Single crystal X-ray diffraction is arguably the most definitive method for molecular structure determination, but the inability to grow suitable single crystals can frustrate conventional X-ray diffraction analysis. We report herein an approach to molecular structure determination that relies on a versatile toolkit of guanidinium organosulfonate hydrogen-bonded host frameworks that form crystalline inclusion compounds with target molecules in a single-step crystallization, complementing the crystalline sponge method that relies on diffusion of the target into the cages of a metal-organic framework. The peculiar properties of the host frameworks enable rapid stoichiometric inclusion of a wide range of target molecules with full occupancy, typically without disorder and accompanying solvent, affording well-refined structures. Moreover, anomalous scattering by the framework sulfur atoms enables reliable assignment of absolute configuration of stereogenic centers. An ever-expanding library of organosulfonates provides a toolkit of frameworks for capturing specific target molecules for their structure determination.
A series of 20 halogen bonded complexes of the types R-Br•••Br − (R is a substituted methyl group) and R´-C≡C-Br•••Br − are investigated at the M06-2X/6-311+G(d,p) level of theory. Computations using a point-charge (PC) model, in which Br − is represented by a point charge in the electronic Hamiltonian, show that the halogen bond energy within this set of complexes is completely described by the interaction energy (ΔE PC) of the point charge. This is demonstrated by an excellent linear correlation between the quantum chemical interaction energy and ΔE PC with a slope of 0.88, a zero intercept, and a correlation coefficient of R 2 = 0.9995. Rigorous separation of ΔE PC into electrostatics and polarization shows the high importance of polarization for the strength of the halogen bond. Within the data set, the electrostatic interaction energy varies between 4 and −18 kcal mol-1 , whereas the polarization energy varies between −4 and −10 kcal mol-1. The electrostatic interaction energy is correlated to the sum of the electron-withdrawing capacities of the substituents. The polarization energy generally decreases with increasing polarizability of the substituents, and polarization is mediated by the covalent bonds. The lower (more favorable) ΔE PC of CBr 4-Br − compared to CF 3 Br•••Br − is found to be determined by polarization as the electrostatic contribution is more favorable for CF 3 Br•••Br −. The results of this study demonstrate that the halogen bond can be described accurately by electrostatics and polarization without any need to consider charge transfer.
A series of 26 hydrogen-bonded complexes between Br− and halogen, oxygen and sulfur hydrogen-bond (HB) donors is investigated at the M06-2X/6–311 + G(2df,2p) level of theory. Analysis using a model in which Br− is replaced by a point charge shows that the interaction energy ($${\Delta E}_{Int}$$ Δ E Int ) of the complexes is accurately reproduced by the scaled interaction energy with the point charge ($${\Delta E}_{Int}^{PC}$$ Δ E Int PC ).This is demonstrated by $${\Delta E}_{Int}=0.86{\Delta E}_{Int}^{PC}$$ Δ E Int = 0.86 Δ E Int PC with a correlation coefficient, R2 =0.999. The only outlier is (Br-H-Br)−, which generally is classified as a strong charge-transfer complex with covalent character rather than a HB complex. $${\Delta E}_{Int}^{PC}$$ Δ E Int PC can be divided rigorously into an electrostatic contribution ($${\Delta E}_{ES}^{PC}$$ Δ E ES PC ) and a polarization contribution ($${\Delta E}_{Pol}^{PC}$$ Δ E Pol PC ).Within the set of HB complexes investigated, the former varies between -7.2 and -32.7 kcal mol−1, whereas the latter varies between -1.6 and -11.5 kcal mol−1. Compared to our previous study of halogen-bonded (XB) complexes between Br− and C–Br XB donors, the electrostatic contribution is generally stronger and the polarization contribution is generally weaker in the HB complexes. However, for both types of bonding, the variation in interaction strength can be reproduced accurately without invoking a charge-transfer term. For the Br−···HF complex, the importance of charge penetration on the variation of the interaction energy with intermolecular distance is investigated. It is shown that the repulsive character of $${\Delta E}_{Int}$$ Δ E Int at short distances in this complex to a large extent can be attributed to charge penetration.
The aromatic hydrocarbons (AHs) fluorobenzene, naphthalene, and 1fluoronaphthalene are introduced as promising alternatives to xenon as propellant for in-space electric propulsion (EP). These storable molecules have similar mass, lower cost, and lower ionization energies compared to xenon, as well as the critical advantage of low post-ionization fragmentation compared to other molecular propellant candidates. The ionization characteristics of AHs are compared with those of xenon and the diamondoid adamantane, previously evaluated as a molecular propellant for EP. Quantum chemical calculations and BEB theory together with 25 eV electron ionization mass spectrometry (EI-MS) measurements have been used to predict the fragmentation of the AHs and adamantane when ionized in a plasma with an electron temperature of 7 eV (a typical electron temperature in EP plasmas). A high fraction (81 − 8 %) of the detected AH ions originate from intact molecules, compared to 3 % for adamantane. indicating extraordinarily low fragmentation for the selected AHs. The ionization potential of the AHs is similar to that of adamantane but lower compared to xenon (8.14–9.2 eV for the AHs, 9.25 for adamantane and 12.13 eV for xenon). BEB calculations have also been used to predict total ionization cross sections. The calculated ionization cross section of the AHs is comparable to that of adamantane but 3–5 times higher than that of xenon, which together with the low ionization potential can contribute to more efficient ionization. The AHs may have the potential to perform better than xenon, despite the absence of fragmentation in xenon.
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