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Spectroscopy
Analytical chemistry optical analysis methods, including UV-visible spectrophotometry, nuclear magnetic resonance spectroscopy, infrared spectrophotometry, etc.
Edited at 2024-01-19 17:03:13
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- Outline
optical analysis method
Infrared spectrophotometry
Overview
Classification of infrared spectral regions
Electromagnetic radiation with wavelength in the range of 0.76um ~ 500um
Near infrared region: 0.76-2.5um: O-H, N-H, C-H frequency doubling absorption
Mid-infrared region: 2.5~25um: vibration, rotation spectrum
Far infrared region: 25~500um: rotation, lattice vibration
Generation of infrared spectrum
Molecular vibrational and rotational energy level transitions
Vibration-rotation spectroscopy
Characteristics of infrared spectrum
Characteristics and fingerprints
Mainly used to identify compounds and determine molecular structure
Representation method of infrared spectrum
The abscissa is wave number or wavelength, the ordinate is percent transmittance, and the absorption peak is downward.
Comparison of IR and UV
Fundamental
Vibration-rotation spectroscopy
simple harmonic motion
Kinetic energy T
Potential energy U
The resonator potential energy curve and the bimolecular potential energy curve at room temperature are similar when the molecule is at the lowest vibrational energy level.
Fundamental frequency peak
Conditions for molecular vibrational energy level transitions
The energy of infrared radiation is equal to the energy difference between the two vibrational energy levels of the molecule
Infrared absorption vibration (the net dipole moment change of molecular vibration is not equal to zero
calculate
Vibration form
stretching vibration
Symmetric stretching vibration
The same direction of movement along the key axis
Asymmetric stretching vibration
opposite direction
bending vibration
In-plane bending vibration
Scissor vibration: the bond angle changes like scissors opening and closing
In-plane rocking vibration: The group as a whole rocks in the plane
out-of-plane bending vibration
out-of-plane rocking
twist
Deformation vibration
Symmetrical deformation vibration
Asymmetric deformation vibration
Fundamental frequency peak and overtone frequency peak
Fundamental frequency peak: ground state to first excited state
Features: The fundamental peak frequency is equal to the basic vibration frequency of a certain group in the molecule It has high intensity and is an important absorption peak in the infrared spectrum.
Overtone peak: transition from ground state to high energy state
Category: Doubling frequency peak, group frequency peak, combined frequency peak, difference frequency peak, etc.
Features: The overtone peak transition probability is small, the intensity is weak, and it is difficult to identify Increased spectral characteristics
Characteristic peaks and related peaks
Characteristic peaks: identify the presence of a certain functional group Correlated peaks: A set of interdependent characteristic peaks produced by a group that support each other.
A set of related peaks identifies a group
Absorption peak peak position
Characteristic area: highly characteristic
Fingerprint area: dense absorption, complex overlap, difficult to identify, reflecting small changes in molecular structure
Vibration degrees of freedom and number of absorption peaks
Vibration degrees of freedom
Number of absorption peaks
reduce causes
Infrared inactive vibration: no dipole moment changes
degenerate
Instrument sensitivity is not high
Absorption strength is too weak
Reasons for the increase
Vibrational coupling: the band splits into two peaks, one higher than the original frequency and one lower than the original frequency
Fermi resonance: vibrational coupling between a doubling frequency peak or a group frequency peak and a fundamental frequency peak
Absorption peak intensity
Absolute Intensity: Molar Absorption Coefficient
Relative Strength
The absorption peak intensity reflects the vibrational transition probability of the energy level of the reaction group. The greater the probability, the greater the band intensity.
