High resolution ro-vibrational analysis of molecules in doublet electronic states: the ν1 fundamental of chlorine dioxide (16O35Cl16O) in the X2B1 electronic ground state.

We report the spectrum of the ν1 fundamental of chlorine dioxide centered in the infrared atmospheric window at 945.592 cm-1 measured with essentially Doppler limited resolution at an instrumental line width of 0.001 cm-1 using the Zürich prototype ZP2001 Bruker IFS 125 HR Fourier transform infrared spectrometer. The ro-vibrational line analysis is carried out with an improved effective Hamiltonian and a newly developed computer code ROVDES for the ro-vibrational spectra of open-shell free radical molecules including spin-rotation interactions. Accurate values of rotational, centrifugal and spin-rotational parameters were determined for 16O35Cl16O in the vibronic ground state X2B1 from more than 3500 ground state combination differences. The 7239 assigned transitions for the ν1 fundamental with Nmax = 76 and Kmaxa = 26 provide a set of 32 accurate effective Hamiltonian parameters for the ν1 fundamental (v1v2v3) = (100) (21 rotational and centrifugal distortion parameters and 11 spin-rotational interaction parameters). This effective Hamiltonian (A - reduction and Ir - representation) reproduces 1703 upper state energies from the experiment with a root-mean-square deviation drms = 1.67 × 10-4 cm-1 and the 7239 transition wavenumbers with drms = 3.45 × 10-4 cm-1. Our results provide a considerable improvement over previous results with which we compare and should provide a benchmark for theoretical studies with applications to atmospheric spectroscopy and laser chemistry, which are discussed in relation to our spectra.

The Gigahertz and Terahertz spectrum of monodeutero-oxirane (c-C2H3DO).

The rotational spectrum of monodeutero-oxirane was analysed as measured using the Zurich Gigahertz (GHz) spectrometer and our highest resolution Fourier Transform Infrared (FTIR) spectrometer system coupled to synchrotron radiation at the Swiss Light Source (SLS). 112 distinct line frequencies have been newly assigned in the GHz range (extended to 120 GHz, compared to previous work extending to only 59 GHz) including rotational states up to J = 23. We have furthermore assigned 398 lines in the far infrared or Terahertz range (0.75-2.10 THz or 25-70 cm-1) including transitions with rotational quantum numbers up to J = 59. The results are discussed in relation to the possible first astrophysical observation of an isotopically chiral molecule and in relation to molecular parity violation.

Nuclear Spin Symmetry Conservation Studied for Symmetric Top Molecules (CH3D, CHD3, CH3F, and CH3Cl) in Supersonic Jet Expansions.

We report results on nuclear spin symmetry conservation studied by high resolution spectroscopy of relative line intensities for the A and E nuclear spin isomers of symmetric top molecules CHD3, CH3D, CH3F, and CH335Cl in supersonic jet expansions with He and Ar as carrier gases. Infrared absorption spectra were measured around 3000 cm-1 by an infrared (lead salt) diode laser and a continuous wave IR-OPO (infrared optical parametric oscillator) locked to a frequency comb. A detailed analysis of the R(2)-line intensities of the CH-stretching fundamental shows that nuclear spin symmetry is conserved for CHD3, CH3F, and CH335Cl during the expansion. For CH3D, a small contribution from nuclear spin symmetry relaxation cannot be excluded completely under our experimental conditions.

Controlling tunneling in ammonia isotopomers.

We report results of full-dimensional variational rovibrational quantum-dynamical computations for several ammonia isotopomers, based on selected potential energy and electric dipole moment hypersurfaces. The variational rovibrational eigenstates have been used as a basis for the solution of the time-dependent Schrödinger equation for nuclear motion including coherent infrared multiphoton excitation. The theoretical and computational framework developed during this study enables the investigation of the coherent inhibition or enhancement of tunneling in ammonia isotopomers by appropriately chosen laser fields. Our quantum-dynamical computations include all vibrational and rotational degrees of freedom and assume neither the alignment nor the orientation of the molecules under investigation. Specific results include accurate rotational-vibrational levels for NH2D, NHD2, NHDMu, and NHDT, probability densities for structural parameters as a function of time from the full-dimensional wavepacket results, time-dependent chirality for the isotopically chiral molecule NHDT, and detailed analyses of the enhancement and inhibition of stereomutation dynamics.

