The intermolecular force is also called van der Waals force.There are three sources of intermolecular force (van der Waals force): ① interaction between permanent dipole moments of polar molecules; ②One polar molecule polarizes the other, producing induced dipole moments and attracting each other. ③The movement of electrons in the molecule produces the instantaneous dipole moment, which makes adjacent molecules instantly polarized, and the latter in turn enhances the instantaneous dipole moment of the original molecule;This coupling produces electrostatic attraction, and the contributions of these three forces are different. Generally, the contribution of the third effect is the largest.[1]
The intermolecular force only exists between molecules orinert gas(noble gas)atomThe force between atoms, also known as van der waals, hasAdditivity, belonging to the secondary key.
Hydrogen bond, weak van der Waals force, hydrophobic force, aromatic ring stacking effect and halogen bond belong tosecondary bond(also called intermolecular weak interaction).
Chinese name
Intermolecular force
Foreign name
intermolecular force
Alias
Van der Waals force
Action object
Functional groups in macromolecular compounds between molecules
General classification
Van der Waals force, hydrogen bond, other non covalent bonds
hydrogen bondIt does not belong to intermolecular force, depending on the definition of "intermolecular force".According to the definition of generalized van der Waals force[Gravitational constantThe term can transform various polarization energies（dipole(dipole)、induce(induced) andhydrogen bondThe hydrogen bond belongs to intermolecular force.According to the traditional definition, the intermolecular force is defined as "the permanence of moleculesdipole(permanent dipole) and instantaneous dipole "then hydrogen bond does not belong to (because hydrogen bond contains at least four interactions, and only three interactions with intermolecular forces haveintersectionHowever, the highest occupied orbit overlaps with the lowest free orbit of another molecule).
Hydrogen bonds can exist both within and between molecules.Secondly, the relationship between hydrogen bond and intermolecular forcequantum mechanicsThe calculation method is also different.In addition, hydrogen bond has high selectivity, not strict saturation and directionality;The intermolecular force does not.In "fold chemistry", multi hydrogen bonds have synergistic effects, inducing linear molecular helices, while intermolecular forces do not have synergistic effects.Super strong hydrogen bond has similarcovalent bondThe essence of (valid bond) is controversial academically and must be distinguished from intermolecular forces.
If the intermolecular force, hydrogen bond and halogen bond are wrongly regarded as equivalent actions, then molecular recognitionDNA structureSimulationprotein structureAccumulation, it is impossible to study.So academically, these electromagnetic interactions are collectively calledsecondary bond。
Force classification
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Definition: Van der Waals force (also called molecular force) is generated frommoleculeoratomElectrostatic interaction between.The empirical equation for energy calculation is: U=B/rtwelve- A/rsix(For 2carbon atomThe parameter value is B=11.5 × 10-6kJ·nmtwelve/mol ；A=5.96 × 10-3kJ·nmsix/mol；A and B have different values between different atoms) When two atoms are close to each otherElectronic cloudWhen they overlap, strong repulsion occurs, and the repulsion force is inversely proportional to the 12th power of distance.The low point is the maximum distance force maintained by van der Waals force, which is calledVander Waals radius。Molecular gravity is inversely proportional to the 6th power of distance, and molecular repulsion is inversely proportional to the 12th power of distance.
Dispersion force(Also called separation force“London Force”）All molecules or atoms exist.Is the force between the instantaneous dipoles of molecules, that is, due to the movement of electrons, the position of the instantaneous electronsNucleusIs asymmetric, that ispositive chargeCenter of gravity andnegative chargeThe center of gravity does not coincide instantaneously, resulting in an instantaneous dipole.Dispersion forceAnd interacting moleculesDeformabilityThe larger the deformability (generally, the larger the molecular weight, the greater the deformability), the greater the dispersion force.Dispersion force and interaction of moleculesIonization potential (i.e. ionization energy)The lower the ionization potential of the molecule (Number of electronsThe more), the greater the dispersion force.The interaction of dispersion forces increases with 1/rsixAnd change.The formula is: IoneAnd ItwoAre the ionization energies of two interacting molecules,αoneandαtwoIt's theirsPolarizability。
Induced force
Square in parentheses
Induced force(induction force)Polar moleculeandNonpolar moleculeThere are inductive forces between polar molecules and between polar molecules.Due to the dipole of polar moleculeelectric fieldInfluence non-polar moleculesElectronic cloudDeformation (that is, the electron cloud is attracted to the positive pole of the dipole of the polar molecule), resulting inElectronic cloudAndNucleushappenrelative displacement The gravity centers of positive and negative charges in non-polar molecules are coincident, but they will no longer coincide after relative displacement, resulting in a dipole of non-polar molecules.The relative displacement of the center of gravity of the charge is called "deformation", and the dipole caused by the deformation is calledInduced dipoleTo distinguish it from the original inherent dipole in polar molecules.The induced dipole and the inherent dipole attract each other. This force generated by the induced dipole is called the induced force.Between polar molecules and polar molecules, exceptOrientation forceIn addition, due to the interaction of polar molecules, each molecule will also be deformed, resulting in an induced dipole.As a result, the molecularDipole distanceIncrease, with both orientation force andInduced force。staycationandanionThere will also be induced force between.
