PChem Ch. 5

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Created by
Naseem Dehaibi

Cards (77)

    • Spectroscopic Methods
    • Infrared Spectroscopy (IR)
    • Nuclear Magnetic Resonance Spectroscopy (NMR)
    • Ultraviolet and Visible Light Absorption Spectroscopy (UV-vis)
    • Microscopy
    • Optical Microscopy
    • Electron Microscopy
    • Transmission Electron Microscopy (TEM)
    • Scanning Electron Microscopy (SEM)
    • Most recent methods
    • Scan Probe Microscopy (SPM)
    • Scanning Tunneling Microscopy (STM)
    • Atomic Force Microscopy (AFM)
  • Thermal Analysis Methods
    • Differential Thermal Analysis; Differential Scanning Calorimetry (DTA/DSC)
    • Phase transition temperatures; enthalpy changes; specific heat
    • Works from -180 to 2400 degrees Celsius
  • Thermal Analysis Methods
    • Thermogravimetry (TGA); Simultaneous Thermal Analysis (STA = TGA + DSC)
    • Mass changes due to evaporation
    • Decomposition and interaction with the atmosphere
    • Works from -150 to 2400 degrees Celsius
  • Thermal Analysis Methods
    • Thermomechanical Analysis (DIL; TMA; DMA; RUL; HMOR)
    • Dimensional changes
    • Deformations
    • Viscoelastic properties
    • Transitions
    • Density
    • -260 to 2800 degrees Celsius
  • Thermal Analysis Methods
    • Thermophysical Properties Methods (TCT; HFM; GHP; LFA)
    • Thermal diffusivity
    • Thermal conductivity
    • Transport properties
    • -125 to 2800 degrees Celsius
  • Thermal Analysis Methods
    • Adiabatic Calorimetry (ARC; MMC; APTAC)
    • Phase transitions
    • Isothermal/scanning calorimetry
    • Thermal stability
    • Reaction behavior
    • 500 degrees Celsius
  • Thermal Analysis Methods
    • Dielectric Analysis (DEA)
    • Ion viscosity
    • Curing behavior
    • Dielectric properties
    • 400 degrees Celsius
  • Evolved Gas Analysis (EGA) is used for the advanced characterization of decomposition/evaporation effects
    • Differential Thermal Analysis; Differential Scanning Calorimetry (DTA/DSC)
    • Thermogravimetry (TGA); Simultaneous Thermal Analysis (STA = TGA + DSC)
    • Thermomechanical Analysis (DIL; TMA; DMA; RUL; HMOR)
  • Molecular Excitation:
    • Electronic: UV-visible spectra
    • Vibrational: IR spectra
    • Rotation: Microwave
    • Nuclear spin orientation in magnet: NMR
  • Formula to quantify molecular spectroscopy:
    • ΔE = hν
    • ν = c/λ
  • Molecular spectroscopy: Molecular response to radiative stimulus is quantized ('geared')
  • Infrared Spectroscopy (IR)
    • Principle:
    • Detects characteristic bands of functional groups through absorption of infrared light.
    • Measures vibrational excitation.
    • A stretching vibration occurs along the line of the bond.
    • Interpretation:
    • IR spectrum interpretation is difficult as the arrangement of organic molecules is complex.
    • Advantages:
    • Provides a unique identification of compounds.
    • Disadvantages:
    • Generally used only in pure samples of fairly small molecules.
  • Infrared Spectroscopy (IR):
    • Regions:
    • The Fingerprint Region (1400–600 cm-1):
    • Single-bond.
    • The Functional Group Region (4000–1500 cm-1):
    • Absorption by:
    • Double-bond.
    • Triple-bond.
    • Bonds of Hydrogen.
    • Bond Strength:
    • Stronger bonds absorb at higher frequencies because the bond is difficult to stretch.
  • Infrared Spectroscopy (IR):
    • Absorption ranges:
    • Carbon-oxygen double bond: 1700 cm-1
    • Oxygen-hydrogen bond: 3400 cm-1
    • Carbon single bond: 1200 cm-1
    • Carbon double bond: 1660 cm-1
    • Carbon triple bond: 2200 cm-1
  • Table showcasing different IR absorption ranges
  • Practice question: Determine the functional group(s) in the compound whose IR spectrum appears here.
  • Structural Characterization of Polymers using FT-IR:
    • Sample Preparation:
    • Polymer samples can be in the form of film, solution, or pellet containing a mixture of the polymer powder and an IR-transparent powder such as potassium bromide (KBr).
    • Bulk samples can be analyzed with reflection or attenuated total reflectance (ATR).
    • Identification:
    • Polymer spectra are unique, and a large number of spectra libraries are available for identification.
    • Measurement of stereoregularity.
    • Assignment of an IR frequency to a particular isomer, for example, with NMR.
  • Structural Characterization of Polymers using FT-IR:
    • Overview:
    • Widely used method to characterize polymers.
    • Qualitatively easy to use, quantitatively more challenging.
  • Additional Characterization using FT-IR:
    • Co-polymer Analysis:
    • Allows for the determination of composition in copolymers.
    • Measurement of Branching:
    • Provides insights into the branching structure of polymers.
    • Characterization of Polymer Blends:
    • Useful for analyzing blends of polymers, providing information about their composition and interactions.
  • FT-IR (Fourier-transform infrared spectroscopy): A method that uses infrared light to analyze the chemical composition of materials by measuring how they interact with this light, producing a unique pattern (spectrum) used to identify chemical bonds and molecular structure.
  • Structural Characterization of Polymers using FT-IR:
    • Extent of Crosslinking:
    • Determines the degree of network formation within the polymer structure.
    • Degree of Crystallinity:
