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Re: Physical properties of isotopes?



As I just pointed out, reaction rates are approximately inversely proportional to the square root of the mass ratio.  So, for example, completely deuterated  glycine (COOH-CH2-NH2) would react at about 97% of the rate of ordinary glycine, glycine with C-13 instead of C-12, at 99% of the rate; glycine that is all deuterated and C-14-substituted, 95%, and so on.  I worked a great deal with isotopically substituted glycine at one time in my life, and I observed no effect on crystallization, hydrolysis, zwitterion behavior as observed spectroscopically.  I clearly observed differences in electron spectroscopy and NMR (that's why I was using isotopic substitution -- to observe these differences and explain them).  This was published in JACS many years ago and is cited in several works on glycine.

Most reaction rate variations are within the experimental error of reaction rate measurements.  Deuterated water reduces the reaction rate to about 95% and completely tritiated water, to about 91%.

Clearly, processes dependent ONLY on molecular mass differences (like mass spectrometry and gaseous diffusion) are going to show differences -- I mean, these processes are used to separate different masses.  When I said insignificant differences, I meant in ordinary macroscopic chemical reactions and physical processes like material strength, melting, boiling, etc.

No I don't need a reference, thank you.

Ruth Weiner, Ph. D.
ruthweiner@aol.com