N. M. Cann

Absolute scale determination for photoabsorption spectra and the calculation of molecular properties using dipole sum-rules

An assessment of the absolute scales of photoabsorption differential oscillator strength (df/dE) spectra which were originally normalized using valence shell TRK (i.e. S(O)) sum-rule normalization is presented for a series of dipole (e,e) measurements for 5 noble gases and 52 small molecules. This comprehensive data set involves previously published absolute high resolution dipole (e,e) spectra of the valence shell discrete region combined with very wide range low resolution measurements in the ionization continuum for each atom or molecule. The absolute scales established for the dipole (e,e) spectra using the S(O) sum-rule in the originally published works (data available on the world wide web at ftp://chem.ubc.ca/pub/cooper or by anonymous ftp - see end of present paper for more details) are assessed by deriving static dipole polarizabilities for each atom and molecule using the S(−2) sum-rule. These values are found to be highly consistent with experimental and theoretical literature values of the static dipole polarizability and in almost all cases well within the estimated ±5% accuracy of the originally published TRK sum-rule normalized absolute photoabsorption differential oscillator strength spectra. Significant errors of 8% and 19% in the previously published absolute oscillator strength scales for CCl4 (G.R. Burton, W.F. Chan, G. Cooper, C.E. Brion, Chem. Phys. 181 (1994) 147) and SiF4 (X. Guo, G. Cooper, W.F. Chan, G.R. Burton, C.E. Brion, Chem. Phys. 161 (1992) 453, 471) respectively are revealed by the dipole polarizability analysis and approproate corrections are recommended. Alternative methods of absolute scale determination for photoabsorption spectra using static or dynamic dipole polarizabilities from refractive index or dielectric constant measurements are also considered. These are found to be particularly useful especially where data are restricted to lower photon energies (< 60 eV) or where phenomena such as low lying inner shells, continuum shape resonances or Cooper minima preclude the use of the valence shell TRK sum-rule. As a result of the assessment of the absolute scales for the published dipole (e,e) spectra and since polarizabilities and refractive index data can typically be obtained with even higher precision (+-1%), these data have been used to further refine the measured differential oscillator strength scales. On this basis, dipole oscillator strength sums S(u) (u = −1, −2, −3, −4, −5, −6, −7, −10) and L(u) (u = −1, −2, −3, −4, −5, −6) are obtained from the df/dE spectra. For many of the systems considered, the presently reported dipole sums are derived molecular properties are more accurate than previously reported values. In fact, for 28 of the 52 small molecules these sum-rule values represent the only available values. The dipole sums can be used to calculate normal Verdet constants which are involved in the Faraday effect. It is also shown that accurate values of the rotationally averaged C6(A,B) dispersion coefficients for the long-range interaction of all possible pairs of atomic and/or molecular species can be obtained from the measured differential oscillator strength spectra. Alternatively, an approximation to C6(A,B), requiring only the S(−2) and L(−2) sums, is shown to provide very reliable estimates of C6(A,B).

An investigation of the influence of solute size and insertion conditions on solvation thermodynamics

In this paper we examine the influence of solute size and insertion conditions on solvent structural changes and excess thermodynamic properties in the infinite dilution limit. A general integral equation approach which can be applied under arbitrary conditions is given and isothermal-isochoric and isothermal-isobaric insertions are discussed in detail. Scaling relationships valid in the large solute limit are determined for both structural and thermodynamic properties. This is done by considering macroscopic thermodynamic relationships and explicit evaluation of low solvent density expansions of pair correlation functions. The hypernetted-chain and reference hypernetted-chain closure approximations are used to obtain numerical results for the insertion of hard sphere solutes of varying diameter into hard sphere, dipolar hard sphere and water-like solvents. The results obtained give a good deal of insight into the nature of solvation of inert solutes. It is shown that for all three solvents the excess pro...

How are completely desolvated ions produced in electrospray ionization: insights from molecular dynamics simulations.

We apply molecular dynamics (MD) simulations to study the final phase of electrospray ionization (ESI), where an ion loses all of its associated solvent molecules. By applying an electric field to a cluster of H(2)O molecules solvating an ion and including a surrounding gas of varying pressure, we demonstrate that collisions with the gas play a major role in removing this final layer of solvent. We make quantitative predictions of the critical velocity required for the cluster to start losing molecules via collisions with gas and propose that this should be important in real ESI experiments. Such collisions have heretofore not been explicitly considered in discussions of the ESI process.

Molecular dynamics study of chiral recognition for the whelk-O1 chiral stationary phase.

In this article, we examine the docking of 10 analytes on the Whelk-O1 stationary phase. A proper representation of analyte flexibility is essential in the docking analysis, and analyte force fields have been developed from a series of B3LYP calculations. Molecular dynamics simulations of a representative Whelk-O1 interface, in the presence of racemic analyte and solvent, form the basis of the analysis of chiral selectivity. The most probable docking arrangements are identified, the energy changes upon docking are evaluated, and separation factors are predicted. From comparisons between the analytes, the mechanism of chiral selectivity is divided into contributions from hydrogen bonding, ring-ring interactions, steric hindrance, and molecular flexibility. We find that both hydrogen bonding and ring-ring interactions are necessary to localize the analyte within the Whelk-O1 cleft region. We also identify one docking mechanism that is often dominant and analyze the conditions that lead to alternate docking modes.

