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We report here investigations on conformational effects in the vibrational and electronic spectra of the propionaldehyde (propanal) molecule using FTIR (600–3200 cm−1) and vacuum ultraviolet (VUV) synchrotron radiation photoabsorption (52 500–85 000 cm−1) spectroscopy respectively. Detailed theoretical calculations (using DFT and TDDFT methodologies) on ground and excited states of the cis and gauche conformers of propanal are performed; a comprehensive spectral analysis of the IR and VUV spectra is presented. A reinvestigation of the IR spectrum reveals several new bands assigned to the gauche conformer based on theoretical calculations. The VUV spectrum exhibits rich Rydberg series structure assigned to ns, np and nd series converging to the first ionization potentials of the two conformers. Earlier assignments of the 3s cis and gauche origins are revised in addition to extending Rydberg series analysis to several higher members. Vibronic bands accompanying the 3s, 4s and 4p Rydberg states are assigned using estimated vibrational frequencies of cis and gauche conformers in the cationic ground state. Simulated potential energy curves of the first few excited states (singlets and triplets) of cis and gauche conformers of propanal help in gaining insights into photodissociation mechanisms and possible conformational effects therein. [ABSTRACT FROM AUTHOR]

The electronic absorption spectrum of N,N-dimethylformamide (DMF) is studied in the 45 000-80 000 cm-1 (5.6-9.9 eV) region using synchrotron radiation. The vacuum ultraviolet (VUV) spectrum comprises mostly of Rydberg series of ns, np, and nd types converging to the first two ionization potentials (IPs). Quantum defect values obtained are consistent with excitation of an electron from the highest occupied molecular orbitals localized on nitrogen (4a″) and oxygen (16a′); in addition, the 3s Rydberg transition converging to the third IP (3a″) is observed at 8.95 eV. A reinvestigation of the infrared spectrum of DMF in the 500-4000 cm-1 region with the help of density functional theory (DFT) calculations establishes the planarity of the ground state and leads to revision of several vibrational assignments. Vertical excited state energies and their valence/Rydberg character are predicted using time dependent DFT calculations; excellent correlation is achieved between theoretical results and experimentally observed spectral features. Potential energy curves of the first few excited states give additional insights into the nature of the excited states and their role in photodissociation dynamics. The absorption spectrum of DMF in the region >63 400 cm-1 (7.85 eV) as well as a complete set of spectral assignments in the VUV region (45 000-80 000 cm-1) is reported for the first time. This work represents a comprehensive study of the absorption spectra of DMF in the VUV and infrared regions. [ABSTRACT FROM AUTHOR]

