Photoionization induced intermolecular proton transfer in the CH ●●● O hydrogen bonded cyclopentanone dimer in the gas phase Arup K. Ghosh, Piyali Chatterjee and Tapas Chakraborty Physical Chemistry Department Indian Association for the Cultivation of Science Jadavpur, Kolkata , India 69 th International Symposium on Molecular Spectroscopy June19 th,2014 1

Photochemistry of cyclopentanone M. Saltmarsh, R. G. W. Norrish, J. Chem. Soc., 445 (1935). S. W. Benson, G. B. Kistiakowsky, J. Amer. Chem. Soc., 64, 80, (1942). F. E. Blacet, A. Miller, J. Am. Chem. Soc., 79, 4327, (1957). R. Srinivasan, Advan. Photochem., 1,83, (1963). R. Bacerra,H. M. Frey, J. Chem. Soc. Faraday Trans.,, 84, 1941, (1988). All the process start with the Norrish type I cleavage ● ● + 2C 2 H 4 CO + hνhν hνhν M. Baba, H. Shinohara, N. Nishi, Chem. Phys., 83, 221, (1984). C. Kosmidis, J. G. Philis, P. Tzallas, Phys. Chem. Chem. Phys. 1, 2945 (1999) T. I. Solling, E. W. G. Diau, C. Kotting, S. de Feyter A. H. Zewail, Chem. Phys. Chem., 3, 79 (2002). hνhν + CH 2 CO hνhν 2 + C2H4C2H4 m/z 56  + n(hν) + C2H5 C2H5  m/z 55 + n(hν)  + + CO m/z 56 n(hν) + + HCO  m/z 55 n(hν)

ionization Intermolecular proton transfer: A new pathway 3 The weakly hydrogen bonded cyclopentanone dimer is formed under supersonic jet conditions. Protonated Cyclopentanone Neutral radical (Cyclopentanone - H atom) CH ∙∙∙O hydrogen-bonded (CP) 2 CPH + (CP-H) ● +

Ionization Scheme: Lock and King, J. Mol. Spectrosc., 36, 53, nm 30,000 35,000 40,000 cm nm 4 I.E. = 9.26 eV Absorption spectrum of Cyclopentanone in the gas phase 3.75eV S0S0 I.E. = (9.26 eV) 3  3.49 eV =10.47 eV Relative Energy (eV) 355 nm266 nm 2  4.66 eV =9.32 eV S 1 (nπ*) (n→3p z ) Rydberg state 4  2.33 eV =9.32 eV 532 nm

C2H2+C2H2+ C2H3+C2H3+ m/z No mass peak due to dimer ion C2H4+C2H4+ m/z 56  + m/z 84 m/z 55 + C3H3+C3H3+ CH 2 CO  + m/z 85  + + Lock and King, J. Mol. Spectrosc. 1970, 36, nm 30,000 35,000 40,000 cm nm 266 nm photoionization mass spectrum S0S0 I.E. (9.26 eV) Relative Energy (eV) 266 nm 2  4.66 eV = 9.32 eV S 1 (nπ*) Rydberg State (n→3p z ) 3.75eV 5

m/z C3H3+C3H3+ C2H3+C2H3+ C2H4+C2H No mass peak due to dimer ion CH 2 CO  + Lock and King, J. Mol. Spectrosc. 1970, 36, nm 30,000 35,000 40,000 cm nm S0S0 I.E. (9.26 eV) 3  3.49 eV =10.47 eV Relative Energy (eV) 355 nm 266 nm 2  4.66 eV =9.32 eV S 1 (nπ*) Rydberg State (n→3p z ) 355 nm photoionization mass spectrum 6 m/z 55 + m/z 56  +  m/z m/z 67 +  + + OH 

m/z CP 2  + CP 2 H + C2H2+C2H2+ C3H3+C3H3+ C2H3+C2H3+ CH 2 CO  + C2H5+C2H5+ Lock and King, J. Mol. Spectrosc. 1970, 36, nm 30,000 35,000 40,000 cm nm 532 nm photoionization mass spectrum S0S0 I.E. = (9.26 eV) 3  3.49 eV =10.47 eV Relative Energy (eV) 355 nm266 nm 2  4.66 eV =9.32 eV S 1 (nπ*) (n→3p z ) Rydberg state 4  2.33 eV =9.32 eV 532 nm 7  m/z 84 + m/z 85 + m/z 55 +  + m/z 56 + m/z 67

8 Intermolecular proton transfer through keto-enol tautomerization Ionic Keto-enol Dimer n(hν) ionization proton transfer dissociation n(hν) ionization CH ∙∙∙O hydrogen-bonded (CP) 2 CPH + (CP-H) ● +

Intermolecular proton transfer: suggested mechanism eV Ionization eV 1.52 eV 1.22 eV TS (Ionic) Products Ionic Diketo Dimer Ionic Keto-enol Dimer 9.32 eV ( 2 x 266 nm) Neutral Diketo Dimer TS (neutral) 1.10 eV Neutral Ketoenol Dimer 0.61 eV 9 DFT/B3LYP/ G(d,p)

10 Summary: Intermolecular proton transfer takes place in the cyclopentanone dimer upon photoionization. The intermolecular proton transfer in the cyclopentanone dimer is dependent on the wavelength of the laser light. We suggest that the intermolecular proton transfer proceeds via a keto–enol tautomerization mechanism in the ionic state.

Acknowledgements: Council of Scientific and Industrial Research (CSIR), India 11

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