Vibrational spectra of triamantane X18H24, iso-tetramantane X22H28 and cyclohexamantane X26H30 (X= C, Si, Ge, Sn)-A theoretical study (original) (raw)

2007, Journal of Molecular Structure: …

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.

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Vibrational spectra of adamantanes X10H16 and diamantanes X14H20 (X= C, Si, Ge, Sn): A theoretical study

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

A density functional theory analysis of Raman and IR spectra of 2-adamantanone

Journal of Molecular Structure: THEOCHEM, 2002

The vibrational force fields for cage-like molecules 2-adamantanone (2-A) and adamantane (A) were calculated by employing density functional theory (DFT) using 6-31G(d,p) basis set with two non-local exchange-correlation functionals BLYP (Becke -Lee-Young -Parr) and the Becke's three-parameter compound functional B3LYP. DFT infrared intensities and the Raman scattering activities were also calculated to simulate the vibrational spectra. The analysis of the two chosen models show that they are comparable in predicting geometry, vibrational frequencies, IR and Raman intensities. The agreement between the predicted and observed IR and Raman spectra is not satisfactory in the region 1500-1200 cm 21 for either of them. The overestimation of the computed intensity connected to the motion of methylene hydrogen atoms is ascribed to the systematic shortcoming of the computational theory. The approach to assignment of the vibrational spectra that combines the comparison of not only calculated and observed frequencies but also of IR and Raman intensities is tested on A and applied to reassignment of the normal modes of 2-A. The scale factors for the force constants of the CyO group were derived. The scaled BLYP and B3LYP force fields reproduced the experimental frequencies of 2-A with an average error of 12 and 10 cm 21 , respectively. The error in reproducing the experimental frequencies of A is 13 cm 21 (BLYP). q

FT-IR and FT-Raman investigation, computed vibrational intensity analysis and computed vibrational frequency analysis on m-Xylol using ab-initio HF and DFT calculations

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2011

The FT-IR and FT-Raman spectra of m-Xylol molecule have been recorded using Bruker IFS 66 V spectrometer in the range 4000-100 cm −1 . The molecular geometry and vibrational frequencies in the ground state are evaluated using the Hartree-fock (HF) and B3LYP with 6-31+G (d, p), 6-31++G (d, p) and 6-311++G (d, p) basis sets. The computed frequencies are scaled using a suitable scale factors to yield good agreement with the observed values. The HF and DFT analysis agree well with experimental observations. Comparison of the fundamental vibrational frequencies with calculated results by HF and B3LYP methods indicate that B3LYP/6-311++G (d, p) is superior to HF/6-31+G (d, p) for molecular vibrational problems. The complete data of this title compound provide some useful information for the study of substituted benzenes. The influences of Methyl groups on the geometry of benzene and its normal modes of vibrations have also been discussed.

Scaling factors for the prediction of vibrational spectra. II. The aniline molecule and several derivatives

The structure of aniline was studied by semiempirical, ab initio, and density functional methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational frequencies of aniline was determined. The normal modes were characterized by the magnitudes and direction of the displacement vectors. Three procedures were used to obtain the scaled frequencies, two of them new, using specific scale factors and scaling equations from the benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost-effective method and procedure of scaling was carried out.

FT-IR and FT-Raman vibrational analysis, ab initio HF and DFT simulations of isocyanic acid 1-naphthyl ester

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2011

The Fourier transform infrared and Fourier transform Raman spectra of isocyanic acid 1-naphthyl ester (C 11 H 7 NO) [ICANE] are recorded in solid phase, the harmonic vibrational frequencies, infrared intensities, Raman activities, bond length, bond angle and dihedral angle are calculated by HF and DFT methods by using different basis set. A detailed vibrational spectral analysis has been carried out and assignments of observed fundamental bands have been proposed on basis of peak positions and relative intensities. The scaled theoretical frequencies showed very good agreement with experimental values. A detailed interpretations of the infrared and Raman spectra of isocyanic acid 1-naphthyl ester are reported, the theoretical spectra for infrared and Raman spectrum of title molecule have been constructed. The effect due to the substitutions of isocyanato group is also investigated. A study on the electronic properties, such as excitation energies and wavelengths, are performed with different solvent by time-dependent DFT (TD-DFT) approach. HOMO and LUMO energies are calculated that these energies show charge transfer occurs within the molecule.

Vibrational analysis of per-fluorinated-triamantane

Chemical Physics Letters, 2008

Triamantane was isolated from petroleum, per-fluorinated and the vibrational behaviour investigated by experimental and theoretical methods. Close agreement was found between the experimental and calculated Raman spectrum. The surface vibrational modes are shifted to lower wavenumbers relative to hydrogenated triamantane, due to the increased mass of the surface groups. The Raman spectrum of the fluorinated molecule more closely resembles that of pure diamond compared with hydrogenated triamantane. The absence of a peak at 1150 cm À1 for fluorinated triamantane suggests that this peak, often seen in the spectrum of nanocrystalline diamond, cannot be attributed to vibrations of diamond nanocrystals.

Vibrational analysis of an electron donor-acceptor substituted adamantane based on ab initio calculations

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.

Normal coordinate analysis and vibrational spectroscopy (FT-IR and FT-Raman) studies of 5-methyl-N-[4-(trifluoromethyl) phenyl]-isoxazole-4-carboxamide using density functional method

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.

Molecular structures and vibrational frequencies of 2-, 3- and 4-pyridine carboxaldehydes by ab initio Hartree–Fock and density functional theory calculations

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2007

The molecular structures, vibrational frequencies and corresponding vibrational assignments of xanthine and its methyl derivatives (caffeine and theobromine) have been calculated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-31G(d, p) basis set level. The calculations were utilized to the C S symmetries of the molecules. The obtained vibrational frequencies and optimised geometric parameters (bond lengths and bond angles) were seen to be well agreement with the experimental data. The used scale factors which have been obtained the ratio of the frequency values of the strongest peaks in the calculated and experimental spectra seem to cause the gained vibrations well corresponding to the experimental ones. Theoretical infrared intensities and Raman activities are also reported. analogues of xanthine prototropic tautomers is rather good approach to experimental study of the prototroic tautomeric equilibrium in nucleotide basis if relative energies of high-energy tautomers are not less than a few kcal/mol [10]. Some authors have described that in the preparation of xanthine and its methyl derivatives some care should be taken since sample handling may affect the water content of these samples which causes pseudopolymorphic transformations affecting their biavailability and stability .

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