Near-equilibrium potential energy surfaces for HBF⁺ and HCO⁺, obtained from high-level calculations beyond fc-CCSD(T), are employed in variational calculations for many rovibrational states of various isotopologues. Calculated effective spectroscopic parameters are in excellent agreement with available experimental data and many predictions are being made, also for line intensities of HBF⁺ and HCO⁺ isotopologues. Combining a difference frequency system with glow discharge and a discharge modulation scheme, six and seven lines of the ν₁ bands for H¹¹BF⁺ and H¹⁰BF⁺, respectively, were observed. Together with data obtained from microwave spectroscopy, the spectroscopic constants of the ν₁ states could be derived through least-squares fitting.

The first studies of the ²A₁-²B₁ electronic band system of the AsH₂ and AsD₂ radicals were made at Sheffield University in the period from1966 to 1968 by Dixon, Duxbury and Lamberton using flash photolysis of arsine and deuterated arsine. The bands have a complex rotational structure associated with that of an asymmetric rotor. Band centres of the 0,v₂,0-0,0,0 progression were identified for v₂'=0 tp v₂'=5, although only the structure of the bands from v₂'=1 to 3 was analysed in detail. After a long time interval in 1986 a low resolution emission spectrum of AsH₂ was recorded by NI et al. However, it was not until 2007 that He and Clouthier studied the electronic transition of jet-cooled AsH₂ using laser induced fluorescence and wavelength-resolved emission. Following on from this in 2009 Zhao and colleagues recorded absorption spectra of the AsH₂ radical by cavity ringdown spectroscopy. Finally in 2012 Grimminger and Clouthier recorded the equivalent transitions in AsD₂ and AsHD. They also carried out ab initio calculations. By comparing the recent spectroscopic results with those of Dixon et al, we wish to show the complementarity of the different methods for understanding the behaviour of AsH₂ and AsD₂ radicals.

The occurence of predissociation in the electronic spectrum of AsH₂ is very dependent upon the magnitude of the spin-orbit coupling parameter of the central atom. Making use of Table 5.6 in "The Spectra and Dynamics of Diatomic Molecules, ELSEVIER" by H. Lefebvre-Brion and R.W. Field, it is possible to appreciate the rapid rate of increase of the spin-orbit constants associated with the heavy central atom in the di-hydrides NH₂, PH₂ and AsH₂. The spin-orbit constants range from 42.7 cm⁻¹ for NH₂, to 191.3 cm⁻¹ for PH₂, and 1178 cm⁻¹ for AsH₂. The effects of spin-orbit coupling may be seen in a plot of the separation of the central ^RQ_0,9 and ^PQ_1,N sub-bands as the value of v₂' increases from 0 to 5. As the value of v₂' increases beyond 2 the spectrum becomes more and more fuzzy as the effects of predissociation become more obvious. This means that unlike the example of the behaviour of PH₂, where the vibronic level pattern can be followed below and above the barrier to linearity, in AsH₂ and AsD₂ the absorption spectrum becomes completely diffuse below the barrier to linearity in the A ²A₁ state. The change in the magnitude of the doublet splittings as v₂' increases may be seen in the plots of the doublet splittings showing the spin-uncoupling as a result of the increase of overall rotation. In the absorption spectrum of SbH₂, recorded in 1967 by T. Barrow in the Chemistry Department at Sheffield University, all the absorption features showed the effects of predissociation, consistent with a spin-orbit constant of 2834 cm⁻¹ for the central atom of SbH₂.

Molecular species of group 14 elements e.g. carbon, silicon, and germanium are well suited to study cumulenic bond properties and to compare experimental results with high level quantum chemical calculations. In our recent investigation of SiC₃Si and GeC₃Ge, a high resolution laser spectrometer has been used to record rotationally resolved spectra of selected isotopologues at 5 μm.
We derived semi-empirical values for Si-C and Ge-C bond distances based on spectroscopic data and corresponding zero-point vibrational corrections calculated at the CCSD(T)/cc-pVXZ level of theory (with X = T and Q). Comparison of semi-empirical structural parameters with those from quantum chemical calculations reveals very good agreement for both molecules. Relativistic effects are found negligible for SiC₃Si and small for GeC₃Ge.

