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mations 01 << ws and w12 ~1~2 << 1, these equations reduce to where N is the number of paramagnetic ions per unit volume; d is the average closest distance of approach between a proton in the bulk and spin S; B3p, the Bessel function of order 3/2; and D = DI + Ds, the self-diffusion coefficient of the bulk protons plus that of the spins S. Equation AS can be integrated in closed form. The result, given by eq 33 of Pfeifer's paper,6l was used to calculate Tlws and Tz,'. and y12Ys2n2s(S + ' ) [7J(0) + 13J(ws)1 (A7) ---1 TzwS 4The term J ( w ) is the spectral density function a t the frequency w and is given by the integral Experimental binary gas-phase diffusion coefficients for some halogenated hydrocarbon compounds diffusing into helium have been determined utilizing the gas chromatography (GC) peak-broadening technique. Certain modifications are described which have improved the speed and accuracy of the technique. A critical discussion of the validity of the method is given. Based mainly on the acquired data, additional parameters have been determined for use in connection with the Fuller-Schettler-Giddings diffusion volume method for estimating binary diffusion coefficients. The relationship between effective collision cross sections and various molecular shape and structural features is discussed.Widespread efforts to characterize numerous diffusion processes more exactly have resulted in an increased need for accurate diffusion data and improved methodology. Gradually more diffusion data are being published, but very little data are presently available for most organic vapors. The major objectives of this investigation were to develop improved experimental apparatus and techniques, to generate reliable data for new systems, and to study the dependence of collision cross sections on complex molecular structure. In contrast to simple systems where temperature variations are of utmost importance, the present complex systems yield more information when studied as a function of structural features such as chain length and substituent position.Experimental. binary diffusion coefficients were determined for thirty-one halogenated hydrocarbons diffusing into helium. From these data additional "atomic diffusion volume" parameters were obtained for use with the method developed by Fuller, Schettler, and Giddingsl for predicting diffusion coefficients. With the exception of two confirmatory determinations, all diffusion coefficients reported here are for systems not previously investigated.

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