RESEARCH
More on Molecules Molecular collisions reveal more about intermolecular attractive forces
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Loss of intensity of a molecular beam as it passes through a gas provides a big clue t o what intermolecular forces are like a n d how strong they are. \ c _1 1~_ a\XUlCV.'Uliil
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directly to molecular collision cross sections. T h e s e in turn give a measure of the long-range (Van der Waals) in termolecular attractive forces. R. B. Bernstein a n d E. W. Rothe of the Uni versity of Michigan told the Division of Physical Chemistry. Studying some 80 molecules, Bern stein and Rothe found the collision cross sections and from them calculated the attractive force constants for these molecules. In all the studies, the molecules' cross sections are relative to that of argon, the reference gas. In their studies, Bernstein and Rothe measured collision cross sections for the interaction of beams of potassium, cesium, a n d cesium chloride mole cules with a variety of gaseous mole cules at pressures in t h e range of 10~5 to 10 4 m m . of mercury. For these molecules, the two researchers esti mated force constants based on litera ture data for refractions and dipole moments of the molecules, and exist ing approximation formulas for dis persion, induction, and dipole-dipole forces. T h e s e calculated force con stants differ from those obtained by cross-section measurements by a signifi cant, b u t nearly constant factor, says Bernstein. However, using the theo retical estimates, h e a n d Rothe calcu lated the relative cross sections and compared them with observed values. For the majority of t h e 80 molecules studied, calculated values agreed with observed values within experimental error of the measurements. For a number of cases, particularly t h e lighter gases—helium, hydrogen, neon, and others—sizable deviations (up to 25%) resulted. Bernstein advances no ex planation for the deviations at pres ent.
John Ε. Willard (center), in the mddst of his usual morning routine, works with graduate chemistry students R. W. Looney (left) and H . L- Benson, Jr.
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society Atvara for Nuciear appli sponsored by the Nuclear-Chicago
JOHN E. WILLARD Most any weekday morning at thte dine atoms activated by radiative neuUniversity of Wisconsin, John Willard tron capture are able to displace hycan be found in the chemistry buildin_g drogen from methane to form methyl looking over the shoulders of his 1 5 iodide. This work has been extended graduate research students. After to other halogens. Activated chlorine lunch, Wiîlard exchanges lab coat for atoms, as another example, in the presbusiness suit, and walks to Bascom Hadl ence of propyl chloride can displace alwhere h e spends the rest of the day a s most any hydrogen atom or organic dean of the graduate school. Such a group . jm the propyl chloride molecule dual role is not unusual at Wisconsin. in order to give a variety of stable comWillard says it assures his not forgettimg binations. what it feels like to b e on the adminWillard has been with the University istered as well as the administering encd. of Wisconsin since 1937. Before that When on the administered end, Wil- he taught for two years at Haverford lard specializes in using radioactive ma- College. He earned his B.S. degree in terials to trace the path of chemical r e - 1930 at Harvard, and received a Ph.D. actions. He's been at this for more in chemistry from Wisconsin in 1935. than 20 years, and believes that Wis- Willard's background led him to the consin chemists h a v e published more Government's atomic bomb project for papers in this field than any other single four years during World War II. Durgroup. And quality as well as quantity ing that time h e worked a t the Univerhas been a hallmark of Willard's worïk. sity of Chicago and the Hanford EnOne result: H e is the winner of t n e gineering Works, Willard's appoint1959 American Chemical Society A w a r d ment as dean of Wisconsin's graduate for Nuclear Applications in Chemistry school came just last year. When not in the lab or at the dean's sponsored by the Nuclear-Chicago Cordesk Willard might b e found engaged pora tion. Willard's interest in "hot atom" chem- in heated tennis b a t d e with one of his istry has contributed a better undea:- four children. He reports that his postanding of the chemical effects of n u - sition as tennis champion of the Willard clear transformation. For instance, has household is under serious threat from group was the first to observe that i o - eldest son Mark. APRIL
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1959
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