# FUNDAMENTALS OF MOLECULAR SPECTROSCOPY PDF

TELEPHONE , FAX British Library Cataloguing in Publication Data. Banwell, C. N.. Fundamentals of molecular spectroscopy-3rd ed. Could anyone provide me with this book (PDF file) under name "Fundamentals of molecular spectroscopy" by Banwell? Don't give me the link because I tried to. Struve, Walter S. Fundamentals of molecular spectroscopy. "A Wiley— Interscience publication." Bibliography: p. 1. Molecular spectroscopy. I. Title. QC

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and Molecular Biology. Cell Division Much more active than normal cells, cancer cells divide blank Methods in Molecular Biology • 16 Enzymes of Molecular. Fundamentals of Molecular Spectroscopy. Prof. Kankan Bhattacharya. Dept. of Physical Chemistry. Indian Association for Cultivation of Science. The Fundamental of Molecular Spectroscopy Cn Banwell - Free ebook download as PDF File .pdf), Text File .txt) or read book online for free.

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Good introductory book. Written in easy to understand English. The book however skips rigorous quantum mechanical derivations which, I believe, is important for thorough understanding.

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If you like books and love to build cool products, we may be looking for you. About Colin N. Add 3 Items to Cart. Anand Yadav rated it liked it Feb 25, No trivia or quizzes yet. There are no discussion topics on this book yet. Banwell and Elaine M. Visit our Beautiful Books page and find lovely books for kids, photography lovers and more. Refresh and try again. It remains an elementary and non-mathematical introduction to molecular spectroscopy that emphasizes the overall unity of the subject and offers a pictorial perception rather than a mathematical description of the principles of spectroscopy.

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Hence a c' configuration gives rise to a 2 E state. The ' and Li. In a nonequivalent a 2 configuration in which two valence electrons occupy different a orbitals aa and ab , the four possible distributions of electron spins are shown in Table 4. The symmetry designation will be given by the direct product of point groups to which MOs aa and ab belong.

For example, if o-a and ab are cg and au orbitals with positive refle-cTion symmetry e. The interesting and important equivalent 7r2 case is typified by ground-state 0 2 , which has the configuration agi s 2 o-u1s 2 0. To choose between these alternatives, we inspect some of the wave functions for the states counted in Table 4. Then the antisymmetrized state 4 3 is g 9 5 5 — 4.

We conclude that the possible term symbols in an equivalent it configuration are 1 E: The nonequivalent 7t 2 configuration can arise when one of two valence electrons is in a nu orbital and the other is in a ng orbital e. Finding the term symbols arising from this configuration is left as an exercise for the reader. The atomic SCF calculations described in Section 2.

The diatomic electronic Hamiltonian in the clamped-nuclei approximation Eqs. The diatomic Hamiltonian 4. One may write a singledeterminant many-electron wave function analogous to Eq. A set of equations analogous to the Hartree-Fock equation 2. The discrepancy between the resulting SCF electronic energy given by Eq. Such a procedure is so cumbersome in diatomics and polyatomics that relatively few such calculations have been performed in species other than atoms [7].

In atoms, the centrosymmetric one-electron Hamiltonian fl i Eq. The latter can be represented in atoms by spherical harmonics Yi,, 0, 0, and the numerical optimization of the Hartree-Fock wave function becomes confined to the radial coordinate.

In nonlinear polyatomics, the molecular orbitals must generally be optimized over a full three-dimensional grid. Compounding this problem is the fact that SCF energies must also be calculated over a grid of molecular geometries bond lengths and bond angles to search for an equilibrium geometry.

A minimal basis set of AOs includes all AOs that are occupied in the separated constituent atoms. The coefficients may be varied to minimize the energy. Parameters in the AOs themselves may also be varied, but these are frequently fixed at values established by prior experience with the same atoms in similar molecules.

STOs Eq. However, numerical calculation of many of the resulting matrix elements , and is slow using ST0s, and the use of Gaussian type orbitals GT0s of the form iGnt.