    • Helps assess the level of molecular ordering within the polymer chains.
  • Structural Characterization of Polymers using FT-IR:
    • Location Sensitivity:
    • The location of an absorbance peak can be sensitive to whether the chemical groups lie in crystalline lamellae or in amorphous regions, providing insights into the structural arrangement.
    • Surface Analysis:
    • Useful for studying surface species and reactions occurring at interfaces, providing information on surface chemistry and interactions.
  • Differences between FT-IR and IR:
    • Both analyze how materials interact with infrared light.
    • The key difference is in data acquisition. IR scans wavelengths sequentially, while FT-IR collects all wavelengths simultaneously using an interferometer, offering faster and more efficient data collection with better signal quality.
  • IR spectrum of PS (polystyrene):
    • C-H Aromatic Tension:
    • 3001.11 - 3081.2 cm-1
    • CH2 Asymmetric and Symmetric Tension:
    • 2923.91 cm-1 (Asymmetric)
    • 2850.40 cm-1 (Symmetric)
    • Aromatic Ring Mono-Substitution:
    • 1728.23 - 1943.19 cm-1
    • Deformation CH2 + C=C of the Aromatic Ring:
    • 1452.28 cm-1
    • Flexion C-H in the Plane:
    • 1069.65 cm-1
  • IR Spectra of Polypropylene
    (a) atactic
    (b) syndiotactic
    (c) isotactic
    1. Differences do not result directly from the differences in configuration
    2. Arise from the effects of configuration upon local chain conformation
    3. From rotation about the C2–C3 bond
  • Raman Overview:
    • Introduction:
    • Another form of vibrational spectroscopy.
    • Raman spectra arise from inelastic scattering of visible light.
  • Raman Overview:
    • Interaction with Sample:
    • Unlike IR, where absorption occurs, Raman involves scattering of light.
    • When monochromatic radiation is incident upon a sample, it can be reflected, absorbed (as in IR), or scattered (which is of interest in Raman).
    • Components of Scattered Light:
    • Light scattered from a molecule has several components:
    • Rayleigh scatter.
    • Stokes Raman scatter.
    • Anti-Stokes Raman scatter.
  • Visual demonstrating different scattering configurations for Raman spectra
  • Raman Scattering:
    • Frequency/Wavelength Change:
    • The change (or lack thereof) in the frequency/wavelength of scattered light provides information.
    • Scattering without a change of frequency of the light is called Rayleigh scattering.
    • Accounts for why the sky is blue (particles in the air scatter blue light more than red).
    • Scattering with a change in frequency of the light is called Raman scattering.
    • The energy of the light changes after interacting with the material.
    • In other words, the color of light will be different.
  • Raman Scattering:
    • Photon Energy:
    • Raman scattered photons of light can be either of higher or lower energy, depending upon the vibrational state of the molecule.
  • Typical Features of Raman Spectrum:
    • Change in Polarizability:
    • Essential prerequisite is a change in the polarizability of the bond when vibrations occur.
    • Complementary Information:
    • Information regarding molecular structure is complementary to that obtained from infrared spectroscopy.
    • Strong Signals for Weak Absorptions:
    • Functional groups that give weak absorptions in the infrared, such as S-S, C=C, and N=N, give strong Raman signals.
  • Typical Features of Raman Spectrum:
    • Non-contact and Non-destructive:
    • Raman spectroscopy is a non-contact, non-destructive technique.
    • No Sample Preparation:
    • No sample preparation is required for Raman spectroscopy.
    • Safe Examination of Samples:
    • Dangerous or delicate samples may be examined in sealed containers using Raman spectroscopy.
  • Visual of Raman spectrum
  • Visual of Raman spectrum
  • NMR (Nuclear Magnetic Resonance):
    • Principle:
    • Nuclear magnetic resonance is a property of the nucleus of an atom, concerned with nuclear spin (I).
    • This is equivalent to the nucleus acting like a miniature magnet.
    • Nuclei with I = ½:
    • Although isotopes can have various values for I (including zero), the most useful for spectroscopy are those nuclei which have I = ½.
    • This includes hydrogen (¹³C and ¹H), as well as ¹⁷O, ¹⁹F, and ²⁹Si.
  • NMR (Nuclear Magnetic Resonance)
    • When a nucleus with I = 1/2 is placed in a magnetic field, it can either align itself with the field (lower energy) or against it (higher energy).
  • NMR:
    • Frequency Variation:
    • As observed in IR and Raman, the different nuclei in NMR absorb/relax at different frequencies.
    • Factors Influencing Frequency:
    • This frequency is not only due to the applied field but also the magnetic effect of nearby nuclei and electrons.
    • Chemical Shifts:
    • This causes the signal to absorb at a slightly different frequency than for a single atom.
    • Frequency changes are termed chemical shifts and are scaled in reference to tetramethylsilane (TMS) as a standard.
  • Typical hydrogen shifts in H-NMR:
  • Information Provided by NMR:
    • Molecular structure
    • Tacticity
    • Branching
    • Co-polymer structure
    • Structural defects
    • Molecular dynamics
    • Molecular interactions
    • Chemical kinetics
    • Molecular identity
  • Practice question: How many signals for each molecule?