Lowest-frequency Vibrational Mode Research Articles

Observation of a dipole-bound excited state in 4-ethynylphenoxide and comparison with the quadrupole-bound excited state in the isoelectronic 4-cyanophenoxide.

Negative ions do not possess Rydberg states but can have Rydberg-like nonvalence excited states near the electron detachment threshold, including dipole-bound states (DBSs) and quadrupole-bound states (QBSs). While DBSs have been studied extensively, quadrupole-bound excited states have been more rarely observed. 4-cyanophenoxide (4CP-) was the first anion observed to possess a quadrupole-bound exited state 20cm-1 below its detachment threshold. Here, we report the observation of a DBS in the isoelectronic 4-ethynylphenoxide anion (4EP-), providing a rare opportunity to compare the behaviors of a dipole-bound and a quadrupole-bound excited state in a pair of very similar anions. Photodetachment spectroscopy (PDS) of cryogenically cooled 4EP- reveals a DBS 76cm-1 below its detachment threshold. Photoelectron spectroscopy (PES) at 266nm shows that the electronic structure of 4EP- and 4CP- is nearly identical. The observed vibrational features in both the PDS and PES, as well as autodetachment from the nonvalence excited states, are also found to be similar for both anions. However, resonant two-photon detachment (R2PD) from the bound vibrational ground state is observed to be very different for the DBS in 4EP- and the QBS in 4CP-. The R2PD spectra reveal that decays take place from both the DBS and QBS to the respective anion ground electronic states within the 5ns detachment laser pulse due to internal conversion followed by intramolecular vibrational redistribution and relaxation, but the decay mechanisms appear to be very different. In the R2PD spectrum of 4EP-, we observe strong threshold electron signals, which are due to detachment, by the second photon, of highly rotationally excited anions resulted from the decay of the DBS. On the other hand, in the R2PD spectrum of 4CP-, we observe well-resolved vibrational peaks due to the three lowest-frequency vibrational modes of 4CP-, which are populated from the decay of the QBS. The different behaviors of the R2PD spectra suggest unexpected differences between the relaxation mechanisms of the dipole-bound and quadrupole-bound excited states.

Hydrogen-bonded contact ion pair in gaseous chloroethane: a multi-reference configuration interaction with singles and doubles (MR-CISD) study including extensivity corrections

Details concerning the energetics and structure of the ion pair in gaseous chloroethane, obtained at the multi-reference configuration interaction with singles and doubles (MR-CISD) level, are given. It is formed in the third excited state (31A′), and it can be classified as a hydrogen-bonded contact ion pair, although from its total binding energy of 3.28 eV (including extensivity corrections, at the MR-CISD + Q level with the aug-cc-pVTZ basis set, and including zero-point energy corrections) only 0.14 eV is due to an underlying hydrogen bond. It is a highly polar structure, with a dipole moment of 9.56 D. As compared to previous systems for which the same type of bond has been observed, it has a much lower hydrogen-bond energy and a much larger distance between the charge centers. The three lowest frequency vibrational modes of the HBCIP correspond to intermolecular cation–anion modes. The structure here obtained brings important structural questions for HCFCs derived from chloroethane.

The millimeter-wave spectrum and astronomical search of succinonitrile and its vibrational excited states ★.