The inconvenience of evaluating the resulting larger number of Hamiltonian matrix elements is more than offset by their efficiency of calculation using GTOs. The configuration interaction technique, which is analogous to that described in Section 2. Te is the energy separation conventionally in cm -1 between the minima in the two electronic state potential energy curves. If the upper electronic state is purely repulsive, its separated-atom asymptote is used to calculate 're. We will see later that the El rotational selection rules are reminiscent of the ones we derived for pure rotational and vibration—rotation spectra although electronic state symmetry must be carefully considered, using the Herzberg diagrams introduced in Section 4.

The term proportional to vanishes due to orthogonality of the electronic states.

## Fundamentals of Molecular Spectroscopy

The Franck-Condon factors do obey a sum rule, however. This implies that summing the vibrational band intensities over an electronic band spectrum allows direct measurement of the averaged electronic transition moment function Me R according to Eq. The g, u subscripts may be dropped to generalize this discussion to heteronuclear diatomics.

The 3 E: The best-known electronic transitions in 12 are the A 1. They are El-forbidden according to the selection rules AS 0 0 , but they gain small El intensity because the large spin—orbit coupling in 1 2 causes considerable admixtures of singlet character into the A and B states and triplet character into the X state [9].

The 3n 2 30E: The Ritz combination principle offers a bruteforce method of assigning y', y" combinations to the bands: For example, [V, 3, 0 — 0 ] and U 3, 1 — 13 2, 1 ] and U7 3, 2 — 2 ] must all equal G' 3 — G' 2 , and this gives a start in organizing the assignment of the spectrum.

This approach is known as a Deslandres analysis Problem 4. A preferable approach is to simplify the spectrum experimentally: Several coupling schemes for spin, orbital, and rotational angular momenta may be identified, depending on the magnitudes of the magnetic field generated by the electrons' orbital motion and the spin—orbit coupling.

Hand's case a describes the majority of molecules with A 0 0 that exhibit small spin—orbit coupling. Since the electronic orbital and spin angular momenta L and S are not mutually strongly coupled, they precess independently about the quantization axis the molecular axis established by the magnetic fields arising from electronic motion. The projections Ah and Eh of L and S respectively along the molecular axis are then conserved, as is their sum Oh.

In contrast, the parts of L and S normal to the molecular axis oscillate rapidly; they are denoted L1 and S1 Fig. The orbital and spin angular momenta L and S precess rapidly about the molecular axis with fixed projections Ah and Eh. The total angular momentum normal to the molecular axis is in effect the rotational angular momentum N, since the normal components 11 and S 1 of L and S fluctuate rapidly and their expectation values are zero.

Interactions between Li, S i and the rotational angular momentum N give rise to Adoubling see text. According to the inequality 1J1 2 J! The second term describes the electronic-rotational interactions, which constitute a small perturbation to the rotational energy in molecular states with A O. This degeneracy is split by the perturbation. This phenomenon is known as A-doubling. No levels appear with J exaggerated.

The corresponding spin—orbital energy in diatomics has the form ASV, where A is a spin—orbital constant [10]. The total energy in excess of the vibronic energy cf. In Hund's case b the magnetic fields due to orbiting electrons are so weak that the electron spin angular momentum S does not precess about the molecular axis. The rotational angular momentum N is directed normal to the molecule axis. In the absence of orbital angular momentum, the local magnetic field is dominated by molecular rotation, with the result that S precesses about J Fig.

The rotational energies to zeroth order are E,.. In Hund's case c the spin—orbit coupling is so large that L and S are mutually coupled to form a resultant J. The rotational energy levels are given by the same expression, 4.

An important distinction between cases a and c is that while the spin—orbit contribution AS22 to Eq. The spin angular momentum S and the rotational angular momentum N precess about the total angular momentum J.

Each N level Eq. Since L and S do not precess independently about the molecular axis in case c Figure 4. The orbital and spin angular momenta L and S are strongly coupled to form a resultant J.. The projection of J. L and S clearly do not have well-defined projections on the molecular axis, and so case c molecular states cannot be strictly characterized in terms of the quantum numbers A and E.

We have seen that no symmetry selection rule exists for Av, but that the vibrational band intensities are proportional to Franck-Condon factors in the Born-Oppenheimer approximation. A complication arises here because we have derived our earlier pure rotational and vibration—rotation selection rules using space-fixed coordinates i. To consider simultaneous rotational and electronic state changes, we must use one set of coordinates consistently.

