The N-methyl acetamide molecules can form dimers with each other through hydrogen bonding interactions between the amide hydrogen and the nitrogen atom of another molecule. The strength of these interactions depends on the solvent in which the molecules are dissolved.
In CCl4, the solvent is a non-polar solvent, which means that the solvent molecules do not have any polar groups that can form interactions with the N-methyl acetamide molecules. As a result, the N-methyl acetamide molecules are free to interact with each other and form dimers. The absence of polar groups in the solvent means that the molecules are not surrounded by any solvent molecules, hence the dimerization is favored.
On the other hand, water is a polar solvent, which means that the water molecules have polar groups that can form interactions with the N-methyl acetamide molecules. The polar nature of water means that the N-methyl acetamide molecules are surrounded by water molecules, which can interact with the polar groups on the N-methyl acetamide molecules and disrupt the hydrogen bonding interactions between the molecules. As a result, the dimerization of N-methyl acetamide molecules is disfavored in aqueous solution, and the molecules are less likely to form dimers.