In this talk we explore the effects of anharmonicity on the zero-point averaged OH bond lengths in hydrogen bonded complexes. Clusters with as many as six HF molecules or water molecules are explored as well as protonated water clusters and complexes of water clusters with F⁻, Cl⁻, Br⁻ and OH⁻. It is shown that there is a universal correlation between the vibrationally averaged OH or HF bond length and the anharmonc OH or HF stretch frequency. This relationship provides an extension to previously investigated correlations between the equilibrium bond lengths and harmonic frequencies and allows one to anticipate OH or HF bond lengths based on measured frequencies. In addition, differences between the R_z and R₀ structures are discussed within the context of these weakly bound complexes.

The breadth of the OH stretching manifold observed in the IR for bulk water is commonly attributed to the thermal population of excited states and the presence of many configurations within the water network. Here, I use carboxylate species as a rigid framework to isolate a single water molecule in the gas phase and cold ion vibrational pre-dissociation spectroscopy to explore excited state contributions to bandwidth. The spectrum of the carboxylate monohydrate exhibits a signature series of peaks in the OH stretching region of this system, providing an archetypal model to study vibrationally adiabatic mode separation. Previous analysis of this behavior accounts for the extensive progression in a Franck-Condon formalism involving displaced vibrationally adiabatic potentialsE. M. Myshakin, K. D. Jordan, E. L. Sibert III, M. A. Johnson J. Chem. Phys. 119, 10138 (2003).^, W.H. Robertson, et al. J. Phys Chem. 107, 6527 (2003)..

Formaldehyde is the smallest organic molecule and is a prime candidate for a thorough investigation regarding the anharmonic approximations made in computationally modeling its infrared spectrum. Mass-selected ion spectroscopy was used to detect mass 30 cations which include of HCOH⁺ and CH₂O⁺. In order to elucidate the differences between the structures of these isomers, infrared spectroscopy was performed on the mass 30 cations using Ar predissociation. Interestingly, several additional spectral features are observed that cannot be explained by the fundamental OH and CH stretch vibrations alone. By including anharmonic coupling between OH and CH stretching and various overtones and combination bands involving lower frequency vibrations, we are able to identify how specific modes couple and lead to the experimentally observed spectral features. We combine straight-forward, ab initio calculations of the anharmonic frequencies of the mass 30 cations with higher order, adiabatic approximations and Fermi resonance models. By including anharmonic effects we are able to confirm that the isomers of the CH₂O⁺·Ar system have substantially different, and thus distinguishable, IR spectra and that many of the features can only be explained with anharmonic treatments.

Dopamine (DA) is an essential neurotransmitter in the central nervous system and it plays integral role in numerous brain functions including behaviour, cognition, emotion, working memory and associated learning.E. Dragicevic, J. Schiemann and B. Liss, Neuroscience, 2015, 284, 798.Y. T. Chien et al. Science, 2010, 330, 1091. In the present work the conformational landscapes of neutral and protonated dopamine have been investigated in the gas phase and in aqueous solution by MP2 and DFT (M06-2X, ωB97X-D, B3LYP and B3LYP-D3) methods. Twenty lowest energy structures of neutral DA were subjected to geometry optimization and the gauche conformer, GIa, was found to be the lowest gas phase structure at the each level of theory in agreement with the experimental rotational spectroscopy.Cabezas etal., J. Phys. Chem. Lett. 2013, 4, 486.ll folded gauche conformers (GI) where lone electron pair of the NH2 group is directed towards the π system of the aromatic ring ( 'non up' ) are found more stable in the gas phase. While in aqueous solution, all those gauche conformers (GII) where lone electron pair of the NH2 group is directed opposite from the π system of the aromatic ring ('up' structures) are stabilized significantly.Nine lowest energy structures, protonated at the amino group, are optimized at the same MP2/aug-cc-pVDZ level of theory. In the most stable gauche structures, g-1 and g+1, mainly electrostatic cation - π interaction is further stabilized by significant dispersion forces as predicted by the substantial differences between the DFT and dispersion corrected DFT-D3 calculations. In aqueous environment the intra-molecular cation- π distance in g-1 and g+1 isomers, slightly increases compared to the gas phase and the magnitude of the cation- π interaction is reduced relative to the gas phase, because solvation of the cation decreases its interaction energy with the π face of aromatic system. The IR intensity of the bound N-H+ stretching mode provides characteristic 'IR spectroscopic signatures' which can reflect the strength of cation- π interaction energy. The CC2 lowest lying S1 ( 1ππ* ) excited state of neutral dopamine is significantly red shifted upon protonation at amino site.