We finally determine how p is influenced by ay molecule-fixed. For definiteness, suppose we are interested in a l E electronic transition which is El-allowed according to the selection rules obtained in Section 4.

This diagram exhibits only R- and P-branch transitions. The-pertinent Herzberg diagram Fig. Recall that we require J 12, and. In homonuclear diatomics an additional symmetry element is needed to classify state symmetries in the p oo h point group, and we can use i moleculefixed for this purpose.

A new complication, peculiar to homonuclear molecules J" o 2 3 Figure 4. Note the absence of the Q 0 and P 1 transitions, since J' in which both nuclei are the same isotope, is the effect of nuclear exchange symmetry on rotational state populations.

The possible combinations of electronic, rotational, and nuclear spin states can now be compiled [10] as shown in Table 4. Fermion nuclei exhibit half-integral spin: According to Table 4. This leads to a nearly 3: Consequently, 16 X3 kJ cannot exist in even - J levels at all.

## Fundamentals Of Molecular Spectroscopy

Such levels can be populated in other electronic states e. Prior to digressing on the subject of nuclear exchange symmetry, we mentioned that a new symmetry element besides o-, molecule-fixed was required to classify electronic-rotational states in homonuclear diatomics. A logical choice is i molecule-fixed , an operation which belongs to D co h but not C. The dipole moment operator p in homonuclear molecules is s under XN [11 ]. As in 1 E transitions for heteronuclear molecules, only R and P branches can appear.

Note that the P 1 and Q 0 lines state. Herzberg's classic Spectra of Diatomic Molecules [10]. Such Herzberg diagrams automatically incorporate the Laporte g u selection rule, since no allowed rotational transitions can be drawn for a 1 E: The Na 2 fluorescence spectra in Fig. The laser linewidth was considerably broader than the energy separation due to A doubling between the a and s 'H. These spectra are excellent examples of the selection rules in Fig. Monochromatic laser excitation of an X' E: According to Fig.

The numbers in boxes give y" for the lower level in fluorescence transitions. Reproduced with permission from W. Demtrbder, M. McClintock, and R. Zare, J. Hence, the y', y" fluorescence bands are doublets in the first fluorescence spectrum, but are singlets in the second one.

Rotational line assignments are easily made by inspection in pure rotational and vibration-rotation spectra Chapter 3.

Since B' and B" are nearly the same in vibration-rotation spectra because B varies weakly with y within a given electronic state , Eqs. For rotational fine structure in electronic band spectra, the P- and R-branch line positions are still given by Eqs. The important physical difference here is that B' and B" are often grossly different in transitions between different electronic states.

For example, He and Be" are 0. The rotational line spacings are smaller for ICI, owing to its larger reduced mass. Horizontal energy scale is in cm -1 for both spectra. Their positions may be analyzed using Eqs.

The vibrational band positions in such spectra e. A strategem for confirming vibrational assignments of electronic band spectra is described later in this section. Huber and Herzberg [14] have compiled molecular constants for over diatomic molecules and molecule-ions, based on critical examination of the literature up to In many applications, it is desirable to know the detailed potential energy curves Ukk R for the upper and lower states.

## Fundamentals of Molecular Spectroscopy

Such information would be required to predict spectral line intensities of heretofore unobserved vibronic transitions e. We are now concerned with the reverse procedure: Given a set of spectroscopically determined vibrational levels, can the detailed potential energy curve Ukk R be reconstructed? For nonpathological potentials, the intuitive answer is yes. When the vibrational levels are equally spaced in energy, Ukk R is a parabola with curvature determined by the level spacing.

Nonuniformities in level spacing manifested by nonvanishing anharmonic constants w ex e , clue , The integral in 4. Rydberg, Klein, and Rees demonstrated that this semiclassical procedure for deriving the allowed vibrational energies E from a given potential U R may be inverted. If the rovibrational energy 4.

Rees showed that these integrals are expressible in closed form [18] when the vibrational energy E I,0 is quadratic in I, but this level of approximation does not yield accurate turning points over a broad range of vibrational energies.

The full potential Ukk R is then constructed by connecting the turning points with a smooth curve. Such RKR calculations furnish the most accurate experimental potential energy curves for diatomics. Their facile execution using established computer codes has largely superseded characterizations of Ukk R by analytic fitted potentials such as the Morse potential Section 3.

