title: An ab initio anharmonic approach to study vibrational spectra of small ammonia clusters.
abstract: Fermi resonance between the N-H stretching (ν1and ν3) and the overtone of N-H bending (2ν4) in ammonia has hindered the interpretation and assignments of experimental spectra of small ammonia clusters. In this work, we carried out anharmonic vibrational calculations using MP2/aug-cc-pVDZ to examine the vibrational spectra of (NH3)n=1-5with a focus on the size evolution. The enhancement of hydrogen bond strength due to cooperative effects will cause ν1and ν4to red-shift and blue-shift, respectively, when the size of the cluster increases. Our calculations show that the energy order of fundamental of ν1and overtone of ν4is reversed between n = 3 and n = 4. Therefore, while the resultant mixed levels do not show remarkable shifts in their peak positions, the main identity of these mixed levels changes and this causes significant re-distribution of their intensities. Furthermore, our ab initio anharmonic calculation scheme can directly evaluate the coupling strength between different N-H stretching and overtone of N-H bending without any experimental parameters, thus leading us to a simpler picture to understand the Fermi resonance in (NH3)n.
authors: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. jlkuo@pub.iams.sinica.edu.tw and Department of Physics, National Taiwan University, Taipei 10617, Taiwan.
authors: Kun-Lin
authors: KL
authors: Ho
authors: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. jlkuo@pub.iams.sinica.edu.tw.
authors: Lo-Yun
authors: LY
authors: Lee
authors: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. asukafujii@m.tohoku.ac.jp.
authors: Marusu
authors: M
authors: Katada
authors: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. asukafujii@m.tohoku.ac.jp.
authors: Asuka
authors: A
authors: Fujii
authors: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. jlkuo@pub.iams.sinica.edu.tw.
authors: Jer-Lai
authors: JL
authors: Kuo