Inductive force and polar moleculesdipole momentIs proportional to the square of.Inductive force and induced molecularDeformabilityIn general, the larger the outer shell of each atomic nucleus in a molecule (the more heavy atoms it contains), the easier it will deform under the external electrostatic force.Interaction with 1/rsixHowever, the induction force is independent of temperature.Its formula:αbyPolarizability。
Orientation force
Orientation force(Orientation force is also called dipole force) Orientation force occurs whenPolar moleculeAnd polar molecules.Because the electrical distribution of polar molecules is uneven, one end is positively charged and the other end is negatively chargeddipole。Therefore, when two polar molecules are close to each other, because their dipoles repel each other and their dipoles attract each other, the two molecules will rotate relatively.suchdipoleThe rotation of the dipole makes the opposite poles of the dipole opposite, which is called "orientation".At this time, because the opposite poles are close and the same poles are far away, the gravitational force is greater than the repulsive force, and the two molecules are close. When they are close to a certain distance, the repulsive force and gravity reach a relative balance.This intermolecular force due to the orientation of polar molecules is called orientation force.Orientation force and moleculardipole momentIs proportional to the square, that is, the greater the polarity of the molecule,Orientation forceThe larger.The orientation force is inversely proportional to the absolute temperature. The higher the temperature is, the weaker the orientation force issixAnd change.The formula is:μone，μtwoTwo moleculedipole moment；R is moleculecentroidDistance between, k is Boltzmann constant, T isThermodynamic temperatureA negative value indicates a decrease in energy.
Relation of three forces
Polar moleculeAnd polar molecules,Orientation force、Induced force、Dispersion forceAll exist;Polar molecules andNonpolar moleculeBetween, there are inductive force and dispersive force;wrongPolarityBetween molecules and nonpolar molecules, there is only dispersion force.The proportion of these three types of forces depends on the polarity andDeformability。The greater the polarity, the more important the role of orientation force;The greater the deformation,Dispersion forceThe more important;Induced forceIt is related to both of these factors.But for most molecules, the dispersion force is the main one.Experiments show that for most molecules, the dispersion force is the main factor;onlydipole momentLarge molecules (such as water),Orientation forceIs the main;The inductive force is usually very small.PolarizabilityαReflect theElectronic cloudWhether it is easy to deform.Although the van der Waals force is only 0.4-4.0kJ/mol, the interaction between a large number of macromolecules will become very stable.For example, the van der Waals force of C-H in benzene is 7 kJ/mol, while in benzenelysozymeVan der Waals force, the sugar binding substrate, was 60kJ/mol, and Van der Waals force was additive.
So the real key word is "distance". We can combine molecular forceIonic bondThink together.
Type of action
Relation between energy and distance
Electrostatic action of charged group
1/r
Ion dipole
1/rtwo
Ion induced dipole
1/rfour
Dipole - Dipole
1/rsixOrientational force chemistry
Dipole - induced dipole
1/rsixInduced force
Induced dipole
1/rsixDispersion force
Non bond repulsion
1/rtwelve—1/rsix
I learned ionic bonding in middle school, andNaCl、CsCl、CaFtwo, CubicZnSSix partiesZnS、rutileTiOtwoOf the six typical compoundscrystalConfiguration is a strong force.
ATP and magnesium ion interaction
In biology, the key is to understandOrganic moleculeIon interaction.Organically formedIons，ElectronegativityThe difference is not so great, and the interaction is not like these typicalIonic compoundThe ionic bond is so large that it is called ionic interaction;But what they have in common isElectrostatic attractionBe formed.
NaCl、CsCl、CaFtwo, cubic ZnS, hexagonal ZnSrutileTiOtwoThe ionic bond energy of the six typical compounds is inversely proportional to the first power of distance2+Interaction with ATP,amino acidInteraction between zwitterions.Ions -dipoleThe ion induced dipole decreases with the fourth power of distance.Therefore, ion interactions in biological molecules (also calledSalt bond）YesWeak interaction, with 1/rtwo—1/rfourAnd decrease.
Van der Waals includesgravitationandrepulsionGravity is inversely proportional to the 6th power of distance, and repulsion is inversely proportional to the 12th power of distance.They areElectrostatic forceEmergence at different levels.
