Characterization of Competing Halogen- and Hydrogen-Bonding Motifs in Simple Mixed Dimers of HCN and HX (X = F, Cl, Br, and I).

This work performs the first systematic comparison of hydrogen- and halogen-bonded configurations of the HCN/HX mixed dimer, where X = F, Cl, Br, and I. Eleven different minima have been characterized for these four heterogeneous dimers near the CCSD(T) complete basis set (CBS) limit. For each complex, two different hydrogen-bonded minima were identified: the global minimum where HX acts as the hydrogen bond donor and a local minimum where HX acts as the hydrogen bond acceptor. A halogen-bonded local minimum was also identified for all but the fluorine mixed dimer. To the best of our knowledge, three of the minima are identified here for the first time. The hydrogen- and halogen-bonded local minima of each complex become more energetically competitive with the global minimum as the atomic radius of the halogen atom increases. CCSD(T) relative energies of the hydrogen-bonded local minima computed near the CBS limit decrease from 4.5 kcal mol-1 for HCN/HF to 2.9, 2.4, and 1.2 kcal mol-1 for X = Cl, Br, and I, respectively. Corresponding relative energies for the halogen-bonded local minima range from 4.0 kcal mol-1 for X = Cl to 2.7 kcal mol-1 for X = Br and to as little as 0.5 kcal mol-1 X = I. Harmonic vibrational frequency shifts reported here suggest that it may be feasible to differentiate between the various minima for X = Cl, Br, and I via spectroscopic analysis, as was the case for the HCN/HF dimer.

Torsional Profiles of Thiophene and Furan Oligomers: Probing the Effects of Heterogeneity and Chain Length.

A systematic analysis of the torsional profiles of 55 unique oligomers composed of two to four thiophene and/or furan rings (n = 2 to 4) has been conducted using three density functional theory (DFT) methods along with MP2 and three different coupled-cluster methods. Two planar or quasi-planar minima were identified for each n = 2 oligomer system. In every case, the torsional angle (τ) between the heteroatoms about the carbon-carbon bond connecting the two rings is at or near 180° for the global minimum and 0° for the local minimum, referred to as anti and syn conformations, respectively. These oligomers have rotational barrier heights ranging from ca. 2 kcal mol-1 for 2,2'-bithiophene to 4 kcal mol-1 for 2,2'-bifuran, based on electronic energies computed near the CCSD(T) complete basis set (CBS) limit. The corresponding rotational barrier for the heterogeneous 2-(2-thienyl)furan counterpart falls approximately halfway between those values. The energy differences between the minima are approximately 2 and 0.4 kcal mol-1 for the homogeneous 2,2'-bifuran and 2,2'-bithiophene, respectively, whereas the energy difference between the planar local and global minima (at τ = 0 and 180°, respectively) is only 0.3 kcal mol-1 for 2-(2-thienyl)furan. Extending these three oligomers by adding one or two additional thiophene and/or furan rings resulted in only minor changes to the torsional profiles when rotating around the same carbon-carbon bond as the two-ring profiles. Relative energy differences between the syn and anti conformations were changed by no more than 0.4 kcal mol-1 for the corresponding n = 3 and 4 oligomers, while the rotational barrier height increased by no more than 0.8 kcal mol-1.

Anchoring the hydrogen sulfide dimer potential energy surface to juxtapose (H2S)2 with (H2O)2.

Twelve stationary points have been characterized on the (H2S)2 potential energy surface using the MP2 and CCSD(T) methods with large, correlation consistent basis sets. To the best of our knowledge, five of the structures have not been identified elsewhere and are presented here for the first time. A similar analysis was performed on the ten, well-known structures of the water dimer in order to facilitate direct comparisons between the corresponding (H2O)2 and (H2S)2 configurations. Harmonic vibrational frequency computations identify three (H2S)2 configurations as minima, four as transition states, and five as higher-order saddle points (ni = 0, ni = 1, and ni ≥ 2, respectively, where ni is the number of imaginary frequencies). The two local minima and four transition state structures identified have electronic energies within 0.73 kJ mol-1 of the global minimum near the CCSD(T) complete basis set (CBS) limit, and the five higher-order saddle points range from 1.90 kJ mol-1 to 4.31 kJ mol-1 above the global minimum at the same level of theory. One of the more substantial differences observed between the H2S and H2O systems is that (H2O)2 has only a single minimum, while the other nine stationary points are significantly higher in energy ranging from 2.15 kJ mol-1 to 14.89 kJ mol-1 above the global minimum near the CCSD(T) CBS limit. For (H2S)2, the electronic dissociation energy of the global minimum is only 7.02 kJ mol-1 at the CCSD(T) CBS limit, approximately three times smaller than the dissociation energy of (H2O)2.