General Valence Force Field Research Articles

Insights into synthesis, structural, energetic, vibrational, anticancer activity and molecular characteristics of 2-((2-aminopyridin-3-yl)methylene)-N-phenylhydrazinecarbothioamide as evaluated using spectroscopic and DFT investigations

Synthesis of 2-((2-aminopyridin-3-yl)methylene)-N-phenylhydrazinecarbothioamide (APHB) was attempted. Elemental analysis and NMR spectra were used to ascertain its formation. Torsional potential energy scans, for all the rotating bonds were made to get approximate initial values of dihedral angles. FT-IR, FT Raman, and UV–Vis spectra were measured for APHB. Their anticancer activity was determined experimentally, for human carcinoma cell lines pertaining to liver, Colorectal, and lung. Barrier heights, around six rotating bonds in APHB were computed. Optimized structure parameters, general valence force field, harmonic vibrational fundamentals, potential energy distribution, infrared and Raman intensities, frontier molecular orbital (FMO) parameters, NLO behaviour, NBO characteristics and molecular electrostatic surface potential (MESP) were determined using DFT/B3LYP/6-311++G(d,p) level of theory. TD-DFT was used to compute absorption maxima (λmax) of electronic transitions for the molecule in DMSO‑d6 solvent. Frontier molecular orbitals were used to understand origin of UV–Vis spectrum and chemical reactivity of the molecule. Good agreement was found between measured and computed structure parameters, IR, Raman and UV–Vis spectra. The rms error between experimental and theoretical vibrational frequencies was 9.5 cm−1, for APHB. All vibrational fundamentals were assigned unambiguously. The computations demonstrated that the molecule was good for NLO applications, which was substantiated by NBO analysis. Existence of bifurcated intramolecular hydrogen bond was predicted for APHB.

Insights into structural and vibrational characteristics of 1-methoxy-4-[2-(phenylsulfonyl)vinyl]benzene: An application of experimental vibrational spectroscopy and density functional theory

Fourier transform infrared and Raman spectra, in the spectral range 4000- 400 and 4000–50 cm−1, respectively, were measured for 1‑methoxy-4-[2-(phenylsulfonyl)vinyl]benzene (MPB). Initial values of torsion angles around five flexible bonds C1-S, S-C15, C17-C19, C22-O and OC30 (see Fig. 1 for numbering), essential for starting geometry optimization, were determined using "bond-pairing method" proposed earlier by us. Optimized structure parameters, harmonic general valence force field, vibrational fundamentals, potential energy distribution and infrared and Raman band intensities were determined using density functional theory, employing B3LYP/ 6–311++G(d,p) formalism. Good agreement was found, between measured and computed quantities investigated here. The r.m.s error between experimental and theoretical vibrational wavenumbers was 8.6 cm−1, for MPB. With the help of PED and eigenvectors, all vibrational fundamentals of the molecule were assigned unambiguously for the first time.

DFT simulation of barrier heights, infrared and Raman spectra, and investigation of vibrational characteristics of 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide and its N-methyl variant

ABSTRACT Fourier transform infrared spectra, for 2-((2-aminopyridin-3-yl) methylene) hydrazinecarbothioamide (APHT) and 2((2-aminopyridin-3-yl) methylene)-N-methylhydrazinecarbothioamide (APMHT), were recorded in the spectral range 4000–400 cm−1. Their Raman counterparts were measured in the spectral region 4000–50 cm−1. Preliminary values of dihedral angles around five rotating bonds C–NH2 pyridyl C–CN, N–N, N–CS and C–NH2 aliphatic in APHT, required for initiating geometry optimization, were obtained by pairing successive bonds and evaluating torsional potential energy for various values of dihedral angle around these bonds in the entire conformational space spanning 0° to 360°. Barrier heights, around five rotating bonds in APHT and six rotating bonds in APMHT were computed, making torsional scans in conformational space from 0° to 360°. This indicated existence of two rotational isomers for APMHT. General valence force field, harmonic vibrational fundamentals, potential energy distribution (PED), along with infrared and Raman intensities were determined using DFT/B3LYP/6-311++G(d,p) level of theory for both the molecules. Good agreement was found between measured and simulated spectra, for APHT and APMHT. This was also true for corresponding Raman spectra. The rms error between experimental and theoretical vibrational frequencies was 9.00 and 6.70 cm−1, for APHT and APMHT, respectively. All vibrational fundamentals were assigned unambiguously, on the basis of computed PED, eigenvectors, and literature range for the first time. These assignments were further supported by comparing with attribution of corresponding bonds in the parent molecule pyridine and a related molecule 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide, wherever possible.

