Lecture 35 Notes: Vibrational Spectroscopy

5.61 Physical Chemistry Lecture #35 1 VIBRATIONAL SPECTROSCOPY As we’ve emphasized many times in this course, within the Born­ Oppenheimer approximation, Harmonic the nuclei move on a potential Approximation energy surface (PES) R determined by the electrons. A + B separated atoms For example, the potential R0 felt by the nuclei in a V(R) diatomic molecule is shown in cartoon form at right. At low energies, the molecule will sit near the bottom of this potential energy surface. In equilibrium bond length this case, no matter what the detailed structure of the potential is, locally the nuclei will “feel” a nearly harmonic potential. Generally, the motion of the nuclei along the PES is called vibrational motion, and clearly at low energies a good model for the nuclear motion is a Harmonic oscillator. Simple Example: Vibrational Spectroscopy of a Diatomic If we just have a diatomic molecule, there is only one degree of freedom (the bond length), and so it is reasonable to model diatomic vibrations using a 1D harmonic oscillator: P̂ 2 1 ˆ 2 P̂ 2 1 + ko R = + mωo 2 Rˆ 2 Ĥ = 2µ 2 2µ 2 where ko is a force constant that measures how stiff the bond is and can be approximately related to the second derivative to the true (anharmonic) PES near equilibrium: ∂ 2V ko ≈ 2 ∂R R 0 Applying Fermi’s Golden Rule, we find that when we irradiate the molecule, the probability of a transition between the ith and fth Harmonic oscialltor states is: W fi ∝ V fi 2 ⎡δ ( Ei − E f − �ω ) + δ ( Ei − E f + �ω )⎤ ⎦ 4� 2 ⎣ 5.61 Physical Chemistry 2 Lecture #34 where ω is the frequency of the light (not to be confused with the frequency of the oscillator, ωo). Because the vibrational eigenstates involve spatial degrees of freedom and not spin, we immediately recognize that it is the electric field (and not magnetic) that is important here. Thus, we can write the transition matrix element as: 2 V fi = ∫ φ f *µ̂ µ iE 0φi dτ 2 = E 0 i ∫ φ f *µ̂ µφi dτ 2 = E 0 i ∫ φ f * eR̂φi dτ 2 Now, we define the component of the electric field, ER, that is along the bond axis which gives 2 2 V fi = E R ∫ φ f * eR̂φi dτ = e2 E R 2 * ∫ φ f R̂φi dτ 2 So the rate of transitions is proportional to the square of the strength of the electric field (first two terms) as well as the square of the transition dipole matrix element (third term). Now, because of what we know about the Harmonic oscillator eigenfunctions, we can simplify this. First, we re­ write the position operator, R, in terms of raising and lowering operators: 2 2 2 V fi = e ER 2 ⇒ V fi = 2 ∫ φf �e 2 ER 2 µω �e 2 ER ( aˆ + + aˆ − ) φi dτ = 2 µω 2 µω � * 2 ( (i + 1) δ f ,i +1 + i δ f ,i −1 ) 2 * ∫ φ f ( aˆ + + aˆ − ) φi dτ i + 1 φi +1 2 iφi−1 where above it should be clarified that in this expression “i" never refers to √­1 – it always refers to the initial quantum number of the system. Thus, we immediately see that a transition will be forbidden unless the initial and final states differ by one quantum of excitation. Further, we see that the transitions become more probable for more highly excited states. That is, Vfi gets bigger as i gets bigger. Combining the explicit expression for the transition matrix element with Fermi’s Golden Rule again gives: e2 2 W fi ∝ ER ( i + 1) δ f ,i +1 + i δ f ,i −1 ⎡⎣δ Ei − E f − �ω + δ Ei − E f + �ω ⎤⎦ 8�µω ( ⇒ W fi ∝ E R 2 {(i + 1) δ ) ( f ,i +1 ) ( ) ⎡⎣δ ( Ei − Ei+1 − �ω ) + δ ( Ei − Ei+1 + �ω )⎤⎦ } + i δ f ,i −1 ⎡⎣δ ( Ei − Ei−1 − �ω ) + δ ( Ei − Ei−1 + �ω )⎤⎦ 5.61 Physical Chemistry ⇒ W fi ∝ ER 2 3 Lecture #34 0 {(i + 1) δ f ,i+1 ⎡⎣δ ( −�ωo − �ω ) + δ ( −�ωo + �ω )⎤⎦ } + i δ f ,i −1 ⎡⎣δ ( �ωo − �ω ) + δ ( �ωo + �ω ) ⎤⎦ 0 ⇒ W fi ∝ ER 2 {(i + 1) δ f ,i +1 + i δ f ,i −1 }δ ( �ωo − �ω ) Thus, we see that a harmonic oscillator will only absorb or emit photons of frequency �ωo , where ωo is the frequency of the oscillator. Thus, if we look at the absorption spectrum, for example, we will see absorption at only one frequency: ωo Light Frequency (ω) Absorption Intensity Molecular force constants are typically on the order