Nonlinear effects Specialized Raman Techniques Strictly speaking the radiation-induced dipole moment should be expressed as M = E + ½E 2 + (1/6)E 3 +... Where and are the first and second hyperpolarizabilities. With conventional CW lasers (10 4 V cm -1 ), the contributions to the M from the and terms are insignificant. If the sample is irradiated by an extremely strong laser pulse (~10 9 V cm -1 ) a number of non-linear effects can be observed.
These include Hyper-Raman Effect. Scattered radiation contains frequencies of 2 (hyper- Rayleigh) and 2 ± M (Stokes and Anti-Stokes hyper-raman). Very weak intensity (~10-12 incident radiation) Advantages of hyper-raman effect different selection rules, possibility of observing more fundamental transitions. A transition is hyper-raman-active if one of the components of the hyper-polarizability tensor,, changes during the vibration. These components transform as the cubic or ternary functions, x 3, (y 2 z x 2 z), etc. Since the symmetries of these functions include those of x, y, and z, all IR transitions are observable in the hyper-raman spectrum, but also several transitions that are Ramanforbidden.
Coherent Anti-Stokes Raman Spectroscopy (CARS) Two co-linear high-energy laser beams, 1, 2, ( 1 > 2 ) interact coherently to produce strong scattered light of frequency (2 1 2 ). If 2 is set to be equal to 1 M then a strong light of frequency 2 1 2 = 2 1 ( 1 M ) = 1 + M is emitted. Advantages of CARS emitted light is coherent and narrow, easily detected without a monochromator emitted light is on the Anti-Stokes side of the pump frequency, whereas any fluorescence is on the Stokes side signals are very strong. Dilute and gaseous samples can be measured. Different selection rules apply. All Raman-active modes are CARS-active plus many modes that are Raman- and IR-inactive.
Simple molecular structures Applications Inferences based on IR and R selection rules AB 2 (linear D h or bent C 2v ) AB 3 (planar D 3h or pyramidal C 3v ) AB 4 (planar D 4h or tetrahedral T d ) etc. XeF 5 anion in NMe 4 + salt has planar pentagonal structure. 12 normal modes In D 5h these are 1A 1 1(R) + 1A 2 2(IR) + 2E 1 1(IR) + 2E 2 1(R) + E 2 2 (inactive)
Raman spectrum shows the three bands at 502 (A 1 1), 422 (E 2 1) and 377 cm -1 (E 2 1).
A more elaborate structure, C 60, with 174 normal modes. Symmetry of molecule is I h. Raman-active modes predicted are 2A g + 8H g Experimental spectrum shows 14 bands, two are shoulders on strong bands and one is very weak.
Symmetry reduction on cordination. Sulfate anion with T d symmetry Four modes A 1 ( 1 ) (R) + E ( 2 ) (R) + 2 T 2 ( 3, 4 ) (IR, R) The two T 2 modes in free SO 4 2 are seen at 1104 ( 3 ) and 613 ( 4 ) cm -1. In [Co(NH 3 )OSO 3 ]Br the effective symmetry of SO 4 2 is reduced to C 3v. In this point group the A 1 and E modes become IR-active. 970 ( 1 ), 438 ( 2 ), ~1040 + ~1130 (from 3 ), and 645 + 604 cm -1 (from 4 ) Use of Correlation Tables, e.g. T d D 2d C 3v S 4 D 2 C 2v C 3 C 2 C s A 1 A 1 A 1 A A A 1 A A A1 A 2 B 2 A 2 B A A 2 A A A2 E A 1 +B 1 E A+B 2A A 1 +A 2 E 2A A1+A2 T 1 A 2 +E A 2 +E A+E B 1 +B 2 +B 3 A 2 +B 1 +B 2 A+E A+2B A1+2A2 T 2 B 2 +E A 1 +E B+E B 1 +B 2 +B 3 B 1 +B 2 +B 3 A+E A+2B 2A1+A2 For a bridging SO 4 2 group the symmetry is reduced to C 2v and the Correlation Table shows that 3 and 4 are each split into three bands.
7Intensities of bands can sometimes aid identification. The CO ligand in metal carbonyls is an excellent IR chromophore since the CO vibrations are much stronger than other bond stretches. In a fragment O C M C O the symmetric CO stretches ( Ú) will have a higher frequency than the antisymmetric (Ú Ú) stretches. (Stretching one CO makes it harder to stretch the other at the same time.) The intensity of an IR band is proportional to the square of the change in molecular dipole moment. Consider I I symm etric antisym m 2 2 ( 2 r cos θ) = 2 = cot an θ ( 2 r sin θ)
Since sym < asymm we can estimate 2 from the intensities. E.g. cis and trans isomers of [CpMo(R)(PPh 3 )(CO) 2 ] + (R= C 4 H 6 O) (square pyramidal, Cp axial) Spectra of two isomers showed I s /I a = 1.44 and 0.32 which correspond to 2 = 79( and 121((cis and trans respectively)
Vibrational chromophores in peptides and proteins A thorough normal coordinate analysis (1958) of N-methylacetamide provides basis for assignments CH 3 CONH(CH 3 ) Amide I (80% CO stretch) 1657 cm -1 Amide II (60% NH in-plane bend; 40% C N stretch) 1507 cm -1 Amide III (40% C N stretch; 30% NH in-plane bend; 20% Me C stretch) 1298 cm -1 and others... Much data available from X-ray and Raman data allows the following correlations to be made Amide I Amide III -Helix 1645 1600 1300 1260 -Sheet 1680 1658 1243 1230 Random coil 1665 1660 1243
Horseradish Peroxidase (HRP) is an Fe-heme protein that catalyses AH 2 + H 2 O 2 ===> A + 2 H 2 O Two intermediates HRP-I and HRP-II HRP(Fe III ) + H 2 O 2 ==> HRP-I HRP-I + AH 2 ==> HRP-II + AH HRP-II + AH ==> HRP(Fe III ) + A HRP-II is low spin Fe IV =O and HRP-I is its %-cation radical. RR spectrum of HRP-II using 406-nm excitation (into the porphyrin %-% * transition).