A study of the eigenvectors of the vibrational modes in crystalline cytidine via high-pressure Raman spectroscopy

Abstract

Raman spectroscopy has been used to study the eigenvectors and eigenvalues of the vibrational modes of crystalline cytidine at 295 K and high pressures by evaluating the logarithmic derivative of the vibrational frequency ω with respect to pressure P: . Crystalline samples of molecular materials have strong intramolecular bonds and weak intermolecular bonds. This hierarchy of bonding strengths causes the vibrational optical modes localized within a molecular unit (“internal” modes) to be relatively high in frequency while the modes in which the molecular units vibrate against each other (“external” modes) have relatively low frequencies. The value of the logarithmic derivative is a useful diagnostic probe of the nature of the eigenvector of the vibrational modes because stretching modes (which are predominantly internal to the molecule) have low logarithmic derivatives while external modes have higher logarithmic derivatives. In crystalline cytidine, the modes at 85.8, 101.4, and 110.6 cm−1 are external in which the molecules of the unit cell vibrate against each other in either translational or librational motions (or some linear combination thereof). All of the modes above 320 cm−1 are predominantly internal stretching modes. The remaining modes below 320 cm−1 include external modes and internal modes, mostly involving either torsional or bending motions of groups of atoms within a molecule.