10/11/2005 1 Solid State Vibrational Spectroscopy November 2011 University of Puerto Rico ENGINEERING RESEARCH CENTER FOR S TRUCTURED O RGANIC P ARTICULATE

10/11/2005

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Solid State Vibrational SpectroscopySolid State Vibrational Spectroscopy

November 2011November 2011

University of Puerto RicoUniversity of Puerto Rico

ENGINEERING RESEARCH CENTER FOR

STRUCTURED ORGANIC PARTICULATE SYSTEMS

RUTGERS UNIVERSITYPURDUE UNIVERSITYNEW JERSEY INSTITUTE OF TECHNOLOGYUNIVERSITY OF PUERTO RICO AT MAYAGÜEZ

NIR Fundamentals and NIR Fundamentals and “a little more…”“a little more…”

Graduate Students –

Yleana M. ColónAndres Román Daniel Mateo

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Electromagnetic SpectrumElectromagnetic Spectrum

NuclearTransitions

SpinOrientation in

MagneticField

MolecularRotations

MolecularVibrations

ValanceElectron

Transitions

InnerShell

Electronic Transitions

-Ray

Radio, TV WavesMicrowaveInfrared

NMRESRFIRMIR

X – Ray

visible

NIR

Ultraviolet

Inte

racti

on

Reg

ion

108 107 106 105 104 103 102 101 1 10-1 10-2 10-3

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 11

Frequency(cm-1)

Wavelength(nm)

12,800 cm-1 (780 nm) 4,000 cm -1 (2500 nm)

Courtesy of Bruker Optics

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SpectroscopySpectroscopy

… is based on the interaction of electromagnetic waves and matter.

Spectral Absorptions

• Microwave Rotation of molecules• IR Fundamental molecular vibrations• NIR Overtones and combinations of IR• UV / Visible Electronic transitions• X-Ray Core electronic transitions in the atom

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Units of spectra- nm, Units of spectra- nm, m, cmm, cm-1-1

Sometimes see cm-1 : 10,000 cm-1 = (1/10,000) cm or

0.0001 cm = 1 m = 1000 nm 6,000 cm-1 = (1/6000) cm or

0.000167cm = 1.67 m = 1670 nm 5,000 cm-1 = (1/5000) cm or 0.0002

cm = 2 m = 2000 nm 4000 cm-1 = (1/4000) cm or 0.00025

cm = 2.5 m = 2500 nm.

1cm = 1 x 107 nm 1 nm = 1 x 10-3

µmWhere cm-1 = 1 x 107 # nm

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What is Infrared Spectroscopy?What is Infrared Spectroscopy?

• Sir Isaac Newton set up an experiment in which a beam of sunlight passed through window shutters into a dark room.

(Algodoo v1.8.5)

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• Much later, Frederic William Herschel, the discoverer of planets and many other celestial objects, imagined the existence of other components of white light, outside the visible region.

• The region after the red part is called Infrared Region.

• Herschel set up an experiment to measure this radiation under the red which is not visible to human eye, thus he used a thermometer.

What is Infrared Spectroscopy? (cont)What is Infrared Spectroscopy? (cont)

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• In March of 1800 Herschel placed a sample of water in the path of the beam, and the difference of temperature was then associated with absorption.

What is Infrared Spectroscopy? (cont)What is Infrared Spectroscopy? (cont)

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Mid-IRMid-IR

• Today, the mid-infrared region is normally defined as the frequency range of 4000 cm-l to 400 cm-1.

• The upper limit is more or less arbitrary, and was originally chosen as a practical limit based on the performance characteristics of early instruments.

• The lower limit, in many cases, is defined by a specific optical component, such as, a beamsplitter with a potassium bromide (KBr) substrate, which has a natural transmission cut-off just below 400 cm-1.

J. Coates, “Vibrational Spectroscopy: Instrumentation for Infrared and Raman Spectroscopy”, Applied Spectroscopy Reviews, 1998, 33(4), 267 – 425.

