Density Functional Theory Studies On Vibrational Spectroscopy Of Several Molecular Assemblied Systems

1. This is the first time to combine the density functional theory (DFT) and two-dimensional (2D) correlation spectroscopic techniques in one results analysis, to our knowledge. The structures and vibrational frequencies of the chiral anti-ferroelectric liquid crystal (AFLC) molecule, 4-(1-methyheptyloxycarbonyl) phenyl-4-(4`-octyloxy) benzoate (MHOCPOOB) and (1-methylheptyl)4-(4'-decyloxy-4-biphenyl) oxymethylene benzoate (MHDBOB) have been calculated using the DFT method with the B3LYP/6-31G(d,p) level. The observed vibrational spectra have been resolved and assigned in detail by comparison to the computed values. The results indicate that the computed and observed spectra are in good agreement with each other. The stable molecular structure obtained with the DFT theory, shows that the two hydrocarbon chains are all-trans zigzag conformer and nearly perpendicular to each other. The orientation of the mesogen part and the hydrocarbon chains for MHOCPOOB and MHDBOB in the Sm-CA* phase are investigated by employing the polarization-angle-dependent infrared spectra in the electric field induced and the 2D correlation spectroscopy. After combining with the DFT method and 2D correlation spectroscopy, we obtained that the transition dipole moments of the CH2 and CH2* stretching modes are oriented at the reverse direction and the transition dipole moments of the CH2 and C=O stretching modes are almost the same orientation. Since the orientation of the CH2 transition dipole moment is nearly perpendicular to the hydrocarbon chain axis, the orientation of the achiral chain and the mesogen core are almost the same direction, and with the chiral chain and the mesogen part are nearly perpendicular to each other. We can deduce from the results above that the two CH2 chains (achiral and chiral) are both a probable all-trans zigzag conformer. The conclusions are quite new and different from the previous results, which show that the hydrocarbon chains of AFLC and/or FLC are in disorder. The obtained stable structure and the assignment of vibrational spectra are helpful in the interpretation of the orientation of the mesogen part and the hydrocarbon chains in the Sm-CA* phase. From the discussion above, the results, which are that the two CH2 chains (achiral and chiral) are both a probable all-trans zigzag conformer and nearly perpendicular to each other in AFLC at the Sm-CA* phase, are normal conclusions. 2. In the second part, the molecular structure and vibrational spectra of 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) were studied by using B3LYP/6-31++G(d,p) method, as well as were investigated by the Fourier transform infrared (FTIR), Fourier transform Raman spectra (FT-Raman) and SERS spectra of BPENB on a new Ag substrates at different concentrations. The assignments of the vibrational spectra on BPENB are well resolved and accurately assigned on the basis of the calculation by using DFT methods for the first time. The results of the optimized molecular structure obtained using B3LYP/6-31++G(d,p) level suggest a strong conjugation and π-electrondelocalization and a planar molecule for BPENB. The two conformations (all-trans and trans-cis) of BPENB have been studied by means of DFT methods. The conversion between two conformations should be rapid and the population of these two conformations is almost the same at the room temperature because of a very small absolute energy difference ΔE0 (0.178kJ/mol with the B3LYP/6-31++G(d,p) methods). According to the results of assignments on FTIR, FT-Raman and SERS bands for BPENB, the B3LYP/6-31++G(d,p) method is very accurate in predicting harmonic vibrational frequencies and the normal modes, as well as the vibrational spectra were well resolved. On the basis of our theoretical calculations and the experimental SERS spectra recorded at different concentrations on a novel Ag substrates, the orientation of BPENB to the Ag substrate surface with the variation in concentration is discussed by employing the SERS surface selection rules. It is concluded that the orientation of BPENB on the new silver substrate is from a significantly tilted to almost flat or complete flat fashion with decreasing concentration and at the lowest concentration (1.0 ×10-9 mol L-1), BPENB adsorbs in a complete flat orientation, whereas at highest concentration (1.0 ×10-4 mol L-1) the orientation is considerably tilted. Additionally, we also suggest a model, which is employed describing the interaction of BPENB with the substrates, is that the interaction between BPENB and the metal surface via the N atom, and the chemisorption takes place but it is not strong. 3. To the best of our knowledge, this is the first report on the structures and vibrational spectra of the thiol and thione tautomers of 4MPY and their dihydrates and dimers with using the DFT method(B3LYP/6-311++G(d,p)) as well as their optimized molecular structures and vibrational frequencies.Comparing the calculated spectra to the experimental spectra (Raman spectra of the bulk and 4MPY in aqueous solutions with different pH values of 1.0, 6.0 and >12, respectively.), the assignments of vibrational spectra are performed on the monomer, dimers and dihydrates. The changes of structure and vibrational spectra on the dihydrates and dimers are also investigated. The results show the followings: (1). Raman spectra of the bulk of 4MPY is the combination of Raman spectra of the thiol and thione tautomers. The theoretical Raman spectra of the thione tautomer are agreement with the experimentals better than the thiol ones. This indicates that the thiol and thione tautomrs are coexisting in the bulk of 4MPY, and that the thione is the main component. (2). The structures and spectra of two tautomers are both greatly impacted on water molecules, due to the strong H-bonds between water molecules and two tautomers, and the influence on the thione tautomer is stronger than on the thiol by H2O. After comparing the theoretical spectra and experimental spectra with different pH values, we can find that the thiol and thione spectra are in better agreements with the raman spectra with pH 1.0 and 6.0, respectively. This result shows that the thiol and thione tautomers are the main component in aqueous solutions with pH 1.0 and 6.0, respectively. (3). The formation of dimer have a some influence on the structures and spectra of the monomer of 4MPY, but this effect is smaller than the waters. From the calculated results, we concluded that the theoretical spectra of the monomers or dimers have a little difference from each other but have a large difference from the raman spectrum of the bulk. One of the thione tautomer is in better agreements with the experimentals.