a) What is the difference between intramolecular and intermolecular isotope effects?
Any effect exerted by the introduction of isotopes are termed isotope effects. Isotope effects can be intermolecular, e.g., upon D. loss from CD4+. versus H. loss from CH4+., or intramolecular, e.g., upon H. loss versus D. loss from CH2D2+..
b) What is the difference between primary and secondary kinetic isotope effects?
Primary kinetic isotope effects are observed if the isotopic bond itself is being broken or formed during the reaction. Secondary kinetic isotope effects are observed if an isotopic label is located adjacent to or remote from the bond that is being broken or formed during the reaction.
c) In general, do isotope effects favor loss/transfer of the heavy isotope containing moiety or do they reduce the rate of their reactions?
The heavy isotope containing moiety reacts slower. However, in rare cases inverse isotope effects can occur. Kinetic isotope effects can be large in case of decomposing metastable ions (kH/kD ≈ 2–4) because these ions possess only small excess energies. Such circumstances make them sensitive to the difference between excess energies of the respective transition states. On the other hand, ions decomposing in the ion source have usually high internal energies, and thus smaller isotope effects are observed (kH/kD ≈ 1–1.5).