Valence Force Constants Research Articles

Theoretical (DFT) and experimental (FT-IR & FT Raman) approach to investigate the molecular geometry and vibrational properties of 2,5- and 2,6-dihydroxytoluenes

Fourier Transform infrared spectra were measured for 2, 5- dihydroxytoluene (5DHT) and 2, 6- dihydroxytoluene (6DHT), in the spectral range 4000–400 cm−1. Fourier Transform Raman spectra were also recorded for the same molecules, in the spectral region 3500–100 cm−1. Torsional potential energy scans were attempted around ring – ortho hydroxyl C-O bonds, and ring-methyl C-C bond, for the two molecules. Barrier heights were computed for the above bonds for both the target molecules. Optimized molecular geometry, general valence force constants, vibrational wave numbers (harmonic), infrared intensities, and Raman scattering intensities were calculated, employing density functional theory and using B3LYP/6-311++G(d,p) level of formalism. Observed and measured frequencies agreed with an rms error 8.6 and 8.3 cm−1, for 5DHT and 6DHT, respectively, on scaling. In addition, experimental infrared and Raman spectra agreed with their simulated counterparts with good accuracy. Using potential energy distribution (PED) and eigenvectors, all fundamentals were assigned unambiguously for both molecules. Geometry optimization was made for trimers of the two molecules at the same level of theory as used for the monomers. Existence of inter-molecular hydrogen bond was predicted.

Investigation on composition-dependent properties of Mg5xAl23−5xO27+5xN5−5x (0 ≤ x ≤ 1): Part II. Mechanical properties via first-principles calculations combined with bond valence models

Due to the application in transparent windows, armors and domes, it is essential to have a deep insight into the mechanical properties of spinel-type MgAlON solid solution. Mg5xAl23−5xO27+5xN5−5x were chosen for the investigation of composition-dependent mechanical properties via combining first-principles calculations with bond valence models. The calculated mechanical properties showed a consistence with experimental values, supporting the validity of the investigation. The simulated elastic constants were used to check the mechanical stability and analyze the shear modulus of Mg5xAl23−5xO27+5xN5−5x. The mechanical properties of Mg5xAl23−5xO27+5xN5−5x weakened with increasing x. The composition-dependent mechanical properties were explored from the perspective of chemical bonds. The results demonstrated that decline trend of the mechanical properties, including hardness, bulk modulus and shear modulus of Mg5xAl23−5xO27+5xN5−5x is mainly impacted by the bonds in tetrahedra, since the linear density of bond valence and bond force constant in tetrahedra decreased dramatically, while those in octahedra were almost unchanged.

Transition-State Vibrational Analysis and Isotope Effects for COMT-Catalyzed Methyl Transfer.

Isotopic partition-function ratios (IPFRs) computed for transition structures (TSs) of the methyl-transfer reaction catalyzed by catechol O-methyltransferase and modeled by hybrid QM/MM methods are analyzed. The ability of smaller Hessians to reproduce trends in α-3H3 and 14Cα IPFRs as obtained using the much larger subset QM/MM Hessians from which they are extracted is investigated critically. A 6-atom-extracted Hessian reproduces perfectly the α-T3 IPFR values from the full-subset Hessians of all the TSs but not the α-14CIPFRs. Average AM1/OPLS-AA harmonic frequencies and mean-square amplitudes are presented for the 12 normal modes of the α-CH3 moiety within the active site of several enzymic transition structures, together with QM/MM potential energy scans along each of these modes to assess the degree of anharmonicity. A novel investigation of ponderal effects upon IPFRs suggests that the value for α-14C tends toward a limiting minimum whereas that for α-T3 tends toward a limiting maximum as the mass of the rest of the system increases. The transition vector is dominated by motions of atoms within the donor and acceptor moieties and is very well described as a simple combination of Walden-inversion “umbrella” bending and asymmetric stretching of the SCα and CαO bonds. The contribution of atoms of the protein residues Met40, Tyr68, and Asp141 to the transition vector is extremely small. Average valence force constants for the COMT TS show significant differences from early BEBOVIB estimates which were used in support of the compression hypothesis for catalysis. There is no correlation between TS IPFRs and the nonbonded distances for close contacts between the S atom of SAM and Tyr68 or between any of the H atoms of the transferring methyl group and either Met40 or Asp141.

