RE: infra-red stretching frequencies



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 From: 	p smith
 Sent: 	Friday, February 28, 2003 3:10 AM
 To: 	chemistry { *at * } ccl.net
 Subject: 	CCL:infra-red stretching frequencies
 Hello,
 	It depends. That is, the cheapest/reliable method depends on the system under
 study. For qualitative purposes on large stable systems, you may use AM1 or PM3
 frequencies scaling them heavily above ~ 1500-2000 cm(-1). However, you couldn't
 expect  reliability e.g. from Hydrogen-bonded complexes in low-frequency region.
 As the cheapest universal tool, DFT B3LYP is recommended, with mandatory
 polarization functions on heavy atoms (and hydrogens for X-H stretch).
 	Also take a look at:
 A.P.Scott and L. Radom, "Harmonic Vibrational Frequencies: An Evaluation of
 Hartree-Fock, Moller-Plesset, Quadratic Configurational Interaction, Density
 Functional Theory, and Semiempirical Scale Factors", J. Phys. Chem. 1996,
 V.100, 16502-16513.
 	Best regards,
 	Serghey.
 ****************************************************************************************
 Serghey I. Kotelevskii,
 Department of Theoretical Chemistry and Astrochemistry,
 Research Institute for Chemistry,
 Kharkiv VNKarazin National University,
 Svobody Square 4,
 UA-61077 Kharkiv, Ukraine
 Hi,
      What is the cheapest method of producing reliable IR stretching
 frequencies computationally i.e. which represents the best compromise
 between cost and accuracy. I know these calculations can be performed
 semi-empirically but I am not sure how accurate the results are.
 Cheers,
             Paul
 p.smith { *at * } student.umist.ac.uk