DOI: 10.1002/asia.201100314 Alkylation Effects on the Energy Transfer of Highly Vibrationally Excited Naphthalene Hsu Chen Hsu, [a] Ming-Tsang Tsai, [a] Yuri A. Dyakov, [a] and Chi-Kung Ni* [a, b] Dedicated to Professor Yuan T. Lee on the occasion of his 75th birthday Introduction Collision-energy transfer of highly vibrationally excited mol- ecules plays an important role in gas-phase photochemical, photophysical, and thermal processes, and has been studied extensively by various experimental methods. [1] Among these studies, a large number of experimental studies have been focused on the methylation effects, i.e., the change of collision-energy transfer properties owing to the replace- ment of a hydrogen atom in the highly vibrationally excited molecule by a methyl group. Toselli and Barker studied the collision-energy loss of highly vibrationally excited toluene and benzene using time- resolved infrared fluorescence. They showed that toluene has larger average-energy-transfer values than that of ben- zene for a number of atomic and diatomic molecular colli- sion partners. [2–3] However, this trend is not always true for polyatomic molecular-collision partners. The same experi- mental method has been used in the studies of collision- energy transfer between highly vibrationally excited methyl- substituted pyrazines by Bae et al. [4] They found the quench- ing probability and the average energy loss per collision, DE, for the collisions between the methyl-substituted pyra- zines and rare gases increase with the number of methyl groups. Similar methylation effects were also observed in the collision-energy transfer between methyl-substituted benzenes and unexcited parent molecules, [5] and in the colli- sions of methyl-substituted pyrazine and pyridine by CO 2 . [6] Hippler et al. studied the energy relaxation of highly vi- brationally excited substituted-cycloheptatrien using UV- transient absorption. [7–8] They showed that in the collisions with rare gases, the average energy transfer increases as molecules change from cycloheptatrien, methylcyclohepta- trien, ethylcycloheptatrien, to iso-propylcycloheptatrien. On the other hand, the average energy transfer for iso-propylcy- cloheptatrien is not always larger than that of ethylcyclohep- tatrien for polyatomic molecular collision partners. Abstract: The energy transfer of highly vibrationally excited isomers of dime- thylnaphthalene and 2-ethylnaphtha- lene in collisions with krypton were in- vestigated using crossed molecular beam/time-of-flight mass spectrometer/ time-sliced velocity map ion imaging techniques at a collision energy of ap- proximately 300 cm 1 . Angular-re- solved energy-transfer distribution functions were obtained directly from the images of inelastic scattering. The results show that alkyl-substituted naphthalenes transfer more vibrational energy to translational energy than un- substituted naphthalene. Alkylation en- hances the V!T energy transfer in the range DE d = 100 ~ 1500 cm 1 by approximately a factor of 2. However, the maximum values of V!T energy transfer for alkyl-substituted naphtha- lenes are about 1500 ~ 2000 cm 1 , which is similar to that of naphthalene. The lack of rotation-like wide-angle motion of the aromatic ring and no enhance- ment in very large V!T energy trans- fer, like supercollisions, indicates that very large V!T energy transfer re- quires special vibrational motions. This transfer cannot be achieved by the low- frequency vibrational motions of alkyl groups. Keywords: alkylation · crossed molecular beam · energy transfer · naphthalene · supercollisions [a] Dr. H. C. Hsu, Dr. M.-T. Tsai, Dr. Y. A. Dyakov, Prof. C.-K. Ni Institute of Atomic and Molecular Sciences Academia Sinica 1, Sec.4, Roosevelt Rd. Taipei, 10617 (Taiwan) Fax: (+ 886) 223620200 E-mail: ckni@po.iams.sinica.edu.tw [b] Prof. C.-K. Ni Also at Department of Chemistry National Tsing Hua University Hsinchu, 30013 (Taiwan) 3048  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 3048 – 3053 FULL PAPERS