Spectroscopic Selection Rules

Spectroscopic Selection Rules E 0 v=4 v=3 v=2 v=1 v=0 d • For a vibrational fundamental (∆v = ±1), the transition will have nonzero intensity in either the infrared or Raman spectrum if the appropriate transition moment is nonzero. • If the transition moment for infrared absorption or Raman scattering is nonzero for a particular normal mode, the mode is said to be allowed or active in the associated kind of spectrum (i.r. or Raman). • If the transition moment for infrared absorption or Raman scattering is zero for a particular normal mode, the mode is said to be forbidden or inactive in the associated kind of spectrum (i.r. or Raman). Spectroscopic Selection Rules Symmetry Analysis of Transition Moment Integrals • An integral of the product of two functions, IfAfBdτ, can be nonzero only if it is invariant under all operations of the molecule’s point group. • This can occur only if the direct product of fAfB is or contains the totally symmetric representation of the point group. • In terms of the symmetry of fA and fB, the direct product of fAfB is or contains the totally symmetric representation only if fA and fB belong to the same symmetry species. Only the direct product of a symmetry species with itself is or contains the totally symmetric representation. Spectroscopic Selection Rules Infrared Activity • For infrared absorption to occur, the normal mode must have an oscillating molecular dipole moment with the same frequency as the oscillating electric field of the radiation. • The transition moment for the fundamental of a normal mode (v = 0 6 v = 1) can be written M(0,1) = Iψ0µψ1dτ where ψ0 and ψ1 are wave functions for the ground and excited vibrational states, and µ is the oscillating electric dipole moment vector as a function of the normal coordinate, Q, for the normal mode. • The vector µ is a resultant of its components µx, µy and µz, so we may write Mx = Iψ0µxψ1dτ My = Iψ0µyψ1dτ Mz =Iψ0µzψ1dτ • If any one of these components is nonzero, the entire transition moment will be nonzero. • We can determine if any of these components is nonzero on the basis of the symmetries of the wave functions and the dipole moment components. Spectroscopic Selection Rules Evaluating the Infrared Transition Moment Expression • If the integral for the transition moment transforms as the totally symmetric representation the vibrational transition will be infrared allowed. • The ground state vibrational wave function ψ0 is totally symmetric for all molecules (except free radicals) and the excited state wave function ψ1 has the symmetry of the normal mode. • Therefore, by itself the product ψ0ψ1 has the symmetry of ψ1. The direct product of the totally symmetric representation with any non-totally symmetric representation is the nontotally symmetric representation (e.g., in C2v, A1 × B1 = B1). • If any component of µ has the same symmetry as ψ1 the product µψ1 will be totally symmetric and the integral will be nonvanishing. • The symmetry species of µx, µy, and µz are the same as the unit vector transformations x, y, and z, as listed in the character table. L A normal mode belonging to the same symmetry species as any of the unit vectors x, y, or z will be active in the infrared spectrum. Spectroscopic Selection Rules Raman Activity I** sample laser polarity scattered radiation Iz incident radiation • The induced moment is related to the field strength of the incident radiation by P = αE where the proportionality constant α is the polarizability, whose magnitude changes as the molecule oscillates. • For a normal mode to be Raman active there must be a nonzero change in the polarizability with the normal coordinate at the equilibrium configuration; i.e., (Mα/MQ)0 … 0. • The polarizability is best expressed as a tensor, so P = αE becomes where αij = αji. • If the change in any one of these components is nonzero [(Mαij/MQ)0 … 0] then the mode will be Raman active. Spectroscopic Selection Rules The Raman Transition Moment Expression • The transition moment for Raman activity of a fundamental may be expressed as P(0,1) = Iψ0αEψ1dτ = EIψ0αψ1dτ • An integral of this form can be written for every component αij, giving components Pij. Pij = EIψ0αijψ1dτ • If there is any component for which Pij … 0, then the entire moment will be nonvanishing (P(0,1) … 0), and the transition will be Raman active. Spectroscopic Activity Evaluating The Raman Transition Moment Expression • The integral will be nonzero if it is totally symmetric. • As before, ψ0 is totally symmetric and ψ1 has the symmetry of the normal mode. • The integral will be nonzero if ψ1 and any component αij have the same symmetry. • The symmetries of the αij components are the same as the binary direct products of vectors, whose transformation properties are listed in the last column of the character table. L A normal mode will be Raman active if it belongs to the same symmetry species as one of the binary direct products of vectors listed in the character table for