Investigation of Structures, FTIR, FT-Raman, In Vivo Anti-Inflammatory, Molecular Docking and Molecular Characteristics of 2-Amino-3-Pyridine Carboxaldehyde and Its Copper(II) Complex Using Experimental and Theoretical Approach

In this work, the FT-Raman and FTIR spectra of synthesized molecules, 2-amino-3-pyridine carboxaldehyde (APC) and Cu(II) complex of 2-amino-3-pyridine carboxaldehyde (Cu(II)APC) were characterized by using density functional theory (DFT). The optimized structural parameters such as harmonic vibrational frequencies, general valence force field, infrared and Raman intensities, frontier molecular orbital parameters, NLO properties, NBO and MESP were calculated using B3LYP functional with 6-311++G(d,p) basis set. Employing MOLVIB 7.0 program, the scaling process was engaged to reach a good agreement between experimental and computed frequencies. The rms error between the scaled calculated and experimental frequencies was found at 9.1 and 10.3 cm– 1 . Unambiguous vibrational assignments were reported by using potential energy distribution (PED) and eigenvectors. APC and Cu(II)APC were evaluated for their anti-inflammatory activity using the carrageenan-induced paw oedema method. Cu(II)APC complex exhibited significant activity compared to standard drug. Molecular docking results support the anti-inflammatory activity of the title compounds against the target protein cyclooxygenase-2 (COX-2) possess the good binding energy values of –4.50 and –7.67 kcal/mol, respectively.

Synthesis, crystal and molecular structure, and characterization of 2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide using spectroscopic (1H and 13C NMR, FT-IR, FT-Raman, UV–Vis) and DFT methods and evaluation of its anticancer activity

2-((2-aminopyridin-3-yl)methylene)-N-ethylhydrazinecarbothioamide was synthesized. It was characterized by making elemental analysis, assisted by experimental 1H NMR, 13C NMR, FT- Raman (4000-50 cm−1), FT-IR (4000-400 cm−1), and UV–Vis (200–400 nm) spectra and evaluating its anticancer activity, for human carcinoma cell lines HeLa (cervical), IMR-32 (neuroblastoma) and A549 (lung). Crystal and molecular structure of the molecule was determined by means of X-ray diffractometry, which showed that it belongs to triclinic crystal system, with space group P-1, having two molecules per unit cell (Z = 2). The parameters of the unit cell are a = 6.0960 (6) Å, b = 7.4119 (8) Å, c = 11.9959 (13) Å, α = 82.1695 (4)°, β = 81.6407 (4)°, γ = 88.3283 (4)° at 100 K. Quantum chemical computations were made using density functional theory (DFT), B3LYP functional and 6–311++G (d,p) basis set in order to determine optimized structure parameters, general valence force field, harmonic vibrational frequencies, potential energy distribution, infrared and Raman intensities, NLO properties, frontier molecular orbital parameters and NBO characteristics. Its time-dependent variant (TD-DFT) was used to calculate the oscillator strengths and absorption maxima (λmax) in DMSO‑d6 as a solvent, of various electronic transitions. There was a good agreement between the theoretical and experimental parameters such as molecular structure parameters, IR, Raman and UV–Vis spectra. The rms error between measured and estimated vibrational frequencies was 6.9 cm−1. The calculations showed that the molecule under investigation was good for NLO applications, which was supported by NBO analysis.

N-substituted dithienopyrroles as electrochemically active monomers: Synthesis, electropolymerization and spectroelectrochemistry of the polymerization products

