1. In this case we would have to draw up a C 2v character table showing the symmetries of all 27 vibrations. 3. Each normal mode of vibration has a fixed frequency. ν (s) O-H 3657 cm-1 IR active 3756 cm-1 1595 cm-1 δ (s) H-O-H IR active, degenerated All IR absorptions result not only in a vibrational excitation but also in transitions For each normal mode, there is a vibrational quantum number. The complex vibrations of a molecule are the superposition of relatively simple vibrations called the normal modes of vibration. 5 In the 3N representation, six of the irreducible representations correspond to translations and rotations of the molecule. Here we will assume a basic familiarity with point groups and discuss how group theory can be used to determine the symmetry properties of molecular vibrations. 3. 2 Bent molecules, such as H 2O, have 3N-6 vibrational modes because 3 of all the modes result in a translation and 3 in a rotation. all pass zero crossings and turning points at the same time. 4. Translational Modes Rotational Modes A mode in which all atoms are moving in the same direction, equivalent to moving the molecule. Normal Modes of Vibration . In order to describe the 3N-6 or 3N-5 different possibilities how non-linear and linear molecules containing N atoms can vibrate, the models of the harmonic and anharmonic oscillators are used. given in section 2. most MX6 species Lots of symmetry ... Vibrational modes Rotational modes Translational modes degrees of freedom # of atoms Symmetry and molecular vibrations A molecular vibration is IR active only if it results in a change in the dipole moment of the molecule A molecular vibration is Raman active There are 3 rotational modes for nonlinear molecules, and 2 rotational modes for linear molecules. For this reason vibrations are not normally included in character tables. All atoms of the molecule move with the same frequency and usually in-phase i.e. The number of fundamental modes of vibration is 27 ( 3 x 11 - 6 = 27). Since the molecular geometry can distort along each of these degrees of freedoms, these constitute vibrational normal modes. A mode in which atoms move to rotate (change the orientation of) the molecule. Vibrational Spectroscopy (IR, Raman) Vibrational spectroscopy. 2. 1. 3 o define the breathing mode vectors on a diagram, Figure 6 o can be represented as an expansion and contraction of the angles involved C 3v E2C 3 3! To a first approximation, [PbI 6] 4-has an octahedral structure and Fig. It is easy to calculate the expected number of normal modes for a molecule made up of N atoms. Each fundamental mode can be excited independently. v"(aH#N#H N-H "in"-plane bends a 3 a 2 a 1 N H H H Figure 6 Setup for the breathing modes o it is clear the breathing modes have the same reducible representation as The irreducible representations of vibrations vib vib = 3N- T- R =(3A 1 +A 2 +2B 1 +3B 2) - (A 1 +A 2 +2B 1 +2B 2) =2A 1 + B 2 Amplitudes, however, depend on masses. Fundamental vibrational modes do not cause translation or rotation of the molecule as a whole. 1 illustrates the six vibrational modes of an octahedral XY 6 molecule: the A 1g, E g, T 2g vibrations are Raman-active, the T 1u ones are infrared-active, the T 2u are silent [].The vibrational modes … every non-linear molecule has 3N-6 vibrations , where N is the number of atoms. For a linear molecule, there are 3 translations and 2 rotations of the system, so the number of normal modes is 3 n – 5. 2. Calculations and comparison with some available experi- mental results are given in section 3. Every atom in a molecule can move in three possible directions relative to a Cartesian coordinate, so for a molecule of n atoms there are 3 n degrees of freedom. These modes of vibration (normal modes) give rise to • absorption bands (IR) SYMMETRY SPECIES - INTERNAL COORDINATES We consider the modes of vibrations of a thirteen-atomic octahedral molecule AB6C6 as shown in fig. 2. Octahedral Group, Oh – common e.g. allowed or forbidden, whether a molecule should have dipole moment, whether a given vibrational mode should be visible in the infrared or not, etc.