Infrared

FIRMIRNIR

104 103 102 101Frequency(cm-1)

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Far IRFar IR• The region below 400 cm-1, is now generally classified as

the far infrared, characterized by low frequency vibrations typically assigned to low energy deformation vibrations and the fundamental stretching modes of heavy atoms.

• There is only one IR-active fundamental vibration that extends beyond 4000 cm-1, and that is the H-F stretching mode of hydrogen fluoride.

• The original NIR work was with extended UV-Vis spectrometers. Indicates that mid and NIR should be considered the same field.

J. Coates, “Vibrational Spectroscopy: Instrumentation for Infrared and Raman Spectroscopy”, Applied Spectroscopy Reviews, 1998, 33(4), 267 – 425.

NIR

Infrared

FIRMIRNIR

104 103 102 101Frequency(cm-1)

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Spectroscopy Provides InformationSpectroscopy Provides Information

• Presence of functional groups

• Variation of functional groups, or elements throughout a surface (chemical information)

• Differences in the crystal structure of compounds

• Qualitative and quantitative analysis

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Mid-IMid-IRR Spectroscopy widely used in: Spectroscopy widely used in:

• Identification of pharmaceutical raw materials and finished products.

• Combination with MS and NMR to determine structure of process impurities and degradation products.

• Characterization of natural products, use of GC/FT-IR. • Forensic analysis, IR-Microscopy. • Environmental analysis: GC/FT-IR.• Surface analysis, diffuse reflectance, attenuated total

reflectance, grazing angle.• Studies of protein structure and dynamics.

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NINIRR Spectroscopy used in: Spectroscopy used in:

• Identification of solid sample forms• Physical characteristic analysis of solid samples

such as particle size and packing density of a material.

• Provide information on moisture content• Monitor process parameters such as flow rates,

blending process end time and even by-products.• Non-invasive remote monitoring of different

processes.• Medical uses such as measurement of the amount

of oxygen content of hemoglobin.

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• The physical origin of molecular vibrations are due to:

- absorption of radiation by a material (MIR and NIR techniques)- scattering of radiation by a material (Raman technique)

Molecular Vibrational SpectroscopyMolecular Vibrational Spectroscopy

• Vibrational frequencies are very sensitive to the structure of the investigated compound

- structure elucidation, finger print spectra

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Hooke’s Law

Vibrating bond

Vibratingspring

F = -k x

F – restoring force exerted by the springk – rate of spring constantx – displacement of the spring from equilibrium

In order to understand the absorption phenomenon, let’s compare a molecule to the vibration of a spring,

m1 m2

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Simple Harmonic OscillatorSimple Harmonic Oscillator

For diatomic molecules it is possible to calculate:

Potential energy

k – force constant of the bond, r – inter nuclear distance during vibration,re – equilibrium inter nuclear distance, q – displacement coordinate

Vibrational frequency

-or-

Wavenumber

m – reduced massc – speed of light

Energy curve for vibrating spring

where,

V – potential energy E – total energyK – kinetic energy

“as a function of position”

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Quantized Vibration TheoryQuantized Vibration Theory

where,h – Plank’s constant0 - vibrational frequencyn – quantum number

Molecular vibrations have:

- Discrete energy values,

- Energy levels are equally spaced,

- Each energy level is defined by n quantum number whose integers values are 0, 1, 2,…

- Only effective for relatively small deformations in the “spring”.

In the harmonic oscillator model, the potential energy well is symmetric.

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Vibration TheoryVibration Theory

On the basis of the equation above it is possible to state the following:

1) The higher the force constant k, i.e., the bond strength, the higher the vibrational frequency (in wavenumbers).


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2) The larger the vibrating atomic mass, the lower the vibrational frequency in wavenumbers.


Vibration TheoryVibration Theory

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+ -

+ -

+ -

H Cl

A Molecule Absorbs Infrared Energy when:A Molecule Absorbs Infrared Energy when:

• Change in dipole moment must occur.