Two new electropolymerizable monomers were synthesized, namely dithieno[3,2-b:2′,3′-d]pyrrole N-functionalized with 4-(2-heptylthiazol-4-yl)phenyl (DTP1) and 4-(5-octylthiophen-2-yl)phenyl (DTP2) groups. Both monomers readily electropolymerize to yield the corresponding polyDTP1 and polyDTP2. Electrochemically determined ionization potential (IP) of both polymers (about 4.75 eV) are higher than IPs of poly(dithieno[3,2-b:2′,3′-d]pyrrole) N-substituted with alkyl or alkylphenyl groups. This finding, corroborated by DFT calculations, suggests that electron accepting nature of (thiazol-4-yl)phenyl and (thiophen-2-yl)phenyl substituents lowers the π-electron density in the dithienopyrrole moiety making the polymers oxidation more difficult. Optical band gaps of polyDTP1 (Eg opt = 1.76 eV) and polyDTP2 (Eg opt = 1.78 eV) are lower than the gaps of poly(dithieno[3,2-b:2′,3′-d]pyrrole) N-substituted with alkyl or alkylphenyl groups. This combined with higher IP value yield higher electron affinity (ǀEAǀ) value. Thus, the obtained new polymers are more difficult to oxidize but easier to reduce as compared to poly(dithieno[3,2-b:2′,3′-d]pyrrole)s studied to date. Again, these findings are in a very good agreement with DFT calculations. As evidenced by UV–vis–NIR spectroelectrochemistry, both polymers undergo classical (for conjugated polymers) oxidation, involving the formation of polarons in the first step and bipolarons in the second one. An interesting feature of the oxidation of polyDTP2 is the highly delocalized nature of bipolarons, indicative of the metallic state (featureless absorption tails extending towards NIR part of the spectrum). In order to elucidate the exact nature of the electrochemical oxidation process detailed Raman spectroelectrochemical investigations of polyDTP2 were carried out supported by the vibrational model calculations using two methods: DFT and General Valence Force Field (GVFF). This combined experimental and theoretical study leads to a conclusion that radical cations (polarons) formed at the first stage of oxidation are formed in the pyrrole ring whereas dications formed in the second stage show classical bipolaron configuration with positive charges located on the thiophene rings.

Vibrational dynamics in dendridic oligoarylamines by Raman spectroscopy and incoherent inelastic neutron scattering.

Vibrational dynamics in triarylamine dendrimers was studied in a complementary way by Raman and infrared (IR) spectroscopies and incoherent inelastic neutron scattering (IINS). Three molecules were investigated, namely, unsubstituted triarylamine dendrimer of the first generation and two dendrimers of the first and second generation, substituted in the crown with butyl groups. To facilitate the assignment of the observed IR and Raman modes as well as the IINS peaks, vibrational models, based on the general valence force field method (GVFF), were calculated for all three compounds studied. A perfect consistency between the calculated and experimental results was found. Moreover, an important complementarity of the vibrational spectroscopies and IINS was established for the investigated dendrimers. The IINS peaks originating mainly from the C-H motions were not restricted by particular selection rules and only dependent on the IINS cross section. To the contrary, Raman and IR bands were imposed by the selection rules and the local geometry of the dendrimers yielding mainly C-C and C-N deformation modes with those of C-H nature of much lower intensity. Raman spectroscopy was also applied to the studies of the oxidation of dendrimers to their cationic forms. A strong Raman resonance effect was observed, since the spectra of the studied compounds, registered at different levels of their oxidation, strongly depended on the position of the excitation line with respect to their electronic spectrum. In particular, the blue (458 nm) excitation line turned out to be insensitive toward the cationic forms yielding very limited spectral information. To the contrary, the use of the red (647 nm) and infrared (1064 nm) excitation lines allowed for an unambiguous monitoring of the spectral changes in dendrimers oxidized to nominally monocationic and tricationic states. The analysis of oxidation-induced spectral changes in the tricationic state indicated that the charge storage configuration predominantly involved one spinless dication of the quinoid bond sequence and one radical cation. However, small numbers of dications were also found in a nominally monocationic state, where only radical cations should have been present. This finding was indicative of some inhomogeneity of the oxidation.

Raman spectroscopic studies on matrix-isolated arsenic and antimony molecules As 4 and Sb 4 in noble gas matrices

The Raman spectra of As 4 and Sb 4 molecules have been studied in neon, argon, krypton, and xenon matrices at 7 K. The vibrational frequencies of the As 4 molecule are up to 17 cm −1 higher than the experimental gas phase data. This evident blue-shift is not caused by matrix effects but originates from an underestimation of the fundamentals in the gas phase as a course of the elevated temperatures. The observed frequencies of As 4 and Sb 4 show a linear dependence toward the matrix host polarizability. Extrapolated values for zero polarizability which best represent a free molecule, are considered fundamental frequencies. The general valence force fields of As 4 and Sb 4 were calculated from the extrapolated frequencies.

