¹³C frequency shifts and the general harmonic force fields of methyl chloride, bromide and iodide

Abstract

General harmonic and hybrid orbital force fields have been calculated for methyl chloride, bromide and iodide using the best available frequency, Coriolis coefficient, and centrifugal distortion data, including the recent laC frequencies for the chloride and iodide. Anharmonicity corrections were made to the fundamental frequencies using Dennison's rule and x values of 0"04 for CH stretching, 0"02 for CH bending, and 0"01 for CX stretching modes. These were the most reasonable values which did not seriously over-compensate for the product rule deficiencies in the 12C and 1~C frequencies.The 13C and CDaX frequency information was input in the form of frequency shifts from the 12CHaX frequencies. All 12 parameters in the most general harmonic force fields are determined with significance for the first time, although the previously uncertain parameters F12 and F13 are only determined with good precision when the 13C frequency data are utilized in addition to the other information. The signs of Fx2, F45 and F46 are in agreement with the predictions of the hybrid orbital model, which are quantitatively obeyed more closely in methyl iodide than in methyl bromide or chloride.There is a slight incompatibility between the centrifugal distortion data and the 13C frequency shifts in the A1 species, attributed here to the unknown effects of higher-order terms on the distortion constants.The effect of using the best present estimate of the equilibrium CH bond length, re(CH) = 1 "085 3,, instead of the previous estimate of re= 1 "095 h, is considered and the changes in the force constants listed.

Quantum Chemical Calculations of the Cl− + CH3I → CH3Cl + I− Potential Energy Surface

“…The calculated frequencies of the CH 3 I reactant and CH 3 Cl product at the different levels of theory agree very well with experiment. 88 For the ECP/d basis, the average deviation is 1%, 2%, 3%, 1%, 2%, and 2% at the MP2, OPBE, OLYP, BhandH, HCTH407, and B97-1 theories, respectively. For the ECP/t basis the average deviation is 3%, 1%, 2%, 2%, 1%, and 1%, respectively.…”

Absolute Raman intensities, force constants, and electro-optical parameters of CH2Cl2, CD2Cl2 and CHDCl2

“…Finally we calculated anharmonicity corrections to the observed wavenumbers (as well as a small wavenumber correction for Fermi resonance between v 1 and 2v 2 in CH~CI~ and CD2CI~) using Dennison's rule and X values of 0.04 for C-H stretch, 0.02 for C-H bend and C-C1 stretch, 0.01 for C-C1 bend and 0.014 for H-C-CI bend [22]. The harmonic wavenumbers o) calculated in a new joint refinement of force constants and electro-optical parameters, and the differences (r are given in table 8.…”

“…The force constants and electro-optical parameters calculated in this fit are also given in tables 6 and 7. In order to break the indeterminations among the force constants we fixed F12 , F14 and F56 to values transferred from the force constants of CHaC1 [22] and CHC13 [23], through the non-redundant internal coordinate representation. 0-507 0"504_+0"01 0"519-+0"008 Symmetry coordinates as defined by Shimanouchi and Suzuki [11 ] ; angular coordinates scaled with rc_~ = 1.09 A.…”

The General Harmonic Force Field of Methyl Chloride

“…This indicates that the errors (observed minus calculated quantities) are on average 2.5 times larger for the MP4 force field. This is only slightly worse than the sum of weighted squares of errors of 121.3 achieved by the previous empirical force field of Duncan et al (1,2) (but note that the latter force constants were fitted to a smaller and less accurate spectroscopic data set than that used here). Analysis of the error vector associated with the MP4 force field shows that the large increase in the sum of weighted squares of errors (compared with the unconstrained fit of the present work) is dominated by the large errors associated with the C-Cl stretching frequencies ( 3 in the symmetric top isotopomers, and 6 in the asymmetric top isotopomers), the D J centrifugal distortion constants, and the ⌬ 2 frequency shift in the 13 CH 3 35 Cl isotopomer.…”

“…(1)(2)(3)(4)(5)(6)) and references cited therein), it is only comparatively recently that the anharmonic corrections required for comparison with empirical vibration frequencies have been quantitatively understood (7,8). These advances in the understanding of the vibrational anharmonicities (including Fermi resonance effects) for the methyl halides (7)(8)(9)(10) and the large number of new and improved high-resolution spectroscopic data now available (in particular axial centrifugal distortion constants D K ) make a recalculation of the empirical general harmonic force field (GHFF) of methyl chloride appropriate at this time.…”