Vibrational analysis of the Raman spectra of polycrystalline cis-2-butene

The vibrational assignments and potential energy distributions for cis-2-butene were examined by transferring appropriate force constants from trans-2-butene and the model systems, cis-1,2- and trans-1,2-difluoroethylene. The model fluoroethylene systems provided both the crucial interaction force constants, which account for effects across the double bond in cis-2-butene, and the skeletal out-of-plane force constants. The zero-order frequencies calculated for cis-2-butene from the transferred potential function agreed quite well with the low temperature Raman spectra of the polycrystalline material.     

A reevaluation of the assignment of the vibrational fundamentals and the rotational analysis of bands in the high-resolution infrared spectra of trans- and cis-1,3,5-hexatriene

Assignments of the vibrational fundamentals of cis- and trans-1,3,5-hexatriene are reevaluated with new infrared and Raman spectra and with quantum chemical predictions of intensities and anharmonic frequencies. The rotational structure is analyzed in the high-resolution (0.0013-0.0018 cm⁻¹) infrared spectra of three C-type bands of the trans isomer and two C-type bands of the cis isomer. The bands for the trans isomer are at 1010.96 cm⁻¹ (ν₁₄), 900.908 cm⁻¹ (ν₁₆), and 683.46 cm⁻¹ (ν₁₇). Ground state (GS) rotational constants have been fitted to the combined ground state combination differences (GSCDs) for the three bands of the trans isomer. The bands for the cis isomer are at 907.70 cm⁻¹ (ν₃₃) and 587.89 cm⁻¹ (ν₃₅). GS rotational constants have been fitted to the combined GSCDs for the two bands of the cis isomer and compared with those obtained from microwave spectroscopy. Small inertial defects in the GSs confirm that both molecules are planar. Upper state rotational constants were fitted for all five bands.     

An ab initio study of the in-plane harmonic force field and frequencies of cis- and trans-glyoxal

The in-plane harmonic force fields of trans- and cis-glyoxal are computed ab initio using a (7,3) basis set contracted to 〈5,3〉. With the trans conformer comparison with experiment is made for both force constants and the derived vibrational frequencies. No such comparison can be made in the case of the cis conformer because only a few assignments of experimental vibrational frequencies have been made, and the calculations thus serve to predict values for the in-plane fundamentals.     

An ab initio study of the in-plane harmonic force fields and fundamental vibration frequencies of cis- and trans-1,3-butadiene

The structure, in-plane force field, and fundamental vibration frequencies of trans- and cis-1,3-butadiene are calculated ab initio using the 4-31G basis set. Using a scaling procedure based on computational results from smaller molecules, the vibration frequencies for the trans-conformer calculated from the ab initio force constant matrix are found on average to be within 2.2% of the experimental values “harmonized” according to Dennison's rule. The values predicted for the cis-conformer, for which experimental spectroscopic data are only now becoming available, should facilitate the complete in-plane assignment of fundamentals in the near future.     

Normal mode analysis of alkenes by the consistent force field

A consistent force field for the simultaneous analysis of vibrational frequencies, conformations and thermodynamic properties of alkenes is applied to the normal mode analysis of cis- and trans-2-butene, 1,2-dimethyl-cyclopentene, cyclohexene, trans,trans,trans-1,5,9-cyclododecatriene, 1,4-cyclohexadiene, and cyclopentene. Special attention is given to the torsional modes of the 2-butenes and to the bending modes of the HCCH group. A number of reassignments are suggested. The simultaneous vibrational analysis of strained and unstrained molecules is achieved by considering nonbonded interactions and their influence on equilibrium conformations and vibrational modes.     

The υ1 fundamental bands of trans- and cis-DONO studied by high-resolution Fourier-transform spectroscopy

The absorption spectrum of deuterated nitrous acid DONO in the region from 2350 to 3000 cm⁻¹ has been recorded at a resolution of 0.003 cm⁻¹ using a Fourier-transform spectrometer. For the first time, 1366 a- and b-type transitions in the υ₁ fundamental band of trans-DONO and 741 b-type transitions in the υ₁ fundamental band of cis-DONO have been assigned. Rotational and centrifugal distortion constants up to sextic order were determined for the v₁ = 1 states of trans- and cis-DONO using non-linear least-squares calculations. Synthetic spectra calculated using the new rovibrational constants obtained for both species reproduce the observed spectra very well. In addition, the infrared transitions of this study were used, together with previously published pure rotational transitions, to determine improved rotational and centrifugal distortion constants of the ground states of trans- and cis-DONO.     

