Theoretical Study of Styrene (Methanol)_n Clusters, n = 1−9. Comparison with Methanol Clusters

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.

Resonant Two-Photon Ionization Spectroscopy of Styrene (Methanol)nClusters,n= 1−9

“…interaction to the styrene π-system. The structures of the SM n clusters, presented and discussed in the companion paper, 31 are consistent with the structural predictions presented here based on the observed spectral shifts and the degree of cluster's fragmentation following ionization. C. Spectra of SM n Clusters, n ) 4-6.…”

“…The calculated structures of these isomers are presented in the companion paper. 31 The spectral shift reverses direction in the single isomer observed for SM 4 . The successive addition of methanol molecules to the SM complex leads to a stable cyclic methanol tetramer in SM 4 that is unable to H-bond to the styrene π-system, consistent with the observed red shift.…”

“…The calculated structures of the SM n clusters are presented and discussed in the companion paper. 31 The third group of peaks observed in the SM 2 mass channel starts with a sharp peak at 182 cm -1 followed by a vdW progression of very low-frequency modes at 189, 199, 204, 213, 232, and 341 cm -1 (corresponding to peaks 1-6 following the SM 3 -I origin in the SM 2 channel in Figure 2). Some of these peaks appear with very small intensities in the SM 3 mass channel, and may be tentatively assigned to one isomer of the SM 3 cluster (SM 3 -I).…”

“…In the companion paper we investigate the structures of the clusters using pair-potential (OPLS, optimized Potential for Liquid Simulation) calculations. 31 The combination of the R2PI spectra and the theoretical calculations provides a reasonably compelling picture of the interactions involved in these systems. Detailed knowledge of these interactions is a prerequisite for a molecular level understanding of the retardation and inhibition effects of methanol on the cationic polymerization of styrene.…”

Monomer, clusters, liquid: an integrated spectroscopic study of methanol condensation

“…Theoretical calculations both complement the experimental work and help interpret the complex spectra. Quantum mechanical computational studies of methanol clusters [22][23][24] have been conducted for clusters up to n = 20, depending on the level of theory and accuracy used, while Monte Carlo methods were employed for clusters 25,26 containing up to n = 256 molecules. These studies find that the stability of the small clusters increases with increasing number of molecules in the clusters, and once the clusters contain 5 to 6 molecules, cluster energy per molecule converges to an asymptotic value.…”