Factors affecting band position
Internal factors
induction effect
conjugation effect
hydrogen bonding effect
Ring tension effect (bond angle effect)
steric hindrance
external factors
Polarity of the solvent: The greater the polarity, the formation of intermolecular hydrogen bonds and the lower group stretching vibration frequency. Try to use non-polar solvents in IR.
instrument temperature
Infrared spectrophotometer
Grating type special spectrophotometer
Light source, absorption cell, monochromator (grating), detector, amplification recording system
The monochromator is placed behind the absorption cell to avoid stray light interference
Radiation source: inert solid, capable of emitting high-intensity continuous infrared radiation
Silicon carbide rod: working temperature 1200~1400 degrees Celsius Long life, large light-emitting area, no need for preheating, disadvantages: long working time, large resistance, electrical contacts require water cooling
Nernst lamp: operating temperature 1750 degrees Celsius, requires preheating Advantages: high luminous intensity, no need for water cooling Disadvantages: expensive, poor mechanical strength
monochromator
Reflection grating: often used in combination with filters or pre-prisms to reduce secondary spectral interference (secondary and tertiary spectrum)
Detector
Vacuum thermocouple: temperature difference converted into potential difference
Golay cell (inflatable detector)
absorption pool
Liquid and gas absorption cells
Solid samples are often evenly compressed with pure potassium bromide and measured directly.
Features
The resolution is better than that of the prism type, and it is cheaper and has lower environmental requirements.
Low sensitivity, slow scanning speed, not suitable for analysis of signals that are too strong or too weak
Fourier transform infrared spectrometer
Light source, interferometer, absorption cell, detector, computer and recording system
Detector: pyroelectric type, photoconductive type
Advantages: fast scanning speed High-resolution high sensitivity High precision Wide range of measurement spectra
Sample preparation
Sample request
Single component, purity greater than 98%
Contains no moisture (crystallization water, free water) to prevent the hydroxyl peak from being interfered with or the salt window from being destroyed
solid sample
Tablet method: KBr tablet method
1～2mg sample and 200mgKBr
Press the oil press into a transparent sample with a diameter of 13mm and a thickness of 1mm.
Measurable complete mid-infrared spectrum of pure KBr
Paste method (ointment method, liquid paraffin method)
After the sample is ground into fine pieces, it is mixed with liquid paraffin or perfluorocarbon and sandwiched between salt tablets.
thin film method
For the determination of polymer compounds, the sample is dissolved in a volatile liquid and applied to a salt tablet.
liquid sample
Clamp method: difficult to evaporate liquids
smear method
High viscosity liquid
liquid pool method
Highly absorbent samples can be measured after appropriate dilution with solvent.
Solvent: chloroform, CS2
Gas sample: After purification, put it directly into the sample pool, and the sample pool must be evacuated.
The relationship between infrared spectrum and molecular structure
Nine sections of traditional Chinese medicine in the infrared spectrum
characteristic frequency
application
Qualitative analysis, purity check
Unsaturation calculation
Spectrum analysis
UV-visible spectrophotometry
Fundamental
The selective absorption of light of a certain wavelength by a substance
Features
High sensitivity, good selectivity, high accuracy, wide application, simple instrumentation, easy operation, and fast analysis speed
Transition type
Common terms
absorption curve
absorption peak
absorption valley
acromion
terminal absorption
Chromophore
Auxiliary chromophore
red shift, blue shift
Additive effect, subtractive effect
absorption band
R belt
K belt
B belt
E-belt
Lambert-Beer law
Conditions: parallel monochromatic light, dilute solution
photometric error
Factors that deviate from Beer’s law
chemical factors
Dissociation, association, solvation, etc.
