Electronic Structure, Molecular Electrostatic Potentials, Vibrational Spectra in Substituted Calix[n]arenes (n=4, 5) from Density Functional Theory

Electronic structure, molecular electrostatic potential, and vibrational frequencies of para-substituted calix[n]arene CX[n]-R (n = 4, S; R = H, NH2, t-Bu, CH2Cl, SO3H, NO2) and their thia analogs (S-CX[n]-R; with R = H and t-Bu) in which sulfur bridges two aromatic rings of CX[n] have been derived from the density functional theory. A rotation around CH2 groups connecting the phenol rings engenders four, namely, cone, partial cone, 1,2-alternate, and 1,3-alternate CX[n]-R conformers. Of these, the cone conformer comprising of large number of O1-H1 center dot center dot center dot O1' interactions turns out to be of lowest energy. Normal vibration analysis reveal the O1-H1 stretching frequency of unsubstituted CX[n] shifts to higher wave-number (blue shift) on substitution of electron-withdrawing (NO2 or SO3H) groups, while electron-donating substituents (NH2, t-Bu) engender a shift of O1-H1 vibration in the opposite direction (red shift). The direction of frequency shifts have been analyzed using natural bond orbital analysis and molecular electrostatic potential (MESP) topography. Furthermore, calculated H-1 NMR chemical shift (delta(H)) in modified CX[n] hosts follow the order: H1 > H3/H5 > H7(a) > H7(b). The delta(H) values in CX[4] are in consonant with the observed H-1 NMR spectra.