Effects of vibrational relaxation on the optical lineshapes in molecular spectra

We derive general expressions for the time dependent optical lineshape of a molecule undergoing vibrational relaxation in a dense medium.     

The vibrational spectra of crystalline pyruvoyl chloride

The complete vibrational spectra of crystalline pyruvoyl chloride have been recorded and analyzed. This compound is an unstable liquid at room temperature. It was synthesized by a known procedure and all sample handling and data gathering were carried out at low temperatures. As an aid to the assignment, pyruvoyl bromide, also an unstable liquid at room temperature, was synthesized and its low temperature i.r. spectra was recorded. A tentative vibrational assignment is proposed and this assignment and the structure of pyruvoyl chloride are compared to those of other molecules of the pyruvate class.     

CH stretching vibrational overtone spectra of tert-butylbenzene, tert-butyl chloride, and tert-butyl iodide

The vapor phase CH stretching vibrational overtone spectra of tert-butylbenzene and tert-butyl chloride are measured in the Delta upsilon(CH) = 2-7 region, while the spectrum of tert-butyl iodide is recorded in the Delta upsilon(CH) = 2-6 region. The overtone spectrum of tert-butylbenzene is too complex to make detailed spectral assignments. Local mode frequencies, omega, and anharmonicities, omegax, are obtained for tert-butyl chloride and tert-butyl iodide. The torsional dependencies of the local mode frequency, delta(omega), and anharmonicity, delta(omega)(x), are calculated for the tert-butyl halides. Nonbonded, through-space intramolecular interactions are observed in the blue-shifting of sterically hindered CH oscillators. Scaling factors are presented for relating ab initio calculated local mode parameters to experimental values for alkyl CH oscillators. Fermi resonances are observed between local mode states and local mode/normal mode combination states in tert-butyl chloride and tert-butyl iodide. Vibrational overtone transition intensities are calculated in the range Delta upsilon(CH) = 3-9 using the harmonically coupled anharmonic oscillator (HCAO) model and ab initio dipole moment functions. The resultant HCAO intensities are compared to experimental intensities at Delta upsilon(CH) = 3.     

Cation migration upon adsorption of methanol in NaY and NaX faujasite systems: a molecular dynamics approach

Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.     

Effects of rapid intramolecular vibrational redistribution on molecular spectra at microwave frequencies

Some molecules with more than 10 atoms and more than two torsional degrees of freedom have state densities sufficient for rapid (10¹⁰ s^?1) intramolecular vibrational redistribution at energies as low as 0.25 kcal/mol. Predicted features of low-resolution microwave (LRMW) band spectra of rapidly relaxing polar prolate molecules are discussed and compared with LRMW spectra of ethyl esters.     

Analysis of the vibrational spectra of chiral liquid crystalline thioesters

Vibrational spectra of (S)-(+)-4-(1-methylheptyloxy)biphenyl 4-pentylphenylthiobenzoate (MHOBS5) were calculated using the density functional theory method, with two different basis sets, the 6-31G* and the 6-31+G*. The force fields were scaled using the scaled quantum-mechanical force field (SQM) procedure. Various sets of scaling factors for the SQM procedure are discussed. Results of simulations were used to analyse the experimental data obtained from infrared and Raman experiments for MHOBS5 and the homologous MHOBS4.     

Raman and infrared spectra of crystalline fluoroform

Raman and far-infrared spectra of crystalline fluoroform at temperatures between 20 and 106 K have been recorded. There is no evidence for any solid-state phase transitions, nor for hydrogen bonding. The rich lattice spectra and crystal field splittings suggest that the unit cell is rather large, and possible structures are discussed.     

Raman and infrared spectra of crystalline chloroform

Raman and far-infrared spectra of polycrystalline samples of chloroform at 20 and 80 K are reported. Crystal field spilttings of the intramolecular fundamentals are observed as well as nine Raman and six infrared lattice modes. Spectra are interpreted in terms of a group theoretical analysis based on the molecular and crystal symmetries.     