Optical factors
non-monochromatic light
Depends on the slit width of the monochromator and the resolution of the prism or grating
stray light
A becomes smaller or false peaks appear
Scattered light and reflected light
A becomes bigger
non-parallel light
A becomes bigger
Measurement error and selection of measurement conditions
Transmittance measurement error
A0.2～0.7 T20%～65% The minimum error is A=0.434
Condition selection
Absorbance range
Wavelength selection: maximum absorption, minimum interference
Color reaction and color condition selection
Color reaction: colorimetric method
Color reaction requirements
Determination of the measurement relationship between the measured object and the non-ferrous substance
The reaction product has high light absorption capacity and sufficient stability
The color of the reaction product is significantly different from the color of the developer
Good color reaction selectivity
Selection of color development conditions
Chromogen dosage
solution acidity
Color development time
temperature
Container
Interference cancellation
The impact of interfering substances
There is absorption at the measured wavelength
Water dissolves into precipitate
Form stable complexes with the ions to be measured or chromogenic agents
Ways to eliminate distractions
Control acidity
Masking agent
The masking agent does not interact with the ions to be measured The masking agent and its complexes with interfering ions do not interfere with the determination.
Select measurement wavelength
Select blank solution
solvent blank
Reagent blank (no test solution added)
Sample blank (add test solution, no developer): suitable for samples with color but no interference from the developer
Separation of interfering substances (pre-treatment)
instrument
The main components
light source
Visible light range of tungsten lamp and tungsten halogen lamp: 350~1000nm
Hydrogen lamp and deuterium lamp: UV region 150~400nm
monochromator
dispersion original
Collimating lens
Slit: directly affects the purity of monochromatic light
absorption pool
Optical glass absorption cell
Quartz absorption pool
Detector: Photoelectric conversion element
Signal processing and display systems
Photometer type
Single beam spectrophotometer
a monochromator
Suitable for measuring absorbance at a given wavelength and cannot scan the entire wavelength range
Double beam spectrophotometer
single wavelength
The monochromator is followed by a beam splitter, and the two beams of light pass through the sample cell and the reference cell respectively.
Obtain full-band scanning absorption spectrum in a short time
Dual wavelength spectrophotometer
Two monochromators, dual wavelength
Qualitative and quantitative analysis
Qualitative methods
Under the same measurement conditions
Absorption spectrum shape Number of absorption peaks The wavelength position, intensity, and absorption coefficient of the absorption peak
Compare absorption spectra
Characteristic data of absorption spectrum: maximum wave field and absorption coefficient
Absorbance or absorption coefficient ratio
Single-component quantitative methods
Quantitative basis: Lambert-Beer law
standard curve method
Precautions
A standard song must have at least 5 to 7 points and cannot be extended at will.
The concentration of the test solution should be within the linear range of the standard curve
The solution to be tested and the reference substance must be measured under the same conditions.
The reason why the song cannot be marked at the origin
Improper selection of blank solution
Color development reaction is not sensitive enough
Absorption cell optical properties are inconsistent
standard control method
Under the same conditions: the same material, the same instrument, the same wavelength, the same thickness of the absorption cell Prerequisite: mark the track past the origin
Absorption coefficient method
Quantitative methods for multicomponent samples
Principle: Absorbance is additive
There is no interference in the absorption spectra of the two components: single component quantification
The absorption spectra of the two components partially overlap
Complete overlap: determination of mixed samples
Solving Linear Equations
Equal absorption point method
Basic conditions
Interfering components have the same absorbance at both wavelengths
The absorbance difference of the measured component in the two wave fields is large enough
relationship to molecular structure
UV absorption spectrum of organic compounds
Influencing factors
Effect of steric hindrance: conjugated chromophore
Cross-ring effect: non-conjugated chromophore, which is beneficial to the interaction between electron orbitals
Solvent effect: K band is red-shifted, R band is blue-shifted, and the fine structure of the absorption peak disappears
System pH impact
structural analysis
No absorption at 220~270nm - aliphatic hydrocarbons, alicyclic hydrocarbons, non-conjugated olefins
Strong absorption at 220~250nm - two conjugated unsaturated bonds
250~350nm weak absorption-carbonyl group
200~250nm strong absorption, 250~290nm medium intensity absorption - stupid group
Strong absorption above 300nm - larger conjugated system
NMR spectroscopy
Overview
Generation of NMR spectra
Nuclear spin energy level transition under strong external magnetic field
NMR spectroscopy, UV, IR comparison
Fundamental
Basic properties of atomic nuclei
Spin and magnetic moment of atomic nucleus
The number of protons and neutrons are both even numbers: spin quantum number (I) = 0
The number of protons and neutrons are both series: I is a positive number
Spin angular momentum P (vector)
Nuclear magnetic moment μ, strength characteristics of magnetic field, vector
The spin quantum number and nuclear magnetic moment are not zero, it is a magnetic nucleus, and there is a nuclear magnetic resonance signal.