of an eV per Å, which leads to vibrational frequencies that are typically between 500­3500 cm­1 and places these absorption features in the infrared. As a result, this form of spectroscopy is traditionally called IR spectroscopy. We associate the spectrum above as arising from all the n→n+1 transitions in the Harmonic oscillator (see left). Of course, most of the time the molecule will start in its ground state, so that the major contribution comes from the 0→1 transition. However, the other transitions occur at the same frequency and also contribute to the absorption. This is the classic paradigm for IR vibrational spectroscopy: each diatomic molecule absorbs radiation only at one frequency that is characteristic of the curvature of the PES near its minimum. Thus, in a collection of different molecules one expects to be able differentiate one from the other by looking for the frequency appropriate to each one. In particular there is 5.61 Physical Chemistry Lecture #34 4 a nice correlation between the “strength” of the bond and the frequency at which it will absorb. Of course, we are not always or even usually interested in diatomics, and even diatomics are not perfect Harmonic oscillators. Thus, there are a number of reasons why IR absorption spectra do not really look like the classic Harmonic oscillator spectrum shown above, but more like: Heterogeneity The primary reason the real spectrum above looks different than the model is because the real spectrum was taken in solution. The model is correct for a single diatomic, or for many, many copies of identical diatomic molecules. However, in solution, ever molecule is just slightly different, because every molecule has a slightly different arrangement of solvent molecules around it. These solvent molecules subtly change the PES, slightly shifting the vibrational frequency of each molecule and also modifying the transition dipole a bit. Thus, while a single hydrogen fluoride molecule might have a spectrum like the model above, a solution with many HF molecules would look something like: 5.61 Physical Chemistry Lecture #34 ωo 5 Light Frequency (ω) Absorption Intensity Going over to the situation where there are 1023 HF molecules and recognizing that our spectra will tend to add the intensity of lines that are closer together than our spectrometer can differentiate, we anticipate that for a diatomic molecule in solution, the vibrational spectrum should look something like: ωo Light Frequency (ω) Absorption Intensity The resulting feature in the spectrum is usually called a lineshape. It primarily reflects the distribution of different environments surrounding your oscillators. Thus, by analyzing the lineshape of a well­known type of vibration (such as a C=O stretch) one can get an idea about the environments those CO groups live in: How polar are the surroundings? Are they near electron withdrawing groups? What conformations give rise to the spectrum? Finally, we should note that vibrational spectra recorded in the gas phase have very narrow linewidths, qualitatively resembling our model above. Anharmonicity 5.61 Physical Chemistry Lecture #34 6 Another reason real spectra differ from our model is that assuming the PES is harmonic is only a model. If we want high accuracy, we need to account for anharmonic terms in the potential: 2 3 4 1 1 1 V ( R ) = mωo 2 ( R − R0 ) + α ( R − R0 ) + β ( R − R0 ) + ... 2 6 24 One can investigate the quantitative effects of the anharmonic terms on the spectrum by performing variational calculations. However, at a basic level there are two ways that anharmonic terms impact vibrational spectra: 1) The energy differences between adjacent states are no longer constant. Clearly, the eigenvalues of an anharmonic Hamiltonian will not be equally spaced – this was a special feature of the Harmonic system. Thus, for a real system we should expect the 0→1 transition to have a slightly different frequency than 1→2, which in turn will be different that 2→3 …. Generally, the higher transitions have lower (i.e. redshifted) energies