Slipchenko Mikhail N, Sartakov Boris G, Vilesov Andrey F, Xantheas Sotiris S
Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
J Phys Chem A. 2007 Aug 9;111(31):7460-71. doi: 10.1021/jp071279+. Epub 2007 May 27.
Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the Møller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.
利用氦滴隔离技术和第一性原理电子结构非谐计算，获得了(NH₃)ₙ团簇（n = 2 - 4）中NH伸缩振动的红外光谱。测量的光谱显示出分辨良好的谱带，这些谱带已被归属为团簇中氨片段的ν₁、ν₃和2ν₄模式。氨二聚体中氢键的形成导致红外强度增加约4倍。在较大的团簇中，每个氢键的红外强度接近二聚体中的值，并接近NH₃晶体中的值。三聚体和四聚体中2ν₄泛频带的强度相对于单体和二聚体增加了10倍，并且与较大团簇中ν₁和ν₃基频带的强度相当。这表明在较大团簇中2ν₄和ν₁模式开始出现强非谐耦合。将实验归属与从二聚体和三聚体团簇的第一性原理电子结构非谐计算中获得的归属进行了比较。非谐计算是在电子结构理论的Møller - Plesset（MP2）水平上进行的，并且基于对振转参数及其对振动光谱和平均结构的影响的二阶微扰评估。一般来说，实验测量的N - H伸缩频率的谱带起源与计算的非谐振动频率之间存在极好的（<20 cm⁻¹）一致性。然而，发现计算高估了团簇中的红外强度约4倍。
