The radical, already detected in the interstellar medium, is also important for nitrile chemistry in Titan's atmosphere.Guélin and Cernicharo, A&A  244 (1991) L21; Loison et al., Icarus 247 (2015) 218uite recently the photoionization spectrum of the radical has been recordedGarcia, Krüger, Gans, Falvo, Coudert, and Loison, J. Chem. Phys. (2017) submittedsing mass selected threshold photoelectron (TPE) spectroscopy and this provided us with the first spectroscopic information about the cation. Modeling such a spectrum requires accounting for the non-rigidity of and for the Renner-Teller effect in . In its ³A" electronic ground state, HCCN is a non-rigid molecule as the potential for the ∠\ceHCC bending angle is very shallow.Koput, J. Phys. Chem. A 106 (2002) 6183ibronic couplings with the same bending angle leads, in the ²Π electronic ground state of , to a strong Renner-Teller effect giving rise to a bent ²A′ and a quasi-linear ²A" state.Zhao, Zhang, and Sun, J. Phys. Chem. A 112 (2008) 12125n this paper the photoionization spectrum of the HCCN radical is simulated. The model developped treats the ∠\ceHCC bending angle as a large amplitude coordinate in both the radical and the cation and accounts for the overall rotation and the Renner-Teller couplings. Gaussian quadrature are used to calculate matrix elements of the three potential energy functions retrieved through ab initio calculations and rovibrational operators going to infinity for the linear configuration are treated rigorously. The HCCN TPE spectrum is computed with the above model calculating all rotational components and choosing the appropriate lineshape. This synthetic spectrum will be shown in the paper and compared with the experimental one.^b

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Footnotes:

Guélin and Cernicharo, A&A  244 (1991) L21; Loison et al., Icarus 247 (2015) 218Q Garcia, Krüger, Gans, Falvo, Coudert, and Loison, J. Chem. Phys. (2017) submittedu Koput, J. Phys. Chem. A 106 (2002) 6183V Zhao, Zhang, and Sun, J. Phys. Chem. A 112 (2008) 12125I

The current CPUFJ. M. Oldham, C. Abeysekera, J. Joalland, L. N. Zack, K. Prozument, I. R. Sims, G. Barrat Park, R. W. Filed and A. G. Suits, J. Chem. Phys. 141, 154202, (2014).Chirped Pulse Uniform Flow) and the new UF-CRDS (Uniform Flow Cavity Ring-Down Spectroscopy) setups relie mostly on the production of a good quality supersonic uniform flow. A supersonic uniform flow is produced by expanding a gas through a Laval nozzle - similar to the nozzles used in aeronautics - linked to a vacuum chamber. The expansion is characterized by an isentropic core where constant very low kinetic temperature (down to 20K) and constant density are observed. The relatively large diameter of the isentropic core associated with homogeneous thermodynamic conditions makes it a relevant tool for low temperature spectroscopy. On the other hand, the length along the axis of the flow of this core (could be longer than 50cm) allows kinetic studies which is one of the main interest of this setup (CRESU techniqueI. Sims, J. L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. Rowe, I. W. Smith, J. Chem. Phys. 100, 4229-4241, (1994).. The formation of a uniform flow requires an extreme accuracy in the design of the shape of the nozzle for a set of defined temperature/density. The design is based on a Matlab program which retrieves the shape of the isentropic core according to the method of characteristics prior to calculate the thickness of the boundary layerD. B. Atkinson and M. A. Smith, Rev. Sci. Instrum. 66, 4434, (1995). Two different approaches are used to test the viability of a new nozzle derived from the program. First, a computational fluid dynamic software (OpenFOAM) models the distribution of the thermodynamic properties of the expansion. Then, fabricated nozzles using 3-D printing are tested based on Pitot measurements and spectroscopic analysesN. Suas-David, V. Kulkarni, A. Benidar, S. Kassi and R. Georges, Chem. Phys. Lett. 659, 209-215, (2016) I will present comparisons of simulation and measured performance for a range of nozzles. We will see how the high level of accuracy of numerical simulations provides a deeper knowledge of the experimental conditions. J. M. Oldham, C. Abeysekera, J. Joalland, L. N. Zack, K. Prozument, I. R. Sims, G. Barrat Park, R. W. Filed and A. G. Suits, J. Chem. Phys. 141, 154202, (2014).( I. Sims, J. L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. Rowe, I. W. Smith, J. Chem. Phys. 100, 4229-4241, (1994).) D. B. Atkinson and M. A. Smith, Rev. Sci. Instrum. 66, 4434, (1995).. N. Suas-David, V. Kulkarni, A. Benidar, S. Kassi and R. Georges, Chem. Phys. Lett. 659, 209-215, (2016).