because of the shape of the molecular PES – rather than tend toward infinity at large distances as the harmonic potential does, a molecular PES tends toward a constant dissociation limit. Thus, the higher eigenstates are lower in energy than they would be for the corresponding harmonic potential. Taking this information, we would then expect a single anharmonic oscillator to have a spectrum something like: ωo Absorption Intensity Light Frequency (ω) 2→ 3 1→ 2 0→ 1 where we note that while the rate of, say, 1→2 is about twice that of 0→1 (because the transition dipole is twice as big) the intensity of 0→1 is greater because the intensity is (Probability of i being occupied)x(Rate of i→f) and at room temperature the system 5.61 Physical Chemistry 7 Lecture #34 spends most of its time in the ground vibrational state. The 1→2, 2→3 … lines in the spectrum are called hot bands. 2) Anharmonicity relaxes the Δn=±1 selection rule. Note that the rules we arrived at were based on the fact that â +φi ∝ φi+1 . This is only true for the Harmonic oscillator states. For anharmonic eigenstates â +φi ∝ φi+1 + ε1φi+2 + ..... . Thus transitions with Δn=±2,±3… will no longer be forbidden for anharmonic oscillators. Rather, in the presence of a bit of anharmonicity, they will be weakly allowed. Combining this observation with point 1) above results in a more complete picture for what the IR spectrum of an anharmonic oscillator should look like: ωo Absorption Intensity 2→ 3 1→ 2 2ωo Light Frequency (ω) 2→ 4 1→ 3 0→ 2 0→ 1 The peaks at around 2ωo are called overtones. Meanwhile, those at around ωo are called fundamentals. Polyatomic Molecules The final difference between the model above and a general IR spectrum is that in chemistry, we are not always dealing with diatomic molecules. For a polyatomic molecule, we can still think of the potential as a Harmonic potential, but it has to be many­dimensional – it has to depend on several variables R1, R2, R3, …. The most general Harmonic potential we can come up with is then of the form: V ( R1 , R2 , R3 ,...) = 12 k11 R12 + 12 k12 R1 R2 + 12 k13 R1 R3 + .... + 12 k21 R2 R1 + 12 k22 R2 2 + 12 k23 R2 R3 + .... + 12 k31 R3 R1 + 12 k32 R3 R2 + 12 k33 R32 + ... where it is important to notice the cross terms involving, say R1 and R2, which couple the different vibrations. At first sight, it seems like we 5.61 Physical Chemistry 8 Lecture #34 can’t solve this Hamiltonian; the only many­dimensional Harmonic potential we would know how to solve would be one that is separable: V� ( R1 , R2 , R3 ,...) = 12 k11 R12 + 12 k22 R2 2 + 12 k33 R32 + ... If the Harmonic potential were of this form, we would be able to write down the eigenstates as products of the 1D eigenstates and get the energies as sums of the 1D eigenenergies. As it turns out, by changing coordinates we can turn a quadratic system with off­diagonal cross terms (like the first potential) into one with no cross terms (like the second). These new coordinates, in terms of which the Hamiltonian separates, are called normal modes and they allow us to reduce a polyatomic molecule to a collection of independent 1D oscillators. First, we note that V can be re­written concisely in matrix notation [Note: it may be useful to consult McQuarrie’s supplement on Matrix Eigenvalue problems if the following seems unfamiliar.]: V ( R1 , R2 , R3 ,...) = 12 R T iK iR ⎛ R1 ⎞ ⎜ ⎟ R R≡⎜ 2⎟ ⎜ R3 ⎟ ⎜ ⎟ ⎝ � ⎠ ⎛ k11 ⎜ k K ≡ ⎜ 21 ⎜ k31 ⎜ ⎝ � k12 k22 k13 k23 k23 � k33 � ... ⎞ ⎟ ... ⎟ ... ⎟ ⎟ �⎠ Now, the Hamiltonian is of the form: P̂ 2 1 ˆ Ĥ = ∑ i + R̂T iK iR 2 µ 2 i i It is convenient to first transform to mass-weighted coordinates: kij P̂ p̂i ≡ i x̂i ≡ µi R̂i k�ij ≡ µi µi µ j in terms of which we can write the Hamiltonian: p̂i 2 1 T � Ĥ = ∑ + xˆ iK ixˆ 2 2 i As is clear from the kinetic energy above, in these coordinates, every degree of freedom has the same reduced mass. Now we perform the normal mode transformation. We want to write: 5.61 Physical Chemistry 9 Lecture #34 � ixˆ = yˆ T iK ′iy x̂T iK ⎛ k11' ⎜ 0 K′ = ⎜ ⎜ 0 ⎜⎜ ⎝ ... 0 k22' 0 ... 