Na3[BN2] and Na2K[BN2]: A Known and a Novel Alkali Metal Dinitridoborate Obtained via Mild Thermal Dehydrogenation

AbstractNa3[BN2] and Na2K[BN2] were obtained as white polycrystalline powders from the reaction of the respective binary mixtures NaNH2:NaBH4 and NaNH2:KBH4 in molar ratio 2:1 at 873 K and 683 K, respectively, in an argon stream. According to the results of thermal analysis measurements, both compounds are thermally stable only up to 954 K (Na3[BN2]) and 712 K (Na2K[BN2]), respectively, decomposing under evolution of alkali metal and nitrogen to yield hexagonal BN as final residue, which was identified from powder patterns. The crystal structure of Na3[BN2] {β‐Li3[BN2] type; P21/c (No. 14); Z = 4} was confirmed and the unit cell parameters redetermined: a = 5.724(1) Å, b = 7.944(1) Å, c = 7.893(1) Å, β = 111.31(1)°. According to X‐ray powder data, Na2K[BN2] crystallizes isotypic to Na2KCuO2 in the tetragonal space group I4/mmm (No. 139) with a = 4.2359(1) Å, c = 10.3014(2) Å and Z = 2. The crystal structure of Na2K[BN2] is composed of linear [N–B–N]3– anions centering elongated M14 rhombic dodecahedra, which are formed by 8 sodium and 6 potassium atoms. The [BN2]@Na8/4K6/6 polyhedra are stacked along [001] and condensed via common tetragonal faces to generate a space‐filling 3D arrangement. The B–N bond lengths for the strictly linear [N–B–N]3– units are 1.357(4) Å. Vibrational spectra of the title compounds were measured and analyzed based on D∞h symmetry of the relevant [N–B–N]3– groups taking into account the site symmetry effects for Na3[BN2]. Both the wavenumbers, as well as the calculated valence force constants f(B–N) = 7.29 N·cm–1 (Na3[BN2]) and 7.33 N·cm–1 (Na2K[BN2]), respectively, are in good agreement with those of the known alkali and alkaline earth dinitridoborates.

Experimental study of the vibrational spectra of (CH3)3GeBr supported by DFT calculations

AbstractThe infrared and Raman spectra of bromotrimethylgermane (BTMG) were recorded afresh to complete the assignment of its vibrational spectra. The vibrational spectrum of BTMG has been predicted from hybrid density functional theory calculations (B3LYP) with several basis sets. The resulting harmonic wavenumbers were scaled by Pulay's scaled quantum mechanical (SQM) and the wavenumber‐linear scaling (WLS) methods to obtain accurate force fields which could aid in the vibrational assignment. Low‐temperature infrared techniques together with Fourier self‐deconvolution (FSD) on the Raman spectrum were used to improve the resolution of the spectra for the modes that could not be observed before. An SQM analysis was carried out to obtain the valence force constants and a set of scale factors that best reproduced the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.

Vibrational Spectra of Cluster Anions. 2. Vibrational Spectra of Compounds with the Cluster Anions [E4]4−: M4E4 (M = K, Rb, Cs; E = Ge, Sn) and β‐Na4Sn4

AbstractVibrational spectra of the compounds M4E4 (M = K, Rb, Cs; E = Ge, Sn) and of β‐Na4Sn4 with the cluster anions [E4]4− were analysed based on the point group ${\bar 4}{\rm 3}m-T_{d}$ of isolated tetrahedranide units. The lower individual ${\bar 4}{\rm 2}m-D_{2d}$ symmetry of the anions in the real structure being more patterned and complex primarily affects the spectra of the tetrahedro‐tetragermanides. ν3(F2) clearly splits both in Raman and IR and in the case of K4Sn4 only in IR. Rb4Sn4 and Cs4Sn4 exhibit very simple spectra with three bands in Raman and one band in IR. The breathing mode ν1(A1) for the quasi isolated [E4]4− cluster appears only in the Raman spectrum and is hardly influenced by the structural environment and by the nature of the alkali metal cations: ν1(A1) = 274 cm−1 ([Ge4]4−) and 183‐187 cm−1 ([Sn4]4−), respectively. The calculated valence force constants fd(E–E) are: [Ge4]4−: fd = 0.89 Ncm−1 (K), 0.87 Ncm−1 (Rb), 0.86 Ncm−1 (Cs) and [Sn4]4−: 0.67 Ncm−1 (Na), 0.66 Ncm−1 (K), 0.67 Ncm−1 (Rb), 0.68 Ncm−1 (Cs). Both, the frequencies and the force constants fit well into the range previously reported.

