Evidence of C-H•••O hydrogen bonds in liquid 4-ethoxybenzaldehyde by NMR and vibrational spectroscopies

Raman, FTIR, and NMR (both C-13 and O-17) spectroscopies are used in a complementary way in order to study the occurrence of C-H . . .O intermolecular hydrogen bonds in liquid 4-ethoxybenzaldehyde (4EtOB), Additional information concerning the structure of the possible dimers is obtained through ab initio calculations, at the B3LYP/6-31G* level. The strongest evidences of the presence of C-H . . .O hydrogen bonds in the liquid phase arise from the temperature and solvent intensity dependence of the two bands observed in the nu (C=O) region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl O-17 NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the nu (C-H) vibrational modes, the (1)J(CH) concentration dependence detected in the NMR spectra, and ab initio results. The experimental observations are consistent with the decrease of the C-H bond length upon hydrogen-bonding, as predicted for the nonstandard blue-shifting hydrogen bonds. Ab initio calculations predict several possible structures for the dimeric species, with nearly identical energies. The calculated dimerization energy is within the -5.1 to -6.5 kJ mol(-1) range, considering both basis set superposition error and zero-point vibrational energy corrections, in agreement with the obtained experimental DeltaH value of -5.7 +/- 0.5 kJ mol(-1).