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doi: 10.1039/b925578hpmid: 20631959
Intermolecular hydrogen bond librational modes in cyclic trimers and tetramers of methanol and t-butyl alcohol isolated at low temperature in pulsed supersonic jet expansions are observed by direct absorption spectroscopy in the far-infrared region. The large amplitude librational modes probe the strength and directionality of the intermolecular hydrogen bonds. In addition, their frequency and intensity is very sensitive to the angle which the alkyl groups form with the hydrogen bonded ring. Theoretical predictions which fail to describe the trends in cluster size, alkylation and symmetry splitting reported in this work are likely to miss important ingredients of the underlying intermolecular interaction. Analysis of the vibrational correlation diagram between planar and puckered tetramer structures circumvents some deficiencies of approximate treatments.
Winnewisser, Brenda P.; Winnewisser, Manfred; Medvedev, Ivan R.; De Lucia, Frank C.; Ross, Stephen C.; Koput, Jacek
doi: 10.1039/b922023bpmid: 20372691
Quantum monodromy has a strong impact on the ro-vibrational energy levels of chain molecules whose bending potential energy function has the form of the bottom of a champagne bottle (i.e. with a hump or punt) around the linear configuration. NCNCS, cyanogen iso-thiocyanate, is a particularly good example of such a molecule and clearly exhibits a distinctive monodromy-induced dislocation of the energy level pattern at the bending-rotation energy at the top of the potential energy hump. Indeed, NCNCS [B. P. Winnewisser et al., Phys. Rev. Lett. 2005, 95, 243002] and the water molecule [N. F. Zobov et al., Chem. Phys. Lett. 2005, 414, 193–197] were the first two molecules for which experimental confirmation of quantum monodromy was obtained. We used the fast scan sub-millimetre spectroscopic technique (FASSST) to extend the measurements and spectral analysis to pure rotational transitions (end-over-end) in bending vibrational states lying well above the monodromy point. The analysis of 9204 lines assigned to 7 vibrational states, presented here, shows that the topological properties of the bending potential function are mapped onto every aspect of the ro-vibrational energy levels involving excitation of the quasi-linear bending vibration. In order to model the large amplitude dynamics of such a molecular system, and also to achieve some insight beyond satisfactory parameters for reproducing the spectrum, we used the generalized semi-rigid bender (GSRB) Hamiltonian, which is described in some detail. This Hamiltonian provides a good description of the energy levels over the seven bending states observed, coming close to experimental accuracy. Due to high J values of the measured rotational transitions (J ≤ 116), the least squares fitting procedure was applied not directly to the measured frequencies, but to effective constants derived from fitting the transition frequencies to a set of polynomials in J(J + 1) yielding effective Beff and Deff constants. The GSRB wave functions are used to show that the expectation values of any quantity which varies with the large amplitude bending coordinate will also have monodromy-induced dislocations. This includes the electric dipole moment components. High level ab initio calculations not only provided the molecular equilibrium structure of NCNCS, but also the electric dipole moment components μa and μb as functions of the large-amplitude bending coordinate. Calculated expectation values of these quantities for individual ro-vibrational levels show the now recognizable monodromy pattern. Finally, a generalization of the quasi-linear parameter γ0 is suggested.
Kunitski, Maksim; Knippenberg, Stefan; Gelin, Maxim; Riehn, Christoph; Dreuw, Andreas; Brutschy, Bernhard
doi: 10.1039/b925388bpmid: 20390150
The ring-puckering vibration in cyclopentene was studied by rotational time-resolved femtosecond degenerate four-wave mixing (fs DFWM) spectroscopy. The fs DFWM spectra of cyclopentene were measured both in a supersonic expansion and in a gas cell at room temperature. The room temperature fs DFWM spectrum has been satisfactorily reproduced by a fitted simulation based on a one-dimensional model for the ring-puckering vibration. This has allowed for the determination of energetic parameters of the ring-puckering motion such as the energy barrier to ring inversion of 274(+12/−20) cm−1 and the equilibrium ring-puckering angle of 24.3°. The derived dependences of the rotational constants A and B on the puckering angle resemble very closely those obtained by microwave spectroscopy. In addition, previous theoretical estimates of the ring inversion barrier of cyclopentene were improved by performing high level ab initio calculations. Zero-point vibrational energy correction was found to be essential for an accurate evaluation of the puckering potential. Altogether, this study provides a proof-of-principle of the applicability of the fs DFWM technique for investigating large amplitude intramolecular motions.
Lüttschwager, Nils O. B.; Wassermann, Tobias N.; Coussan, Stéphane; Suhm, Martin A.
doi: 10.1039/c002345kpmid: 20419173
Proton tunneling between the two equivalent structures of malonaldehyde through a substantial barrier is accelerated by more than a factor of 3 to ≈ 0.24 ps by OH-bend excitation in phase with suitable motions of the molecular backbone. This is derived from a combined FTIR and Raman spectroscopy study in supersonic jets and rare gas matrices and compared to previous theoretical predictions.
