UV Photodissociation of cyclic ketones in the gas phase and cold inert gas matrixes Anamika Mukhopadhyay, Moitrayee Mukherjee, Amit K. Samanta and Tapas Chakraborty Physical Chemistry Department Indian Association for the Cultivation of Science Jadavpur, Calcutta , India 65 th OSU Symposium on Molecular Spectroscopy Date: 21 st June IACS MG17

Photodissociation of cyclic ketones: Products exct : 254 nm + CO Initial step follows Norrish-I reaction + CO 2C 2 H 4 + CO exct : 254 nm ? ?

Molecule of interest 1, 2 cyclohexanedione (1,2 CHD): Considered to be responsible for coffee aroma. Active site of the anticancer drug and anti- tumor steroid Monoenolic form 1,2 CHD Wavelength (nm) Intensity (arb. unit) 3,4 hexanedione Goal of study:  Fragmentation channels in the gas phase  Small chemical substitution  Ring size effect  Comparision with monocarbonyls to explore the role of 2 nd carbonyl on gas phase photochemistry  Environment effect, i.e. if the molecule is taken in a rigid environment

UVC Lamp =254nm channeltron -5 kV Signal + -2 kV e e e Quadrupole mass filter Axial Ionizer Continuous Molecular Beam Turbo pump Diffusion pump Sample pump Computer Gauge Entrance Lens Exit Lens Multipass absorption cell Detector MIR source FTIR Spectrometer Reaction chamber Experimental Scheme for gas phase study

Scheme of UV Photodissociation in Matrix Environment IR beam from an FTIR spectrometer 266 nm light from Nd:YAG laser KBr window cooled at 7K temp ……….. ……….. ….…….. ………... …….…. ………... ………... ………… ………… ………… ………… ………....

λ exct =254 nm light Proposed Channels λ exct =254 nm light + CO Norrish-I + C 2 H nm light CO + CO + 2C 2 H 4 λ exct =254 nm light +

CH 2 CH 2 CH 2 CH 2 C=O + (84) + CO(28) CH 2 CH 2 CH 2 C=O + (70) + H 2 CCO(42) 2 CH 2 CH 2 C=O + (56) Mass spectrometric experiment Before UV exposure After ½ hr UV exposure KE of electron=11 eV C2H4C2H4

A A C B D FTIR Spectrum of 1,2 CHD before and after UV irradiation Before UV exposure After ½ hr UV exposure B

C D 949 C=O C–H 2151 A B Wavenumber(cm -1 ) Identification of final products after UV exposure by FTIR spectroscopy

1 hr UV exposure Before UV exposure 266 nm light Wavenumber (cm -1 ) + Methyl ketene Ethyl ketene ketene Photodissociation of 1,2 CHD in the inert Ar matrix environment ketene Ethyl ketene

Photodissociation of 3-methyl-1,2-CHD: Substituent effect Mass m/z (amu) Before exposure After exposure with 254 nm light Electron kinetic energy=11 eV + CO CO + C 2 H C 2 H No reactivity in matrix with 266nm light

Photodissociation of 3-methyl-1,2-CPN in the inert N 2 matrix environment nm Wavenumber (cm -1 ) Photoreactivity enhanced compared to the six membered diketone (K) (MK) (DMK)

Summary and outlook:  Gas phase photodissociation results ketene, ethylene, carbonmonoxide and cyclopentanone as the major photoproducts of 1,2 CHD by UV exposure of wavelength 254nm  In 7K argon matrix environment, photodissociation channels are drastically altered, and only ketene, methyl and ethyl ketene are produced  An ortho methyl substitution to 1,2 CHD make the molecule resistant to photodissociation. No photodissociation is found in cold Ar-matrix, and in the gas-phase reactivity is much weaker compared to the parent molecule  On the other hand, methyl substituted 1,2-cyclopentanedione show larger photoreactivity both in gas phase and in matrix  Mass spectrometric as well as spectroscopic measurements for identification of the the reaction intermediates of the above mentioned photodissociation reaction are in progress

Thank You