第二课-分子轨道理论.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,钟成,2012,年秋,结构性能关系,几何结构,性质,电子结构,几何结构,电子结构,经典粒子,波粒二象性,锻炼出化学家的眼睛,门外汉,大学水平,硕博士水平,专家水平,Molecular Mechanics,分子力学,Calculation of energy of atoms,force on atoms&their resulting motion,Newtonian mechanics,Use,Improve trial structure by eliminating distortion,steric clashes,finding better conformation.,Study motion of molecule eg rigid body motion of domains etc,Potential Energy,Components,(1)bond length,Bonds behave like spring with equilibrium bond length depending on bond type.Increase or decrease from equilibrium length requires higher energy.,Potential Energy,(2)bond angle,Bond angles have equilibrium value eg 108 for H-C-H,Behave as if sprung.,Increase or decrease in angle requires higher energy.,Potential Energy,(3)torsion angle,Rotation can occur about single bond in A-B-C-D but energy depends on torsion angle(angle between CD&AB viewed along BC).Staggered conformations(angle+60,-60 or 180 are preferred).,.,Potential Energy,(4),van der Waals interactions,Interactions between atoms not near neighbours expressed by Lennard-Jones potential.,Very high repulsive force if atoms closer than sum of van der Waals radii.,Attractive force if distance greater.Because of strong distance dependence,van der Waals interactions become negligible at distances over 15 A,Potential Energy,(5)Electrostatic interactions,All atoms have partial charge eg in C=O C has partial positive charge,O atom partial negative charge.,Two atoms that have the same charge repel one another,those with unlike charge attract.,Dielectric constant to use in uncertain.,Dipoles.In many cases molecules made of neutral groups.Two adjacent atoms have opposite charges&behave like dipole.In this case the potential energy falls off as 1/r3,Electrostatic energy falls off much less quickly than for van der Waals interactions and may not be negligible even at 30A.,Potential Energy,Potential Energy is given by the sum of these contributions:,Hydrogen bonds are usually supposed to arise by electrostatic interactions but occasionally a small extra term is added.,Force fields,A force field is the description of how potential energy depends on parameters,Several force fields are available,AMBER used for proteins and nucleic acids,cvff(consistent valence force field),Force fields differ:,in the precise form of the equations,in values of the constants for each atom type,Energy minimisation,Calculation of how atoms should move to minimise TOTAL potential energy,At minimum,forces on every,atom are zero.,Optimising structure to remove strain&steric clashes,However,in general finds local rather than global minimum.Energy barriers are not overcome even if much lower energy state is possible ie structures may be locked in.Hence not useful as a search strategy.,Molecular dynamics,Energy minimisation gives local minimum,not necessarily global minimum.,Give molecule thermal energy so can explore conformational space&overcome energy barriers.,Give atoms initial velocity random value+direction.Scale velocities so total kinetic energy=3/2kT*number atoms,Solve equation of motion to work out position of atoms at 1 fs.,Molecular dynamics,Higher the temperature the greater and faster the motion&more of conformational space sampled.,Use,(a)to overcome energy barriers to find better structure,(b)explore motion,Quantum,Mechanics,量子力学,H,Y,=E,Y,PSI,sai,波函数,算符,本征值,波函数,BY,=1.7,Y,小明,算符：,小明身高多少,HY,=,?,Y,小明,算符：,小明好不好,Y=0.5g+0.5b,小明,好小明,坏小明,Hg,=,好,g,算符：,小明好不好,好小明,Hb,=,不好,b,算符：,小明好不好,坏小明,HY,=H(,0.5g+0.5b,),=0.5,好,g+0.5,不好,b,但是一次测量只能得到一个值,HY,=,好,g 50%,HY,=,不好,b 50%,Superposition of states,态的叠加,Collapse of the wave,function,波函数的,坍塌,你的测量可以改变对象的状态！,HY,=,好,g,薛定谔的猫,Y=0.5g+0.5b,衰变,未衰变,描述放射性元素的波函数,HY,=,?,衰变了吗？,猫死了吗？,Born-Oppenheimer approximation,the nuclear kinetic energy is neglected,electrons still feel the Coulomb potential of the nuclei clamped at certain positions in space,Potential energy surface,FranckCondon principle,Strokes shift,Reorganization energy,MO theory,H,Y,=E,Y,定性分析,Rule one,:A linear combination of n atomic states will create n MOs.,Overlap between orbitals of comparable energy is more effective than overlap between orbitals of differing energy.,Orbital orientation strongly affects the strength of the resulting bond,Orbital interaction and distance,Excimer,激基复合物,Use simple orbital interaction diagrams to explain each of the following:,MO in conjugation,HOMO-7.26,LUMO 0.512,HOMO-7.3,LUMO-1.1,HOMO-1-10.86,HOMO-6.23 ev,LUMO-0.61 ev,HOMO-1-8.63 ev,HOMO-6.19 ev,LUMO-0.82,Donor and Acceptor,为什么是电子的给体？,-5.48,1.39,-6.33,-0.21,-6.11,0.54,2.25,2.19,2.49,1.436,1.426,1.430,HOMO -5.76,LUMO -0.27,HOMO-5.67,LUMO-3.20,Node=0,Node=1,Node=2,Node=3,benzene-HOMO -6.7,benzene-LUMO 0.1,pyridine-HOMO -6.9,pyridine-LUMO -0.61,Homo,Homo-1,Homo-2,Homo-3,LUMO+3,LUMO+2,LUMO+1,LUMO,dibenzene-HOMO -6.0,dibenzene-LUMO -0.67,benzene-HOMO -6.7,benzene-LUMO 0.1,-9.8,0.6,-6.7,0.1,benzonitrile-LUMO -1.4,benzonitrile-HOMO -7.3,HOMO -5.01708,LUMO 0.364913,HOMO -5.8,LUMO -1.8,对,NN-,二甲基苯腈,ttf-LUMO -0.945073,ttf-HOMO -4.51502,tcnq-LUMO -4.81952,tcnq-HOMO -7.32901,