We have investigated electronic spectra of some quinoline carboxaldehydes dissolved in water by UV-vis measurements in the range of 190-1100 nm and by theoretical calculations with density functional theory (DFT). The geometrical structures of the quinoline carboxaldehydes have been obtained at the B3LYP/6-311++G(d,p) level, while the electronic absorption spectra have been simulated in water by using time-dependent DFT at the same level. Theoretical and experimental spectra agree to each other very well. Keywords: Electronic spectra, Quinoline carboxaldehydes, DFT [1] V. Kucuk, A. Altun, M. Kumru, Spectrochim. Acta Part A 85 (2012)9298. [2] M. Kumru, V. Kucuk, T. Bardakci, Spectrochim. Acta Part A 90(2012)2834. [3] M. Kumru, V. Kucuk, M. Kocademir, Spectrochim. Acta Part A, 96 (2012) 242251. [4] M. Kumru, V. Kucuk, P. Akyürek, Spectrochim. Acta Part A, 113 (2013) 72–79.

A detailed vibrational analysis of the infrared spectra of cyclopentadienone (C5H4=O and C5D4=O) in rare gas matrices has been carried out. Ab initio coupled-cluster anharmonic force field calculations were used to guide the assignments. Flash pyrolysis of o-phenylene sulfite (C6H4O2SO and C6D4O2SO) was used to provide a molecular beam of cyclopentadienone entrained in the rare gas carrier. The beam was interrogated with time-of-flight photoionization mass spectrometry (TOF-PIMS), confirming the clean, intense production of C5H4=O. Matrix isolation infrared spectroscopy was coupled with 355 nm polarized UV for photo-orientation and linear dichroism experiments to determine the symmetries of the vibrations.

When substituted benzenes become a focus of a spectroscopic study there are various well known vibrational labelling schemes present,^1,2 however it was shown in recent works the description of monohalobenzene vibrations in terms of benzene modes (ie. Wilson notation) is questionable in some cases.^3,4 A new scheme is presented which uses the motions of monofluorobenzene vibrations as a basis for labelling vibrational assignments of monosubstituted benzenes.^d The scheme has been successfully applied to the ground and excited states of toluene and its deuterated-methyl group isotopologue.^5,6 Here we present the application of the scheme to fluorobenzene and its fully deuterated analogue. One-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed in order to characterise the fluorobenzene and fluorobenzene-d₅ excited state.
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¹E. B. Wilson Jr., Phys. Rev., 45, 706 (1934) ²G .Varsanyi, Assignments of the Vibrational Spectra of Seven Hundred Benzene Derivatives,Wiley, New York, 1974, Vol. I and II ³I. Pugliesi, N. C. Tonge and M. C. R. Cockett, J. Chem. Phys., 129, 104303 (2008) ⁴A. M. Gardner and T. G. Wright, J. Chem. Phys., 135,114305 (2011) ⁵A. M. Gardner, A. M. Green, V. M. Tame-Reyes, V. H. K. Wilton and T. G. Wright, 138, 134303 (2013) ⁶A. M. Gardner, A. M. Green, V. M. Tame-Reyes, K. L. Reid, J. A. Davies, V. H. K. Wilton and T. G. Wright, manuscript accepted

The consistency of the labelling and assignments of the vibrational frequencies of the chloro- and bromo- monosubstituted benzene molecules is investigated in their first electronically excited states (S₁). The assignments given utilise a recent nomenclature¹ discussed in a previous talk, allowing the ring-localised vibrations to be compared straightforwardly across different monohalosubstituted benzenes. For the S₁ state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed. The assignments of the frequencies include previous work but also the calculated wavenumbers for both fully hydrogenated monohalosubstituted benzenes (-h₅) and the deuterated isotopologues (-d₅) employing time-dependent density functional theory (TDDFT).
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¹A. M. Gardner and T. G. Wright, J. Chem. Phys., 135,114305 (2011)

