Vibrational Spectroscopy for Pharmaceutical Analysis. Part IX. Introduction and Origin of NIR Bands Rodolfo J. Romañach, Ph.D. NIR Fundamentals: Electromagnetic Spectrum. 12,500 cm 1 (800 nm). 4,000 cm 1 (2500 nm). Frequency (cm 1 ). Ray. X – Ray. Ultraviolet. v i s i b l e.
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Part IX. Introduction and Origin of NIR Bands
Rodolfo J. Romañach, Ph.D.
12,500 cm1 (800 nm)
4,000 cm 1 (2500 nm)
Frequency
(cm1)
Ray
X – Ray
Ultraviolet
visible
NIR
MIR
FIR
ESR
NMR
Region
Infrared
Microwave
Radio, TV Waves
NuclearTransitions
InnerShell Electronic
Transitions
ValanceElectron
Transitions
MolecularVibrations
MolecularRotations
SpinOrientation in MagneticField
Interaction
Wavelength
(m)
Courtesy Bruker Optics
Far Infrared: 650 – 25 cm1
Mid Infrared: 4000 – 650 cm1
Near Infrared: 12800 – 4000 cm1(0.8  2.5 m, or 800 2500 nm)
A vibration occurs when the dipole moment of the molecule changes, and the molecule interacts with radiation equal to the frequency of vibration.
Infrared SpectroscopyProcess Applications: Spectroscopy
Excellent analytical method for the study of solids.
Spectra may be obtained in noninvasive manner.
Remote sampling is possible (good for hazardous materials).
Possibility of using it in a wide range of applications (physical and chemical), and viewing relationships difficult to observe by other means.
Laboratory Applications:
Sample preparation is not required leading to significant reductions in analysis time.
Waste and reagents are minimized.
Advantages of NIRThe molecule can be thought of as mass m1 and m2 connected by a spring. At equilibrium, the distance between the two masses is r0. If the molecule is stretched by an amount r = x1 + x2, then a restoring force, F, is produced. If the spring is released, the system will vibrate around the equilibrium position. According to Hooke’s Law, for small deflections the restoring force is proportional to the deflection:
F = k . r
Since the force acts in a direction opposite to the deflection, the proportionality constant, or force constant, k, is negative in sign. The force constant is called the spring constant in the mechanical model, whereas in a molecule the force constant is a measure of the bond strengthbetween the atoms.
Courtesy Bruker Optics
For the harmonic oscillator model, the potential energy well is symmetric. According to quantummechanical principles molecular vibrations can only occur at discrete, equally spaced, vibrational levels, where the energy of the vibration is given by:
Ev=(v + ½) h v = 0, 1, 2, 3, ...
Where h is Planck’s constant and v is the vibrational quantum number. Even in case of v = 0, which is defined as the ground vibrational level, a molecule does vibrate:
Ev= ½ h
Potential energy curve for a harmonic oscillator
Based on Bruker Optics Slide
When absorption occurs, the molecule acquires a clearly defined amount of energy, (E = h ), from the radiation and moves up to the next vibrational level (v = +1).
For a harmonic oscillator, the only transitions permitted by quantum mechanics are up or down to the next vibrational level (v = 1).
Vibration TheoryIf the molecule moves down to the next vibrational level (v = 1), a certain amount of energy is emitted in the form of radiation. This is called emission.
Based on Bruker Optics Slide
 defined amount of energy, (E = h
+

+

+
Change in Dipole Moment during Molecular VibrationsH
Cl
Sometimes see cm1 :
In NIR spectroscopy we are usually interested in observing differences between spectra, and not in the interpretation of NIR spectra.
Normal vibration modes of water. defined amount of energy, (E = h
Overtones and combination bands of water (Fig. 9.1), Near Infrared Spectroscopy, Ed. Siesler, Ozaki, Kawata, Heise, Wiley 2002.
A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, “The Origin of Band Positions and Widths in Near Infrared Spectra”, in Near InfraRed Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Edited K.I. Hildrum, T. Isaakson, T. Naes, and A. Tandberg, Ellis Horwood, 1992.
A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, “The Origin of Band Positions and Widths in Near Infrared Spectra”, in Near InfraRed Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Edited K.I. Hildrum, T. Isaakson, T. Naes, and A. Tandberg, Ellis Horwood, 1992.
A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, “The Origin of Band Positions and Widths in Near Infrared Spectra”, in Near InfraRed Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Edited K.I. Hildrum, T. Isaakson, T. Naes, and A. Tandberg, Ellis Horwood, 1992.