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Footnotes:

E. Dragicevic, J. Schiemann and B. Liss, Neuroscience, 2015, 284, 798.

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Cabezas etal., J. Phys. Chem. Lett. 2013, 4, 486.A

The conformational landscapes of xanthine and its hydrated complex have been investigated by MP2 and DFT methods. The ground state geometry optimization yield five lowest energy conformers of xanth1-(H2O)1 complex at the MP2/6-311++G(d,p) level of theory for the first time. We investigated the low-lying excited states of bare xanthine by means of coupled cluster singles and approximate doubles (CC2) and TDDFT methods and a satisfactory interpretation of the electronic absorption spectra1 is obtained. The difference between the S0-S1 transition energy due to the most stable and the second most stable stable conformation of xanthine was found to be   859 cm⁻¹. One striking feature is the coexistence of the blue and red shift of the vertical excitation energy of the optically bright state S1 of xanthine upon forming complex with a water at C2=O and C6=O carbonyl sites, respectively. The lowest singlet ππ* excited-state of the xanth1-(H2O)1 complex involving C2=O carbonyl are strongly blue shifted which is in agreement with the result of R2PI spectra of singly hydrated xanthine. While for the most stable and the second most stable xanth1-(H2O)1 complexes involving C6=O carbonyl, the lowest singlet ππ* excited-state is red shifted. The effect of hydration on S1 excited state due to bulk water environment was mimicked by a combination of polarizable continuum solvent model (PCM) and conductor like screening model (COSMO), which also shows a blue shift in accordance with the result of electronic absorption spectra in aqueous solution.J. Chen and B.Kohler, Phys. Chem. Chem. Phys., 2012,14,10677-1068.his hypsochromic shift, is expected to be the result of the changes in the π-electron delocalization extent of molecule because of hydrogen bond formation. The optimized structure of xanthine dimer, computed the first time by MP2 and DFT methods. The binding energy of this dimer linked by double N-H…O=C hydrogen bonds was found to be 88 kj/mole at the MP2/6-311++G(d,p) level of theory. Computed IR spectra is found in remarkable agreement with the experiment and the out of phase (C=O)2 stretching mode shows tripling of intensity upon dimerisation. The vertical excitation energy of the optically bright state S1 of xanthine monomer upon forming dimer is shifted towards red as well as blue. J. Chen and B.Kohler, Phys. Chem. Chem. Phys., 2012,14,10677-1068.T

Vibrational spectra are measured for Fe_x⁺(CH₄)_n (x =2,3) (n = 1–3) in the C–H stretching region (2650–3100 cm⁻¹) using photofragment spectroscopy, by monitoring the loss of CH₄. All of the spectra exhibit an intense peak corresponding to the symmetric C–H stretch around 2800 cm⁻¹, which is red shifted by about 100 cm⁻¹ from free methane. The presence of a single peak suggests a nearly equivalent interaction between the methane ligands and the iron center. The peak becomes slightly less red shifted as the number of methane ligands increases. Density functional theory calculations, B3LYP and BPW91, are used to identify possible structures and predict the spectra. Results suggest that the methane(s) bind in a terminal configuration and that the Fe₂⁺complexes are in the octet spin state while the Fe₃⁺ complexes are in the dectet spin state. Lower C-H stretching frequencies are observed for Fe₃⁺ complexes, indicating that the CH₄ interacts more strongly with Fe₃⁺ than Fe₂⁺.