Dinitriles with a saturated hydrocarbon skeleton and a -C≡N group at each end can have large electric dipole moments. Their formation can be related to highly reactive radicals such as CH2CN, C2N or CN. Thus, these saturated dinitriles are potential candidates to be observed in the ISM. Our goal is the investigation of the rotational spectrum of one of the simplest dinitriles N≡C-CH2-CH2-C≡N, succinonitrile, whose actual rotational parameters are not precise enough to allow its detection in the ISM. In addition, the rotational spectra for its vibrational exicted states will be analyzed. The rotational spectra of succinonitrile was measured in the frequency range 72-116.5 GHz using a new broadband millimeter-wave spectrometer based on radio astronomy receivers with Fast Fourier Transform backends. The identification of the vibrational excited states of succinonitrile was supported by high-level ab initio calculations on the harmonic and anharmonic force fields. A total of 459 rotational transitions with maximum values of J and Ka quantum numbers 70 and 14, respectively, were measured for the ground vibrational state of succinonitrile. The analysis allowed us to accurately determine the rotational, quartic and sextic centrifugal distortion constants. Up to eleven vibrational excited states, resulting from the four lowest frequency vibrational modes ν 13, ν 12, ν 24 and ν 23 were identified. In addition to the four fundamental modes, we observed overtones together with some combination states. The rotational parameters for the ground state were employed to unsuccessfully search for succinonitrile in the cold and warm molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30m at 3mm and the Yebes 40m at 1.3cm and 7mm.

Analysis of Low-Frequency Vibrational Modes and Particle Rearrangements in Marginally Jammed Amorphous Solid under Quasi-Static Shear**Supported by the National Natural Science Foundation of China under Grant Nos 11272048 and 51239006, and the European Commission Marie Curie Actions under Grant No IRSES-294976.

We present the numerical simulation results of a model granular assembly formed by spherical particles with Hertzian interaction subjected to a simple shear in the athermal quasi-static limit. The stress-strain curve is shown to separate into smooth, elastic branches followed by a subsequent plastic event. Mode analysis shows that the lowest-frequency vibrational mode is more localized, and eigenvalues and participation ratios of low-frequency modes exhibit similar power-law behavior as the system approaches plastic instability, indicating that the nature of plastic events in the granular system is also a saddle node bifurcation. The analysis of projection and spatial structure shows that over 75% contributions to the non-affine displacement field at a plastic instability come from the lowest-frequency mode, and the lowest-frequency mode is strongly spatially correlated with local plastic rearrangements, inferring that the lowest-frequency mode could be used as a predictor for future plastic rearrangements in the disordered system jammed marginally.

Terahertz radiation induces non-thermal structural changes associated with Fröhlich condensation in a protein crystal.

Whether long-range quantum coherent states could exist in biological systems, and beyond low-temperature regimes where quantum physics is known to be applicable, has been the subject to debate for decades. It was proposed by Fröhlich that vibrational modes within protein molecules can order and condense into a lowest-frequency vibrational mode in a process similar to Bose-Einstein condensation, and thus that macroscopic coherence could potentially be observed in biological systems. Despite the prediction of these so-called Fröhlich condensates almost five decades ago, experimental evidence thereof has been lacking. Here, we present the first experimental observation of Fröhlich condensation in a protein structure. To that end, and to overcome the challenges associated with probing low-frequency molecular vibrations in proteins (which has hampered understanding of their role in proteins' function), we combined terahertz techniques with a highly sensitive X-ray crystallographic method to visualize low-frequency vibrational modes in the protein structure of hen-egg white lysozyme. We found that 0.4 THz electromagnetic radiation induces non-thermal changes in electron density. In particular, we observed a local increase of electron density in a long α-helix motif consistent with a subtle longitudinal compression of the helix. These observed electron density changes occur at a low absorption rate indicating that thermalization of terahertz photons happens on a micro- to milli-second time scale, which is much slower than the expected nanosecond time scale due to damping of delocalized low frequency vibrations. Our analyses show that the micro- to milli-second lifetime of the vibration can only be explained by Fröhlich condensation, a phenomenon predicted almost half a century ago, yet never experimentally confirmed.

