Info. File, ST2). The crystalline environment has significant effect on the structures. The acidbase adduct gets skewed away from planarity within the crystal which within the gas-phase remain co-planar. Non-planar orientation of the acid…base adduct helps in better crystalpacking and allows stronger N-H…O hydrogen bonds at the expense of weakening of the anyways weak C-H…O hydrogen bonds. d(N-H…O) decreases from 1.702 Å in gas-phase to 1.538 Å within the crystal for pyridine…formic acid adduct. However, parallelly d(C-H…O) increases from 2.387 Å to 2.542 Å. Similarly, for pyridazine…formic acid and pyrazine…formic acid the d(N-H…O) decrease from 1.729 Å and 1.748 Å in the gas-phase to 1.434 Å and 1.544 Å within the crystal respectively. The C-H…O hydrogen bond also increases concomitantly from 2.327 Å and 2.375 Å in the gas-phase to 2.709 Å and 2.570 Å within the crystal respectively. Interestingly for the furan…formic acid adducts the O-H…O hydrogen bond is longer in both gas-phase and the crystalline environments corroborating the weaker Hbond accepting ability of furan vis-à-vis the nitrogenous bases.