Double hydrogen bonding of acetic acid studied by femtosecond degenerate four-wave mixing

Abstract

We report for the first time the successful application of rotationally resolving spectroscopy to the acetic acid dimer. We report a study by femtosecond degenerate four-wave mixing (fs DFWM) in a gas cell at room temperature under supersonic jet conditions (ca. 90 K). From the fitting analysis, two centrifugal distortion constants and the sum of the rotational constants (B + C) = 1657.2 ± 1.3 MHz and (B + C) = 1445.2 ± 1.1 MHz were obtained for (CH₃COOH)₂ and (CD₃COOD)₂, respectively. These (B + C) values are in good agreement with the results of ab initio geometry optimizations. A detailed analysis of the experimental spectrum produced constraints for the polarizablity tensor and the molecular asymmetry (B − C). The fs DFWM spectroscopic behaviour of acetic acid is compared with previous results obtained for formic acid.

Keywords:

• Acetic acid
• Double hydrogen bonding
• Femtosecond degenerate four-wave mixing

Acknowledgements

We gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG), the Johann Wolfgang Goethe-Universität Frankfurt/M. and the Fonds der Chemie for financial support. We grieve over the sudden passing of Włodek Jarzęba (Kraków, Poland), who set up the fs DFWM experiment and helped in the theoretical analysis. He was an exceptional scientist and our friend. We thank Daniil Kosov (University of Maryland, USA) for help with the DFT calculations of the acetic acid dimer. P. Hobza and J. Chocholoušová (Prague, Czech Republic) are gratefully acknowledged for communicating detailed results on the MP2 calculations of the acetic acid dimer.