The effect of bromination on the hydrogen bond strength and vibrational spectra of pyridinum perchlorate is studied theoretically. All calculations are performed at the B3LYP/6-311++G** level of theory. The IR spectra of pyridinum perchlorate (PyHClO4) and its ortho bromo substituted derivative2-Br-PyHClO4) are studied experimentally and compared with the theoretical results. In the IR spectrum of PyHClO4 a very strong and broad band is observed at about 2640 cm-1, which is attributed to the NH stretching vibrational mode, NH. However calculations in the gas phase predicts this band at about 2200 cm-1 while calculations in the CCl4 solution gives a value of about 2700 cm- 1 which, after scaling by 0.9659 , is in excellent agreement with that observed experimentally. Substitution of H atoms with Br at the ortho, meta, or para position increases the hydrogen bond strength. For example, the N...O distance in ortho bromo substitution decreases by about 0.029 Å. This effect in different positions is slightly different (see Table 1). Our calculations also indicate that the strength of hydrogen bond in the solution is considerably decreases compared with the gas phase. The effect of solvent is well explained with high dipole moments of these salts. The increasing of hydrogen bond strength upon bromination in the ortho, meta, and para positions is well explained by considering the difference in electron releasing character of H and Br atoms.

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