The author replies to Rich.

I have read the letter by Ronald L. Rich with interest. There are some interesting points to be made, which I will list in point form.

1. Reference 1 cited by Rich is about the boiling point of molecular hydrogen, which has the smallest mass and smallest moment of inertia of any molecule. It is such a special case that it is no surprise that the boiling point changes considerably upon isotopic substitution.
2. It is relevant that while increasing mass by isotopic substitution increases the boiling point of hydrogen, deuteration decreases the boiling points of many hydrocarbon liquids such as cyclohexane. (I am indebted to a colleague for pointing this out to me.) The model given in ref 1 should apply to these liquids as well as to hydrogen, but predicts the wrong mass dependence of the boiling point. It would be useful to collect some data to demonstrate this, for it would seem to undermine the arguments of ref 1 as being too simplified to be applied generally to a range of liquids.
3. Reference 2 cited by Rich makes only slight use of the mass dependence (e.g., Figure 10) within a homologous series, where boiling points generally do increase with molecular mass. However this paper is primarily about symmetry in relation to melting points, rather than boiling points.
4. Reference 3 cited by Rich is the author’s own impressive contribution to the prediction of boiling points, and I wonder whether it could be used to understand the mass dependence of the boiling point of hydrogen, and the opposite mass dependence of the boiling point of hydrocarbons.