Separations are given in A. Data are taken from P. Kusch and M. Hessel, J. Used with permission from F. The band intensities observed in an absorption or fluorescence spectrum should be proportional to these calculated Franck-Condon factors Section 4. If they are not, the vibrational bands in the electronic spectrum may have been incorrectly assigned. In this manner, Zare found it necessary to reassign the y' quantum numbers previously attributed to vibrational bands in the 0E: RKR calculations are, of course, possible only for spectroscopically accessible electronic states for which a reasonable number of the molecular constants in Eq.

For this reason, only the VI: RKR calculations are not applicable to purely repulsive states, and the shapes but not the asymptotes of the repulsive potential energy curves depicted in this chapter are largely conjectural.

A number of potential energy curves is shown for 0 2, , and 02' in Fig. El transitions among these three states are forbidden, in accordance with the general rule that no El transitions exist among states generated from the same electron configuration. The better-characterized excited states i. Since it is not connected to any lower electronic state by El transition, the A 3 E: Berry, S. Rice, and J. Ruedenberg, Rev.

Feinberg, K. Ruedenberg, and E. Mehler, Adv. Quantum Chem. Gottfried, Quantum Mechamics, Vol. Sommerfeld, Ann. Rydberg, Ann. Klein, Z.

Rees, Proc. London A Zemke, K. Verma, and W. Stwalley, Proc. Write down the electron configurations for the ground states of B2, C2, N2, and 0 2. What term symbol corresponds to the ground state in each of these molecules? Using the potential energy curves in Fig. Which of the shown excited states are theoretically accessible by M1 transitions from ground-state N2? In a correctly ordered table, all differences denoted by the same label e.

Determine the vibrational constants co: The absorption band of an allowed El electronic transition originates from the X3Ig- state in 0 2. Rotational fine structure lines occur at the following wave numbers: Is this band shaded to the red or violet?

Is B' greater than or less than B"? These coordinates exhibit the range element is le 22 — p 2 dA dp A traditional motivation for treating polyatomic rotations quantum mechanically is that they form a basis for experimental determination for bond lengths and bond angles in gas-phase molecules.

Microwave spectroscopy, a prolific area in chemical physics since , has provided the most accurate available equilibrium geometries for many polar molecules. A background in polyatomic rotations is also a prerequisite for understanding rotational fine structure in polyatomic vibrational spectra Chapter 6. The shapes of rotational contours i. Rotational contour analysis has thus provided an invaluable means of assigning symmetries to the electronic states involved in such spectra.

We begin this chapter with a derivation of the classical Hamiltonian for a rigid, freely rotating polyatomic molecule. Such a polyatomic rotor may be classified according to its point group symmetry Section 5.

Molecules with lower symmetry e. El selection rules are obtained for pure rotational transitions, and examples are given of structural information yielded by microwave spectra. The origin of this coordinate system coincides with the body's center of mass Fig.

It also contains six off-diagonal terms proportional to obccoy, etc. Since the inertia tensor is necessarily a symmetric matrix according to Eq. In the ellipsoid illustrated, one of the principal axes a lies along the C co axis, while the other two principal axes b and c are any pair of mutually orthogonal axes which are perpendicular to a. Equation 5. Four classes of nonlinear rigid rotors may then be distinguished: In the D2h ethylene molecule, for example, the C2 x , C 2 y , and C2 z twofold axes are principal axes.

All of the principal moments of inertia in ethylene are different in magnitude, so this molecule is an asymmetric top. It is possible in principle for a molecule lacking a Ch axis n 3 to exhibit two equal principal moments; such a molecule is termed an accidental symmetric top.

The frequency resolution afforded by microwave technology can detect such small differences between two moments of inertia that very few molecules pass for accidental symmetric tops under microwave spectroscopy. In symmetric tops, two of the rotational moments of inertia are equal. The third moment is associated with rotation about the axis of highest symmetry called the figure axis. In prolate symmetric tops, the figure axis exhibits the largest rotational constant and the smallest principal moment of inertia.

