Vibrational Transition Moments Research Articles

Determination of the average orientation of low symmetry planar molecules in anisotropic solvent from two quantum chemically calculated vibrational transition moments

Abstract A new procedure combining a known experimental approach with quantum chemical data is proposed for determination of the average orientation of planar molecules (possessing only C s symmetry) oriented in nematic solvents. It is based on quantum chemical calculation of the angle between two vibrational transition moments corresponding to in-plane vibrations giving rise to strong IR absorption bands with different dichroic ratios, which allows one to find the principal orientation axes of the oriented molecule. All other vibrational transition moments are then transformed into this coordinate system in order to obtain an improved description providing consistency between experimental and theoretical determinations of transition moment directions. The correction procedure is illustrated by the example of p -nitrobenzaldehyde.

Chlorinated Guest Orientation and Mobility in Clathrate Structures Formed with Syndiotactic Polystyrene

Syndiotactic polystyrene films uniaxially stretched at different draw ratios, and including different crystalline (δ and γ) phases as well as clathrate phases with 1,2-dichloropropane, 1,2-dichloroethane, and 1-chloropropane, have been studied by combined infrared linear dichroism and X-ray diffraction measurements. Information relative to the orientation and mobility of trans and gauche conformers of these chlorinated guests in their clathrate structures with syndiotactic polystyrene has been achieved by comparison between experimentally evaluated and calculated vibrational transition moment directions of conformationally sensitive infrared peaks of the guest molecules.

Phonon confinement and dispersion curves in finite polyene derivatives

We analyze the infrared and Raman spectra (both experimentally and with the aid of quantum chemical calculations) of a series of polyenals which provide us with the fortunate case of a set of polyene chains with one of the end groups consisting of a CO group which not only does take part in the conjugation but also pulls electrons from the chain making the whole system highly polar thus affecting the vibrational transition moments. In the following we show, for the first time, that it is possible to derive experimental phonon dispersion curves and these prove to be different for each chain length. We support our experimental findings with DFT quantum chemical calculations which reproduce with sufficient accuracy the IR and Raman spectral pattern and at the same time help in disentangling the assignment of the fine structure observed in the experimental spectra.

The rotational Raman spectra and cross sections of H 2O, D 2O, and HDO

We report the experimental rotational Raman spectra of H 2O, and of a mixture of D 2O and HDO in the vapor phase at room temperature, and their interpretation in terms of rotational–vibrational energies, wavefunctions, and transition moments of the molecular polarizability. These transition moments are based on high-level ab initio calculations of the wavelength dependent polarizability surface, and on wavefunctions where the rotational–vibrational coupling is considered in detail. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the rotational Raman spectra can be simulated over the range of temperatures up to 2000 K for H 2O, and up to 300 K for D 2O and HDO.

Vibrational Transition Moment Angles in Isolated Biomolecules: A Structural Tool

Infrared spectroscopy is used extensively in the study of isolated biomolecules, but it becomes less useful as it is applied to systems of increasing complexity. Even if the individual vibrational bands can be resolved spectroscopically, their assignment becomes problematic when they are more closely spaced than can be determined using ab initio methods. We describe a method that helps to alleviate this difficulty by measuring the direction of the vibrational transition moment for each vibrational band. The molecules of interest (adenine and cytosine) are cooled to 0.37 kelvin in liquid helium nanodroplets and oriented in a large dc electric field. A polarized infrared laser is then used to determine the directions of the infrared transition moments relative to the permanent dipole moment. Comparisons with ab initio calculations provide detailed structural information, including experimental evidence for nonplanarity of adenine and three tautomers of cytosine.

High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm−1 in slit jet expansions and at room temperature

The ν2+2ν3 combination band of CH412 near 7510 cm−1 was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. ν2+2ν3 is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of ν2+2ν3 at 7510.3378±0.0010 cm−1, the integrated band strength G=(1.3±0.2)×10−4 pm2 and the vibrational transition moment |μν|=(1.0±0.1)×10−3 D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F1-component of ν2+2ν3 interacting by a strong Coriolis resonance with the IR-active F2-component appears to be a dominant perturber.

Experimental Determination of the Vibrational Transition Moment of NeH+ in the 1Σ+ Ground State

The three lowest vibrational bands 1← 0, 2← 1 and 3← 2 of 20NeH+ in the 1Σ+ ground electronic state are recorded from IR emission of a positive column discharge with a Fourier transform Bruker 120 spectrometer between 1800 and 3000 cm−1. Accurate integrated intensities have been measured for 75 vibration rotation transitions. The diagonalisation procedure first developed by T. Tanaka et al. [J. Mol. Struct.153, 413 (1997)] was applied for the determination of the transition dipole moment of NeH+. With the reported theoretical value of the permanent dipole moment taken as 3.004 D, the vibrational transition moment μ1−0 is determined to be 0.314±0.026D, with one standard deviation.

