Vibrational spectra and non linear optical proprieties of L-histidine oxalate: DFT studies

This paper presents the results of our calculations on the geometric parameters, vibrational spectra and hyperpolarizability of a nonlinear optical material L-histidine oxalate. Due to the lack of sufficiently precise information on geometric structure in literature, theoretical calculations were preceded by re-determination of the crystal X-ray structure. Single crystal of L-histidine oxalate has been growing by slow evaporation of an aqueous solution at room temperature. The compound crystallizes in the non-Centro symmetric space group P2(1)2(1)2(1) of orthorhombic system. The FT-IR and Raman spectra of L-histidine oxalate were recorded and analyzed. The vibrational wave numbers were examined theoretical with the aid of Gaussian98 package of programs using the DFT//B3LYP/6-31G(d) level of theory. The data obtained from vibrational wave number calculations are used to assign vibrational bands obtained in IR and Raman spectroscopy of the studied compound. The geometrical parameters of the title compound are in agreement with the values of similar structures. To investigate microscopic second order non-linear optical NLO behaviour of the examined complex, the electric dipole μ(tot), the polarizability α(tot) and the hyperpolarizability β(tot) were computed using DFT//B3LYP/6-31G(d) method. According to our calculation, the title compound exhibits non-zero β(tot) value revealing microscopic second order NLO behaviour.     

Vibrational anharmonic calculations in solution: performance of various DFT approaches

We report anharmonic spectra calculated for formaldehyde in acetonitrile solution using the quartic force field obtained for various DFT/solvent coupled models. A statistical study has been carried out for each mode by using several classes of DFT functionals and comparing them to the reference ab-initio CCSD(T)/cc-pVQZ calculations. Results lead to the recommended use of hybrid functionals associated with the 6-31+G** basis set and the Polarized Continuum model (PCM) to predict the expected shifts relative to the gas phase.     

Vibrational spectra of 1-methylthymine: matrix isolation, solid state and theoretical studies

The infrared spectra of 1-methylthymine (1-MeT) in argon and nitrogen cryogenic matrices are presented, for the first time. The molecular structure, conformations, vibrational frequencies, infrared intensities and Raman scattering activities of 1-MeT have been calculated by the DFT(B3LYP), MP2 and HF methods using the D95V** basis set. The theoretically predicted intensity pattern of the IR and Raman bands has proved to be of great help in assigning the experimental spectra. Rigorous normal coordinate analysis has been performed, at each level of theory. The unequivocal and complete vibrational assignment for 1-MeT has been made on the basis of the calculated potential energy distribution (PED). Comparison of the experimental matrix isolation spectra with the theoretical results has revealed that the B3LYP method is superior to both the MP2 and HF methods in predicting the frequencies of uracil derivatives. The MP2 method consistently underestimates the frequencies of the out-of-plane gamma(C=O) and gamma(C-H) bending modes, while the HF method yields the reverse order of the frequencies of two nu(C=O) stretching vibrations. Investigation of the frequency shift of several bands, on passing from matrix isolation to solid state spectra, has provided information on the strength of intermolecular hydrogen bonding in the crystal of 1-MeT. Several ambiguities in the earlier assignments of the vibrational spectra of polycrystalline 1-MeT have been clarified.     

Vibrational spectra of methyllithium and its aggregates: a new interpretation from ab initio anharmonic calculations

The complete quartic force field of methyllithium (CH₃Li) is computed at the B3LYP/cc-pVTZ level of theory. The vibrational energy levels calculated from a perturbational and a variational procedure are in agreement with the observed spectra except for the C–Li stretching and the symmetric methyl deformation modes for which a disagreement with the experimental assignment given by Andrews is apparent. This discrepancy between experiment and theory is so large that questions are raised either about a correct characterization of, or correct calculations for the monomeric species CH₃Li. Our theoretical study of methyllithium aggregates (CH₃Li)_n, with n = 2, 3, 4 and 6, gives a new interpretation of the experimental data.     

