Density functional theory , vibrational

Vibrational Spectra Many of the papers quoted below deal with the determination of vibrational spectra. The method of choice is B3-LYP density functional theory. In most cases, MP2 vibrational spectra are less accurate. In order to allow for a comparison between computed frequencies within the harmonic approximation and anharmonic experimental fundamentals, calculated frequencies should be scaled by an empirical factor. This procedure accounts for systematic errors and improves the results considerably. The easiest procedure is to scale all frequencies by the same factor, e.g., 0.963 for B3-LYP/6-31G computed frequencies [95JPC3093]. A more sophisticated but still pragmatic approach is the SQM method [83JA7073], in which the underlying force constants (in internal coordinates) are scaled by different scaling factors. 

A group of investigators recently suggested that the density-functional theory (DFT), which calculates IR and Raman spectra, is a useful tool for direct characterization of the structures of diamondoids with increasing complexity [66]. They applied DFT to calculate Raman spectra whose frequencies and relative intensities were shown to be in excellent agreement with the experimental Raman spectra for C26H30, thus providing direct vibrational proof of its existence. 

The assignment of the TR spectra were based on the known photochemistry of the aryl azides and comparison of the TR spectra vibrational frequencies to those predicted by density functional theory calculations for the likely photochemical intermediates. The good agreement between the experimental TR vibrational... 

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

Amos, R. D., Murray, C. W., Handy, N. C., 1993, Structures and Vibrational Frequencies of FOOF and FONO Using Density Functional Theory , Chem. Phys. Lett., 202, 489. 

Devlin, F. J., Stephens, P. J., 1999, Ab Initio Density Functional Theory Study of the Structure and Vibrational... 

Hutter, J., Liithi, H. P., Diederich, F., 1994, Structures and Vibrational Frequencies of the Carbon Molecules C2-C18 Calculated by Density Functional Theory , J. Am. Chem. Soc., 116, 750. 

Kesyczynski, J. Goodman, L., Kwiatkowski, J. S., 1997, Density Functional Theory and Post-Hartree-Fock Studies on Molecular Structure and Harmonic Vibrational Spectrum of Formaldehyde , Theor. Chem. Acc., 97, 195. 

Martin, J. M. L., El-Yazal, J., Francois, J.-P, 1996, Structure and Vibrational Spectrum of Some Polycyclic Aromatic Compounds Studied by Density Functional Theory. 1. Naphtalene, Azolene, Phenanthrene, and Anthracene , J. Phys. Chem., 100, 15358. 

Stephens, P. J., Devlin, J. F., Chabalowski, C. F., Frisch, M. J, 1994, Ab Initio Calculations of Vibrational Absorption and Circular Dichroism Spectra Using SCF, MP2, and Density Functional Theory Force Fields , J. Phys. 

Wheeless, C. J. M., Zhou, X., Liu, R., 1995, Density Functional Theory Study of Vibrational Spectra. 2. Assignment of Fundamental Vibrational Frequencies of Fulvene , J. Phys. Chem., 99, 12488. 

Kwiatkowski, J. S., and J. Leszczynski. 1996. Molecular Structure and Vibrational IR Spectra of Cytosine and its thio and seleno Analogs by Density Functional Theory and Conventional ab initio Calculations. J. Phys. Chem. 100, 941. 

Of course, experimental methods are used to determine the molecular properties of 1,2,4-triazoles but computational studies, particularly density functional theory (DFT) calculations, are frequently carried out to predict and confirm the experimental findings. Calculation of the fundamental vibrational frequencies using the 6-311G(d,p) basis set has been used to support a comprehensive study of the vibrational spectra of 1,2,4-triazole <2000JST(530)183>. 

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