The vibrational assignment of phenanthridine molecule based on normal coordinate analysis and DFT

AbstractA complete vibrational assignment of phenanthridine C13NH9 has been presented. The infrared (IR) and the Raman spectra of the molecule are analyzed with the help of theoretical prediction of the normal vibrational wavenumbers estimated from normal coordinate analysis (NCA) and density functional theory (DFT) calculations. A general valence force field (GVFF) including 31 parameters (13 diagonal and 18 off‐diagonal) reproduces satisfactorily the in‐plane vibrational signatures of the aforesaid molecule and as well as those for the other related hydrocarbons [phenanthrene and benzo(c)cinnoline]. The bivariate and multivariate data analysis reveals that calculated wavenumbers using GVFF are more accurate than the DFT result. However, DFT yields the relative Raman intensities, which are in good agreement with the experimental ones. The decomposition of the normal mode frequencies into those related to different internal coordinates is also discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Molecular Dynamics Study of Hydrated Imogolite. 1. Vibrational Dynamics of the Nanotube

An atomistic model of imogolite was defined and tested in molecular dynamics simulations. The model is based on the structure by Cradwick et al. (Nature, Phys. Sci. 1972, 240, 187). The interatomic interactions were defined by a generalized valence force field. As the experimental samples are mostly hydrated, water molecules were also included into the model. X-ray diffraction pattern computed from molecular dynamics trajectory well corresponds to the experimental one, thus confirming the validity of structural model. Calculation of the vibrational dynamics of imogolite nanotube shows a good overall agreement between the experimental and the calculated infrared and Raman spectra. The detailed analysis of the vibrational spectra leads to the conclusion that there exist strong couplings between internal coordinates preventing from the complete assignment of the observed bands to a particular coordinate or structural unit. These strong couplings also lead to the appearance of breathing vibrations involving in-phase displacements of all atoms at low frequencies. Origins of the Raman activity of some peaks are analyzed in detail.

Normal coordinate analysis and crystal structure of N, N-dimethyl- N′-(2-chloro-benzoyl)thiourea

In the present paper, the crystal structural and vibrational analysis of the N, N-dimethyl- N′-(2-chloro-benzoyl)thiourea molecule is reported. The molecular structure of the title compound was determined by single crystal X-ray diffraction method. The compound crystallizes monoclinic, space group P2 1/ c with a = 5.6601(13) Å, b = 10.748(2) Å, c = 17.778(4) Å, β = 94.266(5)°, and V = 1078.5(4) Å 3 with Z = 4 for d calc = 1.495 g/cm 3. Calculations of the vibrational frequencies have been carried out on the basis of normal coordinate analysis using simple general valence force field in Wilson's GF matrix method with the SPSIM computer program. With the help of this modern technique we were able to complete the assignment of the vibrational spectrum of the title compound.

Vibrational Spectra of the Closed Form of a Molecule of Indoline Spiroanthraceneoxazine

In an approximation of the generalized valence force field, the frequencies and forms of the normal vibrations of the free molecule of spiroanthraceneoxazine (SAO) are calculated. The vibrational spectra of SAO are measured: the IR absorption spectra in an NaCl disk and the Raman spectra in a KBr disk. The calculated and experimental data were correlated with the aid of a method of refined initial data. The results obtained agree with the calculated and experimental spectra of model and related compounds.

Synthesis and Characterization of Salts Containing the BrO3F2- Anion; A Rare Example of a Bromine(VII) Species