Vibrational spectra of triiodomesitylene: combination of DFT calculations and experimental studies. Effects of the environment

A study of the internal vibrations of triiodomesitylene (TIM) is presented. It is known from X-rays diffraction at 293 K that the molecule has nearly D(3h) symmetry because of the large delocalization of the methyl protons. By using Raman and infrared spectra recorded at room temperature, a first assignment is done by comparing TIM vibrations with those of 1,3,5-triiodo- and 1,3,5-trimethyl-benzene. This assignment is supported by DFT calculations by using the MPW1PW91 functional with the LanL2DZ(d,p) basis set and assuming C(3h) symmetry. The agreement between the calculated and experimental frequencies is very good: always better than 97% for the observed skeletal vibrations. The calculations overestimate the methyl frequencies by 7%, and experiment shows only broad features for these excitations. Because a neutron diffraction study had established that the TIM conformation at 14 K is not exactly trigonal, new theoretical calculations were done with C(s) symmetry. This shows that all previous E' and E' modes of vibration are split by 2-12 cm(-1). This is confirmed by infrared, Raman, and inelastic neutron scattering spectra recorded below 10 K. Apart from two frequencies, all the TIM skeleton vibrations have been detected and assigned by using C(s) symmetry. For the methyl vibrations, experiment has confirmed the splitting of the previously degenerate modes; only some small discrepancies remain in the assignment. This is partly due to the difference of the model conformation used in the calculations and the crystallographic one. All these results confirm that each of the three methyl groups has not only its own tunnel splitting but also a different specific spectroscopic behavior for all the molecular modes.     

Vibrational spectra, NMR and theoretical studies of the enantiomers and rotamers of alpha-cypermethrin

NMR, infrared and Raman vibrational spectra of alpha-cypermethrin have been measured at room temperature. Infrared spectra were also recorded to low temperature. The spectra were analyzed by means of ab initio calculations. The conformational space of both enantiomers and some rotamers A, B and C of alpha-cypermethrin has been scanned using molecular dynamics and complemented with functional density calculations that optimize the geometry of the lowest-energy conformers of each species as obtained in the simulations. The vibrational frequencies and the 1H and 13C NMR chemical shifts were assigned using functional density calculations. The molecular electrostatic potential maps were obtained and analyzed.     

Vibrational resonances in infrared spectra of uracils

Two regions in the i.r. spectra of uracils are analyzed: (a) the NH stretching region in crystals, and (b) the CO stretching region in matrix-isolated monomers. In these regions there is clear evidence of resonances leading to energy splittings and re-distributions of intensifies. These are: (a) a harmonic resonance between the NH stretching in hydrogen-bonded groups and the CH stretching, and (b) an anharmonic (Fermi) resonance between the CO stretching and a combination band involving the N₃H and CO bending vibrations. A theoretical interpretation of the resonances is presented.     

Vibrational transition moment directions of a terminally p-nitrobenzyl substituted long-chain alkanethiol by polarized infrared spectra and DFT calculations

Transition moment directions of the vibrational states of nitro and carbonyl groups of p-nitrobenzyl-16-mercaptohexadecanoate are evaluated by infrared linear dichroism (IR LD) to be further exploited as film orientation markers in self-assembled monolayers (SAMs) that the respective compound forms on metal surfaces. DFT calculations followed by a complete normal coordinate analysis were employed to assist in the vibrational bands assignments. The analysis of the experimental IR LD spectra in conjunction with the step-wise reduction procedure of Thulstrup–Eggers indicated that the transition moment directions of the antisymmetric NO₂ stretching and the carbonyl stretching modes are collinear, and confirmed previous results that those of the symmetric and the antisymmetric NO₂ stretching vibrations are not exactly mutually perpendicular.     

Vibrational spectra and fundamental structural assignments from HF and DFT calculations of methyl benzoate

The Fourier transform Raman and Fourier transform infrared spectra of methyl benzoate (MB) were recorded in the liquid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities, depolarization ratios, reduced masses were calculated by Hartree-Fock (HF) and density functional B3LYP method with the 6-311+G(d,p) basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compound were also performed at HF/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. A detailed interpretations of the infrared and Raman spectra of methyl benzoate is reported. The theoretical spectrograms for FT-IR spectra of the title molecule have been constructed.     

A theoretical anharmonic study of the infrared absorption spectra of FHF-, FDF-, OHF-, and ODF- anions

