A density functional theory analysis of Raman and IR spectra of 2-adamantanone (original) (raw)
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Journal of Molecular Structure: …, 2006
Optimized geometrical parameters are obtained for adamantanes (X 10 H 16 ) and diamantanes (X 14 H 20 ) (XZC, Si, Ge, Sn) using B3LYP method with 6-311G** and/or LANL2DZ basis sets. For carbon compounds Hartree-Fock (HF) and MP2 methods are also used for the study. The harmonic vibrational frequencies are obtained for all the molecules at their respective optimized geometries. The algorithm for the scaled quantum mechanical (SQM) method reported from our lab is modified to include Pulay's scaling procedure. Experimental frequencies of adamantane (XZ C) and four of its isotopomers with different symmetry point groups are fitted to the calculated harmonic frequencies to get suitable scale factors for the diagonal local force constants. These scale factors are used to predict the experimental vibrational frequencies of adamantane and diamantane. The assignments are proposed for all fundamentals of the title compounds based on normal coordinate analysis. q
Journal of Molecular Structure: …, 2007
Optimized geometrical parameters are obtained for triamantane, iso-tetramantane and cyclohexamantane (X = C, Si, Ge, Sn) using B3LYP method with 6-311G** and/or LANL2DZ basis set. For carbon compounds Hartree-Fock (HF) method is also used for the study. The harmonic vibrational frequencies are obtained for all the molecules at their respective optimized geometries. The calculated adamantane frequencies are fitted to the experimental ones of adamantane-d 0 molecule to get the scale factors for C-H stretching frequencies. For all the other modes the scale factors obtained earlier [G. Ramachandran, S. Manogaran, J. Mol. Struct. THEOCHEM, 766 (2006) 125] by fitting all the isotopomers are used. The scale factors thus obtained are used to predict the fundamental frequencies of all carbon compounds and compared to the available experimental Raman frequencies. The assignments are proposed for all fundamentals of the title compounds using normal coordinate analysis.
The prediction of Raman spectra by density functional theory. Preliminary findings
Chemical Physics Letters, 1995
We report the first calculations of Raman vibrational intensities by density functional theory, implemented within the Q-Chem program. Local (S-VWN) and gradient-corrected (B-LYP) DF'I" results are compared with experimental and Hartree-Fock results for the N 2, HF and C2H 6 molecules. Preliminary indications are that local DFT compares less favorably to experiment than either Hartree-Fock or gradient-corrected DF'I'. The Hartree-Fock and B-LYP results are generally similar except for the HF molecule, where B-LYP is somewhat better. For all methods, best results were obtained by augmenting the basis set with diffuse polarization functions.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2014
Theoretical infrared absorption spectra of aromatic ring molecules having up to 102 carbon atoms and with various edge hydrogenations have been obtained using a classic mechanical model and a simpliÐed valence force Ðeld. Force constants have been adapted from those available for smaller molecules. Spectral line intensities are calculated in a double harmonic approximation with e †ective atomic charges obtained using a tight-binding Hamiltonian. Vibrational modes of clusters of like aromatic ring Hu ckel molecules are obtained through introduction of an interlayer force constant. These modes are predicted to occur in the wavelength range between 80 and 400 km. We explore the e †ect of dehydrogenation on these spectra as well as hydrogenation of terminal C atoms in either aromatic or aliphatic form. Many spectra exhibit a quasi-continuum between 6 and 9 km that arises from the overlap of many vibrational modes in this region. With edge groups, we Ðnd a new spectral feature near 16 km and the ali-CH 2 phatic symmetric stretch vibration is found to shift from 3.5 to 3.3 km when these groups are CH 2 present at terminal sites on aromatic carbon skeletons. The relevance of calculated spectra to those of astronomical sources is brieÑy discussed.
