Temperature dependent J coupling and internal motion in acetaldehyde

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Abstract
The indirect proton spin-spin coupling (J) in acetaldehyde has been measured with an accuracy of ±0·01 c.p.s. over the temperature range -119°c to +52°c. The small change of 0·15 c.p.s. is interpreted in terms of the internal motion of the molecule which gives an average of J over its angular thermally excited motion in a three-well potential. The constant and cos 3ϕ terms in J(ϕ) are determined and an estimate of J(ϕ) is made. It is pointed out that the zero point internal vibration in this and similar molecules may be important for the comparison of experimental and theoretical J values. Although J falls with increasing temperature in acetaldehyde it may well rise in other similar molecules.