Presentation on theme: "VIBRATIONAL ANHARMONICITY IN ETHYLENE, METHYL FLUORIDE, AND DICHLOROMETHANE: AN EXPLORATION USING GAUSSIAN 03 NORMAN C. CRAIG, Department of Chemistry."— Presentation transcript:
VIBRATIONAL ANHARMONICITY IN ETHYLENE, METHYL FLUORIDE, AND DICHLOROMETHANE: AN EXPLORATION USING GAUSSIAN 03 NORMAN C. CRAIG, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074, USA MARK M. LAW, Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, U. K.. DONALD C. MCKEAN, School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, U. K.
Vibrational Modes of Methylene Chloride a Symm. Species ModeApprox. Descript.Wavenumber/ cm -1 a1a1 1 sym. CH 2 stretch sym. CH 2 bend sym. CCl 2 stretch sym. CCl 2 bend281.5 a2a2 5 CH 2 twist1153 b1b1 6 asym. CH 2 stretch asym. CH 2 bend b2b2 8 CH 2 rock asym. CCl 2 stretch a Duncan, J.L.; Nivilleni, G. D.; Tullini, F. J. Mol. Spectrosc. 1986, 118, 145.
Comparisons of QC-Calculated Anharmonic Corrections i = i – i (cm -1 ) to Fundamental Frequencies for C 2 H 4 Mode obsd a dtz+ b dcc b mtz+ b mcc b CCSD c dcc ("obs") dcc (calc) 1 A g B 1g B 2u B 3u A g A g A u B 1g B 1u B 2g B 2u B 3u a Observed frequency from D. Van Lerberghe et al., J. Mol. Spectrosc. 1972, 42, 251. b dtz+ = B3LYP/ G**; mtz+ = MP2/ G**; dcc = B3LYP/cc-pVTZ; mcc = MP2/cc-pVTZ. c From a CCSD(T)/cc-pVTZ calculation: Martin, J.M.L.; Lee, T.J.; Taylor, P.R.; Francois, J-P. J. Chem. Phys. 1995, 103, Mode numbers as in this reference. Green: Fermi resonance treated specifically. Red: Fermi resonance not treated. Blue: Anomaly ascribed to the absence of f functions in the G** basis set, when used in MP2 calculations.
Anomalies in Anharmonicity Constants x rs (cm -1 ) in C 2 H 4 r sdtz+dccmtz+mcc Blue: ascribed to the absence of f functions. Red: Fermi resonance not treated.
Methyl Fluoride Experimental anharmonic constants exist for methyl fluoride, a but deficiencies in the G03 code for degenerate modes prevented making the intended comparison between calculations and experiment. a M.M. Law; J.L. Duncan; I.M. Mills J. Mol. Struct. 1992, 260, 323.
Conclusions The Gaussian 03 code for the option anharmonic needs substantial revision if this facility is to be generally useful. The 10-cm -1 window for recognizing resonances is too small. Molecules with degenerate modes are handled improperly. (In talk MI12, the failure to ensure use of Cartesian coordinates in the principal axis system in the vib-rot module of G03 was reported.) B3LYP and MP2 models with triple-zeta quality basis sets can give satisfactory anharmonicity corrections, BUT MP2 calculations using bases without f functions can be quite wrong. Differing anharmonicity corrections can be found for in-phase and out-of- phase stretching vibrations of C-H bonds. In such cases, different scale factors should be used when scaling harmonic QC force fields to reproduce observed spectra. For 1,1-difluorocyclopropane, the G03-based anharmonic analysis of combination tones was correct for the d 0 and d 4 species of C 2v symmetry but incorrect for the d 2 species of C s symmetry.