Rotations in $β$ - ${}^{15}N_{2}$ and $β$ - ${}^{14}N_{2}$ : Magnetic resonance comparison

Recent NMR experiments suggested that the rotations in the rotor solid

β

{}^{14}N_{2}

are different from those in

β

{}^{14}N_{2}

, particularly at low temperatures. Specifically, the orientational average of

P_{2} (cos θ)

p \equiv \frac{〈 (3 {cos}^{2} θ - 1) 〉}{2}

, was determined to be

| p | ≃ 0.1

{}^{15}N_{2}

, while it is known that

| p | ≲ 10^{- 3}

{}^{14}N_{2}

. This would be a surprising isotope effect. We have reexamined this issue by using the quadrupole splitting of dilute

{}^{14}N_{2}

in a

β

{}^{15}N_{2}

host as a sensitive probe of

p

. We find

| p | ≲ 10^{- 3}

, just as in pure

β

{}^{14}N_{2}

. We argue that this value applies to both isotopes in our isotopic mixtures. The second moment of the

{}^{15}N_{2}

resonance agrees reasonably with the intermolecular part of the Van Vleck second moment, indicating again that

p

is small in

β

{}^{15}N_{2}

. We find no evidence that the rotations in

β

{}^{15}N_{2}

differ fromthose in

β

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Rotations inβ-N215andβ-N214: Magnetic resonance comparison