42148-B6
Experimental and Computational Studies of Substituted Benzene Solvation Clusters
1. Carbon and hydrogen NMR spectra (1D and 2D) were obtained for the 1,2,3 – alternate dibenzyl bis-teo (alt) and the cone – dibenzyl bis-teo (cone) calix[6]arene isomer structures. Several of the NMR peaks were not assignable on the basis of general principles. Optimized structures, charge distributions, and ground state energies were calculated using the B3LYP/6-31G(d) method/basis set, with follow-up proton and carbon NMR shifts calculated using the HF/6-31G(d) method/basis set. Calculations were carried out for both isomers, each at three different levels of imposed symmetry, first in the gas phase and then in chloroform solvent. Results indicate that imposed symmetry leads to higher (less favorable) calculated energies and does not improve upon predicted NMR shifts. Consideration of solvent effects improves ground state energies, but other improvements are minimal and not substantial enough to justify the added computational expense of running the solvent calculations. Overall results are consistent with known experimental assignments and are found to be valuable for assigning previously unknown NMR peaks. Net charges, electrostatic forces, and local dipoles—but not bond lengths—are strongly correlated to spectroscopic manifestations of steric compression. A manuscript documenting this work has been submitted to the Journal of Organic Chemistry.
2. Neat fluorobenzene, (H6H5F)n, and mixed fluorobenzene-benzene, (C6H6)m(C6H5F)n, clusters were investigated via resonant two-photon ionization (R2PI) spectroscopy through fluorobenzene's B2 ← A1 000 transition. The fluorobenzene molecule's R2PI spectrum is being analyzed and interpreted with reference to MP2/6-31+(2d,p) and HF/6-31++g(d,p) ground state frequency calculations alongside a CIS/6-31++g(d,p) excited state frequency calculation. The ultraviolet cluster spectra contain no unique sharp features, indicating the presence of multiple isomers for all cluster sizes. Spectra of the mixed clusters were deconvoluted into two ubiquitous features: one red-shifted, the other blue-shifted. In addition to these two, deconvolution of the neat fluorobenzene cluster spectra requires an additional broad underlying feature. As a function of increasing cluster size, all three features tend to broaden, and the ratio of the blue- to red-shifted peak intensities decreases. Ongoing investigation focuses on identifying the physical models that underlie these results.