the molecule. Spectroscopic Selection Rules Normal Modes of SO2: ν1(A1), ν2(A1), ν3(B2) Looking at the character table for C2v, we find the following unit vector and direct product transformation properties: C2v A1 z x2, y2, z2 A2 xy B1 x xz B2 y yz • Both A1 and B2, the species of the three normal modes, have listings for unit vectors and direct products. • Therefore, all three modes are active in both the infrared and Raman spectra. In other words, we should be able to observe the same three frequencies in both the infrared and Raman spectra. Spectroscopic Selection Rules Experimental Identification of Totally Symmetric Modes by Raman Spectroscopy I** sample laser polarity scattered radiation Iz incident radiation The polarizability ratio: • A band for which 0 < ρ < 3/4 is said to be polarized and arises from a totally symmetric vibration. • A band for which ρ = 3/4 is said to be depolarized and arises from a non-totally symmetric vibration. Spectroscopic Selection Rules Normal Modes of XY4: ν1(A1), ν2(E), ν3(T2), ν4(T2) Looking at the character table for Td we find the following unit vector and direct product transformation properties: Td A1 x2 + y2 + z2 A2 E (2z2- x2- y2, x2- y2) T1 T2 (x, y, z) (xy, xz, yz) • All three unit vectors transform degenerately as T2, so only normal modes with T2 symmetry can be infrared active. • For tetrahedral XY4 this means that only ν3 and ν4 can be observed by infrared spectroscopy. • In contrast, A1, E, and T2 all have direct product listings, which means modes with these symmetries will be Raman active. Thus, we should be able to observe all four frequencies by Raman spectroscopy. Spectroscopic Selection Rules Normal Modes of XY4: ν1(A1), ν2(E), ν3(T2), ν4(T2) A1 E 2T2 ν1 ν2 ν3, ν4 Raman Raman i.r., & Raman • Only the Raman spectrum allows observation of all four fundamental frequencies. • The infrared spectrum will show only the frequencies for the two sets of triply degenerate modes, ν3 and ν4 • The ν3 and ν4 modes are coincident in both spectra. This means that we should expect to find a band for ν3 at the same frequency in both the infrared and Raman spectra. The same is true for ν4. Vibrational Frequencies of Methane Mode Frequency (cm–1) Activity ν1 (A1) 3025.5 R (pol) ν2 (E) R 1582.7 ν3 (T2) 3156.8 i.r., R ν4 (T2) 1367.4 i.r., R Spectroscopic Selection Rules Normal Modes and Spectroscopic Activity of XY6 (Oh) Y Y Y X Y Y Y Oh E 8C3 6C2 6C4 3C2 i 6S4 8S6 3σh 6σd Ni 7 1 1 3 3 1 1 1 5 3 χi 3 0 -1 1 -1 -3 -1 0 1 1 Γ3n 21 0 -1 3 -3 -3 -1 0 5 3 Σ ni A1g 21 0 -6 18 -9 -3 -6 0 15 18 48 1 A2g 21 0 6 -18 -9 -3 6 0 15 -18 0 0 Eg 42 0 0 0 -18 -6 0 0 30 0 48 1 T1g 63 0 6 18 9 -9 -6 0 -15 -18 48 1 T2g 63 0 -6 -18 9 -9 6 0 -15 18 48 1 A1u 21 0 -6 18 -9 3 6 0 -15 -18 0 0 A2u 21 0 6 -18 -9 3 -6 0 -15 18 0 0 Eu 42 0 0 0 -18 6 0 0 -30 0 0 0 T1u 63 0 6 18 9 9 6 0 15 18 144 3 T2u 63 0 -6 -18 9 9 -6 0 15 -18 48 1 Γ3n = A1g + Eg + T1g + T2g + 3T1u + T2u Γtrans = T1u Γrot = T1g Γ3n - 6 = A1g + Eg + T2g + 2T1u + T2u h = 48 Spectroscopic Selection Rules Normal Modes and Spectroscopic Activity of XY6 (Oh) Γ3n - 6 = A1g + Eg + T2g + 2T1u + T2u Species νn Transformations Activity A1g ν1 x2 + y2 + z 2 Raman Eg ν2 (2z2- x2- y2, x2- y2) Raman T2g ν5 (xz, yz, xy) Raman 2T1u ν3, ν4 (x, y, z) i.r. T2u ν6 — — • Raman-active modes are not i.r.-active and vice versa. (Mutual exclusion) • Only gerade species A1g, Eg, and T2g are Raman active. • Only the ungerade species T1u is i.r. active. • The degenerate modes that comprise ν6 (T2u) are not active in either Raman or i.r. (Silent modes). Vibrational Frequencies of SF6 Mode Frequency (cm–1) Activity ν1 (A1g) 775 R (pol) ν2 (Eg) R 643 ν3 (T1u) 948 i.r. ν4 (T1u) 615 i.r. ν5 (T2g) 524 R ν6 (T2u) 346 — Spectroscopic Selection Rules General Results • Infrared-active modes can be distributed among no more than three symmetry species, because activity is associated with the transformation properties of the three unit vectors. • Raman-active modes can be distributed among as many as four symmetry species, depending on the point group, because the direct products typically span two to four species. • In centrosymmetric point groups (those that have inversion, i), unit vectors transform as ungerade species, and direct products transform as gerade species. Therefore, infrared-active modes will be Raman inactive, and vice versa, for centrosymmetric molecules. This requirement is known as the rule of mutual exclusion. • Unit vector and direct product transformations do not span all species in some groups. Thus, it is possible to have some normal modes that cannot be observed as fundamentals in either the infrared or Raman spectra. These spectroscopically inactive modes are often called silent modes. • The totally symmetric representation in every point group is associated with one or more direct product transformations. Therefore, normal modes that are totally symmetric will always be Raman active. Totally symmetric modes may or may not be infrared active, depending on the point group.