Ab initio frequencies

We ve included several papers in the References section which perform theoretical and experimental studies of the IR and Raman spectra for these compounds. These compounds were among the earliest ab initio frequency studies of such systems. In addition, in the case of propellane, theoretical predictions of its energy and structure preceded its synthesis. 

Scale factors determined from the CH3Br and CH,C1 ab initio and experimental frequencies were used to scale ab initio frequencies for the complexes and central barrier see text... 

In our first implementation [13, 120, 121, 131] of this idea, we took the transition frequency to be a linear function of this electric field. We determined the coefficients of this linear function by fitting to the ab initio frequencies from water clusters (and in this case the clusters were not surrounded by point charges from the other molecules in the simulation). In the liquid simulation we simply calculate this electric field at every time step and then use this linear map (in this case the electric field was the full Ewald field from the simulation) to determine the frequency. In our later implementation [6, 98] we took the... 

It is also interesting to consider the ensemble of frequencies produced by the map [98], To this end we rerun the simulation and calculate the electric field at every putative H atom from all the point charges out to half the box length. This generates a distribution of fields, which through the map leads to a distribution of frequencies. This distribution can be compared to the histogrammed distribution of actual ab initio frequencies, now from 999 clusters. This... 

Table 3. Measured (Black and Law 2001) and ab initio vibrational frequencies for methyl chloride, C CX.Ab initio frequencies are calculated with GAMESS, using the Hartree-Fock method and 6-31G(d) basis set. The ratio of each measured and model frequency is also shown.

Vibration Measured frequency (cm ) Ab initio frequency (cm ) Meas./A6 init. Ratio Vj,/Vj5 (ab initio)... 

Since no ab initio frequency data for CsKtSi species except for silacyclobutadiene (4) [7] are given in the literature, calculations on the BLYP/6-31G level of theory have been carried out. The relative energies of some selected minima on the CsIi Si hypersurface are shown in Fig. 1. 

Recalling Eq. (5.9), on low-bandpass spectrometers the intensity is a slowly varying function of the energy transferred (°cg ). Therefore, small but seemingly arbitrary changes to the ab initio frequencies will have little impact on the calculated ab initio intensities. 

Ab initio frequencies of normal vibrational modes and, by this also, adiabatic frequencies suffer from the harmonic approximation used in the calculation. Even when applying efficient scaling procedures, there is no guarantee that ab initio frequencies accurately reproduce the exact fundamental frequencies of the experiment. Therefore, one has to ask whether the adiabatic internal frequencies might not be much more meaningful if they would be based on experimental frequencies rather than frequencies calculated within the harmonic approximation. 

For many reactions the calculated structures for potential energy minima are as accurate as those found experimentally. Ab initio and experimental harmonic vibrational frequencies usually agree to within 10-15% at the Hartree-Fock level and 5% at the MP2 level (Hehre et al., 1986). It has been found that Hartree-Fock harmonic frequencies computed with a medium-size basis set ean be scaled by the factor 0.9 to give approximate anharmonic n = 0 — 1 transition frequencies (Hehre et al., 1986). A detailed study has been made of how the computed ab initio frequencies for benzene depend on the size of the basis set and the treatment of electron correlation (Maslen et al., 1992). 

While some relative PA values have been measured experimentally from variable temperature equilibrium experiments, most experimental data have been measured at a single temperature. In this article, all experimental GB data have been adjusted to be consistent with the most recent GB and PA scales, and the entropies used to calculate PAs have been derived from scaled ab initio frequencies when experimental values are not available, Several compilations serve as a source for heats of formation of hydrocarbons and carbocations. Revised, heats of formation are available from the NIST web site and a detailed review of vinyl cations and many other carbocation heats of formation is found in Ref. 14. 

---