Anharmonic vibrational frequencies, equilibrium bond lengths, rotational constants, and vibrational absorption spectra have been calculated for the triatomic anions, FHF(-) and OHF(-), and the heavier isotopomers FDF(-) and ODF(-). The triatomic anions are assumed to maintain a collinear configuration throughout all calculations, so only the symmetric (nu(1)) and asymmetric (nu(3)) stretching modes are considered. The two-dimensional permanent dipole surfaces and potential energy surfaces are then constructed along bond coordinates, using high-level ab initio methods. Fundamental and combination bands are obtained from the vibrational eigenfunctions, resulting in anharmonic frequencies, which can be compared with the available theoretical and experimental data. The agreement is very good, especially for the pure symmetric modes, while the asymmetric ones show larger discrepancies, presumably due to the neglected coupling between stretching and bending modes. Strong inverse anharmonicity is found in the level spacing of the asymmetric modes, for both FHF(-) and OHF(-) anions. The calculated mixed modes (nnu(1)+mnu(3), n, m=0-3) also agree reasonably with the few available experimental data, supporting our model. Based on the vibrational eigenfunctions, isotope effects are also rationalized. Infrared absorption spectra are calculated from the dipole autocorrelation function for FHF(-) and FDF(-), and for OHF(-) and ODF(-). Peak locations and relative intensities are assigned in terms of the fundamental and mixed transitions.     

Matrix infrared spectra and theoretical studies of thorium oxide species: ThOx and Th2Oy

Infrared spectra of three new thorium oxide species have been obtained in argon and neon matrixes. All of the products are experimentally characterized using isotopic oxygen samples with the aid of electronic structure calculations. Ground state thorium atoms react with O(2) to form the ThO(2) molecules, which can dimerize to give Th(2)O(4) products. Th(2)O(4) is predicted to have nonplanar C(2h) symmetry for its closed shell singlet ground state. The rhombus-shaped Th(2)O(2) molecule in the (1)A(g) (D(2h)) ground state is also observed and its formation is proposed via the reaction of Th(2) with O(2). In addition, electron capture of neutral thorium dioxide results in the formation of the ThO(2)(-) anion. It is predicted to have a doublet ground state with a geometry similar to that of the neutral ThO(2) molecule. Electronic structure calculations on the unobserved Th(2)O and Th(2)O(3) molecules are also provided.     

Vibrational frequencies and infrared intensities of the hydrogen-bonded complexes of nitrous acid with ethers: ab initio and DFT studies

The vibrational spectra of the binary complexes formed by HONO-trans and HONO-cis with dimethyl and diethyl ethers have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimisation was made for the complexes studied. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The ab initio and DFT calculations show that the changes in the vibrational characteristics (vibrational frequencies and infrared intensities) upon hydrogen bonding for the hydrogen-bonded complex (CH3)2O...HONO-trans are larger than for the complex (CH3)2O...HONO-cis.     

Vibrational spectra of furan,pyrrole, and thiophene from a density functional theory anharmonic force field

Quartic force fields for furan, pyrrole, and thiophene have been generated using density functional theory (DFT). These were used to evaluate vibrational levels by second-order perturbation theory (PT) and also by the variational method. The results for the fundamental frequencies are in very good agreement with observation. The accuracy of overtones and combination transitions is also good, for those cases where assignments can be made. Second-order PT combines speed and quality whereas the variational approach is inherently more reliable, but converges rather slowly, requiring significant computational effort.     

Improving anharmonic infrared spectra using semiclassically prepared molecular dynamics simulations

Classical molecular dynamics is a convenient method for computing anharmonic infrared spectra of polyatomic molecules and condensed phase systems. However it does not perform well for predicting accurate intensities and it lacks nuclear quantization, two deficiencies that are usually accounted for by empirical scaling factors. In this paper we show on the examples of the trans isomer of nitrous acid and naphthalene that both issues can be alleviated by preparing the initial conditions according to semiclassical quantization based on a normal mode representation. The method correctly reproduces fundamental frequencies obtained with quantum mechanical methods. At increasing temperatures, the effective frequencies are found to follow the same trends as path-integral based methods. In the low-temperature limit, the band intensities predicted by the method are also found to agree with quantum mechanical considerations.     

One-dimensional BeH2 polymers: infrared spectra and theoretical calculations

Laser-ablated beryllium atoms react with H2 upon co-condensation in excess hydrogen and neon to form BeH2 and (BeH2)2, which are identified through isotopic substitution and DFT calculations. Unreacted Be atoms isolated in solid neon or hydrogen are excited to the 1P0 state and react further with H2 to enhance the BeH2 and (BeH2)2 concentrations and produce (BeH2)n polymers. The series of strong infrared-active parallel Be-H-Be bridge-bond stretching modes observed for (BeH2)n polymers suggests one-dimensional structures, and this conclusion is supported by DFT calculations. The computed polymerization energy per BeH2 unit is about 33 kcal/mol.     