Stark spectroscopy with the CO laser: The ν2 fundamental bands of trans- and cis-nitrous acid,HNO2, in the 6-μm region

The ν₂ fundamental bands of trans- and cis-HNO₂ have been studied by the technique of intracavity CO laser Stark spectroscopy. Excited-state rotational constants were determined, and the ν₂-band origins were found to be 1699.760 cm^?1 for the trans isomer and 1640.517 cm^?1 for the cis isomer. The total dipole moments for the ground and excited (v₂ = 1) vibrational states were found to be μ″ = 1.930 D and μ″ = 1.852 D for trans-HNO₂, and μ″ = 1.428 D and μ″ = 1.441 D for cis-HNO₂.     

An ab initio study of the geometries,anharmonic force fields and fundamental vibration frequencies of cis- and trans-formic acid

The geometry, harmonic and anharmonic force fields, and fundamental vibration frequencies of cis- and trans-formic acid are studied ab initio in the 4–31G and (9,5) basis sets. For the more stable trans-conformer (i.e., trans with respect to CH and OH) comparisons are made between the predicted and observed anharmonic frequencies, and between the calculated harmonic force constants and those Redington derived from an analysis of experimental data. In the case of the less stable cis-conformer, for which there is as yet little experimental data, the calculations serve to predict values for the fundamental vibrational frequencies.     

Rotational analysis of bands in the high-resolution infrared spectra of the three species of butadiene-1,4-d2; refinement of the assignments of the vibrational fundamentals

Samples of trans,trans and cis,cis forms of butadiene-1,4-d₂ have been synthesized and found to contain useful amounts of the cis,trans species as a contaminant. Assignments of fundamental frequencies for the three isotopomers of butadiene-1,4-d₂ have been extended and improved from investigations of their Raman spectra as well as their infrared (IR) spectra. High-resolution IR spectra have been recorded for the three isotopomers, and a rotational analysis has been completed for strong bands of each species. Ground state and some upper state rotational constants have been fit. Corresponding ground state moments of inertia compare favorably with equilibrium moments of inertia obtained from B3LYP/6-311++G** theory. Two ¹³C isotopomers are being prepared, and an improved structural analysis of butadiene will soon be available to assess how π-electron delocalization affects its structure.     

Photochemical preparation of nitrosomethanol: Vibrational frequencies,force field,and normal coordinate analysis of the cis and trans isomers

The previously unknown molecule C-nitrosomethanol [H₂C(OH)(NO)] and seven of its isotopic modification [¹³C, ¹⁸OH, ¹⁵N, D₃, D₃-¹⁵N, H₂C(OD)(NO), and HDC(OH)(NO)] have been photolytically prepared from the corresponding methylnitrite isotopes in a low-temperature argon matrix. By irradiating methylnitrite into its $\text{S}{}_1\text{(nπ}{}^1\text{← S}{}_0$ transition at 365 nm, the molecule is transformed into a hydrogen-bonded 1:1 complex between formaldehyde and nitroxyl. Photolysis of this complex produces either the trans (λ_exc = 345 nm) or the cis (λ_exc ≥ 645 nm) isomer of nitrosomethanol. Selective photoisomerization processes permit interconversion of the conformers (cis → trans with λ_exc = 510 nm and trans → cis with λ_exc ≥ 645 nm). The ir spectra (40–170 cm^?1) of cis- and trans-nitrosomethanol were measured and analyzed in conjunction with a normal coordinate analysis based on a constrained valence force field (transferable valence force field approach). The frequencies of cis- and trans-nitrosomethanol were assigned and their force fields determined.     

Absolute line intensities of HONO and DONO in the far-infrared and re-determination of the energy difference between the trans- and cis-species of nitrous acid

Relative line intensities of trans- and cis-HONO and -DONO have been measured using absorption spectra in the far-infrared previously recorded by high-resolution Fourier-transform spectroscopy [A. Dehayem-Kamadjeu, O. Pirali, J. Orphal, I. Kleiner, P.-M. Flaud, J. Mol. Spectrosc. 234 (2005) 182-189]. These relative, experimental line intensities (120 lines for trans-HONO and 94 for cis-HONO, as well as 46 lines for trans-DONO and 31 for cis-DONO) were then least-squares fitted leading to the determination of “relative” permanent dipoles moments (b-component) and their rotational corrections for the trans- and cis-HONO and -DONO species. Then these “relative” permanent dipoles moments and their rotational corrections were scaled to the absolute values derived from Stark effect measurements [M. Allegrini, J.W.C. Johns, A.R.W. McKellar, P. Pinson, J. Mol. Spectrosc. 79 (1980) 446-454] and used to generate “absolute” line intensities. These “absolute” line intensities were used to derive the concentrations of the trans- and cis-species in the absorption cell. It was then possible, assuming thermodynamic equilibrium, to use the ratio of the concentrations of the trans- and cis-species to re-determine the energy differences (ΔE) between the ground vibrational states of trans- and cis-HONO: these energy differences are 99 ± 25 cm⁻¹ for HONO and 136 ± 30 cm⁻¹ for DONO. Finally applying zero-point-energy corrections we report an average value for ΔE_HONO of 107 ± 26 cm⁻¹. This value is in good agreement with previous experimental studies and with recent high-level ab initio calculations.     