h0pt Figure Absolute measurements of the weak transitions require sensitive spectroscopic techniques. With our recently constructed pulsed cavity ring down (CRD) spectrometer, we have recorded the third and fourth vibrational overtone of the OH stretching vibration in a series of simple alcohols: methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), 2-propanol (2-PrOH) and tert-butanol (tBuOH). The CRD setup (in a flow cell configuration) is combined with a conventional FTIR spectrometer to determine the partial pressure of the alcohols from the fundamental transitions of the OH-stretching vibration. The oscillator strengths of the overtone transitions are determined from the integrated absorbances of the overtone spectra and the partial pressures. Furthermore, the oscillator strengths were calculated using vibrational local mode theory with energies and dipole moments calculated at CCSD(T)/aug-cc-pVTZ level of theory. We find a good agreement between the observed and calculated oscillator strengths across the series of alcohols.

While rotational spectra can be rapidly collected, their analysis (especially for complex systems) is seldom straightforward, leading to a bottleneck. The AUTOFIT programSeifert, N.A., Finneran, I.A., Perez, C., Zaleski, D.P., Neill, J.L., Steber, A.L., Suenram, R.D., Lesarri, A., Shipman, S.T., Pate, B.H., J. Mol. Spec. 312, 13-21 (2015)as designed to serve that need by quickly matching rotational constants to spectra with little user input and supervision. This program can potentially be improved by incorporating an optimization algorithm in the search for a solution. The Particle Swarm Optimization Algorithm (PSO) was chosen for implementation. PSO is part of a family of optimization algorithms called heuristic algorithms, which seek approximate best answers. This is ideal for rotational spectra, where an exact match will not be found without incorporating distortion constants, etc., which would otherwise greatly increase the size of the search space. PSO was tested for robustness against five standard fitness functions and then applied to a custom fitness function created for rotational spectra. This talk will explain the Particle Swarm Optimization algorithm and how it works, describe how Autofit was modified to use PSO, discuss the fitness function developed to work with spectroscopic data, and show our current results.

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Footnotes:

Seifert, N.A., Finneran, I.A., Perez, C., Zaleski, D.P., Neill, J.L., Steber, A.L., Suenram, R.D., Lesarri, A., Shipman, S.T., Pate, B.H., J. Mol. Spec. 312, 13-21 (2015)w