Structure of the metal-aqueous electrolyte solution interface

Theoretical results are given for aqueous electrolyte solutions in contact with uncharged metallic surfaces. The metal is modeled as a jellium slab and is treated using local density functional theory. The solution structure is obtained using the reference hypernetted-chain theory. The two phases interact electrostatically and the coupled theories are iterated to obtain fully self-consistent results for the electron density of the metal and surface-particle correlation functions. The metal-induced structure of pure water and aqueous electrolyte solutions as well as the electrostatic potential drop across the interface are discussed in detail. The results are compared with those for ions in simple dipolar solvents. It is found that the water molecules are ordered by the metal field and that the surface-induced solvent structure strongly influences the ion distributions.

Momentum profiles for open shell molecules: studies of the HOMOs of NO, O2 and NO2 by electron momentum spectroscopy and SCF, post-Hartree-Fock and DFT calculations

Abstract Measurements of the outermost valence electron orbital momentum profiles of the open shell molecules NO, O 2 and NO 2 have been obtained using electron momentum spectroscopy (EMS). The presently reported experimental momentum profiles of NO and O 2 display much improved statistics compared with previously published EMS results while the data for the HOMO of NO 2 is the first reported. In the case of NO, the present measurements are considerably different from previous results and these differences appear to be due to the presence of NO 2 impurities in the previous work. The EMS measurements provide a stringent test of basis set effects and the quality of ab initio methods in the description of these open shell systems. The experimental momentum profiles have been compared with theoretical spherically averaged momentum profiles from several basis sets calculated at the level of the target Hartree-Fock approximation (THFA) with a range of basis sets using both unrestricted Hartree-Fock (UHF) and restricted open shell Hartree-Fock (ROHF) methods. Various configuration interaction calculations such as multi-reference singles and doubles configuration interaction (MRSD-CI), averaged coupled pair functional (ACPF) and quasi-degenerate variational perturbation theory (QDVPT) calculations of the full ion-neutral overlap amplitude have also been compared to experiment to investigate the effects of electron correlation and relaxation. The experimental momentum profiles have further been compared to calculations at the level of the target Kohn-Sham approximation (TKSA) using density functional theory (DFT) with the local density approximation and also with gradient corrected exchange-correlation potentials. In addition to momentum profiles, other electronic properties such as total energies, dipole moments, quadrupole moments and values of the electronic spatial extent have been calculated by the various theoretical methods and compared to experimental values.

Rational optimization of the Whelk-O1 chiral stationary phase using molecular dynamics simulations.

Rational in silico optimization of the Whelk-O1 chiral stationary phase (CSP) has been carried out based on the chiral recognition mechanism extracted from previous molecular dynamics simulations [C.F. Zhao, N.M. Cann, Anal. Chem. 80 (2008) 2426] of this CSP. Three modified CSPs have been examined. The first two are designed to increase selectivity by reducing the docking probability of the less retained analyte. The third modified selector reverses the amide bridge to introduce a structural motif found in the popular carbamate-derivatized polysaccharide CSPs [Y. Okamoto, M. Kawashima, K. Hatada, J. Am. Chem. Soc. 106 (1984) 5357]. For each modified selector, an atomistic model has been obtained through extensive ab initio calculations. The effect of selector modification is then evaluated via simulations of the modified interface in the presence of target analytes. Simulation results show that the separation factors are increased for the modified CSPs but in some cases elution orders are reversed. The Whelk-O1 CSP was originally designed to separate naproxen [W.H. Pirkle, C.J. Welch, B. LAmm, J. Org. Chem. 57 (1992) 3854]. With this in mind, molecular dynamics simulations of naproxen are compared for the original, and the modified, selectors.

Charge and intracule densities in singly excited heliumlike ions

The variation of the charge and intracule densities of singly excited states of two‐electron ions with respect to spin multiplicity, nuclear charge, degree of excitation, and angular momentum quantum number is studied systematically. The n 1S, n 3S, n 1P, n 3P, n 1D, and n 3D states with n=3–6 are considered for all the ions from He through Ne8+ using highly accurate explicitly correlated wave functions. Special attention is paid to spin multiplicity differences, that is differences between the densities of a pair of states arising from the same electron configuration of the same ion.

Proline-based chiral stationary phases: A molecular dynamics study of the interfacial structure

Proline chains have generated considerable interest as a possible basis for new selectors in chiral chromatography. In this article, we employ molecular dynamics simulations to examine the interfacial structure of two diproline chiral selectors, one with a terminal trimethylacetyl group and one with a terminal t-butyl carbamate group. The solvents consist of a relatively apolar n-hexane/2-propanol and a polar water/methanol mixture. We begin with electronic structure calculations for the two chiral selectors to assess the energetics of conformational changes, particularly along the backbone where the amide bonds can alternate between cis and trans conformations. Force fields have been developed for the two selectors, based on these ab initio calculations. Molecular dynamics simulations of the selective interfaces are performed to examine the preferred backbone conformations, as a function of end-group and solvent. The full chiral surface includes the diproline selectors, trimethylsilyl end-caps, and silanol groups. Connection is made with selectivity measurements on these interfaces, where significant differences are observed between these two very similar selectors.

Quadrupole oscillator strengths for the helium isoelectronic sequence: n 1S-m 1D, n 3S-m 3D, n 1P-m 1P, and n 3P-m 3P transitions with n <7 and m <7

Quadrupole oscillator strengths (QOS) for He and the isoelectronic ions from Li+ to Ne8+ are reported for all possible n 1S-m 1D, n 3S-m 3D, n 1P-m 1P and n 3P-m 3P transitions involving states with m <7 and n <7. The calculations are based upon explicitly correlated wavefunctions that lead to variational energies only nano-Hartrees above the best literature values. The results extend significantly both the accuracy and range of the QOSs available for two-electron atomic species.