The BrO(3)F(2)(-) anion has been prepared by reaction of BrO(3)F with the fluoride ion donors KF, RbF, CsF, [N(CH(3))(4)][F], and NOF. The BrO(3)F(2)(-) anion is only the fourth Br(VII) species to have been isolated in macroscopic quantities, and it is one of only three oxide fluorides that possess D(3)(h)() symmetry, the others being XeO(3)F(2) and OsO(3)F(2). The fluoride ion acceptor properties of BrO(3)F contrast with those of ClO(3)F, which does not react with the strong fluoride ion donor [N(CH(3))(4)][F] to form the analogous ClO(3)F(2)(-) salt. The single-crystal X-ray structures of [NO](2)[BrO(3)F(2)][F] and [N(CH(3))(4)][BrO(3)F(2)] confirm the D(3)(h)() symmetry of the BrO(3)F(2)(-) anion and provide accurate Br-O (1.593(3)-1.610(6) A) and Br-F (1.849(5)-1.827(4) A) bond lengths. The salt, [NO](2)[BrO(3)F(2)][F], is fully ordered, crystallizing in the monoclinic space group, C2/c, with a = 9.892(3) A, b = 12.862(4) A, c = 10.141(4) A, beta = 90.75(2) degrees , V = 12460(7) A(3), Z = 4, and R(1) = 0.0671 at -173 degrees C, whereas [N(CH(3))(4))][BrO(3)F(2)] exhibits a 2-fold disorder of the anion, crystallizing in the tetragonal space group, P4/nmm, with a = 8.5718(7) A, c = 5.8117(6) A, V = 427.02(7) A(3), Z = 2, and R(1) = 0.0314 at -173 degrees C. The (19)F chemical shift of [N(CH(3))(4))][BrO(3)F(2)] in CH(3)CN is 237.0 ppm and is more deshielded than those of the previously investigated Br(VII) species, BrO(3)F and BrF(6)(+). The vibrational frequencies of the BrO(3)F(2)(-) anion were determined by use of Raman and infrared spectroscopy and were assigned with the aid of electronic structure calculations and by analogy with the vibrational assignments reported for XeO(3)F(2) and OsO(3)F(2). The internal and symmetry force constants of BrO(3)F(2)(-) were determined by use of general valence force field and B-matrix methods, respectively, and are compared with those of XeO(3)F(2), OsO(3)F(2), and the unknown ClO(3)F(2)(-) anion. The instability of ClO(3)F(2)(-) relative to BrO(3)F(2)(-) has been investigated by electronic structure calculations and rationalized in terms of atomic charges, Mayer bond orders, and Mayer valencies, and the enthalpies of fluoride ion attachment to BrO(3)F and ClO(3)F.

FTIR and FT Raman spectra and analysis of poly(4-methyl-1-pentene)

Poly(4-methyl-1-pentene) is the most widely used polymer in industry and medical products. Fourier transform infrared and Raman Spectra of poly(4-methyl-1-pentene) have been recorded in the range of 4000–400 and 4000–100 cm −1, respectively. In the present investigation a detailed assignments of the observed fundamental bands of poly(4-methyl-1-pentene) has been analyzed in terms of peak positions and relative intensities. With the hope of providing more and effective information on the fundamental vibrations, a normal coordinate analysis has also been performed on poly(4-methyl-1-pentene) by assuming Cs symmetry. The simple general valence force field (SGVFF) method has been employed in normal coordinate analysis and the potential energy distribution (PED) has been calculated for each fundamental vibration. The PED contribution corresponding to each of the observed frequencies shows the reliability and accuracy of spectral analysis. The validity of the SGVFF method as a practical tool for complete analysis of vibrational spectra, for poly(4-methyl-1-pentene) is confirmed in the present work.

Physical characteristics, vibrational spectroscopy and normal-coordinate analysis of 2-aminophenol and 2-phenylenediamine complexes

Hg(II) and Cd(II) complexes with 2-aminophenol ( Amph) and 2-phenylenediamine ( Phda) as ligands were prepared and characterized by elemental analysis, electrical conductivity, thermogravimetric analysis (DTA/TG), X-rays, FT-IR and FT-Raman spectra. The complexes formed can be formulated as [M(L) 2· nH 2O]· mH 2O. The electrical conductivity of 0.001 M DMSO solutions revealed the non-electrolytic behavior of the Amph complexes while Phda complexes behave as a 1:2 electrolyte. DTA analysis reveals the presence of two types of water coordinating as aligned and as water of crystallization. Δ E a of the stepwise decomposition was evaluated. X-ray powder diffraction studies on the ligands and their complexes are described. The fundamental frequencies of these complexes have been assigned on the basis of normal coordinate calculations, carried out using a generalized valence force field (GVFF). The proposed assignments are discussed in relation to the group frequencies in structurally related molecules and in terms of the computed potential-energy distributions among the symmetry coordinates.

