Answer to Question #277915 in Physical Chemistry for Kyle

Question

] The 1 H nucleus (i.e., the proton) has a nuclear spin of I=1/2. In the H2 molecule, the coupling of the two nuclear spins is simular to that of the two electrons in the helium atom, i.e., the couplng can be symmetric or anti-symetric with respect to the exchange of the two nuclei. The 1 H nucleus, as well as the electron, is a fermion, i.e., the total wavefuction of the H2 molecule changes its sign if the two 1 H muclei are exchanged. In its ground vibronic (vibrational-electronic) state [ 1Σ𝑔𝑔 + (v=0)], both the vibrational and the electronic wavefunctions are symmetric with respect to the exchange of the two nuclei. Use the information provided above and what you learned in Chem 465, explain why the natural abundance of ortho-H2 and para-H2 is 3:1. The ortho-H2 molecules are those on the J=1,3,5… rotational levels, while para-H2 molecules are those on the J=0,2,4… rotational levels

Answer 1

In the structure of ortho hydrogen, the proton nuclear spins are aligned parallel to each other. While in para hydrogen, the proton nuclear spins are aligned antiparallel to each other. ... - Now, due to antiparallel spin arrangement, para hydrogen has less energy and thus, they are more stable than ortho hydrogen.