PHOTOIONIZATION INVESTIGATION OF CLUSTER FORMATION IN HETEROCYCLIC-COMPOUNDS AND THEIR REACTIONS WITH ACIDIC SPECIES

The formation of homogeneous molecular clusters by six different nitro gen-containing, heterocyclic (Het) compounds has been investigated usi ng multiphoton ionization combined with time-of-flight mass spectromet ry. The substances considered were pyridine, 3-picoline, 4-picoline, p yrazine, 2-methyl pyrazine, and pyridazine, and homogeneous clusters ( Het), with n less than or equal to 15 were observed. None of the clust ers displayed sharp spectral features and consequently the information available was limited to mass spectra. In order to gain some informat ion regarding the properties of these clusters, the effects of introdu cing the Lewis acids BF3, BCl3, 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 boun d complexes (donor-acceptor complexes) with these acids were expected to be formed. It was reasoned that a determination of the number of ac idic molecules that attached themselves to a given cluster would give some insight into che structure of that cluster. The most extensive in vestigation was carried out with BF3 and, surprisingly, not all the he terocyclics listed above formed mixed clusters with this Lewis acid un der our experimental conditions. Our observations indicated the follow ing: (1) the stability of the (Het), homogeneous clusters and the (Het )(n)-(BF3)(m) mixed clusters appears to be related to the pKa value as sociated with the heterocyclics involved; (2) under the conditions use d in this work, the mixed clusters appear to be produced by the additi on of BF3 to homogeneous clusters that had already formed, and (3) the cations of mixed clusters in which n = m are unstable and hence not d etected. 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 goo d agreement with the experimental observations. These calculations sho wed 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 obser vation that mixed-clusters containing only one BF3 molecule appeared t o be preferentially formed. (Int J Mass Spectrom 177 (1998) 197-215) ( C) 1998 Elsevier Science B.V.