The ZZ conformer of 3-methylacrolein is chosen as a model molecule to study the influence of the external pressure on the C-H center dot center dot center dot O intramolecular improper interaction. The effect of the external pressure is modeled by embedding the molecule in helium clusters of variable size. The compression of the molecule encapsulated in the cluster follows from the valence repulsion forces exerted by the helium atoms. Calculations at the level of the MP2/cc-pVTZ approximation show that the improper, blue-shifting, character of the C-H center dot center dot center dot O intramolecular interaction is increased by the compression. (C) 2008 Elsevier B.V. All rights reserved.