Evidence of C−H···O Hydrogen Bonds in Liquid 4-Ethoxybenzaldehyde by NMR and Vibrational Spectroscopies

Raman, FTIR, and NMR (both 13 C and 17 O) 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 ν C O region of the vibrational spectra, as well as from the shift to low magnetic field detected for the carbonyl 17 O NMR peak at higher dilutions. Further evidence is gathered from the changes observed in the ν 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 Δ H value of −5.7 ± 0.5 kJ mol -1 .