The electronic absorption spectrum of diiodomethane in the 30 000-95 000 cm-1 region is investigated using synchrotron radiation, the spectrum in the 50 000-66 500 cm-1 region is reported for the first time. The absorption bands in the 30 000-50 000 cm-1 region are attributed to valence transitions, while the vacuum ultraviolet (VUV) spectrum (50 000-95 000 cm-1) is dominated by several Rydberg series converging to the first four ionization potentials of CH2I2 at 9.46, 9.76, 10.21, and 10.56 eV corresponding to the removal of an electron from the outermost 3b2, 2b1, 1a2 , and 4a1 non-bonding orbitals, respectively. Rydberg series of ns, np, and nd type converging to each of the four ionization potentials are assigned based on a quantum defect analysis. Time dependent density functional theory calculations of excited states support the analysis and help in interpretation of the Rydberg and valence nature of observed transitions. Density functional theory calculations of the neutral and ionic ground state geometries and vibrational frequencies are used to assign the observed vibronic structure. Vibronic features accompanying the Rydberg series are mainly due to excitation of the C-I symmetric stretch (v3) and CH2 wag (v8) modes, with smaller contributions from the C-H symmetric stretch (v1). UV absorption bands are assigned to low lying valence states 11B2, 11B1, 21A1, 31A1, 21B1, and 21B2 and the unusually high underlying intensity in parts of the VUV spectrum is attributed to valence states with high oscillator strength. This is the first report of a comprehensive Rydberg series and vibronic analysis of the VUV absorption spectrum of CH2I2 in the 50 000-85 000 cm-1 region. The VUV absorption spectrum of CD212 which serves to verify and consolidate spectral assignments is also reported here for the first time. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detailed theoretical study of structural, spectroscopic and electron scattering cross section study of glycolaldehyde. • Quantum chemical calculations for ground and excited states are performed using DFT and TD-DFT methods. • In the absence of sufficient experimental data on excited states of glycolaldehyde, we compare VES calculations with the available experimental data for an analogous molecule acetaldehyde. • Maiden theoretical attempt to provide Low-energy (0.1 to 20 eV) scattering cross-sectional data using R-matrix method. • We have detected one π* shape resonance in all the three models viz. SE, SEP and CC model. Detailed theoretical investigations on structural, spectroscopic and electron scattering cross sections of glycolaldehyde, the first ever detected sugar in space are reported in this work. Spectroscopic calculations are performed on the ground and excited states of glycolaldehyde using density functional theory (DFT) and time-dependent density functional theory (TDDFT), respectively. The optimized geometrical parameters and vibrational spectrum are in good agreement with the earlier reported work. Vertical excited state (VES) energies at the TDDFT/B3LYP/aug-cc-PVQZ level of theory along with detailed assignments are reported up to 30 transitions. The TDDFT predicted Rydberg series energies are compared with the Rydberg series of glycolaldehyde converging to its first IP, computed using the standard Rydberg formula and are found to be in good agreement. In the absence of sufficient experimental data on excited states of glycolaldehyde, the theoretical results are validated by comparison of VES calculations performed at the same level of theory with the available experimental data for an analogous molecule viz. acetaldehyde. Triplet excited-state energies for glycolaldehyde are reported here for the first time. Electron impact cross section calculations in the energy range 0.1 to 20 eV are carried out using ab-initio R-matrix method. We have detected one π* shape resonance in all the three models viz. SE, SEP and CC, which may also lead to a possible fragmentation channel wherein a hydrogen atom is removed from the molecule forming a glycolaldehyde dehydrogenated anionic radical (C 2 H 3 O 2 −). The π* shape resonance is correlated with the corresponding molecular orbital electronic structure calculations. Additionally, differential, electronic excitation, ionization and total cross sections are reported here for the first time. [ABSTRACT FROM AUTHOR]

Computational studies of electronically excited states of the acetone molecule [(CH 3 ) 2 CO] and its fully deuterated isotopologue [(CD 3 ) 2 CO] are performed using the time dependent density functional (TDDFT) methodology. In addition to vertical excitation energies for singlet and triplet states, equilibrium geometries and vibrational frequencies of the n =3 Rydberg states (3 s , 3 p and 3 d ) are obtained. This is the first report of geometry optimization and frequency calculations for the 3 p x , 3 p z , 3 d yz , 3 d xy , 3 d xz , 3 d x 2 –y 2 and 3 d z 2 Rydberg states. Results of the geometry optimization indicate that the molecule retains approximate C 2V geometry in most of these excited Rydberg states, with the most significant structural change seen in the CCO bond angle which is found to be reduced from the ground state value. Detailed comparison of the computationally predicted vibrational wavenumbers with experimental studies helps to confirm several of the earlier vibronic assignments while leading to revised/new assignments for some of the bands. The important role of hot bands in analysis of the room temperature photoabsorption spectra of acetone is corroborated by this study. While the vibrational frequencies in excited Rydberg states are overall found to be close to those of the ionic ground state, geometry optimization and vibrational frequency computation for each excited state proves to be very useful to arrive at a consistent set of vibronic assignments. Isotopic substitution helps in consolidating and confirming assignments. An offshoot of this study is the interpretation of the band at ~8.47 eV as the π –3 s Rydberg transition converging to the second ionization potential. [ABSTRACT FROM AUTHOR]