Effect of deuteration on the infrared spectra of crystalline metal formates

The laser Raman and IR spectra of tribromoacetylchloride have been recorded and vibrational assignments are proposed.     

Calculating molecular vibrational spectra beyond the harmonic approximation

We present a new approach for calculating anharmonic corrections to vibrational frequency calculations. The vibrational wavefunction is modelled using translated Hermite functions thus allowing anharmonic effects to be incorporated directly into the wavefunction whilst still retaining the simplicity of the Hermite basis. We combine this new method with an optimised finite-difference grid for computing the necessary third and fourth nuclear derivatives of the energy. We compare our combined approach to existing anharmonic models—vibrational self-consistent field theory (VSCF), vibrational perturbation theory (VPT), and vibrational configuration interaction theory (VCI)—and find that it is more cost effective than these alternatives. This makes our method well-suited to computing anharmonic corrections for frequencies in medium-sized molecules. Contribution of the Mark S. Gordon 65th Birthday Festschrift Issue.     

The laser Raman and infrared spectra of crystalline dibenzyl

The Raman spectrum of crystalline dibenzyl has been excited with a 50-mw He-Ne laser (6328 Å) and thirty one Raman bands have been recorded. The far infrared spec trum which reveals six absorptions in the 10–270 cm⁻¹ region and the infrared spectrum from 300–3200 cm⁻¹ have been observed, and correlations with the Raman frequencies have been established. Assignments of some of the characteristic frequencies have been made, and the implications of the data regarding the geometry of the molecule are discussed.     

Systematic computer-aided interpretation of infrared and raman vibrational spectra based on the crise program

The CRISE computer program is used to correlate wavenumber regions and 6 structural elements containing carbon, hydrogen and oxygen on the basis of 2 standard files with 549 infrared and Raman spectra. The degree of correlation, including score percentages and interfering percentages, is established for different types of intervals in relation to various intensity thresholds. Specific regions (score 100%, interference 0%) proved to be rare, whereas pseudo-specific regions (score < 100%, interference 0%) are normally present. The usefulness of selective regions (score 100%, interference > 0%) is doubtful. The infrared and Raman results for a structural element can differ appreciably, yet neither technique is clearly superior for interpretative purposes.     

Far infrared spectra of solid molecular nitrogen

This paper reports the observation of the far infrared absorption spectrum of a single crystal of N₂, measured over absorption paths of 4 cm (lengthwise) and 2 cm (across the crystal). The crystal chamber, with quartz windows, was immersed in a flow of cold helium gas. The spectrum from 20 to 120 cm^–1 was recorded in the liquid phase, the-phase, and over the full temperature range of the-phase (35.6–2.0 K) with a Fourier transform spectrometer. The spectral resolution, which was not instrument-limited, and the large path allowed the observation of more detailed multiphonon-transition structure in the spectrum of the-phase than has previously been observed.     

Theoretical study on infrared vibrational spectra of boric-acid in gas-phase using density functional methods

Density functional theory (DFT) methods with various exchange-correlation functionals such as SVWN, BVWN, BVWN5, BLYP, B1LYP, B3LYP, B3PW91, and BH and H are employed in a theoretical study of molecular boric-acid in gas-phase. In the calculations, the split valence 6-311++G** and 6-31G* basis sets were used. The geometry, zero-point vibrational energies (ZPVEs), and harmonic infrared vibrational (IR) frequencies are predicted. The calculated C_3h-symmetry geometrical parameters are compared with Hartree–Fock (HF) calculation results and experimental data. IR frequencies predicted by the BLYP, B3LYP, and B3PW91 calculations are in good agreement with experimental data. The frequency calculations presented here also suggest that the C_3h-symmetrical structure corresponds to a minimum in the potential energy surface, but neither is D_3h- or C₃-symmetrical structure.     