Strong intramolecular hydrogen bonds. Part I. Vibrational frequencies of the OH group in some picolinic acid N-oxides predicted from DFT calculated potentials and located in the infrared spectra

Hydrogen bonding in picolinic acid N-oxide (I), its 4-nitro (III), 4-methoxy (IV), 4-amino (V) derivatives and in quinaldic acid N-oxide (II) was characterized by calculations (B3LYP/6-31G(d)) of metric parameters, H-bond energies and one-dimensional proton potential functions with vibrational energy levels. Solvent effects were estimated by the SCRF PCM method of Tomasi and coworkers (J. Tomasi, M. Persico, Chem. Rev. 94 (1994) 2027). The potential functions are strongly asymmetric with the energy minimum placed near the carboxylic oxygen. The inflection near the NO oxygen develops into a second, shallower minimum under the SCRF.

Empirical assignments of the OH stretching and bending modes were made for (I)–(IV). The stretchings of (I, II) and (IV) in various solvents are observed in the region 1600–1300 cm⁻¹, but near 2600 cm⁻¹ for (III). The calculated and observed frequencies are in fairly good agreement with theoretical predictions reflecting the electronic effects of the substituents upon the H-bond strength. The observed trends in the solvent effects upon various parameters characterizing the H-bonding also correspond to predictions.     

Theoretical anharmonic Raman and infrared spectra with vibrational assignments for monofluoroaniline isomers

The ortho-meta-, and para-fluoro substituted anilines are prototype molecules for investigation of the interactions of both the amino group and the fluorine atom with the aromatic ring. The molecular structures, natural atomic charges and theoretical anharmonic Raman and infrared spectra of the three fluoroaniline isomers have been calculated by using the density functional B3LYP method with the extended 6-311++G(df,pd) basis set. The Raman and infrared spectra of 2FA, 3FA, and 4FA have been recorded. The detailed vibrational assignments of the experimental spectra have been made on the basis of the calculated potential energy distributions, PEDs. The effect of fluorine substituent on the aniline ring geometry and charge distribution, the nature of the characteristic “marker bands” and a quenching of intensities of some bands are discussed. It is shown that the frequencies of the NH₂ stretching vibrations depend on the degree of pyramidalization of the C-NH₂ group, in the isomers. In 2FA and 3FA, the NH₂ stretching frequencies are higher than those in 4FA. This corresponds to a more flattened structure of the amino group in 2FA and 3FA, in comparison to 4FA.     

Vibrational spectra of palladium 5-nitrofuryl thiosemicarbazone complexes: experimental and theoretical study

The vibrational spectroscopic behavior of a series of 16 palladium(II) complexes with 8 bioactive nitrofuran containing thiosemicarbazones as ligands has been studied in the solid state. The IR and Raman spectra of these complexes and the free nitrofuran thiosemicarbazone ligands were recorded and analyzed. Experimental spectra were satisfactorily described by density functional theory (DFT) calculations. The combination of experimental and theoretical methods allowed us to perform the characterization of the main vibrations that show the mode of coordination of the thiosemicarbazone moiety to palladium even though these vibration bands are located in spectral regions showing a complicated pattern due to the presence of vibrations of the nitrofuran moiety and combination modes involving furan vibrations. A characteristic vibrational spectroscopic pattern has been defined for Pd(II) 5-nitrofuryl thiosemicarbazone complexes. This systematic knowledge may be useful for the analysis of the spectroscopic behavior of other coordination compounds holding the 5-nitrofuran thiosemicarbazone moiety.     

Vibrational and theoretical studies of urea and magnesium-urea complexes

The molecular structure and IR spectra of urea, H₂NCONH₂, in gas phase and in acetonitrile solution, as well as of the two complexes [MgU₄Cl₂] and [MgU₆]Cl₂ have been observed. The influence of environmental changes to geometry and spectra are shown. Various basis sets have been employed to safeguard the validity of the reported findings, using polarization functions for all calculations to get the correct pyramidal amide configuration. The erroneous low energy of the C_2v symmetry group, after the addition of the ZPVE correction, is discussed. For the solvated urea molecule a reduction of the energy barrier, compared to the gas phase urea, between the two minimum configurations, C₂ and C_s, and the planar geometry, is observed. The lowest energy minimum in acetonitrile is found to be the C₂ symmetry group, while for the two complexes, the local symmetry of urea is C_s or C₂ depending on the complex, or even on the coordination position of urea in the complex. The wagging motion of the amide group is also discussed in all the studied urea species. The computed geometries and most of the spectroscopic results are in good agreement with the available experimental data. Received: 18 July 2000 / Accepted: 31 July 2000 / Published online: 2 November 2000