Photoionization investigation of cluster formation in heterocyclic compounds and their reactions with acidic species

Abstract

The formation of homogeneous molecular clusters by six different nitrogen-containing, heterocyclic (Het) compounds has been investigated using multiphoton ionization combined with time-of-flight mass spectrometry. The substances considered were pyridine, 3-picoline, 4-picoline, pyrazine, 2-methyl pyrazine, and pyridazine, and homogeneous clusters (Het) n with n ≤ 15 were observed. None of the clusters displayed sharp spectral features and consequently the information available was limited to mass spectra. In order to gain some information regarding the properties of these clusters, the effects of introducing the Lewis acids BF 3, BCl 3, and B(OH) 3 and also HF and HCl, into the free-jet expansion were investigated. The heterocyclic compounds listed above are all basic in character and consequently strongly bound complexes (donor–acceptor complexes) with these acids were expected to be formed. It was reasoned that a determination of the number of acidic molecules that attached themselves to a given cluster would give some insight into the structure of that cluster. The most extensive investigation was carried out with BF 3 and, surprisingly, not all the heterocyclics listed above formed mixed clusters with this Lewis acid under our experimental conditions. Our observations indicated the following: (1) the stability of the (Het) n homogeneous clusters and the (Het) n -(BF 3) m mixed clusters appears to be related to the pKa value associated with the heterocyclics involved; (2) under the conditions used in this work, the mixed clusters appear to be produced by the addition of BF 3 to homogeneous clusters that had already formed; and (3) the cations of mixed clusters in which n = m are unstable and hence not detected. The results obtained with the other acids generally confirmed these conclusions and showed that the stability of the cation of the mixed cluster is the main factor influencing its detection. Ab initio calculations were also carried out and the results obtained are in good agreement with the experimental observations. These calculations showed there appears to be several structures for the pyridine dimer with similar stability. Several of these structures have two nonequivalent heterocyclic nitrogen atoms, which may explain the experimental observation that mixed-clusters containing only one BF 3 molecule appeared to be preferentially formed.