Raman spectroscopic studies on matrix-isolated phosphorus molecules P4 and P2

The Raman spectra of P4 and P2 molecules have been studied in nitrogen, argon, krypton, and xenon matrices at 15 K. The vibrational frequencies of the P4 molecule are up to 14 cm−1 higher than the experimental gas phase data. This apparent blue-shift is not caused by matrix effects but rather due to an underestimation of the fundamentals in the gas phase as a course of the elevated temperatures. The observed frequencies confirm a recent theoretical prediction on the basis of high-level calculations. From the observed frequencies of the P4 molecule a general valence force field was calculated.

Vibrational Analysis of Ferrocyanide Complex Ion Based on Density Functional Force Field

Vibrational properties of ferrocyanide complex ion, <TEX>$[Fe(CN)_6]^{4-}$</TEX> , have been studied based on the force constants obtained from the density functional calculations at B3LYP/<TEX>$6-31G^{\ast\ast}$</TEX> level by means of the normal mode analysis using new bond angle and linear angle internal coordinates recently developed. Vibrations of ferrocyanide were manipulated by twenty-three symmetry force constants. The angled bending deformations of C-Fe-C, the linear bending deformations of Fe-C<TEX>${\equiv}$</TEX>N and the stretching vibrations of Fe-C have been quantitatively assigned to the calculated frequencies. The force constants in the internal coordinates employed in the modified Urey-Bradley type potential were evaluated on the density functional force field applied, and better interaction force constants in the internal coordinates have been proposed. The valence force constants in the general quadratic valence force field were also given. The stretch-stretch interaction and stretch-bending interaction constants are not sensitive to the geometrical displacement in the valence force field.

Syntheses and Structures of Donor-Acceptor Complexes of Selenium Dioxide with Pyridine and Trimethylamine

Donor-acceptor (DA) complexes of pyridine (py)·SeO2 and trimethylamine (TMA)·SeO2 were prepared by direct reactions of solid selenium dioxide with the amines. Both are white solids decomposing slowly at room temperature. The former complex decomposes into its components, while the latter one undergoes a redox reaction. The single-crystal structure determination of py·SeO2 has revealed that the chain-like structure of (SeO2)n is destroyed during the formation of the DA bond and the py·SeO2 molecule is bound to the nearest neighbouring molecule through two weak Se···O interactions. More labile TMA·SeO2 complex could not be prepared as single crystals. Analogous molecular structure of both complexes was proved by their Raman spectra. A modified general valence force field was used for the interpretation of Raman spectra of TMA·SeO2 and TMA·SO2.

Preparation, crystal structure, and spectroscopic characterization of [(Se(2)SN(2)Cl](2).

The reaction of [(Me(3)Si)(2)N](2)S with an equimolar amount of SeCl(4) in dioxane at 50 degrees C affords [(Se(2)SN(2))Cl](2) (1) in excellent yield. Crystals of 1 are orthorhombic, space group Pbca, with a = 8.5721(7) A, b = 7.8336(6) A, c = 15.228(1) A, and Z = 8. The crystal structure contains two planar Se(2)SN(2)(*)(+) rings which are linked by intermolecular Se.Se interactions [d(Se-Se) = 3.0690(7) A]. The EI mass spectrum shows Se(2)SN(2)(*)(+) as the fragment of highest mass. Both the (14)N and (77)Se NMR spectra show a single resonance (-52 and 1394 ppm, respectively). The solid [(Se(2)SN(2))Cl](2) gives a strong ESR signal indicating the presence of a Se(2)SN(2)(*)(+) radical. The Raman spectrum was assigned through normal coordinate treatment involving a general valence force field. The vibrational analysis yielded a good agreement between the observed and calculated wavenumbers.

Normal coordinate analyses of three isomeric acetylpyridines and acetophenone

Normal coordinate analyses for 2-, 3- and 4-acetylpyridine and acetophenone were carried out employing a systematic procedure based on a modified valence force field that is comparable to those used previously for different aromatic molecules. The general valence force fields (GVFF) were determined using a set of 41 parameters (25 planar and 16 non-planar) for the above compounds to compare the observed wavenumbers with the calculated values satisfactorily with an average error of 7.0 cm−1 (0.82%) for in-plane and 9.2 cm−1 (1.60%) for out-of-plane modes. On the basis of the calculated potential energy distribution, certain revisions of the previous assignments were made. The decomposition of the normal mode wavenumbers into those related to different internal coordinates is discussed. Copyright © 2000 John Wiley & Sons, Ltd.