Presentation on theme: "VIBRATIONAL ANHARMONICITY IN ETHYLENE, METHYL FLUORIDE, AND DICHLOROMETHANE: AN EXPLORATION USING GAUSSIAN 03 NORMAN C. CRAIG, Department of Chemistry."— Presentation transcript:
1VIBRATIONAL ANHARMONICITY IN ETHYLENE, METHYL FLUORIDE, AND DICHLOROMETHANE: AN EXPLORATION USING GAUSSIAN 03NORMAN C. CRAIG, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074, USAMARK 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.
2Vibrational Modes of Methylene Chloridea Symm. SpeciesModeApprox. Descript.Wavenumber/cm-1a1n1sym. CH2 stretchn2sym. CH2 bend1435.0n3sym. CCl2 stretch712.9n4sym. CCl2 bend281.5a2n5CH2 twist1153b1n6asym. CH2 stretch3055n7asym. CH2 bend898.66b2n8CH2 rockn9asym. CCl2 stretch759.82a Duncan, J.L.; Nivilleni, G. D.; Tullini, F. J. Mol. Spectrosc.1986, 118, 145.
6Comparisons of QC-Calculated Anharmonic Corrections Di = wi – ni (cm-1) to Fundamental Frequencies for C2H4ModenobsdaDdtz+bDdccbDmtz+bDmccbDCCSDcdcc w("obs")(calc)n1 Ag3022.0131.4128.7138.0135.8141.23150.73139.2n5 B1g3083.4136.7134.7134.8136.5143.03218.13194.6n9 B2u3104.9139.7138.2138.4139.1146.23243.13223.0n11 B3u2988.6189.1169.610.3-74.3160.13158.23125.5n2 Ag1625.423.129.024.437.850.41654.41693.0n3 Ag1343.524.225.035.027.128.31368.51282.2n4 Au1025.622.523.3-6.922.120.81048.91067.2n6 B1g122222.64.419.518.81244.61246.7n7 B1u948.814.215.4-50.317.317.9964.2979.0n8 B2g939.915.917.5101514.012.4957.4983.4n10 B2u818.104.22.168.51.11.3828.3836.4n12 B3u1442.534.135.450.639.41477.91479.2a 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.
7Anomalies in Anharmonicity Constants xrs (cm-1) in C2H4 dtz+dccmtz+mcc8 2-5.4-5.98.6-7.18 3-2.1-2.3-18.7-3.28 4-5.8-6.247.9-4.78 65.04.333.06.28 7-0.7-0.6125.3-0.28 81.71.5465.43.27 12-3.1-3.5-24.8-4.57 811 12-69.5-51.6108.8192.3f88881851817803225f1188-179-242-189f4488131127407143f6688252214032Blue: ascribed to the absence of f functions.Red: Fermi resonance not treated.
8Methyl FluorideExperimental 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.
9ConclusionsThe 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 d0 and d4 species of C2v symmetry but incorrect for the d2 species of Cs symmetry.