A study of the minimally exciting topic of agreement between experimental and measured rotational constants of molecules was performed on a set of large molecules with 16-18 heavy atoms (carbon and oxygen). The molecules are: nootkatone (C₁₅H₂₂O), cedrol (C₁₅H₂₆O), ambroxide (C₁₆H₂₈O), sclareolide (C₁₆H₂₂O₂), and dihydroartemisinic acid (C₁₅H₂₄O₂). For this set of molecules we obtained 13C-subsitution structures for six molecules (this includes two conformers of nootkatone). A comparison of theoretical structures and experimental substitution structures was performed in the spirit of the recent work of Grimme and Steinmetz.[1] Our analysis focused the center-of-mass distance of the carbon atoms in the molecules. Four different computational methods were studied: standard DFT (B3LYP), dispersion corrected DFT (B3LYP-D3BJ), hybrid DFT with dispersion correction (B2PLYP-D3), and MP2. A significant difference in these theories is how they handle medium range correlation of electrons that produce dispersion forces. For larger molecules, these dispersion forces produce an overall contraction of the molecule around the center-of-mass. DFT poorly treats this effect and produces structures that are too expanded. MP2 calculations overestimate the correction and produce structures that are too compact. Both dispersion corrected DFT methods produce structures in excellent agreement with experiment. The analysis shows that the difference in computational methods can be described by a linear error in the center-of-mass distance. This makes it possible to correct poorer performing calculations with a single scale factor. We also reexamine the issue of the “Costain error” in substitution structures and show that it is significantly larger in these systems than in the smaller molecules used by Costain to establish the error limits. [1] Stefan Grimme and Marc Steinmetz, “Effects of London dispersion correction in density functional theory on structures of organic molecules in the gas phase”, Phys. Chem. Chem. Phys. 15, 16031-16042 (2013).

I shall present two new variational methods for computing vibrational spectra. Both rely on the Hamiltonian being a sum of products (SOP). To use a variational method one represents wavefunctions in a basis and uses methods of numerical linear algebra to determine the basis function coefficients. A direct product basis has the advantage that it enables one to efficiently calculate the eigenvalues and eigenvectors of the Hamiltonian matrix using an iterative eigensolver. A direct product basis has the crucial disadvantage that the memory cost of a calculation scales exponentially with the number of atoms in the molecule. One of the new methods uses an expanding basis of products of 1D functions and an iterative eigensolver. For ethylene oxide (7 atoms), converged results are obtained with a basis that is many orders of magnitude smaller than the direct product basis with which similar results would be obtained. The second new method uses sum-of-product basis functions stored in canonical polyadic (CP) tensor format and generated by evaluating matrix-vector products. The memory cost scales linearly with the number of atoms in the molecule. Recent improvements make it possible to compute the spectrum of cyclopentadiene (11 atoms).

We have developed a new method for computing numerically exact rovibrational levels of a Van der Waals dimer with flexible monomers and applied it to water dimer, a 12-dimensional cluster. The method uses basis functions that are products of an inter-monomer function and an intra-monomer function. The inter-monomer function is a product of Wigner functions, used to study dimers within the rigid monomer approximation. The intra-monomer functions are monomer vibrational wavefunctions. When the coupling between inter- and intra-monomer coordinates is weak, this new basis is very efficient and only a few monomer vibrational wavefunctions are necessary. The product structure of the basis makes it efficient to use the Lanczos algorithm to calculate eigenvalues and eigenfunctions of the Hamiltonian matrix. In particular, potential matrix-vector products are evaluated, without storing the potential on a full-dimensional grid, by adapting the F-matrix idea previously used to compute rovibrational levels of 5-atom and 6-atom molecules with a contracted basis and an iterative eigensolver.X.-G. Wang and T. Carrington Jr. J. Chem. Phys. 119, 101 (2003) and 129, 234102 (2008).e have obtained numerically exact and converged inter-monomer energy levels and compare these with results obtained using the 6D + 6D adiabatic approach on the CCpol-8sf ab initio potential energy surface.C. Leforestier, K. Szalewicz, and A. van der Avoid, J. Chem. Phys. 137, 014305 (2012).e have also obtained the water bend levels and their shifts. We compare with results of the previous adiabatic calculation and experiment.