Optical, electrical and the related parameters of amorphous Ge–Bi–Se thin films

The related optical and electrical parameters of amorphous Ge–Bi–Se thin films were studied. The dependence of optical and electrical properties on the Bi content was observed in most compositions. At Bi >10 at% the behavior show a switch from p to n type conduction mechanism. The correlation between the optical band gap E g and the average heats of atomization H s were observed. The results indicated that both the number of topological constant N con and the radial and angular N α , N β valence force constants exhibit the same trend with increasing Bi content. On the other hand, the mean bond energy 〈 E〉 increases with increasing Bi content to x=15 at%. It may be concluded that 〈 E〉 is a function of the mean coordination number N co, the type of bonds, the degree of cross-linking and the band energy forming the network.

Ni(N2)4 revisited: an analysis of the Ni–N2 bonding properties of this benchmark system on the basis of UV/Vis, IR and Raman spectroscopy

Matrix isolation of Ni atoms in an N2 matrix leads to the formation of Ni(N2)4. This compound, being isoelectronic with the well known Ni(CO)4, represents an important bench-mark system. It has been characterised experimentally by UV/Vis, IR and Raman spectra. The vibrational spectra give evidence for both a1 modes, three of the four t2 modes, and one of the two e modes of the Td symmetric molecule. The experimental data obtained for Ni(14N2)4 and Ni(15N2)4 were used to determine the valence force constants f(Ni-N) and f(N-N), which are compared with those derived for Ni(N2) and for the corresponding carbonyl complexes Ni(CO) and Ni(CO)4. In addition, several overtones and combination modes of Ni(N2)4 were observed for the first time, providing further valuable information about the bond properties. The data allow for the first time a direct estimate of the Ni-N2 bond energy in Ni(N2)4 (120 kJ mol(-1)), that compares with a value of 148 kJ mol(-1) determined by the same method for Ni(CO)4.

An approach to the structure and vibrational analysis of cis- and trans-3-chlorostyrene through IR/Raman and INS spectroscopies and theoretical ab initio/DFT calculations

Planarity of cis- and trans-3-chlorostyrene is discussed in the light of new data from infrared/Raman and inelastic neutron scattering (INS) spectroscopies and theoretical calculations. Molecular structures have been optimized at different levels of theory, ab initio and DFT, with an assortment of basis sets, 6-31G*, 6-311G** and 6-311++G** concluding that Hartree–Fock and MP2 results predict a non-planar structure while DFT predicts a planar geometry regardless of the basis set. Vibrational spectra have been calculated and compared with experimental data as obtained from IR, Raman and, for the first time, Inelastic Neutron Scattering (INS) providing evidence for the planarity of this system in its ground electronic state. Molecular force fields for the cis and trans conformers of 3-chlorostyrene using the scaled and refinement formalisms are reported. For easier visualisation the valence force constants have also been converted to a so-called pure vibrational force field. These results together with the barrier height calculations to vinyl internal rotation allows conclusions about the extent of localisation of the vinyl double bond.

Raman spectroscopy ofLa2@C80andTi2@C80dimetallofullerenes

This paper is devoted to the Raman spectroscopy of two dimetallofullerenes ${\mathrm{La}}_{2}@{\mathrm{C}}_{80}$ and ${\mathrm{Ti}}_{2}@{\mathrm{C}}_{80}.$ Previous studies of monometallofullerenes $(M@{\mathrm{C}}_{82})$ and dimetallofullerenes ${(M}_{2}@{\mathrm{C}}_{82})$ have demonstrated that Raman spectroscopy is a useful tool to probe the cage-metal bond strength. The latter is a fingerprint of the metal to fullerene charge transfer, which plays an important role in the stabilization of the metallofullerene. In the present study, the metallic ions are trapped inside a ${\mathrm{C}}_{80}$ hollow cage for each metallofullerene, but their group and weight differ. This provides the opportunity to probe in the same study the ability of Raman spectroscopy to determine the ionization state of ions trapped in a cage, which eventually differs only by isomerization. A detailed analysis of the low-frequency part of the Raman spectrum is proposed for both ${\mathrm{La}}_{2}@{\mathrm{C}}_{80}$ and ${\mathrm{Ti}}_{2}@{\mathrm{C}}_{80}.$ Two low-frequency modes at 161 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{La}}_{2}@{\mathrm{C}}_{80}$ and at 196 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{Ti}}_{2}@{\mathrm{C}}_{80}$ are tentatively assigned to a fingerprint of the interaction between the trapped ions and cage. The corresponding values for the metal-cage valence force constant are in favor of an effective charge transfer of three electrons per lanthanum to the cage and of two electrons at most per titanium to the cage. These results are in good agreement with theoretical predictions and with electron energy loss spectroscopy or x-ray observations.