• The dipole moment is a measure of the degree of polarity of molecule (magnitude of the separated charges times the distance between them).

• A measurement of degree of unequal distribution of charges in molecule.

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• HBr does have a dipole change as it stretches, the intensity of the absorption is related to the magnitude of the dipole change. This dipole aligns with the electric field of the beam of light, then the light is absorbed.

Molecular DipoleMolecular Dipole

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Band Intensity in IR and SpectrumBand Intensity in IR and Spectrum

• Band intensity depends on the rate of change of dipole moment during absorption of IR light.

• Stronger bands occur when the change in dipole moment is greatest.

• A spectrum is a plot that shows the absorption or reflection of radiation as wavelength or frequency of the radiation is varied.

A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, in Near Infra-Red Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Ellis Horwood, 1992.

3rd overtone CH3-Sym

3rd overtone CH2-Sym

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Molecules that absorb Infrared energy Molecules that absorb Infrared energy vibrate in two modes:vibrate in two modes:

Stretching is defined as a continuous change in the inter-atomic distance along the axis of the bond between two atoms.

Bending is defined as a change in bond angle

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Molecular SpectroscopyMolecular Spectroscopy

• This situation is simplified considering every functional group in the molecule independently.

• Each functional group has a set of group frequencies which correspond to the normal modes for the group.

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Example: The fundamental vibrations for water, H2O are given in below. Water which is nonlinear, has three fundamental vibrations.

Degrees of FreedomDegrees of Freedom

Molecule Degrees of freedom

Non linearLinear

3N -6 3N- 5

Symmetricstretching

Anti-symmetric stretching

Symmetric Bending

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Molecular VibrationMolecular Vibration

• Hexane C6H14 has 20 atoms (3(20)-6 = 54) normal modes, it is very difficult to analyze each mode.

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NIR bandsNIR bands

• O-H, N-H, C-H, S-H bonds etc., are NIR strong absorbers since they have the strongest overtones as the dipole moment is high

• R-H stretch or R-H stretch / bend form most NIR bands

•The overtone and combination bands are 10 – 100 X less intense than the fundamental bands in mid-IR.

•Differences in spectra are usually very subtle. Instruments have a high signal to noise ratio.

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Combination BandsCombination Bands

• The frequency of a combination is approx. the sum of the frequencies of the individual bands.

• Combinations of fundamentals with overtones are possible as well as well as fundamentals involving two or more vibrations.

• The vibrations must involve the same functional group and have the same symmetry.

C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer. Assn. of Cereal Chemists; 2 nd Ed. (November 15, 2001) .

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NIR & AnharmonicityNIR & Anharmonicity

• A number of bands are observed that cannot be explained on the basis of the harmonic oscillator.

• A more accurate model of a molecule is given by the anharmonic oscillator.

The allowed energy levels for an anharmonic oscillator have to be modified:

Where χ is the anharmonicity constant.• The potential energy curve is represented by an asymmetric

Morse function.

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Morse Potential – Morse Potential – Simple Anharmonic OscillatorSimple Anharmonic Oscillator

Transition Name Rangen=0 n=1 Fundamental mid-IRn=0 n=2 1st Overtone mid-NIRn=0 n=3 2nd Overtone NIRInteraction of two Combination NIRor more differentvibrations

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2876cm-1-C-H2 sym

2930cm-1-C-H2 asym

2962cm-1-C-H3 asym

2885cm-1-C-H3 sym

ExampleExample

Fundamental Vibration

1st Overtone

2nd Overtone

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Calculations of overtones and Calculations of overtones and anharmonicitiesanharmonicities

The wave number position of the fundamental position v1 or an overtone vn of the anharmonic oscillator can be given by:

v0 is not directly accessible from the absorption spectra only the wave number v1, v2 …. may be obtained.

