No abstract

The structures, energetics, and growth pattern of styrene (methanol) n clusters, (SM n ), with n = 2−9 are investigated using a search technique that employs Monte Carlo procedures. A 6−12−1 all-atom potential was developed that accurately reproduces the heat of vaporization, heat capacity, and density of liquid styrene. This potential, in conjunction with the OPLS potential for methanol, yields results strongly correlated with the experimental observations from our R2PI study of the SM n clusters. The progressive addition of methanol molecules to styrene leads to the formation of stable methanol clusters similar to those formed in the absence of styrene, with the exception of the SM3 cluster where the lowest energy structure incorporates the methanol trimer as a hydrogen-bonded chain, rather than as the more stable cyclic structure of M3. For the SM n clusters with n = 4−9, cyclic and branched cyclic methanol structures are found. In the clusters containing 5, 7, and 9 methanol molecules, the methanol subclusters are present on both sides of the plane of the styrene. The nonadditivity and size specificity of observed spectral shifts are explained through the use of a series of compact and expanded structures, with the interaction energy calculated between the styrene and the methanol subcluster (M n ). The results indicate that the spectral shifts correlate with the interaction energies between styrene and M n within the SM n clusters. The modeled cluster structures and simple energetic arguments provide a reasonably compelling picture of the spectral shifts associated with hydrogen bonding interaction among methanol molecules and between styrene and the methanol subclusters.

Well-resolved spectra of styrene−methanol binary clusters SM n , with n = 1−9, have been obtained by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The spectra reveal a rapid increase in complexity with the number of methanol molecules in the cluster, associated with van der Waals modes and isomeric forms. Two distinct isomers are identified for each of the clusters studied with the exception of SM4 and SM8. The progressive addition of methanol molecules to the SM complex leads to the formation of stable cyclic and branched cyclic methanol subclusters within the SM n clusters. The spectral shift of the cluster origin reflects the nature of the intermolecular interactions within the binary cluster. Blue shifts are observed for the SM, SM2, and SM3 clusters and are consistent with hydrogen bonding interactions between the OH groups and the styrene π-system. A remarkable switch in the spectral shift from blue to red is observed at the SM4 cluster and is consistent with the ring structure of the methanol tetramer. Evidence is provided for intracluster dissociative proton-transfer reactions within the S2M n + (n > 2) clusters that generate protonated methanol clusters. These reactions may explain the strong inhibition effects exerted by small concentrations of methanol on the cationic polymerization of styrene.

The styrene-water binary clusters SW(n), with n = 1-5 have been studied by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The structures and energetics of the neutral clusters are investigated using a search technique that employs Monte Carlo procedure. The strong tendency for water molecules to form cyclic hydrogen-bonded structures is clearly observed in the SW(n) structures starting from n =3. The results indicate that the spectral shifts correlate with the interaction energies between styrene and the water subcluster (W(n)) within the SW(n) clusters. Evidence is presented that points to (1) the formation of a covalent bonded styrene radical cation dimer following the 193 nm MPI of styrene neutral clusters, (2) proton transfer from the styrene dimer cation to the water or methanol subcluster, resulting in the formation of protonated water or methanol clusters and a styrene dimer radical, and (3) extensive solvation of the styrene dimer radical within the protonated solvent molecules. The proton-transfer reactions may explain the strong inhibition effects exerted by small concentrations of water or methanol on the cationic polymerization of styrene. These results provide a molecular level view of the inhibition mechanism exerted by protic solvents on the cationic polymerization of styrene.

We present direct evidence for the formation of the covalent bonded styrene (isoprene)(2) oligomer and the isoprene dimer ions following resonance ionization of the gas phase styrene-isoprene binary clusters. The application of resonance ionization to study polymerization reactions in clusters provides new information on the structure and mechanism of formation of the early stages of polymerization and holds considerable promise for the discovery of new initiation mechanisms and for the development of novel materials with unique properties.