A molecular quantum switch based on tunneling in meta-d-phenol C6H4DOH.

We introduce the concept of a molecular quantum switch and demonstrate it with the example of meta-d-phenol, based on recent theoretical and high-resolution spectroscopic results for this molecule. We show that in the regime of tunneling switching with localized low-energy states and delocalized high-energy states the molecular quantum switch can be operated in two different ways: (i) a quasiclassical switching by coherent infrared radiation between the two isomeric structures syn- and anti-m-d-phenol; and (ii) a highly nonclassical switching making use of bistructural quantum superposition states of the syn and anti structures, which can be observed by their time-dependent spectra after preparation.

Intramolecular vibrational energy redistribution in HCCCH2X (X = Cl, Br, I) measured by femtosecond pump-probe experiments in a hollow waveguide.

From the analysis of high resolution overtone spectra it is well established that intramolecular vibrational energy redistribution (IVR) from an initially excited CH-stretching vibration is strongly influenced by its chemical environment. Due to a pronounced Fermi resonance between the CH-stretching and CH-bending vibrations a vibrational energy redistribution on the subpicosecond time scale (∼100 fs) is found for alkyl (sp3) CH-chromophores, whereas this doorway for energy flow is blocked for the acetylenic (sp) CH-stretching vibration because of the much lower CH-bending frequency. From the analysis of the high resolution spectra lifetimes for the initial CH-vibrational excitation of 10-100 ps or longer have been derived. In the present work we have investigated the IVR process for HCCCH2Br, HCCCH2Cl, and HCCCH2I after excitation of the first overtone of the CH-stretching vibration of the CH2X- and the CCH-group by time resolved femtosecond pump-probe experiments in a hollow waveguide. For HCCCH2Br and HCCCH2Cl a clearly different IVR behavior was found for the two different chemical environments. For the excitation of the alkyl CH-chromophore very fast initial relaxation times were found together with a slower relaxation process with τ2 = 15-40 ps, whereas for the acetylenic CH-stretching vibration a relaxation time τ3 = 70-200 ps has been determined. For HCCCH2I also for the excitation of the CCH-group a relatively fast relaxation process with a time constant τ2 = 6 ps could be identified which might result from a not yet identified strong vibrational coupling between the excited first overtone of the acetylenic CH-stretching vibrations with a combination state including the CI-stretching vibration.

Isotope effects on the resonance interactions and vibrational quantum dynamics of fluoroform 12,13CHF3.

We report a comparison of the analysis of the low energy spectrum of 13CHF3 and 12CHF3 from the THz (FIR) range to the ν1 fundamental at high resolution (δ[small nu, Greek, tilde] < 0.001 cm-1 or otherwise Doppler limited) on the basis of FTIR spectra taken both with ordinary light sources and with the synchrotron radiation from the Swiss Light Source. Several vibrational levels are accurately determined including, in particular, the 2ν4 CH-bending overtone and the ν1 CH-stretching fundamental of 13CHF3. Comparison of experimental results with those from accurate full dimensional vibrational calculations allows for a study of the time-dependent quantum dynamics of intramolecular vibrational redistribution (IVR) in the CH chromophore both on short time scales (fs) and longer time scales (ps) when coupling to the lower frequency modes becomes important and where the 12C/13C isotope effects are very large.

On the use of nonrigid-molecular symmetry in nuclear motion computations employing a discrete variable representation: A case study of the bending energy levels of CH5.