Such molecules concentrate most of their nuclear mass along the figure axis the a axis , and tend to be cigar-shaped. Eclipsed and staggered ethane molecules are both prolate symmetric tops, as is 2-butyne Fig.

In oblate symmetric tops, the nuclear mass tends to concentrate at an appreciable distance from the figure axis the c axis , endowing the figure axis with the smallest rotational constant and the largest principal moment of inertia.

Oblate tops e. Linear polyatomic molecules are a special case of prolate symmetric tops, in which the rotational moment about the figure axis the C. For such species e. The classical Hamiltonian 5. By convention, the figure axis is labeled the a axis and the c axis in prolate and oblate tops, respectively.

We will see that this fact considerably complicates the derivation of the quantum mechanical energy levels for an asymmetric top. The space-fixed angular momentum components fx , obey the familiar commutation rules [ix ,: It would also be useful to know whether any spacefixed components of J commute with particular body-fixed components of J, so that commuting sets of observables may be constructed as an aid in visualizing the physical significance of rigid rotor wave functions.

An arbitrary three-dimensional rotation of a rigid body may be described [1] using the Euler angles 4 , 0, x Fig. The body-fixed a, b, and c axes are initially aligned with the space-fixed x, y, and z axes, respectively. The molecule is first rotated by an angle 0 about the space-fixed z axis, causing molecule-fixed axes initially pointing along x and y to be rotated into x' and y', respectively.

The molecule is then rotated by the angle 9 about the new x' axis, displacing the body-fixed axis initially pointing along z into the direction c and the y' axis into direction y". The molecule is finally rotated by the angle x about the c axis, rotating the x' and y" axes into the a and b axes, respectively. The second rotation is a counterclockwise rotation by the angle 0 about the new orientation of the a axis; it rotates the body-fixed c axis by the angle 0 from the space-fixed z axis.

The body is finally rotated counterclockwise by the angle x about the new c axis. The components of the rigid rotor's angular momentum may be specified as projections of J along either the space-fixed axes Jx ,Jy ,. It is not difficult to show geometrically Problem 5.

Use of Eqs. The Hamiltonian 5. In the oblate symmetric top, the rotational Hamiltonian is given by Eq. The c axis is denoted the figure axis. According to the commutation rules obtained in Section 5. These are rarely given explicitly in modern spectroscopy texts, because no knowledge of these functions is necessary to obtain either the energy levels or the selection rules for spectroscopic transitions in symmetric tops.

By combining Eqs. The quantum number M measures the projection of the rotational angular momentum di upon the space-fixed z axis Eq. For given J, the rotational energy is a decreasing increasing function of IK1 for oblate prolate tops. Figure 5. It cannot rotate purely about the c axis: In such a state, one would have J. It is insensitive to the sign of K, which only controls the direction of rotation about the figure axis.

In a prolate symmetric top, the a axis rather than the c axis becomes the figure axis. The body-fixed a, b, c axes depicted in Fig. The mutually commuting set of observables becomes Jz , f a, and J2. With these modifications, the oblate top eigenvalue equations and wave functions 5. Here the rotational constants A and C are fixed, while B the horizontal coordinate is varied continously between A and C.

The oblate and prolate top energies are given by Eqs. In the asymmetric top, the rotational Hamiltonian 5. Since no two body-fixed components of J commute Eq. It is possible to find the rotational eigenstates and energies of an asymmetric top by diagonalizing the Hamiltonian 5.

This has been done in several texts [2], and it will not be repeated here. In this spirit, the energies of the asymmetric rotor may be visualized on a qualitative energy level diagram showing the correlations between the prolate and oblate limits Fig.

The vast majority of molecules are asymmetric tops. Some representative rotational constants are listed for asymmetric, symmetric, and spherical top molecules in Table 5. An intuitive argument will suffice for the selection rule on AK. Since p must be parallel to the figure axis by symmetry in any prolate or oblate top, and since K controls the velocity of rotation about the figure axis, changing K has no effect on the motion of the molecule's permanent dipole moment.

To obtain the selection rules on AJ and AM, we exploit the properties of vector operators. All quantities that transform like vectors under threedimensional rotations have operators exhibiting commutation rules that are identical to those shown by the space-fixed angular momentum operators f,,: The space-fixed angular momentum components fx , fy ,. Since we have from Eq. It is worth remarking that all of the selection rules derived in this section emerged as consequences of vector operator properties; consideration of the specific rotational wave functions 5.