How to predict very large and complex crystal structures

Evolutionary crystal structure prediction proved to be a powerful approach in discovering new materials. Certain limitations are encountered for systems with a large number of degrees of freedom (“large systems”) and complex energy landscapes (“complex systems”). We explore the nature of these limitations and address them with a number of newly developed tools. For large systems a major problem is the lack of diversity: any randomly produced population consists predominantly of high-energy disordered structures, offering virtually no routes toward the ordered ground state. We offer two solutions: first, modified variation operators that favor atoms with higher local order (a function we introduce here), and, second, construction of the first generation non-randomly, using pseudo-subcells with, in general, fractional atomic occupancies. This enhances order and diversity and improves energies of the structures. We introduce an additional variation operator, coordinate mutation, which applies preferentially to low-order (“badly placed”) atoms. Biasing other variation operators by local order is also found to produce improved results. One promising version of coordinate mutation, explored here, displaces atoms along the eigenvector of the lowest-frequency vibrational mode. For complex energy landscapes, the key problem is the possible existence of several energy funnels – in this situation it is possible to get trapped in one funnel (not necessarily containing the ground state). To address this problem, we develop an algorithm incorporating the ideas of abstract “distance” between structures. These new ingredients improve the performance of the evolutionary algorithm USPEX, in terms of efficiency and reliability, for large and complex systems.

Ab Initio Highly Correlated Conformational Analysis of 1,2-Difluorethane and 1,2-Dichloroethane.

Temperature-dependent conformational population calculations for anti and gauche forms of 1,2-dichloroethane and 1,2-difluorethane were carried out at a highly correlated level of theory (MP4(SDTQ) and CCSD(T)) employing good quality basis sets (6-311++G(3df,3pd) and aug-cc-pVQZ) for the determination of gas relative conformational energies, making use of the statistical thermodynamics formalism for the evaluation of the thermal energy correction at the MP2/6-311++G(3df,3pd) and MP2/aug-cc-pVTZ levels. In addition to the standard calculation of thermodynamic partition functions, a treatment of the lowest-frequency vibrational mode as hindered rotation and anharmonic correction to vibrational frequencies was also included. We found a good agreement between ab initio calculated conformational population values and experimental gas-phase electron diffraction data for the 1,2-dicloroethane. However, for the 1,2-difluorethane species, a reasonable agreement with the experimental anti/gauche population ratio obtained from the analysis of gas-phase far-infrared (50-370 cm(-1)) and low-frequency Raman (70-300 cm(-1)) spectra was not obtained. The results reported here indicate that, for 1,2-difluorethane, and probably other substituted alkanes where the gauche effect is of relevance, a more appropriated treatment of the low-frequency modes must be pursued in order to reproduce experimental conformational population data.

Multicoefficient Gaussian-3 Calculation of the Rate Constant for the OH + CH4 Reaction and Its 12C/13C Kinetic Isotope Effect with Emphasis on the Effects of Coordinate System and Torsional Treatment

Rate constants and (12)C/(13)C kinetic isotope effects are calculated by direct dynamics for the OH + CH(4) --> H(2)O + CH(3) reaction. The electronic structure calculations required to generate the implicit potential energy surface were carried out by the high-level multicoefficient Gaussian-3/version-3 (MCG3) method and compared to two other multilevel methods, MC3BB and MC3MPW, and three density functional methods, M06-2X, BB1K, and MPW1K. The rate constants and (12)C/(13)C kinetic isotope effects are shown to depend strongly on the coordinate system used to calculate the frequencies as well as on the method used to account for the torsional anharmonicity of the lowest-frequency vibrational mode of the generalized transition states.

Synthesis and characterization of the iron(III) complexes of tetra-(β,β'-tetramethylene)tetraphenylporphyrin, (TC6TPP)FeCl and (TC6TPP)FeONO2