Infrared spectroscopic study of molecular hydrogen bonding in chiral smetic liquid crystals

We utilize Fourier transform infrared (IR) spectroscopy to probe intramolecular hydrogen bonding in smectic‐C* liquid crystal phases. Infrared spectra of aligned smectic liquid crystal materials vs. temperature and of isotropic liquid crystal mixtures vs. concentration were measured in homologs, both with and without hydrogen bonding. Hydrogen bonding significantly changes the direction and magnitude of the vibrational dipole transition moments, causing marked changes in the IR dichroic absorbance profiles of hydrogen bonded molecular subfragments. A GAUSSIAN94 computation of the directions, magnitudes, and frequencies of the vibrational dipole moments of molecular subfragments shows good agreement with the experimental data. The results show that IR dichroism can be an effective probe of hydrogen bonding in liquid crystal phases.

H2O in stellar atmospheres

We have performed a detailed ab initio computation of the dipole moment surface, the vibrational transition moments, and the spectral lines for the water molecule. A total of 412 vibrational eigenstates were identified below 30 000 cm-1 , corresponding to ≈ 85 000 vibrational transitions. In principle there are many billions of allowed vibration-rotation transitions between these eigenstates. In our most complete test calculations we constructed a list of 3 billion () lines. At room temperature, the computed monochromatic absorption coefficient is in good overall agreement with the hitran data base, while at higher temperatures its value exceeds the hitran-based absorption coefficient by more than an order of magnitude, due to the lack of high excitation lines in hitran. The agreement with the hitemp version of hitran is considerably better than with the standard hitran. By comparing stellar model atmospheric structures and synthetic spectra based on our most extensive line list with results from calculations excluding the huge number of ultra-weak lines, we conclude that when the lines are well chosen, a few times 10 million lines are more than sufficient for all astrophysical purposes. We therefore offer to the community a line list of 100 million lines, easily accessible by anonymous ftp. Finally, we have compared results of synthetic stellar spectra based on this line list with observed ISO spectra and have found good agreement. In particular in the 2-4 μ m region around the strong fundamental bands, the agreement with observations is considerably better than that obtained with other available water line lists.

Infrared Dichroism from the X-Ray Structure of Bacteriorhodopsin

A detailed comparison with the three-dimensional protein structure provides a stringent test of the models and parameters commonly used in determining the orientation of the α-helices from the linear dichroism of the infrared amide bands, particularly in membranes. The order parameters of the amide vibrational transition moments are calculated for the transmembrane α-helices of bacteriorhodopsin by using the crystal structure determined at a resolution of 1.55 Å (PDB accession number 1C3W). The dependence on the angle δ M that the transition moment makes with the peptide carbonyl bond is fit by the expression ( 3 2 S α cos 2 α)cos 2( δ M + β) − ½ S α , where S α (0.91) is the order parameter of the α-helices, α (13°) is the angle that the peptide plane makes with the helix axis, and β (11°) is the angle that the peptide carbonyl bond makes with the projection of the helix axis on the peptide plane. This result is fully consistent with the model of nested axial distributions commonly used in interpreting infrared linear dichroism of proteins. Comparison with experimental infrared dichroic ratios for bacteriorhodopsin yields values of Θ A = 33 ± 1°, Θ I = 39.5 ± 1°, and Θ II = 70 ± 2° for the orientation of the transition moments of the amide A, amide I, and amide II bands, respectively, relative to the helix axis. These estimates are close to those found for model α-helical polypeptides, indicating that side-chain heterogeneity and slight helix imperfections are unlikely to affect the reliability of infrared measurements of helix orientations.

A theoretical study of molecules trapped in an argon matrix: Vibrational energies, and transition moments for low-lying levels and IR bar spectra

Semi-empirical atom-atom potential energy calculations based on pairwise additive interactions are performed and, after applying the Born-Oppenheimer approximation to separate high frequency vibrational modes from low frequency orientational and translational modes, the infrared vibrational spectra of CO2 and N2O monomers trapped in an argon matrix at a temperature of 5 K are determined. It is shown that only a double substitutional site in argon can accommodate N2O, whereas CO2 is trapped in two distinct sites, of single and double substitutional types. The model shows that splitting of the degenerate v 2 mode occurs for both molecules in the double site. In the ground electronic state, the vibrational frequency shifts due to the matrix and the vibrational transition moments for low-lying levels are determined using the contact transformation method, as used for gas phase calculations. Calculated energy levels compare well with observed ones and the theory also predicts some unobserved levels. Moreover, calculations show no significant changes in the dipole moments of both CO2 and N2O trapped molecules.

