[Sona]

Which one has higher dipole moment between p-diacetylbenzene and p-dicyanobenzene?

Is it the first one? due to C-C bond rotation any dihedral angle between two C=O is not necessarily zero. But due to linearity of C≡N it always lie opposite to each other and cancel out the dipole moment.

Is this the correct explanation? please help.

[sjb]

Which one has higher dipole moment between p-diacetylbenzene and p-dicyanobenzene?

Is it the first one? due to C-C bond rotation any dihedral angle between two C=O is not necessarily zero. But due to linearity of C≡N it always lie opposite to each other and cancel out the dipole moment.

Is this the correct explanation? please help.

I suppose at some level you will only be able to tell by measuring this. And then rationalisation of the sort you have made of the result (if indeed the dicyano compound does have the lower dipole moment)

[Sona]

I searched for the values. I couldn't find the value of p-diacetylbenzene, but found that the dipolemoment of p-diformylbenzene.It is 4.57D and for p-dicyanobenzene it is 0.8D. As the structures of p-diacetylbenzene and p-diformylbenzene, from these values can we comment on the dipole moment comparison of p-diacetylbenzene and p-dicyanobenzene? Then what should be the explanation of the observed dipole moments?

[Enthalpy]

[.] Due to linearity of C≡N it always lie opposite to each other and cancel out the dipole moment.

That was my understanding too, and docs tell that p-diacetylbenzene has zero moment:
http://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds10.pdf on page 33
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