We present here theoretical investigations of electronic states and electron scattering cross-sections of the thiazole molecule. Spectroscopic calculations are performed on ground and excited states of thiazole using density functional theory (DFT) and time dependent DFT, while the electron impact scattering calculations are carried out using ab initio R-Matrix method. The optimized geometrical parameters and computed vibrational frequencies of neutral and cationic thiazole molecule are in good agreement with earlier work. Detailed comparison of the present computed vertical excitation energies with earlier experimental studies helps to confirm several of the earlier assigned Rydberg transitions while leading to revised assignments for some of the earlier observed peaks. Extensive electron scattering cross-sectional data; Elastic cross-sections include symmetry decomposition, differential and momentum transfer cross-sections while inelastic cross-sectional data comprises of discrete electronic excitations and ionization cross-sections along with their rate-coefficients. According to literature survey, this is the first report of cross-section data and rate coefficients for inelastic processes. A comparison of the total cross-sectional (TCS) data of thiazole with the TCS data of the isoelectronic thiophene molecule reveals the overall TCS values being higher in thiazole, as expected due to its higher dipole moment. Further, resonant peaks in the TCS are observed to be shifted to lower energies in thiazole as compared to thiophene, which is attributed to the higher electronegativity of the N atom as compared to the CH group in thiophene. For the first time, a comprehensive spectroscopic and electron scattering study of thiazole is summarized in this paper. • Comprehensive spectroscopy and electron scattering study on thiazole. • The geometrical parameters and IR frequencies agree well with earlier work. • Comparison with earlier data; confirm/revised assignments of Rydberg states. • Low energy electron scattering data for Thiazole from 0.1 eV to 20 eV. • This data help to understand various electron induced biochemical processes. [ABSTRACT FROM AUTHOR]

The photo absorption spectrum of carbon disulphide (CS 2 ) is recorded using synchrotron radiation in the 5.5–11.8 eV region. The richly structured spectrum of CS 2 consists of valence and Rydberg transitions up to the first ionization potential (IP) at ~10.1 eV and a few discrete peaks above the first IP. The low lying electronic states of CS 2 are assigned to the singlet or triplet states originating from {2π g →3π u , 7σ g , 8σ g , 6σ u , 4π u , 3π g , 9σ g }, {2π u →3π u , 7σ g } and {5σ u →3π u } excitations. Rydberg series of type nlλ(l=s, p, d, f and λ=σ,π,δ,ϕ) are observed along with associated vibronic structure in a few low lying members. The derived quantum defect values for the s(~2.0), p(~1.6), d(~0.4) and f(~0.1) Rydberg states are consistent with excitation from sulphur non-bonding orbitals. The comprehensive experimental work presented here and the revisit of the study of electronic spectrum of CS 2 gives a clearer picture of the nature and origin of various transitions and helps in interpreting several unassigned peaks. The vibrational features observed above the first IP are attributed to autoionization from a high lying Rydberg state. Theoretical calculations using the time dependent density functional theory method assists in evaluating vertical excitation energies, nature of the transitions and confirming or suggesting electronic assignments of a few low lying valence transitions where ambiguity existed. [ABSTRACT FROM AUTHOR]

We report a comprehensive photoabsorption study of nitrous oxide (N2O) in the vacuum ultraviolet (45,000-95,000cm−1) region using synchrotron radiation. The observed spectrum comprises of a few valence transitions and low lying Rydberg series converging to the two spin-orbit components (2П1/2,3/2) of the ground state of N2O+. Spectral analysis is aided by extensive quantum chemical calculations of vertical excited states, oscillator strengths and potential energy curves using the time dependent density functional theory. Vibronic bands observed in the first absorption system (45,000-60,000cm−1) are assigned to hot band progressions in υ2′ originating from v″=1 or 2. New insights into the assignment of the well-formed progression of bands in the X1Σ+→C1П system (60,000-72,000cm−1) are afforded by consideration of the Renner-Teller interaction. A set of molecular vibrational parameters (ω2=467cm−1, x22=−2.9, ε=−0.24) for the C1П state are derived from a fitting of the experimental data. The 3pπ1Σ+ state at ~77,600cm−1 shows a large quantum defect (0.96) which is explained as arising due to mixed valence-Rydberg character. In the 85,000-95,000cm−1 region, a number of absorption features are observed with greater clarity than in earlier photoabsorption studies and assigned to Rydberg series of type nlλ (n=3,4; l=s,p,d; λ=σ,π,δ) and accompanying vibronic bands. This work has resulted in clarification of several discrepancies in earlier Rydberg series assignments. Additionally, the 3pπ 3Σ− Rydberg state at 85,788cm−1, the valence transition 7σ→3π (1П) at 87,433cm−1 and the 3dλ Rydberg series in the 91,700-92,600cm−1 region are assigned for the first time. [ABSTRACT FROM AUTHOR]