Spatial quantization and energy level splitting of nuclear magnetic moments
Magnetic quantum number m: represents the different orientations of nuclear magnetic moments
Transition conditions
The energy level difference is proportional to the strength of the external magnetic field
The creation of nuclear magnetic resonance
NMR basic equations
Three necessary conditions for the generation of nuclear magnetic resonance
magnetic core
external magnetic field
Energy appropriate magnetic waves
Saturation and relaxation
Boltzmann distribution
Methods to improve NMR sensitivity: increase the difference between the number of high and low energy level nuclei, increase the strength of the external magnetic field, and reduce the operating temperature
Saturation: The number of high and low energy level nuclei is equal
Relaxation: High-energy-level nuclei release energy through non-radiative pathways, returning the nuclei to low-energy levels. This is an essential process for maintaining nuclear magnetic resonance signals.
chemical shift
The production of chemical shifts
shielding effect
Chemical shift: The nuclei of molecules in an inaccessible chemical environment have different resonance frequencies (different absorption peak shifts) due to different shielding effects.
NMR equation (modified)
How to express chemical shifts
Implementing NMR methods
Sweep method
sweeping method
Selection of standard materials
Substances with large shielding constants (positive chemical shifts)
Tetramethylsilane (TMS), displacement constant = 0
Why use TMS as standard material
The chemical environment of 12 hydrogens is the same, and only one peak appears in the NMR hydrogen spectrum.
The shielding constant is very large
The chemical substance is stable, easily soluble in organic solvents, has a low boiling point and is easy to separate from the sample.
display method
Factors affecting proton chemical shift
Electron cloud density outside the nucleus
electrical effect
induction effect
conjugation effect
Magnetic Anisotropy Effect
van der waals effect
hydrogen bonding effect
solvent effect
Pay attention to solvent ratio
In a dilute solution of an inert solvent, the chemical shift does not change much
Chemical shifts of various protons
Spin coupling and spin systems
Spin-spin coupling, spin-spin splitting
Peak splitting rules
Reason: spin coupling between two adjacent groups of hydrogen nuclei
n 1 law, multiple peak area ratio
Conditions for the establishment of n 1 law
The coupling constant J is the same
The proton is weakly coupled to the measured nucleus.
Coupling constant J and its structural relationship
Coupling constant: the distance between multiple peaks, positive or negative, unit is Hz
express
type
Same carbon coupling (coupling): coupling of two protons with different chemical environments on the same carbon
Ortho-carbon coupling (ortho-coupling): the coupling of protons on two adjacent carbons. Hydrogen on adjacent carbons in the same chemical environment does not couple.
