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.     

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.     

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three (iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene (m/z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl₂ group favours the diradical dechlorination mechanism. The single m/z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.     

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.     

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 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.     

Rotational Spectra of cis-HCOOH, trans-HCOOH, and trans-HCOOH

While the rotational spectrum of trans-HCOOH is well known, the cis-rotamer was known previously only from a few microwave transitions. The spectra of cis- and trans-HCOOH and of trans-H¹³COOH have been identified in the far infrared Fourier transform spectrum recorded in the range 20-100 cm⁻¹. The measurements were performed using an 80-cm base length White-type multipass absorption cell in combination with a Bruker IFS 120 HR spectrometer. The White cell was set up for 90 passes (total path length=72 m) and the spectrum was measured with a resolution (FWHM) of 0.0012 cm⁻¹. Submillimeter-wave transitions, just under 1 terahertz, of the same three species of formic acid were measured in the range from 835 to 993 GHz (27.8-33.1 cm⁻¹) and it was possible to assign b-type Q-branch transitions of both cis- and trans-HCOOH in this region, and also transitions of trans-H¹³COOH. These measurements served furthermore as a check on the calibration of the FTIR measurement. Furthermore, new millimeter-wave measurements in the range 172-366 GHz were made, bridging the gap between the MW and FIR lines for cis-HCOOH and providing important data for trans-H¹³COOH. The assigned FTIR, submillimeter- and millimeter-wave data reported in this work were fitted together with previously published rotational transitions to obtain rotational constants, using Watson's A-reduced Hamiltonian, for cis-HCOOH, trans-HCOOH, and trans-H¹³COOH.     

Photoelectron spectra of the gauche and trans conformers of 1,2-bromochloroethane

The HeI photoelectron spectra of the gauche and trans conformers of 1,2-bromochloroethane have been obtained by analysis of time-averaged spectra recorded at different temperatures. Variations in ionization potentials of the two conformers are consistent with data obtained from molecular orbital calculations. Analysis of a HEII spectrum and comparisons with the related molecules 1,2-dichloroethane and 1,2-dibromoethane shows that the ClBr non-bonded interactions include both through-bond and through-space contributions.     

Time-resolved studies on the photoisomerization of a phenylene-silylene-vinylene type compound in its first singlet excited state

In femtosecond laser-flash photolysis experiments, the first singlet excited state of trans-ST, ((E,E)-{1,4-bis(2-dimethylphenylsilyl)ethenyl}benzene) showed a strong S1(π,π^?)-Sn absorption band at 540 nm in acetonitrile and at 550 nm in hexane. The lifetime of this state was determined to be 13.2±2.0 and 11.1±1.5 ps, respectively. Intersystem crossing was shown not to be a principal route for the deactivation of this S1 state of trans-ST. Evidence for this conclusion involved two complementary nanosecond laser-flash photolysis experiments. In one experiment involving direct excitation, no transient absorption spectrum was detected in the 350-650 nm spectral range. Yet, in the second experiment, on triplet sensitization, using xanthone, a transient absorption at 400 nm was tentatively assigned to the triplet state absorption of trans-ST. Photoisomerization was monitored in nanosecond time-resolved bleaching experiments. From these experiments the trans-cis photoisomerization quantum yield was determined to be 0.23 on direct trans-ST excitation. In a xanthone-sensitized stationary-state excitation experiment, the trans-cis isomerization quantum yield was determined to be 0.32. The main deactivation route of trans-ST in its S1 state is repopulation of the ground state directly through internal conversion or with the intermediacy of conformers with twisted geometry.     

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.     

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.     

Structural and Torsional Properties of o-Cresol and o-Cresol-OD as Obtained from Microwave Spectroscopy and ab Initio Calculations

The microwave spectra of o-cresol and of o-cresol-OD were assigned using molecular beam Fourier transform microwave (MB-FTMW) spectrometers in the frequency range of 3-40 GHz. Two conformers of o-cresol were measured where the hydroxy group is syn with respect to the methyl group in one case and anti in the other. The transitions of both conformers were split due to internal rotation of the methyl group. For syn-o-cresol we found the rotational constants A=3249.45242(18) MHz, B=2202.02546(18) MHz, C=1323.66277(16) MHz, and the barrier to internal rotation of the methyl group V₃=7.912(46) kJ mol⁻¹. In the case of anti-o-cresol A=3273.80084(18) MHz, B=2196.26747(18) MHz, C=1325.36424(22) MHz, and V₃=4.4256(14) kJ mol⁻¹ was obtained. Moreover we were able to determine the quartic centrifugal distortion constants, the angle between the internal rotor axes, and the inertial a axes, and, for the deuterated species, additionally the deuterium nuclear quadrupole coupling constants.     

Fourier transform microwave spectroscopy of N,N-dimethylacetamide

The jet-cooled Fourier-transform microwave spectrum of N,N-dimethylacetamide was recorded in the region of 12-24 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Coriolis-like coupling parameters characterizing interaction between internal rotation of methyl groups and the overall rotation were also determined from internal-rotation tunneling splittings of the rotational transitions. The Coriolis-like coupling parameters permitted determination of the barrier heights to internal rotation of the three methyl groups, which were found to be 677, 237, and 183 cm⁻¹ for the C-methyl top, the trans-N-methyl top and the cis-N-methyl top, respectively.     

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.     

