Home  About us  Contact
 

Experimental Frequencies (experimental + frequency)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

Calculation of the vibration frequencies of ,-quartz: The effect of Hamiltonian and basis set

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2004
C. M. Zicovich-Wilson
Abstract The central-zone vibrational spectrum of ,-quartz (SiO2) is calculated by building the Hessian matrix numerically from the analytical gradients of the energy with respect to the atomic coordinates. The nonanalytical part is obtained with a finite field supercell approach for the high-frequency dielectric constant and a Wannier function scheme for the evaluation of Born charges. The results obtained with four different Hamiltonians, namely Hartree,Fock, DFT in its local (LDA) and nonlocal gradient corrected (PBE) approximation, and hybrid B3LYP, are discussed, showing that B3LYP performs far better than LDA and PBE, which in turn provide better results than HF, as the mean absolute difference from experimental frequencies is 6, 18, 21, and 44 cm,1, respectively, when a split valence basis set containing two sets of polarization functions is used. For the LDA results, comparison is possible with previous calculations based on the Density Functional Perturbation Theory and usage of a plane-wave basis set. The effects associated with the use of basis sets of increasing size are also investigated. It turns out that a split valence plus a single set of d polarization functions provides frequencies that differ from the ones obtained with a double set of d functions and a set of f functions on all atoms by on average less than 5 cm,1. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1873,1881, 2004 [source]

Theoretical Simulation of Vibrational Sum-Frequency Generation Spectra from Density Functional Theory: Application to p -Nitrothiophenol and 2,4-Dinitroaniline

CHEMPHYSCHEM, Issue 12 2009
Julien Guthmuller Dr.
Abstract The molecular orientation of adsorbed molecules forming self-assembled monolayers can be determined by combining vibrational sum-frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p -nitrothiophenol and 2,4-dinitroaniline. It is found that a suitable choice of basis set as well as of exchange-correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsorbed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6-311++G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p -nitrothiophenol. Moreover, the investigated XC functionals reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree,Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors. [source]

Theoretical and Experimental Study of the Adsorption of Neutral Glycine on Silica from the Gas Phase

CHEMPHYSCHEM, Issue 6 2005
C. Lomenech Dr.
Abstract The adsorption of neutral glycine onto amorphous silica was investigated both theoretically and experimentally. DFT calculations were performed at the BLYP-631++G** level using a cluster approach. Several possible configurations involving the formation of H bonds between glycine and one, two, or three silanol groups (SiOH) were considered. The most favorable bonding of glycine with one silanol group (45 kJ,mol,1) occurs through the COOH moiety, thus forming a cycle in which the CO group is an H-bond acceptor whereas the acidic OH group is an H-bond donor. With two or three silanol groups, additional H bonds are formed between the amine moiety and the silanol groups, which leads to an increased adsorption energy (70 and 80 kJ,mol,1for two and three silanol groups, respectively). Calculated ,CO, ,HNH, and ,HCHvalues are sensitive to the adsorption mode. A bathochromic shift of ,COas compared to the ,COof free glycine (calculated in the 1755,1790 cm,1range) is found for glycine in interaction with silanol(s). The more H bonds are formed between the COOH moiety and silanol groups, the higher the bathochromic shift. For ,HNH, no shift is found for glycine adsorbed on one and two silanol groups (where the amine is either not bound or an H-bond donor), whereas a bathochromic shift is calculated with three silanols when the amine moiety is an H-bond acceptor. Experimental FTIR spectra performed at room temperature for glycine adsorbed at 160,°C on Aerosil amorphous silica exhibit bands at 1371, 1423, 1630, and 1699 cm,1. The experimental/calculated frequencies have their best correspondence for glycine adsorbed on two silanol groups. It is important to note that the forms giving the best correspondence to experimental frequencies are the most stable ones. [source]

Note on the calculation of analytical Hessians in the zeroth-order regular approximation (ZORA)

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2006
J. H. van Lenthe
Abstract The previously proposed atomic zeroth-order regular approximation (ZORA) approach, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimizations, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]