The recent implementation of the computation of infrared (IR) intensities beyond the double harmonic approximation [1] paved the route to routine calculations of infrared spectra for a wide set of molecular systems. Contrary to common beliefs, second-order perturbation theory is able to deliver results of high accuracy provided that anharmonic resonances are properly managed [1,2]. It has been already shown for several small closed- and open shell molecular systems that the differences between coupled cluster (CC) and DFT anharmonic wavenumbers are mainly due to the harmonic terms, paving the route to introduce effective yet accurate hybrid CC/DFT schemes [2]. In this work we present that hybrid CC/DFT models can be applied also to the IR intensities leading to the simulation of highly accurate fully anharmonic IR spectra for medium-size molecules, including ones of atmospheric interest, showing in all cases good agreement with experiment even in the spectral ranges where non-fundamental transitions are predominant[3]. [1] J. Bloino and V. Barone, J. Chem. Phys. 136, 124108 (2012) [2] V. Barone, M. Biczysko, J. Bloino, Phys. Chem. Chem. Phys., 16, 1759-1787 (2014) [3] I. Carnimeo, C. Puzzarini, N. Tasinato, P. Stoppa, A. P. Charmet, M. Biczysko, C. Cappelli and V. Barone, J. Chem. Phys., 139, 074310 (2013)

The infrared (IR) spectroscopy of the alkyl CH stretch region (2750-3000 cm⁻¹) of a series of bicyclic hydrocarbons and free radicals has been studied under supersonic expansion cooling in the gas phase, and compared with a theoretical model that describes the local mode stretch-bend Fermi resonance interactions. The double resonance method of fluorescence-dip infrared (FDIR) spectroscopy was used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene, tetralin, indene, and indane using the S₀-S₁ origin transition as a monitor of transitions. Resonant ion-dip infrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN) and inden-2-yl methyl (I2M) radicals. The previously developed model Hamiltonian [J. Chem. Phys. 138 064308 (2013)] incorporates cubic stretch-bend coupling with parameters obtained from density functional theory methods. Full dimensional calculations are compared to reduced dimensional Hamiltonian results in which anharmonic CH streches and CH₂ scissor modes are Fermi coupled. Excellent agreement between theoretical results is found. Scale factors of select terms in the reduced dimensional Hamiltonian, obtained by fitting the theoretical Hamiltonian predictions to the experimental spectra, are found to be similar to previous work. The resulting Hamiltonian predicts successfully all the major spectral features considered in this study. A simplified model is introduced in which the CH₂ groups are decoupled. This model enables the assignment of many of the spectral features. The model results are extended to describe the CH stretch spectrum of the chair and twist-boat conformers of cyclohexane. The chair conformer is used to illustrate the shortcomings of the CH₂ coupling model.

The origins of the intensity of the feature in the spectrum of liquid water near 2100 cm⁻¹ are investigated through calculations of the spectra of water clusters. The spectra of thirteen water clusters with six or fewer water molecules are calculated using low-order expansions of the potential and dipole surfaces in internal and normal mode coordinates. The results are dissected to interpret the origins of the intensity in 2100 cm⁻¹ region of the spectra. It is found that the intensity near 2100 cm⁻¹ can be attributed to combination bands involving the HOH bend and intermolecular vibrations that break the hydrogen bonding network. Based on the robustness of a feature near 2100 cm⁻¹ in all of the water clusters studied, it is inferred that a similar assignment can be made for the spectrum of liquid water. When the origins of the intensity near 2100 cm⁻¹ is analyzed from calculations that used internal coordinates to expand the potential and dipole surfaces, the leading contribution to the intensity is attributed to large second derivatives of the dipole moment with respect to the internal coordinates that are excited, or electrical anharmonicity. This picture changes if the derivatives of the potential and dipole surfaces are taken with respect to normal modes. In this representation, the second derivatives of the dipole moment are often vanishingly small, while the mixed third and fourth derivatives of the potential become quite large. Based on this result, mechanical anharmonicity appears to be responsible for the intensity in the 2100 cm⁻¹ region. This strong dependence of the interpretation of the origins of the intensity in the 2100 cm⁻¹ region of the water spectrum is also investigated and discussed.