The molecular force field of 4-fluorostyrene: an insight into its vibrational analysis using inelastic neutron scattering, optical spectroscopies (IR/Raman) and theoretical calculationsElectronic supplementary information (ESI) available: Structural parameters, frequences and force constants of 4-fluorostyrene (4 tables); observed and calculated IR spectra and INS spectra (6 figures). See http://www.rsc.org/suppdata/cp/b2/b212620f/

This paper reports for the first time a molecular force field of the 4-fluorostyrene molecule making a joint use of data from optical spectroscopies (IR/Raman), neutron scattering (INS) and theoretical calculations, ab initio (HF, MP2) and DFT with different basis sets. Harmonic molecular force fields were calculated following two different procedures, scaling and refinement. Also, proposed is a complete set of valence force constants obtained via the so-called pure vibrational force field. In a parallel way, some structural features such as the vinyl torsion barriers, the CC vinyl bond, and Ph-vinyl CC bond, among others, were analysed. A comparison with available experimental data for some standard and related molecules allows conclusions about the aromaticity of these systems.

Darstellung, Schwingungsspektren und Normalkoordinatenanalyse von (n-Bu4N)2[Pt(SCN)n(ox)], n = 2, 4, sowie Kristallstruktur von [(C5H5N)2CH2][Pt(SCN)4(ox)] / Synthesis, Vibrational Spectra and Normal Coordinate Analysis of (n-Bu4N)2[Pt(SCN)n(ox)], n = 2, 4, and Crystal Structure of [(C5H5N)2CH2][Pt(SCN)4(ox)

On exposure of trans-(n-Bu4N)2[Pt(SCN)2(ox)2] to photochemical reduction (n-Bu4N)2- [Pt(SCN)2(ox)] (1) is formed. Further treatment with (SCN)2 yields (n-Bu4N)2- [Pt(SCN)4(ox)] (2). The X-ray structure determination on a single crystal was performed of [(C5H5N)2CH2][Pt(SCN)4 (ox)] (triclinic, space group P1̅, a = 9.823(1), b = 14.850(1), c = 15.799(1) Å , α= 74.304(9), β= 86.268(9), γ= 87.455(9)°, Z = 4). Based on the molecular parameters of the X-ray determination the IR and Raman spectra have been assigned by normal coordinate analysis. The valence force constants of the symmetric axis S-Pt-S and of the asymmetrically coordinated S'-Pt-O* axes are fd(PtS) = 1.71, fd(PtS') = 1.92 (1), fd(PtS') = 2.02 (2) and fd(PtO•) = 2.45 (1) and 2.24 (2) mdyn/Å. Taking into account increments of the trans influence a good agreement between observed and calculated frequencies is achieved. The NMR shifts are δ(195Pt) = 1967.7 (1) and 2930.8 ppm (2)

Vibrational analysis of substituted phenols: Part II. Transferability of valence force constants

A zero-order normal coordinate analysis was made for the in-plane and out-of-plane vibrations of 2,6-dimethylphenol, 2,3-dimethylphenol, 3,5-dimethylphenol, 2,6-dichloro-4-nitrophenol, 2,4-dichloro-6-nitrophenol, 3-chloro-4-hydroxy-5-methoxybenzaldehyde, 3-bromo-4-hydroxy-5-methoxybenzaldehyde and 3-iodo-4-hydroxy-5-methoxybenzaldehyde and for the in-plane vibrations of 2-amino-4-nitrophenol by transferring the force constants obtained beforehand. The transferability of the force constants was demonstrated by the good agreement between the observed and calculated frequencies. On the basis of calculated potential energy distributions and eigenvectors, several assignments suggested by earlier workers have been revised.