At gas pressures low enough to render collisional broadening negligible Chapter 8 , the frequency widths of rotational absorption lines are narrowed to the point where their frequency positions may be determined with high accuracy.

This fact, coupled with the available frequency stability of klystron sources better than one part in , endows microwave spectroscopy with the capability of determining the rotational constants extremely to high precision.

A drawback of obtaining microwave spectra at such low gas pressures 10 -3 to 10 ' torr is loss of sensitivity, since the absorbance Appendix D is proportional to the molecule number density.

This problem is addressed by the technique of Stark-modulated microwave spectroscopy [6]. The waveguide used for microwave propagation through the sample gas is bisected by an insulated, conducting septum running down its length.

Application of a square-wave voltage typical frequency — kHz to the septum generates a modulated electric field in the gas, modifying its rotational energies by the Stark effect.

The rotational line frequencies in the presence of the field are shifted from their zero-field positions, because the level Stark shifts depend on J, K, and M. Hence the absorption lines may be switched in and out of resonance with the incident microwave frequency from the klystron, allowing selective detection of the molecule-specific absorption signal with a lock-in amplifier.

This widely used method greatly enhances the sensitivity and resolution of microwave spectroscopy. As examples of the use of microwave spectroscopy to determine equilibrium geometry, consider the molecules NF3 and CH 3C1, which both have C3 v symmetry.

If the N—F bond length is denoted land the bond angle is 0, the use of Eq. Hence, the microwave spectrum of NF 3 yields no measurement of the rotational constant C.

Since two structural parameters land 0 enter in the rotational moment a single microwave spectrum cannot determine the geometry of NF 3. In this manner, land 0 were determined to be 1. The expression for the moment about the figure axis is analogous to that for NF3 in Eq. In this case, microwave spectra of three isotopic species must be made in principle to determine l 1 2 , and O. The assumption that molecular geometries are insensitive to isotopic substitution is not always justified; the C—H bond distance in CH 3 C1 is in fact some 0.

Hydrogen atom positions appear to be especially prone to isotopic geometry variation. Pauling and E. Townes and A. In this problem, we derive the expressions given in Eqs.

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It is already clear from Fig. Hence, the problem reduces to finding in terms of the Euler angles. The unit vectors a and fis' lie in a tilted plane perpendicular to 5. Obtain the angular momentum commutation relations 5. Classify each of the following molecules by point group and by rotor type spherical, symmetric, or asymmetric.

Which of them will exhibit a microwave spectrum? The microwave spectroscopic bond lengths land bond angles 0 for the C3, molecules PF3 and P35C1 3 are given below. If the N-H bond length is independently known to be 1. Consider a hypothetical molecule AB sC which is known to exhibit C4, geometry it would be an Oh molecule if atoms B and C were identical and all bond lengths were equal. For what H-N-H bond angle would C3. NH3 become an accidental spherical top? Assign the J values for these transitions and calculate the rotational constant B for this isotope.

Determine the directions a, of the principal axes. We have already seen in the preceding chapter that rotations of nonlinear polyatomics about their center of mass may be described in terms of the three Euler angles 4 , 0, and x. Three additional coordinates are required to describe spatial translation of a molecule's center of mass. Hence, there will be 3N — 6 independent vibrational coordinates in a nonlinear polyatomic molecule.

In a linear polyatomic molecule, the orientation may be given in terms of two independent angles 0 and 4. Linear polyatomics therefore exhibit 3N — 5 rather than 3N — 6 independent vibrational coordinates. By their nature, such vibrational coordinates involve collective, oscillatory nuclear motions that leave the molecule's center of mass undisplaced.

It is of interest to know the relative nuclear displacements and vibrational' frequencies associated with these coordinates, because they are instrumental in predicting band positions and intensities in vibrational spectroscopy. An important question arises as to whether vibrational motion occurs in modes that are dynamically uncoupled.

