000 3500 4000

26703053

3646 3732

2000 2500 3000 3500 4000

2834

3123 34733645

3734(Bz)H+(H2O)4→(Bz)+(H2O)4H+

H+(H2O)4Ar→H+(H4O)+Ar

Infrared spectrum of protonated benzene-(water)4 complex

Looks like protonatedWater tetramer

2500 3000 3500 4000

cm-1

2594

2745

3642 3734

2500 3000 3500 4000

2700

2852x10

3582

2400 3000 3600

2740 3457

∆E= 6.6 kcal/mol

∆E= 0 kcal/mol

(Bz)2H+(H2O)Ar→ (Bz)2H+(H2O) + Ar

Infrared spectrum of protonated (benzene)2-(water) complex

2500 3000 3500 4000

cm-1

3645 3733

32953433

3023, 3044 3070, 30942769

2843

3295

3467 x5x10

2500 3000 3500 4000

(Bz)2H+(H2O)3→ (Bz)H+(H2O)3+ Bz

Infrared spectrum of protonated (benzene)2-(water) 3complex

Conclusions

• Various isomers are possible but the location of the proton is determined by the solvation energy

• Structures consist primarily of protonated water clusters interacting with neutral benzene.

Acknowledgement• Michael A. Duncan• Gary E. Douberly• Allen M. Ricks• NSF for funding