Vibrational Spectroscopy of Protonated Imidazole and its Complexes with Water Molecules: Ab Initio Anharmonic Calculations and Experiments
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- 作者：Adeyemi A. Adesokan ; Galina M. Chaban ; Otto Dopfer ; R. Benny Gerber
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：August 9, 2007
- 年：2007
- 卷：111
- 期：31
- 页码：7374 - 7381
- 全文大小：161K
- 年卷期：v.111,no.31(August 9, 2007)
- ISSN：1520-5215
文摘
The results of anharmonic frequency calculations on neutral imidazole (C3N2H4, Im), protonated imidazole(ImH+), and its complexes with water (ImH+)(H2O)n, are presented and compared to gas phase infraredphotodissociation spectroscopy (IRPD) data. Anharmonic frequencies are obtained via ab initio vibrationalself-consistent field (VSCF) calculations taking into account pairwise interactions between the normal modes.The key results are: (1) Prediction of anharmonic vibrational frequencies on an MP2 ab initio potentialenergy surface show excellent agreement with experiment and outstanding improvement over the harmonicfrequencies. For example, the ab initio calculated anharmonic frequency for (ImH+)(H2O)N2 exhibits an overallaverage percentage error of 0.6% from experiment. (2) Anharmonic vibrational frequencies calculated on asemiempirical potential energy surface fitted to ab initio harmonic data represents spectroscopy well, particularlyfor water complexes. As an example, anharmonic frequencies for (ImH+)H2O and (ImH+)(H2O)2 show anoverall average deviation of 1.02% and 1.05% from experiment, respectively. This agreement between theoryand experiment also supports the validity and use of the pairwise approximation used in the calculations. (3)Anharmonic coupling due to hydration effects is found to significantly reduce the vibrational frequencies forthe NH stretch modes. The frequency of the NH stretch is observed to increase with the removal of a watermolecule or replacement of water with N2. This result also indicates the ability of the VSCF method topredict accurate frequencies in a matrix environment. The calculation provides insights into the nature ofanharmonic effects in the potential surface. Analysis of percentage anharmoncity in neutral Im and ImH+shows a higher percentage anharmonicity in the NH and CH stretch modes of neutral Im. Also, we observethat anharmonicity in the NH stretch modes of ImH+ have some contribution from coupling effects, whilethat of neutral Im has no contribution whatsoever from mode-mode coupling. It is concluded that theincorporation of anharmonic effects in the calculation brings theory and experiment into much closer agreementfor these systems.