Remote coupling: separated by more than three chemical bonds
Influencing factors
Magnetism of the atomic nucleus: The greater the magnetism, the greater the J
Molecular Structure
The number of keys in the interval: the more keys, the smaller J
angle
Electron cloud density: the greater the density, the greater J
spin system
Equivalent properties of kernels
Chemical equivalence: a group of nuclei with the same chemical environment, the chemical shifts of chemically equivalent nuclei must be the same
Judgment method
Symmetrical operation, fast movement
magnetic equivalence
Conditions: chemical equivalence, same coupling constants
Classification and naming of spin systems
Classification (strength of coupling): first-level coupling, second-level coupling
name
First level spectrum
Condition: first level coupling NMR equivalence of the same nuclear group
Features: The number of multiple peaks follows the n 1 law The multiplet area ratio is the ratio of the coefficients after the binomial (a b) is expanded to the nth power The center position of the multiplet is the chemical shift, and the peak distance of the multiplet is the coupling constant J
Nuclear magnetic resonance spectrometers and test methods
structure
Magnet or superconducting magnetic field: Provides a constant and uniform strong magnetic field
Radio frequency oscillator: provides fixed frequency electromagnetic waves
RF receiver: senses electrical signals
sample tube
readout system
Pulse Fourier transform nuclear magnetic resonance spectrometer PFT-NMR: fast detection speed and high sensitivity
Sample Preparation
sample
Solvent: Deuterated reagent (sample is easily soluble and does not interfere with the signal): chloroform, acetone, benzene, deuterated reagent of dimethyl sulfoxide
Standard material: Tetramethylsilane TMS, 1%
Analysis of hydrogen nuclear magnetic resonance spectrum
Introduction
Principle: Energy acts on the object to be measured to produce optical radiation, and the light radiation acts on the object to be measured to produce changes.
Properties of electromagnetic radiation
Basic properties: wave-particle duality (wave nature, particulate nature)
Volatility: cannot explain the emission and absorption of radiation
Particulate nature: Light is composed of photons, and the energy of light is concentrated on the particles of light.
electromagnetic spectrum
High radiation area
Gamma rays have the highest energy and originate from nuclear energy level transitions
x-ray inner electron energy level transition
Optical spectral region
UV light
Energy level transitions of outer electrons in atoms and molecules
visible light
Infrared light
Molecular vibrational and rotational energy level transitions
Spectral area
microwave
Molecular rotational energy level transition
electromagnetic waves
Nuclear spin energy level transition
Electromagnetic radiation and matter interaction
absorb
The energy of electromagnetic waves is exactly equal to the energy level difference, and matter absorbs energy
Atomic absorption transition
molecular absorption
UV-visible, infrared absorption spectroscopy
emission
Matter releases energy in the form of photons, producing electromagnetic waves
Atomic emission spectroscopy; fluorescence spectroscopy, phosphorescence spectroscopy, chemiluminescence method
scattering
Photons collide with material molecules and change their direction of motion
Elastic collision, no energy exchange: Rayleigh scattering
Inelastic collision, energy exchange: Raman scattering
Refraction, reflection, interference, diffraction
Optical analysis methods and their classification
Radiation form classification
ll
Basis: wavelength and intensity of emitted light
excited state to ground state
absorption spectrum
Basis: The spectrum produced by a substance absorbing corresponding radiant energy
UV-Vis, IR, NMR spectroscopy
scattering spectrum
Raman scattering
spectral shape
Atomic spectrum
line spectrum
Energy level transition in the outer layer of atoms, line spectrum
continuous spectrum
Blackbody radiation is produced, which is an interference factor and can be used as a continuous light source.
molecular spectrum
band spectrum
Electronic energy level, vibrational energy level, rotational energy level
Small energy level difference, band-like spectrum
Non-spectral method: the internal energy level does not change, only changes in electromagnetic radiation are measured
Spectrometry Instruments
Radiation source, spectroscopic system, sample container, detector, readout device
radiation source
Continuous light source: molecular absorption spectrum: deuterium lamp, tungsten lamp
Line light source: Atomic absorption: metal vapor lamp, hollow cathode lamp
Spectral system (monochromator): entrance and exit slits, collimating lens, dispersion element, focusing lens
sample container
cuvette (absorption cell)
UV: Quartz
Vis: silicate glass
Special device: Nebulizer (Atomic Absorption Spectrophotometry)
Detector
Quantized detector (photon detector)
Thermal detector: for infrared light area
Signal processor and readout device
Amplify the output signal, change DC or AC, etc.