Infrared spectroscopic study of molecular hydrogen bonding in chiral smectic liquid crystals

We report the use of Fourier-transform infrared (IR) spectroscopy to probe intermolecular and intramolecular hydrogen bonding in thermotropic liquid-crystal phases. Infrared spectra of aligned smectic liquid crystal materials vs temperature, and of isotropic liquid-crystal mixtures vs concentration were measured in homologs both with and without hydrogen bonding. Hydrogen bonding significantly changes the direction and magnitude of the vibrational dipole transition moments, causing marked changes in the IR dichroic absorbance profiles of hydrogen-bonded molecular subfragments. A GAUSSIAN94 computation of the directions, magnitudes, and frequencies of the vibrational dipole moments of molecular subfragments shows good agreement with the experimental data. The results show that IR dichroism can be an effective probe of hydrogen bonding in liquid-crystal phases.

Experimental transition dipole moment for the four lowest Δv=1 bands of ArH+ in the Σ+1 fundamental state

Vibration–rotation bands of the Δv=1 sequence of ArH+ are recorded from the emission of a discharge tube with a Fourier spectrometer, between 1800 and 2830 cm−1. The analysis of the relative line intensities of the four lowest transitions made possible the determination of their Herman–Wallis factors. The derived ratios of first- and second-order coefficients of the dipole moment function to the permanent dipole moment M0 are equal to M1/M0=1.89, and M2/M0=0.96. The absolute values of the vibrational transition moments are respectively found, with no need of the ion concentration, equal to 0.194, 0.565, 1.049, and 1.623 debye for the 1–0, 2–1, 3–2, 4–3 transitions, with the assumption of M0 equal to 1.42 debye as experimentally given by K. B. Laughlin et al., Phys. Rev. Lett. 58, 996, (1987).

High-Resolution Infrared Spectrum of the Ring-Puckering Band, ν10, of Diborane

The spectrum of the ν10 band of diborane, arising from the ring-puckering vibration, has been obtained with a spectral resolution of 0.0015 cm−1 in the region 275–400 cm−1. The spectrum of a sample enriched in 10B was recorded as well as one with naturally abundant boron, i.e., 64% 11B2H6, 32% 10B11BH6, and 4% 10B2H6. This mode is the lowest vibrational level of the molecule and is unperturbed, allowing a complete assignment of not only the fundamental bands but also the 2ν10–ν10 hot bands of all three boron isotopomers. The intensities of several hundred lines of the fundamental and hot bands of all isotopomers have been measured and vibrational transition moments have been obtained. Finally, it has been shown that the harmonic approximation does not apply for ν10.

Determination of parameters of the dipole moment of the CO2 molecule

Analytic expressions are obtained for vibrational transition moments of the first, second, and third orders using the eigenfunctions in the second-order perturbation theory. These expressions can be used to solve the inverse electrooptical problem for the 12C16O2 molecule. The resonance interactions were taken into account by solving secular equations. The mixing coefficients for the eigenfunctions were calculated with an accuracy of 0.1%. The experimental data on purely vibrational transition moments 〈μ〉2 used in the solution were obtained by averaging the data available in the literature with the weights that are inversely proportional to the error, resulting in mean-square deviations of (0.1–10)%. Five parameters of the dipole moment of the Σ u symmetry were calculated using 29 values of the transition moments; five parameters of the πu symmetry were calculated using 27 values of the transition moments. The accuracy of the solution of the inverse problem is characterized by the quantity Q=[Σ(δµ i (theor) /δµ i (exp) )2/(n-m)]1/2, where δµ i (theor) is the deviation of calculations from the experiment, δμ (exp) i is the experimental error, n is the number of the experimental data used, and m is the number of parameters smaller than unity.

New High-Resolution Analysis of the 3ν3 and 2ν1 + ν3 Bands of Nitrogen Dioxide (NO2) by Fourier Transform Spectroscopy

