Vibrational Spectroscopy, Chemistry tutorial


Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ). In fact, historically, spectroscopy termed to utilize of visible light dispersed according to its wavelength, for example via a prism. Later the idea was enlarged deeply to include any measurement of a quantity as function of either wavelength or frequency. Therefore it as well can refer to a response to an alternating field or fluctuating frequency (ν). A further extension of the scope of the definition added energy (E) as a variable, once the extremely close relationship E = hν for photons was realized (h is the Planck constant). A plot of the response as a function of wavelength-or more usually frequency-is referred to as a spectrum.

Definitions of Vibration Spectroscopy

A molecular vibration happens when atoms in a molecule are in periodic motion while the molecule as a whole has steady translational and rotational motion. The occurrence of the periodic motion is known as a vibration frequency. A nonlinear molecule through n atoms has 3n-6 normal modes of vibration, while a linear molecule has 3n-5 normal modes of vibration as rotation about its molecular axis can't be examined. A diatomic molecule therefore has one usual mode of vibration.

The normal modes of vibration of polyatomic molecules are independent of each other, each involving simultaneous vibrations of dissimilar parts of the molecule. A molecular vibration is excited whenever the molecule absorbs a quantum of energy, E, analogous to the vibration's frequency, ν, according to the relation E=hν, where h is Planck's constant. A fundamental vibration is  excited  when  one  these quantum  of  energy  is  absorbed  via  the molecule in its ground state. When 2 quanta are absorbed the 1st overtone is excited, and so on to higher overtones.

To a 1st approximation, the motion in a normal vibration can be described as a kind of simple harmonic motion. In this approximation, the vibrational energy is a quadratic function (parabola) with respect to the atomic displacements and the first overtone has twice the frequency of the fundamental. In reality, vibrations are an harmonic and the first overtone has a frequency that is slightly lower than twice that of the fundamental. Excitation of the higher overtones involves progressively less and less additional energy and eventually leads to dissociation of the molecule, as the potential energy of the molecule is more like a Morse potential.

The vibrational states of a molecule can be probed in a variety of ways. The most direct way is through infrared spectroscopy, as vibrational transitions typically require an amount of energy that corresponds to the infrared region of the spectrum. Raman spectroscopy, which typically uses visible light, can also be used to measure vibration frequencies directly.

Vibrational excitation can occur in conjunction with electronic excitation  (vibronic  transition),  giving  vibrational  fine  structure  to electronic  transitions,  particularly with  molecules  in  the  gas state. Simultaneous excitation of a vibration and rotations gives rise to vibration-rotation spectra.

Vibrational coordinates

The coordinate of a normal vibration is a combination of changes in the positions of atoms  in the molecule. When the vibration is excited the coordinate changes sinusoidally with a frequency ν, the frequency of the vibration.

Internal coordinates

 Internal coordinates are of the following types, illustrated with reference to the planar molecule ethylene, Stretching: a change in the length of a bond, such as C-H or C-C Bending:  a change in the angle between two bonds, such as the HCH angle in a methylene group

Rocking: a change in angle between a group of atoms, such as a methylene group and the rest of the molecule. Wagging: a change in angle between the plane of a group of atoms, such as a methylene group and a plane through the rest of the molecule,

Twisting: a change in the angle between the planes of two groups of  atoms,  such  as  a  change  in  the  angle  between  the  two methylene groups.

Out-of-plane: Not present in ethene, but an example is in BF3 when the boron atom moves in and out of the plane of the three fluorine atoms.

In a rocking, wagging or twisting coordinate the angles and bond lengths within the groups involved do not change. Rocking is distinguished from wagging by the fact that the atoms in the group stay in the same plane.

In ethene there are 12 internal coordinates: 4C-H stretching, 1C-C stretching, 2H-C-H bending, 2 CH2 rocking, 2CH2 wagging, 1 twisting. Note that the H-C-C angles can't be utilized as internal coordinates as the angles  at  each  carbon  atom  cannot  all  increase  at  the  same  time.

Symmetry adapted coordinates

Symmetry-adapted coordinates may be created by applying a projection operator to a set of internal coordinates.  The projection operator is constructed with the aid of the character table of the molecular point group. For example, the four (un-normalized) C-H stretching coordinates of the molecule ethene are given by

Qs1 = q1 + q2 + q3 + q4

Qs2 = q1 + q2 - q3 - q4

Qs3 = q1 - q2 + q3 - q4

Qs4 = q1 - q2 - q3 +q4

Where q1-q4 are the internal coordinates for stretching of each of the four C-H bonds.  Illustrations of symmetry-adapted coordinates for most small molecules can be established in Nakamoto.

