氮分子固体中配位环境对其分子振动的影响

Effect of local coordination environment on molecule vibration in N_2-molecule solid

研究了孤立氮分子与处于氮分子固体中氮分子之间的振动频率差异.基于α-N_2晶体结构建立了5种不同氮分子数的氮分子固体团簇模型,采用密度泛函理论计算了孤立自由氮分子及各固体模型中氮分子的振动频率,并对它们的频率进行了比较和讨论.比较发现:受集体效应的影响,处于分子固体模型中的所有氮分子的键长较孤立自由氮分子的键长更短,振动频率更高;就固体模型本身而言,分子数越多,平均振动频率越大,而且,内部氮分子的振动频率总是大于表面氮分子的振动频率,整体来说,频率大小关系为v_(内部)>v_(表面)>v_(孤立).讨论分析认为这种频率差异主要是由于孤立自由氮分子、固体表面和内部分子的配位关系不同引起的;表面分子存在大量配位缺陷,与其相互作用的分子相对较少,氮分子键力较弱,从而频率更低.

The vibration feature in a molecule solid is an important character of its structure.The different vibration frequencies of isolated nitrogen molecule（N2） and nitrogen molecule in the solid state are explored.Five solid-cluster models with the different numbers of nitrogen molecules（N（46）,N（60）,N（76）,N（100）,and N（126）） are constructed on the basis of α-N2 crystal structures.The density functional theory is used to calculate the vibration frequencies of nitrogen molecules.The calculated infrared spectra and average vibration frequencies（AVFs） of the optimized structures for the five models are compared with each other.The results indicate that the AVF of nitrogen molecule in solid model is higher than that of isolated nitrogen molecule due to the collective effect.It is found that the AVF increases with increasing the number of molecules.The AVF of the inner molecules is always higher than that of surface molecules in the solid.On a whole,the vibration frequencies are ordered as v（inner）〉 v（surface） 〉v（isolated） for each case.The local coordination environment is believed to be mainly responsible for the differences in frequency among the isolated,surface and inner molecules.The bond length of molecule in solid is shorter than that in an isolated molecule,thus resulting in a stronger bond force and a higher vibration frequency.Similarly,due to a smaller number of molecules interacting with surface molecules,the bond force between molecules in the solid surface is weaker,thus resulting in a lower vibration frequency than in the inner region of solid.