Sean A. C. McDowell

Molecular polarisabilities - a comparison of density functional theory with standard ab initio methods

Abstract Molecular polarisabilities calculated with density functional theory using both the local density approximation and a gradient corrected functional are contrasted with results obtained using standard ab initio methods. The ab initio methods used are SCF, second-order and fourth-order Moller-Plesset perturbation theory and coupled-cluster methods using Brueckner orbitals. The best agreement with experimental data comes from the coupled-cluster calculations. SCF values are systematically too low, and DFT results are too high.

Blue shifting and red shifting hydrogen bonds: A study of the HArF⋯N2 and HArF⋯P2 complexes

An ab initio computational study of the properties of two weakly bound linear complexes formed between the recently discovered argon-containing compound HArF and the homonuclear diatomic molecules N2 and P2 was undertaken at the MP2/6-311++G(2d,2p) level of theory. The complex N2⋯HArF was found to have a binding energy of 912 cm−1 with respect to the monomer subunits, while the other complex P2⋯HArF was found to have a binding energy of 1130 cm−1. The N2⋯HArF complex exhibits a large harmonic vibrational frequency blue shift of 153 cm−1 for the Ar–H stretching vibration mode, while P2⋯HArF exhibits a large red shift of 116 cm−1 for the same vibrational mode; in both complexes the IR intensity of the Ar–H stretching vibration decreases on complexation. In this study we attempt to explain these frequency shifts.

Blue-shifting hydrogen bonding in N2⋯HKrF

An ab initio computational study of the properties of the weakly bound linear complex N2⋯HKrF was undertaken at three different levels of theory using a 6-311G** basis set. The dimer was found to have a zero-point vibrational energy corrected binding energy of −2.8 kJ mol−1 with respect to the monomer subunits, at the highest level of theory used in this study. This complex exhibits a large harmonic vibrational frequency blue-shift of about 60 cm−1 for the H–Kr stretching mode with a decrease in the infrared intensity for this mode on formation of the complex. This unusual result was rationalized mainly by consideration of the effect of the electrostatic interaction and charge transfer between the HKrF and N2 molecules.

A computational study of the HArF…CO dimer

Abstract An ab initio study of the properties of the linear hydrogen-bonded complex, OC…HArF, was undertaken at the MP2/6-311++G(2d,2p) level of theory. The complex has a binding energy of 21 kJ mol −1 and exhibits a harmonic vibrational frequency blue shift of 10 cm−1 for the Ar–H stretching mode, a 7-fold decrease in the IR intensity of this stretching mode and contraction of the Ar–H bond. These results, which are the opposite of what is expected for normal hydrogen bonds, are explained in terms of charge transfer and electrostatic effects.

Comparison of the intermolecular properties of N2…HArF with N2…HF

An ab initio computational study of the properties of a weakly bound complex formed between the recently discovered argon-containing compound HArF and N2 was undertaken at the MP2/6-311G** level of theory. The complex N2…HArF was found to have a zero-point vibrational energy corrected binding energy of 396 cm−1 with respect to the monomer subunits. This complex exhibits a large harmonic vibrational frequency blue shift of 195 cm−1 for the Ar–H stretching vibration mode with a diminished infrared intensity for this mode on formation of the complex. This surprising result prompted a study to understand the source of the blue shift and the results were compared with corresponding computations for the N2…HF complex, which has the same proton acceptor but which shows the usual red shift of the H–F stretching mode.

A computational study of hydrogen-bonded complexes of HKrCl: N2⋯HKrCl, OC⋯HKrCl, and HF⋯HKrCl

An ab initio computational study of the properties of weakly bound hydrogen-bonded dimers of HKrCl and the small molecules N2, CO, and HF was undertaken at the MP2/6-311++G(2d,2p) level of theory. These complexes, the linear N2⋯HKrCl and OC⋯HKrCl dimers, and the nonlinear HF⋯HKrCl dimer, were found to have large blue shifts of the H–Kr stretching frequency, accompanied by a decrease in the infrared intensity of the stretching mode and compression of the H–Kr bond. We examine the charge density reorganization within the HKrCl molecule on complexation and offer an explanation for these unusual vibrational results.

The effect of atomic ions on model σ-hole bonded complexes of AH3Y (A = C, Si, Ge; Y = F, Cl, Br)

A computational study of ionic X···AH3-Y complexes (X = F(-), Cl(-), Br(-), Li(+), Be(2+); A = C, Si, Ge; Y = F, Cl, Br) predicted optimized structures which are held together by a combination of attractive forces, including ion-dipole and ion-σ-hole electrostatic interactions, and polarization forces. The trends (with variation in the halogen Y) for selected properties were rationalized by considering the electron density shifts due to the ions electric field. Although it has been found previously that the trends for binding energies in neutral complexes follow the sigma-hole strength, the present study found that the dependence on the dipole polarizability of the A-Y bond can explain the trends for binding energies in these more strongly bound ionic complexes.

A computational study of the dihydrogen bonded complexes HBeH⋯HArF and HBeH⋯HKrF

We report an ab initio computational study of the properties of two linear dihydrogen-bonded complexes of HBeH with the recently discovered rare gas compounds HArF and HKrF at the MP2(full)/6-311++G(2d,2p) level of theory. The HBeH ... HArF and HBeH ... HKrF complexes were found to have zero-point energy corrected binding energies of 27 and 12 kJ mol(-1), respectively. Large red shifts of the H-Rg vibrational stretching frequency in both complexes were also predicted. The electron density rearrangement of the rare gas compounds on complexation was also examined. We also consider the relative stabilities of D-containing isotopomers of the complexes by comparison of their computed zero-point vibrational energies.

A theoretical study of H–Ar–Cl

Ab initio calculations at the B3LYP and MP4(SDQ) levels of theory were performed on the noble gas-containing compound H–Ar–Cl. The calculations indicate that the molecule should be metastable with an activation barrier of 73.0 kJ mol−1 for the lowest-energy fragmentation to HCl and Ar. Similar calculations on H–He–Cl and H–Ne–Cl failed to locate stable species. This result suggests that it should be possible to generate and observe H–Ar–Cl at sufficiently low temperatures.

Cooperative and diminutive hydrogen bonding in Y...HCN...HCN and NCH...Y...HCN trimers (Y=BF,CO,N2).

A computational study of the cooperative effect of hydrogen bonding in Y...HCN...HCN and its diminutive effect in NCH...Y...HCN (Y=BF,CO,N2) linear complexes relative to the Y...HCN dimer was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the additional hydrogen bond in Y...HCN...HCN leads to an enhanced Y...HCN dissociation energy, extended H-C bond length, and larger redshift of the H-C stretch relative to Y...HCN, while opposite features are observed in NCH...Y...HCN. The cooperativity is diminished as the hardness of the Y atom directly bonded to the HCN molecule increases. A particularly interesting result is that the small bond contraction and blueshift associated with the H-C bond in BF...HCN is converted to a small bond extension and redshift on the formation of the BF...HCN...HCN trimer.