Spectroscopic and Theoretical Determination of Accurate CH/  Interaction Energies in Benzene-Hydrocarbon Clusters Asuka Fujii, Hiromasa Hayashi, Jae Woo Park, Takaki Kazama, and Naohiko Mikami Department of Chemistry, Graduate School of Science, Tohoku University, Sendai , Japan Seiji Tsuzuki National Institute of Advanced Industrial Science and Technology, Tsukuba , Japan July 21, th OSU International Symposium on Molecular Spectroscopy

What is CH/  interaction? CH/  interaction weak but attractive interaction between C-H and  -electrons origin of the interaction predominance of dispersion ? large contribution of charge transfer ? statistical analyses of crystal structures of organic compounds aromatic rings prefer close contact with C-H groups or

isolated binary clusters in the gas phase ideal systems to characterize weak intermolecular interactions benzene-alkane model clusters for CH/  interaction competition with other interactions (packing force,..etc) Difficulties in CH/  interaction study weak interaction statistical trend in crystal structures dispersion interaction high level theoretical calculations including electron correlations

This work accurate interaction energies (binding energies) determination of jet-cooled benzene-alkane clusters C 6 H 6 - C 2 H 6, C 6 H 6 - C 3 H 8, C 6 H 6 - n-C 4 H 10, C 6 H 6 - i-C 4 H 10, C 6 H 6 - C 6 H 12 laser spectroscopy two-color mass-selected multiphoton ionization (MPI) theoretical calculations ab initio calculations (CCSD(T)/basis set limit) Ref. C 6 H 6 -CH 4, Shibasaki et al., J. Phys. Chem. A110, 4397, 2006 molecular size dependence of the magnitude of CH/  interaction

Experimental determination of the binding energy of the cluster D 0 (S 0 )+IP= IP 1 2 D 0 (S 0 ) D 0 (ion) Bz + X Bz-X S0S0 S1S1 ion   2 : appearance energy of the Bz + fragment D 0 (S 0 )= IP+E E: field ionization correction 135cm -1 ion extraction field of 500V/cm IP: ionization energy of benzene monomer ( 74556cm -1 ) fragmentation(Bz-X) + Bz + + X

Example: C 6 H 6 - C 3 H 8 S 1 -S band of C 6 H 6 – C 3 H 8 monitored by (C 6 H 6 – C 3 H 8 ) + two-color ionization spectrum monitored by (C 6 H 6 ) + fragment appearance energy of the fragment ion Ref. Schauer & Bernstein, JCP 82, 726 (1985) appearance energy ( ) cm -1 binding energy in S kcal/mol

Most stable structures of benzene-alkane clusters Gaussian 03 structure optimization at MP2/aug(d,p)-6-311G** interaction energy at CCSD(T)/basis-set limit C2H6C2H6 C 3 H 8 n-C 4 H 10 i-C 4 H 10 C 6 H interaction energy (kcal/mol) (Helgaker extrapolation + ZPE corrections) on-top structures: interaction between  -electrons and alkane

* (* JPCA, 110, 4397, 2006) Comparison of observed and calculated binding energies CCSD(T) / basis set limit well reproduces the observed values y=x larger alkane shows larger interaction energy

Polarizability and CH/  interaction energy 0.24 kcal/mol: most of attraction comes from dispersion deviation from linear correlation: too large for close contact clear correlation between average polarizability and CH/  interaction energy * (* JPCA, 110, 4397, 2006)

Accurate CH/  interaction energies in the benzene-alkane model clusters were experimentally and theoretically determined. Calculations at the CCSD(T)/basis set limit level well reproduce the observed interaction energies. In the clusters of C1-C4 alkanes, we find linear correlation between average polarizability of the alkane moiety and the interaction energy. Extrapolation to zero polarizability results in nearly zero interaction energy. This means that the attraction in the CH/  interaction essentially comes from dispersion. The CH/  interaction energy in the cyclohexane cluster shows deviation from the linear correlation because the molecular size is too large and only a part of the molecule can closely contact with benzene. Summary