H.W. Siesler, “Basic Principles of Near Infrared Spectroscopy”, In Handbook of Near Infrared Analysis Ed. D.A. Burns and E.W. Ciurczak, 3rd ed., CRC Press, Boca Raton, FLA.

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Fermi Resonance

Is an interaction between transitions of the same symmetry that occur at approximately the same wavenumber as that of a fundamental vibration.

NIR gets complicatedNIR gets complicated

Mid IR spectrum magnesium stearate solid sample

C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer. Assn. of Cereal Chemists; 2nd Ed. (November 15, 2001) .

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• Treats a molecule as if it was made up of a set of equivalent diatomic oscillators:– As the stretching vibrations are excited to high energy levels, the

anharmonicity term χν0 tends to overrule the effect of interbond coupling and the vibrations become uncoupled vibrations and occur as “local modes”.

Local Mode

NIR continues to complicateNIR continues to complicate

C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer. Assn. of Cereal Chemists; 2nd Ed. (November 15, 2001) .

NR spectrum n-pentane liquid sample

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• May lead to the presence of two bands where only one would be expected.

• Resonance between higher order overtone modes and the more intense combination bands.

• Particularly evident for X-H vibrations since interacting energy levels are close together and vibrational anharmonicity is high.

• Provides a complicating effect in NIR spectra, different from the simplifying effect that would be expected from local modes.

Darling-Dennison Resonance

NIR complicates even moreNIR complicates even more

C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer. Assn. of Cereal Chemists; 2nd Ed. (November 15, 2001) . And L. Bokovza in Chapter 2 of Near Infrared Spectroscopy, H. W. Siesler, Y. Ozaki, S. Kawata, H.M. Heise, Wiley, VCH.

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Electronic NIR SpectroscopyElectronic NIR Spectroscopy

• Electronic NIR bands– Involves the change in the electronic state of a molecule

(movement of an electron between different energy levels)

• Electronic transitions are generally of higher energy than vibrational transitions– higher-energy visible and ultraviolet regions of the

spectrum

• Electronic NIR bands are affected by intermolecular interactions and sample state.

C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer Assn of Cereal Chemists; 2nd Ed. (November 15, 2001) .

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Electronic NIR SpectroscopyElectronic NIR Spectroscopy


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The NIR Complicating FactorThe NIR Complicating Factor

1. Multitude of overtone and combination bands produced from only a few vibrations

2. Large number of NIR-active groups (e.g CH, NH, OH, and C=O), each of which contributes its own set of overtone and combination bands

3. Possibility of resonances between vibrational modes. which results in bands that cannot be assigned to "pure vibrations” in the molecule

4. Possibility of several molecular configurations, each of which could produce a slightly different spectrum.

This complications are also an advantage:1. The complexity of NIR spectra help to identify every single difference

(Chemical and Physical).


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The NIR Complicating Factor (The NIR Complicating Factor (CHClCHCl33))


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Understanding Hydrogen Bonding on Understanding Hydrogen Bonding on vibrational spectra vibrational spectra

1st overtone region for O-H bond stretching and free surface water

Miller, C.E. (2001). Chemical Principles of Near-Infrared Technology. In: Williams, P. and Norris, K. Near-Infrared Technology in the Agricultural and Food Industries. 2nd ed. Minnesota, USA: American Association of Cereal Chemists, Inc. St. Paul, p19-36.

Solid minerals

Wavenumber (cm-1)

Resp

onse

Free surface O-H

Hydrogen bonded surface O-H

donor H : acceptor

donor H ----- : acceptor

Lone pair of e-

Hydrogen bond

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MIR and NIR Absorption BandsMIR and NIR Absorption Bands

Typical MID IR Spectra Typical NIR Spectra

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Ab

sorb

an

ce

Wavenumber cm-1

Oleic Acid

NIR

MIR

MIR and NIR Absorption BandsMIR and NIR Absorption Bands


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IR InstrumentationIR Instrumentation

Near IRMid IR

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Advantages of Advantages of Near Infrared Near Infrared Spectroscopy over Mid-IRSpectroscopy over Mid-IR• No sample preparation required leading to significant reductions in

analysis time and waste and reagents.(non-destructive testing). • Possibility of using it in a wide range of applications (physical and

chemical), and viewing relationships difficult to observe by other means.