The laser induced fluorescence ( LIF ) spectrum of the à ²∆ - X̃ ²Π transition was obtained for SiCN generated by laser ablation under supersonic free jet expansion. The vibrational structure of the dispersed fluorescence ( DF ) spectra from single vibronic levels ( SVL's ) was analyzed by numerical diagonalization procedure, in which Renner-Teller ( R-T ), anhamonicity, spin-orbit ( SO ), Herzberg-Teller ( H-T ), Fermi, and Sears interactions have been considered, where the Sears resonance is a second-order interaction combined from SO and H-T interactions with ∆K = ±1, ∆Σ = ±1, and ∆P = 0. Four vibronic levels, (01¹0)   μ  Σ_\frac12⁽⁻⁾, κ  Σ_\frac12⁽⁺⁾, (02⁰0)   μ and κ  Π_\frac12, are almost closed within the four basis functions by R-T and Sears interactions ( i.e. the four-by-four transformation matrix below is close to ortho-normal );

⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ |(0110)   μ  2Σ(−)〉 |(0110)   κ  2Σ(+)〉 |(0200)   μ  2Π\frac12〉 |(0200)   κ  2Π\frac12〉 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ = ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ 0.9 −0.4 0.0 0.0 0.4 0.8 0.3 −0.2 0.2 0.4 −0.8 0.4 0.0 0.0 −0.5 −0.8 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝ | − 〉| +\frac12 〉  | 0; 1, +1 〉 | + 〉| +\frac12 〉  | 0; 1, −1 〉 | + 〉| −\frac12 〉  | +1; 2, 0 〉 | − 〉| −\frac12 〉  | +1; 2, +2 〉 ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠ \notag   ,

(\theequation)

where | Λ 〉| Σ〉| K; v₂, l 〉 = | − 〉| +\frac12 〉| 0; 1, +1 〉 etc. are basis functions of the vibronic Hamiltonian for the numerical diagonalization, and | Λ 〉, | Σ〉, and | K; v₂, l 〉 are basis functions of electronic, electron spin, and two dimensional harmonic oscillator, respectively. The mixing coefficients of the two vibronic levels agree with those obtained from computational studiesV. Brites, A. O. Mitrushchenkov, and C. Léonard, J. Chem. Phys. 138, 104311 (2013); C. Léonard, Private communication. The two levels among the four above, (01¹0)   κ  Σ_\frac12⁽⁺⁾ and (02⁰0)   μ  Π_\frac12, with ∆K = ±1 and ∆P = 0, show typical example of Sears resonance with an almost one-to-one mixing. Even for levels lying at  ∼  1,000 cm⁻¹, some of them are mixed heavily and widely with several levels, and their vibrational quantum numbers are thus meaningless.

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Footnotes:

V. Brites, A. O. Mitrushchenkov, and C. Léonard, J. Chem. Phys. 138, 104311 (2013); C. Léonard, Private communication..

Lanthanide-based materials have unusual electronic properties because of the high number of electronic degrees of freedom arising from partial occupation of 4f orbitals, which make these materials optimal for their utilization in many applications including electronics and catalysis. Electronic spectroscopy of small lanthanide molecules helps us understand the role of these 4f electrons, which are generally considered core-like because of orbital contraction, but are energetically similar to valence electrons. The spectroscopy of small lanthanide-containing molecules is relatively unexplored and to broaden this understanding we have completed the characterization of small cerium, praseodymium, and europium molecules using photoelectron spectroscopy coupled with DFT calculations. The characterization of PrO, EuH, EuO/EuOH, and Ce_xO_y molecules have allowed for the determination of their electron affinity, the assignment of numerous anion to neutral state transitions, modeling of anion/neutral structures and electron orbital occupation.

r0pt Figure A methane molecule (CH₄) has a permanent electric dipole moment (PEDM) in the excited state of the triply-degenerate vibrational modesM. Mizushima and P. Venkateswarlu, J. Chem. Phys. 21, 705 (1953).^,K. Uehara, K. Sakurai and K. Shimoda, J. Phys. Soc. Jpn. 26, 1018 (1969).The rotational dependence of the PEDM was reported in the 2ν₃ bandH. Sasada, K. Suzumura and C. Ishibashi, J. Chem. Phys. 105, 9027 (1996).However, in the ν₃ band, it was only determined on the P(7) E transition which fortunately lies in the tunable range of a 3.4 m He−Ne laser.We have developed a mid−infrared broadband sub−Doppler resolution spectrometer consisting of a difference−frequency−generation source and an optical frequency comb linked to International Atomic Time.This spectrometer enables us to measure the Stark effects of 20 transitions in the ν₃ band of methane from 87.7 to 92.8 THz (29273095 cm⁻¹).The observed linewidth is 0.5 MHz, and the frequency scale is absolutely calibrated.The figure depicts the Stark modulation spectrum of the P(4) E K. Uehara, K. Sakurai and K. Shimoda, J. Phys. Soc. Jpn. 26, 1018 (1969)..H. Sasada, K. Suzumura and C. Ishibashi, J. Chem. Phys. 105, 9027 (1996)..

