The structure of the mobile phase in liquid chromatography plays an important role in the determination of retention behavior on reversed-phase stationary materials. One of the most commonly employed mobile phases is a mixture of methanol and water. In this work, infrared and Raman spectroscopic methods were used to investigate the structure of species formed in methanol/water mixtures. Chemometric methods using multivariate curve resolution by alternating least-squares analysis were used to resolve the overlapped spectra and to determine concentration profiles as a function of composition. The results showed that the structure of these mixtures could be described by a mixture model consisting of four species, namely, methanol, water, and two complexes, methanol/water (1:1) and methanol/water (1:4). The spectral frequencies and concentration profiles found from the Raman and infrared measurements were consistent with one another and with theoretical calculations.

The "soft Coulomb hole" method of Chakravorty and Clementi has been implemented in a Gaussian lobe-orbital (GLO) program to include explicit electron-electron correlation in molecules according to a modified form of Coulomb's law in a program for workstations and personal computers (PCLOBE):Twice as many two-electron integrals must be calculated compared to the usual Hartree-Fock-Roothaan algorithm, but this "correlated self-consistent field (SCF)" method may be embedded within well-known SCF computer codes and yields more than 90% of the correlation energy in run times less than twice that of the restricted Hartree-Fock (RHF) method. A two-parameter model of correlation is calibrated on HF and LiH using the 6-311G * * basis as Gaussian lobe orbitals. The quantum Monte Carlo energies of LiH and HF were fitted to within 150 cal/mol, but the scaling model for first-row atoms is less good for H 2 . This work shows promise as a fast way to modify the Hartree-Fock-Roothaan method for accuracy approaching quantum Monte Carlo methods. This proof-of-concept work offers extension to modeling solvation properties of water solutions in high-pressure liquid chromatography (HPLC).

ABSTRACT:Modeling solvation in high-pressure liquid chromatography (HPLC) requires calculation of anharmonic vibrational frequencies of solvent clusters for a statistical partition function. An efficient computational method that includes electron correlation is highly desirable for large clusters. A modified version of the "soft Coulomb hole" method of Chakravorty and Clementi has recently been implemented in a Gaussian-lobe-orbital (GLO) program (PCLOBE) to include explicit electron-electron correlation in molecules. The soft Coulomb hole is based on a modified form of Coulomb's law:w ϭ scaling parameter.An algorithm has been developed to obtain the parameter "w" from a polynomial in the effective scaling of each primitive Gaussian orbital relative to the best single Gaussian of the H1s orbital. This method yields over 90% of the correlation energy for molecules of low symmetry for which the original formula of Chakravorty and Clementi does not apply. In this work, all the vibrations of the water dimer are treated anharmonically. A quartic perturbation of the harmonic vibrational modes is constrained to be equal to the exact Morse potential eigenvalue based on a three-point fit. This work evaluates the usefulness of fitting a Morse potential to a hydrogen bond vibrational mode and finds it to be slightly better than using MP2 vibrational analysis for this important dimer. A three-point estimate of the depth, D e , of a Morse potential leads to a correction formula for anharmonicity in terms of the perturbed harmonic frequency:When scaled by 0.9141, the harmonic Morse method leads to essentially the same results as scaling the BPW91 local density method by 0.9827.

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