Abstract: The VUV photoabsorption spectra of CHCl3 and CDCl3 in the energy region 6.2–11.8eV (50,000–95,000cm−1) have been investigated using synchrotron radiation from the Indus-1 source. Rydberg series converging to the first four ionization limits at 11.48, 11.91, 12.01 and 12.85eV corresponding to excitation from the 1a2, 4a1, 4e, 3e, orbitals of CHCl3 respectively are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the chlorine lone pair orbitals. Vibrational progressions observed in the region of 72,500–76,500cm−1 have been reassigned to ν3 and combination modes of ν3+ν6 belonging to the 1a2→4p transition in contrast to earlier studies where they were assigned to a ν3 progression superimposed on the 3e→4p Rydberg transition. The assignments are further confirmed based on isotopic substitution studies on CDCl3 whose VUV photoabsorption spectrum is reported here for the first time. The frequencies of the ν3 and ν6 modes in the 4p Rydberg state of CHCl3 (CDCl3) are proposed to be ~454 (409)cm−1 and~130 (129)cm−1 respectively based on the vibronic analysis. DFT calculations of neutral and ionic ground state vibrational frequencies support the vibronic analysis. Experimental spectrum is found to be in good agreement with that predicted by TDDFT calculations. This work presents a consolidated analysis of the VUV photoabsorption spectrum of chloroform. [Copyright &y& Elsevier]

Abstract: The VUV photoabsorption spectrum of acetone-d6 is recorded in the energy region 6–10eV using synchrotron radiation at a resolving power of∼1000. Extensive Rydberg series are observed converging to its first ionization limit (9.708eV). Quantum defect analysis and vibronic analysis of higher Rydberg series members have been reported for the first time. A comparative study of the Rydberg series and accompanying vibronic structure observed in acetone-h6 and acetone-d6 has clarified and consolidated many of the vibronic assignments. The frequency of the ν19′ mode (CO in-plane bend) of the 3py state is obtained as 340±5cm−1 on the basis of observation of an extensive progression of this mode. [Copyright &y& Elsevier]

Abstract: The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8eV at a resolution of ∼4meV at 4eV and ∼10meV at 10eV using synchrotron radiation. The absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd) converging to the first ionization potential of acetone at 9.708eV. Careful consideration of hot band contributions from torsional modes and symmetry selection rules have resulted in an improved set of vibronic assignments as compared to earlier room temperature work. Revised quantum defect values for some of the Rydberg transitions and a few new assignments in the nd series are also reported in this paper. [Copyright &y& Elsevier]

• UV–VUV (4.5–11.8 eV) photoabsorption studies of anisole using synchrotron radiation. • DFT/TDDFT calculations of ground & excited states correlated with observed spectra. • Comprehensive spectral analysis performed; first report of spectrum above 7.8 eV. • IR spectrum reinvestigated; new combination/overtone bands in1700–2700 cm−1 region. • UV photodissociation dynamics reinvestigated using simulated potential energy curves. Photoabsorption studies on anisole (C 6 H 5 OCH 3) are performed in the UV–VUV region (4.5–11.8 eV) using synchrotron radiation; the absorption spectrum in the region > 7.8 eV is reported here for the first time. Quantum chemical calculations are carried out on ground and excited states of anisole using the DFT and TDDFT methodologies and compared with experimental spectra in order to interpret the observed features in terms of the transitions involved, their valence/Rydberg nature and relative intensities. In order to validate the ground state spectral parameters, the infrared spectrum is recorded and assigned in the 600–3500 cm−1 region; a few new vibrational bands observed in the 1700–2700 cm−1 region are assigned to overtone and combination modes. The electronic absorption spectrum in the 4.5–5.0 eV region shows an extensive vibrational progression belonging to the first valence (ππ*) transition, while the 5.0–7.0 eV region is dominated by a broad and intense band peaking at ~6.6 eV which is assigned to two strong overlapping valence transitions originating from the HOMO–1 orbital. Additionally, throughout the 5.0–11.8 eV region, diffuse bands are observed which are attributed to Rydberg series (ns, np and nd) converging to the first four IPs of anisole. Theoretically simulated potential energy curves of ground and first few excited singlet and triplet states help in achieving a better understanding of the spectral features and the UV induced photodissociation mechanisms of anisole. The present studies suggest that photon excitation at 248 nm is likely to result in direct dissociation along a repulsive excited state (3s) in contrast to earlier studies which have attributed it to internal conversion to the ground state. [ABSTRACT FROM AUTHOR]