Discrimination of related concepts
The entry has been revised 62 times because different authors have argued about the subordination between hydrogen bonds and intermolecular forces.
In the traditional definition, the intermolecular force is defined as "the weak electrostatic interaction caused by the permanent dipole and the instantaneous dipole of the molecule".With the deepening of research, many phenomena that can not be explained by the existing mechanism of intermolecular forces have been found.such asHalogen bond, organic mercuryhalideIt was observed that there was a strong covalent interaction between halogen atoms and mercury atoms in the molecule over a long distance, which led to the introduction of the concept of secondary valence forces.
Academically, we no longer use "intermolecular force" to cover all weak interactions, but use more accurate terms“secondary bond”。hydrogen bondVan der WaalsSalt bond, hydrophobic force, aromatic ring stacking effect and halogen bond are collectively referred to as“secondary bond”。
hydrogen bondWhether it belongs to intermolecular force depends on the definition of "intermolecular force". If "intermolecular force" continues to be narrowly defined as "weak electrostatic interaction caused by permanent dipole and instantaneous dipole of molecules".In this way, the properties of hydrogen bond and intermolecular force are not exactly the same,quantum mechanicsThe calculation method is not exactly the same..., more likeJuxtaposition relation，hydrogen bondIt does not belong to intermolecular force.Our current domestic general chemistry textbooks, encyclopedias, etc., are defined in this way, which means Van der Waals in a narrow sense.
If the definition of "molecular force" refers to all molecular interactions (this definition also includes long-range and short-range interactions), thenhydrogen bondIt also belongs to intermolecular force, not onlyhydrogen bondOf,IonsBond forces are also intermolecular forces.In the book "Polymer Interface Science", Professor Zhang believes that the gravitational constant term can combine various polarization energies (dipole, induction and hydrogen bond energies) into one term to calculate. From this perspective, the van der Waals dipole moment interaction coefficient can be expanded to write the electrostatic interaction coefficient.In this way, the generalized van der Waals force about electrostatic force is obtained.This classification of intermolecular interactions has also been reported in some literatures.Professor Xiang Hongwei, the author of Thermophysical and Chemical Properties of Fluids Correspondence State Principle and Its Application, believes that the three types of molecular interaction potential energy are collectively called van der Waals force, includingDirectional force、Induced forceandDispersion force，Directional forceThe gravity generated from the dipole moment includes charge, dipole and fourth order moment. The average potential energy function can be obtained by expanding the Boltzmann weight factor by 1/kT power exponent.charge、dipoleSimilar to the fourth order moments, these types of interactions can be considered to obey Berthelot's law.becauseDispersion forceThere will be no induction, and the actual induction interaction will be corrected according to the electrostatic force ratio.
For HF quantum calculation, Zhang Jishuang and Shen Cheng showed that,hydrogen bondThe formation of at least four different types of interactions, 1. HF dipole moment orientation force;2. The highest occupied orbital of HF molecule overlaps with the lowest free orbital of another molecule, which is called covalent bond.3. IntermolecularElectronic cloudThe repulsion of.4. The induced dipole interaction between molecules is very weak.The essence of hydrogen bond is also electrostatic interaction, mainly the dipole interaction energy and electrostatic interaction energy can be approximately calculated with the generalized van der Waals force formula, and the hydrogen bond interaction is the geometric mean value of the hydrogen bond force obeying Berthelot's law.
From this point of view, the hydrogen bond contains elements composed of "sets" of intermolecular forces, and there is no intersection between the two sets.However, hydrogen bonding also has the characteristic orbital overlap and charge transfer that it does not have.
Super strong hydrogen bond is similar to covalent bond in nature, which is controversial in academia and must be distinguished from intermolecular forces.
Many weak interactions exist both within and between molecules (from the perspective of quantum chemistry);And can be directed toChemical bondConversion.Therefore, the author suggests to use more strict words to collectively refer to“secondary bond”Instead of covering all weak interactions with intermolecular forces.
In addition, due to the controversy, the discrimination of its subordinate concept depends on the basic definition of "intermolecular interaction"University of California, BerkeleyJohn M. Prausnitz pointed out that "our understanding of intermolecular forces is far from enough, and is currently limited to simple ideal situations." The author believes that in basic education, such as middle school education, it is not necessary to strictly distinguish the subordination relationship, and the key is to understand the nature. It is not wrong to say that hydrogen bonds belong to or do not belong to intermolecular forces.If you want to eliminate the test questions about the relationship between intermolecular forces and the concept of hydrogen bonding from the examination (it is meaningless to test who belongs to whom), you can rest!What is more important for students is to understand that,hydrogen bondAnd geometric average relation, not strictSaturationanddirectional, melting and boiling pointsolubilityImpactstabilityAnd π hydrogen bond, etc.Because our understanding of intermolecular forces is far from enough
nature
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For substances with similar composition and structure, the larger the relative molecular weight is, the greater the van der Waals force is. Overcoming the intermolecular gravity to melt and vaporize substances requires more energy, and the higher the melting and boiling points are. However, the melting and boiling points of molecular crystals are often abnormally high in the presence of hydrogen bonds.The distance between gas molecules is large, so the interaction between molecules is small;The existence of liquid and solid is the proof of the mutual attraction between molecules;The difficulty of compressing liquids and solids proves the repulsion between molecules at close distance.