The Journal of Chemical Physics, 2007
The authors propose a new route to vibrational Raman intensities based on analytical derivatives of a fully variational polarizability Lagrangian. The Lagrangian is constructed to recover the negative frequency-dependent polarizability of time-dependent Hartree-Fock or adiabatic ͑hybrid͒ density functional theory at its stationary point. By virtue of the variational principle, first-order polarizability derivatives can be computed without using derivative molecular orbital coefficients. As a result, the intensities of all Raman-active modes within the double harmonic approximation are obtained at approximately the same cost as the frequency-dependent polarizability itself. This corresponds to a reduction of the scaling of computational expense by one power of the system size compared to a force constant calculation and to previous implementations. Since the Raman intensity calculation is independent of the harmonic force constant calculation more, computationally demanding density functionals or basis sets may be used to compute the polarizability gradient without much affecting the total time required to compute a Raman spectrum. As illustrated for fullerene C 60 , the present approach considerably extends the domain of molecular vibrational Raman calculations at the ͑hybrid͒ density functional level. The accuracy of absolute and relative Raman intensities of benzene obtained using the PBE0 hybrid functional is assessed by comparison with experiment.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 2010
The FTIR and FT Raman vibrational spectra of 1,5-methylnaphthalene (1,5-MN) have been recorded using Brunker IFS 66 V Spectrometer in the range 3600-10 cm(-1) in the solid phase. A detailed vibrational spectral analysis has been carried out and assignments of the observed fundamental bands have been proposed on the basis of peak positions and relative intensities. The Optimized molecular geometry, harmonic frequencies, electronic polarizability, atomic charges, dipole moment, rotational constants and several thermodynamic parameters in the ground state were calculated using ab initio Hartree Fock (HF) and density functional B3LYP methods (DFT) with 6-311++ G(d) basis set. With the help of different scaling factors, the observed vibrational wavenumbers in FTIR and FT Raman spectra were analyzed and assigned to different normal modes of the molecule. Most of the modes have wavenumbers in the expected range. The results of the calculations were applied to simulated infrared and Raman s...
Journal of Molecular Structure, 2004
The solid phase FT-IR and FT-Raman, solution phase linear dichroism IR (in nematic liquid crystal), and vapor phase GC/IR spectra of 2-(methylthio)benzonitrile have been recorded in the regions 4000-50, 3500-100, 4000-400, and 4000-650 cm 21 , respectively. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimizations and force field calculations based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311 þ G** method and basis set combinations. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. IR dichroism data revealed an error in band assignment associated with a nCS vibration, which could be eliminated only by introducing independent scaling factors for sulfur, whereas the overall frequency fit was further improved. Simulation of infrared and Raman spectra utilizing the results of these calculations led to excellent overall agreement with the observed spectral patterns, especially with the higher level basis set. The SQM approach applying selective scaling of the DFT force field was shown to be superior to the uniform scaling method in its ability to allow for making modifications in the band assignment, resulting in more accurate simulation of IR and Raman spectra including band polarizations and intensity patterns. q
Ab Initio and DFT Predictions of Infrared Intensities and Raman Activities
The Journal of Physical Chemistry A, 2011
Relative infrared (IR) intensities and relative Raman activities have been computed for vibrations of test molecules, including from two to nine heavy atoms, using second-order Moller-Plesset perturbation theory (MP2), and three hybrid density functionals (B3LYP, M05, and M05-2X). The basis set convergence of vibrational properties is discussed. Our results demonstrate that B3LYP offers the most cost-effective choice for the prediction of molecular vibrational properties, but the predictions of another two tested hybrid functionals are very similar and in very good agreement with experimental data. MP2 shows good performance for the IR intensities, whereas the quality of prediction of the relative Raman activities should be characterized as only moderate. B3LYP calculations of the relative IR intensities using highly compact Sadlej's Z3PolX basis set retain the high accuracy of the more CPU expensive Sadlej's pVTZ and much more expensive aug-cc-pVTZ calculations. Relative Raman activities are more sensitive to basis set effects and require at least Sadlej's pVTZ to obtain quantitative results.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 1997
The Raman and infrared spectra of an electron donor-acceptor substituted adamantane (1-aza-adamant-4-ylidenemalononitrile) and the corresponding donor-only and acceptor-only structures are reported. Vibrational assignments are proposed based on comparison of the spectroscopic data with RHF/6-31G* calculations of frequencies and normal modes for the three substituted adamantanes, as well as previously assigned frequencies of adamantane, 1,l-dicyanoethylene (DCNE), and tetracyanoethylene (TCNE). 0 1997 Elsevier Science B.V.