Using new high-resolution Fourier transform spectra recorded at the University of Denver in the 2-μm region, a new and more extended analysis of the 2ν1 + ν3 and 3ν3 bands of nitrogen dioxide, located at 4179.9374 and 4754.2039 cm−1, respectively, has been performed. The spin–rotation energy levels were satisfactorily reproduced using a theoretical model that takes into account both the Coriolis interactions between the spin–rotation energy levels of the (201) vibrational “bright” state with those of the (220) “dark” state. The interactions between the (003) bright state with the (022) dark state were similarly treated. The spin–rotation resonances within each of the NO2 vibrational states were also taken into account. The precise vibrational energies and the rotational, spin–rotational, and coupling constants were obtained for the two dyads {(220), (201)} and {(022), (003)} of the 14N16O2 interacting states. From the experimental line intensities of the 2ν1 + ν3 and 3ν3 bands, a determination of their vibrational transition moment constants was performed. A comprehensive list of line positions and line intensities of the {2ν1 + 2ν2, 2ν1 + ν3} and the {2ν2 + 2ν3, 3ν3} interacting bands of 14N16O2 was generated. In addition, assuming the harmonic approximation and using the Hamiltonian constants derived in this work and in previous studies (A. Perrin, J.-M. Flaud, A. Goldman, C. Camy-Peyret, W. J. Lafferty, Ph. Arcas, and C. P. Rinsland, J. Quant. Spectrosc. Radiat. Transfer 60, 839–850 (1998)), we have generated synthetic spectra for the {(022), (003)}–{(040), (021), (002)} hot bands at 6.3 μm and for the {(220), (201)}–{(100), (020), (001)} hot bands at 3.5 μm, which are in good agreement with the observed spectra.

Absolute line intensities, vibrational transition moment, and self-broadening coefficients for the 3-0 band of 12C16O

Absolute line intensities, vibrational transition moment, Herman–Wallis factor and self-broadening coefficients for the 3-0 vibration-rotation band of 12C16O are determined from absorption Fourier transform spectra with about 6×10−3cm−1 unapodized resolution. The range of the CO measurements extends from P(20) at 6254cm−1 to R(20) at 6407cm−1. The fitted squared transition dipole moment |μ03|2 and Herman–Wallis factor coefficients C and D are respectively equal to (1.6727±0.0014)×10−7Debye2, and (1.204±0.005)×10−2 and (1.08±0.05)×10−4 when given with one standard deviation. With the absolute uncertainty, |μ03|2 is equal to (1.67±0.1)×10−7Debye2.

The High-Resolution FTIR Spectrum of the ν1 Band of DOBr

Fourier transform infrared spectra of the ν1 bands of DO79Br and DO81Br have been recorded with a resolution of 0.0055 cm−1 in the frequency range of 2510–2800 cm−1. A total of 1901 lines of the A/B hybrid-type bands of both isotopic species have been assigned and fitted to upper state rovibrational constants employing a Watson's S-reduced Hamiltonian up to sextic terms. The Ka ≥ 4 subband transitions were found to be perturbed and were not included in the fit. The unperturbed band centers for ν1 of DO79Br and DO81Br are 2668.79211 ± 0.00006 and 2668.78842 ± 0.00005 cm−1, respectively. The ratio of the vibrational dipole transition moments (μa:μb) was found to be 1.30 ± 0.04 for the band.

Spectroscopy of O3 trapped in rare gas matrices. II. Vibrational energies and transition moments of low-lying levels

The theoretical model described in part I is applied to calculate the vibrational energies and transition moments for low-lying levels of O3 trapped in rare gas matrices. Results are given for molecules trapped in distorted face-centered-cubic (fcc) and hexagonal-closed-packed (hcp) lattice structures. New harmonic and anharmonic constants are determined that lead to matrix dependent calculated energy levels. Changes are significant for harmonic and third-order anharmonic ones. Moreover the symmetry of the potential in which the ozone oxygen nuclei move is shown to be altered. Calculated energy levels compare well with observed ones and allow predictions of unobserved ones. The 2ν3→ν3 fluorescence observed in different rare gas matrices is confirmed. Transition moments hardly differ from one matrix to the other for 2ν3→ν3 and ν1+ν3 transitions although for the latter, it is one order-of-magnitude higher in a double than in a single substitutional site.

Spectroscopy of O3 trapped in rare gas matrices. I. Theoretical model for low-lying vibrational levels

A theoretical model is elaborated which allows methods used in gas phase to be applied to calculate the vibrational energies and transition moments for low-lying levels of O3 trapped in rare gas matrices. The model used in a previous work allowed only one mode to be handled at a time. With the new approach, an overall treatment of low-lying levels is achieved. The trapping site, a single or double substitutional one is distorted to minimize the free energy of the molecule–matrix system. The molecule is considered to be submitted to the net electric field present in the site as the result of the distortion and polarization of the matrix atoms. New harmonic and anharmonic constants that lead to matrix dependent calculated energy levels and transition moments can then be determined. Besides confirmation of two trapping sites, a single (S1) and a double (S2) substitutional site in a distorted face-centered-cubic (fcc) lattice structure, two other S1 sites in argon and krypton in a distorted hexagonal-closed-packed (hcp) lattice structure are shown to be possible. A fit within experimental uncertainty is reached between observed and calculated frequencies for fundamental bands v1, v2, and v3.