Intermolecular forces isotope effect

Studies of the molar volumes of perdeuteriated organic compounds might be expected to be informative about non-bonded intermolecular forces and their manifestations, and such studies might be considered to obviate the necessity of investigating steric isotope effects in reacting systems. The results from non-reacting systems could then be simply applied to the initial and transition states in order to account for a kinetic steric isotope effect. 

In the gas phase, the 180=C=160 band would not be doubled in this way. The only effects of isotopic modification would be to make the symmetric stretch (vj) weakly allowed, and to have slightly more stretching of C=160 in v3, and slightly less in vt. In an asymmetric environment, intermolecular forces on the two ends of the C02 molecule must balance, but the intermolecular force constants can differ. Since in v3, the vibrational amplitude of the C=160 bond is slightly greater than of the C=180 bond, the mode is more sensitive to the intermolecular force constant at the 160 terminus. When 160 is on the end with a higher force constant, the v3 frequency is shifted to higher frequency, while the opposite is true when the lsO is on the other end. 

Most papers dealing with the spectrum of the carbonate ion CO7 neglect to mention the important paper by Decius, Malan and Thompson 42> on the effect of intermolecular forces on molecules in the crystalline state which refer specifically to the out-of-plane bending mode of CO In this paper they derive the dependence of this mode upon the 12C-13C isotopic ratio. Sterzel and Chlorinski 43) also discuss the effect of isotopes upon the CO2" vibrations these two papers should be consulted when assigning C03 -spectra because the modes depend very much upon the t2C-13C ratio. Orville-Thomas 20> has discussed the dependence of the C03 force constants upon the C-O distance, and shows that this leads to a bond intermediate between a single and a double bond. 

Chromatographic separations of the isotopic isomers of molecules are of interest first because they afford a convenient technique for analysis of mixtures, secondly because scale up in the future might afford economically feasible separations of macroscopic amounts of material, and finally because the values of the separation factors and their temperature coefficients are of intrinsic theoretical interest. This last follows because such data are straightforwardly related to the understanding of isotope effects on solution and adsorption processes and of the intermolecular forces which give rise to these effects. We have approached the general problem... 

Equation 1 relates the force fields describing the motions of the molecule in the condensed and in the gaseous phase with the activity ratio. These fields are different owing to the effect of the intermolecular forces which are operative in the condensed phase. The intermolecular forces are exclusively solute-solute forces in the pure state (where the ratio P /P reduces to the vapor pressure isotope effect, VPIE),... 

The theory of Isotope effects In condensed phase systems, especially vapor pressure Isotope effects (VPIE) Is briefly reviewed. It Is pointed out that the VFIE can be enqtloyed as one measure of the effect of Intermolecular forces on the motions of molecules In condensed phases. This Is Illustrated with a number of examples from the recent literature and from our own laboratory. A more detailed description of our recent work on thermodynamic solvent Isotope effects In aqueous systems Is presented. Experiments on vapor pressures, freezing points, and heats of solution and dilution of solutions of electrolytes In HOH and DOD are described. Implications are discussed with respect to the aqueous solvent structure problem. 

The ideal behavior of isotopic mixtures is expected if one assumes that the intermolecular forces between pairs of like molecules of each type and between unlike molecules are all the same, and further assumes the isotopic molecules to have the same size. Both assumptions are reasonable to a first approximation. Highly precise vapor pressure measurements on isotopic mixtures have shown, however, that even these mixtures exhibit small, but still significant, deviations from ideal behavior (Jancso et al. 1993, 1994). Theoretical analysis has demonstrated that the origin of nonideality is closely connected with the difference between the molar volumes of isotopic molecules ( molar volume isotope effect ). 

Ernesti, A., Hutson, J. M. (1997). Non-additive intermolecular forces from the spectroscopy of van der waals trimers A comparison of ar2-hf and ar2-hcl, including h/d isotope effects. Journal of Chemical Physics, 106, 6288. 

The bending mode, 5Fe-N-0 in the metastable state (MSi) of Na2[Fe(CN)s(NO)] shows a " N/ N isotopic shift consistent with an Fe-O-N linkage.Ferric-NO and ferrous-CO complexes of cytochrome P450hor have been studied by resonance Raman spectroscopy. The effects of haem modification on bonding were followed from changes in vNO and vCO. The value of vNO for (ON)Fe(TPP) is very sensitive to intermolecular forces in the solid state. ... 

In this chapter we have tacitly assumed that the intermolecular forces and therefore the interaction parameters e, r are not altered by isotopic effects. Actually isotopic substitution should alter the electronic energies and therefore the intermolecular forces only by a factor of the order rnfM (m mass of the electron, M that of the nucleus). 

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