Vibration-rotation spectroscopy molecular absorption spectrum

E vibration = (V 1/2)hv

Wave number=1/λ=v/c=1/2πc×√K/μ

Wave number=1302/√K/μ’

V₀→V₁

Linear: 3N-5

Nonlinear: 3N-6

The number of fundamental frequency peaks is much smaller than the theoretically calculated number of fundamental vibrations

Near infrared region, weak peaks, increase the characteristics of identifying molecular structures

Frequency octave peak

Combined frequency peak

difference frequency peak

Stretching vibration v

Bending vibration δ

Vibration degrees of freedom = molecular degrees of freedom (3N) - (translational degrees of freedom rotational degrees of freedom)

Each vibrational degree of freedom can be regarded as a basic (fundamental frequency peak) vibration form of the molecule, which has its own characteristic vibration frequency.

4000～1333cm⁻¹(2.5~7.5μm)

Characteristic absorption of major functional groups

Characteristic peaks

The absorption peak is "sparse" and easy to identify → characteristic band area

The low frequency region of 1333~400cm⁻¹ (7.5~25μm) in the infrared spectrum

Stretching vibration and various bending vibrations of C-X (X=C,N,O) single bond

Small changes in the molecular structure will cause obvious changes in the spectrum of the fingerprint area.

effect

Infrared inactive vibration

degenerate

cover

Falling outside the mid-infrared region

The intensity is too weak to be measured

Stronger polar groups (C=O, C-X, etc.) vibrate and have greater absorption intensity.

The dipole moment is related to the symmetry of the molecular structure. The higher the symmetry of the vibration, the smaller the change in the molecular dipole moment during the vibration, and the weaker the band intensity.

Different vibration modes lead to different absorption peak intensities.

When ε>100, it means that the peak is very strong, vs

When ε=20~100, strong peak, s

When ε=10~20, medium strong peak, m

When ε=1~10, weak peak, w

When ε<1, extremely weak peak, vw

s(strong)

m(m)

w(weak)

b(broad peak)

sh (shoulder)

Induction effect (I effect)

Conjugation effect (C effect)

steric hindrance

Ring tension (bond angle tension effect)

ⅴ shifts to low wave number, and the spectrum band becomes wider and stronger

Intramolecular hydrogen bonding (independent of concentration)

Intermolecular hydrogen bonds (concentration dependent)

vibration coupling

Fermi resonance

Polar group: solvent polarity ↑, v↓

v>2700

ⅴ>3000

γ<900

β=1500～1300

v=1300～900

Around 1700

v=about 2200

vC-H

δC-H

vC=C 1695～1540cm⁻¹(1650)

v=C-H 3100～3000cm⁻¹

Substituted alkenes

vC≡C 2270～2100cm⁻¹

v≡C-H 3300cm⁻¹

v=C-H 3100～3000cm⁻¹

vC=C 1600，1500cm⁻¹

Overtone peak 2000～1667cm⁻¹

Substituted aromatics

vO-H 3650～3590cm⁻¹

vC=O 1740～1650cm⁻¹

Phenyl ethers and vinyl ethers

Fatty ether

Acid:vC=O 1710cm⁻¹

Ketones: vC=O 1715cm⁻¹

Aldehydes: vC=O 1725cm⁻¹

Esters: vC=O 1735cm⁻¹

Acid chloride:vC=O 1800cm⁻¹

vO=C-H:2820，2720cm⁻¹Double peaks

vH

δN-H

vC=O 1690～1620cm⁻¹

vas（NO₂） 1565～1540cm⁻¹

vs (NO₂) 1385～1340cm⁻¹

process

light source

monochromator

Detector

computer system

Scan fast

High-resolution

high sensitivity

High precision

Wide measurement spectral range

Dry and water-free

Purity generally needs to be greater than 98%

solid sample

liquid sample

gas sample

Compare with standard material

Check with standard chart

Source and nature of sample

Determine the existence of a certain group

Determine replacement situation and connection method

Compare with standard chart

Feature area first, then fingerprint area

The strongest peak first, then the next strongest peak

Check roughly first, then search carefully

Deny first and then affirm

A set of related peaks confirms the presence of a functional group