The 14-22 μm absorption spectrum of nitrous acid studied by high-resolution Fourier-transform spectroscopy: New analysis of the ν5 and ν6 interacting bands of trans-HONO and first analysis of the ν6 band of cis-HONO

High-resolution Fourier-transform absorption spectra of nitrous acid (HONO) were recorded in the 400-700 cm⁻¹ spectral region. For the trans-HONO isomer a very extensive analysis of the ν₅ (ONO bend) and ν₆ (torsion) interacting fundamental bands of trans-HONO located near 595.620 and 543.879 cm⁻¹, respectively, was performed starting from the results of a previous study [C.M. Deeley, I.M. Mills, L.O. Halonen, J. Kauppinen, Can. J. Phys. 63 (1985) 962-965]. In addition, for the less abundant cis-HONO isomer, the first high-resolution study of the ν₆ band located at 639.7432 cm⁻¹ was achieved, the ν₅ band is presumed to be located near 609.2 cm⁻¹. For both isomers the strong A-type and B-type Coriolis interactions linking the {5¹, 6¹} interacting energy levels were accounted for during the energy level calculations. Finally, using these results a list of positions and of realistic relative line intensities has been generated for both isomers, and a comparison with some ab initio predictions was performed.     

The far-infrared rotational spectrum of nitrous acid (HONO) and its deuterated species (DONO) studied by high-resolution Fourier-transform spectroscopy

In this paper, we present the first high-resolution (0.003 cm⁻¹) absorption measurements of the pure rotational spectra of nitrous acid (trans- and cis-HONO) and its deuterated species (trans- and cis-DONO) in the far-infrared region between 40 and 150 cm⁻¹. The spectra were first assigned based on rotational constants from previous studies in the microwave and mid-infrared regions. New rotational and centrifugal distortion constants were determined for all four species. The accuracy of the principal rotational constants was improved, and several quartic and sextic (and even a few octic) centrifugal distortion constants were obtained for the first time. Synthetic spectra calculated using the new constants of this study reproduce the observed spectra very well.     

Resonant two-photon ionization and mass-analyzed threshold ionization spectroscopy of the selected rotamers of m-methoxyaniline and o-methoxyaniline

We report the resonant two-photon ionization and mass-analyzed threshold ionization (MATI) spectra of m-methoxyaniline and o-methoxyaniline. The vibronic features of m-methoxyaniline are built on 34308 ± 2 and 34495 ± 2 cm⁻¹ corresponding to the origins of the S₁ ← S₀ electronic transition (E₁’s) of the cis and trans rotamers. Analysis of the MATI spectra gives the adiabatic ionization energies (IEs) of 59983 ± 5 and 60879 ± 5 cm⁻¹ for these two species. o-Methoxyaniline is found to have only one stable structure whose E₁ and IE are 33875 ± 2 and 58678 ± 5 cm⁻¹, respectively. Most of the active vibrations of m- and o-methoxyaniline in the electronically excited S₁ and cationic ground D₀ states result from the in-plane ring vibrations. Comparing these data with those of p-methoxyaniline allows us to learn about the vicinal substitution effects resulting from the relative locations of the NH₂ and OCH₃ substituents.     

Rotational spectrum of trans-trans diethyl ether in the ground and three excited vibrational states

We report the results of a comprehensive reinvestigation of the rotational spectrum of diethyl ether based on broadband millimetre-wave spectra recently recorded at The Ohio State University and in Warsaw, covering the frequency region 108-366 GHz. The data set for the ground vibrational state of trans-trans diethyl ether has been extended to over 2000 lines and improved spectroscopic constants have been determined. Rotational spectra in the first excited vibrational states of the three lowest vibrational modes of trans-trans-diethyl ether, ν₂₀, ν₃₉, and ν₁₂ have been assigned. The v₂₀ = 1 and v₃₉ = 1 states are near 100 cm⁻¹ in vibrational term value and are coupled by a strong c-axis Coriolis interaction, which gives rise to many spectacular manifestations in the rotational spectrum. All of these effects have been successfully fitted for a dataset comprising over 3000 transitions, leading to precise determination of the energy difference between these states, (ΔE/hc)=10.400222(5) cm⁻¹. A newly developed software package for assignment and analysis of broadband spectra is described and made available.     

Conformeric fluorescence and phosphorescence of aromatic molecules with a possibility of rotation of chromophoric fragments around single chemical bonds

Absorption, fluorescence and phosphorescence spectra of trans-stilbene,trans-1-phenyl-2- (2-naphthyl)-ethylene, trans-1,2-di(2-naphthyl)-ethylene, trans-1,2-di(1-naphthyl)-ethylene and 1,4-di(2-naphthyl)benzene in liquid and solid solutions are shown to be dependent on the wavelenght of excitation, solvent viscosity and temperature. These effects are explained by the existence of comparatively stable conformers (s-isomers) which are formed due to rotation of aryl fragments around single chemical bonds. Estimated difference in the energies of 0-0 transitions in conformers mounts up to 2000 cm^-1. Conformers with different fluorescence quantum yields were found. Existence of dynamic equilibrium between conformers of diarylethylenes may provide a clue for rationalizing some peculiarities in their photochemical behaviour; it is also suggested that the effects discussed may serve as a tool for determination of the shape and size of free volumes in liquids and solids.     

Reaction products from a discharge of N2 and H2S: The microwave spectrum of NH2SH

Thiohydroxylamine has been identified as one of the reaction products from the discharge reaction of N₂ + H₂S. Both cis and trans conformers have been observed. The rotational spectra have been studied from 56 to 170 GHz for the normal species and several deuterated isotopic species of each conformer. The electric dipole moments of both conformers have been determined. A number of the transitions of the cis conformer exhibit splittings due to the nuclear quadrupole moment of the ¹⁴N nucleus. A least squares fit of the frequency splittings have led to an analysis of the eQq values. Ab initio calculations using a 4-31G basis set both with and without polarization functions have been carried out to aid in the analysis and to provide a final structural comparison with the microwave results.