Weak hydrogen bond interactions in the aniline–alkene (CH2CH(CH2)nH, n=0,1,2) clusters studied by infrared depletion spectroscopy Original Research Article

The NH2 stretching vibrations of the weakly bound clusters of aniline–ethene, aniline–propene and aniline–(1-butene) have been measured by infrared depletion technique. Compared to aniline monomer the frequencies of the NH2 stretching vibrations of the aniline–ethene cluster show small red shifts, suggesting that this cluster is only weakly bound. In contrast the aniline–propene and aniline–butene clusters show notable red shifts (>20 cm−1) for both the NH2 symmetric and anti-symmetric stretching vibrations thus implying the formation of NH–π hydrogen bonds. Ab initio MO calculations were performed at the MP2/6-31G∗∗ level of theory to estimate the red shift of the NH2 stretching vibrations. The calculation suggested a NH–π interaction between the NH bond of aniline and the double bond of the alkene for the aniline–propene and aniline–butene clusters, while a CH–σ type interaction between the CH bond and the lone pair of the nitrogen atom of aniline for the aniline–ethene clusters.