Sublimation Properties of α,ω‐Diamines Revisited from First‐Principles Calculations

Abstract

Sublimation enthalpies of alkane‐α,ω‐diamines exhibit an odd‐even pattern within their homologous series. First‐principles calculations coupled with the quasi‐harmonic approximation for crystals and with the conformation mixing model for the ideal gas are used to explain this phenomenon from the theoretical point of view. Crystals of the odd and even alkane‐α,ω‐diamines distinctly differ in their packing motifs. However, first‐principles calculations indicate that it is a delicate interplay of the cohesive forces, phonons, molecular vibrations and conformational equilibrium which governs the odd‐even pattern of the sublimation enthalpies within the homologous series. High molecular flexibility of the alkane‐α,ω‐diamines predetermines higher sensitivity of the computational model to the quality of the optimized geometries and relative conformational energies. Performance of high‐throughput computational methods, such as the density functional tight binding (DFTB, GFN2‐xTB) and the explicitly correlated dispersion‐corrected Møller‐Plesset perturbative method (MP2C‐F12), are benchmarked against the consistent state‐of‐the‐art calculations of conformational energies and interaction energies, respectively.

Tuning the quasi‐harmonic treatment of crystalline ionic liquids within the density functional theory

“…The basis set pob-tzvp-rev2, refined to minimize the BSSE, then shifts the E el (V) curves to larger volume than the other GTO bases. Such a behavior of periodic DFT calculations has been observed in our recent study for molecular crystals of non-ionic (yet polar) aliphatic diamines, 22 being thus a general rule rather than a particularity for modeling of the ionic liquids. Numerical values of the E el (V) based volumes are listed in Table S1.…”

“…Following the quasi‐harmonic approximation, 34 geometries of the scaled unit‐cells, the vectors of which were gradually scaled by factors ranging from 0.95 to 1.08 with a step 0.01, were reoptimized with the unit‐cell volumes kept fixed to construct the electronic energy–volume E el ( V ) curve. The compression and expansion branches of the curve were then fitted separately by a Murnaghan equation of state 22 . Phonons were calculated for the specified scaled unit‐cells around the minimum of the E el ( V ) curve.…”

“…The compression and expansion branches of the curve were then fitted separately by a Murnaghan equation of state. 22 Phonons were calculated for the specified scaled unit-cells around the minimum of the E el (V) curve. In case of both PBE/PAW and PBE/GTO calculations of phonons, the finite-displacement method, 47 implemented in the Phonopy code, 48 was used and the supercells replicated so that their dimensions exceeded 10 Å in each direction.…”

“…However, such statements should be validated on broader test sets. In our recent study focusing on sublimation of aliphatic α,‐ω‐diamines, 22 we included an assessment of the trends of quasi‐harmonic DFT calculations of structural properties with various plane‐wave and GTOs set‐ups, but without having addressed the basis‐set dependence of the predicted phonons.…”

Statistical Mechanical Approximations to More Efficiently Determine Polymorph Free Energy Differences for Small Organic Molecules

“…The simplicity and low computational cost of harmonic approaches is desirable in a CSP, where 50–200 structures could exist within 2.5 kcal/mol of the global minimum and therefore screening at some level for entropy contributions to stability is useful. Additionally, harmonic approaches can easily be applied with quantum levels of theory, which have successfully predicted polymorph thermodynamic properties. − …”