A high pressure study of the eigenvectors of the infra-red active vibrational modes of crystalline adenosine

Abstract

High-pressure infrared spectroscopy has been used to study the eigenvectors and eigenvalues of the vibrational modes of crystalline adenosine at 298 K by evaluating the logarithmic derivative of the vibrational frequency with respect to pressure: . Crystalline samples of molecular materials such as adenosine have vibrational modes that are localized within a molecular unit (“internal” modes) as well as modes in which the molecular units vibrate against each other (“external” modes). The value of the logarithmic derivative is that it is a diagnostic probe of the nature of the eigenvector of these vibrational modes. Stretching modes, which are predominantly internal to the molecule, have low logarithmic derivatives while external modes have higher logarithmic derivatives. Particular attention is paid to modes in the 800–1000 cm−1 range since modes in that region of the vibrational spectrum are found to be sensitive to the conformation of double-helical DNA. Since the sugar pucker is different for the various conformations of DNA, this fact suggests that these modes involve the motion of atoms in the sugar group. The vibrations of the hydrogen atoms are also of interest to study since the vibrational frequency of hydrogen atoms involved in hydrogen bonds has a negative pressure derivative. Such behavior clearly shows which hydrogen atoms are involved in hydrogen bonding.