An ab innitio study of the structure,in-plane harmonic force field,and fundamental vibration frequencies of cis- and trans-acrolein

The structure, in-plane force field, and fundamental vibration frequencies of trans- and cis-acrolein are computed ab initio using 4-31G and (7,3) basis sets. Using a scaling procedure and spectroscopic masses, the vibration frequencies are calculated from ab initio force constant matrices, and found on average to be within 2.5% of the experimental values in the case of the trans conformer. The values predicted for the cis conformer, for which only a few tentative assignments have been made for spectroscopic observations, should facilitate the complete assignment in the future. Further experimental studies are necessary to resolve the substantial disagreement found between theory and experiment with respect to the length of the central formal carbon-carbon single bond in the series of molecules trans-glyoxal, trans-acrolein, and trans-1,3-butadiene.     

Vibrational spectroscopy of cis- and trans-formic acid in solid argon

Absorption spectra of cis and trans conformers of formic acid (HCOOH) isolated in solid argon are analyzed in the mid-infrared (4000-) and near-infrared (7800-) regions. The HCOOH absorption spectrum reveals matrix-site splitting for the trapped molecule. Narrowband tunable infrared radiation is used to pump a suitable vibrational transition of the trans conformer in order to promote site-selectively the conversion to the cis conformer and separate the spectral features of each site group. Several anharmonic resonances are identified for both conformers. The results of anharmonic vibrational ab initio calculations (CC-VSCF) for the trans and cis conformers of formic acid are reported and compared with the experimental spectra.     

The harmonic force field and structure of chloryl fluoride

The microwave spectra of three isotopic species of chloryl fluoride, FClO₂, previously published by Parent and Gerry (J. Mol. Spectrosc., 49, 343–364 (1974)), have been refit to rotational constants and centrifugal distortion constants using Watson's Hamiltonian in both its A and S reductions. The quartic distortion constants have been combined with the vibrational data of Smith, Begun, and Fletcher (Spectrochim. Acta, 20, 1763–1770 (1964)) to calculate a refined harmonic force field. The rotational constants and force field have been used to calculate a zero-point average structure and an approximate equilibrium structure. Both the force field and structures are in essential agreement with those published earlier.     

Light-induced isomerization and photochemical transformation of methylformate in an argon matrix. Vibrational frequencies,force field,and normal coordinate analysis of trans-methylformate

When methylformate (HCOOCH₃), isolated in an argon matrix, is excited into the $\text{S}{}_1\text{(ππ}{}^1\text{) ← S}{}_0$ transition at 248 nm the prevailing cis conformer is transformed into the trans conformer. In a consecutive photolysis step the latter is decomposed to stable products including a hydrogen-bonded (1:1) complex between CO and methanol, acetic acid, and CO₂ and methane. The efficient photolytic preparation of the unstable trans-methylformate and its deuterated analogs DCOOCH₃, HCOOCD₃, and DCOOCD₃ in an argon matrix allowed measurement of their ir spectra (170–4000 cm^?1) in great detail thus providing an almost complete set of fundamentals together with a number of combination and overtone frequencies. With a normal coordinate analysis based on a constrained valence force field (transferable valence force field approach) the frequencies of trans-methylformate were assigned and compared to those of the remeasured cis rotamer.     

Luminescence and crystal-field fitting of two optical isomeric complexes of europium

FT-IR and Raman vibrational spectra and electronic emission spectra have been recorded for enantiomers of europium complexes with DBM: dibenzoylmethanate 1,2, and TTFA: 2-thenoyltrifluoroacetonate 3,4, employing the chiral ligands L_SS(+)- and L_RR(-)-4,5-pinene bipyridine. Contrary to the previously published X-ray data, where geometrical differences were stated to occur for particular enantiomers, the vibrational (and the emission) spectra of the individual optical isomers of a complex are not distinguishable. Using excitation into the Eu³⁺⁵D₂ multiplet term, the emission intensity is weak from ⁵D₁, whereas a complex structure is observed for the ⁵D₀→⁷F_J transitions. Features in the vibronic sidebands exhibit similar derived vibrational energies to those observed in the Raman spectra. Fittings of 25 4f⁶ crystal-field energy levels of 2 and 4 have been attempted with some approximations concerning the local Eu³⁺ environments. The ⁵D₀ emission lifetimes are monoexponential and are 0.5 (1,2) and 0.9 ms (3,4) at room temperature.     