• In-line monitoring of process.• Spectrum may be used to identify the formulation and to quantify

drug in the formulation.*

*M. Blanco, J. Coello, A. Eustaquio, H Iturriaga, and S. Maspoch, Development and Validation of a Method for the Analysis of a Pharmaceutical Preparation by Near-Infrared Diffuse Reflectance Spectroscopy, Journal of Pharmaceutical Sciences, 1999, 88(5), 551 – 556.

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Infrared EquipmentInfrared Equipment

• Classical (Dispersive)

Reference

Sample

Diffraction Grating

ThermocoupleThermocouple(Detector)(Detector)

Spectrum

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• Modern (Fourier Transform)

Infrared EquipmentInfrared Equipment

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Visualizing the Interaction of Light & Visualizing the Interaction of Light & ParticlesParticles

• No sample preparation in NIR spectroscopy.

• Light interactions with particles.

• Need to learn to visualize the particles and their interaction with light.

J.L. Ramirez, M. Bellamy, R.J. Romañach, AAPS Pharmscitech, 2001, 2(3), article 11.

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Diffuse ReflectanceDiffuse Reflectance

Tramittance

Common NIR TechniquesCommon NIR Techniques

Detector for transmission

Light may be remitted, transmitted & absorbed

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Scattering = reflection + refraction + diffraction. Scattering = reflection + refraction + diffraction.

Isc = Iin (θ, λ, d, n) The intensity of scattered light is a function of the scattering angle, the wavelength λ, particle size d, and the refractive index n.

No interaction undeviated ray

Diffracted rayReflected ray

Transmitted after internal reflection

Transmitted ray

Refracted ray

Contributes to remission

Contributes to transmission

Dahm DJ, Dahm KD. 2001. The Physics of Near-Infrared Scattering. In Williams P, Norris K, editors. Near Infrared Technology in the Agricultural and Food Industries, 2nd ed., Saint Paul: American Association of Cereal Chemists, p 19-37.

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Scattering and Diffuse Scattering and Diffuse ReflectanceReflectance

The radiation that comes back to the entry surface is called diffuse reflectance.

Light propagates by scattering.

As light propagates, both scattering and absorption occur, and the intensity of the radiation is reduced.

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Smaller particle sizes

More remission, less transmissionLarger particle sizes

Less remission, more transmission

Prepared by Martha Barajas Meneses, MS 2006.

Low Scattering

High scattering

Visualizing light interactionVisualizing light interaction

*Multiple path lengths are possible

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Subtle Differences,Valuable Info.Subtle Differences,Valuable Info.

• Cristallinity – high degree of molecular order (narrower bands)

Amorphous – no molecular order (broader bands)

Crystalline sugar

Amorphous sugar

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Particle size effectParticle size effect

Jackeline I. Jerez, Sept. 2009

Changes in spectra due to physical Changes in spectra due to physical properties of a materialproperties of a material

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Changes in spectra due to physical Changes in spectra due to physical properties of a materialproperties of a material

Tablet Packing density

Ropero, J. et al. 2011. Near-Infrared Chemical Imaging Slope as a New Method to Study Tablet Compaction and Tablet Relaxatio. Appl. Spect. 65, 4.