0 0 k33' ... ... ⎞ ⎟ ... ⎟ ... ⎟ ⎟ ... ⎟⎠ Further, we will assume there is a matrix U that transforms from x to y ⇒ y = Uix y T = xT i U T where in the second equality, we recall the general rule that the transpose of a product is the product of the transposes, but in the opposite order. Combining these two equations: � ixˆ = yˆ T iK ′iy = xˆ T iUT iK ′iUixˆ x̂T iK � = UT iK ′iU ⇒K The last equation is a common problem encountered in linear algebra: the � ) and place it in diagonal form (the right quest to take a given matrix ( K � the solution to this problem is hand side). For a symmetric matrix like K � and the well known: the diagonal entries of K ′ are the eigenvalues of K � . The columns of the transformation matrix U are the eigenvectors of K transformed variables y are called the normal modes. These modes are linear combinations of the local degrees of freedom R1, R2, R3, … that we started out with. Thus, while the initial motions might correspond clearly to local stretching of one bond or bending of an angle, the normal modes will generally be complicated mixtures of different molecular motions. We can visualize this in the simple case of two degrees of freedom. The local modes R1, R2 can be thought of as the two orthogonal axes in a plane. Meanwhile, the normal modes y1, y2 are also orthogonal axes, but rotated from the original set: R2 R2 y2 Normal modes Local modes y1 R1 R1 The local modes have interactions between each other: local stretches are coupled to local bends, etc. As a result, the Hamiltonian is not separable in terms of the local modes R1, R2. However, by design the Hamiltonian is separable when written in terms of the normal modes: 5.61 Physical Chemistry 10 Lecture #34 p̂i 2 1 + kii′ ŷi 2 Ĥ = ∑ 2 2 i ’ The eigenvalues, kii , tell us the “stiffness” of the PES in the particular direction yi. Note that the Hamiltonian above is exactly the same Hamiltonian as the one we started with. The coupling terms have simply been rotated away by changing the coordinates. As discussed before, we can immediately interpret the spectra of this Hamiltonian in terms of a sum of many independent oscillators. Thus, for a polyatomic molecule within the harmonic approximation we expect to see lines at each of the normal mode frequencies: ω1 ω2 ω3 ω4 ω5 Light Frequency (ω) Absorption Intensity Where we have noted that the different oscillators will typically also have different transition dipoles. (For obvious reasons, in vibrational spectroscopy the square of the transition dipole is often called the oscillator strength) We can, of course, combine this polyatomic picture with the anharmonicity effects above to get a more general picture that looks like: ω1 ω2 Absorption Intensity 2ω1 ω3 2ω2 ω4 ω5-ω1 ω2+ω3 Light Frequency (ω) ω5 5.61 Physical Chemistry Lecture #34 11 where we predict the existence of various hotbands and overtones for each of the normal mode oscillators in the molecule. Note that while the overtones always involve multiple quanta, the quanta need not come from the same normal mode – hence we expect not only overtones at 2ω1 , but also a combination bands at ω2 + ω3 and ω5 − ω1 . The picture above is qualitatively correct for the IR spectrum of a single molecule. In solution, heterogeneity leads to a smearing out and broadening of the peaks, leading to the complex IR fingerprints we are used to. As should be clear from the above discussion, IR spectra contain a wealth of information about the molecule: the stiffness of each normal mode, the degree of anhormonic effects, the character of the local environment felt by the oscillators …. Of course, in order to extract this information, one must be able to assign the spectrum – i.e. one must be able to distinguish hotbands from overtones and associate the various normal modes (at least qualitatively) with physical motions of the molecule. This task can be extremely challenging – and computation must be used as a guide in many cases – but when it is accomplished, one typically has a very sensitive fingerprint of molecule under consideration.