The potential energy surface (PES), structures and thermal properties of methanol clusters (MeOH) with n = 8-15 were explored by replica-exchange molecular dynamics (REMD) simulations with an empirical model and refined using density functional theory (DFT) methods. For a given size, local minima structures were sampled from REMD trajectories and archived by a newly developed molecular database via a two-stage clustering algorithm (TSCA). Our TSCA utilizes both the topology of O-HO hydrogen bonding networks and the similarity of the shapes to filter out duplicates. The screened molecular database contains only distinct conformers sampled from REMD and their structures are further optimized by the two DFT methods with and without dispersion correction to examine the influence of dispersion on their structures and binding energies. Inspecting different O-HO networks, the binding energies of methanol clusters are highly degenerated. The degeneracy is more significant with the dispersion effect that introduces weaker but more complex C-HO bonds. Based on the structures we have searched, we were able to extract general trends and these datasets can serve as a starting point for further high-level ab initio calculations to reveal the true energy landscape of methanol clusters.

Fermi resonance between the N-H stretching (ν and ν) and the overtone of N-H bending (2ν) 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 (NH) with a focus on the size evolution. The enhancement of hydrogen bond strength due to cooperative effects will cause ν and ν to red-shift and blue-shift, respectively, when the size of the cluster increases. Our calculations show that the energy order of fundamental of ν and overtone of ν is 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 (NH).

Methylamine (MMA) is one of the simplest amines, and the vibrational spectra of its dimer have recently been obtained experimentally. The vibrational spectra of NH stretch modes were well resolved, but the complex features of the CH group could not be fully accounted for even with the assistance of ab initio molecular dynamics (AIMD) with various density functional methods. In this study, we carried out anharmonic vibrational calculations on MMA clusters up to tetramers using MP2/aug-cc-pVDZ to examine vibrational coupling among CH/NH and compute the vibrational spectra of these clusters between 2800 and 3500 cm. We found that the main origin of the complexity between 2800 and 3000 cm was caused by Fermi resonance (FR) between the stretching and bending overtones of the CH group. This spectral feature becomes simpler in trimers and tetramers. Furthermore, Fermi resonance in the NH group is found to be very strong. In the MMA dimer, no noticeable FR features can be found; however, in its trimers and tetramers, the enhancement of hydrogen bond strength due to the cooperative effect will cause the N-H stretching mode to red-shift to revert the energy order of the fundamental of the N-H stretch and overtone of N-H bending between n = 3 and n = 4. Therefore, significant re-distribution of the intensities of the bands at 3200 and 3300 cm should be seen.

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Anharmonic vibrational coupling among N-H stretching fundamental (ν 1 and ν 3 ) and N-H bending overtone (2ν 4 ) vibrations in (NH 3 ) n (n = 1 to 5) are analyzed based a full dimensional Hamiltonian including third and quartic terms. In particular, we examine Fermi resonance between the symmetric N-H stretching (ν 1 ) and N-H bending overtone (2ν 4 ) vibrations. As the cluster size increases, enhancement of the hydrogen bond strength makes ν 1 red-shifted while 2ν 4 blueshifted. These shifts result in the crossing of the frequencies of ν 1 and 2ν 4 levels, and their energy order reverses between n = 3 to n = 4. Because the nature of Fermi resonance, although the zero-order ν 1 and 2ν 4 levels are shifted, the resultant mixed levels do not show remarkable changes in frequency. Instead, the major component of each mixed level largely changes and this causes significant redistribution of the intensity. Our results offer a solution to resolve puzzles on the intensity distribution and assignments of the Fermi mixing bands in the previously reported infrared spectra of (NH 3 ) n .