• Comprehensive IR and VUV photoabsorption studies of n-dodecane. • DFT calculations of ground state geometry and vibrational frequencies. • TDDFT calculation of vertical excited state energies & potential energy curves. • Assignment/interpretation of observed IR & VUV spectra backed by calculations. • First report of VUV absorption spectrum & TDDFT calculations of n-dodecane. We report here a comprehensive spectroscopic study of the absorption spectrum of the n-dodecane molecule using synchrotron radiation based photoabsorption and FTIR spectroscopy. Quantum chemical calculations using the DFT and TDDFT methodologies are used to predict relevant ground and excited state properties and correlate them with the experimental results. In the IR spectrum, assignment of the prominent CH stretching bands in the 2800–3000 cm−1 region are consolidated while a few new vibrational assignments are made in the 700–1500 cm−1 region. The electronic absorption spectrum of n-dodecane lies entirely in the vacuum ultraviolet (VUV) region and consists of a broad continuous absorption band starting from ∼7.5 eV, with no discernible structure. TDDFT calculations of vertical excited states predict that most of the excited states are Rydberg in nature. Potential energy curves of the first few excited states with respect to various bond lengths and bond angles are studied in order to gain additional insights into the nature of the excited states. The broad nature of the absorption is attributed partly to the effect of several conformers as well as hot bands from low frequency modes in the ground state and partly due to overlap of several Rydberg series converging to the first few ionization potentials (IPs). Energy separation between the first and second IP is theoretically predicted to be ∼0.65 eV, while successive separation between the second, third, fourth and fifth IPs is ∼0.03 eV. To the best of our knowledge this is the first report of the VUV absorption spectrum of n-dodecane in the region from 6 to 10 eV, as also the first report of theoretical calculations of its electronically excited states. [ABSTRACT FROM AUTHOR]

Abstract: The UV–VUV photoabsorption spectrum of dibromomethane (CH2Br2) in the energy region 4.3–11.8eV (35,000–95,000cm−1) is investigated using synchrotron radiation. Rydberg series converging to the first four ionization limits at 10.52, 10.74, 11.21 and 11.30eV corresponding to excitations from the 3b1, 2b2, 1a2, and 4a1 orbitals of CH2Br2 are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the bromine lone pair orbitals. Assignments of the ns Rydberg series are revised and the np and nd Rydberg series are assigned for the first time. Observed vibrational features accompanying the 5p and 4d Rydberg states are assigned exclusively to the totally symmetric (a1) –CBr symmetric stretching mode (ν3) in contrast to the earlier assignment to ν3 and –CH2 bending (ν2) modes. The Rydberg and valence transitions observed in the present experiment are found to be in good agreement with the vertical excited states calculated using the TDDFT method. The calculations are further used to infer the valence transitions responsible for the broad intensity pedestals underlying the Rydberg transitions. The assignments are confirmed using isotopic substitution studies on CD2Br2 whose UV–VUV photoabsorption spectrum is reported here for the first time. This work presents a consolidated analysis of the UV–VUV photoabsorption spectrum of dibromomethane. [Copyright &y& Elsevier]