A discrete-variable-representation-based symmetry adaptation algorithm is presented and implemented in the fourth-age quantum-chemical rotational-vibrational code GENIUSH. The utility of the symmetry-adapted version of GENIUSH is demonstrated by the computation of seven-dimensional bend-only vibrational and rovibrational eigenstates of the highly fluxionally symmetric CH5+ molecular ion, a prototypical astructural system. While the numerical results obtained and the symmetry labels of the computed rovibrational states of CH5+ are of considerable utility by themselves, it must also be noted that the present study confirms that the nearly unconstrained motion of the five hydrogen atoms orbiting around the central carbon atom results in highly complex rotational-vibrational quantum dynamics and renders the understanding of the high-resolution spectra of CH5+ extremely challenging.

Tunneling and Parity Violation in Trisulfane (HSSSH): An Almost Ideal Molecule for Detecting Parity Violation in Chiral Molecules.

Measuring the parity-violating energy difference Δpv E between the enantiomers of chiral molecules is a major challenge of current physical-chemical stereochemistry. An important step towards this goal is to identify suitable molecules for such experiments by means of theory. This step has been made by calculations for the complex dynamics of tunneling and electroweak quantum chemistry of parity violation in the "classic" molecule trisulfane, HSSSH, which satisfies the relevant conditions for experiments almost ideally, as the molecule is comparatively simple and parity violation clearly dominates over tunneling in the ground state. At the same time, the barrier for stereomutation is easily overcome by the S-H infrared chromophore.

Wavepacket Dynamics of the Axially Chiral Molecule Cl-O-O-Cl under Coherent Radiative Excitation and Including Electroweak Parity Violation.

We report detailed calculations of the quantum wavepacket dynamics of Cl-O-O-Cl, which serves as a prototype molecule for the stereomutation dynamics of an axially chiral molecule. We include the effects both from electroweak parity violation and from the interaction with a coherent monochromatic laser field. We use the quasiadiabatic channel reaction path Hamiltonian approach to approximately solve the six-dimensional Schrödinger equation describing the vibrational motion, including rotation by an effective Hamiltonian. We calculate time-dependent wave functions based on the time-dependent Schrödinger equation. We study stereomutation dynamics due to tunneling motion and laser-induced population transfer and show results on efficient methods for selectively populating single molecular states in chiral molecules by frequency-modulated laser pulses. We also discuss laser-induced stereomutation (LISM) and a process that may be called resonance Raman induced stereomutation (RRISM). The results are discussed in relation to current experimental attempts to measure the parity violating energy difference ΔpvE between the enantiomers of chiral molecules. Furthermore, we show detailed quantitative simulations of a selection of well-defined parity levels in chiral molecules ("parity isomers") that form the basis of a possible measurement of ΔpvE by the time evolution of parity.

Line shape of amplitude or frequency-modulated spectral profiles including resonator distortions.

We report experiments and an improved method of analysis for any harmonics of frequency-modulated spectral line shapes allowing for very precise determinations of the resonance frequency of single absorption lines for gigahertz spectroscopy in the gas phase. Resonator perturbations are implemented into the formalism of modulation spectroscopy by means of a full complex transmission function being able to model the asymmetrically distorted absorption line shapes for arbitrary modulation depths, modulation frequencies, and resonator reflectivities. Exact equations of the in-phase and the quadrature modulation signal, taking into account a full resonator transmission function, are simultaneously adjusted to two-channel lock-in measurements performed in the gigahertz regime to obtain the spectral line position. The determination of the absorption line position of the rotational transition J' = 7 ← J" = 6 of (16)O(12)C(32)S in the vibrational ground state is investigated while changing the resonator distortions. The results are subjected to the approach proposed here and compared to standard methods known from the literature.

Synchrotron-based rotationally resolved high-resolution FTIR spectroscopy of azulene and the unidentified infrared bands of astronomy.

Chasing the unidentified IR bands: The first rotationally resolved high-resolution infrared spectrum of azulene is reported using synchrotron Fourier transform infrared spectroscopy including a rovibrational analysis of the out-of-plane fundamental ν44. Comparison of azulene, naphthalene, indole, and biphenyl infrared bands leads to coincidences with UIR bands at 12.8 µm with naphthalene and at 13.55 and 14.6 µm with biphenyl bands, but excluding azulene as a strong absorber.

