Room temperature detection of the (H2)2 dimer

The hydrogen dimer, (H2)2, is among the most weakly bound van der Waals complexes and a prototype species for first principles ab initio studies. The detection of the (H2)2 infrared absorption spectrum was reported more than sixty years ago at a temperature of 20 K. Due to the sharp decrease of the (H2)2 abundance with temperature, detection at room temperature was generally considered hardly achievable. Here we report the first room temperature detection of partly resolved rotational structures of (H2)2 by cavity ring down spectroscopy at sub-atmospheric pressures, in the region of the first overtone band of H2 near 1.2 μm. The quantitative analysis of the absorption features observed around ten allowed or forbidden transition frequencies of the monomer provides insight on the structure of this elusive species and a benchmark for future theoretical studies.

Composition of the Water Dimer and the Heterodimers of Water with N2 and O2 in Earth's Atmosphere

The mole fractions χ and number concentrations n of the water dimer and the heterodimers H2O-N2 and H2O-O2 in Earth's atmosphere are reported up to 20 km. The water dimer data is obtained from published values of the equilibrium constant based on the water equation of state. The mixed equilibrium constants for the heterodimers are obtained from the respective second virial coefficients using an approach introduced by Stogryn and Hirschfelder that extracts the components pertaining to pairwise interactions producing bound and metastable dimers. From these calculations, χ and n for the water dimer and the (H2O)(N2) and (H2O)(O2) heterodimers at standard sea level are 1.79(6) × 10-5, 4.77(12) × 10-5 and 9.90(5) × 10-6 and 4.55(15) × 1014, 1.23(3) × 1016 and 2.56(1) × 1015, respectively. Analytical expressions are provided for these quantities for altitudes between 0-20 km and temperatures from 200-300 K. Sea level values of χ and n are given for two specific locations.

On the nature of sub-THz continuum absorption in CO2 gas, its mixture with Ar, and in pure water vapor

Detailed analysis of the unique broadband millimeter-wave (70-360 GHz) collision-induced absorption spectra in pure CO2 and in its mixture with Ar is presented. The nature of the observed continuum absorption is examined using classical trajectory simulation along with statistical physics consideration. Bimolecular continuum is decomposed in the phase space into separate contributions from the so-called free, quasibound, and true bound molecular pairs, the proportions of which greatly vary with temperature. This partitioning is supported by consideration of the second virial coefficient and excluded volume in pure CO2, Ar, and CO2-Ar. Close similarity between collision-induced absorption in the CO2 containing gases and the water vapor continuum in the subterahertz spectral range is demonstrated. This similarity suggests that the physical principles underlying both continuum absorption phenomena have much in common and, therefore, can be used for continuum modeling.

Pure rotational R(0) and R(1) lines of CO in Ar baths: experimental broadening, shifting and mixing parameters in a wide pressure range versus ab initio calculations

The results of a rigorous study of the two first pure rotational transitions of CO perturbed by Ar are presented. The experimental part is based on the use of three different spectrometers covering together the pressure range from 0.02 up to 1500 torr. The measurement results of collisional line shape parameters are supported by fully ab initio calculations, which are in remarkable agreement with retrieved data. A sub-percent uncertainty of line intensity measurements is achieved and the first firm evidence that the resonance spectrum of CO is observed on the continual pedestal is given. We analyze the results of our ab initio calculations on the basis of early analytical theories and demonstrate a good general applicability of the latter to the CO-Ar collisional system.

Reduced-dimensional vibrational models of the water dimer

A model based on the finite-basis representation of a vibrational Hamiltonian expressed in internal coordinates is developed. The model relies on a many-mode, low-order expansion of both the kinetic energy operator and the potential energy surface (PES). Polyad truncations and energy ceilings are used to control the size of the vibrational basis to facilitate accurate computations of the OH stretch and HOH bend intramolecular transitions of the water dimer (H₂ ¹⁶O)₂. Advantages and potential pitfalls of the applied approximations are highlighted. The importance of choices related to the treatment of the kinetic energy operator in reduced-dimensional calculations and the accuracy of different water dimer PESs are discussed. A range of different reduced-dimensional computations are performed to investigate the wavenumber shifts in the intramolecular transitions caused by the coupling between the intra- and intermolecular modes. With the use of symmetry, full 12-dimensional vibrational energy levels of the water dimer are calculated, predicting accurately the experimentally observed intramolecular fundamentals. It is found that one can also predict accurate intramolecular transition wavenumbers for the water dimer by combining a set of computationally inexpensive reduced-dimensional calculations, thereby guiding future effective-Hamiltonian treatments.

Explicit treatment of hydrogen bonds in the universal force field: Validation and application for metal-organic frameworks, hydrates, and host-guest complexes

A straightforward means to include explicit hydrogen bonds within the Universal Force Field (UFF) is presented. Instead of treating hydrogen bonds as non-bonded interaction subjected to electrostatic and Lennard-Jones potentials, we introduce an explicit bond with a negligible bond order, thus maintaining the structural integrity of the H-bonded complexes and avoiding the necessity to assign arbitrary charges to the system. The explicit hydrogen bond changes the coordination number of the acceptor site and the approach is thus most suitable for systems with under-coordinated atoms, such as many metal-organic frameworks; however, it also shows an excellent performance for other systems involving a hydrogen-bonded framework. In particular, it is an excellent means for creating starting structures for molecular dynamics and for investigations employing more sophisticated methods. The approach is validated for the hydrogen bonded complexes in the S22 dataset and then employed for a set of metal-organic frameworks from the Computation-Ready Experimental database and several hydrogen bonded crystals including water ice and clathrates. We show that the direct inclusion of hydrogen bonds reduces the maximum error in predicted cell parameters from 66% to only 14%, and the mean unsigned error is similarly reduced from 14% to only 4%. We posit that with the inclusion of hydrogen bonding, the solvent-mediated breathing of frameworks such as MIL-53 is now accessible to rapid UFF calculations, which will further the aim of rapid computational scanning of metal-organic frameworks while providing better starting points for electronic structure calculations.

Classical calculation of the equilibrium constants for true bound dimers using complete potential energy surface

The present paper aims at deriving classical expressions which permit calculation of the equilibrium constant for weakly interacting molecular pairs using a complete multidimensional potential energy surface. The latter is often available nowadays as a result of the more and more sophisticated and accurate ab initio calculations. The water dimer formation is considered as an example. It is shown that even in case of a rather strongly bound dimer the suggested expression permits obtaining quite reliable estimate for the equilibrium constant. The reliability of our obtained water dimer equilibrium constant is briefly discussed by comparison with the available data based on experimental observations, quantum calculations, and the use of RRHO approximation, provided the latter is restricted to formation of true bound states only.