Pfaffen, Chantal; Frey, Hans-Martin; Ottiger, Philipp; Leutwyler, Samuel; Bachorz, Rafał A.; Klopper, Wim
doi: 10.1039/c002056gpmid: 20485771
The ground-state N–H⋯π interaction of 2-pyridone·benzene (2PY·Bz) has been studied by infrared-UV depletion spectroscopy of the supersonic-jet cooled complex [P. Ottiger et al., J. Phys. Chem. B (2009) 113, 2937]. Here, we investigate the large-amplitude vibrations of 2PY·Bz and its d1-2PY and benzene-d6 isotopologues in the S1 state, using two-color resonant two-photon ionization and UV-holeburning spectroscopies, complemented by RI-CC2 and SCS-RI-CC2 calculations of the S1 state. The latter predict a tilted T-shaped structure with an N–H⋯π hydrogen bond to the benzene ring, similar to the S0 state. The binding energy is predicted to increase by 1.5 kJ mol−1 upon S1 ← S0 excitation, in close agreement with the experimental value of 1.2 kJ mol−1. The vibronic band structure up to 60 cm−1 above the 000 band is dominated by large-amplitude δ tilting excitations, reflecting a change in the tilt angle of the T-shaped complex. The S0 and S1 state δ potentials were fitted to experiment, yielding a single minimum in the S0 state and a double-minimum S1 potential with δmin = ±13 degrees. The second large-amplitude vibration is the θ twisting or benzene internal-rotation mode. Due to the C6 symmetry of the benzene moiety the S0 and S1 state θ potentials are sixfold symmetric. Analysis of the θ band structure reveals that the S0 and S1θ potentials are mutually aligned and that the internal rotation barriers are V6(S0) < 0.2 kJ mol−1 and V6(S1) = 0.10(1) kJ mol−1, in close agreement with the calculations. Weaker excitations of the totally symmetric intermolecular vibrations χ (shear), ω (bend) and σ (stretch) vibrations are also observed. The 2PY intramolecular ν1 overtone, corresponding to an 2PY amide out-of-plane twist distortion, lies ∼30% higher than in bare 2PY, reflecting the hindrance of this motion by the strong N–H⋯π interaction.
van der Avoird, Ad; Podeszwa, Rafał; Szalewicz, Krzysztof; Leforestier, Claude; van Harrevelt, Rob; Bunker, P. R.; Schnell, Melanie; von Helden, Gert; Meijer, Gerard
doi: 10.1039/c002653kpmid: 20485846
An improved intermolecular potential surface for the benzene dimer is constructed from interaction energies computed by symmetry-adapted perturbation theory, SAPT(DFT), with the inclusion of third-order contributions. Twelve characteristic points on the surface have been investigated also using the coupled-cluster method with single, double, and perturbative triple excitations, CCSD(T), and triple-zeta quality basis sets with midbond functions. The SAPT and CCSD(T) results are in close agreement and provide the best representation of these points to date. The potential was used in calculations of vibration–rotation-tunneling (VRT) levels of the dimer by a method appropriate for large amplitude intermolecular motions and tunneling between multiple equivalent minima in the potential. The resulting VRT levels were analyzed with the use of the permutation-inversion full cluster tunneling (FCT) group G576 and a chain of subgroups that starts from the molecular symmetry group Cs(M) of the rigid dimer at its equilibrium Cs geometry and leads to G576 if all possible intermolecular tunneling mechanisms are feasible. Further information was extracted from the calculated wave functions. It was found, in agreement with the experimental data, that for all of the 54 G576 symmetry species (with different nuclear spin statistical weights) the lower VRT states have a tilted T-shape (TT) structure; states with the parallel-displaced structure are higher in energy than the ground state of A+1 symmetry by at least 30 cm−1. The dissociation energy D0 equals 870 cm−1, while the depth De of the TT minimum in the potential is 975 cm−1. Hindered rotation of the cap in the TT structure and tilt tunneling lead to level splittings on the order of 1 cm−1. Also intermolecular vibrations with excitation energies starting at a few cm−1 were identified. A further small, but probably significant, level splitting was assigned to cap turnover, although in scans of the potential surface we could not find a plausible ‘reaction path’ for this process. Rotational constants were extracted from energy levels calculated for total angular momentum J = 0 and 1, and from expectation values of the inertia tensor. Although the end-over-end rotational constant B + C agrees well with the measured microwave spectra, there is disagreement with the measurements concerning the (a)symmetric rotor character of the benzene dimer. It is concluded from calculations for the 54 nuclear spin species that the microwave spectrum should show overlapping contributions from many different species. Another interesting conclusion regards the role of the quantum number K, for a prolate near-symmetric rotor the projection of the total angular momentum on the prolate axis. For the benzene dimer, K has a substantial effect on the energy levels associated with the intermolecular motions of the complex.
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