Dimers and trimers of HF, H₂O, NH₃ and CH₄ with N₂. Ab initio studies on structures and vibrational frequencies

ABSTRACT

High level ab initio calculations have been performed on the trimers of HF, H₂O, NH₃ and CH₄ (XH_n) with N₂ using the Møller-Plesset MP2 method, as well as on respective dimers, using both coupled cluster CCSD(T) and MP2 methods. Guided by structure and stability of the dimers, various structures of trimers were investigated. Dissociation energies D_e are highest for the 2;1 trimers (XH_n)₂-N₂, and smaller for the 1:2 trimers XH_n-(N₂)₂. For dimers and trimers dissociation energies decrease in going from HF to CH₄. For HF to NH₃, dissociation energies of the (XH_n)₂-N₂ trimers lie 100–300 cm⁻¹ above the sum of respective dimer values. Harmonic vibrational frequencies and infrared intensities were calculated using the MP2 method. Frequency shifts are largest for the (XH_n)₂-N₂ complexes. They are negative (red shifted) for stretching modes, and positive (blue shifted) for the bending modes. They decrease in going from HF to CH₄. Infrared intensities for the complexes are usually much enhanced over the monomer values. This applies especially to vibrations corresponding to symmetric stretching modes of (XH_n)₂ and (XH_n)₂-N₂ complexes for H₂O and NH₃. Intensities of intermolecular vibrational modes exceed in many cases the intensities of fundamental modes of the monomers.

GRAPHICAL ABSTRACT

KEYWORDS:

• Dimers of N₂
• HF
• H₂O
• NH₃
• CH₄
• dimers and trimers of HF
• H₂O
• NH₃
• CH₄ with N₂
• structures and dissociation energies
• vibrational frequencies and infrared intensities
• coupled cluster CCSD(T) and Møller-Plesset MP2 methods

Acknowledgements

Long-standing support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Computer time supplied by ComputeCanada is much appreciated.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

*Structures having imaginary frequencies.

^a 104.8 cm⁻¹ with distance and angle optimisation at the CCSD(T)/AV5Z level.

^a 1595 cm⁻¹ from potential energy surfaces [23].

^b From Ref. [13].

^a 1760 cm⁻¹ from potential functions [27].

^b 1743 cm⁻¹ CCSD(T)-F12b/aug-cc-pV5Z [28].

^c taken from Ref. [13].

*Structure having imaginary frequency.

^a 1678 cm⁻¹ MP2/cc-pV5Z [29].

^b 290 cm⁻¹ B3LYP/6-311++G(2d,2p) [5].

^c 2147 cm⁻¹ B3LYP/6-311++G(2d,2p) [5].

^d 1945 cm⁻¹ B3LYP/6-311++G(2d,2p) [5].

^a from Ref. [13].

^b 250.6 cm⁻¹ CCSD(T)-F12a/AVTZ [11].

*Structure having imaginary frequencies.

^a 1020 cm⁻¹ MP2/cc-vV5Z [29].

^b 1080 cm⁻¹ MP2/AVQZ [30].

^c 1034 cm⁻¹ MP2/AVQZ [30].

^a 189 cm⁻¹ CCSD(T)/CBS [34].

*Structures having imaginary frequencies.

^a 141 cm⁻¹ MP4/large basis set [31].

^b 117 cm⁻¹ MP2/5-311+G(2df,2dp) [32].

^c 154 cm⁻¹ MP2/cc-vV5Z [29].

^d 164 cm⁻¹ MP2/CBS [33].

^a Sum of respective additive energies in parentheses.

Note: Frequencies (cm⁻¹) with intensities (km/mol) in parentheses.

Note: Frequencies (cm⁻¹) with intensities (km/mol) in parentheses.

Note: With corresponding monomer frequencies.

^a for the (XH_n)₂-N₂ complex.

^b for the (CH₄)₂ complex.

^c for the CH₄-(N₂)₂ complex.

Additional information

Funding

Long-standing support by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Computer time supplied by ComputeCanada is much appreciated.