Theoretical Investigations of Difluorobenzene Interacting with Ne and Infrared Analysis of 1,1-Dichloroethylene from Synchrotron Data

The intermolecular interactions between two molecules are of great importance in proteins, polypeptides, DNA binding, and drug binding interactions. Dimers of chemical rings such as difluorobenzene with hydrogen containing molecules such as acetylene (HCCH) and atoms such as neon are classic systems for investigating weak interactions; however, the microwave spectrum of 1,2-difluorobenzene...Ne does not behave as expected. Possible explanation of this phenomenon is that there is some large amplitude motion perturbing the spectrum. A computational study was performed to analyze the potential energy surface (PES) of Ne or Ar with ortho, meta, and para difluorobenzene, and reveal the bonding patterns in the weakly bound complexes using the Quantum Theory of Atoms in Molecules (QTAIM). From the PES study, it was shown that all three difluorobenzene (DFB) complexes interacting with Ne have a large global minimum with 1,4-DFB...Ne being largest and located from one end of the fluorine to the other end of fluorine atom across the ring, while 1,3-DFB...Ne has a global minimum that is slightly towards F ends, and finally, 1,2-DFB...Ne has the smallest global minimum region, located from the center of the ring toward the region of the two fluorine atoms. QTAIM analysis provided complimentary results to the PES analysis where it examined the electron density from the ab initio calculations. QTAIM results showed that 1,2-DFB...Ne has the highest amount of the electron density (ρb) at the bond critical point (BCP) while having the shortest Re distance. As the Re distance increases the ρb decreases, and this can be seen going from 1,3-DFB...Ne to 1,4-DFB...Ne. The amount of the electron density at the BCP provided the preliminary information of the strength of these weakly bound complexes. Subsequently, an experimental analysis of far-IR spectrum of 1,1-dichloroethylene was performed to determine the structure of this compound; the success of this method may allow future experimental study to be done on dimers such as 1,2-DFB...Ne. The results of this spectral analysis of an a-type band at 796.02 cm-1 showed that the isotopic molecule (H2C=C35Cl37Cl) was unable to be assigned due to low intensities and that the lines lie beneath the parent molecule (H2C=C35Cl35Cl). Analysis of H2C=C35Cl35Cl was a success with a total of 1601 rovibrational transitions assigned, with an RMS value of 0.000447 cm-1