Biomedical subjects

Sean A C McDowell

Publications and source records attributed to Sean A C McDowell.

7 recordsLinked to original sources

PubMed2005

On the correlation between bond-length change and vibrational frequency shift in hydrogen-bonded complexes: a computational study of Y...HCl dimers (Y = N2, CO, BF).

The H-Cl bond-length change and the harmonic vibrational frequency shift of the H-Cl stretch on formation of the linear isoelectronic Y...H-Cl complexes (Y = N(2), CO, BF) have been determined by ab initio computations at different levels of theory. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the H-Cl bond length from its equilibrium value in the isolated monomer. At the highest level of theory, blue shifts were obtained for BF...HCl and CO...HCl, while red shifts were obtained for FB...HCl, OC...HCl, and N(2)...HCl. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The widely believed correlation between the bond-length change and the sign of the frequency shift obtained on complexation is discussed and found to be unreliable.

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PubMed2005

Redshift and blueshift of the Ar-H vibrational stretching frequency in complexes of FArH and acetylene.

Two planar hydrogen-bonded complexes of FArH and acetylene were found to be stable using second-order Møller-Plesset perturbation theory (MP2) with 6-311++G(2d,2p) basis sets. The more stable complex involves bonding between the F atom of FArH and a H atom of acetylene, while the other isomer is a T-shaped complex with the H atom of FArH bonded to the center of the CC bond of acetylene; the zero-point energy corrected dissociation energies are 29 and 27 kJ mol(-1), respectively. Interestingly, the Ar-H harmonic vibrational stretching frequency is blueshifted in the more stable isomer and redshifted in the less stable form. The electron density rearrangement of FArH on complexation was investigated and used to explain these unusual findings.

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PubMed2005

Comparison of some vibrational features of FArH...Rg and FHc...Rg complexes (Rg=He, Ne, Ar, Kr).

The shift in the harmonic vibrational frequency of the ArH stretch of FArH on formation of the linear FArH...Rg complexes (Rg=He, Ne, Ar, Kr), and of the FH stretch on formation of the linear FH...Rg complexes, has been determined by ab initio computations. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the ArH (FH) bond length from its equilibrium value in the monomer. In the FH...Rg dimers, small blue shifts were obtained for the He and Ne complexes and red shifts for those with Ar and Kr. In the FArH...Rg dimers, blue shifts were obtained for all four complexes. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The bond length changes on complexation are also well predicted by the model. Our computations were restricted to the linear geometry since the objective was to investigate the validity of the perturbation model and to illuminate the causes of the red and blue shifts.

Argon↗

PubMed2004

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.

Argon↗

PubMed2004

Are insertion compounds of CH2CHF and the rare gases stable? A computational study.

Ab initio calculations, using second-order Møller-Plesset perturbation theory with a 6-311+ +G(2d,2p) basis set, predict the stability of two novel compounds of monofluoroethene, CH2CHF, with the rare-gas atoms Ar and Kr. The dissociation energies to the lowest-energy fragmentation products, CH2CHF + Rg (Rg = Ar,Kr), were computed to be -528 and -449 kJ mol(-1), respectively, at the coupled-cluster singles, doubles, and triples level of theory. Possible transition states (at second-order Møller-Plesset theory) via a C-Rg-F bending mode for these fragmentation reactions were also located with barrier heights of about 76 and 106 kJ mol(-1), for the Ar- and Kr-containing species, respectively. However, the Ar-containing species may not exist at all as it is less stable than the fragments CH2CH + F + Ar at the higher level of theory and may possibly dissociate via this route.

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PubMed2004

Intermolecular complexes of HArF and HKrF with CO.

Stable linear weakly bound hydrogen-bonded complexes of HArF and HKrF with the CO molecule have been predicted by ab initio computations at the MP2/6- 311+ +G(2d,2p) level of theory. The complexes, having stabilities in the order, FArH...CO>FKrH...CO>FArH...OC>FKrH...OC are compared. They exhibit unusual blueshifts of the Ar-H (Kr-H) stretching frequency, as well as contraction of the Ar-H (Kr-H) bond, and these effects are rationalized mainly in terms of shifts in the electron density of HArF (HKrF) on complexation, caused mainly by a combination of the intermolecular electrostatic interaction, electron-electron (Pauli) and nuclear-nuclear repulsion and charge density transferred from the CO molecule to the rare-gas-containing molecule.

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PubMed2003

A computational study of the LiH dimer.

A theoretical study of the properties of the linear LiH dimer was undertaken. In this dimer, an unusual type of hydrogen bonding (termed "inverse" hydrogen bonding by some authors), which involves the hydrogen bonded molecule acting as an electron donor (rather than as a proton donor), is exhibited. The optimized geometry, dipole moment, interaction energy, atomic charges, harmonic vibrational frequencies, and frequency shifts for the dimer are computed at the SCF, MP2, and QCISD levels of theory using mainly a 6-31++G(d,p) basis set. We also examined the relative stability of the mono-deuterated isotopomers of linear (LiH)(2), i.e., Li-H...Li-D and Li-D...Li-H. Analysis of the normal vibrational modes, changes in the partial atomic charges, and changes in the vibrational frequencies of LiH on complexation were used to gain insight into the bonding and properties of the linear LiH dimer and its isotopomers.

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