Effects of Electron Correlation and Matrices

The abilities of different types of correlation treatments to properly handle the vibrational frequencies were the subject of a careful inquiry . Frequency shifts of the intramolecular modes of the water dimer, calculated at various levels with the harmonic approximation, are exhibited in Table 3.37. All computed frequencies are higher than the experimental 

The reader may have noted that experi mental spectra of H-bonded species are com monly measured in either the gas phase or in inert gas matrices. Of course, there may be some differences as the molecules of the matrix can interact in various ways with the H-bonded complex. A recent set of measurements provides some estimates as to the perturbations caused by the matrix. Table 3.39 reports in the first row the frequencies of the OH stretches of the free and bridging hydrogens of the proton donor molecule of the water dimer in the gas phase. The next row indicates that a Ne matrix has only a very small effect, perhaps 10 cm . The Ar and Kr matrices produce larger perturbations, reducing the frequencies by about 30 cm . A smaller cluster of Ar atoms, averaging perhaps 50 such atoms yields a result very much like a full Ar matrix. With the single exception of the very small increase for the free OH stretch in the Ne matrix, all matrices and the Ar cluster lower the frequencies of both of the modes studied. 

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