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Footnotes:

X.-G. Wang and T. Carrington Jr. J. Chem. Phys. 119, 101 (2003) and 129, 234102 (2008).W C. Leforestier, K. Szalewicz, and A. van der Avoid, J. Chem. Phys. 137, 014305 (2012).W

Ideally, the cataloguing of spectroscopic linelists would not demand laborious and expensive experiments. Whatever an experiment might achieve, the same information would be attainable by running a calculation on a computer. Kolos and Wolniewicz were the first to demonstrate that calculations on a computer can outperform even the most sophisticated molecular spectroscopic experiments of the time, when their 1964 calculations of the dissociation energies of H₂ and D₂ were found to be more than 1 cm⁻¹ larger than the best experiments by Gerhard Herzberg, suggesting the experiment violated a strict variational principle. As explained in his Nobel Lecture, it took 5 more years for Herzberg to perform an experiment which caught up to the accuracy of the 1964 calculations. Today, numerical solutions to the Schrödinger equation, supplemented with relativistic and higher-order quantum electrodynamics (QED) corrections can provide ro-vibrational spectra for molecules that we strongly believe to be correct, even in the absence of experimental data. Why do we believe these calculated spectra are correct if we do not have experiments against which to test them? All evidence seen so far suggests that corrections due to gravity or other forces are not needed for a computer simulated QED spectrum of ro-vibrational energy transitions to be correct at the precision of typical spectrometers. Therefore a computer-generated spectrum can be considered to be as good as one coming from a more conventional spectrometer, and this has been shown to be true not just for the H₂ energies back in 1964, but now also for several other molecules. So are we at the stage where we can launch an array of calculations, each with just the atomic number changed in the input file, to reproduce the NIST energy level databases? Not quite. But I will show that for the 6e⁻ molecule Li₂, we have reproduced the vibrational spacings to within 0.001 cm⁻¹ of the experimental spectrum, and I will discuss present-day prospects for replacing laborious experiments for spectra of certain systems within the reach of today's "computer spectrometers".

The rovibrational spectrum of the ν₁₄ CH₃-bending mode of dimethyl sulfide (CH₃)₂S was recorded in the 963-987 cm⁻¹spectral region using our sensitive tunable quantum cascade laser spectrometer coupled to a pulsed slit jetP. Asselin, Y. Berger, T. R. Huet, R. Motiyenko, L. Margulès, R. J. Hendricks, M. R. Tarbutt, S. Tokunaga, B. Darquié, Phys. Chem. Chem. Phys. 19, 4576 (2017)P. Asselin, A. Potapov, A. Turner, V. Boudon, L. Bruel, M. A. Gaveau and M. Mons, submitted to J. Phys. Chem. Lett. (2017). The combined use of a high dilution (CH₃)₂S/Ar gas mixture expanded at high backing pressure through a slit nozzle enabled to obtain an efficient rovibrational cooling which narrows the rotational distribution and eliminates hot bands arising from three low frequency modes below 300 cm⁻¹M. L. Senent, C. Puzzarini, R. Domínguez-Gómez, M. Carvajal, and M. Hochlaf, J. Chem. Phys., 140, 124302 (2014) The characteristic PQR band contour of a b₁ symmetry mode centered at 975.29 cm⁻¹was observed and will be compared with theoretical calculations at the CCSD(T)/VTZ level^c (ν₁₄ mode at 986 cm⁻¹) and room temperature experiments at low resolution (974 cm⁻¹)J. W. Ypenburg & H. Gerding, Recueil des Travaux Chimiques des Pays-Bas, 90, 885 (1971) Starting from the accurate set of ground state parameters derived from microwave, millimeter and far-infrared measurements, the rovibrational analysis will be presented and discussed.

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Footnotes:

P. Asselin, Y. Berger, T. R. Huet, R. Motiyenko, L. Margulès, R. J. Hendricks, M. R. Tarbutt, S. Tokunaga, B. Darquié, Phys. Chem. Chem. Phys. 19, 4576 (2017)

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Footnotes:

M. L. Senent, C. Puzzarini, R. Domínguez-Gómez, M. Carvajal, and M. Hochlaf, J. Chem. Phys., 140, 124302 (2014). J. W. Ypenburg & H. Gerding, Recueil des Travaux Chimiques des Pays-Bas, 90, 885 (1971).