NIR spectra of pure lactose tablet at different packing density

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Changes in spectra due to variation in Changes in spectra due to variation in analysisanalysis

Resp

on

se

Wavelength (nm)

1.3 cm

0.3 cm

0.6 cm

0.8 cm

1.0 cm

Probe-sample distance

NIR spectra of pure lactose analyzed at different distances

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Changes in sugar spectra due variation Changes in sugar spectra due variation in temperature in temperature

Wavenumber (cm-1)

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NIR aspects as functions of wavelengthNIR aspects as functions of wavelength

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CDI Lab Scale NIRS system, www.controldevelopment.com

NIR ApplicationsNIR Applications

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Powder & Solids Probe with liquid attachment

Powder and Solids Probe – Courtesy Bruker Optics

Extra-long immersion depth: 12”

Diffuse Reflection Probe Schematic

IR Source

IR Energy Sample

Delivery Fiber Bundle

Collection Fiber Bundle

Reflected IR Energy

Detector

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Diffuse Reflectance ExamplesDiffuse Reflectance Examples

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Diffuse Reflectance for Flowing PowderDiffuse Reflectance for Flowing Powder

I detected = 1/c x Ireflected

Adetected = - log (Rdetected) = - log (Idetected/I0)

= log c + log (I0/Ireflected) = c’ + A

J. Ropero, L. Beach, M. Alcalà, R. Rentas, R.N. Davé, R.J. Romañach, Journal of Pharmaceutical Innovation, J. Pharm. Innov. 2009, 4(4), 187-197.

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TransflectionTransflection

Source

Detector Mirroror Reflector

Analyte

I0

Itrans

Fiber probe for solids

mirror

Courtesy Bruker Optics

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Transflectance using gold plate reflector.Transflectance using gold plate reflector.

M. Blanco, M.A. Romero, “Near infrared transflectance spectroscopy Determination of dexketoprofen in a hydrogel”, Journal of Pharmaceutical and Biomedical Analysis, 30 (2002) 467–472.

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TransmittanceTransmittance

Tablet Sample

Detector Position

IR Beam

Prepared by:

María A. Santos

R.J. Romañach and M.A. Santos, “Content Uniformity Testing with Near Infrared Spectroscopy”, American Pharmaceutical Review, 2003, 6(2), 62 – 67.

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TransmittanceTransmittance

• Transmittance mode preferred since radiation interacts with a greater sample volume.

• Very interesting and often complex interaction between radiation and particles.

• Depth penetration depends on particle size (scattering properties) of particles within the tablet (Iyer, Morris, Drennen, J. Near Infrared Spectrosc., 2002, 10, 233 – 245.).

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• J. Coates, “Vibrational Spectroscopy: Instrumentation for Infrared and Raman Spectroscopy”, Applied Spectroscopy Reviews, 1998, 33(4), 267 – 425.

• A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, in Near Infra-Red Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Ellis Horwood, 1992.

• C.E. Miller, “Chemical Principles of Near Infrared Technology”, Chapter 2 in Near Infrared Technology: In the Agricultural and Food Industry, P. Williams and K. Norris (Editors), Amer. Assn. of Cereal Chemists; 2nd Ed. (November 15, 2001) .

• H.W. Siesler, “Basic Principles of Near Infrared Spectroscopy”, In Handbook of Near Infrared Analysis Ed. D.A. Burns and E.W. Ciurczak, 3rd ed., CRC Press, Boca Raton, FLA.

• M. Blanco, J. Coello, A. Eustaquio, H Iturriaga, and S. Maspoch, Development and Validation of a Method for the Analysis of a Pharmaceutical Preparation by Near-Infrared Diffuse Reflectance Spectroscopy , Journal of Pharmaceutical Sciences, 1999, 88(5), 551 – 556.

• Dahm DJ, Dahm KD. 2001. The Physics of Near-Infrared Scattering. In Williams P, Norris K, editors. Near Infrared Technology in the Agricultural and Food Industries, 2nd ed., Saint Paul: American Association of Cereal Chemists, p 19-37.

Recommended readingRecommended reading

Documents

10/11/2005 1 Solid State Vibrational Spectroscopy November 2011 University of Puerto Rico ENGINEERING RESEARCH CENTER FOR S TRUCTURED O RGANIC P ARTICULATE