Nuclear spin symmetry conservation and relaxation in water ((1)H2(16)O) studied by cavity ring-down (CRD) spectroscopy of supersonic jets.

We report high resolution near-infrared laser spectra of water seeded in a supersonic jet expansion of argon probed by cavity ring-down spectroscopy (CRDS) in the R branch of the 2ν3 band (above 7500 cm(-1)) at several effective temperatures T < 30 K. Our goal is to study nuclear spin symmetry conservation and relaxation. For low mole fractions of water in the gas mixture, we obtained the lowest rotational temperatures and observed nuclear spin symmetry conservation, in agreement with theoretical expectation for inelastic collisions of isolated H2O molecules with Ar and similar to a previous series of experiments with other small molecules in supersonic jet expansions. However, for the highest mole fractions of water, which we used (xH2O < 1.6%), we obtained slightly higher rotational temperatures and observed nuclear spin symmetry relaxation, which cannot be explained by the intramolecular quantum relaxation mechanism in the monomer H2O. The nuclear spin symmetry relaxation observed is, indeed, seen to be related to the formation of water clusters at the early stage of the supersonic jet expansion. Under these conditions, two mechanisms can contribute to nuclear spin symmetry relaxation. The results are discussed in relation to claims of the stability of nuclear spin isomers of H2O in the condensed phase and briefly also to astrophysical spectroscopy.

Global analytical potential energy surface for the electronic ground state of NH3 from high level ab initio calculations.

The analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state developed previously (Marquardt, R.; et al. J. Phys. Chem. B 2005, 109, 8439­8451) is improved by adjustment of parameters to an enlarged set of electronic energies from ab initio calculations using the coupled cluster method with single and double substitutions and a perturbative treatment of connected triple excitations (CCSD(T)) and the method of multireference configuration interaction (MRCI). CCSD(T) data were obtained from an extrapolation of aug-cc-pVXZ results to the basis set limit (CBS), as described in a previous work (Yurchenko, S.N.; et al. J. Chem. Phys 2005, 123, 134308); they cover the region around the NH3 equilibrium structures up to 20,000 hc cm(­1). MRCI energies were computed using the aug-cc-pVQZ basis to describe both low lying singlet dissociation channels. Adjustment was performed simultaneously to energies obtained from the different ab initio methods using a merging strategy that includes 10,000 geometries at the CCSD(T) level and 500 geometries at the MRCI level. Characteristic features of this improved representation are NH3 equilibrium geometry r(eq)(NH(3)) ≈ 101.28 pm, α(eq)(NH(3)) ≈ 107.03°, the inversion barrier at r(inv)(NH(3)) ≈ 99.88 pm and 1774 hc cm(­1) above the NH(3) minimum, and dissociation channel energies 41,051 hc cm(­1) (for NH(3) → ((2)B(2))NH(2) + ((2)S(1/2))H) and 38,450 hc cm(­1) (for NH(3) → ((3)Σ(­))NH +((1)Σ(g)(+))H(2)); the average agreement between calculated and experimental vibrational line positions is 11 cm(­1) for (14)N(1)H(3) in the spectral region up to 5000 cm(­1). A survey of our current knowledge on the vibrational spectroscopy of ammonia and its isotopomers is also given.

Tunneling and tunneling switching dynamics in phenol and its isotopomers from high-resolution FTIR spectroscopy with synchrotron radiation.

Tunneling and chemical reactions by tunneling switching are reported for phenol and ortho-deuterophenol on the basis of high-resolution FTIR spectroscopy. Tunneling splittings are measured for the torsional motion in the ground and several vibrationally excited states of phenol. Tunneling times range from 10 ns to 1 ps, depending on excitation. For more-highly excited torsional levels in ortho-deuterophenol, delocalization and chemical reaction by tunneling switching is found.

Perspectives on parity violation in chiral molecules: theory, spectroscopic experiment and biomolecular homochirality.

