The gas-phase geometries, binding energies (BEs), vibrational spectra, and electron density topological features of methanol (M), water (W), and methanol-water mixed clusters (M(m)W(n), where m = 0-4 and n = 0-4; m + n < or = 4) have been calculated using Hartree-Fock, second-order Møller-Plesset perturbation, and density functional theory with Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional methods. Bader's "atoms in molecules" theory has been used to analyze the hydrogen bonding network. To understand the effect of cooperativity, we have performed natural bond orbital analysis and reduced variational space decomposition analysis. The results show that BEs of methanol and mixed clusters are higher than those of water clusters due to the electron-donating nature of the methyl group. These findings are in accordance with the role of cooperative polarization and cooperative charge transfer in the methanol and mixed clusters. As the size of the cluster increases, the contribution from the cooperative effects also increases. The cooperativity contributes approximately 14 and 24% of stabilization in trimers and tetramers, respectively. The calculated nu(OH) frequencies at MP2/6-311++G(d,p) are in close agreement with the corresponding experimental values.
Structural characterizations using XRD and (13)C NMR spectroscopy of two rodlike mesogens consisting of (i) three phenyl ring core with a polar cyano terminal and (ii) four phenyl ring core with flexible dodecyl terminal chain are presented. The three-ring-core mesogen with cyano terminal exhibits enantiotropic smectic A phase while the four-ring mesogen reveals polymesomorphism and shows enantiotropic nematic, smectic C, and tilted hexatic phases. The molecular organization in the three-ring mesogen is found to be partial bilayer smectic Ad type, and the interdigitation of the molecules in the neighboring layers is attributed to the presence of the polar terminal group. For the four-ring mesogen, the XRD results confirm the existence of the smectic C and the tilted hexatic mesophases. A thermal variation of the layer spacing across the smectic C phase followed by a discrete jump at the transition to the tilted hexatic phase is also observed. The tilt angles have been estimated to be about 45° in the smectic C phase and about 40° in tilted hexatic phase. (13)C NMR results indicate that in the mesophase the molecules are aligned parallel to the magnetic field. From the (13)C-(1)H dipolar couplings determined from the 2D experiments, the overall order parameter for the three-ring mesogen in its smectic A phase has been estimated to be 0.72 while values ranging from 0.88 to 0.44 have been obtained for the four-ring mesogen as it passes from the tilted hexatic to the nematic phase. The orientations of the different rings of the core unit with respect to each other and also with respect to the long axis of the molecule have also been obtained.
The gas-phase geometries, binding energies (BEs), infrared spectra and electron density parameters of benzene (BZ)-water (W) clusters (BZW(n), where n = 1-10) have been calculated using Truhlar's meta hybrid functional, M05-2X, employing 6-31+G** basis set. Both basis set superposition error (BSSE) and zero point energy corrected BEs are in close agreement with the previously reported high level ab initio and experimental values. Among all of the BZW(n) clusters, the same with inverted book conformer of water hexamer has the highest BE when compared to all other water clusters. The Bader's theory of atoms in molecule (AIM) provides evidence for the presence of O-H...pi interactions in all of these clusters. In addition, the roles of C-H...O and lone pair...pi (lp...pi) interactions in the stabilization of BZW(n) clusters are also evident from the AIM analysis. The trend in the electron density at the hydrogen bond critical points varies as O-H...pi < lp...pi < C-H...O. Spectral signatures of these clusters further reinforce the existence of weak H-bond between BZ and W(n) clusters. The red shift in all of these clusters ranges from 13 to 95 cm(-1). The results clearly show that the presence of pi-cloud does not affect the H-bonded network of water clusters except in the case of W(6) ring.
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