Complexes of organic molecules with the main component of earth’s atmosphere are of interest,^a also for a stepwise understanding of the phenomenon of matrix isolation.^b Via its large quadrupole moment, nitrogen binds strongly to polarized OH groups in hydrogen-bonded dimers. Further complexation leads to a smooth spectral transition from free to embedded molecules which we probe in supersonic jets. Results for 1,1,1,3,3,3-hexafluoro-2-propanol,^c methanol,^d t-butyl alcohol,^e and the conformationally more complex ethanol^f are presented and assigned with the help of quantum chemical calculations.

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^aKuma, S., Slipchenko, M. N., Kuyanov, K. E., Momose, T., Vilesov, A. F., Infrared Spectra and Intensities of the H2O and N2 Complexes in the Range of the ν1- and ν3-Bands of Water, J. Phys. Chem. A, 2006, 110, 10046–10052.
^bCoussan, S., Bouteiller, Y., Perchard, J. P., Zheng, W. Q., Rotational Isomerism of Ethanol and Matrix Isolation Infrared Spectroscopy, J. Phys. Chem. A, 1998, 102, 5789–5793.
^cSuhm, M. A., Kollipost, F., Femtisecond single-mole infrared spectroscopy of molecular clusters, Phys. Chem. Chem. Phys., 2013, 15, 10702–10721.
^dLarsen, R. W., Zielke, P., Suhm, M. A., Hydrogen bonded OH stretching modes of methanol clusters: a combined IR and Raman isotopomer study, J. Chem. Phys., 2007, 126, 194307.
^eZimmermann, D., Haber, T., Schaal, H., Suhm, M. A., Hydrogen bonded rings, chains and lassos: The case of ¨ t-butyl alcohol clusters, Mol. Phys., 2001, 99, 413–425.
^fWassermann, T. N., Suhm, M. A., Ethanol Monomers and Dimers Revisited: A Raman Study of Conformational Preferences and Argon Nanocoating Effects, J. Phys. Chem. A, 2010, 114, 8223–8233.

The BeS⁻ anion to neutral ground state transition, X ²Σ⁺ → X ¹Σ⁺ has been studied using photoelectron velocity map imaging spectroscopy. Rotational constants, vibrational intervals and the electron binding energy of BeS⁻ have been determined for the first time. Rotational constants were derived from band contour analyses, as the contours exhibited band head features associated with changes in the rotational angular momenta of ∆N=0, ±1, and ±2. A dipole bound state (DBS) of BeS⁻ was observed 130 cm⁻¹ below the detachment threshold. Autodetachment spectra for the transition to the DBS were rotationally resolved, providing an accurate rotational constant for BeS⁻, v=0. The experimental results were found to be in reasonable agreement with the predictions of high level ab initio calculations.

Surfaces and interfaces play an important role in understanding many chemical process; they also contain molecular configurations and vibrations that are unique compared to those seen in the bulk and gas phases. Sum frequency generated (SFG) vibrational spectroscopy provides an incredibly detailed picture of these interfaces. In particular, the CH stretch region of the spectrum contains an extensive degree of information about the molecular vibrations and arrangements at the surface or interface. The presence of a strong bandwidth SFG signal for the benzene/air interface has generated controversy since it was discovered; since benzene is centrosymmetric, no SFG signal is expected. It has been hypothesized that this signal is primarily a result of bulk contributions that results from electric quadrupole transitions. Our work focuses on testing this conclusion by calculating a theoretical VSF spectrum from pure surface contributions using a mixed quantum/classical local mode Hamiltonian. We take as a starting point our local mode CH/OH stretch Hamiltonian, that was previously used to study alkylbenzenes, benzene-(H₂O)_n, and DPOE-water clusters, and extend it to the condensed phase by including shifts in the intensities and frequencies as a function of the environment. This environment is modeled using a SAPT-based force-field that accurately reproduces the quadrupole for the benzene dimer. A series of independent time-dependent trajectories are used to obtain an ensemble of surface configurations and calculate the appropriate correlation functions. These correlations functions allow us to determine the origins of the VSF signal. Our talk will focus on the challenges of extending our local mode Hamiltonian into the condensed phase.