Normal coordinates

A normal coordinate, Q, might sometimes be constructed straight as a symmetry-adapted   coordinate. This is possible when the normal coordinate belongs exclusively to a particular irreducible illustration of the molecular point group. For instance, the symmetry-adapted coordinates for bond-stretching of the linear carbon dioxide molecule, O=C=O is together normal coordinates:

Symmetric  stretching:  the  sum  of  the  two  C-O  stretching coordinates;  the  two  C-O  bond  lengths change by the  same amount and the carbon atom is stationary. Q = q1 + q2 asymmetric stretching: the difference of the two C-O stretching coordinates; one C-O bond length enhances while the other reduces. Q = q1 - q2

When two or more normal coordinates belong to the similar irreducible illustration of the molecular point group (colloquially, have the similar symmetry) there is 'mixing' and the coefficients of the combination can't be determined a priori. For instance, in the linear molecule hydrogen cyanide, HCN, The 2 stretching vibrations are:

  • Principally C-H stretching by a little C-N stretching; Q1 = q1 + a q2 (a << 1)
  • principally C-N stretching through a little C-H stretching; Q2 = b q1 + q2 (b << 1)

The coefficients  a  and  b  are  establish  via  performing  a  full  normal coordinate analysis via means of the Wilson GF technique.

Newtonian mechanics

Perhaps surprisingly, molecular vibrations can be treated using Newtonian mechanics, to compute the accurate vibration frequencies. The essential assumption is that each vibration can be treated as though it corresponds to a spring. In the harmonic estimate the spring obeys Hooke's law: the force needed to expand the spring is comparative to the extension. The proportionality steady is recognized as a force constant, k. The anharmonic oscillator is considered elsewhere.

Force = -KQ

By Newton's 2nd law of motion this force is as well equivalent to a 'mass', m, times acceleration.

Force = md2Q/dt2

Because this is one and the similar force the ordinary differential equation follows.

md2Q/dt2+KQ = 0

The solution to this equation of easy harmonic motion is

Q(t) = A cos (2πVt);  v = 1/2π √k/m

A is the maximum amplitude of the vibration coordinate Q. It remains to describe the 'mass', m. In a homonuclear diatomic molecule these as N2, it is half the mass of one molecule. In a heteronuclear diatomic molecule, AB, it is the decreased mass, µ specified via

1/μ  = 1/mA + 1/mB

Utilize of the reduced mass ensures that the centre of mass of the molecule isn't influenced by the vibration. In the harmonic approximation the potential energy of the molecule is a quadratic function of the normal coordinate. It follows that the force-constant is equivalent to the second derivative of the potential energy.

F = δ2V/δQ2

When two or more normal vibrations have the similar symmetry a full normal coordinate analysis must be executed. The vibration frequencies, νi are attained from the eigenvalues, λi, of the matrix produce GF. G is a matrix of numbers derived from the masses of the atoms and the geometry of the molecule. F is a matrix obtained from force-constant values.  Factors concerning the determination of the eigenvalues can be originated in.

Quantum mechanics

In the harmonic approximation, the potential energy is a quadratic function of the normal coordinates.

Solving the Schrödinger wave equation, the energy states for each usual coordinate are specified via

En = ( n +1/2) h1/2 π √k/m

where n is a quantum number that can obtain values of 0, 1, 2 ... The dissimilarity in energy  when n changes via 1 are consequently equal to the energy derived using classical mechanics.. Knowing the wave functions, assured selection rules can be formulated. For instance, for a harmonic oscillator transitions are permitted only when the quantum number n transforms via one,

Δn = +-1

But this doesn't relate to an anharmonic oscillator; the observation of overtones is only possible because vibrations are anharmonic. Another consequence of anharmonicity is that transitions such as between states n=2 and n=1 have slightly less energy than transitions between the ground state and 1st excited state. These transitions provide increase to a hot band.


In an infrared spectrum the intensity of an absorption band is proportional to the derivative of the molecular dipole moment through respect to the normal coordinate. The intensity of Raman bands based on polarisability.

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