The polarized infrared and Raman spectra of crystalline ammonium trifluoromethanesulphonate

The infrared and Raman spectra of NH₄CF₃SO₃ were obtained and analysed and an assignment was proposed for the observed bands. The assignment of some anion bands was supported by means of the polarized spectra of monocrystals. The fundamental frequencies of the anion were used for a normal coordinate calculation.     

Size effects in the infrared spectra of NH3 ice nanoparticles studied by a combined molecular dynamics and vibrational exciton approach

Infrared extinction spectra of ammonia ice nanoparticles with radii between 2 and 10 nm show pronounced band shape variations depending on the conditions of particle formation by collisional cooling. We present experimental and theoretical evidence showing that the variations in the region of the nu2 (umbrella) fundamental are due to changes in the particle size. The effect is analyzed in terms of an explicit atomistic model of the particles' structure and vibrational dynamics. An explicit potential function combined with a novel extension of the vibrational exciton approach allows us to simulate extinction spectra for particles containing up to 16,000 atoms. It is shown that the particles formed under the conditions of our experiments consist of a crystalline core surrounded by an amorphous shell with an approximately constant thickness of 1-2 nm. For the nu2 fundamental, this shell gives rise to a broad band [full width at half maximum (FWHM) 72 cm(-1)] blueshifted by about 19 cm(-1) relative to a narrow peak (FWHM of 19 cm(-1)) which arises from the crystalline core.     

Effects of molecular siting and adsorbent heterogeneity on the ideality of adsorption equilibria

The ideal adsorbed solution (IAS) theory is the benchmark for the prediction of mixed-gas adsorption equilibria from pure-component isotherms. In this work, we use atomistic grand canonical Monte Carlo simulations to test the effects of molecular siting and adsorbent energetic heterogeneity on the applicability of the IAS theory. Pure-component isotherms generated by atomistic simulation are used to predict binary isobaric isotherms using the IAS theory. These predicted isotherms are compared with those obtained by a full atomistic simulation of the binary mixture. Binary mixtures of argon, methane, and CF4 in silicalite are found to obey IAS theory, while benzene/methane and cyclohexane/methane in silicalite are nonideal. The mixture of argon and CF4 is ideal despite the large difference in the sizes of the two species. This contradicts previous hypotheses in the literature, which state that mixtures of species of unequal size do not adsorb ideally. The nonideal behavior of the benzene/methane and cyclohexane/methane systems occurs because of adsorbent heterogeneity in these systems, which depends on both sorbent and sorbate. In addition, we use a lattice gas model with parameters derived from atomistic simulation to demonstrate analytically that a sufficiently energetically heterogeneous adsorbent will result in the breakdown of IAS theory even in the absence of interactions between sorbates.     

Effects of complex formation on vibrational circular dichroism spectra

The determination of absolute configurations of chiral compounds using VCD is performed by comparing measured vibrational circular dichroism (VCD) spectra with calculated spectra. The process is based on two facts: the two enantiomers have rotational strengths of opposite sign, and the absolute configuration of the molecule used in the calculation is known. However, calculations on isolated molecules very often predict VCD intensities of very different magnitude or even different signs compared to the spectra measured in solution. Therefore, we have carefully analyzed what type of changes are induced by complexation of a solvent molecule to a solute. In the theoretical example of benzoyl-benzoic acid (in a particular chiral conformation) hydrogen bonded to the achiral NH3, we distinguish six cases, ranging from no or very small changes in the rotational strengths of solute modes (case A) to changes of sign of rotational strengths (case B), changes in magnitude (case C), nonzero rotational strengths for modes of the achiral solvent ("transfer of chirality", case D), large frequency shifts accompanied by giant enhancements of the IR and VCD intensities of modes involved in hydrogen bonding (case E), and emergence of new peaks (case F). In this work, all of these situations will be discussed and their origin will be elucidated. On the basis of our analysis, we advocate that codes for VCD rotational strength calculation should output for each mode i the angle xi(i) between the electric and magnetic transition dipole moments because only "robust modes" with xi far from 90 degrees should be used for the determination of the absolute configuration.