Related experiments
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In 2013, French scientists directly measured the van der Waals force between two atoms for the first time. The experimental method used can be used to establishQuantum logic gateOr it can be used for quantum simulation of condensed matter system.
Atomic dipole: the first direct measurement of van der Waals force between atoms
atomdipole: Scientists measure the van der Waals force between atoms directly for the first time
(Image source: iStockphoto/Icemonkey)
Van der Waals forces between atoms, molecules and surfaces of objects appear in various ways in daily life.For example, spiders and geckos rely on the van der Waals force to climb up the smooth wall, and the proteins in our bodies fold into complex shapes because of the van der Waals force.
Van der Waals is a Dutch scientistJohannes Diderik van der Waals In 1873, he first proposed the concept of van der Waals force to explain the behavior of gases.This force is very weak, and only when atoms or molecules are very close can it be meaningful.The fluctuation of the atomic electron cloud makes the atom have an instantaneous electric dipole moment, thus inducing the generation of nearby atomsElectric dipole momentAs a result, there will be mutual attraction between dipoles.
Indirect measurement
There have been many research results on indirect measurement of van der Waals force between atoms, such as analyzing the net force between macroscopic objects to obtain empirical values, or using spectroscopy to analyzeDiatomic moleculeThe long-range force between two atoms in the.However, there has been a lack of direct measurement of van der Waals force before.
This latest research was conducted by the Laboratoire Charles Fabry (LCF) andLille University (University of Lille)."What we have done is to directly measure the van der Waals force between two independent atoms within a controllable distance, and the distance between the atoms is set by the experimenter. As far as we know, this is the first time to achieve direct measurement," said Thierry Lahaye, a member of the LCF team.
When measuring the force between atoms, it is extremely difficult to control the distance between two ordinary atoms, because the related distance is very small.Research team utilizationRydberg atomTo solve this problem, they are much larger than ordinary atoms.Rydberg atomOne of the electrons in is highexcited state, which means that they have a large instantaneous electric dipole moment, so even at a relatively long distance, there will be a large van der Waals force.At the same time, they also have some unique properties, which enable them to be precisely controlled in the laboratory.
Atomic right and wrong
The experiment first uses two highly focused laser beams to capture tworubidiumAtoms and separate themmicronDistance.Then, a laser beam with a specific wavelength is irradiated on the atom to make the system oscillate between the ground state and one or two Rydberg atoms.The research team found that when the conditions are appropriate, the system will oscillate between the ground state and a pair of Rydberg atoms, at which time the two atoms are respectively at the focus of two lasers.By measuring these oscillations, the researchers calculated the van der Waals force between two Rydberg atoms.
By adjusting the captured laser beam, researchers canRydberg atomClose or pull away.When researchers change the distance R between atoms, the force shows an inverse ratio to the 6th power of R - the result is exactly the same as the expected van der Waals force.
In addition to measuring van der Waals force, the research team also found that two interactingRydberg atomThe evolution of quantum state of is completely coherent.Antoine Browaeys, a member of the LFC team, said that this was "unprecedented in atomic physics".
Similar to quantum logic
The coherent evolution of two interacting atoms is exactly the same as that of a quantum logic gate operating on two qubits.Bravis believes that this shows that the two atoms interacting through the van der Waals force are the ideal system to create a high fidelity quantum gate. "This result allows us toquantum computerAnother step forward. "He said.
In fact, the researchers believe that the long-term significance of their experiment is not to measure van der Waals, but to achieveRydberg atomPrecise control of."This enables us to design small quantum systems and gradually increase the size of quantum systems. It is hopeful that we can start from twoRydberg atomGradually increase to dozens, and we can completely control the interaction between atoms. "Lahaye explained.
Such quantum systems are expected to be applied to quantum information processing or quantum simulation of condensed matter systems (such as quantum magnets).
Those who did not participate in this studyUniversity of MarylandSteven Rolston of the Joint Quantum Research Institute called this achievement an important milestone. He believed that it would help the R&D and manufacturing of quantum information equipment, because it proved thatAtomic qubitVan der Waals' performance was the same as expected.
This research was published in Physical Review Letters.[2]
