AmericanChemicalSociety.com

In this study we focus on mixed neutral aromatic molecular clusters.  The ultimate goal is to interpret and understand the experimental data in the context of theoretical calculations to gain insight into fundamental characteristics of these cluster systems.  For individual clusters, we seek to understand characteristics and properties including: isomerism, structure, delocalization of excitation in the excited state, and the nature of the potential energy surface.  We further seek to understand the evolution of such properties as a function of cluster size—and to deduce how such properties depend on the fundamental nature of the component monomers.

Benzene (C₆H₆—abbreviated as B], 1,3,5-trifluorobenzene (C₆H₃F₃—abbreviated as T], and the mixed dimer (BT) , and trimer (B₂T) clusters were generated in supersonic expansion and were studied via resonant two-photon ionization [R2PI] spectroscopy.  The B monomer spectrum was collected through its B_2u ¬ A_1g transition (forbidden 0⁰₀ origin: 38086 cm^-1); the T monomer and the mixed cluster spectra were collected through 1,3,5-trifluorobenzene's A'₂ ¬ A'₁ transition (forbidden 0⁰₀ origin:  38527 cm^-1).  Spectra were initially collected at a resolution of 1 cm^-1; subsequently, active regions of the spectra were re-scanned at a resolution of 0.3 cm¹.  All spectra were collected over a spectral shift range from 0 – 2276 cm^-1:  for the B monomer, the shift was relative to the B origin at 38086 cm^-1; for all other R2PI spectra, the shift was relative to the T origin at 38527 cm^-1.

For purposes of comparison and analysis, low pressure (< 15 torr) FTIR spectra were collected to evaluate vibrational transitions in the ground electronic state—for both the B and the T monomers.  Three separate electronic structure optimization and frequency calculations were also carried out on each of the monomers:  MP2 and HF calculations were run on the ground electronic states, and a CIS (CI singles) calculation was run on the excited electronic states.  A common basis set was used in all of these calculations:  6-31++g(d,p).

Tiwari et al have recently reported results from their study of the 1,3,5-trifluorobenzene vibronic spectrum A'₂ ¬ A'₁ [Indian Journal of Pure & Applied Physics 2007, 45, 569-572].  Many of their reported frequencies correlate to our measurements but there are notable deviations.  Their experimental technique generates a large number of hot bands that are absent in our spectra; in addition, a number of significant differences related to symmetry assignments remain.

In the nugget published online to accompany this report, the R2PI spectra are shown for B, T, BT, and B₂T.  The BT and B₂T spectra are comprised of a few sharp features—analogous to the B and T monomer spectra; furthermore, none of the vibronic transitions in the cluster spectra appear to contain any clear evidence of van der Waals progressions.  These observations suggest that:  (1) there are very few stable BT and B₂T isomers being probed in the supersonic cluster beam; and (2) excitations within the mixed dimer and trimer are likely localized within the B or T moiety—not delocalized throughout the cluster.

Ongoing work focuses on complete assignment of the T monomer's A'₂ ¬ A'₁ spectrum—making full use of the electronic structure calculation results.  The mixed dimer and trimer assignments will then follow—including detailed evaluation of similarities and differences between the cluster and monomer spectra.  The completed analysis is expected to provide information regarding cluster structure, isomerization, and delocalization of the electronic excitation.  Follow-up MP2 calculations will explore low-energy isomeric structures of the mixed dimer and trimer clusters.