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.

Raman, infrared and force field studies of K 212C 2O 4 · H 2O and K 213C 2O 4 · H 2O in the solid state and in aqueous solution, and of (NH 4) 212C 2O 4 · H 2O and (NH 4) 213C 2O 4 · H 2O in the solid state

The Raman and infrared spectra of solid K 2 12C 2O 4 · H 2O are reported together with, for the first time, the corresponding Raman and infrared spectra of solid K 2 13C 2O 4 · H 2O. Raman spectra of aqueous solutions of both isotopomers are also reported. In the solid state the oxalate anion is planar with D 2 h symmetry in this salt, whereas in aqueous solution the Raman spectra of the anion are best interpreted on the basis of D 2 d symmetry. The Raman spectra of solid (NH 4) 2 12C 2O 4 · H 2O and (NH 4) 2 13C 2O 4 · H 2O, in which the oxalate anion is twisted from planarity by 28° about the CC bond, have also been recorded. Several reassignments have been made. The harmonic force field for the oxalate anion in the D 2 h , D 2 and D 2 d geometries has been determined in part, and approximate values of key valence force constants determined. All the observed band wavenumbers and 12C/ 13C isotopic shifts are well reproduced by the force fields. The potential energy distribution of the totally symmetric normal modes of planar oxalate indicates that each mode consists of extensively mixed symmetry corrdinates and that the labels previously used for the bands seen here at 475 and 879 cm −1 would better be described as ν(CC) and δ s(CO 2), respectively, putting them in the same wavenumber order as ν(NN) and δ s(NO 2) for the isoelectronic and isostructural molecule N 2O 4. The stretching force constants of N 2O 4 and planar C 2O 4 2− are established to be in the order f NN< f CC and f NO> f CO, consistent with the known relative bond lengths.

Raman spectra and lattice-dynamical calculations of natrolite

Polarized single-crystal Raman scattering and powder infrared absorption spectra of Fdd2 orthorhombic natural natrolite (Na1.88 K0.02 Ca0.04 )(Al1.96 Si3.03 O10)⋅2.03 H2O from Khibiny, Kola peninsula, were measured. Using short-range models, lattice-dynamical calculations were performed for an idealized natrolite structure Na4(Al4Si6O20)4H2O containing 46 atoms in the primitive unit cell (Z = 2). By varying the valence force constants, the calculated frequencies in the Raman and IR spectra were fitted to the observed frequencies. On considering their calculated intensities as well, nearly all the observed bands (especially those corresponding to the A1 modes) could be unambiguously assigned and interpreted. The external vibrations of H2O could be correctly assigned using deuter- ated samples. The strongest Raman band at 534 cm-1 corresponds to a breathing mode of the four-membered alumi- nosilicate ring. The calculated bulk modulus (52.7 GPa at zero pressure) is close to the experimental value of 47 ± 6 GPa. The natrolite structure has some advantages upon other zeolites to understand the amorphization mechanism, because samples of this mineral surrounded by a non-penetrating medium show no crystal phase transitions with increasing pressure. Lattice energy minimization calculated with variable unit-cell dimensions shows the crystal structure to become unstable at about 5.5 GPa, thereby apparently explaining the amorphization process at 4-7 GPa. This instability is connected with shear acoustic modes coupled with soft internal framework vibrations.

Quantum chemistry predicted correlations between geometric isomerism (conformation) of OH and NH substituents and typical group frequencies of nucleic acid bases: cytosine

Results of a search for correlations between typical group mode frequencies and conformation of OH and NH substituents is reported. The study is based on quantum chemical data (HF/6-31G (d, p) and MP2 (full)/6-31G (d, p) approximations) of 13 isomers of cytosine. It is closely related to investigations of stability and energetics of all isomers of the more common nucleic acid bases, which revealed correlations between conformation (geometric isomerism) of OH and NH substituents on the one hand and conversion energies, relaxation of internal structural parameters, electric field gradients, etc. on the other hand. In the majority of cases alteration of conformation of these substituents is accompanied by systematic frequency shifts of stretching, in-plane and out-of-plane bending (torsional) modes conventionally assigned to such groups and by alteration of quantum chemically predicted estimates of harmonic valence force constants and structural parameters dominating the mode frequencies. For identification of group modes ‘fractional’ mass (other than ‘natural’ mass) isotope shifts of frequency and normal vectors proved useful. Likewise, estimation of effects of force constant and structural parameter alterations on frequency shifts by first order perturbation theory of the FG problem of partial structures contributed valuable insight into the origin of the shifts.

