Marian A. Lowe

The theory of vibrational circular dichroism: trans-1,2-dideuteriocyclobutane and propylene oxide

Abstract A rigorous theory of vibrational rotational strengths was recently developed by Stephens. We report the first comparisons of calculations using this theory with experimental vibrational circular dichroism data. The theory is implemented using SCF MO electronic wavefunctions and a 4-31G basis set. The molecules studied are trans-l,2-dideuteriocyclobutane and propylene oxide. Encouraging agreement between theory and experiment is obtained.

Scaled quantum mechanical calculation of the vibrational structure of the solvated glycine zwitterion

Abstract The scaled quantum mechanical (SQM) method for calculating the vibrational structure of molecules is applied to the solvated glycine zwitterion. The ab initio force field generated by means of a molecular dynamics calculation [J.S. Alper, H. Dothe and D.F. Coker, Chem. Phys. 153 (1991) 51] was scaled using the frequencies from experimental spectra taken in aqueous solution. Theoretical frequencies, potential energy distributions, intensities, and line shapes were obtained. The theoretical frequencies and normal mode assignments were in excellent agreement with the aqueous spectra. Theoretical frequencies and normal modes calculated by scaling to frequencies obtained from the experimental α-crystal spectra of glycine were in significantly worse agreement with the experimental crystalline values. These results suggest that the calculation has distinguished the aqueous from the crystalline environment.

Scaled quantum mechanical calculation of the vibrational structure of methylthiirane

Abstract The scaled quantum mechanical (SQM) force field method for calculating the vibrational structure of molecules is applied to methylthiirane. Because the vibrational assignment of the experimental spectra is extremely uncertain, the calculation was performed using scale factors transferred from the results of SQM calculations on thiirane and methyloxirane. The resulting SQM vibrational spectrum is obtained without using any experimental information about methylthiirane. The calculated frequencies are in excellent agreement with the experimental Raman and infrared vibrational spectra and allow an assignment of the spectra. Fitting the transferred scale factors to the experimental spectrum improves the agreement.

Comment Comparison of ab initio and scaled quantum mechanical methods for the calculation of vibrational structure: A reply to bose and polavarapu

Abstract Recent criticisms by Bose and Polavarapu of the scaled quantum mechanical (SQM) force field method for the calculation of vibrational structure are discussed. The SQM method can give quantitatively accurate frequencies, force constant matrix elements, and potential energy distributions of molecules, while the corresponding ab initio calculation using the same basis set can fail to give even qualitative agreement with experiment. The SQM method can be employed for molecules whose vibrational assignment is uncertain by transferrring SQM scale factors from related molecules. For molecules without symmetry whose vibrational assignment is certain, the SQM method is equivalent to an ab initio calculation in which all calculated frequencies are scaled using a single multiplicative factor.

Temperature dependence of the magnetic phase transitions of CoCl2·2H2O

Abstract The H - T phase diagram for the antiferromagnet CoCl 2 ·2H 2 O has been measured. The two meta-magnetic transitions are observed to meet at a triple point at 8.8 K and 38.5 kG. A molecular field calculation is in good agreement with experiment.

Scaled quantum mechanical calculations of the vibrational structure of thiirane, thiirene, and their deuterated isotopomers

Abstract The scaled quantum mechanical (SQM) method for calculating the vibrational structure of molecules is applied to thiirane, thiirene, and their deuterated isotopomers. The purpose of these calculations is to test the method against the experimental spectra and recent ab initio calculations of these molecules. It is found that the SQM method implemented with the 6-31 G* basis set at the experimental geometry for the thiiranes and with the 6-31 G* set at the 6-31 G* corrected theoretical geometry for the thiirenes gives excellent agreement with experiment and with the largest of the ab initio calculations.