The reflection (or 'mirror') symmetry of space is among the fundamental symmetries of physics. It is connected to the conservation law for the quantum number parity and a fundamental 'non-observable' property of space (as defined by an absolute 'left-handed' or 'right-handed' coordinate system). The discovery of the violation of this symmetry - the non-conservation of parity or 'parity violation' - in 1956/1957 had an important influence on the further development of physics. In chemistry the mirror symmetry of space is connected to the existence of enantiomers as isomers of chiral ('handed') molecules. These isomers would relate to each other as idealized left or right hand or as image and mirror image and would be energetically exactly equivalent with perfect space inversion symmetry. Parity violation results in an extremely small 'parity violating' energy difference between the ground states of the enantiomers which can be theoretically calculated to be about 100 aeV to 1 feV (equivalent to 10-11 to 10-10 J mol-1), depending on the molecule, but which has not yet been detected experimentally. Its detection remains one of the great challenges of current physical-chemical stereochemistry, with implications also for fundamental problems in physics. In biochemistry and molecular biology one finds a related fundamental question unanswered for more than 100 years: the evolution of 'homochirality', which is the practically exclusive preference of one chiral, enantiomeric form as building blocks in the biopolymers of all known forms of life (the l-amino acids in proteins and d-sugars in DNA, not the reverse d-amino acids or l-sugars). In astrobiology the spectroscopic detection of homochirality could be used as strong evidence for the existence of extraterrestrial life, if any. After a brief conceptual and historical introduction we review the development, current status, and progress along these three lines of research: theory, spectroscopic experiment and the outlook towards an understanding of the evolution of biomolecular homochirality.

Synchrotron-Based Highest Resolution Terahertz Spectroscopy of the ν24 Band System of 1,2-Dithiine (C4H4S2): A Candidate for Measuring the Parity Violating Energy Difference between Enantiomers of Chiral Molecules.

The chiral C2 symmetric molecule 1,2-dithiine (1,2-dithia-3,5-hexadiene, C4H4S2) has been identified as a possible candidate for measuring the parity violating energy difference between enantiomers. We report here the observation and analysis of the low-frequency fundamental ν24 using highest resolution synchrotron-based interferometric Fourier transform infrared (FTIR) spectroscopy in the terahertz range with a band center of ν0 = 6.95375559 THz (ν̃0 = 231.952319 (10) cm-1) and two related hot bands, the (ν13 + ν24) ← ν13 band at ν0 = 6.97256882 THz (ν̃0 = 232.579861 (33) cm-1) and the 2ν24 ← ν24 band at ν0 = 7.01400434 THz (ν̃0 = 233.962001 (14) cm-1). This success in the difficult analyses of the THz spectrum of a complex chiral molecule of importance for fundamental tests on molecular parity violation is enabled by the ideal setup of an FTIR experiment of currently unique resolution with the very stable and bright synchrotron radiation at the Swiss Light Source (SLS).

Global analytical potential energy surface for large amplitude nuclear motions in ammonia.

An analytical, full-dimensional, and global representation of the potential energy surface of NH(3) in the lowest adiabatic electronic state is developed, and parameters are determined by adjustment to ab initio data and thermochemical data for several low-lying dissociation channels. The electronic structure is calculated at the CASPT2 level within an [8,7] active space. The representation is compared to other recently published potential energy surfaces for this molecule. The present representation is distinguished by giving a qualitatively correct description of the potential energy for very large amplitude displacements of the nuclei from equilibrium. Other characteristic features of the present surface are the equilibrium geometries r(eq)(NH(3)) approximately 101.24 pm, r(eq)(NH(2)) approximately 102.60 pm, alpha(eq)(NH(3)) approximately 106.67 degrees, and the inversion barrier at r(inv)(NH(3)) approximately 99.80 pm and 1781 cm(-1) above the NH(3) minimum. The barrier to linearity in NH(2) is 11,914 cm(-1) above the NH(2)((2)B(1)) minimum. While the quartic force field for NH(3) from the present representation is significantly different from that of the other potential energy surfaces, the vibrational structures obtained from perturbation theory are quite similar for all representations studied here.