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A method for calculating multiple long-range interaction potentials between an atom (C in this particular case), and closed-shell molecules is described. This method was recently utilized by Bettens and Collins [J. Chem. Phys. 114, 10342 (2001)] for determining capture rate coefficients for collisions of C ([sup 3]P[sub j]) and Ge ([sup 1]S[sub 0]) with unsaturated hydrocarbons. The method takes account of the physical size of the molecule and exactly treats the electrostatic interaction between the quadrupole of the atom and all electrostatic moments of the molecule. This method requires a molecular density matrix, which can be obtained from an ab initio calculation. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have calculated the capture rate coefficients for collisions of C ([sup 3]P[sub j]) and Ge ([sup 1]S[sub 0]) with unsaturated hydrocarbons and compared them with the experiments of Husain and co-workers [Clary, Haider, Husain, and Kabir, Astrophys. J. 422, 416 (1994); Husain, Ioannou, and Kabir, Z. Phys. Chemie 203, 213 (1998)]. Our findings indicate (i) that all nine electronic surfaces for the C ([sup 3]P) reactions must contribute to the observed reaction rate coefficients, (ii) a central multipole approach is inappropriate for calculating the capture rate coefficients involving the larger hydrocarbons, (iii) the trends observed in the measured rate coefficients for the Ge and C reactions can simply be understood in terms of the physical size of the hydrocarbons, and (iv) the electrostatic interaction plays a significant role in the determination of capture rate coefficients for reactions involving C ([sup 3]P) and highly polar species as well as acetylene. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]