The synthesis, NMR and EPR spectroscopic investigation as well as two crystal structures of ( TC 6 TPP ) FeX , where X - = chloride and nitrate are reported. The crystal structure of ( TC 6 TPP ) FeCl reveals an almost equal mixture of saddled and ruffled distortion of the porphyrin as judged by the coefficients of the lowest-frequency vibrational modes (calculated from Normal-Coordinate Structural Decomposition), while ( TC 6 TPP ) FeONO 2 is mainly saddled and more distorted overall. This difference in core structure indicates high conformational flexibility of the TC 6 TPP porphyrin ligand. Overall, both ( TC 6 TPP ) FeX structures have smaller deviation from planarity as compared to five coordinate ( OMTPP ) FeCl and ( OETPP ) FeCl . Therefore, the nature and number of peripheral substituents as well as the axial ligand(s) control geometry and conformation of the porphyrins and fine-tune their spectroscopic properties. EPR data (4.2 K) indicate a predominantly high-spin ( S = 5/2, 97.3%) ground state for ( TC 6 TPP ) FeCl and less pure high-spin state ( S = 5/2, 80%) for ( TC 6 TPP ) FeONO 2. The NMR results support an ideally saddled structure or rapid switching between saddled and ruffled conformations of ( TC 6 TPP ) FeX in solution. The flexibility of the porphyrin core was addressed by using dynamic NMR spectroscopy. The following kinetic parameters for ring inversion were obtained: Δ H ‡ = 24(1) kJ . mol −1, Δ S ‡ = −37(3) J . mol −1. K −1 and Δ H ‡ = 36(1) kJ . mol −1, Δ S ‡ = 20(4) J . mol −1. K −1 for ( TC 6 TPP ) FeCl and ( TC 6 TPP ) FeONO 2, respectively. This results in low free energies of activation, Δ G 298‡ = 35(2) and 30(2) kJ.mol−1, respectively, indicating extremely high flexibility of the porphyrin core in solution (kex298 > 4.2 × 106 and 3.8 × 107 s−1).

Photodissociation of Ar-H2O at 121.6 nm: high lying metastable rotational levels of Ar-OH

The total kinetic energy release spectra of H atoms from the photolysis at 121.6nm of water entrained in an argon jet show a number of peaks with widths of 100–150cm−1. The relative intensity of these peaks varies with the physical conditions, which suggests that they should be attributed to molecular clusters. Calculation of the energy levels of Ar-OH with a known potential shows that there exists a wide range of states in which the OH moiety is excited to levels of the A 2Σ+ state with high internal angular momentum, but which are remarkably stable to dissociation to Ar + OH. Indeed their stability increases with increasing rotation, even though their internal energy is many times the binding energy of the complex. The lowest frequency vibrational mode of the complex (v < 10 cm−1) is a nutation of the OH rotational axis in the potential field of the argon atom. On this basis the broad peaks in the observed spectra have been simulated as unresolved bands involving these metastable levels. The calculated lifetimes for these levels are up to a nanosecond and more, so that in principle transitions involving them may give rise to resolvable spectra. It is also concluded from a comparison of spectra with and without clustering that photodissociation of Ar-H2O leads exclusively to levels of Ar-OH(A). This is attributed to the blocking by the argon atom of the conical intersections which would be the pathways to Ar-OH(X), and which dominate in the dissociation of bare H2O to give OH(X).

Size dependence of confined acoustic phonons in CuCl nanocrystals

Persistent spectral hole burning spectroscopy was used to study the size dependence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCl. It is found that the energies of the confined acoustic phonons in the nanocrystals in glass and KCl are almost the same, but twice larger than those in NaCl, which were obtained from the Stokes-side acoustic phonon holes with the same excitation energies. The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition. However, the energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencies of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition. This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shape, and the boundary condition of the nanocrystals.

Brillouin-scattering determination of vibrational relaxation in dichloromethane

A Brillouin-scattering experiment was performed on dichloromethane to investigate the vibrational relaxation in the liquid. A comparison of the experimental results with theoretical predictions indicates that there is a single. relaxation of all but the lowest-frequency vibrational mode.

Microwave spectrum and conformation of triformylmethane

The microwave spectra of two isotopic species of triformylmethane have been assigned. A single conformation is observed in the vapour phase having a planar keto–enol structure with an intramolecular hydrogen bond. This structure is similar to malonaldehyde and its 2-substituted methyl and nitro derivatives which have recently been studied. The isotopic shift and dipole moment evidence suggest that this conformation has the HCO group on carbon 2 in a position such that CO is cis to CC. Some information on the lowest frequency vibrational modes of triformyl-methane has also been obtained.