If it does, an isolated molecule initially having several quanta of vibrational energy in a particular mode will not spontaneously redistribute this energy into some of its other modes, even though such a process may conserve energy. Hence, the form of the modes has implications not only for vibrational structure as manifested by energy levels and selection rules in infrared spectra , but also for understanding dynamical processes like vibrational energy transfer in collisions and intramolecular vibrational relaxation IVR.

These phenomena are currently well-pursued research areas, and can only be understood with a seasoned physical appreciation of vibrational modes. In molecules with harmonic potential energy functions, vibrational motion occurs in normal modes that are mutually uncoupled. In molecules with sufficient symmetry, the use of group theory simplifies the procedure of obtaining the normal mode frequencies and coordinates.

We obtain El selection rules for vibrational transitions in polyatomics, and consider the rotational fine structure of vibrational bands.

We finally treat breakdown of the normal mode approximation in real molecules, and discuss the local mode formulation of vibrational motion in polyatomics. In analogy to what was done for diatomics Eq. If we now make the harmonic approximation by ignoring third- and higher order terms in Eq. Note that motion in any mass-weighted coordinate th is coupled to motion in all other coordinates j, when bu 0 0, so that polyatomic vibrations generally involve all of the nuclei moving simultaneously in collective motions.

Writing the trial solubecause the function sin t tions ni in the form of Eq. The simultaneous equations 6. It turns out that six of the eigenvalues of 6. The only approximation we have used in this treatment was to break off the Taylor series 6. In a full three-dimensional treatment, such a molecule would have three translations, two rotations, and four vibrational modes.

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Constraining the nuclei to one-dimensional motion along the axis will eliminate two of the translations, both rotations, and two bending vibrational modes which involve nuclear displacements perpendicular to the axis. The nuclear masses and Cartesian coordinates are shown in Fig. A more accurate potential could also include some interaction between the end atoms, and so depend on 3 — as well. We can gain considerably more insight by recasting this treatment in the form of a matrix eigenvalue problem, because then we can exploit several wellestablished theorems from matrix algebra [2].

The coefficients bii in the potential energy function 6. Each of the roots 2k of this secular determinant can be substituted back into Eqs. All of the information about the form of the vibrational motions is contained in the normalized eigenvectors lk , whose elements reflect the relative displacements in each of the mass-weighted coordinates.

The number Ck in Eq. Finding the roots 21 , , 2 3N of the secular determinant 6. Each of the column vectors lk Eq. Lengths of arrows give relative displacements in mass-weighted not Cartesian coordinates. In a similar way, the normalized eigenvectors for the other two modes can be derived as 6. It is apparent from Eq. It is clear from the form of eigenvector 12 that mode 2 is an asymmetric stretching mode Fig. Since all of the nuclei move in mode 2, 11 2 depends on mB as well as on mA.

The potential energy function 6. Under this linear transformation, 2V remains harmonic, because Eq. We may appreciate the physical significance of such normal coordinates by substituting Eqs. This separation of motion into noninteracting normal coordinates is possible only if V contains no cubic or higher-order terms in Eq. Anharmonicity will inevitably couple motion between different vibrational modes, and then the concept of normal modes will break down.

In the normal mode approximation, no vibrational energy redistribution can take place in an isolated molecule. The formal solutions to the second-order differential equations 6. These frequencies are identical to those found by diagonalizing the secular determinant 6.

To find the actual form of the normal coordinates Q, we note that from Eq. In fact, only 3N — 6 such coordinates are required to fully specify the potential 3N — 5 in linear molecules: Such a truncated set of 3N — 6 3N — 5 generalized coordinates is called an internal coordinate basis, and is commonly denoted S. To illustrate how an internal coordinate basis may be used to evaluate normal modes, we consider the bent H 20 molecule in Fig.The use of Eq. The way we have labeled the NaH potential energy curves in Fig.

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No trivia or quizzes yet. Berry, S. Other Editions 2. Constraining the nuclei to one-dimensional motion along the axis will eliminate two of the translations, both rotations, and two bending vibrational modes which involve nuclear displacements perpendicular to the axis. Some of the group operations transform the vectors x, y into linear combinations of x and y, so that x, y form a basis for a two-dimensional JR of C op t,.

With atomic orbital basis sets of sufficient size, close agreement is obtained with experiment. The diatomic electronic Hamiltonian in the clamped-nuclei approximation Eqs.