We carried out a comprehensive SCF MRD-CI ab initio study of the electronic structure of O₂ and O₂⁺. Potential energy curves (PECs) of about 150 electronic states of O₂ and about 100 of O₂⁺, as well as a number of states of O₂⁺⁺ were computed. The cc-pVQZ basis set augmented with diffuse functions was employed. Spectroscopic parameters (T_e, T_v, ω_e, ω_ex_e, B_e, D_e, D₀, μ, IP, etc.) are reported. A preliminary sample of the results will be presented. The electronic absorption spectrum of O₂ has proved difficult to analyze/interpret due to the unusually large number of electronic states which arise from the peculiar open-shell structure of both the oxygen atomic fragments and the O₂ molecule. For instance, there are 62 valence molecular electronic states which correlate to the six lowest dissociation limits resulting from the three valence O atom fragment states (³P, ¹D, ¹S). In addition, there are several nlλ Rydberg series converging to the X²Π_g ground ionic state and to the lowest two excited states of the cation, a⁴Π_ui and A²Π_u. Furthermore, a number of interactions of various types among several electronic states result in rovibronic perturbations which manifest themselves, e.g., as irregular vibronic structure, hence severely complicating the assignment of the absorption features and the analysis and interpretation of the spectrum. An overview of the electronic states and spectroscopy of O₂ will be presented. A chief motivation of this study of O₂ was to try to provide a theoretical insight on the nature, energetic position, shape, and dissociation asymptotes, of electronic states located in the 4 eV energy region encompassed between the O₂⁺ ground state X²Π_g (IP=12.07 eV) and the first excited state of the cation a⁴Π_ui (IP=16.10 eV). This in order to aid in the interpretation of experimental data related to the mechanism(s) of the neutral dissociation of the O₂^** (Rydberg) superexcited states, which competes with autoionization. We are currently striving to compute PECs of relatively highly excited states of O₂ located in the 12-16 eV energy region which might help to visualize possible pathways for the neutral XUV photodissociation of the I, I^′ and I^{′′ 3}Π_u superexcited states of O₂ leading to the O(³P) + O^*(³S, ⁵S) dissociation limits.

The structure-function relationship in proteins is widely recognized, motivating numerous investigations of isolated neutral and ionic polypeptides that generally employ conformation specific, multidimensional UV and IR spectroscopies. This data taken in conjunction with computed harmonic frequencies has provided a snapshot of the underlying molecular physics at play in many polypeptides, but few experiments have been able to probe the energetics of these systems. In this study, we use vibrational spectroscopy to measure the gas-phase enthalpy change for isomerization between two conformations of the dipeptide N-acetyl glycine methyl amide (NAGMA). A two-stage oven source is implemented producing a gas-phase equilibrium distribution of NAGMA molecules that is flash frozen upon pickup by He nanodroplets. Using polarization spectroscopy, the IR spectrum is assigned to a mixture of two conformers having intramolecular hydrogen bonds made up of either five- or seven-membered rings, C₅ and C₇, respectively. The interconversion enthalpy, obtained from the van’t Hoff relation, is 4.52±0.12 kJ/mol for isomerization from the C₇ to the C₅-conformer. This experimental measurement is compared to computations employing a broad range of theoretical methods.

The pure rotational spectrum of lead monochloride, PbCl, has been measured and analyzed using chirped pulse and cavity Fourier transform microwave (CP-FTMW and FTMW) spectrometers equipped with an ablation source. Refined parameters of an effective Hamiltonian including fine and hyperfine interactions similar to those previously reported by Fink et al. [1] were determined. Dynamically-weighted, explicitly-correlated MRCI-F12 calculations [2] were performed for both PbF and the valence isoelectronic PbCl to predict potential energy curves (PEC). Spin-orbit coupling was included in the calculations, which is known to split the X₁²Π_1/2 and X₂²Π_3/2 components of the ground electronic state by roughly 8280 cm⁻¹in both lead halide systems. Calculated rotational levels were obtained using the PECs and compared with experiment including previously published results for PbF [3]. References: 1- K. Ziebarth, K. D. Setzer, O. Shestakov,1 and E. H. Fink, J. Mol. Spec. 191, 108 (1998). 2- B. J. Barker et al. J. Chem. Phys. 137, 214313 (2012). 3- R. J. Mawhorter et al. Phys. Rev. A 84, 022508 (2011).