Microwave spectrum,structure, and nuclear quadrupole coupling of cis-1-chlorobutadiene-1,3

The microwave spectra of the two natural isotopic species of cis-1-chlorobutadiene-1,3, CH³⁵ClCHCHCH₂ and CH³⁷ClCHCHCH₂, together with all monosubstituted species with deuterium or ¹³C isotopes have been measured and assigned and the complete substitution structure has been determined. The spectral region investigated was between 18 and 40 GHz. The molecule was found to be planar and the following values were found for the principal parameters: r(CC)_chlorovinyl = 1.327(6), $\text{r(}\text{CC}\text{)}{}_{\mathrm{<mtext>vinyl</mtext>}}\text{= 1.343(2)}\text{A}\text{?}$, <(CCC)_chlorovinyl = 126.5(3)° and <(CCC)_vinyl = 123.0(7)°. The nuclear quadrupole coupling constants, χ_aa and χ_bb, for the parent molecule were calculated and further used to estimate the symmetry of the field gradient around the CCl bond. Force-field calculations were used to predict the centrifugal distortion constants and inertial defect of some isotopic species. Thermodynamic functions were calculated for cis- and trans-1-chlorobutadiene-1,3 and used to estimate the energy difference between them.     

Photosensitivity and grating development in trans-4-stilbenemethanol-doped poly(methyl methacrylate) materials

This paper reports the photosensitivity of poly(methyl methacrylate) (PMMA) and its copolymer doped with trans-4-stilbenemethanol. UV irradiation of the doped-PMMA at 325 nm induced the trans- to cis-isomerization of the dopant. This process was confirmed by ¹H NMR spectra of trans-4-stilbenemethanol in CDCL₃ solvent before and after irradiation. The isomerization can be initiated by the irradiation with an intensity of 0.62 mW/cm². Photo-induced refractive index change of −0.0024 was obtained when a PMMA copolymer film doped with 5.1 wt% dopant was exposed to 325 nm light. Lorentz-Lorenz equation was used to estimate the refractive index of a trans-4-stilbenemethanol-PMMA composite and a trans-4-stilbenemethanol-PMMA copolymer composite from the mole refraction and van der Waals volume of each component. A slight elevation of molecular packing coefficient (K) for PMMA and its copolymer containing the dopant implies a denser aggregation as compared to the polymer without the dopant. Long period gratings were created in doped-PMMA films and doped-PMMA copolymer fibers using amplitude mask technique. Gratings were confirmed by microscopic observation and diffraction patterns.     

Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy and ab initio calculations of N-methylformamide

Rotational spectra of both trans and cis forms of the N-methylformamide normal as well as deuterated (HCONDCH₃, referred to as N-D) species were observed by Fourier transform microwave spectroscopy in the frequency region from 5 to 118 GHz. Samples were prepared in the form of a beam by a pulsed jet valve maintained at 50 °C and were introduced in a high-vacuum cavity cell, with either Ne or Ar as a carrier gas at a backing pressure of 100 kPa. The observed spectra were analyzed to yield molecular parameters including rotational constants and barrier, V₃, to CH₃ internal-rotation: 53.9 (6) and 301 (4) cm⁻¹ for the trans and cis forms of the normal species, respectively, and 41.9 (6) and 309 (4) cm⁻¹ for the trans and cis forms of the N-D species, respectively. Spectra of four trans isotopologues with ¹³C, ¹⁵N, or ¹⁸O singly-substituted in the internal-rotation A state were observed and analyzed to derive the r_s structure of the trans form. For comparison with the experimental data, ab initio calculations were carried out at MP2/6-31G∗∗ level to derive molecular structure, potential barrier to CH₃ internal rotation, and the energy difference between the cis and trans forms. An extensive coupling was found between the CH₃ internal rotation and N-H out-of-plane bending, suggesting that the potential function for the CH₃ internal-rotation deviates considerably from a simple cos(3α) form. The effects of the V₆ term is briefly discussed.     

Vibrational assignments and force field calculations for trimethylarsenic dichloride,dibromide and their deuterated analogs

Raman and infrared spectra have been recorded for trimethylarsenic dichloride, dibromide and their deuterated analogs and vibrational assignments were made for the skeletal modes of these compounds. The spectra of the dichloride and its deuterated analog were interpreted in terms of a trigonal bipyramidal structure, D_3h symmetry, whereas the spectra of the dibromides were interpreted using the ionic model, [(CH₃)₃AsBr]⁺Br⁻, having C_3v symmetry. Using a generalized valence force potential field, normal coordinate analyses data were obtained for the dichloride and the dibromide. Included in this data are potential energy distributions for a set of symmetry coordinates. The force fields obtained for the dichloride and dibromide were used to calculate frequencies for the corresponding deuterated compounds. The discrepancies between the observed and calculated frequencies for the deuterated compounds are discussed in terms of the assumptions made.