对称性与光谱学-第六章-1.pptVIP

双原子分子的振动-转动光谱 振动能级的改变必然伴随着转动能级的改变，每条振动谱带都由许多谱线组成。 振动和转动的总能量为： Infrared Spectroscopy Vibrational transition is infrared active because of interaction of radiation with the: molecular dipole moment, m. There must be a change in this dipole moment This is the transition dipole moment Probability is related to transition moment integral Transition is forbidden if TM = 0 Only non zero if direct product: yf m yi contains the totally symmetric representation. IE all numbers for c in representation are +1 The ground state yi is always totally symmetric Dipole moment transforms as Tx, Ty and Tz. The excited state transforms the same as the vectors that describe the vibrational mode. Raman Spectroscopy Raman effect depends on change in polarisability a. Measures how easily electron cloud can be distorted How easy it is to induce a dipole THIS IS NOT AN ABSORPTION Usually driven by a laser at w1. Scattered light at w2. Can be Stokes(lower energy) or Anti-Stokes shifted Much weaker effect than direct absorption. Analysis of Vibrational Modes: POCl3 1) Highest frequencies probably stretches. 2) P-Cl stretches probably of similar frequency. 3)Double bonds have higher frequency than similar single bonds. A1 modes first. P=O – highest frequency Then P-Cl stretch, then deformation. 581 similar to P-Cl stretch so assym. stretch. Remaining modes must therefore be deformations now we can use SALCs to look more closely at the normal modes Two stretching vibrations exist that transform as A1 and B2. These are linear combinations of the two vectors along the bonds. We can determine what these look like by using symmetry adapted linear combinations (SALCs) of the two stretching vectors. Our intuition tells us that we might have a symmetric and an anti-symmetric stretching vibration A1 and B2 (2) Symmetry Adapted Linear Combinations S O O r2 r1 C2v r1 E C2 s(xz) s(yz) r1 r2 r2 r1 Pick a generating vector eg: r1 How does this transform under symmetry op