Principle and application of infrared spectrum

Definition and generation of infrared absorption spectrum the vibrational energy of molecules is larger than the rotational energy. when the vibrational energy level transitions, it is inevitably accompanied by the rotational energy level transitions, so the pure vibrational spectrum cannot be measured, and only the vibrational-rotational spectrum of molecules can be obtained, which is called infrared absorption spectrum.

Infrared absorption spectrum is also a kind of molecular absorption spectrum. when the sample is irradiated by infrared light with continuously changing frequency, the molecules absorb some frequencies of radiation, and its vibration or rotation motion causes the net change of dipole moment, resulting in the transition of molecular vibration and rotation energy level from the ground state to the excited state, which weakens the transmitted light intensity corresponding to these absorption regions. record the curve of the relationship between the percent transmittance of infrared light and the wave number or wavelength to obtain the infrared spectrum.

Fundamental principle

First, the conditions of infrared absorption. one, when a molecule vibrates, it must be accompanied by the change of instantaneous dipole moment. symmetric molecules: no dipole moment, no resonance caused by radiation, no infrared activity. such as N2, O2, Cl2, etc. asymmetric molecule: having dipole moment and infrared activity. two, only when the frequency of infrared radiation irradiating the molecule is the same as the frequency of a certain vibration mode of the molecule, after the molecule absorbs energy, it will jump from the ground state vibration energy level to the higher energy vibration energy level, thus the corresponding absorption band will appear on the spectrum. Second, the vibration types of molecules. telescopic vibration: change of bond length, including symmetric and asymmetric telescopic vibration. bending vibration: key angle changes, including scissor vibration, plane swing, non-plane swing and torsional vibration.

Factors affecting group absorption frequency

Influence of external conditions on absorption peak position: state effect and solvent effect. effect of molecular structure on group absorption band; one, induction effect: usually, the electron-withdrawing group increases the absorption wave number of adjacent groups, while the electron-donating group decreases the wave number.  two, conjugation effect: the conjugation of groups with electron-withdrawing groups increases the bond force constant of groups, so the absorption frequency of groups increases, and the conjugation of groups with electron-donating groups decreases the bond force constant of groups, so the absorption frequency of groups decreases. when there are both induction effect and conjugation effect, if the two effects are consistent, the two effects will strengthen each other, but they are inconsistent, depending on the strong effect. three, dipole field effect: groups close to each other act through space. four, tension effect: the stretching vibration wave number of the double bond outside the ring increases with the decrease of the ring. five, hydrogen bond effect: the formation of hydrogen bond makes the stretching vibration wave number move to low wave number, and the absorption intensity is enhanced. six, steric hindrance: conjugation is limited by steric hindrance, and the group absorption is close to the normal value. seven, the influence of vibration coupling tautomerism.

Analysis of infrared absorption spectrometry

One, check whether the spectrogram meets the requirements; two, understand the sample source, physical and chemical properties of the sample, other analysis data, recrystallization solvent and purity of the sample; three, eliminate possible "false bands"; four, If the molecular formula can be written according to other analysis data, the molecular unsaturation should be calculated first.