Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Journal Article · Thu Aug 09 04:00:00 EDT 2007 · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

DOI:https://doi.org/10.1021/jp071279+· OSTI ID:915703

Slipchenko, Mikhail N; Sartakov, Boris G; Vilesov, Andrey F; Xantheas, Sotiris S

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 v₁, v₃, and 2v₄ 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 four. In the larger clusters the infrared intensity per hydrogen bond is close to the one for dimers and approaches the value in the NH₃ crystal. The intensity of the 2v₄ 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 v₁ and v₃ fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2v₄ and v₁ 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 four. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 915703
• Report Number(s): PNNL-SA-53939; KC0301020
• Journal Information: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Journal Name: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory Journal Issue: 31 Vol. 111; ISSN 1089-5639; ISSN JPCAFH
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

08 HYDROGEN
AMMONIA
DIMERS
ELECTRONIC STRUCTURE
EVALUATION
HELIUM
HYDROGEN
INFRARED SPECTRA
MONOMERS
SPECTRA
SPECTROSCOPY