Vibrational Spectra of the Cluster Anions [E4]4- in themetallic Sodium and Barium Compounds Na4E4 and Ba2E4 (E = Si, Ge)

The vibrational spectra of the cluster anions [E4]4– (E = Si, Ge) in the metallic compounds Ba2E4 and Na4E4 have been measured and assigned based on the Td symmetry of the discrete tetrahedranide anion. Due to the lower site-symmetries in the respective crystals all degenerate modes are split, but to different extends. The characteristic breathing frequency ν(E–E) of the [E4]4– cluster appears exclusively in the Raman spectrum and is almost unaffected by the nature of counterions: ν(E–E) = 486 cm–1 (E = Si) and 276 and 278 (Ge), respectively. The calculated valence force constants fd (Si–Si) = 1.17 Ncm–1 (Na); 1.15 Ncm–1 (Ba) and fd (Ge–Ge) = 0.98 Ncm–1 (Na); 0.94 Ncm–1 (Ba) are in good agreement with those previously reported.

Influence of Compression upon Kinetic Isotope Effects for SN2 Methyl Transfer: A Computational Reappraisal

Secondary α-D3 kinetic isotope effects (KIEs) have been calculated using ab initio HF and B3LYP methods with 6-31G and 6-31G* bases for four methyl transfer reactions of the general type R3N + CH3NR3+. Comparison of R = CH3 (B) with R = H (A) serves to investigate the effect of varying the nucleophile and nucleofuge by alkylation of the entering and leaving amine moieties. The changes in KIE, transition-structure (TS) looseness, and energy barrier do not accord with the generalization of Wolfe and co-workers (J. Am. Chem. Soc. 1993, 115, 10147). Reactions C and D are intramolecular methyl transfers between the bridgeheads of inside-methylated [1.1.1]cryptand and 1,7-diazabicyclo[5.5.5]heptadecane, respectively, in which the N···N distances are significantly smaller than in B. Comparison of B with either C or D serves to investigate the effect of compression along the N−C−N axis. The energy barriers for C and D are markedly lower than for B, and, although their TS's are tighter than for B, their reactant complexes (RC's) are even tighter. Progress from RC to TS in the compressed systems is accompanied by a decrease in strain. The inverse α-D3 KIEs for C and D are dominated by zero-point energy changes, but the contribution from the CH stretching modes is less inverse than that for B. The more inverse B3LYP KIEs for C and D arise because there is a significant increase in the relaxed valence force constant for bending the H−Cα−Nlg angle in going from RC to TS. These calculated results are consistent with Schowen's compression hypothesis for enzymatic methyl transfer.

Quantum-Chemical Studies of Fluoroethanes: Vibrational Assignments, Isolated CH Stretching Frequencies, Valence Force Constants, and Bond Length Relationships

Harmonic force fields for the series of fluoroethanes, derived from HF and B3LYP calculations with the 6-311G** basis set, are scaled individually for each molecule and for each type of CH bond. Reassignments are made in the νCH region with the aid of earlier correlations between CH bond length and six observed isolated CH stretching frequencies νisCH. Eleven more νisCH values are obtained from the spectra through the determination of scale factors. Estimation of the remaining three undetermined νisCH values is discussed. Problems of assignment remain both in the νCH region and also between 1200 and 1000 cm-1, which call for further study. Improved predictions are made of unobserved frequencies in the less abundant conformers of the 1,2-, 1,1,2-, and 1,1,2,2- fluoroethanes. νCH scale factors vary significantly according to the number of fluorine atoms attached to the same carbon and also to a lesser degree among those of the same type. Scale factors for other types of motion also vary both between molecules and also within a given molecule. Changes in the factor for torsional motion are particularly large. For the CH, CF, and CC bonds, unscaled valence force constants are given, and their relation to νisCH, bond lengths, and bond length “offset” values is discussed. Evidence for variations in the latter is reviewed. An unscaled νisCH value reflects very closely the corresponding CH stretching force constant in any of the groups methyl, methylene, and methine.