A global frequency and intensity analysis of the infrared tetrad located in the 600−1500 cm⁻¹ region was carried out using the tensorial formalism developed in Dijon for X₂Y₄ asymmetric-top moleculesRaballand W, Rotger M, Boudon V, Loëte M. J Mol Spectrosc 2003;217:239-48.nd a program suite called D_2hTDS (now part of the XTDS/SPVIEW spectroscopic software)Wenger Ch, Boudon V, Rotger M, Champion JP, Sanzharov M. J Mol Spectrosc 2008;251:102-13. It relied on spectroscopic information available in the literature and retrieved from absorption spectra recorded in Brussels using a Bruker IFS 120 to 125 HR upgraded Fourier transform spectrometer, in the frame of either the present or previous workRotger M, Boudon V, Vander Auwera J. J Quant Spectrosc Radiat Transf 2008;109:952-62. In particular, 645 and 131 lines intensities have been respectively measured for the weak ν₁₀ and ν₄ bands. Including the Coriolis interactions affecting the upper vibrational levels 10¹, 7¹, 4¹ and 12¹, a total of 10737 line positions and 1870 line intensities have been assigned and fitted with global root mean square deviations of 2.6 ×10⁻⁴ cm⁻¹ and 2.4 %, respectively. Relying on the results of the present work and available in the literature, a list of parameters for 65420 lines in the ν₁₀, ν₇, ν₄ and ν₁₂ bands of ¹²C₂H₄ was generated. The present work provides an obvious improvement over HITRAN and GEISA for the ν₁₀ band (see figure), and a marginally better modeling for the ν₇ band (and for the ν₄ band hidden beneath it). To the best of our knowledge, this is the first time that a global intensity analysis is carried out in this range of the ethylene spectrum.

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Footnotes:

Raballand W, Rotger M, Boudon V, Loëte M. J Mol Spectrosc 2003;217:239-48.a Wenger Ch, Boudon V, Rotger M, Champion JP, Sanzharov M. J Mol Spectrosc 2008;251:102-13.. Rotger M, Boudon V, Vander Auwera J. J Quant Spectrosc Radiat Transf 2008;109:952-62..

High resolution stimulated Raman spectra of ¹³C₂H₄ in the regions of the ν₂ and ν₃ Raman active modes have been recorded at at two temperatures (145 and 296 K) based on the quasi continuous-wave (cw) stimulated Raman spectrometer at Instituto de Estructura de la Materia (CSIC) in Madrid. A tensorial formalism adapted to X₂Y₄ planar asymmetric tops with D_2h symmetry has been developed in DijonRaballand W, Rotger M, Boudon V, Loëte M. J Mol Spectrosc 2003;217:239-48.nd a program suite called D_2hTDS (now part of the XTDS/SPVIEW spectroscopic softwareWenger Ch, Boudon V, Rotger M, Champion JP, Sanzharov M. J Mol Spectrosc 2008;251:102-13.as proposed to calculate their high-resolution spectra. The effective Hamiltonian operator, involving a polyad structure, and transition moment (dipole moment and polarizability) operators can be systematically expanded to carry out global analyses of many rovibrational bands. A total of 103 and 51 lines corresponding to ν₂ and ν₃ Raman active modes have been assigned and fitted in frequency with a global root mean square deviation of 0.54 ×10⁻³ cm⁻¹ and 0.36 ×10⁻³ cm⁻¹, respectively. The figures below shows the stimulated Raman spectrum of the ν₂ and ν₃ bands of ¹³C₂H₄, compared to the simulation at 296 K.

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Footnotes:

Raballand W, Rotger M, Boudon V, Loëte M. J Mol Spectrosc 2003;217:239-48.a Wenger Ch, Boudon V, Rotger M, Champion JP, Sanzharov M. J Mol Spectrosc 2008;251:102-13.w